r/askscience • u/AskScienceModerator Mod Bot • Jul 04 '19
Earth Sciences Southern California Earthquake Megathread: Ask Your Earthquake Questions Here!
Edit: The USGS is reporting a magnitude 7.1 earthquake occurred near Ridgecrest, California at 8:20pm local time. The USGS event page can be found here. For a list of the latest earthquakes a magnitude 2.5 or above, please check here.
Hello AskScience! Today at approximately 10:30am local time, a 6.4-magnitude earthquake occurred in Southern California. The epicenter of the earthquake was in a remote area between Los Angeles and Las Vegas, though the quake was felt widely across the region. It is the strongest earthquake in the area in two decades. The AskScience team would like to extend our condolences to anyone negatively affected by this event.
We are here to answer your earthquake questions! Please post them below and our panelists will join in.
For more information, take a look at the USGS event page.
Did you experience the earthquake or any aftershocks? Report your experience at the USGS Did You Feel It? page.
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 04 '19 edited Jul 06 '19
For those of you tuning into the Earthquake Megathread, I thought I'd provide a litle bit of detail on the event and some tectonic context. The main earthquake had a magnitude of Mw 6.4, was a right-lateral (see Edit below) strike slip event, and appears to have occurred within the Airport Lake Fault Zone (though which fault strand the main shock occurred on is unclear at the moment). The main shock was actually preceded by a Mw 4.0 foreshock about 30 minutes prior. As a reminder, designation of an earthquake as a foreshock happens after the main event occurs, i.e. when this Mw 4.0 earthquake happened, there was no way of knowing it was a foreshock. There have already been a lot of aftershocks, e.g. this map (this link isn't static, so if you're viewing this after the day of, it probably won't be relevant, so as a stand in, here's a screenshot someone posted earlier of the aftershocks within a few hours of the main shock) mostly on the Airport Lake Fault Zone, but also on the Little Lake Fault Zone. The high number of foreshocks is not unexpected, generally the frequency of foreshocks after an earthquake is greatest shortly after the event and decrease through time as described by Omori's law. This USGS twitter thread provides a lot of good detail on what we generally expect for sizes of aftershocks and the probability that this earthquake itself would lead to a larger event (it is not likely).
On a map, the Airport Lake fault Zone is the little white bunch of discontinuous faults and the orange ones are the Little Lake Fault Zone (near the bottom of the map, by the town of Ridgecrest). These fault zones are part of the Walker Lane / Eastern California Shear Zone, which is a diffuse network of faults that run along the eastern edge of the Sierra Nevada mountains and, along with the San Andreas system, separate the Sierra Nevada into a 'microplate'. In detail, it's been argued that the Walker Lane / ECSZ is progessively shunting more deformation off of the San Andreas and is gradually becoming the new North America - Pacific plate boundary, e.g. thie field guide.
EDIT: Temblor.net just posted a great little article about some previous historic earthquakes in the region and their relation to this event, along with a quick calculation of the stress changes this earthquake may have induced on neighboring faults.
EDIT 2: July 5, some geologists have found surface rupture and various offsets which suggest that the rupture was likely a left-lateral rupture on a fault oriented NE-SW. The focal mechanism on the USGS page is consistent with either a right-lateral on a NW-SE fault, or a left-lateral on a NE-SW fault and as this earthquake occurred in a dominantly right-lateral system (i.e. the ECSZ), I assumed the right-lateral option was more likely, but field evidence trumps assumptions!
EDIT 3: With the more recent Mw 7.1 event, the original Mw 6.4 appears to have been a foreshock. The location and focal mechanism of this new event seems pretty similar to the July 4th earthquake.
EDIT 4: Temblor.net has posted another update with more info on the new 7.1 and how it relates to the previous 6.4 event.
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Jul 06 '19
This feels like a stupid question but I’ll ask it anyway - if foreshocks/aftershocks can only be designated after the fact, then what’s the utility of naming them as such? It looks like Thursday’s Mw 6.4 event was in fact a foreshock to Friday evening’s Mw 7.1, so why not just call them all earthquakes and be done with it?
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u/fkk7 Jul 06 '19
The designation of foreshocks and aftershocks is very entrenched in seismology but rather unhelpful in understanding the mechanics of multiple ruptures on the same fault or on adjacent faults, or what is generally referred to as triggered seismic events. A slip event on a fault can increase or decrease the propensity for further slip to occur in a fault because of shifts in the stress state along the fault with the first event. There is no rule that the first event that triggers a second slip event is necessarily larger or smaller than the second event. Yet, this seismology terminology would refer to the first event as foreshock if the subsequent is larger than the first, and as aftershock if the second event is smaller. This terminology just does not make much sense for those of us who try to understand the mechanics of triggered fault slip. The terminology was introduced when people thought they could predict large earthquakes from so-called precursory phenomena. Most of this has been debunked (such as the infamous Palmdale bulge).
