Just after 8am on the morning of October 15th (Philippines time) a large earthquake, M 7.1, occurred beneath the island of Bohol in the Philippines. The earthquake caused significant damage and has caused, at the time of this writing, almost 90 confirmed deaths. The earthquake was relatively shallow, originating only 20 km beneath the surface, which makes the damage relatively severe. However, Bohol is a relatively sparsely inhabited part of the Philippines. The nearest large urban area, Cebu City, is about 50 km away. Had this earthquake occurred nearer to major population centers the death toll could have been much higher.
Interestingly, according to CNN many of the injuries were due to “falling rubble” as opposed to outright building collapse. This is consistent with the distance between the epicenter and the major population centers. What this means is that, in this earthquake, a trained population provided with Earthquake Warning might have experienced a drastically reduced death toll. People’s gut reaction to feeling an earthquake is often to run out of the building they are in, and this can be a deadly mistake. Running out of a building during a quake is quite dangerous, as it exposes you to hazards from weak facade elements that commonly fall outward from a building, even if the building does not totally collapse. In the M 6.5 San Simeon earthquake, two people died in Paso Robles when they ran out of a building and were hit by a brick facade falling off the front of the building, while the building itself remained standing. The proper response, which can be trained, is to shelter in place within a building by dropping under a sturdy table or chair, covering your head and neck, and holding on to the furniture to prevent it shifting away from you. With that training, and a properly designed earthquake warning system, survivability in such events is drastically improved.
The shallow depth of this earthquake is particularly interesting because, at first blush, this earthquake might appear to be related to the Philippine Trench which resides just offshore to the east of the Philippines. This trench, like all subduction-type plate boundaries, produces shallow-angle thrust earthquakes which, at this depth, can produce tsunamis like the one seen in Japan in 2011. However, this earthquake differs from “normal” subduction events in two key ways: first, it is farther from the trench than we would expect given the depth of the earthquake; and second, the mechanism, though it is a thrust earthquake, is rotated and tilted differently to what we would expect in a subduction event. The latter difference is more academic than practical, but the shallow depth directly affects the impact of this earthquake. In the first figure to the left, we see a “normal” profile of depth of earthquakes vs. distance from the trench, this one taken from the Kurile Trench north of Japan. Notice how earthquakes get deeper the farther away from the trench they are.
The second figure shows the depth profile for the northern part of the Philippine Trench. This is for an older earthquake, so the star is not relevant to today’s earthquake (these figures are no longer produced by USGS so I used an older event). Notice the mess: there is no appreciable trend in depth vs. distance from the trench. This is because the tectonic regime in the Philippines is much more complicated. There are many faults, large and small, throughout the region that are separate from the Philippine Trench. This means earthquakes can occur at almost any depth up to 100 km, and in almost any location. This is a seismologist’s way of saying “Here Be Dragons,” that is, we don’t really know where the hazard is beyond just saying “everywhere.” From an earthquake warning perspective this changes a lot of our deployment strategy. In California, where the hazards are largely known and geographically localized, we can deploy a network to watch the hazardous areas (i.e., around the known faults) and warning everyone, regardless of whether they are near a fault or not. By contrast, in places like the Philippines where the hazard is either geographically distributed or just poorly known, we have to deploy the network around the population centers to protect them from an earthquake that could come from anywhere, even right under the cities. This is akin to the concept of “zone defense”, and while it works, it is less effective than a fault-centered strategy in that it tends to slightly reduce available warning times. There isn’t much we can do about it, that’s just the nature of trying to defend against “dragons.”
Early this morning, Pacific Time (4:30 in the afternoon of Sept. 24th in Pakistan), a major earthquake, currently estimated at Magnitude 7.7, struck a sparsely populated region of Pakistan. The earthquake was both large and very shallow: although the epicenter was 20 km (12 miles) underground, preliminary slip inversions indicate that the majority of the fault motion was between the surface and about 10 km (6 miles) depth. Although the area is remote and reports are sparse, given these facts we can reasonably expect this earthquake to have severe consequences. Current reports have injuries in the range of 100-200, but the USGS PAGER report for this earthquake (http://earthquake.usgs.gov/earthquakes/eventpage/usb000jyiv#pager) indicates a likely death toll around 1000.
