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Parkfield Research

T. V. McEvilly, R. W. Clymer, R. Nadeau, A. Kirkpatrick, L. Johnson, V. Korneev, E. Karageorgi

Overview

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 ( -1 < M < 2) with high-resolution locations and source mechanisms.

Analyses of the 13+ 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-half 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 creeping fault segments. 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 are expanding the network to improve its view of the drilling target zone on the fault surface.

Moment - Magnitude Relationship

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 if 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 M>2 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 a methodology for estimating seismic moment, calibrating the HRSN moments with the regional NCSN preferred magnitude catalog, producing a seamless relationship from M<0 to M>6. Data from this calibration set are being used in current research requiring accurately merged catalogs (e.g., Burgmann et al., 2000; Wiemer and Wyss, 2000).

Earthquake Physics and Fossil Earthquakes

Since last year several manuscripts in the community have surfaced which attempt to explain the striking scaling relation of recurrence intervals with 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 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 small scales are extremely high, 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) or creep-slip (Beeler, 2000) 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) (Figure 13.1a) 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 (Figure 13.1b) and strong, scale-dependent, heterogeneity of fault strength. (Wenk et al., 2000; Nadeau et al., 2000).


  
Figure 13.1: Pseudotachylite (PT) structures on mesoscopic (a) and microscopic (b) scales. (a) Hand sample of an isolated pseudotachylite vein in a highly cataclastic gneiss. Note its planar geometry, manifest by the 3-dimensional cut of the sample. This is typical of PTs found in the study area. This vein corresponds to an earthquake of -1 Mw, penny for scale. (b) Microstructures of microlites in plane polarized light. Skeletal crystals of plagioclase nucleate preferentially on fragments. They are surrounded by a fine groundmass, mainly consisting of biotite. Large fragments are plagioclase and quartz. These structures are indicative of very rapid energy release and nearly instantaneous melting, providing additional evidence for the episodic (earthquake) origins of these PTs. (c) Source area (A) verses seismic moment for repeating earthquakes and PTs. Data points of 567 repeating earthquake sequences (over 2700 earthquakes) -1 < Mw < 6 from the central San Andreas fault system (including parts of the Hayward and Calaveras faults) (solid circles) and 290 pseudotachylite veins from the Peninsular Ranges (open squares) are shown. The fit to the combined data set is shown as a thick solid line. Homogeneous, circular fault model representations are given by the 1 and 100 bar stress drop lines (dashed). Typical earthquake stress drops appear to overestimate the source areas of these small events, by up to 3 orders of magnitude for the smaller events.
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Cluster Signature of Active Faults

As part of a more ambitious exploration of the extent to which clustering of repeating and highly similar microearthquakes characterizes active fault zones, we have devised a Cluster Signature (CS) measure of such behavior for any segment of a seismogenic fault. This characteristic helps provide the underlying recurrence statistic with which fault slip rate can be estimated, applying the methodology developed at Parkfield. We have embarked upon a Calibration/Extrapolation/Characterization of slip rate along the San Andreas Fault from Parkfield to San Francisco over the past 16 years, using the NCSN event catalog. Initial results are fascinating, revealing portions of the fault extending over many tens of kilometers that exhibit coherent pulsing in slip rate. In addition, the pulses appear closely related to the occurrence of the Loma Prieta earthquake in 1989, with quiescence prior to, and pulsing onset subsequent to Loma Prieta. An expected exponential decay of slip rate is seen in the LP aftershock zone following the main event. Figure 13.2 illustrates the coherency in slip rate variations after LP on 2 segments of the SAF SE of the LP rupture and separated from each other by over 75 km. Also notable is the observed quiescence prior to LP on its adjacent SJB segment. This research is testing the utility of the repeating earthquake statistic and the waveform similarity characterizations in defining fault properties and fault segment boundaries based more quantitatively on what appears to be a robust measure of fault slip rate.


  
Figure 13.2: Slip rate histories for two segments of the San Andreas fault obtained by applying to the NCSN catalog the method devised and calibrated with the high-resolution HRSN data at Parkfield. (Top) Occurrence timelines for 42 repeating sequences on a 12 km section of the fault at San Juan Bautista. (2nd Panel) Cumulative slip for the fault segment derived from the cumulative slip of all repeated events occurring on the segment and scaled by the number of sequences. (3rd Panel) Slip rate variations from the differences in cumulative slip averaged over a 1.2 year time window. Note the pulse-like slip rate variation at   3 year intervals and the related clustering of event occurrences in the top panel. The vertical line is the time of the Loma Prieta earthquake, centered about 50 km northwest of the SJB segment. The three arrows are times of the slow silent earthquakes at SJB reported by Linde et al. (1996). (Bottom Panel) Results of the same analysis applied to a segment 75 km to the southeast of SJB. Three-year pulsing is again visible, perhaps advanced slightly from the SJB pattern, and the depressed slip rate seen at SJB prior to the LP earthquake is not present on this more distant segment.
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Fault-zone Wave Propagation

