The HRSN at Parkfield (Chapter 6) 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 M 4.7-5.0 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.
Over the past year, data and previously derived theoretical and empirical relationships from Parkfield have served as a basis for investigations on a variety of topics by BSL researchers and collaborators at the Department of Terrestrial Magnetism (Carnegie) and Lawrence Berkeley National Laboratory (LBNL).
Johnson and Nadeau (2002) developed an empirically based earthquake asperity model that explains previously determined earthquake scaling relationships from characteristically repeating earthquake sequences (CS) at Parkfield. Their model suggests fault strength to be highly heterogeneous.
Korneev et al. (2002) have used Fault Zone Guided Waves (FZGW) from HRSN recorded microearthquakes to image the structure of the innermost fault zone using FZGW attenuation.
Niu et al. (2002) have used Parkfield CS events as highly repeating illumination sources to reveal the stress-induced migration of scatterers of seismic energy. By examining the systematics of temporal changes in the coda arrivals of CS events and stress changes inferred from the evolution of deformation at Parkfield, they infer that stress-induced redistribution of fluids along fractures in or adjacent to the fault are taking place.
Using the scaling (Tr-Mo) of CS recurrence intervals (Tr) with seismic moments (Mo) (Nadeau and Johnson, 1998) and the its calibration with intermediate scale geodetic measurements at Parkfield, Schmidt et al. (2002) and Nadeau and McEvilly (2002) are mapping areas of deep fault slip and slip rates along the East Bay Area Hayward Fault and along the central creeping section of the San Andreas Fault (SAF) in California.
In ongoing investigations of the Tr-Mo scaling, BSL researchers have extended the range in Mo over which scaling occurs to over 15 orders of magnitude in Mo (Figure 15.1). This relationship serves as a basis for empirical determinations of earthquake source parameters of area (A), seismic slip (d), and stress drop (Nadeau and Johnson, 1998). These determinations involve relatively few model assumptions and are independent of existing models relating the shape and spectra of seismic waveforms to the mechanics of earthquake sources. Results of the CS based approach have implications that are significantly different from the standard models derived from the waveform base methods.
Recently, several independent research groups external to Berkeley have attempted to explain the discrepancy between CS based and standard model scaling of source parameters by providing alternative interpretations to that of Nadeau and Johnson for the observed Tr-Mo (e.g. Anooshehpoor and Brune, 2001; Sammis and Rice, 2001; Beeler et al., 2001; Chen and Sammis, 2002).
These interpretations involve mechanisms in which loading of the CS patches is magnitude dependent. For example, Beeler et al. have proposed a creep-slip mechanism to explain the Tr-Mo discrepancy at small Mo ( to dyne-cm); however, their model does not appear to provide a satisfactory explanation for Tr-Mo at intermediate Mo earthquakes (approx. magnitudes 1.5 to 5) nor for very small events (magnitudes 0) (Figure 15.1).
Interpretations based on slip shielding arguments such as those of Sammis and Rice and Anooshehpoor and Brune require CS to be located adjacent to or embedded within large locked patches. However, locations of CS appear to be widely distributed and usually well away from large locked zones (e.g. in the central creeping section of the SAF). Furthermore their relationships in the Tr-Mo scaling generally remain consistent with that expected from local geodetic loading rates regardless of proximity to large locked zones.
A number of other recent studies have recognized the significance of the CS systematics and scaling found at Parkfield and have explored their implications from a variety of perspectives. For example with respect to high-precision relative relocations (Waldhauser and Ellsworth, 2002; Rubin, 2002; Schaff et al., 2002), and with respect to earthquake physics, seismicity variations and deformation rates along strike-slip and subducting faults in California and Japan (Matsuzawa et al., 2002; Seno, 2002).
Another principal focus of BSL's recent research at Parkfield has been the detailed analysis and monitoring of the characteristics of microseismicity with in the drilling and penetration zone of the SAFOD component of the NSF initiative EarthScope. Of particular interest is the evolution of fault zone deformation and detailed seismic structure immediately surrounding the repeating SAFOD M2 target zone, and the recurrence behavior and size of the two potential M2 targets (separated by 70 m).
Using a 3-dimensional double-difference code developed by Alberto Michelini in Italy and a preexisting 3-D cubic splines interpolated velocity model developed from HRSN data (Michelini and McEvilly, 1991), we have been able to resolve the relative seismic structure in the target zone in great detail (Nadeau et al., 2001) (Figure 15.2). The relocations indicate that the subhorizontally drilled portion of the SAFOD hole may need to penetrate a seismically active (and the existence of CS imply actively slipping) strand some 300m to the SW before entering the M2 target region.
CS exist on both strands and slip rates on the strands inferred from the Tr of the CS in the strands indicate that both are slipping at about 10 to 15 mm/yr. This suggests a distinct possibility of shearing of the deep borehole casing on the SW strand which needs to be taken into account if long term monitoring of the local target is to take place.
The Tr-Mo relationship and ongoing monitoring of the Tr's of the CS local to the M2 target(s) can also be used to help estimate the expected occurrence time of the next M2 repeat (Figure 15.3). This information will be helpful in the planning of SAF penetration and monitoring efforts, as well as for testing of earthquake recurrence forecast models, and for evaluating the conditions surrounding the M2 target(s) leading up to failure.
The SAFOD experiment will also measure deformation along the deep hole which will provide a direct calibration of slip rates at depth with CS Tr's near the target zone. This will provide ground truth for interpretation of the Tr-Mo relationship, and is crucial for establishing an accurate model of CS recurrence behavior, for interpreting Tr-Mo based source parameter scaling relationships, for the extrapolation of fault and earthquake physics based on the Tr-Mo scaling, and for application of the CS deep fault slip rate method to slipping faults generally.
In regards to the drilling operation, the calibration will also provide a better picture of the M2 target size by providing a more accurate estimate of the partitioning of Mo from the expected M2 event (Mo=dA) into its seismic slip (d) and rupture area (A) dimensions. Currently, estimates of the dimensions of the M2 target(s) vary significantly depending on the expected stress drop of the M2 events on the patch. Figure 15.4 shows the 2 potential M2 targets and estimated target sizes based on 30 bar stress drop (from a standard constant stress drop model) and 2700 bar stress drop suggested by the Tr-Mo scaling (Nadeau and Johnson, 1998).
More information about the SAFOD project is available on the Web at: http://www.icdp-online.de/html/sites/sanandreas/objectives/objectives.html and http://www.icdp-online.de/html/sites/sanandreas/objectives/proposal.html
We appreciate support for this project by the USGS NEHRP through grant numbers 01HQGR0057 and 02HQGR0067 and by the NSF through award number 9814605.
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Nadeau, R.M. and T.V. McEvilly, Periodic Pulsing of the Tectonic Plate Boundary in Central California, in prep., 2002.
Niu, F., P.G. Silver, R.M. Nadeau and T.V. McEvilly, Stress-Induced Migration of Seismic Scatterers Associated with the 1993 Parkfield Aseismic Transient, Science, submitted, 2002.
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