The last major earthquake in Pakistan, the 2005 M 7.6 Kashmir Earthquake, killed over 70,000 people. The magnitude of that death toll, especially compared with comparable earthquakes in the Western world, highlight the importance of stringent building codes and strict compliance with those codes in preventing earthquake deaths. Pakistani homes and buildings are almost all constructed of unreinforced masonry (URM), usually cinder blocks cemented with mortar and little or no steel reinforcement. These buildings may as well be mud huts in an earthquake, for all they can resist the shaking. In situations like this, earthquake warning is of minimal value. The first order of business needs to be long-term retrofitting, reinforcement and, in many cases, outright reconstruction of the building stock. Only then does earthquake warning start to provide a significant benefit, as it does in the West.
Photo of the reported new island off the Pakistani coast. Dimensions are reported to be about 1/2 mile across, ~50 feet high.
One of the more amazing reports coming out of Pakistan is that a new island has appeared off the coast of Gwadar, some 200 miles from the epicenter. As strange as it sounds, this is not unprecedented: following a M 8 earthquake near the Gwadar coast in 1945, three small islands were reported to appear, accompanied by “fireballs.” This suggests, and the local geology strongly supports the hypothesis, that these islands were mud volcanoes triggered by the earthquake, and that the fireballs were ignitions of methane gas being released from the sediments in the bottom of the ocean, which in that area is rich in shale that can store methane in large quantities. The fact that this earthquake was hundreds of miles away adds some interest to the story, because it suggests that, if this earthquake is a mud volcano, it was triggered by relatively mild shaking. Keep in mind that this report, like almost any news coming out of the region, is uncorroborated and may be untrue. The photo to the right shows the reported island, and it’s more or less all we have to go on at this point. We will learn more about the effects of this earthquake in the days and weeks to come, but one thing is certain: life in large portions of Pakistan will be very hard for many people, for a very long time.
At around 7:30 this evening (about 2:30 tomorrow afternoon in New Zealand) a M 6.8 earthquake shook the South Island near Blenheim, a medium-size city of about 30,000 on the north end of the island. At this time there are no reports of damage or casualties, and as this is New Zealand’s sparsely-populated wine country, the damage from this event is not likely to be significant.
A tectonic map of New Zealand showing motion of the Pacific Plate relative to the Australian Plate, and the country's major faults.
The earthquake was a right-lateral strike-slip event, very similar to the sort of earthquakes that occur here on the San Andreas Fault. This quake occurred in the Marlborough Fault Zone, which is a series of nearly parallel faults that span the Cook Strait that separates the North and South Islands. The faults splay out from the Alpine Fault to the southwest, so the seismic hazard in the Marlborough region is very similar to that in the San Francisco Bay Area, where the San Andreas splays off into the seven major faults that span the Bay (starting from the sea and moving inland, the San Gregorio, San Andreas, Hayward-Rodgers Creek, Calaveras, Concord-Green Valley and Greenville faults).
What’s interesting about the earthquakes on the Marlborough Fault Zone is that, despite plate motions in this area that are both strike-slip and convergent (see map), the earthquakes in this area are mostly pure strike-slip. This is an example of “stress partitioning”, which is a phenomenon in which oblique convergent plate motion is partitioned onto two different fault systems. In this case, the Marlborough Fault Zone accommodates all the strike-slip motion between the Australian and Pacific plates, while the convergent motion is accommodated on the Hikurangi Trench, which has large, mostly thrust-mechanism earthquakes. The earthquakes on the Hikurangi Trench are similar in nature to the great (M 8 and larger) earthquakes that have hit over the last decade in Sumatra, Chile and most recently Japan. Since these two faults are loosely related, activity on the Marlborough Fault System may have a slight effect on the stress condition on the Hikurangi trench, though it is difficult to say. In addition, there have been a number of moderate earthquakes in the M 6 range in the Cook Strait over the past few months, and while the Marlborough Region is sparsely populated, on the other side of the Cook Strait is Wellington, New Zealand’s capital and second-most-populous urban region.
Around 10am today, a M4.7 earthquake struck the Coachella Valley. The quake was felt throughout the valley, rattled nerves but caused no reported structural damage. The felt reports for this earthquake, which appear in the map to the left, cap out at about Intensity V, which corresponds to very light damage and moderate shaking felt. As of this writing, the earthquake has generated over 200 aftershocks, including one M3.0 and four more of M2.5 or greater.
