Fault Zone Head Waves Research Articles

Imaging the deep structure of the San Andreas Fault south of Hollister with joint analysis of fault zone head and directParrivals

SUMMARY We perform a joint inversion of arrival time data generated by direct P and fault zone (FZ) head waves in the San Andreas Fault south of Hollister, CA, to obtain a high-resolution local velocity structure. The incorporation of head waves allows us to obtain a sharp image of the overall velocity contrast across the fault as a function of depth, while the use of near-fault data allows us to resolve internal variations in the FZ structure. The data consist of over 9800 direct P and over 2700 head wave arrival times from 450 events at up to 54 stations of a dense temporary seismic array and the permanent northern California seismic network in the area. One set of inversions is performed upon the whole data set, and five inversion sets are performed on various data subsets in an effort to resolve details of the FZ structure. The results imply a strong contrast of P-wave velocities across the fault of ∼50 per cent in the shallow section, and lower contrasts of 10‐20 per cent below 3 km, with the southwest being the side with faster velocities. The presence of a shallow low velocity zone around the fault, which could corresponds to the damage structures imaged in trapped wave studies, is detected by inversions using subsets of the data made up of only stations close to the fault. The faster southwest side of the fault shows the development of a shallow low velocity FZ layer in inversions using instruments closer and closer to the fault (<5 and <2 km). Such a feature is not present in results of inversions using only stations at greater distances from the fault. On the slower northeast side of the fault, the presence of a low velocity shallow layer is only detected in the inversions using the stations within 2 km of the fault. We interpret this asymmetry across the fault as a possible indication of a preferred propagation direction of earthquake ruptures in the region. Using events from different portions of the fault, the head wave inversions also resolve small-scale features of the fault visible in the surface geology and relocated seismicity.

High‐resolution imaging of fault zone structures

Damaged fault zone (FZ) rocks with high crack density are expected to produce several indicative wave propagation signals. These include scattering, anisotropy, nonlinearity, and FZ guided head and trapped waves. These signals can be used to obtain high‐resolution imaging of the FZ structures and to track possible temporal evolution of FZ properties. Results associated with systematic analysis of such signals recorded at several large strike‐slip FZs can be summarized as follows: The observed FZ trapped waves are generated by relatively shallow structures that extend only over the top ∼3−−4km of the crust. The shallow trapping structure in the North Anatolian fault (NAF) is surrounded by broader anisotropic and scattering zones that are also confined primarily to the top 3 km. Systematic analyses of anisotropy and scattering around the NAF do not show precursory temporal evolution of properties. However, the scattering results show clear co‐seismic changes and post‐seismic logarithmic recovery. These effects are likely to reflect mostly changes in properties of the shallow crust in response to strong shakings of nearby major earthquakes. Analyses of FZ head waves along the San Andreas Fault point to the existence of material interfaces that extend to the bottom of the seismogenic zone.

Discontinuity of the Mozumi–Sukenobu fault low-velocity zone, central Japan, inferred from 3-D finite-difference simulation of fault zone waves excited by explosive sources

We delineate shallow structures of the Mozumi–Sukenobu fault, central Japan, using fault zone waves generated by near-surface explosions and detected by a seismometer array. Two explosive sources, S1 and S2, were placed at a distance of about 2 km from the array, and the other two, S3 and S4, were at a distance of about 4 km. Fault zone head waves and fault zone trapped waves following direct P wave arrivals were clearly identified in the seismograms recorded by a linear seismometer array deployed across the fault in a research tunnel at a depth of 300 m. Synthetic waveforms generated by a 3-D finite-difference (3-D FD) method were compared with observed fault zone waves up to 25 Hz. The best fitting model indicates a 200-m-wide low-velocity zone extending at least to shot site S1 located 2 km east of the seismic array with a 20% decrease in the P wave velocity relative to the wall rock. The width of the low-velocity zone is consistent with the fault zone defined by direct geological observation in the research tunnel. However, the low-velocity zone should disappear just to the east of the site S1 to explain the observed fault zone waves for shot S3 and S4 located 4 km east of the seismometer array. Yet the observation and the simulation show notable trapped wave excitation even though shots S3 and S4 are outside the fault zone. These results indicate that (1) the effective waveguide for seismic waves along the fault does not exist east of source site S1 although the surface traces of the fault are observed in this region, and (2) considerable trapped waves can be excited by sources well outside the fault zone. These results highlight the along-strike variability in fault zone structure.

3-D finite-difference simulation of seismic fault zone waves—Application to the fault zone structure of the Mozumi-Sukenobu fault, central Japan—

Fault zone waves have the potential to be a powerful tool to reveal the fine structure of a fault zone down to the seismogenic depth. Seismic fault zone waves include head waves, trapped waves and direct body waves propagating in the fault zone. 3-D numerical simulation is necessary to interpret the waveforms in the presence of low-velocity zones with relatively complex fault structure. We computed finite difference (FD) synthetic seismograms to fit the seismograms of explosions, which contain frequencies up to 25 Hz, recorded by a linear seismometer array across the Mozumi-Sukenobu fault, central Japan. We find fault zone head waves, direct P waves propagating within the low-velocity zone and wave trains following the direct P waves associated with the fault for both observed and synthetic waveforms. Thus, modelling of fault zone waves is expected to determine details of complex fault zone structure.

Joint inversion of fault zone head waves and direct P arrivals for crustal structure near major faults

A newly recognized class of seismic phases, namely fault zone head waves arising from refraction at a transfault velocity contrast, provides additional constraints to direct P waves in inversions of near‐fault arrival time data for earthquake location and velocity structure. Incorporation of fault zone head waves in the inversion process increases the available data and broadens the spatial coverage of ray paths between sources and receivers. Equally important, the explicit recognition of fault zone head waves eliminates a source of error resulting from misidentifying head wave first arrivals as direct P waves. A simple algorithm for the joint inversion of fault zone head waves and direct P phases is illustrated with observed and synthetic data. Application to a small data set recorded by the borehole seismic network at Parkfield, California, provides separate seismic velocity depth profiles for the crustal blocks on the two sides of the San Andreas fault. For depths less than 3 km the obtained velocity contrast is 10–20%. For greater depths it decreases to 3–7%. Numerical tests with synthetic data show that for sources with known locations and for well located local earthquakes (randomized mislocation errors of up to 600 m) proper use of fault zone head waves can significantly improve the accuracy of the fitted velocity structure. The results of this work motivate the inclusion of fault zone head waves in state‐of‐the‐art joint structure‐hypocenter inversions using high quality near‐fault data.

San Andreas Fault Zone Head Waves Near Parkfield, California

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.