Empirical Low‐Frequency Earthquakes Synthesized From Tectonic Tremor Records

Abstract

Tectonic tremor and low‐frequency earthquakes (LFEs) are two different representations of the high‐frequency (>1 Hz) components of broadband slow earthquakes, which have been discovered in various tectonic regions. Although LFEs are considered building blocks of tremor, it is difficult to find constituent LFEs for some long‐duration tremor sequences. Here we introduce a new scheme to synthesize an impulsive response from a tremor source, or empirical LFE (eLFE), from complex tremor sequences. The eLFE is constructed by maximizing the absolute amplitude (L∞ norm) of the normalized and stacked waveforms of many tremor sequences at many stations. This problem includes a highly multimodal objective function, which is maximized via a combined optimization method that incorporates a genetic algorithm and iterative improvements. We first apply this method to several validation tests using synthetic tremor waveforms and real LFEs, and then tremor activities where few nearby LFEs have been detected. One example is the Okayama tremor, which has been known to be extraordinarily sensitive to tidal stress. The eLFE is helpful in determining the accurate hypocentral location and recovering the long‐term comprehensive tremor activity via a matched‐filter analysis. The well‐constrained depth of the Okayama tremor sources is less than 30 km, which suggests that the subducting Philippine Sea Plate is very shallow at this distant location from the trench axis. This eLFE scheme highlights the fact that tremor sequences may be composed of random fluctuations that are not necessarily impulsive and emphasizes the importance of characterizing such randomness to better understand slow earthquake behavior.

A Single‐Station Method for Seismic Detection of Slow Earthquakes: Applications to Japan and the Mexican Subduction Zone

“…Here, we focus on the deep part, where the tectonic tremor events are distributed within a belt‐like zone that possesses several gaps from the Bungo Channel in the west to the deep part of the Suruga Trough in the east (Figures 1a and 1b). Although LFEs and VLFEs have only been detected in parts of the tectonic tremor distribution, stacking the tectonic tremor waveforms has enabled the extraction of impulsive signals, such as LFEs in the high‐frequency band and VLFEs in the low‐frequency band, even where only tectonic tremors occur (Ide, 2021; Ide & Yabe, 2014). We therefore assume that the tectonic tremor distribution represents that of slow earthquakes and compare our results with the tectonic tremor catalog compiled by Mizuno and Ide (2019).…”

Recent advances in earthquake seismology using machine learning

“…Even for the template-matching problem, large-scale dataset requires the parallel computation of many GPUs (Ross et al 2019a). Furthermore, template matching using synthetic or empirical synthetic waveforms (Chamberlain and Townend 2018;Ide 2021) potentially contributes to better catalog development. Hence, an efficient similar waveform search is an important technique in catalog development.…”

Revisiting Seismic Energy of Shallow Tremors: Amplifications due to Site and Propagation Path Effects Near the Nankai Trough

“…Tremors can be considered successive occurrences of LFEs (Brown et al, 2009;Ide, 2021;Shelly et al, 2007). Swarms of LFEs/tremors and VLFEs during geodetic slow earthquakes (slow slip events) have often been observed (e.g., Bartlow et al, 2011;Itoh et al, 2022;Obara et al, 2004;Rogers & Dragert, 2003).…”