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CAGiversary!
http://www.accuweather.com/mt-news-blogs.asp?blog=andrews&partner=&pgUrl=/mtweb/content/andrews/archives/2009/03/bombay_beach_callifornia_earthquake_swarma.asp
Thursday, March 26, 2009 10:41 AM
Do Southern California Quakes Foretell `The Big One`
Bombay Beach, California Earthquake Swarm--a Little Background
A "swarm", or series, of earthquakes, most of them small, has taken place this late March in Southern California along the infamous San Andreas Fault Zone. While none has been severe, the earthquake swarm raises interesting questions in a region that is always bracing for the "big one"; that is, a catastrophic shake affecting the lives and property of millions.
A swarm of earthquakes has been defined as "a series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time." This swarm began on March 21, 2009 beneath the floor of the Salton Sea off Bombay Beach, California. The swarm continued through at least March 24. Strongest of the tremors was a Magnitude 4.8 shake that happened on the 23rd with an epicenter about 2.5 miles south of Bombay Beach. While the M4.8 quake was widely felt, by far most of the quakes had magnitudes below M2.0, meaning that they were unlikely to be felt by most (if not all) people. Focal depth, which represents the starting point within the Earth of the rupture triggering the shaking, was mostly between 2 and 5 miles underground.
Plotted on a map, the epicenters (surface location above the rock break) were alined in a northeast-southwest direction, or "strike." And the M4.8 quake showed fault movement along this same strike. This movement implied that the rock rupture was "strike-slip." A strike-slip fault can be defined as "a fault along which the slip motion is parallel to the strike of the fault." In this instance, the southeastern side of the fault shifted northeastward with respect to the northwestern side--this was "left-lateral" fault slip. For reference, the many fault strands of the San Andreas Fault Zone, which stretches most of the length of California, are all right-lateral strike-slip faults.
This earthquake swarm happened within an area know to students of earthquakes (seismologists) as the Brawley Seismic Zone, which is not a single fault; rather, it is a complex series of breaks in the Earth's crust between the southern end of the southern San Andreas Fault and the northern Imperial Fault, which reaches southward into Mexico. These two great faults, with their many strands and splinter faults, have the overall task of taking western California and Baja California northward versus the rest of North America ("right-lateral" "strike-slip" motion, that is).
The Imperial Valley Fault has a few strong historical quakes which are known by seismologists for their offset of tree rows in the agricultural Imperial Valley. But it is the San Andreas Fault that has both the history of that and the potential for the "big one"--disastrous shaking of magnitude up to, say, M8.0.
Northern California had the San Andreas break with catastrophic effect in 1906. In the south, however, the region nearest the Salton Sea has not had a great earthquake for at least 300 years. Farther north, there was a great quake (magnitude estimated at M7.9) having an epicenter at Fort Tejon near the modern-day Grapevine--the busy highway bottleneck between Southern California and the San Joaquin Valley.
The Fort Tejon earthquake of 1857 was triggered by a right-lateral strike-slip displacement of up to 30 feet/9 meters along the San Andreas Fault. The tear in the Earth's surface stretched about 215 miles/350 km along the fault. Rivers were, in a few instances, thrown over their banks, as were waters of the Tulare Lake, which once filled a part of the San Joaquin Valley.
While it killed only two people and did relatively little damage in a region sparsely settled at the time, such a violent shaking would, today, have a catastrophic effect upon the state. Damage would reach far into the billions of dollars.
So it's been over 160 years since the Fort Tejon Earthquake and even longer for strands of the San Andreas Fault Zone nearest the Salton Sea and the Bombay Beach Earthquake Swarm. Could this swarm of quakes mean that some major break in the crust is in the making? A question that seismologists are no doubt bearing in mind, for it is known that major quakes can sometimes be preceded by swarms of much weaker shakes. Unlike the weather, however, forecasting earthquakes is a science in its infancy. So, in the end, there may be no way of knowing with any confidence that a major rupture of the southern San Andreas Fault will happen before the fact.
Thanks to the Southern California Earthquake Data Center (SCDEC) for the information that helped me in the building of this story.
