Temporal Activity Modulation of Deep Very Low Frequency Earthquakes in Shikoku, Southwest Japan

Abstract

We investigated long‐term changes in the activity of deep very low frequency earthquakes (VLFEs) in western Shikoku, southwest part of the Nankai subduction zone in Japan for 13 years by the matched‐filter technique. VLFE activity is expected to be a proxy of interplate slips. In the Bungo channel, where long‐term slow slip events (SSEs) occurred frequently, the cumulative number of detected VLFEs increased rapidly in 2010 and 2014, which were modulated by long‐term SSEs. In the neighboring inland region near the Bungo channel, the cumulative number increased steeply every 6 months. This stepwise change was accompanied by episodic tremors and slips. Deep VLFE activity in western Shikoku has been low since the latter half of 2014. This decade‐scale quiescence may be attributed to the change in interplate coupling strength in the Nankai subduction zone.

Repeating VLFEs During ETS Events in Cascadia Track Slow Slip and Continue Throughout Inter‐ETS Period

“…In comparison with the matched filter VLFE catalog in Japan, the present study does agree with the findings that there is an overall increase in the number of VLFEs during the ETS events. However, only in one region of three in the Baba et al () study is there a reasonable amount of inter‐ETS VLFE. We do not find that these are conflicting findings but rather consistent in that inter‐ETS VLFE likely is a result of slow earthquake processes occurring at smaller magnitudes.…”

“…However, in the 2014 ETS event, VLFEs and tremor were spatiotemporally asynchronous (Hutchison & Ghosh, ). Baba et al () used a matched filter method, similar to that applied herein, and found a strong correlation between long‐term SSEs and ETS events and the number of VLFEs, though this correlation was not ubiquitously observed across all three regions included in the study. One region behaved differently than the neighboring regions in that the number of VLFEs did not correlate as clearly with ETS events and long‐term SSEs, though tremor near the area did show a correlation, suggesting that different VLFE source regions may behave differently.…”

Asthenosphere Flow Modulated by Megathrust Earthquake Cycles

“…The magnitude of earthquakes is in part controlled by the rheology of the surrounding rocks, as megathrust earthquakes preferentially initiate and propagate in cold, strong rocks (Blanpied et al, 1995;Scholz, 2002Scholz, , 1998. A wide range of fault slip styles have now been identified downdip megathrusts, including all of creep, slow-slip events, very-low frequency earthquakes, tsunami earthquakes, typical megathrust earthquakes, and giant ruptures (Araki et al, 2017;Baba et al, 2018;Ide, 2012;A. Kato & Nakagawa, 2014;Nakamura, 2017;Nakano et al, 2018;Obara et al, 2004;Plourde et al, 2015;Radiguet et al, 2012;Rogers & Dragert, 2003;Suenaga et al, 2016;Toh et al, 2018;Wallace & Beavan, 2006).…”

Characteristic activities of slow earthquakes in Japan

“… 18 ) The stacking analysis of broadband seismograms with reference to the tremor time revealed that the VLFE distribution, including weak-amplitude events, covered the entire tremor source zone. 30 ) Deep VLFEs have been comprehensively detected in western Shikoku, and long-term temporal variations have been observed with respect to the VLFE seismicity 31 ) via matched filter analysis based on the synthetic waveforms calculated from the focal mechanisms of deep VLFEs. 30 )…”

“…Deep VLFEs were triggered by the Bungo Channel long-term SSEs. Baba et al 31 ) detected deep VLFEs in western Shikoku using the matched filter method and revealed a rapid increase in VLFE seismicity with respect to the long-term SSEs observed in 2010 and 2014. The number and activated area of the triggered VLFEs during the SSE in 2010 were larger than those of the SSE in 2014 because of the larger moment of the former.…”