There is an interesting discussion regarding this quake on the newsgroup sci.geo.earthquakes. The thread is begun with one by Gerard Fryer, rather an authority on things seismological. He posts:

<spurred a workshop at next month's Soc. Exploration Geophysicists. Now
we have an even more unusual event: an M 6 thrust in the middle of the
basin: http://earthquake.usgs.gov/eqcenter/recenteqsus/Quakes/usslav.php

Most think the earlier earthquake was a slump, but after this one
(which is probably tectonic) they may change their minds.>>>

Further on in the thread, other posters have provided additional interesting links.

Michael Williams in Arroyo Grande, CA USA