Links between earthquakes and other geologic activity

Nature Geoscience (August volume 6(8), pp. 585-672) has a fairly long section ("a Web Focus") and a number of papers on geologic activity associated with or triggered by earthquakes. The introductory editorial reflects that in 1835 Charles Darwin voyaging on the Beagle experienced a large earthquake near Concepcion, Chile, and noted that within the hour a train of volcanoes in the Andes spouted out a dark column of smoke (though it would take a journey into Darwin's notes to determine whether he thought this was volcanic gas or perhaps debris from landslides. The implication in the editorial is that it was the former).
   

Illustration of the elastic rebound part of volcanic arc
subsidence after a megathrust earthquake

The first paper in this section (by Sigurjon Jonsson) summarizes the deflation of volcanic areas in response to the 2011 Tohoku and 2010 Maule (Chile) earthquakes. Both settings are at subduction zones (see figure), and the volcanoes that subsided were on the overriding plate. Prior to the earthquake, strain accumulates and compresses the overriding plate. During and after the earthquake, the overriding plate extends and subsides. However, subsidence beyond that which can be explained by this process is observed.
     In the case of the Tohoku earthquake, Takada and Fukushima documented 5-15 cm of subsidence at a distance of 150-200 km from the rupture earthquake, but no volcanic eruptions. They suggest that subsidence is caused by sinking of magma reservoirs and their warm host rocks through the colder surrounding crust. Prichard and colleagues noted that two earthquakes (1906, 1960) were followed by eruptions in the Andes within a year, but that no eruptions have been clearly associated with the 2010 earthquake. They were, however, able to document the 15 cm of subsidence, and suggest that hydrothermal fluids were released from hydrothermal systems surrounding the volcanoes in Chile during the 2010 quake, and that the escape of these fluids caused the volcanic areas to deflate.
     A second example of a proposed connection between earthquakes and geologic activity is more controversial: the Lusi mud volcano eruption. In 2006, mud erupted through and around a drill hole, flooding towns and displacing thousands of people.  Paul Davis summarizes a paper by Lupi et al. that proposes that the 2006 Lusi mud eruption in Indonesia (still continuing) was triggered by a M6.3 earthquake two days prior to the eruption and 275 km away.  Lupi et al. argue that strains, which are unarguably small at such a distance in homogeneous media,  were amplified by a downward concave layer of shale that acted as a parabolic reflector. Their simulations suggest that the stresses could have been about 100 kPa, five times higher than original estimates of 21 kPa. Such pressures, the assert, could have liquified the mud that resides at depth, resulting in the eruption of mud through the drill hole. This conclusion remains controversial (see discussion by R.J. Davies, et al., Earth and Planetary Science Letters, 272, 627-638, 2008).
     For a third example, Fischer et al. examine subduction zone earthquakes as triggers of submarine hydrocarbon seepage.  Offshore of Pakistan, the Arabian Plate subducts beneath the Eurasian plate. This is a region of intense seismicity, in particular a major earthquake (M8.1) occurred there in 1945. It occurred in an area where gas hydrates (methane clathrates) are present, and leakage of hydrocarbon gas is known to occur here. Methane and sulfates both occur in the ocean with sulfate being stable above about 5 mbsf, and methane at greater depths. The concentration of both goes to nearly zero at a depth known as the sulfate-methane transition (SMT). In a complicated chemical reaction, sulphate is consumed through anaerobic oxidation of methane (CH4 + SO24􏰀 ! HCO􏰀3 + HS􏰀 + H2O). Barium, being present in sea water, is precipitated at the SMT in so-called "barite fronts" and the abundance of barite can be used to reconstruct changes in upward methane flux.  The authors calculated that it would take approximately 38-91 years to produce the observed barite enrichments. This leads them to conclude that the barite production could have been initiated by the 1945 earthquake and an accompanying increase in methane flux due to release from the hydrates. If confirmed, submarine gas release triggered by earthquakes needs to be added to the list of processes that can add methane to the hydrosphere, and possibly to the atmosphere, in the carbon budget.




References: Takada, Y., and Fukushima, Y., Nature Geoscience, 6, 637-641, 2013.
Pritchard, M.E., Jay, J.A., Aron, F., Henderson, S.T., and Lara, L.E., Subsidence at southern Andes volcanoes induced by the 2010 Maule, Chile earthquake, Nature Geoscience, 6, 632-626, 2013.
Lupi, M., Saenger, E.H., Fuchs, F., and Miller, S.A., Lusi mud eruption triggered by geometric focusing of seismic waves, Nature Geoscience, 6, 642-646, 2013.
Fischer, D., et al., Subduction zone earthquake as potential trigger of submarine hydrocarbon seepage, Nature Geoscience, 6, 647-651, 2013.

