Microearthquake Imaging of the Parkfield Asperity
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Abstract:
Microearthquake data from a downhole seismometer network on the San Andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the Parkfield characteristic earthquakes. The source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most. Furthermore, the magnitude-frequency statistics suggest that at local magnitude 0.6, the cumulative number of small events begins to fall off the usual Gutenberg-Richter (b = -1) relation, in which the number of events increases exponentially with decreasing magnitude. The downhole seismometer data establish a baseline from which the evolution of the earthquake process at Parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical Gutenberg-Richter relation may be operating for the smallest of Parkfield microearthquakes.Keywords:
Microearthquake
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Asperity (geotechnical engineering)
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abstract An array of moveable seismographic trailers was deployed at three sites along the northern section of the “Big Bend” in the San Andreas fault in southern California. The three sites monitored were the Carrizo Plains, Frazier Park, and Lake Hughes areas. Effective observation times at each site ranged from 38 to 69 days. The microearthquake activity rates observed were 0.3 events/day, 3.0 events/day, and 1.9 events/day, respectively, based on the number of located events plus the number of unlocated events with S-P ≦ 3.0 sec. The majority of the activity does not appear to be directly associated with the San Andreas fault. A comparison of the activity rates observed in this study with the results of a survey conducted in the same areas by Brune and Allen (1967), indicates more than an order of magnitude increase in activity rate in the Lake Hughes area and nearly the same levels of activity at the Carrizo Plains and Frazier Park sites.
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Microearthquake seismograms from the borehole seismic network on the San Andreas fault near Parkfield, California, provide three lines of evidence that first P arrivals are "head" waves refracted along the cross-fault material contrast. First, the travel time difference between these arrivals and secondary phases identified as direct P waves scales linearly with the source-receiver distance. Second, these arrivals have the emergent wave character associated in theory and practice with refracted head waves instead of the sharp first breaks associated with direct P arrivals. Third, the first motion polarities of the emergent arrivals are reversed from those of the direct P waves as predicted by the theory of fault zone head waves for slip on the San Andreas fault. The presence of fault zone head waves in local seismic network data may help account for scatter in earthquake locations and source mechanisms. The fault zone head waves indicate that the velocity contrast across the San Andreas fault near Parkfield is approximately 4 percent. Further studies of these waves may provide a way of assessing changes in the physical state of the fault system.
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We hypothesize that highly stressed asperities may be defined by mapping anomalously low b values. Along the San Andreas fault near Parkfield the asperity under Middle Mountain, with its b =0.46, can be distinguished from all other parts of the fault surface. Likewise, along the Calaveras fault the northern asperity of the Morgan Hill 1984 ( M 6.2) rupture can be identified by its low b of 0.5 as a high stress patch along the fault. We add further evidence to the observations that the b value of the frequency‐magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the Parkfield segment of the San Andreas and along the Calaveras fault. In both of these areas, b values above and below 5 km depth are ∼1.2 and 0.8, respectively. We propose that probabilistic recurrence times Tr , based on the seismicity parameters a and b , should be calculated from their values within asperities only, instead of from the values of the entire rupture area of the maximum expected earthquake. The strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone, which slips along passively when an asperity breaks. Therefore no information on Tr is contained in the passive fault segments, only in the asperities. At Parkfield the probabilistic estimates of Tr derived from the data in the whole rupture and in the asperity only are 72 (−18/+24) and 23 (−12/+18) years, respectively, compared to the historically observed repeat time of 22 years. At Morgan Hill the Tr estimates are 122 (−46/+76) and 78 (−47/+110) years, respectively, compared to the observed repeat time of 72 years.
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The Parkfield segment of the San Andreas fault shows seismic quiescence for M > 2.0 earthquakes since early to mid-1986. The rate decreases by 45 to 70 per cent depending on the magnitude band. We interpret this change as real because it is concentrated in the larger magnitudes and cannot be explained by a reasonable magnitude shift. The rate decrease is present in the Parkfield segment of the fault between 35.6 and 36.1°N, and no other rate change as significant has been observed during the time when high quality data were available (since 1975). We interpret this observation as a seismic quiescence precursor to the next Parkfield mainshock. Although information on the expected duration of precursory quiescence is weak, we estimate that the Parkfield earthquake should occur in the interval 1990 February to 1992 February. Precursory seismic quiescence, which lasted approximately 1.5 yr, was found in, and possibly around, the source volume of an earthquake with M = 3.6 which occurred on 1986 August 29 in the Parkfield area. This observation demonstrates that the quiescence hypothesis is applicable to the Parkfield segment of the San Andreas fault. The false alarm rate was estimated in a detailed analysis as a function of alarm level for the fault segments surrounding the M = 5.0 Stone Canyon (1982 August) and M = 3.6 Parkfield (1986 August) mainshocks. For these two events, the duration and statistical significance of false alarms as a function of space, time and magnitude band was determined. We propose that in this tectonic environment, and with the data characteristics of these earthquake catalogues, attempts to predict mainshocks based on the quiescence hypothesis may be successful and generate one to two false alarms covering about 10 per cent of the time-space dimensions of the catalogue.
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Characteristics of microearthquake occurrence along the locked‐to‐creeping transition of the San Andreas fault at Parkfield in central California are reviewed for evidence that fault zone fluids play a critical role in slip dynamics there. Previous studies at Parkfield have defined a low‐velocity, anisotropic, attenuating fault zone and a high V p / V s ratio in the nucleation zone of the repeating M 6 earthquakes. Most of the ongoing seismicity is organized into a temporally‐evolving checkerboard pattern of alternating high and low seismic slip regions divided on the fault zone at 3–6 km spacings. Much of the seismicity is further confined to a few hundred small (20–30 m radius) cells of densely clustered microearthquakes that exhibit periodic recurrence. Space‐time development of small (tens to hundreds of meters), transient (minutes to days) earthquake sequences reveals diffusivelike outward spreading along the fault zone. There is some evidence for anomalous source mechanisms in sequence‐initiating events, possibly indicative of hydraulic fracturing. The data are consistent with a model in which some microearthquake dusters and confined sequences occur by the cyclic pressurization of fluid within localized patches of the fault zone. The consequent modulation of the effective normal stress leads to fluid‐driven slip manifested both as the highly periodic earthquake clusters and as localized earthquake sequences observed at Parkfield.
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Since 1985, a focused earthquake prediction experiment has been in progress along the San Andreas fault near the town of Parkfield in central California. Parkfield has experienced six moderate earthquakes since 1857 at average intervals of 22 years, the most recent a magnitude 6 event in 1966. The probability of another moderate earthquake soon appears high, but studies assigning it a 95% chance of occurring before 1993 now appear to have been over‐simplified. The identification of a Parkfield fault “segment” was initially based on geometric features in the surface trace of the San Andreas fault, but more recent microearthquake studies have demonstrated that those features do not extend to seismogenic depths. On the other hand, geodetic measurements are consistent with the existence of a “locked” patch on the fault beneath Parkfield that has presently accumulated a slip deficit equal to the slip in the 1966 earthquake. A magnitude 4.7 earthquake in October 1992 brought the Parkfield experiment to its highest level of alert, with a 72‐hour public warning that there was a 37% chance of a magnitude 6 event. However, this warning proved to be a false alarm. Most data collected at Parkfield indicate that strain is accumulating at a constant rate on this part of the San Andreas fault, but some interesting departures from this behavior have been recorded. Here we outline the scientific arguments bearing on when the next Parkfield earthquake is likely to occur and summarize geophysical observations to date.
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