Tectonic geomorphology of the San Andreas Fault zone from high resolution topography: An example from the Cholame segment

High resolution topographic data along fault zones are important aids in the delineation of recently active breaks. A 15 km-long portion of the south-central San Andreas Fault (SAF) along the southern Cholame segment contains well preserved tectonic landforms such as benches, troughs, scarps, and aligned ridges that indicate recurring earthquake slip. Recently acquired LiDAR topographic data along the entire southern SAF (“B4” project) have shot densities of 3–4 m^− 2. Computed from the LiDAR returns, Digital Elevation Models (DEMs) of 0.25 to 0.5 m resolution using local binning with inverse distance weighting and 0.8 m or larger search radii depict the tectonic landforms at paleoseismic sites well enough to assess them confidently. Mapping of recently active breaks using a LiDAR-only based approach compares well with aerial photographic and field based methods. The fault zone varies in width from meters to nearly 1 km and is comprised of numerous en echelon meter to kilometer-length overlapping sub parallel fault surfaces bounding differentially moving blocks that elongate parallel to the SAF. The semantic variations of what constitutes “active” and the importance of secondary traces influence the breadth and complexity of the resulting fault trace maps.     

Shear wave anisotropy in the crust around the San Andreas fault near Parkfield: spatial and temporal analysis

We systematically analysed shear wave splitting (SWS) for seismic data observed at a temporary array and two permanent networks around the San Andreas Fault (SAF) Observatory at Depth. The purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the SAF in this segment and its temporal behaviour in relation to the occurrence of the 2004 Parkfield M 6.0 earthquake. The dense coverage of the networks, the accurate locations of earthquakes and the high-resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the SAF zone. The results show that the primary fast polarization directions (PDs) in the region including the SAF zone and the northeast side of the fault are NW–SE, nearly parallel or subparallel to the SAF strike. Some measurements on the southwest side of the fault are oriented to the NNE–SSW direction, approximately parallel to the direction of local maximum horizontal compressive stress. There are also a few areas in which the observed fast PDs do not fit into this general pattern. The strong spatial variations in both the measured fast PDs and time delays reveal the extreme complexity of shear wave anisotropy in the area. The top 2–3 km of the crust appears to contribute the most to the observed time delays; however substantial anisotropy could extend to as deep as 7–8 km in the region. The average time delay in the region is about 0.06 s. We also analysed temporal patterns of SWS parameters in a nearly 4-yr period around the 2004 Parkfield main shock based on similar events. The results show that there are no appreciable precursory, coseismic, or post-seismic temporal changes of SWS in a region near the rupture of an M 6.0 earthquake, about 15 km away from its epicentre.     

Recurrence-time and frequency-slip statistics of slip events on the creeping section of the San Andreas fault in central California

An important question in seismic hazard assessments is the frequency-size and recurrence interval statistics at a point on a fault. Does a point on a fault obey the same statistics as earthquakes in a region do? This is a difficult question to answer because the number of repetitive earthquakes on a particular fault that have been observed is small. In order to overcome this difficulty we consider slip events on the creeping section of the San Andreas fault in central California. Sequences of up to 100 events are obtained from creepmeter records. We compare the statistical distribution of recurrence times with the Brownian passage-time, lognormal, and Weibull distributions and using goodness-of-fit tests find that the Weibull is the preferred distribution. We also consider the frequency–amplitude distribution of slip events. We find that the data clearly do not obey a Gutenberg–Richter distribution. Instead there is a uniform distribution of event sizes for a large fraction of events.     

Amplification of ground motion and waveform complexity in fault zones: examples from the San Andreas and Calaveras Faults

Summary. P -wave seismograms at ranges less than 10 km are synthesized by asymptotic ray theory and by summation of Gaussian beams for point sources located in a low-velocity wedge surrounding a fault. The computations are performed using models of the wedge inferred from the analysis of reflection and refraction experiments across the San Andreas and Hayward-Calaveras faults. Calculations in these models show that the 10–20Hz vertical displacements of earthquakes located at 3–10km depth are amplified by up to an order of magnitude in a 1–2km wide region centred on the fault trace compared to displacements predicted by laterally homogeneous models of the crust. This amplification is not cancelled by high attentuation in the fault zone and compensates for the reduction in amplitudes directly above the source predicted from the radiation pattern of a strike-slip earthquake. Depending on the source depth of the earthquake and the structure and velocity contrast of the wedge, multiple triplications in the travel-time curve of direct P - and S -waves will occur at stations in the fault zone. A wedge model successfully predicts the triplications observed in the P waveforms of aftershocks of the Coyote Lake earthquake recorded in the fault zone, showing that body waves from microearthquakes can be used to determine the three-dimensional velocity structure of the fault zone. The amplification, waveform complexity, and distortion of ray paths introduced by the low- velocity wedge suggest that its effects should be included in the interpretation of strong ground motions and travel times observed in the fault zone. For realistic models of the wedge, asymptotically approximate methods of calculating the body waveforms are strictly valid for frequencies greater than 20Hz. Numerical methods may be necessary to calculate accurately the wavefield at lower frequencies.