This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Thursday, July 31, 2014

Los Angeles geyser on Sunset Boulevard!

Back in the 1970's I used to run on the UCLA track near Sunset Boulevard. Two days ago, a 93-year-old water pipe and a 58-year-old pipe broke under Sunset Boulevard near the track, sending a pulsating geyser of water high into the air.  You can view a video of it here (the video symbol in the center of the photo doesn't work because it's just a frame grabbed from the CNN video).

The track was flooded, as well as newly rennovated ($136 million)Pauley Pavilion, the home of UCLA basketball named in honor of the famous coach of winning teams back in the 1970's. At its peak, the broken pipes were sending 35,000 gallons of water per minute onto the streets, with estimates of 20 million gallons released before the flood was brought under control. Maybe the tartan track will survive, the basketball court is questionable. Firefighters had to rescue some people trapped in a parking structure
Flooded track and athletic field at UCLA

If you watch the video, you'll see that the jet is strongly pulsating. This is likely due to an effect known as a "water hammer." The pipeline was a high pressure line, and these lines are subject to very destructive forces due to the water hammer effect (sometimes called a hydraulic shock). These are pressure surges that arise when the water changes direction or momentum.  In the news, you'll see reports that the pipeline had to be shut down gradually--that's because they had to minimize the potential for water hammers. If a pipe is shut off suddenly at the downstream end (where the vent is on Sunset Boulevard), the mass of water upstream is still moving and therefore can build up high pressure.  Such shocks can cause further breakage in the pipelines. (This is common in noisy old water/steam heaters in buildings.)

Photo of Pauley Pavillion
basketball court by Jason McIntyre
I found an interesting set of numbers on Wiki about this effect: "In hydroelectric generating stations, the water travelling along the tunnel or pipeline may be prevented from entering a turbine by closing a valve. However, if, for example, there is 14 km of tunnel of 7.7 m diameter, full of water travelling at 3.75 m/s,[3] that represents approximately 8000 Megajoules of kinetic energy that must be arrested. This arresting is frequently achieved by a surge shaft[4] open at the top, into which the water flows; as the water rises up the shaft, its kinetic energy is converted into potential energy, which decelerates the water in the tunnel."

See the Wiki article for more on water hammers. 

Wednesday, July 9, 2014

Gorgeous Air New Zealand plane! (And, how much can Dreamliner wings flex?)

The new Air New Zealand Dreamliner; photo from CNN.com here
This strays from "Geology In Motion," but I can't resist--the Boeing 787 "Dreamliner" is truly a beautiful plane in flight! It's wings can flex up to 26' (150% of max load).  All aircraft are required by the FAA to be able to withstand at least three seconds of 150% maximum loads (on all structures). In January, 1995, a 777's wings deflected 24' at 154% max load (I couldn't find the actual data to check the facts--I'm using www. flightglobal.com.) Boeing actually did a break test, which you can see in this Boeing produced video. They do not say how flexed it was when it failed, however, only that it was beyond 150%! Here's a cool video (in German) of a lab test showing the flex in a way that you can actually see-it's huge--definitely worth watching this one all the way to the end to see the failure! Here's an explanation that I found on this aviation.stackexchange.com site:

"The amount of flex is really a product of the material. The wing requires a specified ultimate strength; with metal, that translates into a given amount of flex. This can be varied within limits, but it is really the material, its stiffness to yield point ratio, and its fatigue properties, that control how much flex you are going to end up with. CFRP is a very different material, and has much less stiffness for the same yield point, and has essentially no fatigue problems. This is beneficial in that it provides a smoother ride in turbulence; the wing acting essentially like a giant leaf spring. There is some lift lost due to the nature of the curvature, though. However, this is relatively small."

Monday, July 7, 2014

Super-typhoon Neoguri ("racoon") approaches Okinawa

Super-typhoon Neoguri, first super-typhoon of 2014
imaged on July 6 (?) by NOAA/EPA
A quote from my (hard working scientist) friend on Okinawa sent on Monday night, PDT: "The storm has been here since yesterday night. So far nothing comparable to the big storm last year. That one was only category 3 by the time it reached Okinawa, but a typhoon's power is concentrated in a narrow ring around the eye, and last year we were right there in the eye.  The current storm might be stronger but we are only exposed to the outer arms, at least so far, and the effects have been mild. The sound was terrifying last year; now it is merely annoying....I should be working, of course, but I have found that it is not easy for me to work during a typhoon. Perhaps I should try some cooking. I need some pasta sauce, and I have got all the ingredients in the fridge!"

Three inches of rain PER HOUR??? I wonder for how many hours!! Waves up to 14 meters (45')? I have friends on Okinawa and  wish them well (and also asked them to send a first hand report!) The storm is expected to work its way up to mainland Japan by Wednesday. The highest danger is for Miyako-jima, in the center of the archipelago.
     As I write this (Monday a.m. PDT) gusts of up to 270 km/hour (160 miles per hour) are expected, and the Japanese national weather agency is saying that this may be the worst storm in decades. This is the first storm of hurricane season there, and it is apparently hitting rather early in the season. The US evacuated some of its plane from Okinawa in advance of the storm.

Projected path and conditions, from the Japan Meteorological Agency
In my last post, I started by pondering the effect of El Nino on droughts in Japan, but did not address typhoon. But, according to research led by Ryuzaburo Yamamoto at Kyoto University and the Japan Weather Association, El Nino increases the strength of typhoons and increases typhoon-related damage in Japan. The conclusion was based on a study of typhoons over the 48-year period between 1951 and 1999. El Nino's push warm water toward the coast of Peru. Therefore El Nino storms travel further than non-El Nino storms across the Pacific toward Japan, giving them more time over warm waters before reaching Japan.

Damage from typhoons in such years is, on average, three times greater than in La Nina years, even though the average number (16.1) is less than in La Nina years (18.2). Pressures in the center of the typhoons, a measure of their strength, are, however, lower in El Nino years, producing stronger typhoons. The average number of days in which the strength (as measured by the low core pressures) was 46.3 days for El Nino and only 26.9 days for La Nina years. Average storm radius was 235.9 km vs. 180.4 in La Nina years, another measure of the effect of El Nino.

In summary, here, in the last figure, is the Accuweather forecast for the west Pacific for 2014.

Friday, June 20, 2014

Is an El Nino in the offing?

