Results associated with systematic analysis of such signals at several large strike-slip fault zones can be summarized as follows: The observed fault zone trapped waves at several fault and rupture zones are generated by relatively shallow structures that extend generally only over the top ~3-4 km of the crust [ Ben-Zion et al., GJI, 2003 ; Peng et al., GJI, 2003 ; Lewis et al., GJI, 2005 ]. The shallow trapping structures are characterized by about 100m-wide fault zone layers with strong reduction of seismic velocity (e.g., 30-40%) and strong attenuation (e.g., Q values of 10-30). The shallow trapping structure in the North Anatolian fault is surrounded by broader anisotropic and scattering zones that are also confined primarily to the top 3 km [ Peng and Ben-Zion, GJI, 2004 ]. The shallow structure responsible for generating both fault zone trapped waves and anisotropic effects may be related to the top part of a flower-type structure of the fault zone. It contains highly damaged materials with intense microcracks and resides above the active portion of seismogenic crust where earthquakes nucleate. Such structure significantly increases the seismic shaking hazard near the fault zone, since seismic sources external to the fault zone are capable of generating strong motion amplification in the shallow trapping structure.

~18000 earthquakes recorded along the Karadere-Duzce branch of the north Anatolian fault after the 1999 Izmit and Duzce earthquakes in Turkey. The rest panels show subsequent analysis of fault zone signals based on this data set [Ben-Zion et al., 2003, 2007; Peng and Ben-Zion, 2004, 2005, 2006].

Our recent work in this direction focuses on using fault zone head waves (FZHW) to image the fault interface at seismogenic depth. Major plate boundary faults such as the San Andreas fault often juxtapose blocks with different elastic properties. A sharp material contrast is expected to refract seismic energy along the interface, generating emergent first arriving waves with opposite polarity from that of the following direct P waves that are recorded by near-fault stations on the slower block. Systematic analysis of near-fault data along the Calaveras fault [ Zhao and Peng, GRL, 2008 ] and Hayward fault [Ohlendorf et al., in prep.] in northern California, and around the Parkfield section of the San Andreas fault [Zhao et al., in prep] in central California have found clear FZHWs in each segment, with velocity contrast ranging from 2 to 15%. Our next step is to perform detailed waveform analysis of FZHW to obtain high-resolution information of fault zone structure at seismogenic depth.

Stacked traces at stations CCO and CMH around the Calaveras fault showing the moveout of the fault zone head waves [Zhao and Peng, 2008].

Waveforms generated by repeating earthquakes showing temporal changes after the 1984 M6.2 Morgan Hill earthquake [Zhao and Peng, in prep].

Recent developments in passive imaging via auto- and cross-correlation of ambient seismic wavefields (e.g., seismic noise, earthquake coda waves) provide an exciting opportunity for mapping spatio-temporal variations of the Earth’s properties with unprecedented resolutions. The basic idea is that cross-correlation or deconvolution of diffuse seismic wavefields recorded at two stations results in the Green’s function between them. Because ambient wavefields exist at all times in many regions, they provide an ideal source for continuous monitoring of temporal changes in the upper crust.

Currently we are developing multi-scale passive-imaging techniques to observe temporal changes of material properties in the upper crust associated with the occurrence of major earthquakes. We are using various signals and approaches to extract site response in the shallow crust and within active fault zones, and use them to monitor temporal changes associated with large earthquakes at nearby and teleseismic distances [ Wu et al., GJI, 2009 ; Chao and Peng, GJI, 2009, submitted; Wu et al., in prep.; Zhao et al., in prep].

Temporal changes of spectral ratios for the strong motion data at stations VO and FP before and after the occurrence of the 1999 Mw7.1 Duzce, Turkey, earthquake [Wu et al., GJI, 2009].

(Left) An example showing surface reflected waves recorded at the borehole station CHY. (Right) Temporal changes of time delays for each component versus the occurrence time before and after the 1999 M6.4 Chia-Yi earthquake [Chao and Peng, GJI, submitted].

Recent studies by our group and others have found that in addition to occurring in protracted ETS episodes, NVT along the San Andreas fault system in California [ Gomberg et al., Science, 2008 ; Peng et al., GRL, 2008 ; Peng et al., JGR, submitted; Ghosh et al., JGR, submitted], and beneath the central range in Taiwan [ Peng and Chao, GJI, 2008; Chao and Peng, in prep] can be triggered during the passage of surface waves of large teleseismic events. A general emerging feature from these studies is that dynamic stresses on the order of a few to a few tens of KPa are sufficient to trigger NVT. Because the lithostatic stresses at the depth where tremor occurs are several orders of magnitude larger than the stresses associated with teleseismic waves, it is hypothesized that the existence of near-lithostatic fluid pressure could significantly reduce the effective stresses and hence make tremor more susceptible to those subtle external forcing than regular earthquakes. Hence, NVT signals provide an exciting new tool for tracking the aseismic process that may be important for better understanding of loading and releasing of tectonic stresses, earthquake nucleation, and physical basis of earthquake predicibility.

Nov-volcanic tremor around the Parkfield section of the San Andreas fault triggered by the surface waves of the 2002 Mw7.8 Denali Fault earthquake [Peng et al., GRL, 2008].

Nov-volcanic tremor beneath the central Range in Taiwan triggered by the surface waves of the 2001 Mw7.8 Kunlun earthquake [Peng and Chao, GJI, 2008].