The dam and the Sichuan Earthquake

I get Science via Pony Express plowing through snow over the Continental Divide, I suspect, so I didn't see this news article until a couple of days ago:

A Human Trigger for the Great Quake of Sichuan?

The article is behind a paywall, so I'll try to summarize it: Last year's devastating M 7.9 earthquake in Sichuan, China may have been triggered by the Zipingpu Dam. At least, that was implied by an AGU talk (which I didn't see) by Christian Klose, and by an independent article in the Chinese journal Geology and Seismology. Klose's argument is that the weight of water changed stresses on the fault in exactly the wrong way: it increased the shear stresses (which make the fault more likely to slide), and decreased the normal stresses (which prevent slip). His evidence includes the type of fault slip (especially the initial slip) and the depth of the majority of the aftershocks. I don't have quite the right background to evaluate his evidence (especially based only on the abstract), but Science reporters Richard Kerr and Richard Stone describe the reaction at AGU like this: "Klose's listeners were intrigued but far from convinced. They wanted to hear more details about changing water levels and local, lower-level seismicity."

This isn't the typical fluid-induced earthquake, caused by an increase in pore fluid pressure. The Sichuan earthquake nucleated 20 km below the ground surface, and that's an awfully long distance from the water source near the surface. It sounds as though the size of the reservoir, the location of the reservoir compared to the fault, and the direction of fault slip may have combined in just the wrong way... if the reservoir was what made the fault slip then.

The dam would still be a trigger, rather than a cause, for the earthquake. The cause, ultimately, is the collision of India with Asia. (As Klose states in his abstract, "This region has been tectonically loaded for >10kyr.") But the dam could explain some of the surprising things about the earthquake - for instance, why a fault that was thought to slip every 2000 to 10,000 years would have gone now, with such horrible consequences.

It's intriguing and scary... and terrible. The earthquake was tragic enough. If humans were responsible for triggering it... well, I don't want to think about it.

(Except that I'm teaching a course inspired by John McPhee's book The Control of Nature in May, and I'm trying to decide which possible case studies to use this time.)

(H/T to Sciencewoman, who told me to watch for this news article... two weeks ago. I've finally seen it!)

Spooky geology

Happy day-before-Halloween. (All Hallow's Eve Eve?)

I've got a ridiculous amount of candy (though I'm in town now, and I'm not calibrated for trick-or-treaters other than the Small Human). I've got a costume for the Small Human. I don't have a costume for myself, however. (Small Human wants me to go as a bat. Given today's stories about the causes of white-nose syndrome, though, I don't have the heart for it. Poor little guys.)

A few years ago, I encouraged my intro class to dress up as something geological. I forgot to do it this year (and it worked better when the class suggested the idea), but I got to wondering...

What would be the scariest possible geologic costume?

I think I would have to go with liquefaction. There's an earthquake, the ground turns to quicksand, buildings and people sink into the ground, and then the sand solidifies around you so you can't breath.



And then the dogs come out and eat your head.


(Images of the 1692 Port Royal earthquake from http://www.longjohnsilvertrust.co.uk/projects/henrymorgan.htm .)

Braaaaaaaains.

I'm not sure exactly how the costume would work, though. I could encase myself in sandy plaster and stop breathing... but maybe that wouldn't be a good idea.

On second thought, maybe I'll go as some obscure geology jargon that deserves to be re-animated. Nothing like a good word to eat a brain.

Braaaaaaaaains....

Earthquake in Pakistan

BBC: Scores dead after Pakistan earthquake
Scientific American 60-second-science post

There were two moderately large (M6.4, M6.1) earthquakes within twelve hours:



The earlier one had an epicenter right beside a populated valley - Khanozai, Pakistan. (Unfortunately.)



The second one had an epicenter beneath some spectacular plunging folds (WOGE-worthy, except that I'm posting the picture out of turn):



Both earthquakes had similar focal mechanisms. North-south compression (along the India/Asia collision), earth-west extension, along a strike-slip fault (again, common in Asia - see Tapponier et al., 1982, for example). Based on the aftershock locations, I would guess the NW-striking solution is the real fault. I'm curious about the relationship of the seismogenic faults to the plunging folds, though. I don't have a quick explanation... and I'm just starting to talk about faults in class this week. I would love to show the Google Earth images, but the folds show up so well on the satellite images that I should be prepared to answer questions about them in a semi-coherent way.

