December 17, 2012

Watch the ground ripple in Long Beach

Posted by Austin Elliott

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