An intensive and ongoing effort by the SAFOD target event location
working group is underway with the following goals: 1) the characterization
of the detailed velocity and seismicity structure in the crustal volume
containing the SAFOD main hole and 2) to determine the most accurate
estimates of the absolute locations of SAFOD's target events.
The HRSN data
play a key role in this effort by providing low noise and high sensitivity
seismic waveforms from active and passive sources and by providing a backbone of
earthquake and tremor detection and waveforms from the numerous
microearthquakes and tremors that are occurring in the general vicinity of SAFOD.
In a special section of Geophysical Research Letters from May of 2004,
several papers make significant use of the HRSN data for characterizing
the SAFOD area and illustrate the role that the
HRSN data have played in the SAFOD effort over the past year (e.g., Oye et al.,
2004; Roecker et al., 2004; Thurber et al., 2004; Nadeau et al., 2004).
In addition to the data collection, Berkeley's other contributions also include detailed
monitoring of the repeating M2 SAFOD target sequences, and in particular, this year,
their evolving pattern of behavior following the 28 September 2004, M6 Parkfield
earthquake (Figure 6.4).
Upper right panel shows a 40 km along fault depth section of the San Andreas
Fault that includes the epicenters of the 1966 and 2004 Parkfield M6 earthquakes (box
and star respectively),
HRSN catalog locations of seismicity in the region preceding the 2004 M6
event (black points) and the region containing the SAFOD target sequences of
repeating M2 earthquakes (small box). Zoom box of the SAFOD region shows that
the SAFOD target region actually contains numerous sites of repeating microearthquakes
(circles and diamonds). The diamonds labeled NW and SE are the two primary
M2 sequences being considered as targets for close-in monitoring and eventual
penetration. Large circle labeled SW is also an M2 repeating site that lies
2-300 meters to the southwest of the primary targets and lies between the SAFOD
drill site and the primary targets. Lower panel shows waveforms from the target sequence
events occurring since 1984 and into the period immediately following the Parkfield
mainshock. Waveforms on the left are from surface NCSN station PCA (VHZ channel) and
the three on the right are from the HRSN. The much higher sampling rate and borehole
sensors of the HRSN are critical in identifying and discriminating between repeats of the
primary targets that are located only 70 m apart and whose waveforms among the NCSN
recordings are virtually indistinguishable.
Both the primary target (NW and SE) and SW sequences experienced several
repeats during the year following the 2004 M6 (for comparison, prior to the M6 the recurrence times
of sequence members was generally once every 3 years).
During January of 2005 (between
SAFOD's Phase 1 and Phase 2 drilling periods) a string of seismic sensors was deployed
down the SAFOD hole and recorded one of the post-M6 SE sequence repeats. This close-in
conjunction with recordings from the HRSN, NCSN, the temporary PASO deployment and the
SAFOD pilot hole string led to some surprises regarding the accuracy of the
absolute location estimates for the primary target sequences, and provided critical
information for guiding drilling during phase 2 in the summer of 2005.
Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
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