Deep very low frequency earthquakes in Shikoku, Kii and Tokai areas from Broadband Campaign for Deep Extent of Nankai Slow Eathquake (BC-DENSE)

Abstract:

Very low frequency earthquake (VLFE) is one type of slow earthquakes observed at a period range of ~20­–50 s. To observe small signal of deep VLFEs in the Nankai subduction zone, we have installed broadband seismometers in three different stages. In the first and second stage in 2015 and 2017, we installed 6 stations in total in Shikoku and Kyushu regions. In the third stage, we installed 6, 4 and 4 stations in Shikoku, Kii and Tokai regions from September 2019 to March 2020 as a part of “Science of Slow Earthquakes” project. The seismometers are either CMG-3T (100 s or 120 s) or Trillium QA/PA (120 s). The data is transferred to a remote server almost in real time. We call these stations BC-DENSE after Broadband Campaign for Deep Extent of Nankai Slow Earthquakes.

In this study, we detected VLFEs using these datasets as well as data of permanent broadband F-net stations operated by NIED. The method is similar to GRiD MT (Tsuruoka et al. 2009; Takeo et al. 2010), but we chose lower threshold of variance reduction (15%). To suppress false detection, we discarded the event when variance reduction assuming plane wave propagation exceeds the variance reduction assuming CMT event. We also discarded the event when the summation of the uncertainties of P and T axes exceeds 40 degree. As a result, we detected and determined centroid moment tensor for 453 events from January 2018 to July 2020 in the western Shikoku region, for 71 events in 2020 in the Kii region and 5 events in 2020 in the Tokai region. The CMT solutions are consistent with the subduction plate interface, and the moment magnitude is around 3.0–3.3, slightly smaller than the range in previous study by Ito et al. (2009).

We here focus on the result in the Shikoku region. The VLFE activity is mostly episodic corresponding to the episodic tremor and short-term slow slip event (SSE), whereas it becomes intermittent during the long-term SSE from late 2018 to early 2019. We could also determine tremor locations for about 70% of the time windows with VLFEs, and found that the distance between tremor and VLFE epicenters are less than ~10 km, similar to the location uncertainty. We will modify detection procedure to enhance detectability in Kii and Tokai regions, continue monitoring these datasets and determine detailed source parameters of VLFEs to understand the dynamics of interplate slip in the future.