Article,
Migrating tremors illuminate complex deformation beneath the seismogenic San Andreas fault
Nature, 463 (7281): 648--652 (Feb 4, 2010)
DOI: 10.1038/nature08755
Abstract
The San Andreas fault is one of the most extensively studied faults
in the world, yet its physical character and deformation mode beneath
the relatively shallow earthquake-generating portion remain largely
unconstrained. Tectonic 'non-volcanic' tremor, a recently discovered
seismic signal probably generated by shear slip on the deep extension
of some major faults can provide new insight into the deep fate of
such faults, including that of the San Andreas fault near Parkfield,
California. Here I examine continuous seismic data from mid-2001
to 2008, identifying tremor and decomposing the signal into different
families of activity based on the shape and timing of the waveforms
at multiple stations. This approach allows differentiation between
activities from nearby patches of the deep fault and begins to unveil
rich and complex patterns of tremor occurrence. I find that tremor
exhibits nearly continuous migration, with the most extensive episodes
propagating more than 20 kilometres along fault strike at rates of
15-80 kilometres per hour. This suggests that the San Andreas fault
remains a localized through-going structure, at least to the base
of the crust, in this area. Tremor rates and recurrence behaviour
changed markedly in the wake of the 2004 magnitude-6.0 Parkfield
earthquake, but these changes were far from uniform within the tremor
zone, probably reflecting heterogeneous fault properties and static
and dynamic stresses decaying away from the rupture. The systematic
recurrence of tremor demonstrated here suggests the potential to
monitor detailed time-varying deformation on this portion of the
deep San Andreas fault, deformation which unsteadily loads the shallower
zone that last ruptured in the 1857 magnitude-7.9 Fort Tejon earthquake.
