Installation and performance of the Albuquerque Seismological Laboratory small-aperture posthole array

The Global Seismographic Network (GSN) has been used extensively by seismologists to characterize large earthquakes and image deep earth structure. While some of the networks design goals have been met, the seismological community has suggested that the incorporation of small-aperture seismic arrays at select sites may improve performance of the network and enable new observations. As a pilot study for this concept, we have created a 500 m aperture, 9-element broadband seismic array around the GSN station ANMO (Albuquerque, New Mexico) at the U.S. Geological Survey Albuquerque Seismological Laboratory (ASL). The array was formed by supplementing the secondary borehole seismometer (90 m depth) at ANMO with eight additional 2.6 m posthole sites. Each stations seismometer was oriented using a fiber optic gyroscope to within 2.0 of north. Data quality, particularly on the vertical components is excellent with median power levels closely tracking the secondary sensor at ANMO at frequencies lower than 1 Hz. Horizontal component data is more variable at low-frequencies (< 0.02 Hz) with the type of installation and local geography appearing to strongly influence the amount of tilt induced noise. Throughout the manuscript, we pose several fundamental questions related to the variability and precision to which we can measure the seismic wavefield that we seek to address with data from this array. Additionally, we calculate the array response and show a few examples of using the array to obtain back azimuths of a local event and a continuous narrowband noise source. The apparent velocity of the event across the array is then used to infer the local P-wave velocity of ASL. Near real-time data collected from the array along with co-located meteorological, magnetic, and infrasound data are freely available in near-real time from the Incorporated Research Institutions for Seismology data management center.