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“We Missed It”

“We’d like to think we know about all of the faults of that size and their prehistory, but here we missed it,” Dr. [Ross] Stein said.

“The geologists in this area are the very best — people aren’t asleep at the wheel,” he said. “But there are real opportunities for young scientists to come in and learn how to do this better.”–New York Times story on Ridgecrest earthquake

We missed it?  As one who has worked in this area, GG didn’t feel that way, though he was never asked beforehand if a M7 was possible there.  There were mapped scarps in very young alluvium along a pretty well established seismic lineament. That this could be one connected fault seemed pretty darn obvious, but close study was always a challenge due to the presence of the China Lake Naval Weapons Center.  It even had a name–the Airport Lake fault zone.  And frankly, there are many others like this kicking around in the west.

There is in point of fact a very long list of geoscientists “missing it” out there, including most prominently these:

  • When GG was an undergraduate he was taught that all earthquakes in California with a magnitude above about 6 would produce ground rupture.  This was then followed in short order by the Coalinga earthquake (1983, M6.7), the Whittier Narrows earthquake (1987 M5.9), the Loma Prieta earthquake (1989 M6.9), and the Northridge earthquake (1994, M6.7), none of which produced the kind of dramatic surface rupture expected. (While there was some surface deformation in Loma Prieta, it isn’t clear that any of it was from the main fault). Frankly, the peculiar relation between the surface rupture and fault rupture of the 1952 Kern County (Arvin-Tehachapi) earthquake should have been a hint that surface rupture wasn’t a given.
  • Seismic hazard assessments assumed that the biggest earthquake you could get associated with slip on a fault was related to the length of that fault.  Then we got the Landers (1992 M7.3) earthquake, which ruptured several unconnected but similar faults. This should have been seen coming, though, as the Dixie Valley/Fairview Peak earthquakes in 1954 demonstrated much the same kind of behavior. A related misjudgment was that big faults were segmented and thus there was a maximum earthquake that could be inferred from past ruptures. Tohoku (M9.1, 2011) underscored that as a bad interpretation.
  • Seismologists often would say that earthquakes don’t trigger distant earthquakes because the finite stress changes don’t go out that far. The Landers event triggered seismicity as much as 1250 km away, mainly (it seems) from the dynamic stresses associated with the surface waves from that event. This has now been observed in other large events. There are suggestions that other stress transfer mechanisms might be out there that led, for instance, to the Little Skull Mountain earthquake and the much later Hector Mine (M7.1) earthquake after Landers.
  • Not as clearly stated but clearly in the mindset of seismologists was that big earthquakes are of one dominant motion.  So while Landers was on several faults, they were all pretty much strike-slip faults and the feeling was they were connected at depth. But we then got the Kaikoura earthquake (M7.8, 2016) (among others), which spectacularly lit up a large number of individual faults with wildly different styles of slip. Frankly, the Big Bear earthquake (M6.3) that shortly followed Landers but was a totally separate and very different orientation should have hinted that very complex earthquakes were possible.

So frankly having a seismic zone with scattered preserved scarps in an alluviating environment be the hints of a through-going fault is hardly a shock.  GG thinks that a better interview target would have been Egill Hauksson, who has studied the seismicity of the Coso region in particular (something that Ross Stein had not prior to this event) to see if he felt that this was “missed.”

Given all this, what are some of the under-appreciated hazards out there? After all, the Big One is supposed to be a rerun of the 1857 Ft. Tejon earthquake. GG thinks worse could be out there.  You want a really big one? What if the Malibu Coast, Hollywood Hills, Raymond Hills and Sierra Madre faults all went as one event?  They all are doing the same sort of thing, but hazard mappers consider each to be independent.  And while that is probably true for the average surface rupturing earthquake (as, for instance, 1971 San Fernando was separate from the kinematically similar and adjacent Northridge earthquake), that is no guarantee. Maybe you wouldn’t exceed M8, but a rupture like that would pound LA like nothing else. Or maybe multiple segments of the Wasatch Fault go as one (though frankly even the one segment in Salt Lake City would be devastating). There are no end of partially buried, poorly studied structures across the whole of the Basin and Range. Lots of stuff could be hiding in the forests of the Cascades as well.

