Subsections

Correlated Changes in Nonvolcanic Tremor, Seismic Velocity and Fault Displacement Associated with the 2003 San Simeon and 2004 Parkfield Earthquakes.

R.M. Nadeau, F. Brenguier $^{1,2}$ M. Campillo $^{2}$ C. Hadziioannou $^{2}$ N.M. Shapiro, $^{1}$ E. Larose $^{2}$
$^{1}$ (Sismologie, Institut de Physique du Globe de Paris & CNRS, Paris, France)
$^{2}$ (Laboratoire de Geophysique Interne et Tectonophysique & CNRS, Grenoble, France)

Introduction

Figure 2.73: 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.
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Since their discovery in 2002 (Obara, 2002), deep ($\sim$ 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).

Continuous Borehole Seismic Data

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.

Data Sets

Seismic Velocity Change

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 measurement accuracy.

Tremor Activity

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

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.

Results

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 ($\sim$ 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.

Acknowledgements

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).

References

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