The earthquake appears to have been associated with the Buck Ridge fault, a sub-fault of the San Jacinto fault zone. The San Jacinto is one of the major faults threatening the Coachella Valley (the other being the San Andreas fault), and this section of the San Jacinto has produced M5 earthquakes twice in recent memory: the 2001 and 2005 Anza earthquakes were both in this same area. There have also been a few earthquakes centered somewhat south of this location, closer to Borrego Springs.
This earthquake displays a pretty typical aftershock sequence, though there appears to have been a M2.7 foreshock, as well as several smaller quakes, about 20 minutes before the M4.7 mainshock. A good rule of thumb is that the largest aftershock should be about one magnitude unit less than the mainshock, so in this earthquake we would expect no aftershocks larger than about M3.7. For earthquakes like today’s, the aftershocks should subside to the point of not being noticeable after no more than a few days. For larger earthquakes, that subsiding takes a lot longer. Today is coincidentally the two-year anniversary of the M9.0 Tohoku earthquake in Japan, and they are still feeling aftershocks from that earthquake!
SWS has 16 stations installed in the Valley, and they all performed as expected for this earthquake. Because of local variations in geology and topography, an earthquake’s felt intensity can vary significantly in different parts of the Valley. Our sensors account for these variations. The QuakeGuard systems are tuned to trigger at Intensity V, and today’s event was borderline. That means most of the stations triggered and a few did not, but all stations took the correct action for their local site conditions.
To the right is a map showing the warning times that the Valley would have been afforded for this earthquake, had the CREWS system been in place today. Warning times range from just under 5 seconds, for parts of the Valley closest to the epicenter, to nearly 15 seconds at the North end of the Valley. The reason CREWS can provide so much warning is because we designed it to handle an earthquake in the Valley itself. That is a much more difficult earthquake, and we designed the system to provide warnings even in that worst-case scenario. Compared to that, today’s earthquake is so far from the Valley that it’s easy for CREWS to handle, and the warning times are as good as they are because of the effort that went into the design. In other words, planning for the worst-case scenario pays dividends even if the earthquake is not the worst case!
On Monday evening, Pacific Time, the Global Seismograph Network recorded a M 5.1 earthquake in North Korea. Turns out, this was not an earthquake at all, but a North Korean nuclear test. Although North Korea admitted to having conducted the test, we didn’t really need them to tell us, because we can use some basic seismological understanding to tell that it wasn’t an earthquake at all.
Since you’re reading this on an Earthquake Warning blog, you already know that earthquakes emit both P-waves and S-waves, the latter of which are both stronger and slower than the former. S-waves are slower than P-waves because of the fundamental physics of wave propagation in a solid body, so that doesn’t change regardless of whether we’re talking about seismic waves in the earth, or a hammer hitting an iron bar, or any other physical waves passing through a solid. But the fact that S-waves are stronger than P-waves is unique to earthquakes. Why is that?
Let’s for a moment imagine an earthquake as a result of two blocks sliding past one another. They stick for a moment because of friction, and then they jerk past one another in an uneven fashion. These “jerks” are earthquakes. The motion of the blocks when they slide past one another is a “shear” or sideways motion. Because of this, earthquakes generate S-waves, which are shear waves, much more efficiently than P-waves, which are pressure waves.
Contrast that for a moment with an explosion (doesn’t have to be nuclear, conventional explosions work the same way): the motion of the ground around the explosion is all outward, in a pressure wave. So explosions, nukes included, generate P-waves much more efficiently than S-waves. So the situation is the opposite of earthquakes: the P-wave is larger and the S-wave is smaller (well, as a practical matter they’re about the same size because of certain complications, but never mind). Here we have a simple, elegant means of discriminating earthquakes from explosions. What’s more, based on statistics gathered over many years of nuclear tests (at the Nevada Test Site as well as other locations around the world), we can make a rough estimate of the yield of the bomb from the apparent magnitude. This particular bomb yield was around 6 kilotons TNT equivalent. Not a large bomb by nuclear standards (Little Boy, the bomb dropped on Hiroshima, yielded around 16 kilotons), but larger than any of North Korea’s prior nuclear tests.