The earthquakes are also right at the place where the folds near the plate boundary make a sharp bend:



(By the way, does the apparent strike of the fault line up with the boundary of that basin by Kandahar, Afghanistan? It's the brown area west of the earthquakes; I don't have the cities layer turned on in my image.)

I'm not sure how that affects the faulting, either.

Reference: Tapponier, P., Peltzer, G., Le Dain, A.Y., Armigo, R., and Cobbold, P., 1982, Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine: Geology, v. 10, p. 611-616.

Edit: There's discussion of landslide potential from these two earthquakes at Dave's Landslide Blog: post #1; post #2.

THAT wasn't the search result I needed...

I'm talking about earthquakes in my intro class tomorrow. Most likely, I will tell the story about my experience in 1989 (sitting in a doorway shouting "is this an earthquake?" - which, by the way, is not the currently recommended behavior). But part of me would like to show the GPS monitoring that's being done right now. (Not that I'm quite prepared to explain vectors to my intro students... though, come to think of it, the GPS map would be a great example for anyone teaching about vectors.)

I've had the Earthscope Plate Boundary Observatory vectors plugged into Google Earth for quite some time, but I didn't remember where I'd found it. So I did what I always do: I googled it.

And my blog showed up as the second hit.

So, for anyone who goes looking for "continuous GPS google earth," here are the sites you really want:

Google Earth plug-in
GPS products from the Plate Boundary Observatory
Earthscope: Plate Boundary Observatory

(And yes, I'm doing this because I know I'll lose track of the sites again, and I figure I can always search my own blog.)

M 5.4 Los Angeles quakes - maps from google earth

Andrew and Jim Repka have already talked about today's LA basin earthquake. (And they got through to the moment tensor solution: a combination of right-lateral strike-slip and thrust, a snapshot of the tectonics of the LA region all in one moderate earthquake rupture.)

Here are maps from Google Earth and the USGS. Faults are color-coded based on how recently they are known to have had earthquakes. Red is historic, orange is younger than around 10,000 years old, and green and blue are somewhat older, but still young enough to worry a nervous geologist.



Here's the same information, but zoomed in:



It's not on a mapped fault, but that isn't surprising: the thrust earthquakes in LA (such as the 1994 Northridge EQ) tend to be on "blind" thrusts - faults that don't cut the surface. (They do, however, tend to raise the ground surface and create hills and mountains. Notice the topography around the aftershocks.)

It's also between two segments of mapped faults, and near the end of one of them. The geometry of the earthquake rupture makes sense there - the rocks near the tip of the right-hand fault (orange on map) should be getting compressed. If the orange fault is active. Which I'm willing to bet it is. (On a geologic time scale, at least. I can't work out possible stress changes in my head, so I'm not making an earthquake prediction.)

Links: Sichuan earthquake and age of geoscience profs

I don't have internet at the new house yet, I'm going into the field tomorrow, and I'm leaving Sunday for the Teaching Intro Geology workshop. So I won't be doing much blogging for a while (unless I end up trying to blog from the workshop).

In the meantime, some news that I wish I had time to say more about:

1) GSA Today has an article about the tectonics of the Sichuan earthquake, by Clark Burchfiel, Leigh Royden, and collaborators. It's open access, so you can see it for yourself. The article explains Burchfiel's comments that the earthquake was larger than he would have expected - both estimates of exhumation rates in the mountains and GPS measurements of active deformation suggest that earthquakes of that size occur only every 2000 to 10,000 years. In other words, although the earthquake makes sense in the broad context of plate tectonics, the techniques used to study active tectonics would not have predicted this to be an especially dangerous area.

2) AGI's Geoscience Currents series has just released the geoscience faculty age distribution for 2008. Fewer than 200 faculty members are younger than 36; the numbers of faculty become dominated by associate professors (rather than assistant professors) between age 41 and 45. The flyer states that "The low numbers of faculty under the age of 40 likely reflects the growing tendency for geoscientists to take post-doctoral fellowships prior to entering the faculty ranks." I suspect that cause and effect are switched here, and that many geoscience post-docs would happily trade their jobs for faculty positions if given the opportunity. (As an aside, when I got my Ph.D. at age 26, I told myself that I would give myself ten years to find a job, and if I wasn't employed by age 36, I would do something else with my life. I know I finished grad school at a frighteningly young age, but it's still disconcerting to see that my upper age limit is on the low end of the age range for new hires!)

The graph is also worrisome for young women who wish to be both geoscience professors and mothers - waiting until after tenure (as I tried to do my first time around) is not an option for most women, and that means dealing with a young child in the stressful, highly uncertain world of post-docs and visiting instructors and assistant professors. (Or not having kids at all. But nobody expects men to give up the possibility of kids in order to be scientists.)

