Tomographic Rayleigh wave group velocities in the Central Valley, California, centered on the Sacramento/San Joaquin Delta

Abstract

If shaking from a local or regional earthquake in the San Francisco Bay region were to rupture levees in the Sacramento/San Joaquin Delta, then brackish water from San Francisco Bay would contaminate the water in the Delta: the source of freshwater for about half of California. As a prelude to a full shear‐wave velocity model that can be used in computer simulations and further seismic hazard analysis, we report on the use of ambient noise tomography to build a fundamental mode, Rayleigh wave group velocity model for the region around the Sacramento/San Joaquin Delta in the western Central Valley, California. Recordings from the vertical component of about 31 stations were processed to compute the spatial distribution of Rayleigh wave group velocities. Complex coherency between pairs of stations was stacked over 8 months to more than a year. Dispersion curves were determined from 4 to about 18 s. We calculated average group velocities for each period and inverted for deviations from the average for a matrix of cells that covered the study area. Smoothing using the first difference is applied. Cells of the model were about 5.6 km in either dimension. Checkerboard tests of resolution, which are dependent on station density, suggest that the resolving ability of the array is reasonably good within the middle of the array with resolution between 0.2 and 0.4°. Overall, low velocities in the middle of each image reflect the deeper sedimentary syncline in the Central Valley. In detail, the model shows several centers of low velocity that may be associated with gross geologic features such as faulting along the western margin of the Central Valley, oil and gas reservoirs, and large crosscutting features like the Stockton arch. At shorter periods around 5.5 s, the model's western boundary between low and high velocities closely follows regional fault geometry and the edge of a residual isostatic gravity low. In the eastern part of the valley, the boundaries of the low‐velocity zone and gravity anomaly are better aligned at longer periods (around 10.5 s) suggesting that the eastern edge of the gravity low is associated with deeper structure. There is a strong correspondence between a low in gravity near the Kirby Hills fault and low velocities from the ambient noise tomography. At longer periods, higher velocities creep in from the east and narrow the overall dimension defined by the lower velocities. Overall, there is a strong correspondence between the shape and location of low velocities in the Rayleigh wave velocity images, and geological and geophysical features.

Shear-wave Velocity Model from Rayleigh Wave Group Velocities Centered on the Sacramento/San Joaquin Delta

“…Each cell is about 5 km in width (0.05°). Figure 3 (from Fletcher et al 2016) shows the weights used in the inversion for Rayleigh wave velocities. The weights are determined from the time spent in each cell and reflect the uneven array geometry.…”

“…6 (Fletcher et al 2016). The Rayleigh wave sensitivities are a function of period with the shortest period being more sensitive to the upper few kilometers.…”

“…1). The time period covered is between 2006 and 2013 with various configurations to the number of stations and arrays being used (Fletcher et al 2016). Ambient ground motion data spanning from 8 months to over a year were processed to create dispersion curves between all pairs of stations and then inverted for a 3-D grid of Rayleigh wave velocities as shown in Fig.…”

“…The western margin of the Great Valley appears controlled by active local faults including the Pittsburgh and Kirby Hills faults just east of the Montezuma Slough, the Greenville fault just east of the Calaveras and the Midland and west Tracy faults which are located in the Delta. Fletcher et al (2016) processed ground motion recordings for ambient noise tomography using an array of 14 broadband seismometers in or near the Delta together with broadband stations from the Berkeley Digital Seismic Network and the Transportable Array of Earthscope; a total of 31 stations (see Fig. 1).…”

“…To this end, Fletcher et al (2016) used ambient noise tomography to determine the spatial distribution of Rayleigh wave group velocities (Campillo and Paul 2003;Shapiro et al 2005;Fletcher et al 2016) in the Central Valley, CA centered near the Delta. In this paper, we invert the Rayleigh group velocities to build a 3-D shear-wave velocity model.…”