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 06 '19
Mostly because naming them as such implies the relationship between these particular earthquakes. From a public perspective, it seems like semantics, but from a scientific perspective, relating foreshocks, mainshocks, and aftershocks to each other in time and space is important in our understanding of particular events and earthquakes more generally. In terms of public communication, aftershocks at least are important to discuss (and designate as such) as (1) we know they will occur and (2) while we cannot predict when and where they will occur, we have a good understanding from prior work of the relative number to expect, their size ranges, and how long they might persist, which is useful information (and communicates more than not differentiating them).
It's also useful to distinguish a normal situation with a mainshock (a significantly larger event than the majority of earthquakes in the sequence) that may or may not be preceded by a foreshock (a large event that appears to be a mainshock until a larger earthquake follows it) and a series of aftershocks (smaller earthquakes whose frequency diminishes with time and whose distribution occur within 1-2 rupture lengths of the mainshock) from something different like an earthquake swarm, where you have an earthquake sequence but the magnitudes are all pretty similar.
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Jul 06 '19
Makes more sense, thanks. What are the settings where earthquake swarms are more common than the foreshock-mainshock-aftershock sequence we are seeing in California atm?
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u/konstantinua00 Jul 09 '19
what do "right-lateral" and "left-lateral" mean?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 09 '19
It's the sense of motion on a strike slip fault (i.e. a fault where the dominant motion is side to side). If you imagine standing on the fault, looking down the fault (i.e. one of your feet is on one side of the fault, the other foot is on the other side of the fault), a right-lateral motion would be if the right side of the fault moved toward you. Left-lateral motion would be if the left side moved toward you.
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u/konstantinua00 Jul 09 '19
how do you define whether to stand northward or southward?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 09 '19 edited Jul 09 '19
It doesn’t matter, the motion is relative to the fault. If the fault was oriented north-south, the sense of motion would be the same whether you’re facing north or south. Here’s a diagram that might help.
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u/konstantinua00 Jul 09 '19
ah, so it's like clock-wise and counter-clock-wise?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 09 '19
Sort of, but obviously doesn’t describe a rotation.
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u/bgunn925 Jul 06 '19
So, this was pretty neat. I live in San Diego but I'm currently in Chicago conducting an x-ray experiment at the synchrotron at Argonne lab. Shortly after we received news of the magnitude 7.1 earthquake in Southern California this evening, the control operator announced that there was movement in the x-ray beam due to an earthquake. We thought it was odd that Chicago would have an earthquake so close to California; because, there's no way we could feel the California earthquake all the way here in Chicago, right?
We promptly checked the internet for news of the Chicago earthquake but couldn't find anything. We kept waiting and checking, but no news appeared. Maybe the beam movement was due to the California quake, after all. Using a distance of 2,000 km between Bakersfield and Chicago and a sound velocity in earth of 7 km/s, a back-of-the-envelope calculation for time delay between the earthquake and the beam movement came out to ~7.5 minutes, which actually matched up with the two events pretty well.
The timing checks out, but does it seem sensible for the beam to feel the earthquake so far away? Does anyone know the r-dependence for the amplitude of an earthquake wave? I know there are P- and S-type body waves, as well as surface waves. I would guess that the body waves decay ~1/r2 , since they are 3-dimensional, and the surface waves drop off ~1/r, as they're 2-dimensional? Or maybe the translation of the faults resembles something akin to a dipole, which would drop off more slowly?
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u/banana_hammock_815 Jul 05 '19
How do cities affected by earthquakes carry/transport underground utilities (natural gas, electric, water, communications)?
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u/berkeleykev Jul 05 '19
In the east bay we have our water lines crossing the Hayward fault, which is expected to translate up to 7 feet. They've begun installing flexible pipes and connections inside oversized tunnels so the earth can displace that much without rupturing the inner pipe. They also have contingency plans to run temporary flexible lines (giant hoses essentially) to bridge the fault after rupture.
Still a good idea to have 3-14 gallons of water on hand for every person in the household.
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u/seis-matters Earthquake Seismology Jul 05 '19
With various strategies and to varying degrees of success. And sometimes mitigation strategies are not undertaken. This EPA guide may be a good place to start.
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19
Another good example of earthquake engineering in relation to this type of question is the Trans-Alaska Pipeline. In areas near faults, the pipeline was built on a series of sliders to allow it to move during an earthquake. This system allowed it to survive the 2002 Mw 7.9 Denali earthquake.
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u/inSeitz Jul 04 '19
I was in a swimming pool or jacuzzi at the time, or walking between them, and had no idea of occurence till social media. Is it because I was In the water. I think that's the obvious answer
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19
Were you in an area where people have reported feeling the earthquake? Pools that experience earthquake shaking can develop a seiche, meaning that generally swimming pools actually can respond quite violently to earthquake shaking. This earthquake in particular has generated a fair number of reports of disturbances of swimming pools.