The record of La Nina's and weak El Ninos that
have occurred since the
last powerful El Nino in 1997-1998. From the
Nikkei Asian Review cited in the text.
The Nikkei Asian Review had an interesting article on June 12 saying that, although it is too early for certainty, there are hints of an El Nino this winter, and perhaps an El Nino that will be as strong as the one of 1997-1998. Since an El Nino means a cooler summer in Japan, it could portend a problem for rice growers in Japan.
     What has caused this speculation? Normally the trade winds blow from east to west, but in January and February there were two strong westerly bursts, followed by two "slightly less powerful" ones in March and April. If such bursts continue and develop into a reversal of the trade winds, an El Nino will occur. Warm surface waters of the Pacific will be pushed easterly toward the west coast of South America.
     Although highly speculative at present, a switch back to El Nino conditions may have significance in the bigger picture. La Nina conditions have permitted storage of heat in the deep waters of the Pacific. Storage of heat in the ocean takes it away from the atmosphere, keeping global warming in check. The haitus in global warming in recent years may be due to this string of La Nina events. A switch back to El Nino conditions, particularly if they last a decade or more as is common, could result in a resumption of global warming conditions. Here's a link to a Slate article on the possibility, and here's Cliff Mass's comments about it, as well as the quality of forecasts made in April, the time of these two articles.
Typical El Nino weather conditions. From here.
     The June 5 ENSO Diagnostic Discussion issued by the Climate Prediction Center says that the chance of El Nino is 70% during the Northern Hemisphere summer and 80% during the fall and winter.
     The Asian Review article also notes that because of budget cutbacks, 24 of NOAA's 55 ocean buoys in the tropical Pacific are unable to operate and send data needed for monitoring the situation.

Monday, May 26, 2014

Massive mudslide in Colorado on Sunday, Memorial Day weekend

The Sunday, May 24, Mesa County Mudslide. Dimensions
are uncertain, but at least 0.5 mile wide, 2-3 miles long, and
250 feet deep. Photo by Aaron Ontiveroz, Denver Post
as published here.
UPDATED: May 26 and May 28, 2014

Heavy rain again (after Oso and Afghanistan) appears to have triggered a massive mudslide, this time in the Grand Mesa country of Colorado. The location is 11 miles southeast of Collbran, about 40 miles east of Grand Junction. The area is remote, cell coverage is sparce to nonexistent, and news is just starting to break of this event. More than 3/4" of rain hit the area on Sunday.  Three men, locals who had gone into the area to investigate the possibility of a smaller slide when they noticed problems with their irrigation water, are missing.

Update May 26: Weather.com meteorologist Jon Erdman said that Grand Junction picked up 0.42 inches of rain on Sunday, and that higher totals atop Grand Mesa above the slide were likely Heavy rain is expected toward the end of the week and into the weekend, making the slide area still very dangerous.

Location of Collbran ("A") relative to Grand Junction
This slide (dimensions in figure caption) is significantly larger than the Oso, WA, landslide which measured about 1500 feet wide, 4400 feet long, and 30-70 feet deep. The area is on U.S. Forest Service and private land. No structures or major were involved. The sheriff has reported that the person who reported the slide heard a sound like a freight train, and that "the slide came down with so much force and velocity that it came to a hill and went up and over a hill and then came back down--a significant hill." The area remains unstable as of this writing (2:30 PDT, Monday).

Another photo of the mudslide taken by Aaron Ontiveroz
as found here.
The area where the landslide climbed a hill is at the
upper right of this image. It is at a sharp bend in
the path of the flow. Photo also by Aaron Ontiveroz,
Denver Post.
I couldn't find any references to previous slides in this area, but did find a 2013 paper entitled "Characteristics of Landslides in Western Colorado, USA", focused around the Somerset-McClure Pass area only about 50 km away (as the crow flies, a lot longer by any access roads as they are separated by the Grand Mesa National Forest and some rugged country.) The authors of this paper are N.R. Regmi, J.R. Giardino, and J.D. Vitek, and it is published in Landslides, on-line 05 June 2013. The Colorado Geological Survey also has an extensive website and inventory program that can be viewed here. It is painfully clear that western Colorado has major landslide problems.

Update 5/26: Geologist Jonathan White, with the Colorado Geological Survey, said that another slide seems inevitable because of the buildup of water in a depression created by the big slide. The depression that he's referring to may be (???) at the top of the slide in the lower photo. Water has been flowing into the depression already, but White says that it's impossible to tell when the next slide could occur--perhaps even years from now when people have forgotten the danger. The terrain is too unstable for any work to drain the water, and Jonathan Godt, a geologist with the USGS says that "practical engineering measures for things of this size are pretty limited."

Update, May 28: The last photo in this set shows the mobility of the flow as it climbed up and over the ridge in the upper right where it took a sharp right turn in the flow path. From a topo map of the area, it looks like the elevation of the headwall at its crest is roughly 9700 feet, and that the elevation where the slide began the right-angled turn is about 8000 feet. This drop of 1700 feet elevation occurred over about 1.5 miles. It looks like the topped a ridge that is a couple of hundred feet higher than the valley bottom (where it spilled over the ridge on the right side as viewed in the photo, left side when viewed from the flow direction). Since the slide was reported to be several hundred feet thick, it's not clear how much of this spill-over was due to the energy of the flow versus the thickness of it as it approached the ridge. Note the erosion line of the trees in the extreme upper right of the photo and how it appears to be close to the elevation of the top of the overtopped ridge. This suggests to me that the overtopping had a significant contribution just due to the huge thickness of the flow.

There is also a very unusual change in texture between the material that did not go up the hill (streamwise right) and the material on the proximal side of the hill (rough vs. smooth?), and I'm wondering if bigger particles in the flow got left behind as the more fluid finer-grained material climbed the hill? Did the flow climb the hill on its left side (toward the top of the photo) and then fall back toward the valley on its right side (the smooth area on the right side of the ridge)? Is it possible that the "slide" changed to a muddy flow at this point--certainly there is a change in the texture of the slide downstream from this area, as pointed out today by Dave Petley on his AGU landslide blog. Field observations will be needed to clarify many questions.

After making the first turn to the right, the material then turned to the left, spread out around and over a region of ridges (emanating from the hill on the streamwise right side of the flow in the middle of the photo), and eventually exited from the main channel, narrowly missing a developed site of some sort (natural gas, fracking facility??).  I'm not sure what this is, and the WWW is full of speculation. The elevation of the toe of the slide is about 7300 feet, making the total drop of the order of 2400'.

