The HRSN network at Parkfield was installed in 1987 to provide a direct test of two hypotheses critical to our understanding of the physics of the earthquake process, with implications for earthquake hazard reduction and the possibilities for short-term earthquake prediction - major goals of the NEHRP:
1) That the earthquake nucleation process produces stress-driven perturbations in physical properties of the rocks in the incipient focal region that are measurable, and
2) That the nucleation process involves progressive and systematic failure that should be observable in the ultralow-magnitude microseismicity () with high-resolution locations and source mechanisms.
Analyses of the 14+ years of Parkfield monitoring data have revealed significant and unambiguous departures from stationarity both in the seismicity characteristics and in wave propagation details. Within the presumed M6 nucleation zone we also have found a high Vp/Vs anomaly at depth, where the three M4.7-5.0 sequences occurred in 1992-94. Synchronous changes well above noise levels have also been seen among several independent parameters, including seismicity rates, average focal depth, S-wave coda velocities, characteristic sequence recurrence intervals, fault creep and water levels in monitoring wells. We have been able to localize the S-coda travel-time changes to the shallow part of the fault zone and demonstrate with numerical modeling the likely role of fluids in the phenomenon. We can connect the changes in seismicity to slip-rate variations evident in other (strain, water level) monitored phenomena. Based on the ubiquitous clusters of repeating microearthquakes, scaling laws have been developed that can be projected to fit earthquakes up to M6, and they predict unprecedented high stress drops and melting on the fault surface for the smallest events. Exhumed fault-zone rocks provide independent evidence for such source conditions. This hypothesis is being debated vigorously in the current literature. Recurrence interval variations in the characteristic event sequences (about one-third of the microearthquake population) have been used to map fault slip rate at depth on the fault surface, and this technique appears to be applicable to other types of faults. Along the way in this exciting discovery process we have challenged the conventional 'constant stress drop' source model, affirmed characteristic earthquake occurrence and developed four- dimensional maps of fault-zone microearthquake processes at the unprecedented scale of a few meters. The significance of these findings lies in their apparent coupling and inter-relationships, from which models for fault-zone process can be fabricated and tested with time. A more fundamental contribution of the project is its production of a continuous baseline, at very high resolution, of both the microearthquake pathology and the subtle changes in wave propagation, providing to the seismological community a dynamic earthquake laboratory available nowhere else. This unique body of observations and analyses has also provided much of the impetus for Parkfield as the preferred site for deep drilling into an active seismogenic fault zone (the SAFOD project), and we have upgraded and expanded the network to improve its view of the drilling target zone on the fault surface.
Scaling laws for earthquake source properties, statistical description of earthquake occurrence, precise monitoring, forecasting, estimating fault slip rates from repeating microearthquakes, or virtually any careful analysis of the earthquake process all demand an accurate estimation of earthquake size. The difficulty is that these kinds of investigations must operate at the M 0 level to acquire sufficient data over the lifetime of a realistic study For example, at M 6 on the Parkfield San Andreas segment, definition of the recurrence statistics for the repeating rupture of the fault requires data spanning centuries, while at M 0, the same number of events can be seen in 2-3 years, and there are hundreds of such repeating sequences on the fault segment. The Parkfield event archive has become the de-facto calibration data set for extending new methods of microearthquake analysis to other segments on active faults. There are few if any complete catalogs of events with self-consistent moment-magnitude relations at the microearthquake level that can be integrated with conventional data sets at so that the above-mentioned studies can be projected into the vast resource of well-established data bases of larger events. For the Parkfield data we have compiled and carefully tested methodologies for estimating seismic moment and for calibrating the HRSN moments with the regional NCSN preferred magnitude catalog using order statistics, and have produced a seamless relationship from to .
Data from this calibration set are being used in current collaborative efforts with researchers at USC (C.G. Sammis) and the Univ. of Alaska (M. Wyss) requiring accurately merged catalogs to explore the relationship between fractal dimension, D, and b-values on locked and creeping sections of the San Andreas Fault (Figure 14.1) (Sammis et al., 2001). The data are also being used to refine the empirical scaling relationship between magnitude, repeat times and slip loading rates (Nadeau and Johnson, 1998 and Nadeau and McEvilly, 1999) and to calibrate and integrate estimates of slip rate at depth from repeating earthquakes at Parkfield with surface deformation and repeating earthquake data elsewhere on the central San Andreas, Calaveras and Hayward faults (Nadeau and McEvilly, 2000; Bürgmann et al., 2000; Section on Fault Slip Kinematics from Earthquakes (this report)).
Since last year several manuscripts in the community have surfaced which attempt to explain the striking scaling relation of repeat times from characteristic microearthquake sequences with characteristic earthquake size. The original scaling for Parkfield microearthquakes was first published in a peer reviewed journal by Nadeau and McEvilly (1997), and later developed in more detail and extended to include larger repeating earthquakes by Nadeau and Johnson (1998). These analyses present strong evidence in support of a highly heterogeneous fault zone with scale-dependent stress drops where stress drops on the small scale are either extremely high (on relatively few small locked patches) or extremely low (on the fault surrounding the patches), while on the large scale, averaged over large regions, they are small. Subsequent research publications are taking issue with this model, presenting alternative fault-zone processes such as load-shielding (Anooshehpoor and Brune, 2000; Sammis and Rice, 2000), creep-slip (Beeler, 2000) or rupture overlap (Schaff et al., 2001) to avoid the high stress drops we hypothesize for the small events.