Fault-zone 'Guided' Waves

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 model for the fault zone, and 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). To investigate FZGW more quantitatively, we have begun to characterize the distribution throughout the fault zone of source-receiver paths that produce strong FZGW signals from earthquakes. The goal of this research is to be able to first determine the patterns of generation and propagation of FZGW, to characterize the wavefield in terms of velocity 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 illumination sources for imaging temporal changes in FZGW propagation in search of evidence relating to processes of fault healing or large event nucleation. Figure 13.3 shows the type of data set that can be constructed from the HRSN waveforms. In the Parkfield archive there are thousands of microearthquakes available with which to build a receiver gather of a pattern of sources for any component of motion throughout the ten stations. Stacking of the traces is 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 appears to be the plunging NW edge of the M6 asperity - the green region under station MMN (bottom panel Figure 13.3).


  
Figure 13.3: (Top) A receiver gather for station MMN, horizontal component, of 544 earthquake sources at depths 3.3 to 3.8 km (see swath shown in bottom panel) along a 25 km stretch of the SAF at Parkfield. Traces are stacked into 100 m bins, a legitimate procedure due to the uniform focal mechanisms. Note the spatial variation in the relative generation of the fault-zone guided wave (FZGW) along strike, and the corresponding drop in S phase amplitudes. (Bottom) Ratio of the S-coda to S-phase energy as recorded at station MMN for earthquakes through-out the fault zone. The position of the dots represents the earthquakes hypocenters projected along the fault surface. The dark grey dots indicates earthquakes whose coda energy is less than 20 S-phase energy as recorded at MMN, Medium grey indicates a ratio of greater than 35 the color change with the onset of the strong FZGW illustrates the significance of the color change. This type of display provides an easy assessment of travel paths to MMN from events through-out the fault zone which generate strong guided wave energy. A similar plot for earthquakes recorded at station EAD (not shown) provides a reverse profile image of wave propagation through the zone which shows a converse relationship (light grey to the NW and dark grey to the SE). This argues for a source of the FZGW which is located, in depth, along the NW edge of the M6 Parkfield asperity at the transition between creeping and locked behavior (roughly in in the light grey region under MMN).
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Modeling Changing Travel-times

The Vibroseis monitoring investigation reported significant travel-time changes in the coda of S for paths crossing the fault zone southeast from the epicenter of the 1966 M6 earthquake. Progressively decreasing travel times in the anomalous region reached 50 msec or more by the end of the study. Changes in frequency content and polarization were also found and those effects, too, could be localized to the zone of common nucleation and rupture onset for the previous M6 earthquakes, and, possibly, the region of slip initiation for the great earthquake of 1857. The temporal pattern in these variations appears to be synchronous with changes in deformation and seismicity measured independently (Nadeau and McEvilly, 1999). Because similar variations are not seen in the waveforms recorded from microearthquakes in the same part of the fault, Karageorgi et al. (1997) conclude that changing fluid conditions in the uppermost section of the fault zone in response to deeper, tectonic stress perturbations are the likely cause of the temporal variations. Korneev and McEvilly (2000) modeled the variations numerically, and successfully explain the observations as interaction (reflection and transmission) of the shallow wavefield with a 200-meter-wide low-velocity fault zone in which the velocity increases by 6 we hypothesize, to hydrological changes accompanying a significant pulse in slip rate and seismicity that was evident in independent data. We will pursue the modeling of wave propagation influenced by the fault zone in a more complex and 3-D medium.

Improved 3-D Velocity Model

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 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. We have a suite of new models using various permutations of grid spacings and data set combinations. The gross features of these models are similar with each other and with the 1991 model, however, the new models include a larger geographic scope and more earthquakes and additional auxilliary 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 SW of the fault trace and the USGS locations. The salient features in velocity models and the Vp/Vs ratio are not significantly different from the 1991 inversion results. This study is being prepared for publication.

References

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.

Burgmann, 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.

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?, submitted to Seism. Res. Lett., 2000.

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.

Sammis, C.G. and J.R. Rice, Repeating Earthquakes as Low-Stress-Drop Events at a Border Between Locked and Creeping Fault Patches, submitted to Seism. Res. Lett., 2000.

Wenk, H.-R. L.R. Johnson, and L. Ratschbacher, Pseudotachylites in the Eastern Peninsular Ranges of California, Tectonophysics, 321, 253-277, 2000.

Wiemer, S. and M. Wyss, Combined Mapping of the Earthquake Size Distribution and Stress Tensor Orientation Offers New Insight into Properties of Faults, submitted to Science, 2000.


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Next: Source Rupture of the Up: Ongoing Research Projects Previous: Hayward Fault & Bridges

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