Friday, May 23, 2014

Camelopardalid meteor shower

In the lower right half of the image you can see the shape
of a giraffe and the location of the Camelopardalids meteor
shower tonight. Courtesy Science@NASA.
In the early hours of the morning May 24 (3-4 a.m. EDT and midnight and 1 a.m. PDT), a meteor shower from Comet 209P/LINEAR will dazzle observers with up to 1000 meteors per hour. This is a new shower, occurring five days before Comet 209P/LINEAR makes its closest approach to the earth (8 million kilometers) on May 29. The earth passes through the comets orbit, which is strewn with debris shed from the comet on previous passes. Amateur and professional observers are excited about the potential for this to be a "meteor storm", levels of 1,000 per hour, and many will be at telescopes or out on the lawn with pencil and paper counting them.
Plot of the Earth's path through the meteor shower
by Jeremie Vaubaillon.

Comet 209P/LINEAR was discovered on Feb. 3, 2004, by the Lincoln Near-Earth Asteroid Research (LINEAR) research project. It orbits around the sun with a period of roughly 5 years, with an aphelion out near Jupiter's orbit. As a result, calculations show that its orbit has been perturbed by the gravitational pull of Jupiter over the past few centuries, at least as far back as 1798. Most particles in the shower are smaller than a grain of sand and burn up high in the atmosphere.

Scientists are being cautious, predicting a few hundred meteors per hour to be on the safe side, but almost all of them express hope for the storm-level of 1000 per hour. Comet 209P/LINEAR is a small comet, and has in recent passes near the earth, a fairly low dust production. Observers in the United States and southern Canada are in the best position to see the shower.  The moon is a waning crescent, just four days from the dark new phase and will not be a hindrance.

Fred Whipple first developed the idea that comets were "dirty snowballs" orbiting the sun.The meteoroids are formed when a comet passes by the sun and some of the ice (water, methane, ammonia or other volatiles) sublimates, releasing the small silicate particles bound in it. The meteoroids spread out around the comet, eventually, after many passes by the sun, filling in the entire orbit.

Here is a link to Mikhail Maslov's website on the 2014 meteor shower, and here is a post by Robert Lunsford.

Sunday, May 4, 2014

Afghanistan landslide of May 2, 2014

The Badakhshan landslide, showing its extreme mobility
Photo by Blal Sarwary as posted on Dave Petley's Landslide Blog
Links to sources are in adjacent text.
Our thoughts and condolences are with the survivors of the devastating landslide in Afghanistan.

On his Landslide Blog, David Petley has drawn from the Twitter feed of Bilal Sarwary, a BBC reporter on the spot where the landslide occurred in the Argo District of Badakhshan Province in Northeast Afghanistan was a failure in deposits of windblown sand (loess) after a period of heavy rainfall and flooding. With no hope of recovering the bodies of victims, Afghanistan has declared the site a mass grave and dedicated Sunday as a day of national morning.

While Petley conservatively says that "is now believed to have killed at least 350 people," the news media are reporting as many as 2000 deaths. Even the conservative value makes it the worst landslide to date in 2014, and the death toll is bound to increase as reports of missing people come in. There are about 4,000 survivors in need of care. The landslide is believed to have occurred in two phases, separated by enough time that people from adjacent villages had arrived to help with rescue efforts, only to be caught by the second landslide. Note that the Oso landslide in the U.S. two months ago also had two phases, but these were separated only by about 4 minutes.

Location of the landslide
From Cnn.com here
Petley based his conclusion that the landslide had occurred in loess on the high mobility of the landslide shown in the photo above, and the observation that there are no large boulders visible in the photo, an indication that the sediments were probably wind-laid. Landslides in loess are not unknown in this part of the world, though much more reported from China than Afghanistan. Loess covers huge areas of the world (see map), including some that are prone to large earthquakes (up to M8.5), such as this part of Asia. Loess covers 6.6% of the total area of China. In the middle part of the Yellow River in China, it can be 1000' thick. Chinese scientists have pioneered may of the studies of loess failure.

Map of location of loess deposits from here.
Loess was deposited during the Pleistocene by winds blowing across deposits of the receding glaciers. The climate was dry and, in many areas of the world, such as China and Afghanistan, has remained relatively arid since the loess was accumulated.  It is highly porous and has a weak cohesion, giving it rather unusual geological engineering properties.* It can contain up to 20% water in so-called "macropores", see photo. The mosaic structure of loess gives it strength, but the existence of macropores makes the strength vulnerable under seismic shaking or heavy rains. When loess fails due to heavy rains (or over-irrigation), it is because the pores fill with water, destroying the strength of the soils. However, even relatively dry loess has been known to fail.
Macropores in loess, Tianshui, Gansu Province
from the Zhang reference cited at *

There are a number of ways in which landslides can be triggered in loess areas. Zhou et al. (2002)** have classified loess landslides according to three "indicators": (1) the landslide material; (2) the position of the surface along which the slide moves; and (3) the mechanism of movement. Landslide materials can be either loess alone, or loess and underlying soil/rock, such as laterite, which is common in this part of the world. The development surface can be solely within the loess, or within the loess and underlying soil/rocks. The mechanism of movement can be slow gliding, or "collapsing-gliding." They point out that (in China, but I'm assuming that this applies to Afghanistan), landslides occur in (a) the rainy seasons of July, August and September, and (b) the ice melting period of March, April and May. This Afghanistan slide, if triggered by heavy rains, has occurred early compared to to the (Chinese) rainy season.

Water affects/causes landslides in three different ways: rainfall, surface water, and ground water. Both rainfall and surface runoff can increase the water content of a hillslope, and even saturate it. When saturated, e.g., below the water table, the cohesive force of the soil and the frictional force of layers within it can greatly decrease. Surface water also erodes the topography, particularly in tectonically active areas of the world. This leads to unstable, steep slopes prone to landsliding. Changes in groundwater affect the mechanical characteristics of soil. For example, when water content of soil increase up to 35%, shear-resistance decreases by 60% (Zhou et al., page 160). Ground water can also dissolve components of the soil, changing its composition and strength. 

