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

Sunday, July 21, 2013

Gansu, China, rain, mudslides, and now a shallow earthquake: are they related? And, was that shallow earthquake notice right?

A map showing the location of the December 16,
1920 M 7.8 earthquake
From Wiki here 
An alert has just come out over the USGS earthquake network that there was a very shallow (1 km depth) magnitude 5.9 earthquake in Gansu province, China, 13 km east of the city Chabu (or, later reported as 156 km west of Tianshui.). Interestingly, two other agencies give very different estimates of the magnitude and depth: a 6.3 at 10 km depth by GFZ (at the Helmholtz Centre in Potsdam) and a 6.1 at 15 km depth according to EMSC (the European Seismological Centre), as reported here. And, while I was checking this on the EMSC site the numbers changed before my eyes to a M6.0 at 10 km depth! It would have been interesting to see how this all sorts out--if the USGS depth of 1 km is correct, this is a very unusual earthquake, and I wonder if it is actually an event associated with a major landslide rather than a fault zone (this was the case on May 18, 1980 when the north flank of Mount St. Helens failed into a big landslide). However, when I checked into the USGS site here, the depth is now listed at 9.8 km.  Makes me wonder what that earlier bulletin was all about!

The epicenter appears to have been Dingxi City. Dingxi is a "prefecture level city" in the southeast of Gansu province. It's area covers 20,300 sq. km, and the population is reported as 2.7 million people (in 1 urban district, 6 counties, and 119 towns). It was an important city in early development of some of China's cultures because the Wei River, one of the Yellow River's biggest tributaries, flows here.  The surrounding terrain hills and ravines cut into the loess deposits, weak sandy deposits, possibly water-saturated from the recent rains. This does not bode well for damage.

 The earthquake occurred near midnight UTC on July 21. This follows, by only 8 days, a major landslide that reportedly trapped at least 100 tourists after a landslide cut off a road during a week of storms that have flooded rivers and triggered mudslides. At least 86 people were reported to have died by Chinese state media. Mudslides and floods are common in the mountainous areas of China, but this year seems to have been an especially bad one in many areas of Asia.

Map showing the relation of todays earthquake to
the major city of Tianshui.
In 2010, a deadly mudslide killed approximately 1500 people. After heavy rains, water built up behind a dam of debris that blocked a small river north of the city of Zhugqu. The dam broke, sending nearly 2 million cubic meters of mud and rocks through the town in a surge tens of feet high. A major problem in this area is that the forested areas have shrunk (by as much as 30% according to Wiki), and the reserve of timber has diminshed by 25% due to harvesting (these are 2010 numbers, and are relative to a base in the 1950's.) It is a region of massive construction of hydroelectric projects.

Gansu is also a region with many earthquakes.  In 1879, an earthquake with an estimated magnitude of 8.0 and Mercalli intensity XI (extreme) killed an estimated 220,000 people. This earthquake was preceded by foreshocks for a few days. The earthquake also triggered landslides that dammed local rivers up to 40-120 m.

Today's earthquake appears to be followed by numerous aftershocks, though it is difficult to tell where they are relative to the initial location given as 13 km east of Chabu. Some are reported at a distance of 156 km from Tianshui in Gansu province, a city of 3.5 million people.

Here is a site that is updating and comparing locations of today's quake, and that also gives earthquake locations since 1900 in this region.

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