Questions about earthquakes in SoCal
April 5, 2010 10:10 PM Subscribe
SoCal filter: So I was pitching my tent in Anza Borrego when all the sudden it felt like I was kneeling on a trampoline. Was this (Mexicali Quake) a typical earthquake for the area?
Being a boy from the Northeast who has never encountered anything like this before, this was my first experience with an earthquake. I was pitching my tent in Anza Borrego (right near Yaqui Well for those of you who know the area) and all the sudden it felt like I was kneeling on a trampoline, or a big vat of Jello; the ground seemed to bounce up and down at a speed of about 3 HZ, but it felt like a very pure sine-wave, not what I would have imagined at all.
I always thought of earthquakes as being much more abrupt/sudden/jarring and destructive. Is this a typical quake? Is my perception of its intensity tainted because I was out in the middle of nowhere? (eg - no buildings to shake, no glass to break)
A couple of times in the following 2 hours I heard low rumblings coming from the hills around me (for the sound geeks out there, think a band of noise from 20-50Hz), but was told by fellow campers that it was probably more likely that it was the Air Force testing bombs at the Carrizo Impact Area.
Tonight I'm staying at a motel in El Centro, CA (about 10 miles from the epicenter) and I'm pretty sure that I just felt an aftershock (around 9:15pm local time) that made everything in my motel room shake.
Can someone who knows more about these things enlighten me about the nature of earthquakes in this area and what they feel like?
Being a boy from the Northeast who has never encountered anything like this before, this was my first experience with an earthquake. I was pitching my tent in Anza Borrego (right near Yaqui Well for those of you who know the area) and all the sudden it felt like I was kneeling on a trampoline, or a big vat of Jello; the ground seemed to bounce up and down at a speed of about 3 HZ, but it felt like a very pure sine-wave, not what I would have imagined at all.
I always thought of earthquakes as being much more abrupt/sudden/jarring and destructive. Is this a typical quake? Is my perception of its intensity tainted because I was out in the middle of nowhere? (eg - no buildings to shake, no glass to break)
A couple of times in the following 2 hours I heard low rumblings coming from the hills around me (for the sound geeks out there, think a band of noise from 20-50Hz), but was told by fellow campers that it was probably more likely that it was the Air Force testing bombs at the Carrizo Impact Area.
Tonight I'm staying at a motel in El Centro, CA (about 10 miles from the epicenter) and I'm pretty sure that I just felt an aftershock (around 9:15pm local time) that made everything in my motel room shake.
Can someone who knows more about these things enlighten me about the nature of earthquakes in this area and what they feel like?
I'm pretty sure that I just felt an aftershock (around 9:15pm local time) that made everything in my motel room shake.
Possibly, as there were several 2s and 3s in that general area in the last hour.
posted by Cool Papa Bell at 10:24 PM on April 5, 2010
Possibly, as there were several 2s and 3s in that general area in the last hour.
posted by Cool Papa Bell at 10:24 PM on April 5, 2010
I've been through a lot of quakes, and this one was unusual in that it was much more of a rolling feeling -- and lasted a lot longer -- than most of the others I've been through. Most of the previous quakes I've felt were more of the jolting type that you were expecting; this one was definitely more of a gentle rolling back and forth, so much so that most of the people I know who felt it at first thought they were having a spell of vertigo rather than experiencing an earthquake.
posted by OolooKitty at 10:29 PM on April 5, 2010
posted by OolooKitty at 10:29 PM on April 5, 2010
I've lived in southern California for 33 years, and this was much more of a "roller" than we usually get. Most of the earthquakes I've been through are more jarring, more shaky, more like what you'd expect. This one felt a lot longer and a lot smoother, more like being on a boat in rough waters, and frankly made me feel a little big seasick.
posted by infinitywaltz at 10:30 PM on April 5, 2010
posted by infinitywaltz at 10:30 PM on April 5, 2010
Yeah as CPB says, it really does take all kinds. A few years ago there was a very small quake, but almost directly under the westside of L.A. It was so abrupt it was more like a very loud doorslam than anything else.
