Layer: Relocated Earthquakes

A companion to Active Faults, Known or Suspected, this layer includes relocated earthquake hypocenters for events greater than magnitude 1.0 with depths shallower than 25 km throughout Washington State. Included for each event are calculated latitude and longitude coordinates, a calculated depth, an observed date and time, and a determined magnitude. Earthquake data were obtained from the Pacific Northwest Seismic Network (PNSN) catalog and have been analyzed using the earthquake hypocenter relocation program, hypoDD. The hypoDD program uses the double-difference earthquake relocation algorithm of Waldhauser & Ellsworth (2000) to relocate earthquakes clusters relative to one another, in some cases revealing previously unobserved linear trends. The double-difference method reduces the error of predicted travel times for event raypaths. Linear trends between earthquake locations within a region have been used to locate unmapped faults. For more information on hypoDD please visit http://www.ldeo.columbia.edu/~felixw/hypoDD.html.

Unfortunately, not all earthquakes in WA state were able to be relocated due to insufficient occurences or recordings (lack of seismic stations) of surrounding earthquake events.

This information is meant only as a general guide to areas prone to earthquakes. It is not a substitute for a site-specific investigation to assess the level of risk for any development project. It cannot be used to determine the presence or absence of faults beneath any specific locality. Such a determination requires a site-specific geotechnical investigation performed by a qualified practitioner.