We use a dataset collected from the Oregon Array for Teleseismic Study (OATS) operating from May 2003 to May 2006. The OATS array extends northwest-southeast across Oregon from the coast to the McDemitt Caldera (Figure 2.47). The dataset was complemented by data from 9 permanent networks and a temporary deployment: Berkeley Digital Seismograph Network (BDSN), Cascade Chain Volcano Monitoring (CC), Global Seismograph Network (GSN), Laser Interferometer Gravitational-Wave Experiment (LIGO), Princeton Earth Physics Project-Indiana (PEPP), Pacific Northwest Regional Seismic Network (PNSN), USArray Transportable Network (TA), University of Oregon Regional Network (UO), the United States National Seismic Network (USNSN), and the temporary deployment of the Wallowa Mountains Experiment. A total of 52 stations were used (Figure 2.47). The data from seismic events with epicentral distance greater than 30and magnitude 6.0 and above from July 19th, 2003 to Nov. 10th, 2004 were inspected for all stations. For the S-velocity inversion, a total of 95 events (Figure 2.47) with clear S and SKS phases were recorded at 45 stations and a total of 2148 rays were used. For the P-velocity inversion, a total of 78 events with clear P and PKiKP phases were recorded at 46 stations and a total of 2101 rays were used. We follow a similar inversion procedure as described in Allen, et al., 2002.
Here we present the vertical slices through our S- and P- wave velocity models along the OATS array where both models have the highest resolution (Figure 2.48a and b). The better ray coverage available from shear-wave arrivals means the Vs model has greater resolution than the Vp model. We therefore focus on the Vs model. The most prominent feature in our tomographic models is the high velocity anomaly which dips and extends down to a depth of km. We interpret this feature as the subducted Juan de Fuca plate. Resolution tests show that we are able to resolve any slab to a depth of km. The second prominent feature is the low velocity body immediately beneath the slab extending to a depth of at least km. This layer has a similar geometry as the slab: a dip of to the east and a thickness of km. The amplitude of this low velocity anomaly is estimated to be up to for Vs. Resolution tests show that the low velocity layer is required by data and is not an artifact of the inversion.
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