Basically, when we look as geologists at the Earth, we are seeing only the top surface of a deforming medium.  That top surface is constantly being modified by other processes (mainly erosion, deposition and urbanization). Toss in that major earthquake faults are not razor sharp planes penetrating the earth but are a complex creation of a network of smaller faults that have coalesced in some manner and you expect it to be hard to pick out all the big faults. Even adding subsurface information (which is often quite deficient in these areas) and faults can hide. Go farther east and it gets even hazier as recurrence times get really long and so hints of past activity hide from view. Frankly, there are probably some truly great misses out there; Ridgecrest really isn’t that far off the mark from what we might have expected.

Coso Concerns

Update 7/14/19. Things are steadily quieting down in this area, though there are still a lot of small (M<2.5) quakes just west of the rhyolite domes. This spot and the area near Little Cactus Flat to the north remain the most active areas outside of the original ruptures.

Update 7/11/19. While the number of quakes in this area is declining, there was a M4.3 that also had a large non-double couple mechanism–according to Caltech.  The USGS-NEIC also estimated a solution and got something much more like regular fault slip. Which indicates that getting mechanisms for very shallow M4s can be tweaky.  While more action is now farther north, those events look more fault like–though those mechanisms are also from NEIC, so could be NEIC’s procedures tend towards double-couple solutions more than CIT’s. And as an aside, it is a bit surprising how little activity has been at Mammoth–it is an area that has had seismicity triggered by surface waves in the past, but has remained fairly quiet this go round.

Original post: One thing GG has kind of been looking for is whether the M7.1 Ridgecrest event is triggering things near the Coso volcanic field. And it seems there is something worth being concerned about going on.

CosoEQs

Annotated version of USGS seismicity map (past week M2.5 and above). Band of orange (past day’s) events about 3 km west of rhyolite domes noted.

Seismicity in this area is traditionally shallow, meaning above 5 km depth (Monastero et al., 2005). The tight cluster of orange dots include 2 M4+ earthquakes.  This area is at the west edge of a seismic discontinuity at about 5 km depth inferred to represent the top of a magma chamber (Wilson et al., 2003). While there has certainly been seismicity in this region before, given the proximity to fairly recent volcanic activity, one has to wonder if there is magma on the move.  Supporting that are the focal mechanisms for the two M4 earthquakes, both of which have substantial non-double couple components (indeed, the mechanism for one looks very much like a diking event). Given that all these events are being located in the top 2 km (probably relative to sea level, so top 3 km of crust), this could get pretty interesting pretty fast.

As background, the central core of the Coso volcanic field are silica-rich rhyolites that appear as blister-like bodies in the image above.  Surrounding this core area that overlies the seismically inferred magma body are basaltic eruptions (like Red Cone, in lower left corner). The troubling seismicity is directly on the road into the geothermal area from Coso Junction to the west.

An overview of the M7.1 with the first InSAR image of the 7.1 rupture is at Temblor.com. This also discusses seismicity in this area, but with less consideration of volcanic activity.

Avoiding Volcanoes (Book Review)

Thirty nine years ago today 57 people died when Mt. St. Helens blew out its side early on a Sunday morning. Just why those lives were lost–and how it could have been far worse–are questions addressed in Steve Olson’s 2016 book,  Eruption: The Untold Story of Mount St. Helens

It would seem easy to avoid volcanoes.  We know where they are, that they are dangerous.  Yet as we have seen recently in Hawaii, we somehow can’t find it in ourselves to stay away and so, when the inevitable happens, we find lives threatened, ruined or ended and property destroyed. The 1980 eruption of Mt. St. Helens is often described as unpredictable and thus the loss of lives hardly a surprise, but the reality was different.

Olson’s book takes a curious path, wandering through the origins of the Weyerhaeuser empire and the national forest system before reaching the tales from the eruption itself. Reading these when expecting more immediate stories of death or survival, destruction and salvation can feel like assembling the blank parts of a jigsaw puzzle–you don’t feel like you are seeing the picture you are seeking and wondering why you should bother with the effort.