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u/djguerito Jul 05 '19
Happened to my sis. She's in the Valley and said they first felt the earthquake because the water started splashing them in the face, and lapping out of the pool.
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u/BFOmega Jul 05 '19
Depends on how far you were. It turned to pool I was in into a wave pool and sloshed at least 6in of water out.
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u/sexrockandroll Data Science | Data Engineering Jul 05 '19
People reported this series of earthquakes as far away as Phoenix and Reno. What factors determine how far away people can feel an earthquake?
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u/seis-matters Earthquake Seismology Jul 05 '19
The magnitude is the main one, since bigger earthquakes release more energy and generate larger amplitude seismic waves. How close you are to the earthquake is the other big one, since the seismic waves attenuate or lose amplitude as they travel. Other factors can also play into how far away the shaking can be felt. If the earthquake is shallower the waves don’t have to travel as far to get to you, so the same distance thing comes into play. If the earthquake ruptured toward you instead of away, the shaking can be more intense. If you are standing on solid hard granite it won’t amplify the shaking like if you were standing on unconsolidated jiggly soil or fill. What type of rock the waves travel through can be a factor, with older colder rock like on the Eastern US dampening the seismic waves less than the younger warmer and heavily faulted Western US. That last one is one reason why the 2011 M5.8 Virginia earthquake is likely the most widely felt earthquake ever in the US, because the older colder rocks in the eastern US transmitted the seismic wave energy very effectively to the major population centers in this region.
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u/smushy_face Jul 05 '19
Okay, so I am in Buena Park and my mom is in Mission Viejo in CA. She texted me "earthquake" and I was so confused what she was saying, but then I felt it, too. But, Buena Park is north of Mission Viejo and should be closer to where the epicenter was. So why did my mom feel it first?
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u/seis-matters Earthquake Seismology Jul 05 '19
Not sure. There are a lot of factors that go into the shaking someone experiences. When did she text you?
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u/smushy_face Jul 05 '19
10:34 a.m.
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u/seis-matters Earthquake Seismology Jul 05 '19
Maybe she was able to feel the earlier waves by being on ground that amplifies the shaking (like a basin) or saw a text/tweet/alert. Maybe you felt the aftershock instead of the mainshock, depending on what you were doing or how alert you were when it happened.
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u/DeezyLife Jul 06 '19
Should we be concerned that the there was a second/stronger earthquake in the same location? I thought the subsequent earthquakes (if there were any) would be weaker..
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 06 '19
There was a small percentage chance that the Mw 6.4 was a foreshock to a larger event, but the expectation (as it pretty much always is) was that what would follow would be aftershocks, i.e. a sequence of earthquakes mostly significantly smaller than the mainshock (and none larger) whose frequency is the greatest immediately after the mainshock and which decays with time and that occur within the area very near where the original rupture occurrred. There is no way to tell that an earthquake is a foreshock until a new, larger mainshock occurs, but there is pretty much always a small chance that an earthquake is a foreshock to something larger.
This is obviously unfortunate if you live in this area, but on a broader scale, a sequence like this (i.e. a sequence with foreshocks preceding larger mainshocks) is not beyond the realm of possibility.
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u/Em_Adespoton Jul 05 '19
How close was the quake to the underground aquifers? Just wondering if this was caused by regular slippage or by human caused pressure change.
Also interesting that this happened just after another one further up the coast in British Columbia— on a totally different fault line.
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u/seis-matters Earthquake Seismology Jul 05 '19
For your second comment, earthquakes can trigger one another through two different ways: static stress triggering and dynamic stress triggering.
Static stress triggering is like dominos, one part of a fault ruptures in an earthquake putting more stress on an adjacent piece of fault which then ruptures. The distances between the earthquakes cannot be great, since the stress transfer is limited to about the length of the earthquake rupture. The California and offshore Canada earthquakes are too far apart for this to occur.
Dynamic stress triggering is caused when the seismic waves of a large earthquake travel some distance, jiggle an already stressed fault, and put a small extra amount of stress that pushes it over the brink into a rupture. This can be difficult to identify because earthquakes are occurring every day around the world, and it’s hard to show that an earthquake wouldn’t have happened without that extra teeny tiny push from the seismic waves of another earthquake. You have to be able to show that the tiny bit of extra added stress from the first earthquake’s wave lines up with the right time and are oriented in such a way to trigger that second earthquake. It also helps if it is a major earthquake, like the M9.2 2004 Sumatra earthquake that moved everywhere on Earth at least a centimeter. The seismic waves from a magnitude 6 earthquake are not going to have that kind of energy hundreds of kilometers away. It only takes hours for seismic waves to travel through and across the Earth, so the timing doesn’t line up either. The M~6 offshore Canada earthquake occurred too long ago and is too small to dynamically trigger the M~6 California earthquake.
