High‐resolution deep tectonic tremor locations beneath the San Andreas Fault near Cholame, California, using the double‐pair double‐difference location method

Abstract

We determine high‐precision deep tectonic tremor locations beneath the San Andreas Fault (SAF) near Cholame, California, region by using a newly developed double‐pair double‐difference location method and a new 3‐D Vs model. Compared to the previous results, newly determined tremor locations are more consistent with known low‐frequency earthquake locations and are more concentrated along the SAF. New tremor locations near Cholame show clear contrast in the predominant depth of the tremor activity across the SAF and also show contrasts in the degree of periodic behavior both along and across the SAF. They collectively define a very special tremor generation region (VSTR) near the intersection corner between the SAF and the adjacent San Juan Fault (SJF). In the VSTR, new locations reveal some isolated tremor clusters, which may occur on multiple deep fault patches. For these isolated clusters, some are closer to the SAF and some are closer to the SJF, implying that both the SAF and SJF may extend to the ductile lower crust. Possible interaction between the SAF and SJF in the lower crust beneath the VSTR, combined with the trapped high‐pressure fluids, may explain the strong tremor activity, the isolated tremor clusters, and the characteristic tremor occurrence patterns and depth distributions in this region.

Complex Migration of Tremor Near Cholame, CA, Resolved by Seismic Array Analysis

“…About 1% E of the time windows we analyzed pass these criteria. In agreement with previous studies (Guo et al, 2017;H. Zhang et al, 2010), we find major tremor-producing areas in Parkfield and near the intersection between the SAF and the SJFZ.…”

“…However, much of Parkfield tremor is not associated with LFE activity (H. Zhang et al., 2010). Additionally, Parkfield LFEs are predominantly located on the fault, while some of the tremor apparently occurs off‐fault (Guo et al., 2017). The August 2018 transient features tremors propagating both along the SAF strike and along its depth.…”

Geodetic Measurements of Slow‐Slip Events Southeast of Parkfield, CA

“…While the locations of the tremor sources appear to be located in a wide cloud around the SAF (Figure 1a), they are largely consistent with being located on the fault interface since the perpendicular distances from the fault are normally distributed about the fault interface with a standard of deviation of 4.6 km (supporting information Figure S1). Further, Guo et al (2017) show that by using a local 3‐D velocity model and implementing a double‐difference relocation scheme, these tremors concentrate more strongly about the inferred fault interface. However, since the spatial distribution of tremors does not appear to be strongly elongated along fault (Figure 1a), and the distribution of tremor distances from the fault are too wide to be consistent with tremors only being located directly on the fault (supporting information Figure S1), we cannot rule out the case that at least some of the tremor may be generated by off‐fault sources which are distinct from those which generate the Cholame LFEs.…”

Modeling Low‐Frequency Earthquake Recurrence Patterns

“…Shelly and Johnson () suggest that LFE patterns are mainly controlled by the source depth, with the rising temperature with depth causing the brittle to ductile transition. However, previous studies have also found across‐ and along‐fault variations of LFE patterns (Guilhem & Nadeau, ; Wu et al, ) possibly caused by relative variations of quartz and feldspar at different locations (Solum et al, ), presence of serpentines (Ellsworth, ; Kirby et al, ), and intersection with San Juan Fault at the Cholame segment (Guo et al, ; Zhang et al, ). In this study, the chaotic LFE sources, which require relatively higher brittle to ductile friction ratio according to the model, are generally observed at shallower depths but do not have a clear dependence on the along‐fault position (Figure ).…”