The P-to-S ratio technique has been used for more than half a century to monitor nuclear tests. In fact, the Worldwide Standard Seismograph Network (WWSSN), which was the first global seismic network, was developed and deployed in the 1960′s, not for the sake of science, but for the express purpose of monitoring global nuclear testing! These days we have significantly more sophisticated means of discriminating nuclear bombs from earthquakes. These techniques involve gathering and integrating data from all over the world to arrive at a description of an event called a “moment tensor.” The most common way of looking at moment tensors is in the form of a “beach ball diagram”, three of which are shown in the figure to the left. The topmost beach ball looks just like a real beach ball, with two black panels and two white panels. This is the moment tensor for a typical earthquake, and the important thing is they are always half shaded and half white (sometimes other colors are used instead of black).
The beach ball in the middle is the moment tensor for an explosion: all black. The black color means that the initial motion of the earth was outward, away from the source. If the beach ball had been all white, that would indicate an implosion, with initial motion inward, toward the source. Seismologists can use these moment tensors to tell with a very high degree of accuracy whether a given signal is caused by an earthquake or an explosion. Or…
Something else! The bottommost beach ball looks pretty funny, kind of like a band of black goes around the middle. This is the beach ball from a very specific event: the collapse of the Crandall Canyon Mine in Utah, in 2007. This collapse led to the tragic deaths of six miners and three rescue workers in a subsequent cave-in, and led to a high-profile lawsuit against the owner of the mine. The owner claimed that a M 4 “earthquake” which coincided with the collapse caused the tragedy, and since it was an act of God he was not responsible. However, a team of seismologists, including Professor Doug Dreger of UC Berkeley, used the event’s moment tensor to prove conclusively that the M 4 event was the mine collapse itself and not an earthquake. The beach ball here is reproduced from a 2008 paper in Seismological Research Letters by Sean Ford (then Prof. Dreger’s grad student), Prof. Dreger, and Bill Walter at Lawrence Livermore National Lab.
EDIT: The beach ball at the bottom is not the entire moment tensor, but the part of the moment tensor that highlights the signal of the mine collapse. The complete moment tensor would be all white, like an implosion.
Last week saw a flurry of news articles covering a new paper in the journal Nature. The headline invariably went something like this:
“San Andreas Fault Capable of Producing a Megaquake!”
Well, they weren’t all that sensationalistic, but reading the articles one was left with this feeling of impending doom, as if Lex Luthor were about to knock California into the Pacific Ocean. The truth is, the Nature paper in question did not really address the San Andreas Fault at all, other than by way of saying “here’s a place where our work might be relevant” in the conclusion of the paper.
This paper was actually primarily about the 1999 Chi-Chi, Taiwan and 2011 Tohoku, Japan earthquakes. Both earthquakes were large, and both were notable for rupturing through regions of their respective faults which were not thought to support earthquakes. A fault that doesn’t cause earthquakes? How does that work?
It turns out that we see two types of fault behavior in nature. The most well-known type is called “locked” behavior, in which the fault does not slip at all, except during earthquakes. This is the behavior most people think of when they think of faults: long periods of nothing at all, interspersed with isolated large earthquakes that make the fault slip several feet at a time. The second type is called “creeping” behavior, and in this mode a fault does not cause earthquakes, but is constantly creeping along very slowly, about at the rate fingernails grow. The San Andreas Fault has a “creeping” section about at its midpoint, between San Juan Bautista and Parkfield, CA. It has been thought that this creeping section could not support earthquakes, so seismologists have historically considered only M7.8 “Northern San Andreas” and “Southern San Andreas” earthquakes likely, with very little probability of a M8.1 “wall-to-wall rupture” of the entire San Andreas Fault from Mendocino all the way to the Salton Sea.
The observation that led to the Nature paper, however, was that both the Tohoku and the Chi-Chi earthquake appeared to rupture through fully creeping fault segments, which had not been thought possible. To solve this mystery, the paper’s authors constructed a computer model of those faults and simulated earthquakes under a particular hypothesis involving water in the fault. Their computer models behaved in a way that matches the observations in Japan and Taiwan, so we have a plausible mechanism to explain how creeping segments rupture during an earthquake. Does this mean we need to re-evaluate the probability of a wall-to-wall earthquake on the San Andreas? I’d say, not quite yet.
There are a couple of lines of evidence that are still missing before I would feel comfortable hanging my hat on this scenario. First of all, the hypothesis put forward in the Nature paper requires rather specific geophysical properties on the fault, and we have not really sampled the creeping section of the San Andreas to look for those conditions. Second, we have a geologic record of earthquakes on the San Andreas Fault going back about 1000 years, and to my knowledge there is no evidence of a single earthquake rupturing both the Southern and Northern sections at the same time. Since this would be a very large event, it’s safe to assume that the evidence would be quite easy to spot if it were there, so I’m confident that such an event hasn’t happened in the past millennium.