StHelens1982

Mt. St. Helens from closest pubobservation point, Sept. 1982, showing the blowdown forest and steam coming off the growing dome in the crater. (c) Craig Jones

But just as the big blank areas on a jigsaw are critical to the final image, these background stories are essential to the author’s main focus on those killed or nearly killed by the eruption and how they found themselves at risk. Read More…

Fear of Every Geology Prof…

…is death on a class trip.  Going to places with unstable footing and exposure is often part of seeing geology that clarifies understanding, but it carries real risks. For GG, the most terrifying site is Toroweap Point in Grand Canyon National Park where, every time he visits, he breathes a sign of relief when the same number of students pile back into vehicles that had piled out of them. That site has 3000′ of vertical cliff to punish the unwary, but it doesn’t take that much for a fatality, as an environmental studies class from Briar Cliff University found out when they lost a classmate to a 100′ fall.

While family and friends grieve, another discussion is probably going on, if not now then soon. Should the school curtail field expeditions? Given the growing number of deaths by selfie, what is the role (and responsibility) of the instructor who takes students to places with hazards? Should the school dictate what is and is not an acceptable risk? Should students sign waivers, and if so, are they really enforceable?

Geoscience education benefits immensely from seeing what you are studying in the field. And the greatest hazard in field trips is generally the drive to the field or working on roadcuts near highways. But the drama of a fatal fall is more damning in some ways.  GG hopes that future students will get to experience the field safely, hopefully mainly by recognizing and avoiding hazardous situations on their own and with the guidance of an instructor rather than by being blocked from accessing important or memorable sites by fearful administrators.

Reaping Restraint

GG has written a few times about the state of oil and gas regulations here in Colorado. A lot of that is now changing as Senate Bill 181 has passed the Colorado Legislature and heading for the governor’s desk, where it is expected to be signed.

As is typical these days, the public debate was overheated.  Claims that passing this legislation would end petroleum development and cripple the Colorado economy were broadcast in commercials, while some advocates felt that the bill didn’t go far enough and were upset when amendments loosened some of the language of the original bill. Others felt that this was overturning the voters’ rejection of proposition 112 last fall (e.g., comments here). Votes in the legislature were along party lines.

So is this the death knell of oil and gas in Colorado? Best to see what passed rather than rely on public pronouncements. So let’s look at what is in here.

Read More…

Spring is Here…

…and while in other places flowers are blooming and trees are leafing out, in the Rockies it is fall.

Falling rocks, that is:

Rockfall2019detail

This is a spot on the road between Boulder and Nederland a little above Boulder Falls that GG has always been very wary of.  Fortunately it appears nobody got hit by a rock, though once you start having rockfalls in a spot there is an increased risk of more.  Probably the state highway department will have a close look in the near future.

Springtime is a big time for rockfalls as solid freezes of the winter give way to freeze-thaw and bigger temperature swings on canyon walls. Occasionally cars get crushed, usually cars parked under steep rocky slopes. More hazardous are rockfalls into dwellings.

The Biggest Disasters

GG recently commented on Lucy Jones’s [no relation] book on the Big Ones, disasters out of proportion to recent experience. An LA Times article on concerns that dams in the Los Angeles basin are not up to dealing with a superstorm brings up an interesting question: how big can you go? Forty days and forty nights?

For seismologists, the magic equation has often been the Gutenberg-Richter equation which basically says that the log of the number of earthquakes of a given magnitude over a specified time is inversely proportional to the magnitude (so log N = a + bM, where N is the number of earthquakes of magnitude M and a and b describe the distribution in some area). The rate of decrease in number of earthquakes with increasing magnitude, the b-value, is close to -1.  So say you have 10 M5 earthquakes in a year, you expect to have one M6. You’d then expect over 10 years to have 100 M5s, 10 M6s, and 1 M7.

If you keep playing this game, you might say that in 100 years you should see a M8, and in a thousand a M9, and in ten thousand a M10. And this is where seismologists see a problem: physically, a M10 is probably impossible (and if the area we’re concerning ourselves with is anything less than a quarter of the globe, it is certainly impossible).

Read More…