A Mw5.0 earthquake occurred in southern California on Wednesday, Sept. 29 at
3:54 p.m. located 17 miles northeast of Arvin in the southern Sierra Nevada.
Because this occurred just over a day and less than 200 km southeast of the
Mw6.0 Parkfield earthquake, there
may be a correlation between the events.
We have in the past, such as July 1986 and June-July 1992 seen moderate to
large earthquakes over wide areas of California.
The proximity of these two earthquakes in time and space, while not
completely understood, is consistent with our growing understanding
that earthquakes can sometimes be triggered by other earthquakes.
Because investigations of triggered earthquakes remain in their
infancy, we cannot quantify precise probabilities of further
triggered events beyond saying "very low." The precise nature of the
relationship (if any) between the two earthquakes is also unclear,
and may be illuminated by future in-depth analysis of the data. As
with any M5.0 earthquake, however, the Mw5.0 mainshock has begun
generating its own local aftershocks.
The Mw5.0 had a strike-slip focal mechanism,
with left-lateral motion on a nodal plane striking N65W. This is
opposite motion to what is normally observed for San Andreas type
earthquakes. The depth of the event is
about 6.5 km.
These earthquakes accommodate crustal thinning within the southern
Sierra Nevada. Extension occurs in both north-south and east-west
direction, causing gradual oblate flattening of the range. The
extensional deformation is associated with areas of high potential
energy, and may be seismic expression of buoyancy-driven thinning of
the Sierran lithosphere.
This earthquake today occurred about 8 km to the north of a
that occurred 25 January 2003. The 2003 M4.3 event was part of a large
swarm of earthquakes that consisted of more than 1000 smaller events.