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Fault Slip Kinematics using Characteristic Microearthquakes

R.M. Nadeau, D.C. Templeton, T.V. McEvilly, and R. Bürgmann

Introduction

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. Synchronous with these changes have been changes in the repeat times of characteristic microearthquakes sequences , $T_r$, which have been related to slip loading rate variations at depth and which have been used to provide a 4-D image of the evolution of slip rates along the 25 km segment of the SAF spanning the presumed nucleation region of the next Parkfield M6 earthquake (Nadeau and McEvilly, 1999).

Our ongoing research is showing that this technique is applicable to other faults in Central California, and its more widespread application is revealing unexpected attributes of the kinematics of deformation along these faults and on scales ranging from a few meters to 100's of km.

$T_r$, is also a particularly well constrained observable that can be theoretically related to a variety of physical processes linked to earthquakes, fault structure, and fault deformation (e.g. overall fault strength, stress drops, fault heterogeneity, fault deformation at depth, slip loading of locked segments, fault healing rates, 'in-situ' friction, and event-event triggering). However to provide accurate measurements of these processes in an absolute sense, accurate calibrations between $T_r$ and various constitutive properties within the fault core (e.g. composition, texture, temperature, stress, slip rate and fluid conditions) are needed. Currently such calibrations must be inferred from surface observations, laboratory experiments and seismic behavior. Ideally, however, they would come from 'in situ' measurement of the fault zone-an expensive proposition.

The National Science Foundation, through its EarthScope initiative (SAFOD component), has recently proposed careful scientific drilling into the seismogenic zone of the active San Andreas (SA) fault at Parkfield. Scientific drilling would sample fault zone properties directly and, as proposed, at the site of a repeating earthquake sequence.

This effort promises to significantly enhance our understanding of the physics of of this portion of the SAF and to provide the calibration necessary for relating $T_r$ to the constitutive properties and processes responsible for earthquakes and fault deformation at Parkfield.

At the same time EarthScope, through its PBO and InSAR components, has proposed to increase the detail and extent of deformation measurements along a major plate boundary and throughout the country which should greatly improve estimates of fault slip from surface and space based observations.

This information, together with the ongoing discovery of widely distributed repeating earthquake sequences in central CA make it feasible to extrapolate (infer) the physical conditions- relative to those found at Parkfield-on 100's of km of fault in central California and elsewhere.

Small repeating earthquakes continue to be discovered in other regions of the world and on other fault types (e.g. convergent boundaries off the coast of Japan (Matsuzawa et al., 1999) and on normal faults in Greece (Dimitrief and McCloskey, 2000)). This suggests the possibility of inferring fault conditions elsewhere.

In the U.S., it is conceivable that a collateral benefit of the USARRAY component of EarthScope will be the discovery of a significant number of new repeating earthquake sequences in regions where low magnitude seismicity has not been studied in detail. This has the potential to greatly expand the geographic coverage where inferential methods based on $T_r$ can be applied.

The following subsections summarize the current state of repeating earthquake studies at Berkeley, and give examples of how such studies are already changing our understanding of the kinematics of fault deformation.

East-Bay Faults

The method of determining slip rates from earthquake recurrence has been developed in our research program using data from a borehole network along the active San Andreas fault zone at Parkfield, CA. Studies at very high resolution of microearthquakes occurring there since 1987 reveal a systematic organization in space and time, dominated by clustering of virtually identical, regularly recurring microearthquakes ('characteristic events') on patches meters to 10's of meters in dimension and located within the fault core. (Nadeau and McEvilly, 1997; 1999). At Parkfield, nearly half of the 5000+ events in the 1987-1998 catalog exhibit this trait and the sites where the repeating activity occurs are distributed throughout the slipping fault surface.

Nadeau and McEvilly (1999) showed that it is possible under reasonable assumptions, to infer the spatial distribution of variations in slip-rate on the fault surface from changes in the recurrence intervals of the characteristic event sequences. They found, for example, that during a 26-months period of greatly increased activity (M4.2, 4.6, 4.7 and 5.0 events and their aftershocks) accompanied increases exceeding 50 $\%$ in previously stable recurrence intervals (and equivalently, inferred slip rates) occurred. Langbein et al. (1999) also argue for a similar slip rate increase at Parkfield based on evidence from surface deformation measurements.

Another example of application of the repeating earthquake method having comparably good agreement with surface deformation can be found in Bürgmann et al. (2000). In this application, earthquake waveforms recorded by the surface regional NCSN along the northern segment of the Hayward fault were used.

A long enough time base of waveform data recorded on the deep borehole Hayward Fault Network has also now become available which should allow practical application of the repeating quake method to repeating sequences of much lower magnitude and consequent greater time resolution (see Hayward Fault research section, this report).