It also varies widely based on where you are when it happens. A lot of people seem freaked out about this recent quake, but I literally didn't know it had happened until I saw it on twitter. I think because I was driving when it occurred. On the other hand there was a smaller one about two years ago, when I happened to be at work on the 18th floor. Newer tall buildings in SoCal are literally sitting on rollers, so they roll back and forth by design. I was freaked out the rest of the day.
If it makes you feel better, there hasn't been a quake that caused serious damage to SoCal since Northridge in 1994. Of course you could also think of that as "we're due," but big ones are rare, and we are very prepared here.
posted by drjimmy11 at 10:31 PM on April 5, 2010
It also varies widely based on where you are when it happens. A lot of people seem freaked out about this recent quake, but I literally didn't know it had happened until I saw it on twitter. I think because I was driving when it occurred. On the other hand there was a smaller one about two years ago, when I happened to be at work on the 18th floor. Newer tall buildings in SoCal are literally sitting on rollers, so they roll back and forth by design. I was freaked out the rest of the day.
If it makes you feel better, there hasn't been a quake that caused serious damage to SoCal since Northridge in 1994. Of course you could also think of that as "we're due," but big ones are rare, and we are very prepared here.
posted by drjimmy11 at 10:31 PM on April 5, 2010
nthing the "much more rolling and gentle than usual". Also much LONGER than usual... I was starting to question my sobriety after 30 seconds or so of that.
posted by flaterik at 10:42 PM on April 5, 2010 [1 favorite]
posted by flaterik at 10:42 PM on April 5, 2010 [1 favorite]
Yeah, it really, really depends, as everyone else is saying. You never know how a quake is going to screw up your decor.
In '94, my house was a couple miles from the epicenter in Northridge and the shaking was such that the bookshelves in my room, which faced roughly south, were completely emptied by the shaking, while the bookshelves in the living room, which faced roughly east, looked completely undisturbed - not a single book even moved. And as if that wasn't weird enough, all but 2-3 inches of the water sloshed out of the fishtank in the kitchen, but the tank remained upright. My grandparents experienced the same quake from just the other side of the Sepulveda Pass and described it as being much more of a gentle rolling, like you experienced, and being relatively long. I wasn't fully awake at the time, granted, but to me it felt short but extremely violent. It varies widely depending on what kind of quake it is (how the fault is moving), and what kind of ground it goes through (and how much) to get to you, what kind of structure you're in (if any), etc.
And, yeah, a lot of things sound like quakes once you start listening for it. My high school had a very old radiator heating system and every winter the first time they turned on the radiators I'd reflexively want to take cover.
You'll have a real southern CA story to tell your fancy-pants back-east associates :) Stay calm, stay safe, and know where your exits and covers are. Cool Papa Bell's link will help you determine how paranoid you really are :)
posted by little light-giver at 10:48 PM on April 5, 2010 [1 favorite]
In '94, my house was a couple miles from the epicenter in Northridge and the shaking was such that the bookshelves in my room, which faced roughly south, were completely emptied by the shaking, while the bookshelves in the living room, which faced roughly east, looked completely undisturbed - not a single book even moved. And as if that wasn't weird enough, all but 2-3 inches of the water sloshed out of the fishtank in the kitchen, but the tank remained upright. My grandparents experienced the same quake from just the other side of the Sepulveda Pass and described it as being much more of a gentle rolling, like you experienced, and being relatively long. I wasn't fully awake at the time, granted, but to me it felt short but extremely violent. It varies widely depending on what kind of quake it is (how the fault is moving), and what kind of ground it goes through (and how much) to get to you, what kind of structure you're in (if any), etc.
And, yeah, a lot of things sound like quakes once you start listening for it. My high school had a very old radiator heating system and every winter the first time they turned on the radiators I'd reflexively want to take cover.
You'll have a real southern CA story to tell your fancy-pants back-east associates :) Stay calm, stay safe, and know where your exits and covers are. Cool Papa Bell's link will help you determine how paranoid you really are :)
posted by little light-giver at 10:48 PM on April 5, 2010 [1 favorite]
You're describing the difference between P waves and S waves. There's also probably an explanation for why you would feel one but not the other. For one thing, there is this shadow zone, but I think the waves you did feel wouldn't travel far enough to reach the shadow zone either. Maybe a geologist can inform us.
posted by salvia at 10:58 PM on April 5, 2010
posted by salvia at 10:58 PM on April 5, 2010
Best answer: My quick, non-science-y answer: I have experienced many earthquakes in the L.A. area and they mostly just felt like vibrations - some strong and very obvious, some so weak that I just thought it was a truck passing. I've never experienced the rolling motion, but I have only ever experienced earthquakes indoors, except for one where I was in a car. My aunt told me a story once about being out in a big field and actually seeing the ground roll. So cool.
