For those who come hoping to see material related to the Grumpy Geophysicist’s trade book on the Sierra Nevada, The Mountains that Remade America, here are a few quick pointers.
GG will give a talk at the Rocky Mountain Map Society on September 10, 2019, at the Denver Public Library, fifth floor, Gates Room, 5:30 pm and open to the public.
“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.
In previous entries, we’ve examined the emplacement of oceanic/forearc affinity (POR) schists and the igneous activity from a similar timeframe. Here we will consider what was deforming when and how. There are four main pieces to this puzzle: the termination of ongoing thrusting of the Sevier, Eastern Sierra belts, and Maria belts, the emergence of thrusting in southeastern Arizona and New Mexico, the appearance of extensional faulting, and the beginnings of Laramide shortening in the Colorado Plateau and Southern Rockies.
The outline version is that thrusting in the north-south trending Sevier foreland fold-and-thrust belt shutdown by about 90 Ma in southern Nevada but continued for another 30+ million years farther north. Northwest-trending retroarc thrusts probably continued to be active in southeastern California until 80 Ma and possibly 75 Ma. Rock uplift and extensional shear zones between ~75 and ~65 Ma in several localities may reflect extension of the crust in the Sevier hinterland, but some kind of intra-continental convection is hard to rule out (e.g., Wernicke and Getty, 1997). Closer to the coast, right-lateral strike-slip deformation in the dying Sierran arc reflects some obliquity to convergence at the plate margin. As time passes, Laramide-style basement-cored uplifts begin to emerge, perhaps including structures very close to the Sevier thrust front in the Kingman arch and associated uplift of the southwestern Colorado Plateau. Thrusting appears to have accompanied magmatism in expanding eastward across southern Arizona.
There is a lot here and yet GG is confident he’s missed some important papers–feel free to point some out in the comments.
Seems like every week or two, somebody is complaining about peer review–it is a barrier to scientific communication, it empowers gatekeepers, it destroys careers, etc. Now GG doesn’t buy into that (baby and bathwater territory, in his view), but perhaps as an exercise, what would happen if we outlawed peer review?
So you write up some science and want it shared with other scientists. Let’s consider your options: email, blog post, paper server, journal.
So to start with, you email all the colleagues in your field with your paper–how does that go? Probably your email looks like spam to a bunch, but maybe you have cultivated connections well and several colleagues read the paper and want to incorporate its results in their work. How shall they cite it? Should they include your article with theirs, a kind of blockchain sort of thing? Hmmm, this doesn’t sound promising…
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.
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.
All the attention on the Ridgecrest earthquakes has returned the Eastern California Shear Zone and its rather more obscure relative, the Walker Lane. Among the news articles out there is a pointer back to an article on the notion that the plate boundary will shift in to these fault systems sometime in the geologic future. This then results in western Nevada eventually facing an ocean–or, more extravagantly, Salt Lake City as a coastal town. So let’s clarify our terms, understand when and why these faults have emerged, and then set out to consider what it takes to move a chunk of continent onto an oceanic plate.
First off, there are two named parts to any potential new plate boundary (well, actually, they are already taking up about a quarter of the plate motion): the Eastern California Shear Zone and the Walker Lane. The Eastern California Shear Zone is a well-defined feature in the Mojave Desert south of the Garlock fault and named recently in 1990 by Dokka and Travis. The Walker Lane, in contrast, has had multiple incarnations: it was originally defined by Locke et al. (1940) on the basis of more confused topography along a swath of ground running from Las Vegas through the Walker Lake area to north of Reno. At times it was thought to be as old as Jurassic. John Stewart redefined it in a 1988 book chapter, and subsequent workers have now generally drawn the Walker Lane as kind of hugging the east side of the Sierra out about 150-200 km into the Basin and Range, removing the Las Vegas Valley Shear Zone from being within the Walker Lane and, generally, dating the strike-slip motion within the last 9 million years, with ~4 Ma seemingly a good guess (Andrew and Walker, 2009).
The northern edge is more of a mystery and the names for it more inconsistent. The Central Nevada Seismic Zone takes off from the Walker Lane to the northeast as a more dominantly normal fault system. Strike-slip faulting cuts into the Sierra on a Northern California Fault Zone (Wesnousky, 2005) and might connect to the coast (e.g., Unruh et al., 2003) or towards the Cascades arc (e.g., Waldien et al., 2019), and some have carried deformation northward through central and eastern Oregon and Washington (e.g., Pezzopane and Weldon, 1993). If we really are making a new transform boundary, its northern extent sure isn’t obvious.
