North-Central New Mexico Seismicity Updates Using the Los Alamos Seismic Network

The seismicity of north-central New Mexico is unique and poorly understood. This is due partially to the complexity of the local tectonics of the Pajarito Plateau and partially to the limited regional-scale seismic station coverage. The Los Alamos Seismic Network (LASN), operated by Los Alamos National Laboratory (LANL), provides the primary earthquake monitoring for the region. Although LASN's spatial extent has grown to roughly 600 sq km over the last 10 years by the addition of 10 stations to the original 7, it is still best suited for local seismicity characterization relevant to LANL's Seismic Hazards Program. Nonetheless LASN has detected more than 2500 small earthquakes from 1973 to present. Roughly 900 of these earthquakes have been located within 150 km of LANL. On average, about 20-25 of these local earthquakes per year are recorded, most with magnitudes less than 1.5, that are not listed in other network catalogs. In general, the seismicity occurs at a steady rate and is spatially diffuse and scattered. More of the observed seismicity occurs to the north than to the south. Some earthquakes are associated with diverse, poorly understood tectonic features such as the Pajarito fault system, the Rio Grande rift, Valles caldera and the Nacimiento Uplift, but most are not related to known or well-mapped faults. There have also been numerous earthquake swarms superimposed on this background seismicity. Other than several large swarms that occurred near the Albuquerque Volcanoes, few appear to be associated with magma movement. Although residual magma is believed to exist beneath Valles Caldera, that area is remarkably quiet seismically. Recent station expansions and upgrades, though, have allowed the detection of very small caldera-related events. Continued improvements include plans to install a fourth station in the caldera and to upgrade outdated short-period analog UHF stations with broadband digital equipment. We will present the history of LASN from 1973 to present, including a description of the current LASN station configurations, station coverage and instrument improvements that have allowed detection of smaller, more distant events. We will also present an updated LASN earthquake catalog for north-central New Mexico and review the historic, pre-LASN seismicity of the region as well.