Now, here is my science-tastic answer for why you felt what you did, and why it is not a really crazy unusual phenomenon, just a neat one! (N.B.: I am a first-year seismology graduate student.)
For one thing, there is this shadow zone, but I think the waves you did feel wouldn't travel far enough to reach the shadow zone either. Maybe a geologist can inform us.
No, definitely not far enough away to be experiencing shadow zone effects. (Although low-velocity layers can produce local shadow zones, but only for seismic waves in certain orientations - there shouldn't have been any crazy structure between the OP and the quake to create a shadow zone at his location. I can elaborate more about shadow zones if people want, but I don't want to derail.)
However, Salvia, you are right to note that there are different types of seismic waves, which do behave differently. In addition to P and S waves, there are surface waves. Surface waves are exactly what they sound like: they are strongest at the surface of the earth, and they cause the famous "ground roll" sensation. They can indeed look astonishingly sinusoidal on real seismograms! See, for example, this image. ("Teleseism" is seismologist speak for "earthquake that happened far away.")
P waves are the smallest amplitude and they make that initial shock that you may feel in an earthquake. S waves have a larger amplitude, and surface waves travel slowest but have the largest amplitudes, so often surface waves are what people feel most strongly and what produces the most damage. Rayleigh waves have a long-period rolling motion similar to what you describe.
Now the question is, if surface waves are so common (they are), why do people expect sharp, sudden earthquakes instead of long, gentle ground roll? Well, I don't know exactly why this earthquake may have had more surface wave action than usual, if that's the case. However, I think that people in the L.A. area expect the sharp, sudden earthquake because they are used to being closer to the epicenter - L.A. usually feels earthquakes that are at most tens of miles away, but this one was 200+! Even in Anza-Borrego you may have been 100 miles away.
The reason why this matters goes back to the velocity differences between wave types. P, S, and surface waves all have different velocities, but the difference between their arrival time is not much close to the epicenter, so they all arrive more or less at once. Short, high-intensity shaking. However, the farther away you get from the epicenter, the more the different wave types spread out. The energy is more spread out so the shaking lasts longer but is gentler, and the surface waves have separated out from the body (P and S) waves so you feel them more distinctly.
-----------------------
Okay. That was a lot of stuff already. I just want to add in one more note, which is not directly related the OP's question but may be interesting, as it relates to some things that other people have brought up. But feel free to stop reading here, I know this is getting long!
As far as remarks on the varying length of earthquakes - yes, people in different locations relative to the quake may experience quite different shaking duration! Of course, shaking duration depends primarily on the duration of rupture, but directivity also matters. Let's say that in an earthquake, a big chunk of land moved to the north. That directs more seismic energy to the north - but because both the seismic waves and the land are moving in the same direction, the seismic waves kind of get squished and pile up, so people to the north experience shorter but higher-amplitude shaking. On the other hand, the wave train going south gets stretched out, because the rupture area is moving in a different direction, so people to the south feel longer but lower-amplitude shaking. This is analogous to the Doppler effect, where a siren moving toward you sounds higher-pitched and shorter, while the siren moving away seems lower-pitched and more drawn out.
I don't know what the rupture direction was for this earthquake, unfortunately, so I can't say much about this specific case; I don't know whether directivity played a role, but it's definitely possible. I suspect that directivity was meaningful in the scenario that little light-giver described.
posted by mandanza at 11:52 PM on April 5, 2010 [2 favorites]
Now, here is my science-tastic answer for why you felt what you did, and why it is not a really crazy unusual phenomenon, just a neat one! (N.B.: I am a first-year seismology graduate student.)
For one thing, there is this shadow zone, but I think the waves you did feel wouldn't travel far enough to reach the shadow zone either. Maybe a geologist can inform us.
