| Summary: | University deployed two arrays of three sonobuoys over the Blanco
fracture zone and two arrays of four sonobuoys over the Gorda ridge
to detect and locate microearthquakes. Microearthquake activity predicted
by plate tectonic theory for the Blanco fracture zone and Gorda
ridge was observed by these arrays to originate from these features.
Microearthquake activity of one event per hour was observed
along the southern flank of the ridge associated with the Blanco fracture
zone near 128°10'W. Similar seismicity was observed in
Cascadia Gap and associated with the fault scarp and basin of the gap.
Cascadia Gap, which is located near the center of the Blanco fracture
zone, exhibits ridge-like seismic activity based on a b-value of 1.5
determined from microearthquake observations.
The seismic activity on the Gorda ridge, including swarm events,
averaged 3.5 events per hour. The microearthquakes originated from
the median valley floor, valley walls, and on top of the crestal hills.
Other events, which could not be located, appeared to originate from
the surrounding hills with a predominance of events from west of the
Gorda-Blanco intersection. Focal depths at the intersection area are
6.5 to 9 km below a 3.5 km datum, while those farther to the south at
42°41'N range from 2.5 to 3 km below the datum. A composite fault
plane solution for the events on the Gorda ridge indicates high angle
faulting on the eastern valley wall with the eastern side down.
Since microearthquake and earthquake activity are usually
closely associated, the data obtained by this study suggests that the
20-30 km eastward offset of large earthquake epicenters from the
topographic features is due to an incorrect use of excessively high
lithospheric velocities for seismic stations lying predominantly to the
east. === Graduation date: 1976
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