Coincidentally however, ongoing work at Berkeley correlating the energetics of formation of fossil earthquakes (i.e. pseudotachylites) with repeating earthquakes at Parkfield and elsewhere on the central SAF system has evolved to the point where strong arguments can be made based on direct field observation of features of exhumed fault zones - the pseudotachylites - to support the stress drop scaling and strong, scale-dependent, heterogeneity of fault strength. (Wenk et al., 2000; Nadeau et al., 2001).
In addition an asperity model of an earthquake has now been developed that explains the empirical scaling relationships for the repeating earthquakes (Johnson and Nadeau, 2001). The model also leads to differential equations that can be solved to yield a complete static model of an earthquake-equations for scalar seismic moment, the radius of the asperity patch, and the radius of the displacement shadow surrounding the patch are all specified as functions of a displacement deficit that accumulates between repeated rupture. The model predicts a repeat time scaling for the earthquakes that is in agreement with the empirical results for repeating earthquakes and, assuming that the asperity patches are distributed on the fault surface in a random fractal manner, leads to a frequency-size distribution of earthquakes that agrees with the Gutenberg-Richter formula and a simple relationship between the value and fractal dimension, D. The model has also shown that the basic features of the theoretical model can be simulated with numerical calculations employing the boundary integral method, and simulation procedures are currently in development.
There has been a lot made of so-called fault-zone guided waves (FZGW). Much of it has been directed toward modeling wave propagation in relatively simple, vertical low-velocity structures in order to match discrete observations of the late, low frequency arrivals sometimes recorded near the fault trace. We are approaching this problem from a somewhat different direction, using the extensive observations of these waves in the Parkfield network, the 3-D P- and S-velocity models for the fault zone, our Vibroseis results that place an apparent strain-related zone of changing wave propagation parameters within the shallow (the upper 3 km) part of the fault zone (Karageorgi et al., 1992, 1997; Korneev et al., 2000), and a tomographic inversion of the 3-component (3-C) amplitudes of low frequency FZGW arrivals relative to 3-C S-phase arrivals. The tomographic inversion has greatly facilitated the characterization of the distribution of FZGW generation throughout the fault zone. It uses source-receiver paths from the thousands of microearthquakes at Parkfield to borehole stations of the High Resolution Seismic Network (HRSN).
The goal of this research is to first determine the patterns of generation and propagation of FZGW, to characterize the wavefield in terms of velocity attenuation and particle motion relative to the fault zone, and to model the phenomenon numerically using new 3-D guided-wave algorithms under development. We are also using the numerous and widely distributed sites of repeating earthquakes as repeating illumination sources for imaging temporal changes in FZGW propagation in search of evidence relating to processes of fault healing or large event nucleation.
By taking advantage of the thousands of microearthquakes in the Parkfield archive, we enhance the signal to noise ratio of the energy of propagation by stacking traces from events which are close neighbors. This approach has proven to be very effective because of the uniform source mechanism common to neighboring events on the fault. Our initial work is suggesting that the strong generation as well as the propagation of typical low-frequency FZGW in the coda of S is controlled by a well-defined feature within the fault zone that is high Q and appears to be the plunging NW edge of the M6 asperity. The velocity, Q and FZGW generation characteristics suggest that this zone is a region of dewatering caused by fracture closure and/or fault-normal compression.
In a project carried out by Ann Kirkpatrick, we explored the degree of improvement possible over the Michelini and McEvilly (1991) 3-D P- and S-wave velocity models estimated early in the Parkfield project, when there were only 169 events used in the inversion. Now, with another decade of data, it is possible to build a much more extensive data set. About 4800 and 2100 P and S arrival times, respectively, were selected for uniform raypath illumination throughout the study volume. The gross features of the new model are similar to the 1991 model; however, the new model includes a larger geographic scope and more earthquakes and additional auxiliary data sets. These additional data primarily help to fix the edges of the model and to extend the model in the along fault direction (both NW and SE). As a result, the event locations on the ends of the network have straightened out significantly (including the those in the vicinity of the SAFOD drilling site). The apparent dip of the events is reduced somewhat, but the hypocenters are still biased to the SW of the fault trace and the USGS locations.
