Matched filter technique is often used to detect microearthquakes such as deep low-frequency (DLF) earthquakes. It compares correlation coefficients (CC) between waveforms of template earthquakes and the observed data. Conventionally, the sum of CC at multiple seismic stations is used as an index to detect the DLF earthquakes. A major disadvantage of conventional method is drastically reduced detection accuracy when there are too few seismic stations. A new matched filter method proposed in this study can accurately detect microearthquakes using only a single station. It adopts mutual information (MI) in addition to CC to measure the similarity between the template and target waveforms. The method uses the product of MI and CC (MICC) as an index to detect DLF earthquakes. This index shows a distinct peak corresponding to an earthquake signal in a synthetic data set consisting of artificial noise and the waveform of a DLF earthquake.
Application of this single-station method to field observations of Kirishima volcano, one of the most active volcanoes in Japan, detected a total of 354 DLF earthquakes from the data in December 2010, whereas the catalog of the Japan Meteorological Agency shows only two. The catalog of DLF earthquakes constructed here shows similar temporal behavior to that found by conventional matched filter method using the sum of the CC of the six stations near the volcano. The proposed method successfully identified approximately 80% of the earthquakes in the conventionally constructed catalogs. These results suggest that the proposed method can greatly contribute to the accurate cataloging of DLF earthquakes using only a single seismic station.