No, definitely not far enough away to be experiencing shadow zone effects. (Although low-velocity layers can produce local shadow zones, but only for seismic waves in certain orientations - there shouldn't have been any crazy structure between the OP and the quake to create a shadow zone at his location. I can elaborate more about shadow zones if people want, but I don't want to derail.)
However, Salvia, you are right to note that there are different types of seismic waves, which do behave differently. In addition to P and S waves, there are surface waves. Surface waves are exactly what they sound like: they are strongest at the surface of the earth, and they cause the famous "ground roll" sensation. They can indeed look astonishingly sinusoidal on real seismograms! See, for example, this image. ("Teleseism" is seismologist speak for "earthquake that happened far away.")
P waves are the smallest amplitude and they make that initial shock that you may feel in an earthquake. S waves have a larger amplitude, and surface waves travel slowest but have the largest amplitudes, so often surface waves are what people feel most strongly and what produces the most damage. Rayleigh waves have a long-period rolling motion similar to what you describe.
Now the question is, if surface waves are so common (they are), why do people expect sharp, sudden earthquakes instead of long, gentle ground roll? Well, I don't know exactly why this earthquake may have had more surface wave action than usual, if that's the case. However, I think that people in the L.A. area expect the sharp, sudden earthquake because they are used to being closer to the epicenter - L.A. usually feels earthquakes that are at most tens of miles away, but this one was 200+! Even in Anza-Borrego you may have been 100 miles away.
The reason why this matters goes back to the velocity differences between wave types. P, S, and surface waves all have different velocities, but the difference between their arrival time is not much close to the epicenter, so they all arrive more or less at once. Short, high-intensity shaking. However, the farther away you get from the epicenter, the more the different wave types spread out. The energy is more spread out so the shaking lasts longer but is gentler, and the surface waves have separated out from the body (P and S) waves so you feel them more distinctly.
-----------------------
Okay. That was a lot of stuff already. I just want to add in one more note, which is not directly related the OP's question but may be interesting, as it relates to some things that other people have brought up. But feel free to stop reading here, I know this is getting long!
As far as remarks on the varying length of earthquakes - yes, people in different locations relative to the quake may experience quite different shaking duration! Of course, shaking duration depends primarily on the duration of rupture, but directivity also matters. Let's say that in an earthquake, a big chunk of land moved to the north. That directs more seismic energy to the north - but because both the seismic waves and the land are moving in the same direction, the seismic waves kind of get squished and pile up, so people to the north experience shorter but higher-amplitude shaking. On the other hand, the wave train going south gets stretched out, because the rupture area is moving in a different direction, so people to the south feel longer but lower-amplitude shaking. This is analogous to the Doppler effect, where a siren moving toward you sounds higher-pitched and shorter, while the siren moving away seems lower-pitched and more drawn out.
I don't know what the rupture direction was for this earthquake, unfortunately, so I can't say much about this specific case; I don't know whether directivity played a role, but it's definitely possible. I suspect that directivity was meaningful in the scenario that little light-giver described.
posted by mandanza at 11:52 PM on April 5, 2010 [2 favorites]
http://earthquake.usgs.gov/earthquakes/recenteqsus/Quakes/ci14607652.php
posted by neuron at 9:27 AM on April 6, 2010
posted by neuron at 9:27 AM on April 6, 2010
Summer of '08 I felt a strong, jolting earthquake while in Arcadia, CA (somewhat NE of LA). That one lasted a dozen seconds or so and literally sounded like a ton of people running down the street outside; before the shaking and sound crescendoed to an improbable amount and the entire apartment shook, I really did think it was a marathon or something going by outside. It had a very pronounced left, right, left, right motion and feel to it. It scared the crap out of me; I'd never been in an earthquake before and had thought they'd been just a generalized shaking, not this pull one way, then the other thing.
But it turns out different earthquakes feel different. It's hard to describe, but there are two different kinds of shaking as far as I mentally categorize them, and while I have no idea whether these descriptions match up to the physical reality of the situation, they're good for visualizing the distinction between "shaking" and "pull one way, then the other." With the "pull one way, then the other" quake above, I had the impression of being right on the fault line and part of the building was going one way, and then the other side lurching another way. It's very upsetting. But with generalized "shaking" it feels like the fault is somewhere else, further away, and you're solidly on one side so everything is shaking in a uniform, vibrational kind of way.