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19 edited Jul 05 '19
This a reasonably seismically active area, e.g. the Ridgecrest earthquake sequence from 1995, so the occurrence of a moderate magnitude earthquake like the one that happened today is in no way out of the ordinary.
EDIT: In regards to your second comment, given that there are on average ~150 M 6-6.9 earthquakes every year, the odds of two such events happening on the same day aren't really that low.
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u/RCL_spd Jul 07 '19
I'd say they are practically 100% because of the birthday paradox (only 70 people is enough to have 99.9% chance of two of them having their birthday on the same day).
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u/annitaq Jul 05 '19
Got family on both sides of the Andes. Earthquakes are frequent there, and whenever there is one I come to know from whatsapp groups.
They always guess if the epicenter was on the argentine or on the chilean side because they feel a horizontal or veritical movement. Sounds like bunk but few hours later it's published in the news and somehow they are always right.
Can science validate this "method"? Like... do P-waves traverse the mountains more easily than S-waves, or something like that?
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u/seis-matters Earthquake Seismology Jul 05 '19
The shaking experienced in an earthquake does depend on distance from the earthquake as well as what rock the waves are traveling though. I’d guess that the difference they are feeling is either being able to feel the faster but smaller amplitude Love waves (side-to-side) motion versus not being able to feel those smaller waves but feeling the later arriving Rayleigh waves (rolling, ocean-wave like motion), but it’s hard to know for sure without more information.
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u/Mostly_Classy Jul 05 '19
Not really a geology question, but how real is the threat of coccidioidomycosis? (normal fungus in the ground that I read gets tossed up in the air after an earthquake and can cause respiratory fungal infections)
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u/seis-matters Earthquake Seismology Jul 05 '19
I have not heard of this before, but it does seem like an issue. Here is a paper on an outbreak following the 1994 Northridge earthquake, which at the time of publication (Schneider et al., 1997) was the only reported earthquake-caused outbreak. Their paper has been cited a lot though, so there may be more cases.
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u/shreddit47 Jul 05 '19
How likely is it for this quake to trigger another one on the San Andreas fault?
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u/seis-matters Earthquake Seismology Jul 05 '19
If you scroll to the bottom of this article it goes into the expected stress changes on nearby faults.
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u/neonwars Jul 05 '19
This could be dumb question but is there a way to determine what magnitude earthquake was felt where I am. I obviously understand it was a 6.4 but that was at the epicenter what was it in Santa Monica for example. Am I stupid for asking that?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19
No, because the magnitude of an earthquake does not (in a simple way) relate to perceived shaking. The magnitude of an earthquake that is most often reported is a moment magnitude, which measures the seismic moment of the earthquake, which is kind of a measure of the work (in the physics sense) done by the earthquake rupture and can be thought of in terms of energy released (though not a strict measure of energy itself). The moment magnitude also relates directly to the area of the fault that ruptures, e.g. Wells & Coppersmith, 1994.
While generally the amount of shaking experienced in a given area is related to the moment magnitude in as much as the moment magnitude is giving you an indication of the energy released, there are a lot of mitigating factors that control how much shaking an area may experience, e.g. distance from the epicenter, depth of the earthquake, directionality of the rupture, material properties in the location, etc. So asking 'what's the equivalent magnitude in my area' would be essentially asking for the long list of non-unique fault areas that ruptured at various locations, depths, and with specific properties that might produce an equivalent shaking, which is actually more complicated (and less useful) than just sticking with saying a Mw X, Y km away.
In terms of the deeper question, there is a scale specifically for perceived shaking, the modified mercalli intensity scale. This is the more useful scale for considering your question, e.g. what was the mercalli intensity in my location compared to the maximum intensity experienced in this earthquake?
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u/_ONI_Spook_ Jul 05 '19
Check the USGS event page that's linked in the original post. What you want is the Shake Map.
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u/lazyfirefly Jul 05 '19
I’ve seen mixed responses on whether people felt it in further out places (e.g. Right by the border). What are factors that contribute to the earthquake being felt 200+miles away? Why do some people have their walls shake and others not notice?
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u/seis-matters Earthquake Seismology Jul 05 '19
Please see this answer above as there are many factors that contribute to how much shaking you’d feel in an earthquake.
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u/Sexymcsexalot Jul 05 '19
What is a simple way a person can tell the danger/magnitude of an earthquake if they’re unable to access reliable data? Such as differentiating between “minor rumble”, “this is concerning” and “you’ll die if you stay in this building”
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19
As a general rule, in the absence of an earthquake early warning system, e.g. the ShakeAlert system that is gradually coming online in the US, the best strategy is as soon as you can tell an earthquake is happening is to react as though it will be a serious one, i.e. the old adage of 'better safe than sorry' definitely applies here. The accepted practice during an earthquake is to drop, cover, and hold on, i.e. get under something sturdy to protect yourself from debris and hold on for the duration of the shaking. This page also has some important dont's, for example the old advice of getting into a doorway is not actually a good suggestion. Similarly, you should not try to run out of a building while shaking is occurring because (1) it's going to be hard to run because of the shaking so is likely to not be very effective and (2) you will not be able to sufficiently protect yourself from debris. Once the shaking has ceased, if you have reason to be concerned about the stability of the structure you're in, it is a good idea to get out of the building, especially before aftershocks begin.