Being a scientist means never having to say “never,” so I won’t say it’ll never happen. It is a remote possibility, and it doesn’t keep me up at night. Even if it did happen, the effect on San Francisco or LA would not be much different from a Northern or Southern scenario. The shaking would not be any worse in any given area, it would just be more widespread.
The difference is that both metro areas would be hit at once, so it does have some ramifications for our system design. For example, we are initially establishing two datacenters for the California-wide earthquake warning network: one in the San Francisco area, and one in the Los Angeles area. If one goes down the other can pick up the slack and the system will continue to function, but a wall-to-wall event could bring down both datacenters at once. So it’s prudent to place a third or even fourth datacenter away from the San Andreas Fault. That was our plan long before this paper came out, so in a sense nothing has changed. The system is designed to be robust, and it feels good to know that even if the doomsaying turns out to be true, we are prepared to handle it.
The original paper is (subscription required for the link):
Noda and Lapusta (2012), Stable creeping fault segments can become destructive as a result of dynamic weakening, Nature, doi:10.1038/nature11703
A couple of significant earthquakes happened in different parts of the world last week. Although they were both around M 7, and both caused a small number of deaths, their tectonics were very different.
First up, the M 7.4 off the Pacific coast of Guatemala, which sadly killed around 30 people. This earthquake is very simple: a vanilla subduction event on a pretty vanilla stretch of subduction zone known as the Middle America Trench, where the Cocos plate is diving beneath, here, the Caribbean plate. The only wrinkle in this area is the presence of the Motagua Fault, the strike-slip boundary between the North American and Caribbean plates, but that fault appears not to have been involved in this event. A few hundred kilometers north of this location, the Middle America Trench exhibits a seismic gap, known as the Guerrero Gap, where a major subduction earthquake (M 8+) is thought to be overdue, and threatens the Mexican coastline as well as the capital, Mexico City. But down here in Guatemala, there have been a number of M 7+ earthquakes and there is no particular sense of an overdue “big one.” In fact, the Motagua fault is much more hazardous to Guatemala, as it runs onshore unlike the subduction zone. If last week’s M 7.4 had occurred on the Motagua fault, the damage might have been much worse because the epicenter might have been much closer to Guatemala City. It is scenarios like that, as opposed to the earthquake that actually happened, that drive our system design for the Earthquake Warning System: large events, close to population centers, with limited communications and power infrastructure and sparse sensor networks. Our requirements for rapid processing, failsafe behavior, and single-station regional solutions mean that we can protect people even for a major event on the Motagua fault.
Next up is a M 6.8 in Burma. This was a shallow strike-slip earthquake that unfortunately took about a dozen lives in the epicentral region. The tectonics of this part of Southeast Asia stand in stark contrast to the simplicity of Central America: it pretty much doesn’t get more complicated than this. The overarching story here is: blame it on India. See, about 25-50 million years ago, the Indian plate slammed into Asia at the blazing-fast speed of 20 centimeters per year. That may not sound very fast, but in plate tectonic terms that’s doing 100 mph in a 25 zone. The most obvious result of that collision (which is still ongoing, by the way!) is the Himalaya mountain range, the tallest on Earth. But the collision was so vigorous, and the energy to be dissipated so great, that the impact has had an ongoing effect on all of Southeast Asia, which is being squeezed out of the way of the crash like so much plate tectonic gore. In effect, the entire Southeast Asian peninsula is a splatter of continental crust that is extending south and east away from the Himalayas. This can be seen quite easily in the figure to the left, which shows GPS-derived plate motions in this region. This phenomenon has been dubbed “escape tectonics,” and the net effect here is that the entire region is riddled with faults. In fact, it’s exceedingly hard to discern any real “plates” in this area, since the typical definition of a plate is a piece of crust that moves together as a unit. Since different parts of Southeast Asia are spreading in different directions, earth scientists have taken to referring to the tectonics in terms of “microplates” or even blocks, which are really small tectonic units.
