An effective method for small event detection: match and locate (M&L)

Detection of low magnitude event is critical and challenging in seismology. We develop a new method, named the match and locate (M&L) method, for small event detection. The M&L method employs some template events and detects small events through stacking cross-correlograms between waveforms of the template events and potential small event signals in the continuous waveforms over multiple stations and components, but the stacking is performed after making relative traveltime corrections based on the relative locations of the template event and potential small event scanning through a 3-D region around the template. Compared to the current methods of small event detection, the M&L method places event detection to a lower magnitude level and extends the capability of detecting small events that have large distance separations from the template. The method has little dependence on the accuracy of the velocity models used, and, at the same time, provides high-precision location information of the detected small-magnitude events. We demonstrate the effectiveness of the M&L method and its advantage over the matched filter method using examples of scaled-down earthquakes occurring in the Japan Island and foreshock detection before the 2011 M-w 9.0 Tohoku earthquake. In the foreshock detection, the M&L method detects four times more events (1427) than the templates and 9 per cent (134) more than the matched filter under the same detection threshold. Up to 41 per cent (580) of the detected events are not located at the template locations with the largest separation of 9.4 km. Based on the identified foreshocks, we observe five sequences of foreshock migration along the trench-parallel direction toward the epicentre of the M-w 9.0 main shock.