Seismotectonics near the San Juan Islands

June 4, 2012

by John Vidale

 This study was just published in April in BSSA.  Here is a map of the faults in the San Juan Islands.  The red lines are faults at the surface, the black dots locate earthquakes, and the beach balls indicate focal mechanisms.

Unfortunately, while one can see suggestions in the alignment of hypocenters, focal mechanisms, and surface faults of throughgoing east-west and northwest-southeast faults, it is ambivalent.  The seismicity is too sparse and the connection between surface faulting and earthquakes at depth is too tenuous for us to glean the most active through-going structures from this information.  Determination of the valuable information of which patches are capable of breaking in major earthquakes is simply not yet possible.  Guessing which faults are now loaded and ready to go and which have ruptured too recently to be fully loaded is way beyond our means for now.

This illustrates that for much of the Washington-Oregon region, we have hints of patterns, but the details of the fault locations and mechanisms are not yet clear and require further study and accumulation of more earthquake recordings, and possibly ancillary information such as LIDAR to find the faults and trenching to estimate the record of past earthquakes could also help.