Passive seismic imaging with directive ambient noise: application to surface waves and the San Andreas Fault in Parkfield, CA

Abstract Three-dimensional finite difference simulations of elastic waves in the San Bernardino Valley were performed for two hypothetical earthquakes on the San Andreas fault: a point source with moment magnitude M5 and an extended rupture with M6.5. A method is presented for incorporating a source with arbitrary focal mechanism in the grid. Synthetics from the 3-D simulations are compared with those derived from 2-D (vertical cross section) and 1-D (flat-layered) models. The synthetic seismograms from the 3-D and 2-D simulations exhibit large surface waves produced by conversion of incident S waves at the edge of the basin. Seismograms from the flat-layered model do not contain these converted surface waves and underestimate the duration of shaking. The seismograms from the 3-D simulations have larger amplitude coda than do the seismograms from the 2-D case because of the presence of off-azimuth surface wave arrivals in the 3-D simulations that are not included in the 2-D simulations. Snapshots of the wavefield of the 3-D simulation show that these off-azimuth arrivals represent surface waves reflected from the edges of the basin. The anelastic attenuation of the sediments is a key parameter controlling the overall duration of motion. Some of the coda energy at rock sites near the basin edges represents leakage of surface wave energy out of the basin. For the M6.5 earthquake simulation, the largest ground velocities occur where surface waves reflected from the edge of the basin interfere constructively with the trapped waves that follow the direct S-wave. Maps of maximum ground velocity are produced for two directions of rupture propagation. The largest velocities occur in localized portions of the basin. The location of the largest velocities changes with the rupture propagation direction. Contours of maximum shaking are also dependent on asperity positions and radiation pattern.

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The July 2019 Ridgecrest, California, Earthquake Sequence Recorded by Creepmeters: Negligible Epicentral Afterslip and Prolonged Triggered Slip at Teleseismic Distances

Seismological Research Letters ◽

10.1785/0220190293 ◽

2020 ◽

Vol 91 (2A) ◽

pp. 707-720 ◽

Cited By ~ 3

Author(s):

Roger Bilham ◽

Bryan Castillo

Keyword(s):

Surface Waves ◽

Travel Time ◽

San Andreas Fault ◽

Surface Strain ◽

Earthquake Sequence ◽

Fault Creep ◽

San Andreas ◽

Garlock Fault ◽

Growing Body ◽

Epicentral Region

Abstract We report sequential triggered slip at 271–384 km distances on the San Andreas, Superstition Hills, and Imperial faults with an apparent travel-time speed of 2.2 ± 0.1 km/s, following the passage of surface waves from the 4 July 2019 (17:33:49 UTC) Mw 6.4 and 6 July 2019 (03:19:53 UTC) Mw 7.1 Ridgecrest earthquakes. Slip on remote faults was not triggered instantaneously but developed over several minutes, increasing in duration with distance. Maximum slip amplitudes varied from 10 μm to 5 mm within minutes of slip nucleation, but on the southernmost San Andreas fault slip continued for two months and was followed on 16 September 2019 by a swarm of microearthquakes (Mw≤3.8) near Bombay Beach. These observations add to a growing body of evidence that fault creep may result in delayed triggered seismicity. Displacements across surface faults in the southern epicentral region and on the Garlock fault in the months following the Ridgecrest earthquakes were negligible (<1.1 mm), and they are interpreted to characterize surface strain adjustments in the epicentral region, rather than to result from discrete slip on surface faults.

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Monitoring of fields using body and surface waves reconstructed from passive seismic ambient noise

10.1190/segam2019-3216217.1 ◽

2019 ◽

Author(s):

Florent Brenguier ◽

Aurélien Mordret ◽

Richard Lynch ◽

Roméo Courbis ◽

Xander Campbell ◽

...

Keyword(s):

Surface Waves ◽

Ambient Noise ◽

Seismic Ambient Noise ◽

Passive Seismic

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