Research ArticleGEOPHYSICS

Excitation of San Andreas tremors by thermal instabilities below the seismogenic zone

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Science Advances  04 Sep 2020:
Vol. 6, no. 36, eabb2057
DOI: 10.1126/sciadv.abb2057
  • Fig. 1 Deep LFEs and slow slip along the San Andreas fault.

    (A) LFE (10) (yellow circles) and Mw ≥ 3 seismicity from the Northern California Earthquake Data Center (NCEDC) catalog (gray circles) for the period 2004–2017. The 10 LFE families above 20 km depth are marked with a black contour. The Mw 6.0 Parkfield earthquakes of 1966 and 2004 are marked by the white and red stars, respectively. The displacements (black vectors) at GPS sites (white triangles) are due to a simulated large slow-slip event of Mw 4.72, whose slip distribution is shown in Figure 3B. (B) Temporal behavior of the shallowest 10 LFE families northwest of Parkfield. (C) Vertical cross section along the San Andreas fault. The background color, associated with a correlation coefficient, indicates the likely location of a slow-slip event driving the shallowest LFE (12).

  • Fig. 2 Thermal instabilities in a spring-slider assembly.

    (A) Schematic strength profile showing the velocity-weakening shallow crust above a velocity-strengthening mid-crust and a ductile lower crust. A velocity-strengthening, temperature-weakening fault patch undergoes shear heating in the active shear layer of thickness w. Temperature diffuses to the bath temperature Tb through a fault zone of thickness W. (B) Dependence of steady-state friction on velocity and temperature. (C) Repeat cycles among velocity and temperature, and shear stress and temperature. (D) Repeat cycles among age of contact and temperature, and velocity and temperature. Different phases are indexed ordinally and labeled with different colors. (E) Evolution of velocity and temperature during three slow-slip events. (F) Corresponding evolution of the age of contact and shear stress.

  • Fig. 3 Numerical simulations of slow-slip events on the San Andreas fault.

    (A) Surface displacements produced by two slow-slip events representing partial ruptures of the unstable temperature-weakening area. (B) Slip distribution of slow-slip events with contour lines for cumulative slip of 10, 20, and 30 mm. The black contour is for the slow-slip event of Mw 4.72 shown in Fig. 1. (C) Simulated fault-parallel surface displacements for GPS stations P292, P293, P289, and P291. (D) Slip history at the center of the unstable patch. (D) Temporal evolution of slip velocity (black) and temperature (gray) at the patch center.

  • Fig. 4 Spatiotemporal evolution of slip velocity and temperature along the San Andreas fault.

    (A) Horizontal velocity profile A-A′ at 17-km depth showing a complex sequence of slow-slip events. The inset locates the profile. Light gray and dark lines indicate contour of velocity of 1 and 3 nm/s, respectively. Dark profiles indicate velocity above 3 nm/s. (B) Evolution of maximum velocity anywhere on the fault. (C) Evolution of temperature on profile A-A′. The contours are the same as in (A). (D) Evolution of maximum temperature anywhere on the fault.

  • Fig. 5 Recurrence pattern of Parkfield LFE and simulated events.

    (A) Histograms of recurrence times for simulated slow-slip events (bars) and for the 10 shallow LFE sources (thick profile). (B) Histograms of event duration for simulated slow-slip events and the same LFE sources. (C) Moment-duration scaling for the catalog of simulated slow-slip events. The scaling for linear (solid line) and cubic (dashed line) relationships is shown for reference. Two clusters of events can be determined based on the peak velocity, below (squares) or above (triangles) Vpeak = 1.6 × 10−8 m/s, associated with different moment/duration scaling.

  • Fig. 6 Schematic of mechanical properties of the San Andreas fault in the brittle regime.

    (A) Depth dependence of the steady-state frictional properties ab that control the velocity dependence. (B) Depth dependence of the steady-state frictional properties aQbH that control the temperature dependence. Thermal instabilities may occur in a temperature-weakening domain, depending on stability conditions. (C) Fault fabric at depth and seismic cycle behavior. (D) Horizontal section of a shear zone at the root of the San Andreas fault with multistranded slip surfaces. (E) Details of the fault core with primary slip surfaces associated with shear heating and pseudotachylyte injection veins resulting from partial melting during slow-slip events.

Supplementary Materials

  • Supplementary Materials

    Excitation of San Andreas tremors by thermal instabilities below the seismogenic zone

    Lifeng Wang and Sylvain Barbot

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