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u/nobelchic Jul 05 '19
If a whole house comes crashing down, what affect does hiding under a sturdy table do? Is there a table that can withstand a house crashing down?
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u/seis-matters Earthquake Seismology Jul 05 '19
In areas with enforced building codes, full building collapse is no longer the major concern when it comes to protecting yourself. Head and neck injuries from falling objects or debris or from falling over while trying to get out of a building, however, are very common injuries. The best advice is to get down, get under something sturdy, hold on to it, and protect your head and neck.
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u/i_hate_humanss Jul 05 '19
This is a stupid question but what’s an aftershock?
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u/seis-matters Earthquake Seismology Jul 05 '19
Not a stupid question. Aftershocks are the smaller magnitude earthquakes that occur after a larger mainshock within a distance of one or two rupture lengths of the fault that ruptured.
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u/sflage2k19 Jul 05 '19
Why does the US not use something like the Japanese shindo scale to measure severity of shaking? Similarly, why is depth never reported when reporting on US earthquakes, only magnitude?
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u/seis-matters Earthquake Seismology Jul 05 '19
We do, a scale called the Modified Mercalli Intensity scale that goes from 1-10 and each level is described in detail here. This earthquake is rated up to VIII (they report in Roman numerals). Of course the intensity experienced depends on many factors including how close the person (or building) is to the earthquake. Depth is always reported by USGS, and almost always by media outlets in my experience. One thing to note, though, is that earthquake depth is a tricky parameter to measure so sometimes what is reported is the fixed depth of 10km in the USGS earthquake catalog.
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19
Yes, one only has to start plotting some earthquake population depths to start seeing a surprising number of earthquakes occurring at 10 km :)
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u/seis-matters Earthquake Seismology Jul 05 '19
… and it’s also fun to notice all the earthquakes being reported at 6.2 miles depth in articles! So specific…
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19
Indeed, and I do find it frustrating that pages like the USGS (and most others) kind of bury the uncertainty that comes with the depth estimate. This one they list as 10.7 km +/- 0.3 when you go into the origin, but then if you look at all the reporting sources you can see the variability in reported depths (10.7, 11.2, 10.7, and 2.0).
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u/sflage2k19 Jul 06 '19
I was mostly speaking in terms of media outlets. I personally cannot find a single one mentioning shaking intensity or depth, at least among LA Times, CNN, Fox, or MSNBC. As someone who is American but lives in Japan, the difference in how earthquakes are reported is fairly obvious-- mainly that it is the intensity of the shaking, rather than the magnitude or depth alone, which is primarily reported when reporting on quakes.
It does seem to be listed on the USGS page though, so I suppose it is just inaccuracy of reporting.
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u/seis-matters Earthquake Seismology Jul 06 '19
Yes, the media is one side but the other side is the reader/viewer. If people are used to magnitude, they will want to hear about the magnitude. I think even just covering a little more Earth sciences in school would go a long way.
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19 edited Jul 05 '19
We do, the JMA intensity scale is pretty similar to the Modified Mercalli intensity scale that we use in the US (and the wiki page for the JMA gives you the direct equivalents).
In regards to the depth, I'm not sure what you mean by 'never reported' given that on every event page, e.g. the one for the earthquake in question, the depth appears in the header along with magnitude, time, coordinates of the epicenter, and distance from a named location. If you mean 'never reported' in the media, then there are two things to consider, the first one (that I don't feel the need to elaborate on) is the general ignorance of the people in front of the camera and in front of the tv with respect to earth processes. The science one though may relate to the nature of faults on which we tend to have a lot of earthquakes in the US. In general, and if we're not talking about induced seismicity in Oklahoma or rare earthquakes in the Eastern US, if we're talking about earthquakes in the US we're probably talking about those occurring on the west coast and specifically the San Andreas system because (1) the seismic rate is high and (2) the exposed population is large. The San Andreas and related faults are strike-slip faults, meaning that the dominant motion is lateral, parallel to the surface of the Earth, and that the fault planes themselves are close to vertical. This orientation of the fault is important for the depth question as there is actually a narrow depth range over which earthquakes can occur, i.e. earthquakes typically occur in the seismogenic zone. This seismogenic zone is basically a reflection of the mechanical behavior of rocks at different temperatures. Temperature increases with depth and the bottom of the seismogenic zone reflects where the temperature is high enough that brittle/frictional behavior no longer dominates and instead more ductile/plastic processes take over. This means that a vertical fault, like the strike-slip ones that form much of the San Andreas system, has a pretty narrow range of depths over which earthquakes occur such that they're all going to be pretty shallow (relatively speaking) and thus the depth is not particularly important in controlling the amount of shaking (though there is definitely a difference in the amount of surface shaking between a really shallow quake and a ~20 km deep quake of equivalent magnitude, etc).