Mud volcanoes on Mars?

Mounds on Mars in Acidalia Planitia, Mars. From
Oehler and Allen, Evidence for pervasive mud volcanism
in Acidalia Planitia, Mars, Icarus, 208(2),pp. 636-657, 2010.

Since the days of Viking exploration, there has been speculation that mud volcanism occurs on Mars. Candidate features populate the Northern Plains (see review in the paper referenced in the figure caption to the left).  On Earth, mud volcanism is triggered by tectonic compression or generation of hydrocarbons (esp. methane, CH4).  Neither process appears to be prominent on Mars.  Mechanisms that have been proposed include: (1) dewatering of debris flows; impact-related, seismic shaking causing liquefaction; sedimentation with compaction and degassing; and sublimation of CH4 or CO2 clathrates.  The mounds in Acidalia are on a very large scale, and Oehler and Allen favor an explanation that includes the basin's unique geologic setting.  Acidalia Planitia sits where large quantities of sediments were deposited from outflow channels.  It was a "depocenter" for accumulation of mud and fluids from this sedimentation.  The mounds may be attributable to large overpressure developed in response to the rapid outflow deposition, "perhaps aided by regional triggers for fluid expulsion related to events such as tectonic or hydrothermal pulses, destabilization of clathrates, or sublimation of a frozen body of water."  They could account for a significant release of gas, and the process may have created long-lived conduits for upwelling groundwaters.

In a recent article in Earth and Planetary Science Letters (v. 304, pp. 511-519, 2011), Pondrelli et al. reported on possible mud volcanoes within Firsoff impact crater. The mounts are on the crater floor, and appear as isolated or composit cones 100-500 m in diameter, and tens of meters high.  More than 1/3 have subcircular depressions on their apices, 5-39 m deep, interpreted as vents. The mounds themselves are meter-sized boulders embedded in a finer-grained matrix, a mud breccia. The mounds are located on or near faults and are aligned with fractures, suggesting larger pathways for fluid migration along faults related to the impact that produced the crater. The authors speculate that methane was involved in the process of forming the mounds.

Hydrocarbon gases estimated to have been 40% of the discharges from the BP oil well blowout

NASA Terra Satellite image of sun on the oil slick off of the
Mississippi Delta (highlighted by the tree-like structure in green)
on May 24, 2010

The BP oil well blowout last summer caused release of at least 50-100 million barrels of oil into the Gulf of Mexico.   In other units, this was somewhere between 7 and 14 million tons of oil.  This release was unusual geologically because the discharge rate was high compared to natural rates, it was extended (84 days), and it was deep (nearly 1500 meters).  It was also unique because it contained another 40% by weight dissolved hydrocarbon gases, mostly methane and pentane. I featured an article on natural vs. Deepwater Horizon seep rates by Cutler Cleveland on this earlier post. 

In a paper just released on-line by Nature Geoscience, Joye et al. have quantified this estimate of the amount of gas released, where it was released, and some of the implications.  The reference is Nature (published on-line) 13 February, 2011, DOI: 10.1038/NGE01067.  (I'll try to remember to come back and put in the formal publication reference!). I really like this paper--the authors dealt with some of the most atrocious scientific units possible to unravel this story: tons (or is it tonnes?), barrels, bopd's (barrels of oil per day), gallons ('a standard barrel of oil is 42 US gallons or 159 litres), BOE's (barrel of oil equivalent, in energy units of kilojoules or, in volume units, cubic meters or cubic feet),.... Definitely enough to drive an undergraduate chemistry class nuts for a few homework sets!  Well done!

The bottom line is that hydrocarbon inputs into the gulf were 5,000-10,000 tons of carbon per day from this single well, and that "contrasts starkly" with inputs from natural seepage (220-550 t carbon per day over the entire 70,000 square kilometers of the Gulf of Mexico. The hydrocarbon gases seemed to appear in discrete layers between 1,000 and 1,300 m depth, where concentrations were up to 75,000 times background levels. The ultimate fate of these hydrocarbons is uncertain at present. However, if the geologic record of the past is an indication, it is possible that bacterial activity and consumption of the methane could lead to formation of oxygen-depleted bottom waters.

Why does natural gas explode?

Last night there was a terrible gas line explosion in Philadelphia, captured on screen by a bystander (see video here).  The line was apparently being repaired when the explosion occurred. One worker was killed and three others are in critical condition.