Cruel malware hoax targeting Chinese earthquake fears

Amongst the many e-mails that piled up in my work account, I saw a number with subject headings such as "1000000 dead in Chinese earthquake" or "Terrible earthquake devastated Beijing." I didn't know the senders, I hadn't seen anything about a new earthquake in China on airport televisions, and I hadn't received any messages from the USGS Earthquake Notification Service about a big earthquake in China. (Tonga, yes; China, no.) So I deleted all the messages, figuring they were probably some kind of potentially dangerous spam.

And sure enough, they are. There's information about them on various internet security blogs. This one (mxlogic) has information about the specific malware involved: it's a worm, and only infects your computer if you click on the link in the e-mail, and then attempt to watch the fake video on the linked page.

At least one of my spam e-mails came from a geologic-sounding address, so I figure it's useful to pass the word around: the message is fake. There hasn't been a devastating earthquake in Beijing. (Thank goodness.)

After the tragedy of the Sichuan earthquake, this is a particularly cruel hoax.

Earthquakes, volcanoes, and landslides in Japan

The rural setting of Saturday's M 6.8 earthquake in northeastern Honshu, Japan* has fortunately meant that many fewer deaths occurred than during the 1995 Kobe earthquake. (From news reports and photos, it looks like landslides were responsible for a lot of the destruction. Dave's landslide blog has photos.)

So the surface is where the destruction is, but I'm also interested in what's going on deeper down. If you use the Google Earth plug-ins for the USGS real-time earthquakes and the Smithsonian Institution's volcanoes, you can see where the fault is compared to the volcanic arc:



It's in the arc. Right in the middle of the arc. One volcano, Kurikoma, lies above the aftershock zone. And it's acting up. (News report here.)

So here are my somewhat random thoughts:

- The earthquake was along a thrust fault. I know of arcs that are extensionally collapsing, which involves normal faults. (The Marianas arc is a good example.) I know of arcs with strike-slip faults running through the line of volcanoes. (The Sumatra fault, for instance, is like this - there's a strike-slip fault that runs along the arc, so the plate movement is partitioned into two zones - subduction at the trench that caused the 2004 tsunami, and strike-slip movement within the arc.) And I know of studies (including one of mine) arguing that thrust faults moved during the growth of ancient plutons. But modern thrust faults in the middle of arcs - not thrust faulting partitioned into the wedge above the subduction zone, or in the back-arc (like in parts of the Andes) - I'm not sure if I've seen a thrust earthquake in an arc before. I know there are thrust earthquakes in the Sea of Japan, and that there is compression between Japan and Asia, so the fault mechanism isn't surprising. But the location is. Is this a case where the magmatism created a weaker zone in the crust and allowed thrust faulting to take place?

- Did the earthquake (either due to stress changes, or due to the passage of seismic waves) cause Kurikoma to start acting up?

- What's going to happen next?

* The Japan Meteorological Agency measured the earthquake as magnitude 7.2, but I don't know which magnitude scale they're using for it. I would rather use moment magnitude consistently to think about earthquake size, and I know that's what the USGS is reporting, so I'll stick with that.

NPR interview about aftershock prediction

NPR has an interview with Walter Mooney of the USGS about expectations for aftershocks of the Eastern Sichuan earthquake. It's a good explanation of what seismologists can predict (things like the size of aftershocks) and what they can't (the exact time of aftershocks). Mooney also talks about the stress changes that can trigger later earthquakes - science that's been developed in the nineteen years since I was in Loma Prieta (and heard Mooney, amongst other people, talk about what to expect).

All that knowledge doesn't help when villages are buried by landslides by the shaking, however. It sounds like the most difficult problem, as with natural hazards from California to Indonesia to New Orleans, is that there are a lot of people in the world, and most of them aren't able to choose a perfectly geologically stable place to live.

NPR's China earthquake coverage

NPR had a crew in Chengdu, the large city near the May 12 earthquake, when the earthquake struck. They have a recording of the earthquake itself (scroll down and look on the left side of the page for a link), and a blog with stories before they go on the air. (It's hard to read, as a parent - so many kids in schools, and left in apartment buildings with grandparents...) Back in the US, NPR interviewed USGS seismologist Lucy Jones about the earthquake (and how it was the same size as the great San Francisco earthquake of 1906).