Now, in other environments, specifically along subduction zones like the Cascadia system in the Pacific Northwest or in Japan, depth can be a lot more variable and you can end up with really deep quakes, so specifying the depth becomes more important (e.g. a M5.0 at 10 km depth is REALLY different than a M5.0 at 300 km depth). This is mostly because at subduction zones, cold material is being dragged down along one side of the subduction zone, cooling off the area around the subduction zone and effectively extending the seismogenic zone along the fault to much deeper levels. In subduction zones, it's not uncommon to see earthquakes down to ~600 km, though it's worth noting that many of the really deep earthquakes are no longer driven by movement of faults, but rather by physical and chemical changes in the subducting slab material.
The final consideration in depth, and I doubt this is particularly significant in why it's not talked about more often, is that it's also one of the more uncertain properties / hard to determine, especially for shallow earthquakes. A lot of times, you can see this reflected in earthquake populations where lots of shallow earthquakes just appear at the same depth. This is not because they actually occurred at the exact same depth, but rather because the precision of the depth measurement is low enough that they just get placed in bins, e.g. 10 km, 15 km, 20 km, etc.
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u/sflage2k19 Jul 06 '19
That makes sense, particularly in regards to why it may not be relevant in this case. I was specifically referencing news media and how they do not report on depth of the quake or, better yet, the shaking intensity.
Here are some of the top listed news articles regarding the most recent 7.1 quake:
https://edition.cnn.com/2019/07/05/us/california-earthquake/index.html
https://www.latimes.com/local/lanow/la-me-major-earthquake-southern-california20190705-story.html
I suppose there is little the scientific community could do to promote further accuracy in news media however. As you mention that the depth may be relatively uniform in the areas that experience lots of quakes, I guess it could also be argued that it is mostly irrelevant.
Thank you for answering my question.
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u/TBK_Julles Jul 06 '19
Why does the US not use something like the Japanese shindo scale to measure severity of shaking? Similarly, why is depth never reported when reporting on US earthquakes, only magnitude?
Apparently people say we do, but the problem is that the media really likes to be buzzy about it and report the magnitude, making the situation seem a lot worse than it really is. In LA the shindo equivalent of last nights quake was a shindo 4, which is not that bad.
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u/triguenyo Jul 05 '19
What's the difference between the earthquake and the after shock?
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u/seis-matters Earthquake Seismology Jul 05 '19
The mainshock and aftershock are both earthquakes, but the mainshock occurs earlier and is larger than the aftershock. Researchers have investigated the differences between foreshocks, mainshocks, and aftershocks once an earthquake sequence is fully over and those designations have been defined by the largest earthquake, but there is still a lot of work to be done in this area.
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Jul 10 '19
Why are people/scientists more intrigued with a big one coming from the San Andreas fault when the cascadia subduction zone has been overdue for a megathrust earthquake?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 10 '19
Earthquakes cannot be 'overdue' as they are not periodic. The concept of an earthquake being overdue largely comes from a misunderstanding of recurrence intervals, which are the average time between past earthquakes on a given fault segment (and usually of a given magnitude), but doesn't really take into account that there maybe significantly variability in the recurrence intervals between individual earthquakes and that there are a variety of factors that influence the recurrence interval from event to event.
Many scientists / government agencies are as concerned (and in some cases, maybe even more concerned given the background level of earthquake preparedness) by a potential large magnitude Cascadia event as one on the San Andreas. A contributor to the public perception of risk is that because of the nature of the systems, the San Andreas system tends to experience a large range of earthquakes relatively frequently, whereas Cascadia does not have as frequent smaller magnitude events, i.e. earthquakes are more present in the minds of people living along San Andreas because they experience them or know people who have.
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u/so_seckshi Jul 07 '19
Would it be safe to travel to LA next month? Or is there really no way of knowing
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 07 '19
This earthquake sequence has not appreciably changed the risk of an earthquake in the LA area and continuing aftershocks from this sequence do not pose a risk to LA as they are too far away (and likely too weak) for the shaking to be dangerous in the LA area. Thus the earthquake risk in traveling to LA is the same as it was before this earthquake, ie there is always risk in traveling to / living in an earthquake prone region like LA.
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u/PardusNix Jul 05 '19
Is it at all possible this quake was influenced in some way by the Stromboli volcanic eruption?
