Graduate Thesis Or Dissertation
 

Microearthquake studies of the Blanco fracture zone and Gorda Ridge using Sonobuoy arrays

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4q77ft81p

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  • 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.
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file details Jones_IIIPaulRoy1976.pdf 2017-08-08 Pubblico Scaricare