Watch how the ground really moves during an earthquake

Back in 2011, geologists working for Signal Hill Petroleum and NodalSeismic conducted an extremely detailed survey to measure the 3D rock structure beneath their Long Beach oilfield. To do so, they deployed a series of seismic arrays consisting of 5,300 nodes spaced about 330 feet (100 meters) apart. And then the earthquakes hit. The results can now be seen in this remarkable video — what shows a series of seismic waves rippling through several Californian cities.


This video, which was produced in collaboration with seismologists from Caltech and Berkeley, clearly shows the earthquake propagating from the epicenter and across the city in a coherent succession of seismic waves. Though earthquakes might feel completely chaotic, this visualization clearly shows that they're anything but.

Illustration for article titled Watch how the ground really moves during an earthquake

The 12-minute video features four different quakes in both real time and in slow motion (the individual earthquakes start at 0:45, 2:20, 6:00, and 8:35). And as you'll see in the video, the sensors used by the seismologists are so sensitive that they even pick up human activity, including traffic.

Writing in Trembling Earth, Austin Elliott explains what's happening:

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.

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.

Read more about this research here.

Image via Trembling Earth/Caltech.


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Corpore Metal

It's a bit like ripples in a pond, with reflections and echoes as the seismic waves bounce off harder rock and soil or transmitted with attenuation through fault lines.