Very low frequency earthquakes detection and characterization in the Southern Ryukyu subduction zone using a new time-differential template matching method (VLFE_DRL)

Very low frequency earthquakes (VLFE) are seismic events whose sources emit significantly more energy at low frequencies than at high frequencies, when compared to standard earthquakes. Such original seismicity activity requires specific methods to be exhaustively detected and located. 

The VLFE_DRL (VLFE Detection and Relative Location) method has been developed in this respect and consists of two main steps. Its first step aims at detecting events that share similar low frequency waveforms with the ones of known earthquakes. Recorded waveforms of catalogued earthquakes are taken as templates for several stations. These templates are match-filtered on a low-frequency bandwidth to the continuous records of the corresponding stations. Among the stations, one is chosen as a reference, and the others are paired with it. For each pair, a time delay is allowed to maximize the correlation at the non-reference station. It enables the detection of events that are not collocated with the chosen templates. Moreover, thanks to the delays obtained with the different pairs, the events can be located relatively to their templates. The second step of the method aims at identifying VLFEs among the detected events, based on their high-frequency contents. Events detected by the same template are gathered in so-called “families” of similar earthquakes (both in location and mechanism). Within each family, the relative stress drop between each detected earthquake and the event with the largest stress drop is then computed. Events with abnormally low relative stress drops are identified as VLFEs.

The VLFE_DRL method can be applied iteratively : waveforms of detected events can then be used as templates. It allows the detection of events that are further from the catalogued earthquakes, increasing the likelihood of identifying abnormal events. The method has been applied iteratively in the Southern Ryukyu subduction zone, known for its VLFE activity. Between 1996 and 2024, its application detected and located several hundreds of VLFEs. This database of VLFEs waveforms is now used to extract their moment rate functions, in order to explore their magnitude-duration scaling law.