It's good coverage - sensitive to the human tragedy and careful about describing the geology and engineering. And there are photos on the blog that give a sense of the topography - steep hillsides with one-lane dirt roads and thick vegetation. Difficult terrain to get through - it's easy to see why rescuers have only reached the epicenter today.

Tectonics of the May 12 Sichuan earthquake

The M 7.9 earthquake that struck Sichuan province in China is already tragic, with at least 10,000 people dead. It's China's worst earthquake since the 1976 Tangshan earthquake - the earthquake that killed at least a quarter of a million people, and maybe many more. MIT's tectonicist Clark Burchfield is quoted in the NY Times as being surprised that an earthquake of that magnitude (which requires a lot of fault to have moved at once) occurred in this particular spot. But although the size might be larger than one would expect, the type of earthquake isn't. This isn't a classic plate boundary earthquake, but it's part of the way that continents respond to continent-continent collisions.

In the big picture, India is moving northward and colliding with Asia. That's what created the immense mountains of the Himalayas, and the highlands of the Tibetan Plateau, in the first place. This earthquake, however, was not on the plate boundary itself. It was off on the eastern edge of the Himalayas, on the boundary between the high country and the Sichuan basin.

Here's the USGS earthquake location map. The pink lines represent the collisional plate boundaries - the boundary between India and geologic Asia runs along the southern end of the Himalayas. This earthquake was within Asia proper.


But continents are... well, continents are kind of wimpy, actually, in terms of deformation. They're made of rock that's easier to break than oceanic crust is. And on top of that, they're thick. And when they run into one another, the continental crust gets even thicker. And that thick, weak crust can break within itself - it doesn't behave like the classic rigid plates that "plate tectonics" is named for.

The crust of Asia breaks in a number of different ways. Parts of it are squeezed out, especially to the east, where it can run over the subducting Pacific plate like a semi over a squirrel. Parts of it actually stretch east-west, creating features like Lake Baikal up in Russia. Parts are pushed over each other, making the height of the Himalayas and Tibet possible. And parts are squeezed over other parts of Asia. And that's what's happening on the northwestern side of the Sichuan basin - the edge of the Tibetan Plateau is running over eastern Asia, in this case, along a classic thrust fault, sloping about 30 degrees down into the ground beneath the edge of the mountains.

You can also see the movement of Tibet in this map of GPS velocities:



(From Gan, Zhang, Sun, and Sun, 2006?))

Each of the arrows represents the movement of a GPS monitoring site compared to the rest of Eurasia. The southern Himalayas are moving north; the Tien Shan mountains, north of the Tarim basin (white, north of the Himalayas) are moving much more slowly, and the eastern Himalayas... are moving east. Or even south.

And at the Sichuan basin, that green blob east of the Himalayas, the GPS velocities drop. There's a change in movement. And that M 7.9 earthquake took place where the change occurs.

The Google earth image (from the USGS Google earth KML) of the earthquake and its aftershocks shows the setting closer-up:



It looks like the earthquake was on the basin-bounding fault. It looks like there are ridges in the basin running parallel to the fault, too - ridges that are cut by rivers. They are probably actively growing folds... which means there are probably active thrust faults running underneath the basin itself, not just under the mountains.

Those faults, at least, don't seem to have moved.

Dear NPR: let's talk earthquakes

NPR's Bryant Park Project features a story about last week's M 5.4 earthquake in Illinois. And, well, they should have interviewed Maria instead, because her post explaining earthquakes in Illinois is really good. And NPR's story, ummm, isn't.

What did NPR get right? Well, yes, there have been big earthquakes on the New Madrid seismic zone. Really big earthquakes. And the 1811-1812 quakes (which supposedly rang church bells in Boston) are worth telling people about.

But...

The California activity — along what is known as the San Andreas fault — is a result of the North American plate riding over the Pacific plates and continually building stress, Gibson says. The New Madrid fault, on the other hand, is an "ancient scar" where two very old plates are slowly pulling apart.

Ok. Problem #1: California. California is on a strike-slip plate boundary (as Julian's photos show very nicely). The Pacific plate slides past the North American plate. Mostly. Except where the boundary isn't oriented in quite the right direction. But it's not a subduction zone, as implied by "riding over." (You need to go north, near the Oregon border, to find one of those. And up there, North America slides over the teeny-tiny Juan de Fuca plate, not the Pacific plate.)

