The Trembling Earth

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Tag Archives: seismic waves

Watch the ground ripple in Long Beach

As the seismic waves from a whole host of little earthquakes in L.A. rippled through the basin in 2011, an astonishingly dense array of seismometers deployed in Long Beach captured them in unprecedented detail. Local oil and gas company Signal Hill Petroleum deployed the monitoring instruments in order to conduct an extremely detailed survey of the 3D rock structure beneath their oil fields. Researchers from Caltech and Berkeley struck an agreement with the oil company to share the data for academic research into the earthquake process and details of fault behavior. One of the results of this research is the amazing video below, in which we can see the elastic (seismic) waves of several earthquakes as they propagate from the hypocenter and rock the city block by block. Note that the initial playback is in real time, not sped up or slowed down. Skip around to see each of the 4 quakes without watching all 12 minutes: the individual quakes start at 0:45, 2:20, 6:00, and 8:35.

The seismometers of this network–in this case relatively inexpensive geophones, measuring vertical ground velocity–are located a mere 100 meters apart, creating a network with several instruments per city block! Because of this amazingly dense coverage, we can see the great gory detail of waves of motion moving through the rock underfoot.

In the videos, they have drawn the trace of the Newport-Inglewood Fault, a notable northwest striking strike-slip fault (the source of the 1933 Long Beach earthquake). One of the most notable features of the wavefields displayed in the videos is how drastically this fault zone alters the propagating waves.

Seismic waves from a nearby M2.5 earthquake ripple across the city of Long Beach in this visualization of an unprecedented dense array of seismometers.

Seismic waves from a nearby M2.5 earthquake ripple across the city of Long Beach in this visualization of an unprecedented dense array of seismometers.

When they travel along the fault, they speed up in the fault zone, likely due to alignment of mineral grains and rock structural boundaries in the direction of slip. When the waves have to cross the fault, they get held back and slowed down, forming an irregular jog or knick in the wavefield. This hold-up is probably partially due to that same alignment of grains, now traveling along their short axes, but it’s also due in part to “microslip” along the fault. As the rock on one side bends with elastic waves, the fault accommodates a bit of slip before letting the wave propagate past. The researchers are studying this effect as well, and have begun to map out regions of slip on the N-I fault during adjacent temblors.

It’s rather beautiful, really, to see that this mapped fault has a real physical effect, validating its presence and importance. It’s also endlessly fascinating to watch the details of real seismic waves passing beneath the city of Long Beach. This is how the ground moves in an earthquake.

More info about the research coming out of this awesome data can be found here:

http://www.gps.caltech.edu/~clay/LB3D/LB3D.html

Thanks to Chris Rowan and Cristoph Grützner for bringing this one to my attention.

Update: If you’re now hooked on this kind of visualization, fret not: the Incorporated Research Institutions for Seismology produce these regularly using seismic data from the US Array. Though not at a block-by-block resolution, the animations come from impressive coverage on a spectacularly dense instrumental array, in which you can see the imperceptible seismic waves from distant earthquakes roll beneath the U.S.

Check them out after big quakes! http://www.iris.edu/spud/gmv

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Animation of Japan quake traversing the U.S.

The data-consolidating institutional consortium IRIS–the Incorporated Research Institutions for Seismology–has a spectacular resource to visualize actual seismic waves propagating around the Earth, that everyone should check out.

Here is an animation they put together (they do this for every significant quake) displaying ground motion at recording stations set up around the U.S. You can see the dramatic passage of the seismic waves from the 8.9 quake, and you get glimpses of the waves that have circled the Earth from the other side and are passing back through heading back towards the epicenter. This is a visualization of actual recorded data–note the scale bar on the seismogram at the bottom: nearly a centimeter of ground motion all across the U.S.!

The data are from the “USArray,” a travelling deployment of seismometers meant not to detect earthquakes but to probe the interior of the Earth using seismic waves, like a giant ultrasound. Of course the array thoroughly detects earthquakes, but its density is overkill for the location and characterization of individual seismic events–that’s why it doesn’t matter that it’s migrating across the country.

Obviously right now the array predominantly occupies the great plains and is just entering the midwest. In this animation the amplitude of vertical motion is colored on a red-blue scale and the direction of motion at any given station is indicated in real time (well, sped up) as a little line directed outward from the station. It’s a pretty good way to visualize 3D ground motion if I do say so myself. The seismogram visible on the bottom is a representative one chosen near the center of the array (highlighted in yellow on the map). Those of you in the midwest–look how much was going on beneath your feet yesterday!