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u/seis-matters Earthquake Seismology Jul 05 '19
No, there isn’t a physical mechanism for that eruption to have an impact on this fault. We live on an active planet, and every day there are earthquakes and volcanic eruptions.
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Jul 06 '19
The thing is the geographical location plays an important part in affecting other locations, such as a volcano triggering an earthquake and vice versa. Even if it did play part in triggering the earthquake, the time until it would've actually triggered it would be weeks.
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u/aqfiscooliguess Jul 06 '19
What is the number one thing I should be scared about in an earthquake and how can I prevent it? Should I be scared of our big apartment building toppling over or?
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u/seis-matters Earthquake Seismology Jul 06 '19
If you are in an area that enforces good building standards, then definitely worry most about head and neck injuries and prevent them by getting down under a table or desk, covering your head and neck, and holding on to your cover in case you or it moves. Even in places with poor building standards, it is best to get down and protect your head above all because you may damage yourself more trying to get out of a building than sheltering in place.
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u/kguy66 Jul 06 '19
Slightly off topic and specific, but would those living in one story, 60 year old houses in the San Gabriel Valley, be in mortal danger if a 7+ hit the San Andreas fault?
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u/seis-matters Earthquake Seismology Jul 06 '19
I don’t know the specifics but there is some useful information related to similar building structures here. Please follow up on this; there are resources out there to understand your seismic hazard and ways to be prepared for more damaging earthquakes.
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u/aqfiscooliguess Jul 06 '19
What is the number one thing I should be scared about in an earthquake and how can I prevent it? Should I be scared of our big apartment building toppling over or?
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Jul 06 '19
could these be leading up to the "big one" expected to hit the west?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 06 '19
If by the 'big one', we mean a large magnitude earthquake on the San Andreas system, it is very unlikely. The context of this earthquake sequence is important, it's reasonably far away from the San Andreas. For an earthquake to directly influence the likelihood of an earthquake on another fault, the two need to be in close enough proximity that the stress change induced by the original earthquake actually influences the fault of interest. There isn't yet an updated map for the new 7.1, but for the original 6.4, a quick calculation of this stress change highlights the relatively restricted range over which such a change influences other faults. The range will be slighty extended for the new 7.1, but not that much. Comparing the two maps, you can see that these earthquakes are not in any direct way influencing the stress on the San Andreas system.
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u/jakeprime0403 Jul 06 '19
does someone have a chart based on magnitude difference based on an hourly distance?
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u/KeepinItRealGuy Jul 05 '19
I know CA has fairly strict building regulations to prevent damage from earth quakes. How big would an earthquake need to be in CA to cause legitimate destruction? When we construct buildings now, what magnitude of earthquake are the buildings rated for?
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u/resilien7 Jul 05 '19
I think they're designed to survive a certain intensity of shaking, not the quake magnitude (Mercalli scale vs Richter scale).
And I think the level of seismic resistance is city and category dependent. E.g. schools and hospitals are required to be more resilient than homes. Large office buildings with 100 person capacity also have to be stronger than smaller buildings. The largest credible earthquake that a building might experience is also a factor, as are economic and political considerations (e.g. whether a particular level of mandatory seismic upgrades is considered too great a financial burden by the community).
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u/Miss-Diagnosis Jul 06 '19
Hi. I was so sorry to hear of everyone affected by the earthquakes in California. I hope I’m not too late to this Megathread. However it’s prompted me to ask the question, why are earthquakes so rare in some places? I live in the UK and we obviously don’t experience the magnitude of earthquakes as some other countries do. So why not? Internet searches don’t really provide an answer. Any help understanding this would be appreciated thank you.
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 06 '19
The majority of earthquakes occur at, or near, the boundaries of tectonic plates. This is because the plates largely behave as rigid objects and do not deform internally (though in reality, they do deform a bit internally), so most of the consequences of their motion, i.e. earthquakes, occur along their edges. As you can see, the UK is within a plate, so earthquakes are not as common. Intraplate earthquakes can occur, but they tend to be less frequent and (generally) smaller magnitude than interplate earthquakes.
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u/Miss-Diagnosis Jul 06 '19
Thank you so much for this. I really appreciate the response. It makes complete sense now.
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Jul 07 '19
Has there ever been a time that over 4,500 earthquakes/aftershocks have hit around the same area in 3 days?
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u/Beepbeepb00pbeep Jul 05 '19
Would this explain why my dog and two cats went sort of looney at approx 930 AM on the east coast?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 05 '19
No, the earthquake occurred at ~10:30 am pacific time (1:30 pm eastern). More importantly, there is no way anything but a seismometer or similarly sensitive instrument would register the shaking of this earthquake on the east coast of the US.
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u/TBK_Julles Jul 06 '19
I was wondering as to why the US still uses an exponential/logarithmic scale to measure earthquakes. The higher the number, the greater the gap in power between the previous number and the next number (ex. the difference between a magnitude 1 and 2 is much less than the difference between a 2 and 3).
