Recurring Slow Slip Events and Earthquake Nucleation in the Source Region of the M 7 Ibaraki‐Oki Earthquakes Revealed by Earthquake Swarm and Foreshock Activity

Abstract

Slow slip events (SSEs) on the plate interface are closely related to the occurrence of earthquakes and often trigger earthquake swarms in subduction zones. Moreover, some SSEs, accompanied by intensive foreshocks, precede large interplate earthquakes. Therefore, detecting and monitoring SSEs is important for assessing the potential of future large earthquakes. However, there are many SSEs not followed by large earthquakes, and it is unclear whether these can be distinguished from SSEs preceding large earthquakes. Here we use the epidemic‐type aftershock sequence (ETAS) model and matched‐filter technique to examine the spatial‐temporal distribution of earthquake swarms and foreshocks at Ibaraki‐Oki in the Japan Trench. We found that 19 swarm sequences repeatedly occurred during the period 1982–2008 at the same location as the foreshock sequences of the 1982 and 2008 M 7 Ibaraki‐Oki earthquakes. Both the foreshock and swarm sequences contain repeating earthquakes and have anomalously high seismicity rates inexplicable by the ETAS model, suggesting the recurrence of SSEs. The foreshock sequences in 1982 and 2008 contain more events inexplicable by the ETAS model than the swarm sequences. The fault slip of repeating earthquakes in the 2008 foreshock sequence was also larger than those of the swarm sequences, and the slip rate showed an abrupt increase 12 hr before the 2008 M 7 event. Our results imply that the SSEs that preceded the M 7 events had larger seismic moments than the other SSEs. These large SSEs might be related to the nucleation phase of the M 7 earthquakes.

Scaled Energy Estimation for Shallow Slow Earthquakes

“…Such coexistence of ordinary earthquakes and tremors at similar depth have not been observed in the Nankai subduction zone but observed in the Japan Trench. Some clusters of shallow tremor (Ito et al, ) and VLFEs (Y. Asano et al, ; Matsuzawa et al, ) have been reported along the Japan Trench, as well as earthquake swarms (Nishikawa & Ide, , ).…”

The slow earthquake spectrum in the Japan Trench illuminated by the S-net seafloor observatories

“…In the Japan Trench, VLFs have been detected using onshore broadband seismometers (7), and SSEs and tectonic tremors preceding the Tohoku-Oki earthquake have been inferred from oceanbottom pressure gauge and seismograph observations (2,3). Other indicators of aseismic slip are from observations of repetitive ruptures on the same fault patches, called repeating earthquakes ("repeaters") (8), and increases in the seismicity rate without a distinguishable mainshock, called earthquake swarms (9). Previous studies (8)(9)(10) have reported recurrent SSEs in the Japan Trench based on analyses of repeating earthquakes and earthquake swarms.…”

Tremor and Inferred Slow Slip Associated With Afterslip of the 2011 Tohoku Earthquake

“…Most of their epicenters were located in the slightly downdip side of the network. While the tremor region largely coincides with the VLFE region (Matsuzawa et al, ), it appears almost complementary to the regions of repeating earthquakes (Uchida & Matsuzawa, ) and regular earthquake swarms (Nishikawa & Ide, ), and it lies between the coseismic and afterslip regions (Figure ). Figure shows examples of tremor seismograms recorded by the broadband OBSs where we can clearly see tremor signals in the high‐frequency band (2–8 Hz) as well as possible VLFE signals in the low‐frequency band (0.02–0.05 Hz).…”

“…Geophysical Research Letters repeating earthquakes (Uchida & Matsuzawa, 2013) and regular earthquake swarms (Nishikawa & Ide, 2018), and it lies between the coseismic and afterslip regions (Figure 1). Figure 2 shows examples of tremor seismograms recorded by the broadband OBSs where we can clearly see tremor signals in the high-frequency band (2-8 Hz) as well as possible VLFE signals in the low-frequency band (0.02-0.05 Hz).…”

“…Distribution of tremor off the coast of the Tohoku region. (a) Map showing tremor events (colored circles), VLFEs (purple stars;Matsuzawa et al, 2015), repeating earthquakes (M4 or greater, orange crosses;Uchida & Matsuzawa, 2013), earthquake swarms (red crosses;Nishikawa & Ide, 2018), and regular earthquakes in the Japan Meteorological Agency (JMA) catalog (light blue dots) during the observation period. Tremor circles are colored according to duration as noted in the legend.…”