ALBERT

Beschreibung: The 2004 M w  6 Parkfield, California, earthquake was preceded by a 4-year period of anomalously high seismicity adjacent to, but not on, the San Andreas fault. The rate of small events ( M w 〈3) at distances between 1.5 and 20 km from the fault plane and at depths 〉8 km, increased from 6 events per year prior to 2000 to 20 events per year between the 2000 and the 2004 earthquake. This increase in seismicity coincided with an increase in the rate of nonvolcanic tremor, which, if tremor is indicative of creep on the fault plane, suggests that creep may have driven the enhanced seismicity. Coulomb stress-transfer calculations predict the observed spatial pattern of the seismicity, and thus support a causal relation between creep at the base of the fault zone and off-fault seismicity. In particular, an observed southeast-striking lineation of enhanced seismicity is shown to be a direct consequence of a deepening boundary between the crust and mantle southeast of Parkfield, as evidenced by a deepening of the tremor and low-frequency earthquakes. Other evidence for a causal link between deep creep and off-fault seismicity is the observation that off-fault seismicity before and after the 2004 earthquake occurred in the same location. This is expected if the foreshocks are driven by an episode of deep creep and the aftershocks are driven by afterslip, both occurring on the same deep extension of the fault plane.

Beschreibung: A previous study that presented the static solution for an asperity model of an earthquake is extended to solve the dynamic problem that develops when failure occurs on the boundary of an asperity patch and then spreads over the surrounding displacement shadow region. The boundary integral equation method is coupled with basic constitutive equations for failure and friction to solve the dynamic problem, with different parameters used for the strong asperity patch and weak shadow region. No friction, displacement-weakening friction, and velocity-strengthening friction are all investigated. Depending on the type and amount of friction that is present, the dynamic solutions for slip on the fault exhibit a range of different features, including overshoot of the static solution and oscillation, rupture front velocities that may be greater than or less than the S velocity and change with position, and either total or partial release of the static moment. Common characteristics of the solutions are that failure on the asperity patch is almost independent of failure on the shadow region and that the displacement deficit on the shadow region is released by propagating slip pulses. The stress concentrations of the asperity model are sufficient to produce nonlinear elastic effects in a region extending outward from the fault to distances comparable with the dimensions of the shadow region. Beginning with the solutions for slip on the fault, waveforms are simulated for an earthquake of magnitude M (sub W) 1.44 and compared with data recorded at a distance of 8.65 km. Simulations that contain both source and propagation effects are capable of explaining most of the basic features of the observational data, including general agreement with the shape of the waveforms in the time domain, the levels and slopes of the spectra at low frequencies (less than 10 Hz) and at high frequencies (greater than 100 Hz), and some of the interference effects present in both the time and frequency domains.

Beschreibung: Repeating earthquakes (REs) are sequences of events that have nearly identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth. We investigate the occurrence of these REs along faults in central California to determine which faults exhibit creep and the spatiotemporal distribution of this creep. At the juncture of the San Andreas and southern Calaveras-Paicines faults, both faults as well as a smaller secondary fault, the Quien Sabe fault, are observed to produce REs over the observation period of March 1984 through May 2005. REs in this area reflect a heterogeneous creep distribution along the fault plane with significant variations in time. Cumulative slip over the observation period at individual sequence locations is determined to range from 5.5-58.2 cm on the San Andreas fault, from 4.8-14.1 cm on the southern Calaveras-Paicines fault, and from 4.9-24.8 cm on the Quien Sabe fault. Creep at depth appears to mimic the behaviors seen for creep on the surface in that evidence of steady slip, triggered slip, and episodic slip phenomena are also observed in the RE sequences. For comparison, we investigate the occurrence of REs west of the San Andreas fault within the southern Coast Range. Events within these RE sequences occurred only minutes to weeks apart from each other and then did not repeat again over the observation period, suggesting that REs in this area are not produced by steady aseismic creep of the surrounding fault surface.

Beschreibung: We used borehole seismic records of four repeating-earthquake clusters and two explosions to investigate the coseismic changes of the scattering wave field of the 28 September 2004 M 6 Parkfield earthquake. We found systematic changes in P-wave and S-wave coda recorded from repeating events that occurred before and immediately after the earthquake. Applying the coda wave interferometry technique allowed us to determine that the observed changes are caused by a localized change in the scattering field. We further developed a technique based on decorrelation indexes calculated from running time windows to locate migrating scatterers. Synthetic tests showed that the technique is relatively insensitive to changes in background velocity of the medium and source location and thus can be applied to records of loosely colocated clusters. We found a localized change of material property within the fault zone at approximately 3 km depth beneath the Middle Mountain area. The change is shown most clearly in the P-to-S scattering mode of the active source data and both the P-to-S and S-to-S scattering modes of the repeating earthquakes data, suggesting that the observed scattering property change is a result of charge or discharge of fluids in fractures caused by the 2004 Parkfield earthquake. The same scatterer(s) was found to respond to the 1993 aseismic stress/strain transient event, acting like an in situ stress-strain meter at seismogenic depth. Four-dimensional time-lapse imaging of the scattering wave field thus provides an effective way to monitor the subsurface stress-strain field.

Beschreibung: Stress perturbations influence earthquake recurrence and are of fundamental importance to understanding the earthquake cycle and determining earthquake hazard. The large population of repeating earthquakes on the San Andreas fault at Parkfield, California, provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes. Using 187 M -0.4 to approximately 1.7 repeating earthquake sequences from the High Resolution Seismic Network catalog, we find that the time to recurrence of repeating events subsequent to nearby M 4-5 earthquakes is shortened, suggesting triggering by major events. The triggering effect is found to be most evident within a distance of approximately 5 km, corresponding to static coseismic stress changes of 〉0.6-26.6 kPa, and decays with distance. We also find coherently reduced recurrence intervals from 1993 to 1998. This enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s.