We use data from a temporary passive seismic array to illuminate the deep structure of the Ruby Mountains metamorphic core complex (RMCC). Despite decades of geologic mapping and geophysical exploration, the relative importance of lateral crustal flow, diapirism, and brittle faulting in the formation of the RMCC has remained unclear. Our Ruby Mountains Seismic Experiment (RMSE) utilized 50 passive broadband stations from 2010 to 2012 spaced at 5-10 km along three ~100-km-long intersecting profiles as part of the Earthscope Flexible Array program. Common conversion point stacks of our P-wave receiver functions show a fairly flat Moho at 32 ± 2 km depth throughout most of the study area but reaching 40 km in a narrow, north-south crustal welt 20-50 km west of the exposed RMCC. Our shear-wave splitting analysis shows that fast directions of polarization rotate clockwise from west to east across our study area, broadly matching regional studies and models that placed the anisotropy below the lithosphere. However, because our north-south crustal welt coincides with WE polarizations, the observed splitting may include a component of crustal anisotropy. We integrate our observations with older magnetotelluric and seismic refraction and reflection data to support a model of asymmetric crustal flow during formation of the RMCC at the edge of a preexisting orogenic plateau.

The Intermountain West (IMW) region is bounded by the Sierra Nevada Mountains to the west and the Great Plains to the east. Tectonically, the region is dominated by active extension and has moderate to high seismic hazard. Both paleoseismic and historical records include M>7 surface-rupturing earthquakes. The region is also the location of frequent moderate-size (M 5–6) earthquakes. In this article, we focus on the eastern IMW and its six regional seismic networks. We document recent and historical seismicity, describe the evolution of the regional networks, and clarify the rationale for sustained and improved seismic monitoring. Although absolute population is relatively low compared with other parts of the country, the IMW is experiencing rapid growth. Beyond population, there is significant seismic risk posed to major transportation and energy corridors, nuclear generation and storage facilities, dams, national laboratories, military bases, and other critical facilities. Despite the relatively high seismic hazard and increasing risk, seismic monitoring varies from excellent to skeletal, with some seismically active regions having minimal seismographic coverage. Clear monitoring needs for the IMW include increased station density, replacement of outdated seismic equipment, and more stable funding.

While most earthquakes in New Mexico are due to tectonic forces associated with the Rio Grande Rift and the Socorro Magma Body, in recent years seismic activity has increased in areas where fluid injection is ongoing, raising the possibility that these earthquakes are induced by human activity. The two regions of New Mexico that have seen the largest rise in activity are the Raton Basin, which spans the New Mexico-Colorado border in northeastern New Mexico, and the Permian Basin, which spans the New Mexico-Texas border in southeastern New Mexico. I will discuss the recorded history of seismicity in the Permian Basin in New Mexico, as well as past and ongoing efforts to better understand its causes.

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