Zhou et al** point out the potentially large influence of humans on inducing landslides. Remembering that this article was written in 2002, so that the statistics are not current, they report that in the 20 years preceding their report, population had reached 81.49 million, accounting for 7.8% of the nation's population. With this increase of population, "irrational human activities" such as excavating foothills, filling hillsides, and over-cultivating accelerated natural disasters. Because industry is relatively sparse in these loess areas, the output of industry and agriculture was below the national average, accounting for 6% of that of the nation. These areas tend to be poor, and frequent natural disasters "are major factors hindering the social and economic development of [the] loess area." On the borders of the loess plateau, flat ground is rare and slopes are universally leveled to build houses, and many people live in cave-houses cut into the slopes. All of these activities reduce the support at the bottom of the slope and decrease its stability. Vegetation, that normally stabilizes a slope, has been heavily impacted since the 1950's by deforestation. 

ASIDE: Although there appears not to have been an earthquake that triggered the Afghanistan landslide, it is worth keeping in mind that this huge region in Asia is subject to major earthquakes, which commonly trigger landslides if they are greater than M6, and wet weather.

COMMENT ADDED APRIL 6: John SanFilipo has commented that "There was a small (?) quake just west of the slide area near Rostaq reported on 12 April. Thanks, John! It will be interesting to see if anyone looks for a cause and effect.

On December 16, 1920, the M7.8 Haiyuan earthquake (also called the 1920 Gansu earthquake and estimated by some to have been as high as M8.7) killed at least 100,000 people, probably double this number according to the USGS. A Chinese paper in existence at the time Ningxia Daily, reported 240,000 killed. Strong trembling occurred for 10 minutes. Reportedly, 500,000 houses and cave dwellings collapsed.* The shaking occurred over an area of about 50,000 square kilometers, most of which is covered by loess. The landslides triggered by the earthquake blocked rivers and buried villages and farmlands.

There are several prominent consequences of earthquakes. (1) Fissures are produced both near the fault and far away from it. Near the fault, the fissures are primarily related to tectonic deformation and are spread on both sides of the fault. However, in zones of weak shaking far away from the fault (all the way through much of the loess region in China) fissures can be formed by subsidence in the loess caused by the loss of strength of the loess when it is shaken (liquefaction). (2) Landslides occurred in three types of areas in the Haiyuan quake: (1) near or in the high intensity shaking zone. However, because the soil cover was thin in this region, the landslides were quite small. (2) In an area of moderate shaking, more than 1000 sq. km. in area, there were 650 landslides reported (see the Zhang ref below), and 41 of them dammed rivers to form so-called "barrier" or "quake" lakes. These slides were big because the area was hilly and covered with loess ranging from 20-50 m (66-165') in thickness. (3) More distal regions where the damage caused by the landslides was more severe than that caused by the shaking of the quake itself. This area covered another 1000 sq. km.

*Zhang, Zhenzhong, Geological disasters in loess areas during the 1920 Haiyuan earthquake, China, GeoJournal, 36 (2/3) 269-274, 1995.

Zhou, Jin-xing, Zhu Chun-yun, Zheng Jing-ming, Wang Xiao-hui, Liu Zhou-hong, Landslide disaster in the loess area of China, Journal of Forestry Research 13(2), 157-161, 2002.

Friday, April 18, 2014

Mexico's 7.2 earthquake and it's early warning system

Seismic record from the Guerrero April 18 earthquake
From Earthquake-Report.com 
Note on May 4: I am very grateful to Jenda Johnson who pointed out an obvious typo--I had reversed P and S waves in the third paragraph! Sometimes the mechanics of doing this blog just numb out the brain! Thanks, Jenda!

A magnitude 7.2 earthquake struck in the Acapulco/Mexico City region early this morning, a Good Friday holiday morning when many residents had apparently slept in or gone away on vacation. The earthquake lasted about 30 seconds. The epicenter was in the state of Guerrero, north of Acapulco. The U.S. Geological Survey has an automatic damage estimator here. The USGS estimates deaths between 1-100, and economic losses between 1-100 million; Max Wyss's estimator at WAPMER predicts 0-50 killed. In spite of damage reports there have been no reports (10 hours later) of fatalities, so there are likely to be very few if any as more information comes in. Why?

One reason that many buildings in the area are built to be quake-resistent because of the history of previous earthquakes. But another reason is that Mexico has an early warning system for earthquakes, and the news has reported that a warning went out about 2 minutes before the quake (I have not been able to independently verify this).

Mexico instituted a Seismic Alert System (SAS) for Mexico city as an experimental project in August 1991. By monitoring the arrival of the faster compressional ("P") waves, warnings can be issued before the arrival of the stronger shear ("S) waves. The system gives, on average, about 60 seconds warning for earthquakes generated in the Guerrero Gap. The quake was the result of a thrust motion where the Cocos plate is being subducted below the North American plate at a rate of about 65 mm/year. The Guerrero Seismic Gap is a ~200 km long segment of this plate boundary that has experienced no significant earthquakes since 1911 (M7.6 at that time). It is thought that an earthquake of magnitude greater than 8 is possible if the entire gap were to rupture at the same time.

The development of the SAS was sponsored by the Mexico City Government, beginning operation in 1991. By the end of the first year, it was experimenting with providing warning to some public elementary schools, and was opened as a public service on commercial radio stations in 1993 after a successful alert that gave 65-73 seconds advance warning during two Guerrero earthquakes (M5.8 and M6) on May 14, 1993. Extensive planning for dissemination and education for the public followed. By 1998 the SAS detected 681 seismic events, 12 of which were strong enough to trigger the general early warning signals in Mexico City, one false one, and one earthquake well detected, but not warned. In the one false alarm, phenomena that were feared--such as panic that could cause injuries--did not occur, but it was realized that many members of the public had not been trained, and training of the public remained a high priority.

The advantages of an early warning system are numerous: Casualties and fatalities are reduced by making people aware that strong ground shaking is imminent. Tsunami calculations can be initiated earlier. Traffic such as trains or subways can be stopped or slowed. The disadvantages or risks are the alerts may not be quick enough  in areas close to the focus of the earthquake and subjected to strong shaking, that there can be false alarms, and that the technique is not good if there are multiple earthquakes close in time or location. Hence, there are tradeoffs between speed and accuracy. Continuous citizen education and awareness must be maintained, and a wide variety of channels of communication must be used to ensure wide dissemination. The private sector must be incorporated into the early warning system so that appropriate services/operations can be shut down for safety.

The Polar Vortex: Good riddance!

