Friday, February 19, 2010

The Redlands Earthquake Swarm: California Has Her Faults; Lots of Them!

A swarm of small and moderate earthquakes has been shaking things up in Southern California in and around the town of Redlands. The largest, on February 13, was magnitude 4.1, with five additional events between 3.1 and 3.6 in the days that followed. Well over a hundred smaller quakes have been detected as well. Swarms are interesting and exciting if you live on top of them, but they don't usually signify bigger things; the earth is shifting a bit, and that is often the end of it. On much rarer occasions, they may be a sign of an impending larger event, a foreshadowing disturbance of the earth similar to the cracks and pops a tree branch makes before it breaks (disclaimer: I am NOT making ANY kind of prediction; I don't have any access to data, nor do I have the background to do so, and seismologists know better than to try and make a short-term prediction of any quake). My message for the day is to remind people who live in earthquake country that they should ALWAYS be prepared for a large seismic event.

The view from Santa Rosa Peak in the Peninsular Ranges reveals the parallel valleys of the San Jacinto fault. The perspective is towards the west; the San Andreas fault is some twenty miles to the east of the peak

There is another opportunity here: a lot of people, when they hear of quakes in California, think San Andreas Fault. Many California residents who have lived here all their lives cannot think of the name of another fault in the state, but our landscape is literally crisscrossed by active faults (look at the map above; most of the brown lines are active faults). Most earthquakes in CA happen on faults other than the San Andreas, and this week's swarm is no exception. It appears to be taking place on one what is arguably the most active fault in the state, the San Jacinto fault.

The San Jacinto fault is certainly part of the San Andreas system. It splits off from the San Andreas at the east end of the San Gabriel Mountains, and runs roughly parallel to the San Andreas for 140 miles south into the Imperial Valley. It has the same type of motion, right lateral (features on the opposite side of the fault have been shifted to the observer's right). Since it began moving a few million years ago, something like 15 miles of lateral motion has taken place.

The San Andreas fault is justly famous for several devastating earthquakes, including the 1906 San Francisco event that killed 3,000 people, and the 1989 Loma Prieta quake (the World Series quake). It also produced a magnitude 8 event in southern California in 1857. But most of the time the fault is quiet (in a menacing way; it is storing up stress). Compare this with the history of the San Jacinto system (courtesy of Wikipedia):

1890 - Magnitude 6.5 that occurred in the "San Jacinto or Elsinore Fault region".

1892 - Another magnitude 6.5 occurred in the same region as the 1890 earthquake.

1899 San Jacinto Earthquake - Magnitude 6.4 earthquake destroys San Jacinto and Hemet.

1918 San Jacinto Earthquake - Magnitude 6.9 earthquake strikes the same area that was damaged by an earthquake 19 years earlier, with an epicenter roughly 10 mi NW of the previous earthquake.

1923 North San Jacinto Fault Earthquake - Magnitude 6.3 earthquake damaged the San Bernardino and Redlands area. Last time the fault, which runs under the I-215/I-10 interchange, ruptured in this area.

1937 Terwilliger Valley Earthquake - Magnitude 6.0

1942 Fish Creek Mountains Earthquake - Magnitude 6.3

1954 Arroyo Salada Earthquake - Magnitude 6.2

1968 Borrego Mountain Earthquake - Magnitude 6.5

1987 Superstition Hills Earthquake - Magnitude 6.6 (Note: some consider it to have occurred on a fault completely unrelated to the San Jacinto Fault Zone)

These quakes are considerably smaller than the 1906 event, by a factor of 32 or more (it takes the energy of 32 magnitude six quakes to equal the energy of a single magnitude 7 quake; a magnitude 8 quake is 32 times more powerful than a magnitude 7 quake and more than a thousand times more powerful than a six). But they clearly happen more often . The message is clear: to live in California, we must be prepared not just for the BIG, HUGE ONE, but also lots of lesser BIG ONES.

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