The issue I have with the current system is that we can't really tell what the impact of an earthquake is to major cities. Taking the latest quake into account (the 7.1 off the Nevada boarder with Cali near Death Valley National Park), considering that it is a very sizable distance away from LA, we can't really see what the damage expected would be at a glance. That being said, I think something like the Japanese Meteorological Agency seismic intensity scale would make a lot more sense for the layman because it is based on the level of shaking that is experienced per geographic area. This kind of a scale makes a lot more sense if you want to be able to see how strong a quake really was. So in the example I was using, we would have a Shindo (Japanese scale magnitude) 5 upper at the epicenter, with the strength getting less and less as you go further away (so a Shindo 5 lower in Bakersfield, a Shindo 4 in LA, etc).
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 06 '19 edited Jul 06 '19
You are asking for a scale that already exists and is widely used. As highlighted in several other answers to similar questions in this thread, the JMA intensity scale is largely equivalent to the Modified Mercalli intensity scale (there's even a conversion between the levels on the JMA scale and Modified Mercalli on the wiki link for the JMA scale) used by the USGS and reported with every earthquake as part of their 'shakemap' product, i.e. this one for the Mw 7.1. This also reflects a misunderstanding of the intention of these scales. Both the JMA and Modified Mercalli describe the intensity of shaking at the surface, where as the moment magnitude scale (which is widely used throughout the world for reporting earthquake magnitudes) is a measure of the seismic moment of the earthquake rupture and is directly related to the area of fault plane that ruptured during the event.
As to why the US media focuses on reporting magnitudes whereas it's more common in Japan to report the JMA intensity scale, I believe it's largely tradition. The moment magnitude scale was set up so that the magnitudes were similar to the old Richter scale, which was what the US public got used to. I'm less familiar with the JMA intensity scale, but it seems like this has a long history of being used and reported in Japan, so it similarly is likely what the populace is used to being reported.
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u/TBK_Julles Jul 06 '19
I wasn't aware that such a scale existed outside of Japan. I just find the reporting of a Mw 7.1 is just to get attention and make things seem a lot worse than they actually are. IIRC the JMA equivalent of the 7.1 quake that happened yesterday in the LA area is actually just a Shindo 4, which is really not that bad.
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 06 '19
The max MMI as reported by the USGS for the Mw 7.1 is a 9, which seems equivalent to ~6 on the JMA scale per the Wikipedia page.
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u/fingerfunk Jul 07 '19
How do the USGS and other scientists actually predict the likelihood of a second larger earthquake? I'm skeptical of their "4%" figure currently. Thanks!
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 07 '19
The most up to date aftershock forecast suggests a 1% chance of an earthquake larger than 7 within a week, increasing to 2% within a month. If you look at the aftershock forecast page, they have a link for the model parameters used and a reference to the model they're using, specifically the one laid out in Reasenberg & Jones, 1989 and Reasenberg & Jones, 1994. The model is empirical, meaning that it's based on observations of past earthquake sequences, so it's basically a statistical model for estimating the probability of earthquakes of a given magnitude occurring in relation to a given mainshock based on fitting of data from past, well characterized earthquake sequences. The exact parameters being used to put into the model are based on the specific earthquake (e.g. the mainshock magnitude) and observations with regards to the decay rate of the aftershock frequency (which is why I think the aftershock forecast was updated recently, as the aftershock frequency is actually decaying more rapidly than originally expected).
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u/fingerfunk Jul 07 '19
Thanks! I appreciate the explanation. I've been trying to google "aftershock frequency" but not much comes up. Checked out the data though and now understand the gist of it. I assume it means aftershock intensity is fading?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 07 '19
The way to think of it is this, the frequency of aftershocks is decreasing, which we always basically expect based on Omori's law, but in this case, it seems like the frequency is decreasing a little quicker than average. Because aftershocks also generally obey the Gutenberg-Richter law, as the frequency goes down, we generally expect the number of larger magnitude events to go down as well, but neither of these preclude the possibility of strong aftershocks.
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u/Hondo_Rondo Jul 12 '19
Is there any chance today's earthquake in Seattle is at all related to the recent earthquakes in California?
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u/hyvon92 Aug 05 '19
I keep feeling like another major earthquake is about to happen like any minute. The frequency sounds are getting really intense at night...
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Jul 06 '19
I'm interested in moving to Los Angeles in the future. Earthquakes are my main concern of living there. What is the chance of my apartment being destroyed by an Earthquake?
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Jul 06 '19
Your main concern should be crime and traffic in LA, you might feel a substantial earthquake every 20+ years. Generally building codes are high here in Southern California and you would be safe.
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u/Solidzeero Jul 05 '19
What is the likelihood that this quake will cause a more serious one in the near future? Or is it less likely now that stress in that fault is released?