I was in Chicago earlier this week, and it was freezing cold once again. My friends in Illinois have had a miserable winter and it wasn't letting go easily. As I started to look at why it was so cold there mid-April, I discovered that I had a post that I started in the winter and didn't finish. It was about the "polar vortex," and I realized that I don't know very much about this thing. So, belatedly, here's my introduction to myself about the polar vortex. For more details and the references from which I took this material, see Skepticalscience, an excellent resource on climate, and the weather.com post here.
     We live in a portion of the atmosphere called the troposphere, and most of us have heard of the stratosphere, the layer of the atmosphere above the troposphere. The boundary between the two is the tropopause, and it's altitude varies considerably with the seasons. We in North America also live in the mid-latitudes, a region of mild temperatures that extends very very roughly between 30 and 60 degrees latitude. North of this (in the northern hemisphere, the reverse in the southern hemisphere) is the very cold polar air. The boundary between the two is the Polar Front, a collision zone between the warm moist and cold dry air.
Typical polar vortex position on the edge of the
polar high (not shown). Graphic from weather.com.
     The collision zone between the two big air masses results in very high winds at high altitudes at the top of the troposphere. In the northern hemisphere, this is referred to as the Polar jet stream. It is strongest in the winter when the temperature contrast is the greatest between the polar air (because there is no sunlight) and the temperate air of the mid-latitudes (which may seem cold, but is nothing like the Polar air!)
Distorted polar vortex (from same weather.com site as above.
      The polar regions are areas of high atmospheric pressure covering the north and south poles. On the borders of the polar highs are polar vortices (sometimes called polar cyclones or polar lows). These are permanent areas of cold, low pressure in the upper atmosphere that draw their energy from the temperature difference between the cold polar air and the warmer air of the midlatitudes. They are, therefore, strongest in the winter.    There are typically two polar vortices in the northern hemisphere--one near Baffin Island and the other over northeast Siberia.The air in them spins counter-clockwise. (Only one of these is shown in the graphics here.)
    The polar vortex is contained by our jet stream (typically at around 35,000 feet altitude where airplanes fly).  The jet stream is normally rather loopy, an instability known as Rossby waves, that arises because the Coriolis effect has a different magnitude at different latitudes. When one of loopy parts of the jet stream tokes an unusually deep plunge southward into the midwest, it brings air from the polar vortex and freezing temperatures southward. Note in both of the graphics that the main location of the low pressure zone stayed up north over Baffin Island, it's permanent home.
     The cold air aloft in the polar vortex sinks to the ground, displacing the lighter warmer (winter) air normally there. Rossby waves migrate, typically to the east, and so the disturbance moved on out of the midwest. Things  warm up, and Chicago goes back to balmy 30+ degree nights in the winter!

Tuesday, April 8, 2014

USGS updates information on Oso landslide

Here is an update on the Oso landslide from the USGS.

Wednesday, April 2, 2014

E-an Zen: 1928-2014

My great friend and mentor of many decades died on Saturday a.m. after a long battle with cancer.

His life was so rich that I'd like to share the tribute to him written by Andrew Alden on Geology.about.com:

Andrew, thank you for this wonderful tribute and for preserving my description of being in the field with him!

E-an, whereever you are, we all miss you!

"One of America's unsung senior geologists, E-an Zen, died on 29 March at the age of 85. Born in China, he emigrated to the U.S. and earned a doctorate in 1955 from Harvard. A 30-year career followed at the U.S. Geological Survey, then 23 more years on the faculty of the University of Maryland. He was basically a mineralogist, but his field skills were formidable and he made large contributions to Appalachian geology, metamorphic petrology, and mapping of northern Rockies. Anyone who's looked into the literature of those fields has read his papers. He earned his full share of awards: membership in the National Academy of Sciences, the Geological Society of America's Day Medal, the Mineralogical Society of America's Roebling Medal, the Geological Society of London's Coke Medal, and more.
In 1991 Susan Werner Kieffer, no slouch herself, recalled fieldwork with Zen: "I pride myself on being fit, but when I'm in the field with E-an, I'm always so out of breath that I can't talk, and thus I'm subjected to questions. For example, I was recently subjected to 3–4 days of questions about granites, migmatites, structural geology, epidote, and eucalyptus while working with E-an at the Cooma Granite in Australia. . . . I was so out of breath and confused by the rocks we were in that I wasn't providing him any feedback. E-an could sense my frustration and, with the sensitivity so characteristic of the man, politely changed the questions: to ones about scientific ethics, education, literacy, policy, religion, or philosophy—subjects about which he is deeply concerned."
Those wider concerns marked Zen's tenure as president of the GSA in the early 1990s. In hisPresidential Address of 1992, published in GSA Today, he told his audience, "Science is too important to be left to the scientists. Geology directly impinges on human welfare and so cannot be an ivory-tower science. Conservation of the environment, discovery and recovery of Earth's resources, avoidance of natural hazards, disposal of wastes, forecasting of global change, decisions on land use, equity for the future—these and other issues need geological knowledge both for technical resolution and for guiding public policy. Public policy needs public support; we ignore the public at our peril." He went on to discuss scientific literacy, ethics, education and geologists' obligation to do public outreach.
I wasn't there that day, but I recall being impressed when I read his words, and I continue to take his ideas seriously in my work here on About.com."

