Scanning for Unusual Seismicity in the Mendocino Triple Junction Region

Anomalous seismic activity with wide-ranging behavior has been detected in the vicinity of the Mendocino Triple Junction (MTJ) and offshore transform faults. Among those unusual earthquakes are non-volcanic tremors, repeating earthquakes and slow-rupture or low-stress-drop earthquakes. These unusual events together with 'typical' earthquakes provide clues regarding the mechanics of faulting in the offshore region. We have compiled a catalog of unusual earthquakes looking for relationships between them, the fault structures and 'typical' earthquakes. We present broadband observations of these events over a multiple year period ending in 2007 as well as their characteristics that allow one to detect and recognize them. One difficulty in the study of seismicity of the region is that events located far offshore may go undetected or they may have poor locations with large uncertainty. Another difficulty is that there is a class of events that have either low stress drop or are have slow rupture processes that make detection difficult. To improve our monitoring capability in the region we have implemented a low-frequency continuous waveform scanning method (Tajima et al., 2002 and Kawakatsu, 1998) to detect, and locate events in the offshore region, as well as compute the seismic moment tensor. Using waveforms of a few known slow earthquakes as references, we also perform a cross-correlation analysis over multiple years of bandpass filtered continuous seismic data recorded at several broadband stations in northern California with the goal of detecting unknown slow-rupture event similar in waveform to known slow events. We will show results of these wavefield scanning efforts in the Mendocino transform fault. Finally, the continuous scanning method we are implementing offers improved response time for rapid characterization of earthquake and tsunami hazard from offshore earthquakes.