Why don’t the earthquakes line up with the San Andreas fault?

1 February 2016  |  Quake Insights

Magnitude-2.7 shock near Hollister highlights a string of quakes lighting up the San Andreas fault

A M=2.7 quake occurred on 1 Feb 2016 with a strike-slip mechanism parallel to the San Andreas, one of about 30 quakes in the past month on the same trend, about a mile west of the San Andreas. At 99, this area has the highest Seismic Hazard Rank anywhere in the US, because the active San Andreas and Calaveras faults merge here, and so ruptures on either fault could strongly shake the region from Morgan Hill, Gilroy, San Juan Bautista, Hollister, and Paicines. But why don’t the quakes line up with the San Andreas fault?

Temblor map with the pointer hovering over today’s M=2.7 quake. The seismic hazard rank here is as high as it gets anywhere in the United States.

It turns out that the San Andreas is not vertically inclined here, probably because the fault is bending about 10° in a clockwise sense from its orientation (or ‘strike’) to the south. Careful relocation of small shocks by Janet Watt and others published in the journal Tectonics in 2014 reveal its geometry. Here is a cross-section through the San Andreas fault (SAF), Calaveras fault (CF), and Quien Sabe (QS). For the Calaveras fault, nothing is clear, but the San Andreas quakes reveal a ‘dip’ or inclination of 75°. Today’s quake lies close to this section, in which B is to the southwest and B’ is to the northwest, with the section bisecting the town of Hollister.

Seismicity cross-section with fault interpretations from Watt, J. T., D. A. Ponce, R. W. Graymer, R. C. Jachens, and R. W. Simpson (2014), Subsurface geometry of the San Andreas-Calaveras fault junction: Influence of serpentinite and the Coast Range Ophiolite, Tectonics, 33, 2025–2044, doi:10.1002/2014TC003561.

One can also examine a longer record of M≥2 quakes in this area and see that the pattern over the past month is typical of the past 15 years. Seismicity always lies to the west of the fault, and we should assume that the highly-active San Andreas is the culprit for these small quakes.

USGS ANSS catalog map of M≥2 earthquakes since 2000 in the vicinity of today’s quake show the same alignments as seen in the past month.

Ross Stein and Volkan Sevilgen, Temblor

Data from USGS, California Geological Survey, and Watt et al (Tectonics, 2014)

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