To further improve resolution of the hypocenter distribution at Parkfield, we are collaborating with Alberto Michelini to extend the double-difference (DD) relative relocation method (Waldhauser et al, 1999) to the 3-D case. In a focused study of the target zone of the SAFOD deep drilling project, we found the comparable measures of resolution for the three commonly used high-resolution location methods - catalog solutions from the NCSN using a 1-D velocity model, catalog solutions from the HRSN using a 3-D velocity model, and DD relative locations based on HRSN arrival times, and HRSN waveform-correlation data - based on the scatter of events in repeating sequences, to be 1000 m, 180 m and 6 m, respectively (Figure 14.2). It appears the SAFOD exercises aimed at hypocenters will need to rely upon the cross-correlation method for accurate relative locations, and that the constellation of events within the drilling target will have to be precisely fixed using direct travel time measurements made to or from the borehole at various depths.
Another notable finding of the focused study has been the discovery of two subparallel, seismically active fault strands separated by about 300 m and oriented NW-SE-perpendicular to the horizontal drilling direction planned by SAFOD. The M2 repeating earthquake sites targeted for SAF penetration occur on the NE strand-furtherest away from the drill rig-indicating that the horizontal component of drilling will have to penetrate the active NW strand first and remain intact during drilling into the NE strand and during subsequent phases of the experiment, thus introducing many complications to the planned scenario.
Anooshehpoor, A. and J.N. Brune, Quasi-Static Slip-Rate Shielding by Locked and Creeping Zones as an Explanation for Small Repeating Earthquakes at Parkfield, submitted to Bull. Seism. Soc. Am., 2000.
Beeler, N.M., A simple stick-slip and creep-slip model for repeating earthquakes and its implication for micro-earthquakes at Parkfield, submitted to Bull. Seism. Soc. Am., 2000.
Bürgmann, R., D. Schmidt, R.M. Nadeau, M. d'Alessio, E. Fielding, D. Manaker, T.V. McEvilly, and M.H. Murray, Earthquake Potential along the Northern Hayward Fault, California, Science, 289, 1178-1182, 2000.
Johnson, L.R. and R.M. Nadeau, Asperity Model of an Earthquake-Static Problem, Bull. Seism. Soc. Am., in press 2001.
Karageorgi, E., R. Clymer and T.V. McEvilly, Seismological studies at Parkfield. II. Search for temporal variations in wave propagation using Vibroseis, Bull. Seism. Soc. Am., 82, 82, 1388-1415, 1992.
Karageorgi, E.D., T.V. McEvilly and R.W. Clymer, Seismological Studies at Parkfield IV: Variations in controlled-source waveform parameters and their correlation with seismic activity, 1987-1994, Bull. Seism. Soc. Am., 87, 39-49, 1997.
Korneev, V.A., T.V. McEvilly and E.D. Karageorgi, Seismological Studies at Parkfield VIII: Modeling the Observed Controlled-Source Waveform Changes, Bull. Seism. Soc. Am., 90, 702-708, 2000.
Michelini, A. and T.V. McEvilly, Seismological studies at Parkfield I: Simultaneous inversion for velocity structure and hypocenters using B-splines parameterization, Bull. Seism. Soc. Am., 81, 524-552, 1991.
Nadeau, R.M. and L. R. Johnson, Seismological Studies at Parkfield VI: Moment Release Rates and Estimates of Source Parameters for Small Repeating Earthquakes, Bull. Seism. Soc. Am., 88, 790-814, 1998.
Nadeau, R.M., L.R. Johnson and H.-R. Wenk, Are Pseudotachylites Fossil Earthquakes?, in revision, Seism. Res. Lett., 2001.
Nadeau, R.M. and T. V. McEvilly, Seismological Studies at Parkfield V: Characteristic microearthquake sequences as fault-zone drilling targets, Bull. Seism. Soc. Am., 87, 1463-1472, 1997.
Nadeau, R.M. and T.V. McEvilly, Fault slip rates at depth from recurrence intervals of repeating microearthquakes, Science, 285, 718-721, 1999.
Nadeau, R.M. and T.V. McEvilly, Spatial and Temporal Heterogeneity of Fault Slip from Repeating Micro-Earthquakes along the San Andreas Fault in Central California, EOS Trans., Am. Geophys. Union., 81, F919, 2000.
Sammis, C.G. and J.R. Rice, Repeating Earthquakes as Low-Stress-Drop Events at a Border Between Locked and Creeping Fault Patches, Bull. Seism. Soc. Am., 91, 532-537, 2001.
Sammis, C.G., M. Wyss, R.M. Nadeau, and S. Wiemer, Comparison Between Seismicity on Creeping and Locked Patches of the San Andreas Fault Near Parkfield, CA: Fractal Dimension and b-value, submitted to J. Geophys. Res., 2001.
Schaff, D.P., G.H.R. Bokelmann, G.C. Beroza, F. Waldhauser and W.L. Ellsworth, High Resolution Image of Calaveras Fault Seismicity, submitted to J. Geophys. Res., 2001.
Waldhauser, F., W.L. Elsworth, and A. Cole, Slip-parallel seismic lineations along the northern Hayward fault, California, Geophys. Res. Lett., 26, 3525-3528, 1999.
Wenk, H.-R., L.R. Johnson, and L. Ratschbacher, Pseudotachylites in the Eastern Peninsular Ranges of California, Tectonophysics, 321, 253-277, 2000.