This year we initiated a computationally intensive search for characteristic sequences using the NCSN catalog on the southern Hayward, Calaveras, and Mission crossover faults (HF,CF, MF, respectively) where seismicity resembles the Parkfield clustering, but at a lower rate.

This stage of processing characterizes the waveform similarity of all the event pairs separated by 7.5 km or less. Even at reduced detection completeness (M 1.3) of the NCSN, we see large numbers of highly similar and repeating events (coherency $>$ 0.95) distributed widely along all three faults, indicating the presence of substantial repeating earthquake activity (Figure 15.1).

Using NCSN surface data we estimate the fractions of potential repeaters to be about 10$\%$ 15$\%$ and 25$\%$, for the northern HF, southern HF/MF and CF segments respectively. The lowest fractions are explainable in part by lower slip rates and higher magnitude thresholds, since under these conditions recurrence intervals may be longer than observation times. At Parkfield, where the slip rate is much higher and using comparable NCSN data, the fraction of potential repeaters is about 45$\%$.

Figure: 15.1 Example repeating sequences along the Hayward, Calaveras and Mission faults. Characteristic repeaters are numerous, widespread and distributed in time for all the regions examined so far. Variations in the rates of repetition are consistent with known variations in slip rate both along strike and through time. For example, sequences NW of Morgan Hill (MH) typically show a steadily decreasing repeat rate with time since the 1984 MH magnitude 6. We also plan on examining changes in repeat rates and micro-quake inferred slip as they relate to the occurrence of Loma Prieta (LP) and other significant near by events.
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In our prototype analysis on the northern HF, we showed that sufficient information was available to resolve spatially varying features of slip using the NCSN but that monitoring of temporal variations was not practical with the limited resolution of the surface data set on such a slowly moving fault. To the South and East, on the faster moving MF and CF, we are finding sufficient rates of quake repetition to allow us to use NCSN data for monitoring transients as is currently being achieved on the faster slipping SAF to the West (Nadeau and McEvilly, 2000; and following section, this report).

To date earthquakes either identified as members of repeating earthquake sequences (about 200) or quakes having similar waveforms with cross-correlation coefficients greater than 0.98 at multiple stations (candidate repeaters, over 1800 quakes) have been identified. Future efforts will focus on completing the search for similar and repeating earthquakes on all of the HF, MF and CF seismicity, confirming their characteristic nature, grouping the repeaters into sequences, and inverting the resulting recurrence interval statistics into estimates of slip rates at depth and through time along these faults. Subsequent efforts are planned to determine the kinematics of slip through joint modeling of surface GPS, creep and space based data so that geologic questions regarding the details of slip in the complex of East Bay faults (particularly the Mission cross over region) can be investigated at much higher resolution.

SAF: Parkfield to Loma Prieta

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 (Nadeau and McEvilly, 1999). We have embarked upon a Calibration/Extrapolation/Characterization of slip rate along the San Andreas Fault from South of Parkfield to the Southern extent of rupture of the 1989 Loma Prieta (LP) earthquake 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,

In Figure 15.2 relative quiescence on the northern end of the segment is observed prior to LP, after which slip rates increase markedly-the LP locked segment appears to have exerted control on the SAF slip rates at distances greater than 30 km from the termination of its rupture. Also of interest is the fact that LP occurred coincidentally with the initiation of an ongoing pulsing cycle as seen further to the South (60 to 100 km).

Following LP, the periodic pulsing observed to the South (between 60 and 100 km) appears to extend northward and shows some correspondence to the occurrence of 3 slow silent quakes in the SJB area. The pulsing observed after LP also appears to be superposed on an exponential decay of slip rate corresponding to aftershock decay in the zone adjacent to and following the LP main event (upper right hand time history panel).

In the middle of the examined fault stretch, quasi-periodic pulsing of slip rates with amplitude variations exceeding 50-100 $\%$ are pronounced with the north central portion repeating with about 3 year periodicity and 1.6 year periodicity (less visible in this black and white rendering) in the south central portion.

From 60 to about 20 km, a migration pulse can be inferred (when color contours are used). On this segment the SAF appears to act as a strain guide for the transference of the tectonic slip-load at a rate of about 25 km/yr. The migration pattern also appears to repeat itself as part of the more frequent (1.6 yr.) pulsing pattern.

To the South at Parkfield, pulsing is more difficult to define, although considerable slip rate change is observed, in accordance with observations of surface slip-rate change in that area (Langbein et al., 1999). Also noteworthy at Parkfield is a distinct slow down in the slip rates beginning in about 1998 indicating a transition to a phase of quiescence perhaps preparatory to occurrence of the anticipated M6 there-similar to the quiescence observed prior to LP and to the $M_w$ 5.1 event at SJB to the North.

This research is testing the utility of the repeating earthquake statistic and the waveform similarity characterizations in defining fault kinematic processes and fault segment boundaries based on the $T_r$ of characteristic repeating sequences-what appears to be a robust quantitative measure of fault slip rate at depth.

