From October 1983 to May 1984, the southern Sierra Nevada range was the site of a swarm of about 2000 earthquakes, 35 of which were of magnitude M_L 3.0 or greater. The swarm started in the Durrwood Meadows area, about 32 km (20 miles) north of Lake Isabella, on October 19, 1983, with a magnitude 4.0 earthquake. Some fifty-six hours later, the largest quake of the swarm, magnitude 4.9, struck the area. Three days later, another magnitude 4.0 quake followed. All three of these quakes were at a depth of roughly 5 km and exhibited almost pure normal faulting, along a north-south strike -- parallel to the large, but inactive, Kern River Fault (to the west). As the activity continued on into 1984, the "center" of the swarm migrated northward, and eastward.
Earthquake swarms are often connected to magmatic activity, but despite the recent basaltic lava flows in the Golden Trout Creek volcanic center to the north, magmatic activity was deemed unlikely due to the narrow range of earthquake depths and focal mechanisms.
A significant (greater in magnitude) earthquake sequence occurred north of the Durrwood Meadows area in 1868, likely on the same seismic belt. Four years after that, the Owens Valley fault ruptured in a magnitude 8 earthquake (the epicenter of which is north of the clickable map area) with a focal mechanism similar to those typical of the southern Sierra Nevada seismic belt. Whether this was in any way due to the 1868 earthquake is unknown, but because of the possible correlation, activity in the Sierra Nevada range -- as is any activity near a fault capable of producing a major earthquake -- is monitored carefully. It is unlikely, however, that the Durrwood swarm could have acted as such a trigger. Still, the swarm generated a lot of interest and gave us a few more clues about the extent of seismic hazard in the Sierra Nevada range.





REFERENCES