Tuesday, April 1, 2014

UPDATED: Chile earthquake and tsunami on April 1; 900,000+ evacuated

Map of South American
subduction zone and significan
earthquakes. From
Susan Bilek reference listed
below at **.
A powerful earthquake off shore of Chile has generated a tsunami that has already produced six-to-seven-foot waves that have struck the beaches. The earthquake was centered about 60 miles northwest of Iquique and at a depth of about 12-13 miles. A tsunami warning has been issued for Colombia, Panama and Costa Rica, and geoscientists are working to determine the magnitude of tsunami waves at Hawaii and as far north as North America. UPDATE: Tuesday afternoon--The government of Chile has reported over 900,000 evacuated; CNN has reported a million. The major problem appears to be serious structural damage to poorly built homes.
      Chile has been subjected to powerful earthquakes several times over the past century: November 11, 1922, a magnitude 8.5; May 22, 1960 a magnitude 9.5; February 27, 2010 a magnitude 8.8, and today, a magnitude 8.0 or 8.2 (magnitudes are being revised as I write this). Charles Darwin experienced the 1835 earthquake in southern Chile in 1835, one that had a magnitude of 8.1 or 8.2. It triggered a tsunami that destroyed Talcahuano and devastated Concepcion. In this tsunami, a schooner was swept 200 meters inland. The earthquake took place in the middle of the day and inhabitants had time to run into the hills so the death toll was fortuitously low. Historical records going back to the 1500's suggest other great earthquakes.
Geometry of the plates around South America
From Wiki here
     Why is Chile so prone to these quakes? Off the west coast of Chile, the Nazca plate is diving down (being subducted) below the South American plate.  The Nazca plate is relatively young, having formed when the now-defunct Farallon plate split about 22.8 Mya split into the Nazca and Cocos plates. The plate is being subducted at a rate of 3.7 cm/year, one of the fastest motions of any tectonic plate. It dives so deeply under the South American plate that it even influences the geology and geography of Bolivia far inland to the east. The 1994 Bolivia earthquake of magnitude 8.2 occurred on this place and is renowned as the strongest earthquake occurring deeper than 300 km.
Susan Bilek's model of the role of heterogeneities
in subduction zone dynamics.
    In a nice review paper**, Susan Bilek discussed how the heterogeneity of the subducted plate causes there to be a wide variety of rupture modes along this zone, ranging from "magnitude >8 events during one century followed by smaller ones in other time periods, as well as unusual tsunami events." Her idea of the effect of heterogeneity is shown on the figure attached. The idea is that as various geographic features on the subducting plate--such as seamounts and ridges--enter the subduction zone, they change the friction in the zone. This acts, along with variations in the thickness of sediments in the overriding plate and in pore pressure in the sediments, to produce variability in the slip mechanisms along the fault.

**Bilek, Susan L., Invited Review Paper: Seismicity along the South American subduction zone: Review of large earthquakes, tsunamis, and subduction zone complexity, Tectonophysics, 495 (2010), 2-14.

Sunday, March 23, 2014

Major landslide in the state of Washington, USA: UPDATE(S)

Oso, Washington, is at the marker. Arlington is to
the left, Darlington is to the right by the magnifier bar.
The slide at Oso cut off Highway 530 between
Arlington an Darlington.
**Chapter 4 in my book "The Dynamics of Disaster" is titled "The Flying Carpet of Elm" and discusses the factors that influence landslides. It also discusses the mother of all landslides, a slide that occurred 50 million years ago in Wyoming, covering 1300 square miles and traveling more than 30 miles.

Monday a.m. Update and correction: The Seattle Times reports "108 reports of missing people." CNN.COM has reported "Washington landslide: 8 dead, 108 missing." Emergency managers are saying that they have a list of those reported missing but that it does not mean all of them were killed. I thank a Washington reader for pointing out this difference.

Cliff Mass has a post on March 24 that describes the meteorological conditions leading up to the landslide.

The SeattleTimes is providing excellent coverage.

Take I-5 north from Seattle about 50 miles through Everett toward Arlington, and turn east onto Highway 530, which takes you south of Mount Baker. Along this road is the small town of Oso, population about 200. At about 11:00 this morning, a massive slide of mud, rocks and trees travelled a mile down near Oso, taking down at least 6 homes, killing at least three people and trapping others. (As of Monday morning, 18 are still missing.) Three more are reported in critical condition.The slide was at the 29400 block of SR 530 near milepost 37, between the cities of Arlington and Darrington. It landed in the path of the Stillaguamish River, reducing its level at one spot from about 3.1 feet to 0.9 feet, indicating that the slide appreciably blocked the river. The state hydrologist reported that 15-20 feet of debris blocked the river, and that its flow had been reduced to about 1,000 cubic feet per second. Other reports have said that the slide is 135 feet and 180 feet deep.
Image of the landslide from Seattle Pi
     In situations like this where an earth slide blocks a river, the concern is that water will pond behind the blockage forming a lake, and that the blockage--a dam--will suddenly collapse and release the water catastrophically. The National Weather Service has said that for this to occur the water would have to be blocked for 36 hours, and then released within an hours time.
     Snohomish county officials have advised residents downstream of the slide to evacuate their homes as "It is going to break loose and the question is how and where" (John Pennington with the Department of Emergency Management (quoted from King5 news here).
     The National Weather Service has issued a flash flood watch through Snohomish County through Sunday afternoon. The ground is saturated with water from recent rains, flash flooding is possible, and the saturated ground combined with rain is believed to be the cause of the Oso mudslide. Flood alerts have been issued both upstream and downstream of the slide.
Image showing the abundance of water at the base of the slide.
From Seattle Pi
     UPDATE: Several serious hazards remain in this area. The first is the landslide material itself. It is composed of a lot of fine grained materials, sand and clay, that form a nasty, hazardous substance called "quick sand" or, depending on particle size, "quick clay." Quick clay was the cause of the landslide from which the title of my book chapter, The Flying Carpet of Elm, was taken. I'm not sure of the exact geology of the Oso area, but it looks like the materials are a watery mixture of very fine particles. These materials, if undisturbed, can be very strong, forming the slopes on which homes and other buildings were built in the area. But, when disturbed, in an instant under certain conditions of stress, the state of the materials changes to liquid. Although some of the water visible in the images (to the right) may be from the dammed river, it looks like a lot of the water came from within the landslide itself.
     The second major hazard arises from the partially blocked river. These landslide dams are not strong and eventually, sooner or later, the water will either erode through the toe of the dam (the best outcome) or the dam will break (worst outcome). Until equilibrium is restored, downstream residents and infrastructure are at risk. Apparently a major bridge on Highway I-5 is being watched carefully because the pilings holding it up are old and not as deep as would be built under newer bridges.

Here are more references from Dave Petley's AGU Landslide Blog.

Friday, March 14, 2014

Happy Pi Day!

Young folks celebrating Pi Day at San Francisco's
Exploratorium. Photo from CNN.com here.
When I saw the wonderful faces of these students celebrating Pi day at the, yes again I'll use the word, "wonderful" Exploratorium in San Francisco, my mind went back to life in the 1950's and the science teacher who inspired me. So, I thought that I'd put up a photo of a 9th grade exam that I've saved all these decades, and hope that these kids get the inspiration that I got from that teacher (in spite of the fact that I wasn't a boy and couldn't be made a knight!!).
     It was 1959, the space race was in full swing, and mimeograp'ing was the technology of the day for producing student exams.  This was a general science class and was one of the physics components. The purple ink questions have long faded away, but perhaps you can guess them from the answers:

32 ft/sec^2
32 ft/sec^2
100 ft/sec
(1/2 provided your initial velocity is zero)
320 ft/sec
160 ft/sec
1600 ft
200 sec
49,000 m
980 m/sec
102.4 ft/sec
100 lbs
37 degrees
45 degrees....

