Detection Method of Low-Frequency Earthquakes in the Non-Volcanic Tremor Beneath the San Andreas Fault

Recent studies showed the occurrence of non-volcanic tremor in the Cascadia Subduction zone, in the southwest Japan and along the San Andreas Fault. In the southwest Japan the non-volcanic tremor was accompanied by low-frequency earthquakes (LFEs). Using a method developed on tremor at Mt. Etna volcano, we apply a detection procedure to look for LFEs in the non-volcanic tremor recorded during 2004-2006 along the San Andreas Fault, by a seismic network comprising 13 3-C stations. This method is composed of two steps, trigger detection and trigger selection. The former is based on three algorithms, standard, adaptive standard and adaptive recursive STA/LTA, that evaluate the ratio between short- and long-term energy density to find amplitude transients. The main differences between them consist in the manual or automatic (adaptive) selection of the window length of the STA and LTA and in the rectangular or decaying exponential impulse response (standard and recursive, respectively). Using these algorithms we obtain a trigger list for each station. Trigger selection consists of selecting from these lists the triggers common at many of the stations which are characterised by similar spectral content of the seismic signal following the triggers and a time distribution of triggers consistent with the distribution of the stations. Applying this procedure we were able to find small amplitude transients related to LFE activity.