ALBERT

Description: [1]  Along creeping sections of the San Andreas and other faults, small asperities in the fault zone load and fail in characteristic repeating earthquake sequences which can be used as subsurface creepmeters. Here, we use these virtual creepmeters to examine and compare deep slip rates on both the northwestern end of the creeping section of the San Andreas Fault near San Juan Bautista and on the nearby sub-parallel Sargent Fault. While creep on the San Andreas increased dramatically due to staticstress changes in response to the 1989 Loma Prieta earthquake, the Sargent showed very little immediate response, consistent with Loma Prieta finite slip models that put this section of the fault in a region of less than three bar Coulomb stress increase. After about ten years, the San Andreas creep rate fell back close to the interseismic rate and variations in creep became coherent in time with the Sargent, indicating a mutual driving force in the system.

Description: We investigate temporal variations in the polarization of surface waves determined using ambient seismic noise cross-correlations between station pairs at the time of the Mw 6.0 Parkfield earthquake of September 28, 2004. We use data recorded by the High Resolution Seismic Network's 3-component seismometers located along the San Andreas Fault. Our results show strong variations in azimuthal surface wave polarizations, Ψ, for the paths containing station VARB, one of the closest stations to the San Andreas Fault, synchronous with the Parkfield earthquake. Concerning the other station pair, only smooth temporal variations of Ψ are observed. Two principal contributions to these changes in Ψ are identified and separated. They are: (1) slow and weak variations due to seasonal changes in the incident direction of seismic noise; and (2) strong and rapid rotations synchronous with the Parkfield earthquake for paths containing station VARB. Strong shifts in Ψ are interpreted in terms of changes in crack-induced anisotropy due to the co-seismic rotation of the stress field. Because these changes are only observed on paths containing station VARB, the anisotropic layer responsible for the changes is most likely localized around VARB in the shallow crust. These results suggest that the polarization of surface waves may be very sensitive to changes in the orientations of distributed cracks and that implementation of our technique on a routine basis may prove useful for monitoring stress changes deep within seismogenic zones.

Description: The Hayward and Calaveras Faults, two strike-slip faults of the San Andreas System located in the East San Francisco Bay Area, are commonly considered independent structures for seismic hazard assessment. We use InSAR to show that surface creep on the Hayward Fault continues 15 km farther south than previously known, revealing new potential for rupture and damage south of Fremont. The extended trace of the Hayward Fault, also illuminated by shallow repeating micro-earthquakes, documents a surface connection with the Calaveras Fault. At depths greater than 3-5 km, repeating micro-earthquakes located 10 km north of the surface connection highlight the 3-D wedge geometry of the junction. Our new model of the Hayward and Calaveras Faults argues that they should be treated as a single system with potential for earthquake ruptures generating events with magnitudes greater than 7, posing a higher seismic hazard to the East San Francisco Bay Area than previously considered.

Description: The Central segment of San Andreas Fault (CSAF) is characterized by a nearly continuous right-lateral aseismic slip. However, observations of the creep rate obtained using small Characteristically Repeating Earthquakes (CREs) show pulses of creep along the CSAF, which may indicate spatially and temporally variable seismic hazard along the CSAF. Therefore, the goal of this study is to obtain a high resolution time-dependent model of creep along the CSAF to examine this hypothesis. To this end, we apply a time-dependent creep modeling approach, which combines InSAR surface deformation time series and observations of fault creep obtained from CREs. The SAR dataset includes C-band scenes acquired by the ERS-2 and Envisat satellites between 2003 and 2011. The resulting creep rate distribution implies a peak rate up to 32 mm/yr along the central part of the CSAF. Afterslip due to the 2004 Parkfield earthquake on the southeastern segment of the CSAF is also manifest in the model and there is clear evidence of creep pulsing along strike and depth of the CSAF. Estimated annual rate of slip deficit accumulation is equivalent to a magnitude 5.6-5.7 earthquake. Taking advantage of the time-dependence of our model, we also refine the scaling relationship, which associates the released seismic moment due to a CRE event with the amount of creep on the fault, surrounding the CRE patches. This study provides the first kinematic model of creep pulsing, constrained using geodetic and seismic data, which can enhance time-dependent seismic hazard maps and improve earthquake operational forecast models.

Description: 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.

Description: 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.

Description: 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.

Description: 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.