Thursday, March 26, 2009 10:41 AM
Do Southern California Quakes Foretell `The Big One`
Bombay Beach, California Earthquake Swarm--a Little Background
A "swarm", or series, of earthquakes, most of them small, has taken place this late March in Southern California along the infamous San Andreas Fault Zone. While none has been severe, the earthquake swarm raises interesting questions in a region that is always bracing for the "big one"; that is, a catastrophic shake affecting the lives and property of millions.
A swarm of earthquakes has been defined as "a series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time." This swarm began on March 21, 2009 beneath the floor of the Salton Sea off Bombay Beach, California. The swarm continued through at least March 24. Strongest of the tremors was a Magnitude 4.8 shake that happened on the 23rd with an epicenter about 2.5 miles south of Bombay Beach. While the M4.8 quake was widely felt, by far most of the quakes had magnitudes below M2.0, meaning that they were unlikely to be felt by most (if not all) people. Focal depth, which represents the starting point within the Earth of the rupture triggering the shaking, was mostly between 2 and 5 miles underground.
Plotted on a map, the epicenters (surface location above the rock break) were alined in a northeast-southwest direction, or "strike." And the M4.8 quake showed fault movement along this same strike. This movement implied that the rock rupture was "strike-slip." A strike-slip fault can be defined as "a fault along which the slip motion is parallel to the strike of the fault." In this instance, the southeastern side of the fault shifted northeastward with respect to the northwestern side--this was "left-lateral" fault slip. For reference, the many fault strands of the San Andreas Fault Zone, which stretches most of the length of California, are all right-lateral strike-slip faults.
This earthquake swarm happened within an area know to students of earthquakes (seismologists) as the Brawley Seismic Zone, which is not a single fault; rather, it is a complex series of breaks in the Earth's crust between the southern end of the southern San Andreas Fault and the northern Imperial Fault, which reaches southward into Mexico. These two great faults, with their many strands and splinter faults, have the overall task of taking western California and Baja California northward versus the rest of North America ("right-lateral" "strike-slip" motion, that is).
The Imperial Valley Fault has a few strong historical quakes which are known by seismologists for their offset of tree rows in the agricultural Imperial Valley. But it is the San Andreas Fault that has both the history of that and the potential for the "big one"--disastrous shaking of magnitude up to, say, M8.0.
Northern California had the San Andreas break with catastrophic effect in 1906. In the south, however, the region nearest the Salton Sea has not had a great earthquake for at least 300 years. Farther north, there was a great quake (magnitude estimated at M7.9) having an epicenter at Fort Tejon near the modern-day Grapevine--the busy highway bottleneck between Southern California and the San Joaquin Valley.
The Fort Tejon earthquake of 1857 was triggered by a right-lateral strike-slip displacement of up to 30 feet/9 meters along the San Andreas Fault. The tear in the Earth's surface stretched about 215 miles/350 km along the fault. Rivers were, in a few instances, thrown over their banks, as were waters of the Tulare Lake, which once filled a part of the San Joaquin Valley.
While it killed only two people and did relatively little damage in a region sparsely settled at the time, such a violent shaking would, today, have a catastrophic effect upon the state. Damage would reach far into the billions of dollars.
So it's been over 160 years since the Fort Tejon Earthquake and even longer for strands of the San Andreas Fault Zone nearest the Salton Sea and the Bombay Beach Earthquake Swarm. Could this swarm of quakes mean that some major break in the crust is in the making? A question that seismologists are no doubt bearing in mind, for it is known that major quakes can sometimes be preceded by swarms of much weaker shakes. Unlike the weather, however, forecasting earthquakes is a science in its infancy. So, in the end, there may be no way of knowing with any confidence that a major rupture of the southern San Andreas Fault will happen before the fact.
Thanks to the Southern California Earthquake Data Center (SCDEC) for the information that helped me in the building of this story.
ing roasting weather for a major portion of the year. It gets plenty hot enough here as it is. Arizona during the same months? No thanks. I'll take the vanishingly small threat of seismic event fatality over the guaranteed quality of life reduction. Every year Arizona tallies up a few dozen fatalities due to heat exposure. The last time an earthquake killed anyone within a two-hundred mile radius of me was fifteen years ago. (The interchange named for the motorcycle cop who wasn't able to stop in time before going off the collapsed section is just a few miles from where I now live.)
