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Colocated explosive sources often produce correlated seismic waveforms. Multichannel correlation detectors identify these signals by scanning template waveforms recorded from known reference events against "target" data to find similar waveforms. This screening problem is challenged at thresholds required to monitor smaller explosions, often because non-target signals falsely trigger such detectors. Therefore, it is generally unclear what thresholds will reliably identify a target explosion while screening non-target background seismicity. Here, we estimate threshold magnitudes for hypothetical explosions located at the North Korean nuclear test site over six months of 2010, by processing International Monitoring System (IMS) array data with a multichannel waveform correlation detector. Our method (1) accounts for low amplitude background seismicity that falsely triggers correlation detectors but is unidentifiable with conventional power beams, (2) adapts to diurnally variable noise levels and (3) uses source-receiver reciprocity concepts to estimate thresholds for explosions spatially separated from the template source. Furthermore, we find that underground explosions with body wave magnitudes mb = 1.66 are detectable at the IMS array USRK with probability 0.99, when using template waveforms consisting only of P -waves, without false alarms. We conservatively find that these thresholds also increase by up to a magnitude unit for sources locatedmore » 4 km or more from the Feb.12, 2013 announced nuclear test.« less

Allan (Al) R. Sanford, professor of geophysics, emeritus, at New Mexico Tech (NMT) passed away quietly at his home in Woodinville, Washington, on 9 August 2016. He was 89. Al was a pioneer in the recording and interpretation of microearthquake seismograms. For decades, he was the leading authority concerning seismicity across New Mexico and within the Rio Grande rift (RGR). He published numerous papers with graduate students and colleagues on the unique, high‐amplitude reflected phases seen on Socorro‐area microearthquake records. Al and his students eventually attributed these phases to the presence of an extensive midcrustal magmatic sill now known as the Socorro Magma Body (SMB). Figure 1. Allan R. Sanford (1927–2016) Al was born in Pasadena, California. He attended Pomona College, but then entered the Navy to train as a radar specialist during the final 18 months of World War II. Back at Pomona following the war, he completed a B.S. degree in physics in 1949. He then spent time as the “computer” on a seismic crew prospecting for oil across the western United States. This experience led him to graduate school at California Institute of Technology where he gained knowledge in seismology while working for C. F. Richter and he completed his doctorate in geophysics under the guidance of C. H. Dix in 1958. Al was employed as a professor at NMT in Socorro from 1957 to 1997 where he taught courses in exploration geophysics and earthquake seismology, and supervised the research projects of about 50 masters students and about one dozen doctoral students. He joined Seismological Society of …