So why is strike-slip faulting a relatively late arrival in this area where deformation extends back 15 or 25 million years?
Update 7/7: The real time aftershock forecaster has now dropped the probability of a M7+ to under 1% in the coming week. Lucy Jones’s twitter feed notes the decreasing rate of earthquakes drives the predicted aftershock rates down fairly quickly as well. The basis for this is are statistical analyses of earthquakes in the past; it doesn’t really include the more challenging suggestions of changes over years in the stress field as the lower crust and/or mantle relax (e.g., Landers and Hector Mine quakes, Freed and Lin, Nature, 2001; later papers highlight difficulties in modeling this (e.g., Freed et al., EPSL, 2010)).
11% chance of another huge earthquake in Southern California, scientists say
The odds that Southern California will experience another earthquake of magnitude 7 or greater in the next week are now nearly 11%, according to preliminary estimates from seismologists.
Contrast with this:
The USGS estimates the chance of more aftershocks as follows: Within the next 1 Week until 2019-07-13 15:00:00 (UTC):
- The chance of an earthquake of magnitude 7 or higher is 3 %, such an earthquake is possible but with a low probability.
While often the media goes in search of irrelevant discrepancies (“A says magnitude 5.9, B says 6.1–who is lying!?”), this difference is quite striking. First, Dr. Lucy Jones’s main research focus over her career has been the statistics of aftershock (and possible foreshock) sequences; although her USGS affiliation ended a couple years back, you’d kind of expect the survey to be relying on her work.
In general the probability of an event being a foreshock has typically been represented as a 1 in 20 probability. And that probability has to go down the larger the earthquake–after all, if we saw a M9.5 event, there is no room to go higher. So why is Dr. Jones [again, no relation to GG] saying there is an 11% chance of a M7 or higher? Or was she misquoted? Frankly it is hard to say; her twitter feed is a bit contradictory:
Like any quake, today’s M7.1 has a 1 in 20 of being followed by something even bigger. Smaller quakes – M5s are likely and a M6 is quite possible.
— Dr. Lucy Jones (@DrLucyJones) July 6, 2019
Yes, we estimate that there’s about a 1 in 10 chance that Searles Valley will see another M7. That is a 9 in 10 chance that tonight’s M7.1 was the largest.
— Dr. Lucy Jones (@DrLucyJones) July 6, 2019
As GG cannot find much in the way of explanation, the best he can do is guess. So here goes. It is possible there are two things that are contributing to the somewhat higher probability of a large EQ to come. One is that this is a very active sequence, which is more characteristic of areas with volcanic activity. Such sequences have a higher chance of behaving like a swarm, where there are a lot of earthquakes near the high end. So perhaps a b-value calculation has led Dr. Jones to estimate a higher chance of large aftershocks. The second might be calculations of the changes in stresses on nearby faults. The Airport Lake fault zone terminates at a “step over” to the Little Lake fault to the north; it is likely that fault has had an increase in stress on it. Similarly, the Blackwater fault to the south could well have had a stress increase (though that seems somewhat less likely because of the left-lateral part of the original M6.4 sequence). That the LA Times story quotes Dr. Jones thinking that a M6+ EQ in the Owens Valley seems plausible suggests the second explanation might be more likely.
What does the future hold? Hard to say: in some ways, this event resembles the Landers earthquake, which probably had a role in setting up the Hector Mine event a few years later. As noted last night, the northern termination of the fault has shifted into complex geology of the stopover to the Little Lake fault; it appears that the north-south striking normal faults are picking up slip from Airport Lake to the north. Right now USGS and Caltech are locating a number of earthquakes into Death Valley–whether these are real or simply artifacts of auto pickers confusing two events remains to be seen, but Death Valley has seen very little seismicity in the instrumental record and so a real upswing would be of interest. The events near “Rose” in Rose Valley are near the Coso Geothermal Field, which is a fairly active volcanic zone. It is unsurprising that some earthquakes would be triggered there; whether this seismicity could lead to some changes in that system is an interesting question.
