Modulation of seismic attenuation at Parkfield, before and after the 2004 M6 earthquake, and one example from the Geysers geothermal field.

Abstract

Seismic attenuation is generally thought to be a constant, or a simple monotonic function of frequency, and generally not a function of time. Examples of exceptions include attenuation enhancement due to shallow earthquake-induced damage, and fluctuations due to fluid diffusion. In reality, seismic attenuation fluctuates continuously in time at all frequencies, and the presence of cracks, their density and connectivity, as well as the presence and saturation of fluids, play a central role in defining such behavior. Due to multiple mechanisms, the crack density within a fault’s damage zone varies throughout the seismic cycle. Moreover, non-tectonic stress loads, seasonal or tidal, can change the crack density of crustal rocks, and leave detectable signatures on seismic attenuation. A strong signature can also be left on the crustal attenuation by a stress transfer from a nearby fault. Here we show that attenuation time histories from the San Andreas Fault (SAF) at Parkfield are affected by seasonal loading cycles, as well as by 1.5–3 year periodic variations of creep rates, consistent with published results that 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 mainshock, the modulation of seismic attenuation is clearly 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 for over one year on the SW side of the SAF, until the San Simeon earthquake, whereas it decreased steadily on the NE side of the SAF, roughly for the 6 months prior to the event. Random fluctuations are observed up to the 2004 M6 Parkfield mainshock, when rebounds in opposite directions are observed, in which attenuation decreased on the SW side, and increased on the NE side. Another example of changes of attenuation with time is given for the Geysers geothermal field, where a large data set of earthquake recordings from a dense temporal deployment are analyzed and results are given in terms of 1/Q(f,t).