Problem #2: New Madrid as an active rift? The failed rift that is now the New Madrid seismic zone tried to tear North America apart 700 million years ago. It's not an active rift now. It failed. No ocean basin. No separate continents. Just lingering effects like low elevations and lots of old faults, just waiting to be reactivated. Maria's analogy is perfect: they are old injuries that react to unrelated events. In this case, I would guess that the fault was driven by some east-west compression of the North American plate, between the Mid-Atlantic ridge on the east and the San Andreas fault on the west. (The focal mechanism says that the earthquake involved strike-slip motion with east-west compression - consistent with reactivation, but not with active rifting.)

So. Seismic hazard? Yes. (And so are many other areas with old normal faults: one of the other big earthquakes of the 1800's was in Charleston, South Carolina.) Future beach-front property? Not without a lot of sea-level rise.

And should we care? Well, if the New Madrid seismic zone were actively building stress, we might have a better handle on the risks there. We could use continuous GPS to monitor the ground deformation. We could make a decent guess about the likely size of future ruptures, and the size of future earthquakes. And then it might be possible to decide just how crucial it would be to retrofit old buildings. But as it is, we've mostly got history, rather than theory, as a guide. And in most places, human history is too short to give a good feel for earthquake risk.

Biggest EQ in the eastern Basin & Range in 25 years...

It's only a Mw 6.0. That means it's an order of magnitude smaller than the Borah Peak (Idaho) earthquake of 1983 (M 6.9) and the Hebgen Lake (Montana) earthquake of 1959 (M 7.3).

But, still, this morning there was a M 6 earthquake just east of Wells, Nevada. From the maps, it looks like it could be on a range-bounding fault on the next range east of the East Humboldt Range.



If there's a surface rupture, it should be a great place to visit - just south of I-80, on an alluvial fan, not much vegetation to obscure the geology.

The topography of the Basin and Range is controlled by normal faults: they have dropped the basins and lifted the ranges, and made the entire state of Nevada look, from space, like a washboard. But current deformation seems to be most intense near the California-Nevada border, where there is both strike-slip movement (part of the greater Pacific-North America plate boundary) and east-west extension. On the eastern side of the Basin and Range, there have been major earthquakes north of the Snake River Plain/Yellowstone area. The Wasatch Fault/Hurricane Fault, on the eastern edge of the Basin and Range, is considered potentially hazardous, but less so than the faults on the western side.

This earthquake is a couple hours drive from Salt Lake City, about 250 km from the Wasatch Fault. I'll be curious to see what the people who have been doing GPS monitoring of the Basin and Range have to say. Did they have continuous GPS stations in the area before the earthquake occurred? And did they see anything - anything - that could have told them that the fault was getting ready to go?

Edit: The USGS has added to its summary page. They think the earthquake occurred on a west-dipping fault, associated with the fault that bounds the Pequop Mountains (the range east of the earthquake). (The focal mechanism "beach ball" diagram has two possible solutions: one east-dipping fault, and one west-dipping fault.) That would make the rupture even more interesting to visit - it's very close to the east side of the hills between the Pequop and East Humboldt ranges. I want to go there and see for myself which side went up.

Edit 2: There's quite a bit of damage in Wells. Links from Salt Lake Tribune and KUTV News (with slideshow and video). There were lots of brick facades in downtown Wells - the damage looks pretty dramatic.

Subduction zones from the side

The USGS earthquake site is one of my favorite sites on the net. Maybe it's just that I'm a hopeless seismic rubber-necker. But maybe it's the wealth of information the USGS makes available.

Each major earthquake has its own set of pages, with tabs for general information and maps and scientific information. And lately, they've added a number of new types of scientific information. There are more beachball diagrams, both for current earthquakes and for historic earthquakes. (The southern California earthquakes have particularly detailed information from the Southern California Earthquake Center.)

And then, for some earthquakes, there are diagrams like this (from the Feb. 20, 2008 Mw 7.5 earthquake off of Sumatra):


Look! It's a subduction zone! Ok, yes, I knew that. But it's nice to see the earthquake foci in cross-sectional view. The subducting plate coming in from the left is nearly flat, until it reaches the islands of Nias and Simeulue, and then it heads down into the mantle. The big triangular area of orange and yellow dots probably represents the thrust faults in the accretionary wedge. (I wonder if the earthquakes near the surface on Sumatra are on the strike-slip fault that runs down the island, or if they are volcanic earthquakes? Does anyone know?)

There are black and white images like this, of Wadati-Benioff zones, in all sorts of textbooks. But it's nice to be able to see recent earthquakes plotted in this context.

