Seismic Q and Velocity Structure for the Magma-hydrothermal System of the Valles Caldera, New Mexico

نویسنده

  • MORSE ROBERTS
چکیده

The Valles Caldera lies at the intersection of the Jemez Lineament and the western edge of the Rio Grande Rift in northern New Mexico. The caldera and its immediate surroundings comprise one of the world's largest and most complex volcanic centers, with a long history of eruptive activity extending up to geologically recent times. In this thesis we present the results of two different forward modeling methods used to determine the local 2-dimensional seismic structure directly beneath the caldera. In the first method we modeled observed P-wave delays by ray-tracing. In the second method we modeled spectral amplitude variations using the Aki-Larner discrete wavenumber spectral synthesis technique. We used six portable seismic event recorders to collect teleseismic data at sites placed along a line bisecting the caldera. The azimuth of the array was approximately on-line to seismically active regions to the northwest and southeast, so that our observations would be for plane waves arriving nearly parallel to the plane used to specify a 2-D vertical cross-section of the caldera directly beneath the array line. This design minimized the effects of 3-D structure in the recorded data. Two separate deployments of the six instruments along the same line yielded 24 high quality teleseismic signals for on-azimuth events recorded at most of the sites. The most useful data were obtained on the second array, which had 4 sites on the resurgent dome and 1 each on the northwest and southeast ring fractures. Using spectral ratio techniques to characterize the amplitude variations for the vertical and radial components across the array, we found that the amplitudes for both components were much lower inside the caldera than at the ring fracture sites. This dominant behavior did not vary significantly with incidence angle or direction of the events. We modeled this dominant behavior using synthetic spectral displacement solutions for the P-SV scattering problem in irregularly layered models, solved using the Aki-Larner method. The model which best fit the data required a zone of strong attenuation and low velocity approximately 16 km wide, 4 km high and 1 km deep, centered beneath the caldera. We interpret this feature as a zone of highly fractured and partially water-saturated material created by the deformation which must have accompanied caldera collapse and dome resurgence, and by subsequent hydrothermal activity. For each recorded event we also measured P-wave delays across the caldera, relative to the earliest site. The dominant behavior in these observations was that the delays are much stronger at the dome sites for southeastern events than for northwestern events. We used a standard ray-tracing method to model these observations and found that a large lens-shaped low velocity inclusion was required at significant depth beneath the caldera. We constrained the inclusion's width to be about 17 km and its average depth to be 10-13 km. We could not constrain its maximum height and velocity, but minimum values for these were found to be 8 km and 3.5 km/sec, respectively. We interpret this feature as a zone of partial melt related to the original Valles magma chamber, which has cooled considerably since the caldera and resurgent dome were created. Thesis Supervisor: Prof. Keiiti Aki Title: Professor of Geophysics This thesis is dedicated to the memory of my father, Victor Jamison Roberts, and to the future of my son, Alexander Collins Roberts.

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تاریخ انتشار 2010