I highly encourage you to explore this site and the vast collection of Ground Motion Visualizations (GMVs) they have. Pick your favorite earthquake and watch it ripple through the U.S.!

Enjoy!

Japan quake felt over >2500km radius

One of the many remarkable features about this planet’s largest quakes (like the one that just happened in Japan) is their truly global effect. Be it the tsunami or seismic waves perceptible and imperceptible, most parts of the planet have been touched significantly by the 8.9.

Let’s start with perceptible seismic waves, i.e., ones people don’t need sensitive instruments to detect. The elastic energy released by this earthquake was enough to set the ground rippling at frequencies and amplitudes people can percieve at distances of greater than 2,500 kilometers (over 1,500 miles).

Shaking intensity as reported online by citizens of east Asia. Data points are colored by intensity and sized by city population. The USGS Did-You-Feel-It page (click image for link) contains the original data and plots of response time and attenuation with distance.

Note the felt reports in Taiwan and Beijing. This affected radius is quite comparable to the huge swaths of the globe that felt both the M8.8 Maule, Chile event last year (most of South America) and the 2004 9.1 Sumatra, Indonesia quake (most of southeast Asia and India):

Felt reports from the other M~9 earthquakes that have occurred since the advent of the USGS's Did You Feel It surveys.

Not only did the perceptible shaking stretch far around the globe in all of these cases, but sizeable elastic waves detected only by sensitive seismometers rippled through the solid crust (and bounced around deep in the mantle), observably circling the globe several times.

Seismic events larger than M~6.5 tend to be recorded globally on broadband seismometers–those that can detect a broad range of frequencies, from sharp local jolts to the long, slow undulations of large, distant quakes. I’ll have another post on seismic waves and attenuation later, for now suffice it to demonstrate the following: Below is a seismic record from northern California showing waves from the 7.2 “foreshock” in Japan on March 9.

Northern California seismic recording of a 7.2 in Japan on March 8 (9th in Japan). Read this like a book; each line represents 15 minutes of recording, so where the oscillations last >15 minutes the records overlap, making it a bit messy. Basically, the ground oscillated detectably in California from the distant quake. The amplitude of ground motion is small, and the frequency of the largest waves is 1 per ~30 seconds, meaning it takes the ground 30 seconds to oscillate one way then back the other, slower than your bedroom fan... no wonder we can't sense it without seismometers.

Compare that to the Mar 11 M8.9 in virtually the same location recorded on the same instrument:

Magnitude 8.9 earthquake in Japan recorded in northern California. The amplitude of ground motion is literally off the charts, but since the frequency is relatively low, we couldn't feel the waves passing though they certainly were.

Now, check out yesterday’s:

March 11 seismic record section from northern California

At the top of the record section is the messy, high-amplitude disruption from the initial passage of the 8.9’s surface waves. Just before 12:00 UTC you can see the dense green squiggle that represents the arrival of seismic waves from a 7.1 aftershock in Japan, with the early high-frequency waves superimposed on the low frequency oscillations continuing from the 8.9.

Several large Japanese aftershocks are apparent throughout the next few hours (in the low- to mid- M6 range, represented by occasional dense, high-frequency squiggles). The record in the background continues to undulate irregularly as elastic waves reverberate through the Earth, bouncing off of the dense metal core, refracting and reflecting off of every boundary or irregularity they come to. Imagine someone jumping into an undisturbed swimming pool–or try it yourself!–and watch as the initial coherent rings of waves bounce off the walls and turn the pool surface into a sloshy, ripply mess. That’s what has happened to the planet.

At around 02:00 UTC another extended period of oscillation begins. This likely represents the (much-diminished) seismic waves from the original earthquake re-circling the globe. Hopefully someone with more seismology expertise than myself will soon put together a nifty stacked plot like this one from 2004 illustrating this repetition.

Of course, the tsunami still has the Pacific ocean sloshing about, roiling in turbulent currents where it reaches the shallow topography of coastlines.

You can see just how turbulent and dangerous this ocean-wide surge of water can be in this video from the coast of northern California, taken as the tsunami arrived. Pay attention to the speed with which the water rises and falls.

Many thanks to Aron Meltzner for doing some superb video sleuthing and pointing me to many of the tsunami clips. I may post a compilation of more footage from all around the Pacific in the near future.

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