Rock of the (mid) week: Wasatch Fault rock

When I first started planning work for Advanced Structural Geology, I considered focusing the entire course on faults and shear zones. In part, I was trying to make up for things that I worried were deficiencies of my own - I still remember staring blankly at my field camp TA as he sat on the Darby Thrust, asking leading questions until I finally realized that the stratigraphic section was not in the correct order.

Now, I've got some spectacular exposures of minor faults in my backyard. But they aren't quite the same thing. So I decided I wanted to take students to see some of the rocks found in major fault zones.

My brittle example was the Wasatch Fault, the big normal fault east of Salt Lake City. It's active, and there are good geomorphic signs that it's capable of earthquakes, including a nicely offset glacial moraine at the mouth of Little Cottonwood Canyon.

This outcrop, of an actual slip surface, is on the edge of Provo. The rock is limestone, but it's been broken into tiny pieces, and then cemented back together by more calcite. The striations on the surface may be slickenlines, but they may also be scratches from the equipment that stripped this surface bare (in an aborted plan to build a ski lift).

Is this rock the result of earthquakes on the fault? In the past, the only dead-certain evidence for earthquakes on exhumed fault rock was pseudotachylite, rock that melted and then froze during an earthquake. But at last fall's GSA meeting, during a session on Co-Seismic Fault Zone Structures, a number of groups suggested that maybe cataclasites form during earthquakes, as well.

Serpentine rheology and subduction zone earthquakes

There’s something about subduction zones. Maybe it’s the potential for magnitude 9+ earthquakes and major tsunamis... and for silent earthquakes, and slow slip events, and postseismic deformation. Maybe it’s the explosive volcanoes. Maybe it’s the accretion of stuff scraped off the downgoing plate. Maybe it’s my old fondness for minerals that look purple under a microscope. Maybe I’ve just been seduced by the name.

Anyway, although I’ve worked in rocks that have been down in subduction zones, I don’t have a very good idea about the nature of the faults that generate the immense earthquakes. I work on land, and on rocks that have made it to the earth’s surface; the subducting plate starts on the ocean floor, and goes down from there. So I don’t have a mental image of what’s involved – not like I have of, say, the Wasatch normal fault, or the Snake Range decollement.

An article in last week’s Science by Nadege Hilairet and co-authors, about experimental deformation of serpentine, gives me a better idea.

Minerals can deform in a number of different ways, depending on the temperature, the stress they experience, and the rate of deformation. At low temperatures, things deform elastically - they change shape in response to the stress until either the stress goes away, or they break. At higher temperatures, chemical bonds break and reform within the crystals, allowing the crystals to change shape without physically breaking. This can lead to some major differences in behavior when the rocks are stressed – at low temperatures, the release of elastic energy when rocks break and slip gives us earthquakes. At high temperatures, rocks behave more like Silly Putty – they’re elastic at short time scales (and can transmit seismic waves), but over longer times, they change shape at a rate that’s proportional to the stress they feel.

Different minerals deform by different mechanisms. Some change shape ridiculously easily – rock salt, for instance, flows so easily that it is used to equalize pressures in rock deformation experiments. Others, such as olivine, are strong even at quite high temperatures – that is one reason why the uppermost part of the mantle, the mantle lithosphere, is the strongest part of most tectonic plates.

And then there’s serpentine, the hydrous magnesium-iron-aluminum silicate that forms from adding water to mantle rock. It’s the mineral that dominates the state rock of California (and the rock found in the core across the San Andreas Fault). And it is, presumably, a mineral that forms in the mantle wedge above the subducting plate, as the downgoing plate loses its water. (Some subduction zones also have evidence that it really is there – seismic waves traveling through the overriding plate are slowed by a material that has the right seismic velocity and other elastic properties to be serpentinite.)

Serpentine is soft and slippery at surface conditions, so one might expect it to be weak in subduction zones, as well. But serpentine is brittle in low-pressure experiments (below 0.7 GPa, which would be equivalent to a depth in the neighborhood of 20 to 25 km) – near the surface, it breaks. The authors, however, managed to do deformation experiments at pressures of 1 to 4 GPa (equivalent to around 33 to 120 km – for a subduction zone beneath a continent, we’re talking about depths from the average base of continental crust to the base of the mantle lithosphere – though at a subduction zone, I’m not sure what is typically found at those depths).