Then there is the comment: "You deserve a medal. If you were a boy you should be made a knight." Gzsh,
shouldn't have at least told me I could be a Dame?

And, in pencil on the left
side of the astronaut sketch written
a bout ten years later when I
had done an internship at NASA,
someone wrote in "An official of
the NASA says there are no
provisions as yet for a woman
astronaut. The exploration
rockets, however, he says,
do provide for 120 pounds
of recreational equipment."

Times have changed, kids, go for it
on Pi Day!! You are great!

Monday, March 3, 2014

Corvettes and sinkholes: what is a sinkhole?

1962 Black Corvette
from Roscoe-Restoration.com
A few weeks ago, a sinkhole swallowed eight valuable Corvettes at the National Corvette Museum in Bowling Green, Kentucky, and today the news is that the recovery of the cars has started. Here, from CNN.com, is a list of the cars:
-- a 1962 "Black Corvette"
-- a 1984 PPG pace car
-- a 2009 ZR1 "Blue Devil"
-- the 1992 white "1 Millionth Corvette"
-- a 1993 ruby red "40th Anniversary Corvette"
-- a 2001 Mallett Hammer Z06 Corvette
-- the 2009 white "1.5 Millionth Corvette"
-- a 1993 ZR-1 Spyder
The museum estimated millions of dollars in damage. The sinkhole was approximately 40 feet in diameter and 20 feet deep.
    What causes sinkholes? A glimpse at this fascinating map of Kentucky groundwater flow routes confirms the well-known fact that sinkholes are not uncommon there, given features with such names as "Sinking Creek", "Auburn Bluehole," and "Lost River Rise."
Simplified geology of Kentucky
from Geology.about.com.
     Bowling Green lies in the south-central western portion of Kentucky in rocks of Mississippian age (359-323 million years ago; comprise about the lower (oldest) 2/3 of the Carboniferous rocks). In Kentucky, the Carboniferous Series rocks (359-299 m.y. ago) contain massive amounts of coal, and are so abundant that they are subdivided into the Mississipian and Pennsylvanian. They are the thickest in the Appalachian Basin in the eastern portion of Kentucky, and the Illinois Basin in the west.
      The Mississipian rocks of western Kentucky are comprised mostly of limestones, shales and sandstones. The limestones contain a oil reservoirs underground and where exposed at the surface, the limestone is quarried--the Reed quarry producing more limestone than any other quarry in the U.S. The limestone also includes Mammoth Cave, part of the Mammoth Cave-Flintridge system, the longest cave system in the world. These limestones were deposited in shallow seas.
Karst features from UTexas here.
     Limestone is dominantly composed of calcium carbonate, CaCO3. This mineral is quite soluble in water containing CO2 through the reaction:

CaCO3 + CO2 + H2O → Ca(HCO3)2

There is a similar reaction for aragonite, a magnesium containing carbonate that is another common component of limestone. The dissolution of calcite and aragonite produces caves underground. The caves are often connected through fissures leading to extensive networks. As the dissolution proceeds over time, the caves approach the surface and when the surface rocks or soils can no longer support the load of trees or human structures, they collapse, producing sinkholes. Notice also the sinking streams on the illustration, and the name of the stream "Sinking Creek" mentioned above.


Tuesday, February 11, 2014


Ice storms are very bad for trees! NOAA image from here.
(I have no idea how blogger put the "U" on this post, nor any idea how to get rid of it....Grrrr....)

The CNN  headline today is "Forecast: Historic, crippling, catastrophic ice: Atlanta prepares for the worst." It is well known that freezing rain storms occur frequently in the southeastern part of the U.S. They are beautiful, but dangerous and costly.

And, they are not all that rare. Montreal, Quebec, typically receives freezing rain more than a dozen times a year. In 1998 the great North American ice storm of January 5-9 was one of the most damaging and costly ice storms in North American history, causing massive power outages on the east coast. Eastern Canada bore the brunt of the storm. Millions were without power for days to weeks to even months. 35 people died, a significant number from carbon monoxide poisoning from generators they used to try to keep themselves warm. The effort to reconstruct the power grid led to the biggest deployment of Canadian military personnel since the Korean War.
What makes an ice storm? The attached graphic from Gay and Davis summarizes the types of precipitation nicely and, when I read their paper, I learned a new word: "hydrometeor." It is "any water or ice particles that have formed in the atmosphere or at the Earth's surface as a result of condensation or sublimation." Examples are clouds, fog, rain, snow, hail, dew, rime, glaze, blowing snow and blowing spray.

Vertical temperature profiles in the atmosphere and
the kind of storms that they produce. From Gay and Davis,
1993 here.
The graphs shown here summarize the general conditions under which snow, sleet, freezing rain, and rain land on the ground within the context of the atmospheric temperature distribution.** Consider the situation when a warm front moves in.  If warm front isn't too strong, the atmosphere remains cold (below freezing) throughout, and precipitation falls in the form of snow. But, as a warm front moves in, an inversion layer develops with cold air near the surface under the warm air aloft. If snow starts falling aloft and encounters this warm air, the snowflakes melt. A mixture of frozen and unfrozen "hydrometeors" develops in the warm layer (left side of the Figure shown here). As these hydrometeors fall into the near-surface cold layer, they get supercooled. Any icy snowflakes that didn't melt as they traveled through the warm layer become efficient sites for refreezing, and a mixture of snow, ice, and some liquid falls to the ground, i.e., sleet. As the warm layer develops (gets warmer and thicker), all of the snowflakes melt as they travel through it. Without nearby ice particles to serve as nuclei, these become supercooled as they fall through the cold layer near the ground, i.e., they are supercooled liquid. When they land on cold ground, they freeze, producing freezing rain. If the liquid droplets formed in the warm layer reach ground that is above freezing temperature, the precipitation is cold rain.

As the warm front develops, it is common to see a sequence of precipitation progress from snow to sleet to freezing rain to rain.  The reverse situation occurs with cold front events in the southern Plain states.

A few factlets from Wiki: The thickest recorded ice accumulation from a single ice storm in the U.S. is 8 inches (northern Idaho, January 1961). In February 1994 a severe ice storm caused over $1 billion damage in the southeast.

