Application of Subspace Detectors to 2012 Sumatera Earthquake Sequence

Abstract

In this report the aftershock sequence of the 2012 Sumatra earthquake is used to study the performance of subspace detectors (Harris, 2006) to detect and classify the events. The ground truth used for this study is 1222 aftershock solutions from the LLNL database drawn mostly from the Reviewed Event Bulletin (REB). Subspace detectors are built using several spawning strategies with waveforms recorded by the Makanchi Array (MKAR).Because we desire the subspace detectors to have a very low false alarm rate, the subspace templates are required to be about 50 seconds long (most of the P-bundle). For the same reason, the power detections used to create the subspace templates are restricted to have high SNR. Because of these restrictions, none of the subspace detectors are able to reproduce the entire ground truth catalog. They do have a very low false alarm rate, however, and so can reliably be used as single-array detection classifiers.Because of their reliability, in a suitably designed system, these detectors could be used to segregate detections from regions of high seismicity such as aftershock sequences, and allow processing of those detections in isolation from routine seismicity. It may also be possible to use these detectors to process groups of detections simultaneously. This leads to the concept of "analyst workload reduction factor"; a measure of the reduction in effort achieved by processing N detections in M groups instead of individually. If M is much smaller than N, a significant reduction factor is achieved.We use three subspace detector spawning strategies.1. In the first strategy, correlators are spawned directly from power detections and are allowed to run to the end of the sequence. Correlation clustered detections are then used to create multirank subspace detectors which are run against the entire sequence. 2. In the second strategy, power detectors are run for the entire duration of the sequence. Nearly the entire set of detections is used to create one or more subspace detectors with a high energy capture value. 3. The final strategy attempted was to build subspace detectors as in (2) but restricted to only the first day's detections.The second strategy was the most successful. Four subspace detectors produced 781 detections for the 44-day period with at most a few false detections. Operationally, this system is not practical, but we LLNL-TR-6438632 present a method for creating the detectors incrementally which we believe would maintain a high sensitivity with few false detections, while being able to provide detections without a delay for template creation.