Internal deformation of the Gorda Plate observed by hydroacoustic monitoring

On August 29, 1991, the National Oceanic and Atmospheric Administration VENTS Program began monitoring the northeast Pacific Ocean plate boundaries using continuously recorded data from the U.S. Navy's Sound Surveillance System (SOSUS) hydrophone arrays. Routine detection and location of small (2.4 less than or equal to M less than or equal to 3.5) earthquakes revealed a NNE-SSW trending band of seismicity through the center of the Gorda Plate. The majority of the earthquakes in the band were not detected by land-based seismic networks. This band of microearthquakes was active from the initiation of SOSUS monitoring in August 1991 through July 1992, at a rate of similar to 20 events per week. Since August 1992, however, the activity has effectively ceased. Some of the microearthquakes in the band are likely aftershocks to three large (M-W > 6) mid Gorda Plate earthquakes that occurred 1-2 months prior to the beginning of SOSUS monitoring. The hydroacoustically detected events in the band, however, do not occur in the proximity of the M-W > 6 events, and the activity level increased several months afterward, suggesting the microseismicity band represents a more general pattern of deformation within the Gorda Plate. The cessation of the midplate activity follows the occurrence of a large earthquake sequence in the adjacent Cascadia Subduction Zone in April 1992. It is proposed that the band of microseismicity and the three large midplate events reflect an accumulation of stress in the Gorda Plate, with the eventual termination of this pattern due to stress reduction associated with movement in the adjacent subduction zone. Stress field modeling of the region yields results consistent with this interpretation.