Time histories of seismic attenuation from the San Andreas fault at Parkfield

During the seismic cycle, in nature and as well as in lab samples, the crack density of rocks varies substantially, as stressed rocks approach a critical state and eventually fail (Vasseur et al, 2017; Nur, 1972; Gupta, 1973) . At Earth scales, small periodical stress variations such as seasonal loading/unloading and tides (Johnson_etal_2017) are constantly being superimposed on the tectonic loading stress of crustal rocks, inducing periodic changes in crack porosity, pore-fluid pressure, and saturation, that should leave a signature on crustal attenuation. However, results from seismic techniques applied thus far have been too noisy, or lacked sufficient resolution, to yield meaningful measurements. Here we use a new technique that shows that seismic attenuation on the creeping section of the San Andreas Fault (SAF) at Parkfield is modulated by recognizable periodicities mostly due to tides, as well as to longer period fluctuations in creep rates (between 1.5 and 3-4 years) that have been previously observed (Nadeau sensitive to periodic stress perturbations well below 100 Pa, more than one order of magnitude smaller than the largest of all periodic stress fluctuations, due to water/snow loading/unloading (Johnson earthquake, we observe changes in anelastic attenuation on both sides of the SAF. and McEvilly, 2004; Turner et al., 2015) . Our analysis is et al., 2017) . Before and after the 2004 M6 Parkfield main Frequency-dependent precursors with opposite signs are seen on the two sides of the fault, reflecting the fact that prior to the earthquake, the Pacific side of the SAF was under decreasing compressional stress, whereas the North-American side of the fault was experiencing increasing compression. Coseismic and post-seismic stress relaxation cause anomalies of opposite signs on the two sides of the SAF at Parkfield, opposite to the pre-seismic ones. Due to rock damage, pre-2008 fluctuations show enhanced sensitivity to seasonal stresses and solid tides (Gao eta., 2000) , with amplitudes modulated by decreasing slip rate through healing. Post-2008 fluctuations indicate close-to-fault medium healing.