The folks who might most want to make sure their earthquake kits are ready and there isn’t anything ready to fall on them (bookcases, heavy pictures) are probably in Rose Valley and Owens Valley and then possibly down to Barstow. And, quite possibly, the Sierra foothills communities in the general vicinity of Porterville–not because a fault would be close but because ground shaking transmits pretty well across the Sierra.
Update 7/5/19 ~ 9:30-10:30 pm PDT. Well, looks like the rest of the Airport Lake fault zone ruptured this evening (that is the alignment of the northwest trending limb of seismicity from the earlier sequence). This probably is the farthest edge of this fault zone. It might well load the Little Lake fault, which is kind of the next right-lateral system to the north. So if there is a further progression, rupturing the Little Lake fault would be the next logical domino to fall. But that isn’t terribly likely. No doubt the naval weapons center has a bit of a mess as the rupture tracked right past their main facility. What will be interesting will be the focal mechanisms at the northwest end; the NW-SE trending folds of the White Hills anticline might produce some thrust mechanisms, or the oblique-normal Coso Wash fault might produce normal faulting mechanisms. [The M5.5 aftershock in the northwest corner is an oblique normal mechanism on–probably–a north-south trending fault, which looks likely to be the Coso Wash fault].
The southeast end isn’t home free either; the Blackwater fault on the south side of the Garlock probably has seen some increase in stress from these events. It is less clear if that is true of the Garlock fault. But given the complexity at each end of the rupture of this M7.1, stress transfer is probably complex.
And listening to the media is a bit disheartening. “People in LA are used to this.” Um, well, they are 150 miles from the event and any directivity with this event (it appears to have been a bidirectional rupture from the aftershocks) would affect places like Barstow or perhaps Porterville far more than LA. The long rolling motion at those distances is generally not a problem (it does produce sloshing pools). Sleep in a house or not? Certainly depends on the house: if the gas is off and the house is on its foundation and there are no structural issues, you could stay in the house, but if you aren’t certain of all those, staying out would be for the best. As Lucy Jones [not related to GG] is fond of relating, there is about a 5% chance there could be a larger quake, so some caution is warranted. They were saying “there is not a continuous stream of ambulances”–but there are not too many in Ridgecrest to start with, so the absence of a stream is hardly a surprise even if there are numerous injuries. Asking a resident of Ridgecrest “did you get an alert”? Simple answer is no: the town was too close for the alert system to give a useful warning. But a good question is whether there was a warning at other communities–it sounded like the system didn’t trigger on the M6.4–shaking from the 7.1 could be of concern in some areas.
And now we have speculation on triggering distant events. This is where things get a little shaky. Lucy Jones, when interviewed, downplayed remote triggering, instead emphasizing that triggered large aftershocks are usually pretty close–but the Little Skull Mountain earthquake in southern Nevada looked a lot like it was triggered by the Landers earthquake, so this is a bit less certain. Generally triggered seismicity occurs in areas with magma or geothermal systems–Mammoth Lakes area and Yellowstone have had earthquakes (generally pretty small ones) triggered by the shear and surface waves of distant events, so it isn’t impossible to trigger something at a distance. Right now not much such seismicity is showing up, but then the systems right now are pretty heavily hit by aftershocks near Ridgecrest.
Original post: So SoCal finally got an earthquake above M6. GG suspects the residents of Ridgecrest are tired of hearing in news reports about how they live in a remote rural area. (If your vision of “rural” are widely separated farm houses with crops or cattle in between, this ain’t it–the main business is the China Lake Naval Weapons Center and, to a far lesser degree, tourists on their way somewhere else). But let’s take a brief look at the seismicity associated with this event, for this is a curious area.
First off, this is hardly a new thing in this area. In the mid-1990s there were several earthquakes very near this spot--a northwest trending band to the NW and a southwest-trending band to the SW. This sequence appears to illuminate both, with the southwest trending band appearing to be a left-lateral strike-slip fault and the northwest trending band a right-lateral strike-slip fault. The seismicity extends to a mapped Quaternary fault; it seems likely that the “crack” reported and filled by Caltrans had a couple inches of left-lateral offset (there are some photos on Twitter showing the white line on the edge of the road having been offset). There is no mapped fault on the northwest trending leg, but the existing mapping in the area wasn’t focused on identifying such minor faults. We’ll learn shortly whether there was surface rupture in that direction.
So why is this curious? Generally the swarms in the area have been on one system or the other; this includes both, and for that reason is a great reminder of the peculiar tectonics of this region.