Detection of deep low-frequency earthquakes in the Nankai subduction zone over 11 years using a matched filter technique

Abstract

To improve our understanding of the long-term behavior of low-frequency earthquakes (LFEs) along the tremor belt of the Nankai subduction zone, we applied a matched filter technique to continuous seismic data recorded by a dense and highly sensitive seismic network over an 11 year window, April 2004 to August 2015. We detected a total of ~510,000 LFEs, or ~23× the number of LFEs in the JMA catalog for the same period. During long-term slow slip events (SSEs) in the Bungo Channel, a series of migrating LFE bursts intermittently occurred along the fault-strike direction, with slow hypocenter propagation. Elastic energy released by long-term SSEs appears to control the extent of LFE activity. We identify slowly migrating fronts of LFEs during major episodic tremor and slip (ETS) events, which extend over distances of up to 100 km and follow diffusion-like patterns of spatial evolution with a diffusion coefficient of ~10 4 m 2 /s. This migration pattern closely matches the spatio-temporal evolution of tectonic tremors reported by previous studies. At shorter distances, up to 15 km, we discovered rapid diffusion-like migration of LFEs with a coefficient of ~10 5 m 2 /s. We also recognize that rapid migration of LFEs occurred intermittently in many streaks during major ETS episodes. These observations suggest that slow slip transients contain a multitude of smaller, temporally clustered fault slip events whose evolution is controlled by a diffusional process.

Empirical Low‐Frequency Earthquakes Synthesized From Tectonic Tremor Records

“…Case B: Figure S3 in Supporting Information S1 shows another example of the eLFE waveforms compared with the original LFE record (event B) that occurred on 4 September 2008 at 05:26:12.4 (JST). This example used 442 M > −1 events from the catalog of Kato and Nakagawa (2020). There was a clear improvement in the LFE signal, with easily detectable P ‐ and S ‐waves in the eLFE waveforms.…”

“…We analyze the LFE events detected by Kato and Nakagawa (2020) using MF analysis with two template LFEs selected from the JMA LFE catalog (Figure 1). Since each waveform is sufficiently similar to the template LFE, this is essentially a validation test, which is similar to Test 1, with a known answer: the summation of all of the detected waveforms aligned at the timing of the maximum correlation with the template.…”

“…Figure 5 shows the normalized original LFE records (black traces). We selected 536 M > 0 events from the catalog of Kato and Nakagawa (2020), and prepared the corresponding waveforms for the three components at 10 stations, as shown in Figure 5. We applied a 2–8 Hz bandpass filter to the original velocity records.…”

“…There was a steady increase to the maximum in each case, with a gradually decreasing rate, and a random mutation or permutation sometimes accelerating the improvement. We checked whether each record was correctly time shifted based on the origin time of each LFE, which was measured by maximizing the network cross correlation (Gibbons & Ringdal, 2006) with event A since the catalog of Kato and Nakagawa (2020) lacks accurate timing information. The τ i values are often close to the timing of the LFE for the maximum S ( τ ), and 55.8% are within two samples (Figure 6b).…”

“…The events detected by a given template LFE are grouped as one LFE family. Using MF analysis with a LFE template is now a popular method for studying the spatial and temporal distribution of LFEs and tectonic tremor (Bostock et al., 2012; Chamberlain et al., 2014; Frank et al., 2013; Kato & Nakagawa, 2020; Shelly, 2009; Sweet et al., 2014).…”