Modulation of Seismic Attenuation at Parkfield, Before and After the 2004 M6 Earthquake

Abstract

The crack density within a fault's damage zone is thought to vary as seismic rupture is approached, as well as in the postseismic period. Moreover, external stress loads, seasonal or tidal, may also change the crack density in rocks, and all such processes can leave detectable signatures on seismic attenuation. Here we show that attenuation time histories from the San Andreas Fault at Parkfield are affected by seasonal loading cycles, as well as by 1.5–3‐year periodic variations of creep rates, consistent with Turner et al. (2015, https://doi.org/10.1002/2015JB011998), who documented a broad spectral peak, between 1.5 and 4 years, of the spectra calculated over the activity of repeating earthquakes, and over InSAR time series. After the Parkfield main shock, we see a clear modulation between seismic attenuation correlated to tidal forces. Opposite attenuation trends are seen on the two sides of the fault up to the M6.5 2003 San Simeon earthquake, when attenuation changed discontinuously, in the same directions of the relative trends. Attenuation increased steadily of over one year on the SW side of the San Andreas Fault, until the San Simeon earthquake, whereas it decreased steadily on the NE side of the San Andreas Fault, roughly for the six months prior to the event. Random fluctuations are observed up to the 2004 M6 Parkfield main shock, when rebounds in opposite directions are observed, in which attenuation decreased on the SW side, and increased on the NE side.

Seismic Attenuation Monitoring of a Critically Stressed San Andreas Fault

“…Additionally, positive anomalies observed on both sides of the SAF correspond with reduction of compression induced by a sequence of three M 4.2 earthquakes that occurred in 1994 near the SAF (Figure 2, line “(d),” and Figure 3d). Another example, consistent with the results by Malagnini et al (2019), is the regional reduction in compressional stress caused by the 2003 M 6.5 San Simeon earthquake (Figure 2, line “(f),” and Figure 3f). Stress changes induced by the San Simeon earthquake are clearly seen in the total seismic attenuation at Parkfield (Figure 2).…”

“…We base our approach on the results by Malagnini et al (2019), who used Random Vibration Theory (Cartwright & Longuet-Higgins, 1956) to enhance the signal-to-noise ratios of their results by substituting the Fourier amplitude of a specific time history at a specific frequency with the peak value of a version of the same waveform, narrow-band-filtered around a frequency of interest. After filtering seismograms around a set of central frequencies f ck , we cast the observed peak amplitudes (A n ) at all individual frequencies in a matrix form (one independent matrix for each central frequency, with no cross-frequency smoothing):…”

“…Nadeau and Johnson (1998) compiled a catalog of repeating earthquakes belonging to 165 different clusters, whose waveforms within an individual family of earthquakes were almost identical (correlation coefficients >0.98). To maximize the signal‐to‐noise ratios of their results, Malagnini et al (2019) used only waveforms from repeating earthquakes. Figure 1a shows a map of the study region, with the HRSN station locations and hypocenters of 1,811 repeating earthquakes used in this study.…”

“…We base our approach on the results by Malagnini et al (2019), who used Random Vibration Theory (Cartwright & Longuet‐Higgins, 1956) to enhance the signal‐to‐noise ratios of their results by substituting the Fourier amplitude of a specific time history at a specific frequency with the peak value of a version of the same waveform, narrow‐band‐filtered around a frequency of interest. After filtering seismograms around a set of central frequencies f c k , we cast the observed peak amplitudes ( A n ) at all individual frequencies in a matrix form (one independent matrix for each central frequency, with no cross‐frequency smoothing): where t is the median origin time of the specific set of earthquakes used for the inversion of (), r ij is the hypocentral distance between the i th recording site and the j th earthquake, and r 0 is an arbitrary reference distance (see below).…”

Preseismic Fault Creep and Elastic Wave Amplitude Precursors Scale With Lab Earthquake Magnitude for the Continuum of Tectonic Failure Modes

“…Part of the difficulty is that without advanced knowledge of an impending earthquake's location, one cannot focus efforts to search for so called precursor‐temporal changes in rock (or other) properties prior to failure. However, precursory variations in seismic velocity and amplitude anomalies have been observed in some cases (Crampin et al, ; Malagnini et al, ; Niu et al, ; Whitcomb et al, ) and lab work suggests that they might occur for the full spectrum of earthquake failure modes, from slow slip to elastodynamic earthquakes (Kaproth & Marone, ; Main & Meredith, ; Scuderi et al, ). Generally, precursors in nature are thought to arise from pore fluid‐modulated variations in effective modulus of fault zones (Sammonds et al, ; Yamashita & Tsutsumi, ).…”