Low-Velocity Damage Zone on the San Andreas Fault at Depth near SAFOD Site at Parkfield Delineated by Fault-Zone Trapped Waves

Abstract Two instances of fault zone trapped seismic waves have been observed. At an active normal fault in crystalline rock near Oroville in northern California, trapped waves were excited with a surface source and recorded at five near-fault borehole depths with an oriented three-component borehole seismic sonde. At Parkfield, California, a borehole seismometer at Middle Mountain recorded at least two instances of the fundamental and first higher mode seismic waves of the San Andreas fault zone. At Oroville recorded particle motions indicate the presence of both Love and Rayleigh normal modes. The Love-wave dispersion relation derived for an idealized wave guide with velocity structure determined by body-wave travel-time inversion yields seismograms of the fundamental mode that are consistent with the observed Love-wave amplitude and frequency. Applying a similar velocity model to the Parkfield observations, we obtain a good fit to the trapped wavefield amplitude, frequency, dispersion, and mode time separation for an asymmetric San Andreas fault zone structure—a high-velocity half-space to the southwest, a low-velocity fault zone, a transition zone containing the borehole seismometer, and an intermediate velocity half-space to the northeast. In the Parkfield borehole seismic data set, the locations (depth and offset normal to fault plane) of natural seismic events which do or do not excite trapped waves are roughly consistent with our model of the low velocity zone. We conclude that it is feasible to obtain near-surface borehole records of fault zone trapped waves. Because trapped modes are excited only by events close to the fault zone proper—thereby fixing these events in space relative to the fault—a wider investigation of possible trapped mode waveforms recorded by a borehole seismic network could lead to a much refined body-wave/tomographic velocity model of the fault and to a weighting of events as a function of offset from the fault plane. It is an open question whether near-surface sensors exist in a stable enough seismic environment to use trapped modes as an earth monitoring device.

Download Full-text

Spatial-temporal characterization of the San Andreas Fault by fault-zone trapped waves at seismic experiment site, Parkfield, California

Earthquake Science ◽

10.29382/eqs-2021-0014 ◽

2021 ◽

Vol 34 (0) ◽

pp. 1-25

Author(s):

Yong-Gang Li ◽

Keyword(s):

Fault Zone ◽

San Andreas Fault ◽

Trapped Waves ◽

San Andreas ◽

Seismic Experiment

Download Full-text

An Electron Backscatter Diffraction and Cathode-Luminescence Study of Microstructures in the Damage Zone and the Actively Creeping Core of the San Andreas Fault Zone

10.1002/essoar.10509030.1 ◽

2021 ◽

Author(s):

Alan Boyle ◽

Jafar Hadizadeh

Keyword(s):

Fault Zone ◽

San Andreas Fault ◽

Electron Backscatter Diffraction ◽

Damage Zone ◽

Cathode Luminescence ◽

Luminescence Study ◽

San Andreas ◽

Electron Backscatter ◽

Backscatter Diffraction

Download Full-text

Imaging fine seismic structures of faults with fault-zone trapped waves

10.5194/egusphere-egu2020-3296 ◽

2020 ◽

Author(s):

Hui Su ◽

Yuanze Zhou

Keyword(s):

Fault Zone ◽

Earthquake Prediction ◽

Transport Theory ◽

Damage Zone ◽

Fault Zones ◽

Physical Parameters ◽

Trapped Waves ◽

Low Velocity ◽

Velocity Models ◽

Before And After

A fault is a low-velocity zone with widely distributed scatterers compared to the surrounding uniform materials because of the highly damaged rocks in its core. When seismic waves travel through faults, they will reflect on boundaries multiply and be trapped in the fault zones which cause the energy redistribution and generate coda waves with complicated characteristics after the direct P- and S- waves. The coda is named fault-zone trapped waves (FZTWs) (Li et al., 1990). The amplitude and duration characteristics of FZTWs (Li et al., 2016) can be used to constrain the geometric features of the fault and the physical parameters of the scatterers, so the fine structure of the fault can be finally obtained. We observed some FZTWs at several Hi-net stations in Japan, which were generated by low magnitude aftershocks following large earthquakes. Relatively strong FZTWs can be recorded by the seismic stations near or on the fault where the events happened. In this study, we simulate the theoretic envelops of FZTWs with radiative transport theory (Sanborn et al., 2017) for possible velocity models with scatterers described with von Karman distribution (Sato et al., 2012). While the theoretical envelops of FZTWs fit the observed ones well, &#160;the fine fault model is determined. The FZTWs from different events before and after the main shock can be used to determine the physical properties of faults and their adjoint area varied in the seismogenic process, then we can deeply understand the fault evolutions before and after earthquakes. The varying properties of faults can provide a new perspective for earthquake preparation and a new reference for earthquake prediction and promotes the development of earthquake prediction. Li, Y. G., R. D. Catchings, and M. R. Goldman. 2016, Subsurface Fault Damage Zone of the 2014Mw 6.0 South Napa, California, Earthquake Viewed from Fault&#8208;Zone Trapped Waves. Bulletin of the Seismological Society of America, 106, no. 6,2747-2763. doi: 10.1785/0120160039. Li, Y. G., P. Leary, K. Aki, and P. Malin. 1990, Seismic Trapped Modes in the Oroville and San-Andreas Fault Zones. Science, 249, no. 4970,763-766. doi: 10.1126/science.249.4970.763. Sanborn, C. J., V. F. Cormier, and M. Fitzpatrick. 2017, Combined Effects of Deterministic and Statistical Structure on High-frequency Regional Seismograms. Geophysical Journal International, 210, no. 2,1143-1159. doi: 10.1093/gji/ggx219. Sato H., Fehler M.C. 2012, Seismic Wave Propagation and Scattering in the Heterogeneous Earth, 2nd edn, Springer-Verlag.

Download Full-text