NEW INSIGHTS INTO FAULT KINEMATICS AND BASIN STRUCTURE IN THE SOUTHERN ALBUQUERQUE BASIN FROM THE DENSE PASSIVE SEVILLETA SEISMIC EXPERIMENT

The geology of the southern Albuquerque Basin records a structurally complex history, including multiple Phanerozoic orogenies, Rio Grande Rift extension, and ongoing uplift from a mid-crustal magma body. The 2015 Sevilleta seismic experiment augmented the long-term, regionally sparse seismograph network with 801 vertical-component 10-Hz geophone nodes deployed at 250-350 m spacing for 10 days across the ~1600 km² Sevilleta National Wildlife Refuge north of Socorro, NM. We used this dense array to determine the velocity structure of the primary rift basin and upper crust, test rift fault kinematic models, and investigate seismicity associated with the Socorro magma body. Cross-correlation of 3-7 s period ambient noise across the array reveals the 3D shear-wave velocity structure of the basin and upper crust. Slower S-wave velocities along the Rio Grande Rift axis at 3 and 5 km depth coincide with rift-fill sediments and higher velocities in this depth range coincide with uplifted basement blocks flanking the basin. Below the basins, at 8-10 km depth, this relationship is inverted: higher S-wave velocities occur along the rift axis than the rift flanks, possibly due to intrusive rocks. Using signals from teleseismic earthquakes recorded across the array at ~0.6-4 s period, we created virtual reflection profiles that can be compared with traditional reflection sections, but without the logistical and financial costs typically associated with active source experiments. Comparisons to legacy active source reflection (COCORP) imaging show agreement with structural geometries in the rift basin and upper crust and highlight additional signals that could be targets for reprocessing the legacy data. The extended footprint of the Sevilleta array compared to the legacy profiles provides additional constraints on Quaternary fault geometries at depth. The data also show high amplitude surface wave conversions originating from high impedance contrast across the Loma Pelada fault on the western basin boundary. This feature suggests that the Loma Pelada fault acts as the primary basin-bounding structure separating Quaternary alluvium deposits to the east from Paleozoic and Precambrian basement rocks to the west, with ~4 km vertical offset.