Seismic modeling of the Valles caldera, Jemez Mountains, New Mexico
Abstract
This study presents results from seismic modeling of the Valles caldera complex, Jemez Mountains, northern New Mexico, from refraction/wide-angle reflection data acquired during the Jemez Tomography Experiment (JTEX) in 1993 and 1995. Data from three seismic profiles were collected and modeled and results are presented here. These data are centered on the Valles caldera, a silicic volcanic system with active volcanism over the past 16 Ma. At least two major instances of explosive volcanism produced the dominant caldera structures and subsequent volcanism produced a number of rhyolitic domes. This study utilized JTEX seismic data to develop three detailed seismic velocity models to aid in the understanding of the shallow structure of the caldera complex. Data were collected using conventional explosive sources and Vibroseis sources were utilized within the Jemez Mountains. A unique method, the source-offset technique, was used during acquisition; this method utilizes the mobility of the Vibroseis source to increase the number of seismograms. Advantages of this technique include decreased seismograph station spacing and increased coherency in arrivals. Numerous severe shallow crustal heterogeneities present throughout the Jemez Mountains required the use of variable grid spacing for modeling of the seismic data. Due to the variable grid spacing, traditional resolution estimates from damped least-squares inversion gave misleading results. Thus, an alternate method for displaying the resolution of a model is detailed: the resolution spread diagram. This diagram displays model resolution as a measurement of distance which is related to the area of the model contributing to each model parameter. Velocity modeling of the Jemez Mountains gave three detailed models of the shallow velocity structure. Large quantities of fill material are evident and the collapsed caldera floor was determined to reside at approximately 3500 m in the southeast portion of the caldera. These velocity models correlate well with observed Bouguer gravity anomaly data, which allows us to extend our interpretation to the lower crust and upper mantle. A modest amount of crustal thickening under the Jemez Mountains is indicated from this work.
Degree
Ph.D.
Advisors
Braile, Purdue University.
Subject Area
Geophysics
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