**This discussion is from David Gay and Robert Davis, "Freezing rain and sleet climatology of the southeastern USA," Climate Research, vol. 3, 209-220, 1993. Notably, they comment that at the time this paper was written, relatively little was known about freezing rain and sleet climatology.

Sunday, February 9, 2014

Meteorite impact craters and their rays

Martian impact crater formed between July 2010 and
May 2012. NASA image ESP-034285_1835
NASA just released this beautiful image of a fresh Martian impact crater. The image came from HiRISE on NASA's Mars Reconnaissance Orbiter taken on November 19, 2013. The age range was pinpointed through the orbiter's "Context Camera" that revealed a change in appearance at that site between July 2010 and May 2012. The crater is about 30 m in diameter, and the ejecta extends out to 15 km. The blue color in this image is attributed by the HiRISE team to removal of reddish dust in the area. Alternatively, I'm wondering if it sue to the veneer of fresh excavated ejecta covering the reddish dust.

In discussing this with a colleague, I pointed out that many of the studies of impact ejecta processes date back to the 1960's and 1970's, and were in the context of where to send an astronaut to explore on the Moon.  If you wanted to sample material from deep in the crust, it would be too hazardous for an astronaut to climb down the walls of an impact crater (believe me, having scrambled around the walls of Meteor Crater in Arizona many times, you do not want to be wearing a space suit while climbing down into an impact crater!). One thought was that you could sample the ejecta by going to the rays of a crater. For example, from this source:

"Lunar crater rays are those obvious bright streaks of material that we can see extending radially away from many impact craters. Historically, they were once regarded as salt deposits from evaporated water (early 1900s) and volcanic ash or dust streaks (late 1940s). Beginning in the 1960s, with the pioneering work of Eugene Shoemaker, rays were recognized as fragmental material ejected from primary and secondary craters during impact events. Their formation was an important mechanism for moving rocks around the lunar surface and rays were considered when planning the Apollo landing sites. A ray from Copernicus crater crosses the Apollo 12 site in Oceanus Procellarum. Rays of North Ray and South Ray craters cross near the Apollo 16 site in the Descartes Highlands and a ray from Tycho crater can be traced across the Apollo 17 site in the Taurus-Littrow Valley on the eastern edge of Mare Serenitatis. There is still much debate over how much ejecta comes from the primary impact site or by secondary craters that mix local bedrock into ray material."

In a 1971 article, Verne Obereck concluded that the bright rays "only reflect local excavation of mare substrate material by myriads of small secondary or tertiary impact craters:"

Observations of high resolution photographs of part of one of the prominent rays of the lunar crater Copernicus show that there is a concentration of small bright rayed and haloed craters within the ray. These craters contribute to the overall ray brightness; they have been measured and their surface distribution has been mapped. Sixty-two percent of the bright craters can be identified from study of high resolution photographs as concentric impact craters. These craters contain in their ejecta blankets, rocks from the lunar substrate that are brighter than the adjacent mare surface. It is concluded that the brightness of the large ray from the crater Copernicus is due to the composite effect of many small concentric impact craters with rocky ejecta blankets. If this is the dominant mechanism for the production of other rays from Copernicus and other large lunar craters, then rays may not contain significant amounts of ejecta from the central crater or from large secondary craters. They may in fact only reflect local excavation of mare substrate material by myriads of small secondary or tertiary impact craters.

Recently, Valery Shuvalov proposed a ray production mechanism based on a large supercomputer simulation. In this simulation, the hypothesis was that rays result from interaction between the shock wave associated with a developing crater and nonuniformities in the target surface. The results of a simulation of the formation of a crater by a 5-km diameter asteroid on the Moon at an impact velocity of 15 km/s are shown in the adjacent figure. This impact would have produced a crater approximately the size of Tycho, a famous rayed crater on the Moon. The target and projectile material were both assumed to have the mechanical properties of granite.

When the shock wave from the developing primary crater hits a depression (preexisting small crater) a jet of material is spalled off the wall of the small crater proximal to the primary crater (upper left in the simulation sequence shown).  In contrast to the effect of a depression on ray formation, a ray-suppressing effect is seen if there is a nearby elevation.

Monday, February 3, 2014

Eruption of Mount Sinabung, in North Sumatra, Indonesia

Photo from CNN.Com by Einsar Bakkara/AP
in the cited article in text
(if I read the credit correctly)
Mount Sinabung, an Indonesian volcano dormant since 1600 came to life in 2010 and, on Saturday, spewed forth pyroclastic flows that killed at least 14 people. Tragically, it appears that these people had been evacuated last summer and only the day before this eruption, had been allowed to return to their villages. The Wiki site for Mount_Sinabung appears to be updated in a timely way, so I won't go into details here.
     It is difficult to tell what the source of the erupted material is in detail, but from photos of the volcano (a classic beautifully conical stratovolcano) and the lack of any indication of lateral bulges on the flank, a good assumption is that the flows are originating in a summit crater. A question/assumption, is whether they are being driven by volatiles (presumably H3) from magma or whether or not groundwater is involved. According to the Wiki article, in late December, a lava dome had formed on the summit.
     The eruption gas/ash material from lava domes results in eruptions known as "Pelean" or "Merapi"-type pyroclastic flows. Two processes contribute to the high-velocities observed from such eruptions: gravitational collapse (supplemented by heating and expansion of entrained air), and sudden expansion of pressurized gases from inside the domes. If gravity controls the energy transfer, then areas affected can be predicted on the basis of topography. If gas expansion adds a significant contribution, which is likely in the proximal region around a dome, then velocities beyond those acquired by acceleration in a gravitational field, exist, and these imply that much larger areas are at risk than might be predicted from the gravitational forces alone.
     In 1993, Jonathan Fink and I published a paper "Estimate of pyroclastic flow velocities resulting from explosive decompression of lava domes," Nature, v. 363, pp. 612-615, 1993.  In this paper we examined the two processes above, and concluded that the decompression process produces velocities comparable to those acquired by gravitational accelerations. In snapshots, such as that in the photo in this post, my guess is that the flow is clearly already some distance down the slopes of the volcano where it has assume the classic profile of a dense gravitational flow with air entrainment. More proximal regions have already been hit, and are, apparently, where the casualties have occurred. With the complicated sequence of recurring explosions/eruptions from the summit, it may never be possible to reconstruct the dynamics of the flows in the proximal region.