Long story short, this area is a great big mess, tectonically speaking. There are a many known, large faults close together, and likely many more unrecognized faults waiting to rupture. The trouble with this is that you never know where the next big quake is going to come from. Well, you never know that anyway, but we know where the major threats are in places like California or Guatemala, and we can focus on protecting against those. This strategy won’t work in Southeast Asia, because there are just too many threats, known and unknown. Here again, SWS technology has an advantage over traditional networks. Although threat-based deployment strategies won’t work well here, SWS sensors can produce regional warnings from single stations. That means we can apply an asset-based deployment strategy without losing too much warning time, since we don’t have to wait for three or four stations to detect the shaking.
Although we are focusing on a warning system for California, the design decisions we’ve taken here make the system very versatile, and well-suited to deployments worldwide, even in relatively remote locations.
This evening, a Moment Magnitude 7.7 Earthquake struck the Queen Charlotte Islands off the west coast of Canada. Thankfully given the size, this earthquake struck a very sparsely populated region, and it appears that it did only minimal damage and there are no reported deaths as of this writing. The event was shallow (less than 20 km deep), and caused a small tsunami, so we will have to wait to see the effects of that tsunami in the coming hours.
The mechanism is consistent with a subduction zone earthquake, and it is near a major plate boundary. So… subduction earthquake, in the style of Sumatra and the Tohoku earthquake of last year, yes? Like the looming Cascadia earthquake south of this location, right? Stick a fork in, we’re done.
Er… not quite. Some things seem a bit off about this earthquake. The biggest one is that the Queen Charlotte Fault Zone (QCFZ), which extends from the Cascadia Subduction Zone in the south, to the Aleutian Trench in Alaska, is not a subduction plate boundary. It’s strike-slip, like the San Andreas Fault. The figure at the left (from Irving, et al., Tectonophysics, Nov. 2000) shows the plate motion of the Pacific Plate relative to North America (PAC and NAM in seismology lingo), and you can see it’s almost parallel to the QCFZ. Note: almost parallel. This will come into play a bit later.
The second issue is that, although this is a thrust earthquake, which is the type we expect to see in subduction zones, it is very steeply dipping (almost 45 degrees), whereas most subduction earthquakes have relatively shallow dips (more like 30 degrees or less). That is, most subduction zones have faults that are nearly horizontal, and this fault seems to be much more vertical.
Here’s what I think: this is a transform fault masquerading as a subduction zone! Perhaps it decided to dress up for Halloween. Possibly this is the West Coast’s answer to The Frankenstorm. Maybe nature just likes to keep us seismologists on our toes. The thing is, there is a small amount of convergent motion between PAC and NAM, as mentioned earlier. This convergent motion has to go somewhere. In some places, like Sumatra and Japan, the oblique motion partitions onto two large faults. In Sumatra the convergent motion is expressed on the Sunda Trench (where the 2004 earthquake struck) and the strike-slip motion is on the inland Sumatra Fault. Here, there is no one large subduction fault to take up the convergent motions, so they are accommodated on a series of thrust faults to the east of the main QCFZ.
I think it was one of these thrust faults that ruptured this evening, and that’s a good thing. Some people might wonder if this earthquake is a preview of an impending megathrust earthquake in the style of Japan or Sumatra. As we unfortunately learned thanks to an Italian court, a seismologist should never say never, but I think that the tectonic setting here is wrong for a megathrust. Surely a M 7.7 is nothing to sneeze at, but the likelihood of a larger, M 8.5+ earthquake in this region is pretty low.
This morning Imperial County was shaken up by a series of earthquakes, including a M 5.4, another M 5.3, and, so far, six earthquakes between M 4 and M 5, as well as numerous earthquakes between M 3 and 4. This is a traditional swarm, with multiple large shocks with no obvious mainshock-aftershock relationship to the smaller events (as opposed to the double event on the Elsinore Fault earlier this month).
Such swarms are relatively common in the Brawley Seismic Zone, between the southern end of the Salton Sea and the Mexican border. In this region, the San Andreas and San Jacinto Faults disperse into a broad zone of deformation that can best be described as a series of blocks rotating clockwise (see the map on the left). Note that this is a very “cartoony” view of the situation, and in reality these blocks are not as clean and simple as that. Because they are rough and broken up, it is not thought that the Brawley Seismic Zone itself can support a major earthquake, and consequently the blocks generate swarms of small to moderate earthquakes where they rub past one another. A similar swarm to today’s occurred in 2005, with maximum magnitudes around M 4.5.
