R.M. Nadeau, F. Brenguier
(Sismologie, Institut de Physique du Globe de Paris & CNRS,
(Laboratoire de Geophysique Interne et Tectonophysique
& CNRS, Grenoble, France)
Seismic velocity changes (upper time history),
surface displacements from GPS (upper, curve following Parkfield event) and tremor
activity (lower time history) near Parkfield. The displacement
curve represents the postseismic fault parallel displacements along
the San Andreas fault as measured by GPS at station POMM. The tremor
rates are averaged over a centered 30 day moving time window.
Since their discovery in 2002 (Obara, 2002), deep ( 15-40 km) nonvolcanic tremors
(long-duration seismic signals with no clear P or S waves)
have generally been found in transition zones between freely slipping
and locked fault (Rogers and Dragert, 2003; Nadeau and Dolenc, 2005;
Brudzinski and Allen, 2007; Payero et al.; 2008). In most cases, the
tremors occur in subduction zones; hence, fluids from dehydration
processes are believed to play an important role in tremor generation.
Changes in the rate of tremor activity also often correlate with
transient fault deformation (slow-slip events)
and with dynamic stress changes from tides and
surface waves of teleseismic (distant) earthquakes
(Gomberg et al., 2007, Rubinstein et al., 2008).
These associations suggest that a better understanding of the
mechanisms responsible for tremors may provide important clues
to the rheology of deep fault zones and processes responsible for
generating large earthquakes.
Reported here is a recently discovered correlation
between changes in nonvolcanic tremor activity and changes in
seismic velocity and fault deformation along the
central San Andreas fault occurring conjunctively
with the 22 December 2003, M6.5 San Simeon and 28 September
2004, M6.0 Parkfield earthquakes (Brenguier et al., 2008b).
To monitor variations in seismic velocities and nonvolcanic tremor,
we analyzed more than 5 years
(January, 2002 to October, 2007) of continuous seismic data from
the 13 borehole seismic stations of the Berkeley High Resolution
Seismic Network (HRSN). The analyses also spanned the
two strongest earthquakes occurring within 100 km of
Parkfield, CA: the Mw 6.5 San Simeon Earthquake of 22 December 2003,
whose epicenter was located 60 km west of Parkfield, and the Mw 6.0
Parkfield Earthquake of 28 September 2004. Sensor depths of the HRSN
stations range between 60 and 300 m, thus reducing locally generated
noise and effects of temperature variations and precipitation.
For every possible pair combination of stations we computed the daily
cross-correlation of seismic noise using the procedure of (Brenguier et al., 2007), yielding
91 x 2140 days = 194,740 cross-correlation and autocorrelation time
functions. A Reference Green Function (RGF) was computed for each
station pair by stacking the daily cross-correlations for the entire
2140 day period (Brenguier et al., 2008b). The velocity changes were then determined by
measuring time delays between the RGF and 30 day stacks of cross-
correlation functions in the frequency range 0.1 to 0.9 Hz (Brenguier
et al., 2008a, 2008b).
By measuring the slope of the travel time
shifts, dt, as function of time, t, we then estimated the relative time
perturbation (dt/t), which is the opposite value of the medium's
relative velocity change (dv/v). Finally, following Brenguier et al.
(2008a), we averaged the relative time delays over all station pairs to increase the
During the study period, 1577 tremor events ranging in
duration from 3 to 21 minutes were detected using 3 to 8 Hz filtered
continuous records from the HRSN and root-mean-square envelope techniques
(Obara, 2002; Brenguier et al., 2008b). In total, 8962 minutes of
tremor activity was detected. These tremors are estimated to have
occurred between 20 and 40 km depth, indicating that they are related to
deep processes along the fault zone (Nadeau and Dolenc, 2005). A 30 day averaged rate
history of tremor activity was then generated from these data for
comparison to the seismic velocity and deformation data (Figure 2.73).
Surface deformation measurements used in this study (primarily GPS) were
obtained over the internet from the USGS web site and selected and processed as
described in Brenguier et al. (2008b). More detail concerning the USGS
deformation and GPS networks and related surface deformation measurements can be found
at, http://earthquake.usgs.gov/research/parkfield/deform.php and http://quake.usgs.gov/research/deformation/twocolor/pkf_continuous_gps.html.
The correlated evolution of the nonvolcanic
tremors, seismic velocities, and fault zone deformation suggests the
presence of two physical mechanisms generating changes in
crustal and fault zone properties: 1) shallow damage within the fault
zone and near surface layers from strong ground shaking by the mainshocks
and 2) coseismic stress change and postseismic relaxation
extending down to nonvolcanic tremor depths ( 30 km). The results also
demonstrate that measurements of velocity change from
seismic noise analysis can be useful for studying the
continuous time evolution of stress in the vicinity of
seismogenic faults and tremor zones.
The HRSN is funded by USGS grant 07HQAG0014.
Research support was from ANR (France) under
05CATT01001, PRECORSIS, ANR06-CEXC005, and COHERSIS, from NSF under
EAR0537641 and EAR0544730 and from USGS grant 06HQGR0167.
This collaboration arose from discussions at the 2nd joint BSL/IPGP workshop on Seismology and Seismotectonics: BSL and IPGP research perspectives, held at Berkeley Seismological Laboratory (BSL) on December 16-17, 2007 and co-sponsored by BSL and the Project International de Cooperation Scientifique (PICS) awarded to the 'Departement de Sismologie' at the `Institut de Physique du Globe de Paris' (IPGP).
Brenguier F., N.M. Shapiro, M. Campillo, A. Nercessian, V. Ferrazzini,
3-D surface wave tomography of the Piton de la Fournaise volcano using
seismic noise correlations, Geophys. Res. Lett., 34, L02305,
Brenguier F., N.M. Shapiro, M. Campillo, V. Ferrazzini, Z. Duputel, O.
Coutant, A. Nercessian, Towards forecasting volcanic eruptions using
seismic noise, Nature Geoscience, 1, 126-130, 2008a.
Brenguier F., M. Campillo, C. Hadziioannou, N.M. Shapiro, R.M.
Nadeau and E. Larose, Postseismic Relaxation Along the San Andreas Fault
at Parkfield from Continuous Seismological Observations, Science,
in press, 2008b.
Brudzinski, M. and R.M. Allen, Segmentation in Episodic Tremor
and Slip All Along Cascadia, Geology, 35, 907-910, 2007.
Gomberg, J., J.L. Rubinstein, Z. Peng, K.C. Creager and J.E. Vidale, P.
Bodin, Widespread Triggering of Non-Volcanic Tremor in California, Science, 319, 173, 2008.
Nadeau, R.M. and D. Dolenc, Nonvolcanic Tremors Deep Beneath
the San Andreas Fault, Science, 307, 389, 2005.
Obara, K., Nonvolcanic Deep Tremor Associated with Subduction in
Southwest Japan, Science, 296, 1679-1681, 2002.
Payero, J. S., V. Kostoglodov, N. Shapiro, T. Mikumo, A. Iglesias, X.
Peres-Campos and R. W. Clayton (2008), Nonvolcanic Tremor Observed in
the Mexican Subduction Zone, Geophys. Res. Lett., 35,
L07305, doi:10.1029/2007GRL32877, 2008.
Rogers, G. and H. Dragert, Episodic Tremor and Slip on the Cascadia
Subduction Zone: The Chatter of Silent Slip, Science, 300,
Rubinstein, J. L., M. La Rocca, J.E. Vidale, K.C. Creager and A.G. Wech
(2007), Tidal Modulation of Non-Volcanic Tremor, Science, 319,
Berkeley Seismological Laboratory
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