Figure: 15.2 Quasi-periodic pulsing of the the central San Andreas Fault (SAF). An 175 km stretch of the creeping SAF between the SE terminus of the magnitude 7.1 Loma Prieta (LP) quake and the NW terminus of the magnitude 8 1857 Fort Tejon rupture. Left panel: Along strike map view. Shown are the cities of Parkfield, Hollister, San Juan Bautista (SJB) (open circles); hypocenters of M5+ activity since 1984 along the 175 km fault stretch (stars); the location of Slow-Silent-earthquakes near SJB (labeled SSEQ); and the sites of 515 repeating microearthquake sequences (small filled circles), comprised of 2594 repeated quakes ranging in magnitude from -0.5 up to 3.5. Middle panel: Grey contours showing slip-rate changes through time along this stretch of the SAF as percentage of the 1984 to 1999 average. The thick cross in the lower-left of the panel shows smoothing windows in time and along strike. Symbol, LP, and vertical thick line indicate the time of the Loma Prieta earthquake. Other significant events ($>$M4.5) occurring on this stretch of the SAF are shown in the contour plots as short black vertical lines of length keyed to magnitude and at their approximate times of occurrence. The data presentation is causal and smoothed with a 0.8 yr. time window so that initiation of change is accurate while actual window average values are delayed by 1/2 the length of the smoothing window (i.e. 0.4 yrs.). Right series of panels: Shown are time histories of slip rates averaged over short segments of the SAF. The thin horizontal lines in the middle panel delineate the space-time data regions used and the light grey arrows connect slip histories with their corresponding segments.
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San Andreas, Calaveras Juncture

The juncture between the San Andreas fault and the southern Calaveras fault is a highly complex area where subsurface seismicity does not always follow surface fault traces and where secondary sub-parallel faults, such as the Quien Sabe fault zone, may play an active role in accommodating local shear deformation. The goal of this investigation is to determine the slip rate distribution of these faults at the juncture using repeating earthquake defined subsurface slip rates and surface geodetic measurements. To date, we have determined initial subsurface slip rates using the previously derived empirical formula of Nadeau and McEvilly, (1999) which combine recurrence intervals and moment magnitudes of repeating earthquakes to determine slip rates at depth of individual repeating earthquake sequences. Initial sequence estimates of slip rates for the San Andreas, southern Calaveras and Quien Sabe fault zone are found to be between 12 and 24 mm/yr, 4 and 13 mm/yr, and 4 and 10 mm/yr, respectively. Figures 15.3 and 15.4 show the location of these clusters with respect to background seismicity and surface fault traces. Due to the low rate of secular slip, the magnitude threshold of the NCSN catalog, and the limited observation time (17 years), we have only identified pairs of repeating earthquakes on the southern Calaveras fault and Quien Sabe fault zone. On the San Andreas fault, which has a higher rate of secular slip, we are able to see earthquakes that have recurrence intervals of as little as 2 years. Nevertheless, the simple fact that we find repeating earthquakes on these faults is important because it indicates that they are slipping at depth even though there may be no surface expression of this slip. For example, relatively high subsurface slip rates were found on the Quien Sabe fault zone but surface slip rate estimates there are only 1 mm/yr +/- 1mm (Bryant, 1985). In the future, the distribution of subsurface slip rate estimates will be improved by using relocated seismicity and through identification of a greater number of repeating earthquake sites. The slip rate inversion will use the improved subsurface slip rates and surface deformation data for a more accurate image of fault kinematics in the area.

Figure: 15.3 Location Map. Map of study area showing surface fault traces and local seismicity from 1984 to 2001 and map view of cross sections shown on Figure 15.4.
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Figure: 15.4 Cross Sections. Locations of repeating earthquake sequences, indicated by the gray filled circles, shown with respect to background seismicity for the San Andreas, Calaveras and Quien Sabe fault Zone. Fault perpendicular cross sections are to the left and fault parallel cross sections to the right.
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References

Bryant, W.A., Faults in the southern Hollister area, San Benito counties, California, California Division of Mines and Geology Fault Evaluation Report 164, 1985.

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.

Dimitrief, A. and J. McCloskey, A search for Repeating Microearthquakes in Western Greece (abstract), EOS Trans. Am. Geophys. U. 81, F924, 2000.

Langbein, J., R. Gwyther, R. Hart, and M.T. Gladwin, Slip rate increase at Parkfield in 1993 detected by high-precision EDM and borehole tensor strainmeters, Geophys. Res. Lett., 26, 2529-2532, 1999.

Matsuzawa, T., T. Igarashi and A. Hasegawa, Characteristic Small Earthquake Sequence off Sanriku, Japan (abstract), EOS Trans. Am. Geophys. U. 80, F724, 1999.

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.


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