Although it is unusual to see M 5+ earthquakes in these swarms, it is not unheard-of. Today’s events caused some minor damage in the town of Brawley, but so far no reported injuries. Because these events occurred so far from the Coachella Valley, the warning times from CREWS for these events range from about 20 to 40 seconds in the Coachella Valley. However, an ICREWS system could have provided 0.8 seconds warning in Brawley, and up to 10 seconds to the rest of Imperial County.
Over the last 24 hours there have been over 30 minor and light earthquakes in Southern California, including two M4.5 events last night and this morning, as well as a M3.4 this morning. The two mainshocks of roughly equal size are suggestive of a swarm of activity, but to me it actually looks like two distinct (but related) events, each with its own aftershock sequence. It is possible that the first earthquake changed the stress on the fault in such a way as to trigger the second one.
This is not unusual behavior. Most often an earthquake will trigger other earthquakes which are smaller than the original one; we call these triggered earthquakes “aftershocks” since they are smaller than the “mainshock.” About 5% of the time, an earthquake will trigger an even larger event shortly thereafter on the same fault. We then call the larger earthquake the mainshock, and the earlier earthquake is labeled a “foreshock.” It so happens that in this case, the first earthquake triggered a second earthquake of almost the same size, so is it a foreshock-mainshock sequence, or a mainshock-aftershock sequence? The difference is perhaps semantic.
The earthquakes occurred on the Whittier Fault, which is the northernmost segment of the larger Elsinore Fault System. This fault is thought to be capable of a M 7 earthquake, and directly threatens the LA basin due to its proximity to heavily urbanized areas. Although there have been significant earthquakes in this area in the past (the 2008 M5.5 Chino Hills earthquake and the 1987 M5.9 Whittier Narrows earthquake), these occurred off the main trace of the fault. It appears based on location and source mechanism that today’s events occurred on the main trace, but one should not infer too much from this. The fault is considered relatively hazardous in the most recent USGS report, but the recent earthquakes neither increase nor decrease that hazard appreciably.
These earthquakes highlight an important aspect of Earthquake Warning System (EWS) design. When designing an EWS, it is tempting to consider the “most damaging” earthquake as the design case. In the Bay Area, this is a 1906 repeat M~8.0 event, propagating southward along the Northern San Andreas Fault from Mendocino into San Francisco and beyond. In Southern California, the most damaging earthquake (also M~8.0) would propagate northward from the Salton Sea to Palmdale, along the Southern San Andreas Fault. These earthquakes certainly are threatening, but there are two shortcomings to focusing on them:
- They are rare. The EWS is much more likely to experience smaller (M6-7) earthquakes than a M~8 earthquake.
- They are far away. Great earthquakes project their damage far from the fault and the epicenter, and in these scenarios the EWS has a lot of time to figure out what’s going on before the damaging waves arrive in LA or San Francisco.
If one really wants to design a warning system to save the most lives and the most money, the focus should be on earthquakes on the Whittier Fault as today’s events were, or on the Hayward Fault in the San Francisco Bay Area. These faults are right up against heavily populated areas, and are considered likely to generate moderate or large earthquakes that could cause much damage in the epicentral region. Because they are so close to people and property, earthquakes on these faults give the EWS very little time to react. So if you can design your EWS to deal with these earthquakes, the more distant great earthquakes become a more tractable problem just by virtue of having lots of extra time to figure them out.
SWS has been hard at work designing just such an EWS: one that reacts as quickly as technically and scientifically possible to any earthquake, so that warnings can be issued even for close-in epicenters like today’s event. The map to the left shows the LA basin and nearby faults, as well as the epicenter of the second 4.5 event as a red circle. The blue triangles are seismic stations in a “Los Angeles Regional Earthquake Warning System” or LAREWS, and the green square is the Los Angeles Emergency Operations Center. The black contours show how much warning time would be available at any point on this map, if LAREWS has been operational during these earthquakes. For example, the LA EOC would have received about 12 seconds of warning from LAREWS for this event. These warning times would be the same even for a larger earthquake with the same epicenter. As the map shows, there is some warning available everywhere on the map, even right at the epicenter!
This, then, is the benefit of designing an EWS from the ground up to cope with worst-case close-in earthquakes: we leave no-one behind. There is no “shadow zone” or “blind zone” in which clients would be left without a warning. Everyone is covered by this system.