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How to mislocate an earthquake [nerdy]

One of the bread-and-butter things seismologists do is locate earthquakes. There are kind of two main flavors of this: one is global and the other is local/regional.  GG doesn’t do global relocations (but will point out that depth of such locations often relies on the presence of depth phases like pP reflections from the surface or the relative strength of surface waves) but has done local event locations.  And there are gotchas out there that often aren’t as appreciated as they should be because all too often data is pitched into an inversion code and the resulting output is accepted as correct.

We’ll start with some simple things and move into somewhat complex stuff a bit but will stop short of the really ugly problems of locating events in a 3-D structure that is in part determined by those same travel times.

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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.

The 11% Prediction…

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.

And the chances that a quake will surpass the 7.1 temblor that struck near Ridgecrest on Friday night are roughly 8% to 9%, said Caltech seismologist Lucy Jones.

Los Angeles Times

Contrast with this:

Aftershock Forecast

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.

USGS National Earthquake Information Center

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.

Except…

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:

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.

Screen Shot 2019-07-06 at 11.22.14 AM.png

10:30 am 7/5 – 10:30 am 7/6 PDT. Mainshock (M7.1) in center of band of seismicity. Aftershocks define the rupture more or less, which extends from the Garlock fault at lower right into the Airport Lake area near the blue dot. USGS Latest Earthquakes page.

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.

Curious Quakes

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.

RidgecrestEQ2019a

M2.5 and greater EQs from past week (6/29-7/5/19) from USGS as of 10:45 am PDT. Mainshock is blue.

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.

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Is Oklahoma Now OK?

Its been quite awhile since we checked in on the seismicity in Oklahoma.  As we’ll see, on the whole the news is good, but there are a couple of things worth watching…

First, the number of quakes has steadily dwindled…

OK_EQs_2015-2018

This is what you’d hope to see with decreases in wastewater injection.  Some of this is regulatory, but a big piece is because the low price of oil made the more water-rich (an thus injection-heavy) fields less attractive.

If instead we look at seismic moment, things are somewhat less clear:

OK_EQ_Moment_15-18

Now first off you see the big drop in increase of seismic moment starting in late 2016; that rate has continued to the end of 2018 (the red curve is new since the last post). But curiously it hasn’t dropped: the M4.6 in April of 2018 offsets the seemingly slowing rate since then–a straight line from the end of 2016 through early 2018 projects right to where the cumulative seismic moment stood at the end of 2018. At present it seems the moment release rate is pretty constant. For this to coexist with a decreasing number of earthquakes means that earthquakes are getting larger even as they are less frequent.

What this means is that while things are a lot better, they might not be improving as much as you could hope.

If you want to see some reporting on how others view this, Tulsa World had a good end-of-year review. The data used here is from the USGS’s earthquake catalog.

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).

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