Seismo Blog

(Anxiously) Waiting for the Big One

2009-03-25 00:00:00 -0700


Seismologists are - like many other scientists - a curious and a restless bunch. Pronounced patience is usually not one of their prime virtues. However, in their job of earthquake monitoring, they sometimes resemble Vladimir and Estragon, the lead characters in Samuel Beckett's "Waiting for Godot." But instead of expecting a mysterious fellow named Godot who never shows up, seismologists know exactly what to expect: A major temblor will come someday and wreak havoc in earthquake prone areas like California, Japan, Greece, or the Hindu Kush. And many a time this "Waiting for the Big One" is no less demanding than the dialogue of Beckett's characters: We seismologists know that our Godot will come, but we don't know when.

However, with events starting last weekend, earthquake researchers in Southern California are on alert. Forty-two smaller earthquakes near the hamlet of Bombay Beach on the eastern shore of the Salton Sea preceded a magnitude 4.8 event, which rattled the area early Tuesday morning. This moderate earthquake was widely felt in the sparsely populated region at the southern end of our state. Several dozen aftershocks followed, the largest having a magnitude of 3.1.

Map showing many recent small earthquakes near Bombay Beach, CA. The 3/24/09 Mw 4.8 quake is also shown.

Map showing many recent earthquakes near Bombay Beach, CA. (Courtesy of USGS)

Why did this earthquake series put our colleagues in the Southland on higher alert? The temblors occured exactly at the southernmost tip of the San Andreas Fault (see blog November 4, 2008). Looking north from there towards Los Angeles, this section of California's major earthquake fault has not ruptured for many decades. This means that a lot of tectonic energy is currently stored in this part of the fault. It contains so much stress, that its possible rupture was the scenario for the biggest earthquake drill ever held in our state. During "The Great Southern California Shake Out" last November, the scenario assumed that the San Andreas Fault would start rupturing near Bombay Beach and would not stop for 120 miles, all the while sending out its destructive waves (see blog November 10, 2008).

Although seismologists operating the Southern California Seismic Network in Pasadena monitor the most recent shocks and aftershocks very carefully, no one can predict what will happen. Are those recent earthquakes starting to loosen up this locked section of the San Andreas fault - and thereby leading to the expected megarupture? Or was it just a cough of the Earth's crust, which will blow away without consequence, like the many sandstorms in the deserts surrounding the Salton Sea? (hra034).

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