Natural gas is comprised mostly of methane, CH4, with up to 20% other hydrocarbons such as ethane, C2H6. Methane is notorious for causing explosions, and I covered some of the physics of explosion in a previous post. Gases are flammable only under certain conditions.  The "lower explosive limit (LEL)" is the composition of a mixture (with oxygen in most cases) that contains the smallest amount of methane possible for combustion, and the "upper explosive limit (UEL)" contains the highest.  Below and above these limits there is either too little methane (LEL), or too little oxygen (above the UEL).  There is a quantitative difference between the flammability limit and the explosive limit, but for all practical purposes these terms can be used interchangeably.

There are two types of combustion: deflagration, when the combustion zone (where reactions are taking place) travels at a velocity less than the speed of sound in the unreacted mixture, and detonation, where the combusion zone travels at a velocity greater than the speed of sound in the unreacted mixture.  An explosion occurs when the container in which the reaction takes place bursts.  Control of the gas and vapor concentrations is a major issue in occupational safety and health.

Coal Mine Explosion in New Zealand

The entrance to the Pike River Coal mine.  Photo/NZPA
published by the nzherald.co.nz, Nov. 19, 2010

On November 19, 1 man was killed and 27 miners are missing due to an explosion in the Pike River coal mine on the south island of New Zealand.  The mine has been under development since the 1970's, and is excavating from the largest hard-coking coal deposit in New Zealand. It is in an environmentally sensitive area, and has been lauded as an exemplary development in a sensitive area.  The tunnel, seen at the left, goes nearly 2.5 km nearly horizontally into the mountain.  The trapped miners may only be a few hundred feet underground.

From this Wiki site

Dangers in coal mines arise from at least three factors: the presence of both methane and carbon dioxide gases, and coal dust.  In this particular area of New Zealand, active faults produce a seismic risk as well.  Other causes of accidents are rock bursts, collapse of natural or artificial pillars, flooding, malfunctioning of equipment, and improper use of explosives in the mining process.  Methane gas is nasty stuff: an asphyxiant, flammable and potentially explosive.

The combustion of methane, CH4, is exothermic.  When combined with two O2 molecules, it decomposes to form CO2 (gas) and 2 H2O (liquid) molecules, releasing 891 kJ of energy. It is both flammable and explosive.  When mixed with oxygen and nitrogen in air in certain mixtures (orange region on the ternary diagram to the right), it will explode. The blue line represents air. If there is less than about 5% methane in the mixture it is too lean to explode (the LEL point on the diagram), and if there is more than about 15%, it is too rich to explode (the UEL point).

The process of mining coal generates an enormous amount of coal dust.  When high concentrations of this are suspended in air, and there is even a small ignition source, the nearly instantaneous reaction of the fine coal particles with oxygen can produce an explosion.  The ignition source does not need to be a flame; it can be an electrostatic discharge, friction, or sparks from machinery.  Dust explosions are not restricted to coal mines, but have occurred around grain silos, flour mills, and metal works (aluminum, titanium).  They are intentionally part of thermobaric weapons; see my older post here.

The BP Well Operation and Their Problem with 'ice' Clogging

Image source: unknown

British Petroleum engineers have successfully installed a cover on the leaking well and are trying to siphon oil to surface ships.  They have been proceeding cautiously because their previous attempt with this procedure got clogged with a nasty form of ice called "methane hydrate" or "methane clathrate".

Water ice is capable of absorbing an amazing amount of some gases into its structure in "cages".  A typical clathrate cage is shown in the inset of this image of a methane clathrate burning.  Yes, you can set ice on fire!. The gas burns off leaving the ice structures behind. The gas molecules are stored in cages of H2O molecules, and it's quite amazing how much gas can be stored in these cages.

Clathrates are a major reservoirs of natural gas globally, as shown by the map of clathrate deposits.  The dissociation of this weird form of ice has been implicated in everything from the disappearance of ships in the Bermuda Triangle to sudden climate change.  It's been proposed that sudden "burps" of methane gases from clathrates in the Bermuda Triangle have changed the density of the water column so that ships that were floating on the water suddenly found themselves floating on "gassy water" and lost buoyancy, sinking to the bottom. (One thing nice about this blog is that I don't have to provide rigorous referencing!!)  55 million years ago, the Paleocene/Eocene boundary, there was a dramatic climate change that some have argued was due to a sudden methane influx into the atmosphere due to decomposition of clathrates.  The cause of decomposition is unknown, but theories relevant to the current climate crisis are abundant.  I found this article by Galvin Schmidt to be a thoughtful analysis.