The ones I've felt since then (now living in Burbank, north of LA), have mostly been just general shaking in the "could have mistook it for vertigo" category: just a slight swaying. Sometimes I'll think an earthquake is coming on and it's just a construction guy rolling something heavy down the walkway to remodel one of the apartments. Sometimes I'm not entirely sure it wasn't a strong gust of wind instead, since I'm on the third (top) floor of the apartment building. It's not always easy to tell: sometimes my husband and I will say to the other, "Earthquake?" and the other won't have even noticed anything. But every now and then it goes on long enough that you can say, okay, that was just a mild earthquake.
I just remembered a few weeks before the Baja quake, I woke up at 4AM or so and an earthquake happened the next minute. I'm not entirely sure it wasn't a smaller earthquake that woke me up to begin with. That one was short but had relatively strong shaking -- it never got quite to the point of anything falling off of a table even, but it was strong enough I worried it would get there. That one felt a bit trampoline-y, but I was lying on a bed so that might have been why.
The earthquake in Baja the other day was very noticeable even in Burbank; it felt like strong vertigo but things like our birds' cages rattled and the quake lasted seemingly a long time. That one was in the "general shaking" category, but even that was a bit different sensation than the other quakes; as other people have said, they're called "rolling" quakes and that's what they feel like. It felt like a slowed-down, simmering version of the other quakes. It was strong, but it was easy to stay calm during it because it didn't feel so sharp and fast. Even our birds didn't freak out during that one -- they just seemed a bit anxious and puzzled -- and they usually do freak out during earthquakes.
So yes, the one you experienced is a bit different of a sensation than usual, at least compared to the ones I've experienced in the past couple years.
posted by Nattie at 12:21 AM on April 7, 2010
But it turns out different earthquakes feel different. It's hard to describe, but there are two different kinds of shaking as far as I mentally categorize them, and while I have no idea whether these descriptions match up to the physical reality of the situation, they're good for visualizing the distinction between "shaking" and "pull one way, then the other." With the "pull one way, then the other" quake above, I had the impression of being right on the fault line and part of the building was going one way, and then the other side lurching another way. It's very upsetting. But with generalized "shaking" it feels like the fault is somewhere else, further away, and you're solidly on one side so everything is shaking in a uniform, vibrational kind of way.
The ones I've felt since then (now living in Burbank, north of LA), have mostly been just general shaking in the "could have mistook it for vertigo" category: just a slight swaying. Sometimes I'll think an earthquake is coming on and it's just a construction guy rolling something heavy down the walkway to remodel one of the apartments. Sometimes I'm not entirely sure it wasn't a strong gust of wind instead, since I'm on the third (top) floor of the apartment building. It's not always easy to tell: sometimes my husband and I will say to the other, "Earthquake?" and the other won't have even noticed anything. But every now and then it goes on long enough that you can say, okay, that was just a mild earthquake.
I just remembered a few weeks before the Baja quake, I woke up at 4AM or so and an earthquake happened the next minute. I'm not entirely sure it wasn't a smaller earthquake that woke me up to begin with. That one was short but had relatively strong shaking -- it never got quite to the point of anything falling off of a table even, but it was strong enough I worried it would get there. That one felt a bit trampoline-y, but I was lying on a bed so that might have been why.
The earthquake in Baja the other day was very noticeable even in Burbank; it felt like strong vertigo but things like our birds' cages rattled and the quake lasted seemingly a long time. That one was in the "general shaking" category, but even that was a bit different sensation than the other quakes; as other people have said, they're called "rolling" quakes and that's what they feel like. It felt like a slowed-down, simmering version of the other quakes. It was strong, but it was easy to stay calm during it because it didn't feel so sharp and fast. Even our birds didn't freak out during that one -- they just seemed a bit anxious and puzzled -- and they usually do freak out during earthquakes.
So yes, the one you experienced is a bit different of a sensation than usual, at least compared to the ones I've experienced in the past couple years.
posted by Nattie at 12:21 AM on April 7, 2010
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According to most reports, this felt like a very large, very long and very "rolling" version of the "normal" quake experienced in SoCal. But your mileage will certainly vary, by quite a bit.
posted by Cool Papa Bell at 10:22 PM on April 5, 2010