They found that serpentine behaves differently at different pressures, presumably because different deformation mechanisms are at work. At both pressures, the behavior could be modeled as power-law creep. Power-law creep means that the strain rate is related to stress taken to some power n – that is, the higher the stress, the faster the strain rate, and the higher the value of n, the more sensitive strain rate is to increases in stress. Power-law creep is typical of deformation mechanisms involving movement of dislocations (defects in a crystal lattice). But at lower pressures, the stress exponent was higher – increasing stress really made deformation happen more quickly – which is typical of sliding along grain boundaries. At higher pressures, the behavior was closer to that of a viscous material (though still with viscosities that change with stress).

Ok, nice. So why, exactly, is this interesting? Well, for one thing, I usually think about the effect of temperature on deformation mechanisms, but I don’t usually see discussions of the role of pressure. Temperature’s important because intracrystalline deformation involves breaking chemical bonds, and higher temperatures generally make it easier to break and re-form bonds. In fact, I had always thought about the great depths of subduction-zone earthquakes as a function of the cold temperatures in the subducting slab. (The rock descends faster than heat can flow into it – among other things, that makes high pressure/low temperature metamorphic rocks possible.) So it’s interesting to see rheology controlled by pressure as well.

For another, the experiments show that, at depths in the range of approximately 30 to 100 km, serpentine is much weaker than most other likely minerals (pyroxene, plagioclase, and wet and dry olivine – the likely constitutents of oceanic crust and mantle). Is the likelihood of large earthquakes on subduction zones governed by the presence or absence of serpentine, rather than age of subducting crust or rate of plate movement? This would seem difficult to test, but if the seismic velocity and Poisson ratio of serpentinites are unusual, maybe tests are possible.

And what about those weird not-really-earthquake events that take place? Silent earthquakes, like the ones along the Cascadia subduction zone beneath Vancouver Island? Or afterslip – slow movement after a major earthquake, like occurred after the 2004 Boxing Day earthquake off Sumatra? (GPS data suggests that, during the month after the earthquake, slow slip equivalent to a M 8.7 earthquake occurred (Subarya et al., 2006).) The authors calculate that stresses in serpentine should relax on the same time scales described for slow earthquakes and afterslip, and imply that serpentine could explain some of the things that have been observed. There are some problems with that explanation, though. Modeling of afterslip following the Sumatra earthquake (Subarya et al., 2006), placed the slip in the same area as the initial earthquake, at depths shallower than 25 to 50 km, depending on the model they used to invert their ground deformation data. And Hilairet et al. cite studies saying that Sumatra has no evidence of serpentinization. So...the pieces don’t all fit together. Not perfectly.

This study, plus the recent study that found talc in the San Andreas Fault, are interesting because they tie together mineral behavior, rock mechanics, and seismology. The combination seems fruitful. I’ll be interested to see how the modelers who study triggered earthquakes work with these data.

References:

Hilairet, N., Reynard, B., Wang, Y., Daniel, I., Merkel, S., Nishiyama, N., and Petitgirard, S., 2007, High-pressure creep of serpentine, interseismic deformation, and initiation of subduction: Science, v. 318, p. 1910-1913.

Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J-P., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., and McCaffrey, R., 2006, Plate-boundary deformation associated with the great Sumatra-Andaman earthquake: Nature, v. 440, p. 46-51, doi:10.1038.

Interesting post about Japan earthquake early warning system

Sismordia has posted a really interesting description of Japan's earthquake early warning system. At the end, she asks for comments:

Call for comments: Are you living in Japan, or do you know people who are? Have you (or they) had to react to an Earthquake Early Warning yet? If so, please let me know how it worked, what went through your mind, what you did to prepare for the shaking, how you felt afterwards.

I'm curious, too, so if you know anyone who has experienced the system, please go over to her blog and describe your experiences.

Anniversaries and missed opportunities

The Loma Prieta earthquake was eighteen years ago today.

And I mentioned it in one class already (as "the Big Enough One," in response to a question about the maximum possible size of an earthquake in a different tectonic setting). And I totally forget the anniversary.

Perhaps it's not that important to remember any more. I mean... the college freshmen were born in 1989.

Human memory really isn't long enough to handle geology, is it?

Not dead, but shaken

I really had no one but myself to blame.

Consider this: I chose my graduate school after a lovely late-March visit. But it wasn’t the California spring sunshine that sold me. Nope. It was a drive up Page Mill Road with two first-year grad students. A drive to the San Andreas fault.

When we got back, they pointed to the old, unreinforced sandstone masonry of Geology Corner. “They haven’t gotten around to reinforcing this side of the Quad yet,” they said. “So this could all fall down in an earthquake.”

