Robert M. Nadeau and Aurélie Guilhem
Nonvolcanic tremors have generally been observed in transition zones between freely
slipping and locked faults, and in many locations, tremor
activity is also seen to increase with detectable transient fault zone deformation
(slow-slip events) (Liu and Rice, 2007). These
associations suggest that tremor activity is
closely related to the processes responsible for generating
earthquakes. We analyzed tremor
activity in the Parkfield region of California between 27 July 2001 and
21 February 2009, during a period when two
strong earthquakes occurred in the region: the 6.5 San Simeon earthquake
in 2003 and the 6.0 Parkfield earthquake in 2004 (Figure 2.24).
Study region with 1250 well-located tremors (black dots).
Thirty kilometer square boxes (black) define the Monarch Peak (MP) and
Cholame tremor zones. Color contours give regional shear-stress change
at 20 km depth from the Parkfield earthquake (green segment) along the
San Andreas Fault (SAF). The thrust type San Simeon earthquake rupture
is represented by the grey rectangle and hatched line labeled SS. The
currently locked Cholame segment is 63 km long (solid portion of
arrow) and is believed capable of rupturing on its own in an M7
earthquake. The grey lines within the Cholame box bound the west quadrant
where quasi-periodic episodes predominate. White star indicates the epicenter
and gray stars the foreshocks of the 1857 Ft. Tejon earthquake.
Using borehole seismic data from the High Resolution Seismic Network
(HRSN) at Parkfield, CA, 2198 tremor events (12,547 minutes of activity)
were detected during the period (Figure 2.25), with
event activity lasting from 3 to nearly 21 minutes. Significant activity also occurred below
the detection threshold. The average seismic energy
released during the tremors was typically equivalent to a 0.5
earthquake, and energies ranged over 1.5 units.
Data from five seismographic networks (BP, BK, NC, CI, and TA) were combined to locate the tremors
(Figure 2.25). Approximately 90% of the tremors occurred adjacent
to the locked Cholame segment of the San Andreas fault (SAF). An
additional 5-10% occurred 65 km to the northwest in the vicinity of Monarch
Peak, CA. Tremors in both zones occurred at between 15 and 30 km depths.
A sudden increase and subsequent decay of tremor activity
(aftertremors) began immediately after the 2003 San Simeon and 2004
Parkfield earthquakes (Figure 2.25A).
In addition, approximately 3 weeks before the Parkfield earthquake, an unusually
strong episode of activity (foretremor) lasting 5 days also occurred
(Nadeau and Guilhem, 2009).
The decay of tremor activity after the Parkfield earthquake continued until
mid-2006 when annual rates began again to increase, reaching 181% of the pre-San
Simeon level during the final year of our analysis (Figure 2.25A).
Before the Parkfield earthquake, tremor activity in the
Monarch Peak tremor zone (Figure 2.24) was low
(Figure 2.25B). However, beginning 10 days after the Parkfield
event, activity in this zone increased significantly and has
continued at an elevated rate.
After the Parkfield earthquake, persistent episodes of quasi-periodic
tremor also emerged that are reminiscent of episodic tremor
observed in some subduction zones (Figure 2.25A) (Rogers and Dragert, 2003).
The episodic component of the tremors are most persistent and regular in
the western quadrant of the Cholame tremor zone that is bounded by the
SAF to the northeast and by the seismic-aseismic transition into the
locked Cholame segment to the southeast (Figures 2.24,
2.25C and D,). This localization of tremor behavior suggests that there are
differences in the process generating tremor in different subregions of
the SAF and that tremor in the Cholame zone
is distributed both normal-to and along-strike of the SAF at depth.
Time histories of tremor activity. (A) Fifteen-day (grey) and
45-day (black) smoothed rate histories for all tremors in the study
area. Times of the San Simeon and Parkfield earthquakes are SS and PF,
respectively. Intense foretremor (Fore-Tremor) activity occurred 3
weeks before PF. Fifteen- and 45-day peak values just after PF are
31.1 and 18.9 minutes/day, respectively. White bars are interpreted
times of quasi-periodic episodes. Label ? is time of a weak or missing
episode. (B) History of tremor in the Monarch Peak zone (45-day
smoothed). (C) History since 2005 of activity (5-day smoothed) in the
western quadrant of the Cholame zone (Figure 2.24) (423 tremors; 2835 cumulative
minutes of activity) showing strong episodic
behavior. (D) Same as (C) for Cholame tremors outside the west
quadrant (416 tremors; 2423 cumulative minutes) showing significantly
less episodic behavior.
Static Coulomb- and shear-stress changes in the tremor zones from
the 2003 San Simeon and 2004 Parkfield earthquakes on SAF aligned planes
were small (6 to 14 kPa), but increases in tremor rates correspond with
the increases in Coulomb- and shear- stresses.
Static normal-stress changes from the earthquakes do not clearly
correspond to the tremor rate changes, suggesting that normal-stress
change may not play a significant role in stimulating the SAF tremor.
The Coulomb stress changes that stimulated the SAF tremors
are roughly an order of magnitude smaller than those typically reported
for the triggering of earthquakes (King et al., 1994). This suggests that tremors are
a more sensitive indicator of stress change than earthquakes.
The periodic tremor and persistent elevated activity after the
Parkfield quake are not consistent with expectations of
exponentially decaying post-seismic stress after an earthquake (Savage and Langbein, 2008).
Because the SAF tremor changes have persisted for over 4 years since
Parkfield, they could be signaling a shift in the
process of deformation and stress accumulation beneath this hazardous portion of
the SAF. Faulting from the great 7.8 1857 Ft. Tejon earthquake
appears to have propagated from the
Monarch Peak area southeastward along the SAF for 350 km, through
Parkfield and the Cholame tremor zone (Toppozada et al., 2002)
(Figure 2.24). The Cholame segment is now fully locked
and last ruptured 152 years ago as part of the
great 1857 event.
The HRSN is funded by USGS grant 07HQAG0014.
Research was supported was by USGS grants 06HQGR0167 and 08HQGR0100,
and by NSF grants EAR0537641 and EAR0544730.
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Berkeley Seismological Laboratory
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