We all laughed.

So, as I said, it wasn’t as if I hadn’t been warned.

On October 17, 1989, I had been a student at Stanford for three weeks. It was a beautiful afternoon, and I had spent it avoiding doing my homework, as any new grad student who hasn’t yet figured out that she isn’t still in college might do. Around 5 pm, I had finally settled down to work a little before supper.

And then the desk started to shake.

“Oh, cool,” I thought. “Maybe this is an earthquake!” I considered getting under my desk.

My desk started moving across the floor. I decided that maybe the door was a better choice.

I couldn’t stand in the door, so I sat and wedged myself in, feet on one side of the frame, back on the other. As the floor rolled like a ship in high seas, I yelled: “Is this an earthquake? Is this an earthquake?”

Then I heard the sound of glass breaking, and then all was still. Well, except for the fire alarm going off. I knew what that meant: time to evacuate the building. So I did.

It was hard to piece together the news that we heard over the car radio. The Bay Bridge had collapsed. San Francisco was burning. The aqueduct to Hetch Hetchy was broken. (The last one, fortunately, was untrue, and the fire in the city was restricted to the Marina District.) I did what any good geology student would do: I grabbed all of my camping equipment and prepared to survive for as long as necessary.

We slept on the lawn, afraid of aftershocks and gas leaks. The next morning, we snuck into a dorm to take a shower (because, you know, if there’s still water somewhere, one should always take a shower before the water is gone!), and then waited for news. Classes were cancelled; building inspectors had to decide what was safe and what was damaged.

By the end of the week, the “damaged” list included both my house (built in, I believe, the 1910’s) and Geology Corner.

I slept on at least three different couches that week. I learned everything I now know about earthquakes when the Geophysics Department decided to turn the event into a Teachable Moment. (It was a good idea, really. Though they still had a functioning building.) I missed getting to see Naomi Oreskes defend her dissertation. I packed my personal belongings into boxes in a moving truck, and my geology belongings into another set of boxes. It took a week to find a new place to live, and longer to find the location of the trailer which housed the new geology grad student offices. The post office never would forward my mail, because I had lived at a group mail address. For all I know, the Stanford Post Office is still throwing away my catalogues.

The aftershocks woke me up at night for the rest of the year.

We were lucky, in many ways. Neither my home nor my office (nor, for that matter, the Earthquake Engineering Building) was seismically sound. My house had a brick foundation; after the earthquake, it bulged in ways that a brick wall shouldn’t. (It was later condemned and torn down.) The sandstone of the Quad hadn’t held up particularly well in 1906, and it spent the 90’s braced so that the keystones would not collapse. (It was fixed and reinforced... eventually.) But nothing actually fell down on campus, except a chimney on another house, and a lot of bookcases. Further from the epicenter, a double-decker freeway in Oakland had collapsed, and people were killed.

But I survived with nothing worse than a lot of stress, and a good story to tell in class.

Indonesian earthquakes: just foreshocks of the Really Humongous One?

Kerry Sieh thinks the M8.4 and other recent earthquakes off the coast of Sumatra might be essentially foreshocks to a larger, M9+ earthquake:

"No one can say whether it will be in 30 seconds or 30 years," he said. "But what happened the other day, I think is quite possibly a sequence of smaller earthquakes leading up to the bigger one."

(Quote from yahoo's news, but I've seen essentially the same report other places.)

He's published a lot of stuff about the Sumatra earthquakes since 2004, and I haven't read any of it, so I'm not sure which of several reasons he may have for his statement. (He has worked out the paleoseismic history of the subduction zone, so he may be thinking about recurrence intervals - typical times between major earthquakes on a given fault zone. He could be thinking about seismic gaps - areas of the fault that haven't had as many earthquakes, and are therefore probably more prone to slip. He could be thinking about the M 8.4 earthquake as a foreshock - a smaller earthquake that precedes a larger earthquake. He could be thinking about stress changes as a result of the 2004 M9 earthquake - something similar to the work that's been done on the North Anatolian Fault in Turkey, except on a subduction zone.)

[Edit: I suppose Sieh could also be making a far more basic point: the subduction zone is capable of larger earthquakes. The news may simply be a good opportunity to remind people of that fact, just like little earthquakes in LA or SF are a good opportunity to remind people that California is earthquake country.)

It feels like studies of earthquake mechanics are on the verge of really understanding the process better. It's exciting... though for the sake of Sumatra and people living beside the Indian Ocean, I hope Sieh is wrong.