Dissipative Intraplate Faulting During the 2016 M_w6.2 Tottori, Japan Earthquake

Abstract

The 2016 Mw 6.2 Tottori earthquake occurred on 21 October 2016 and produced thousands of aftershocks. Here we analyze high‐resolution‐relocated seismicity together with source properties of the mainshock to better understand the rupture process and energy budget. We use a matched‐filter algorithm to detect and precisely locate >10,000 previously unidentified aftershocks, which delineate a network of sharp subparallel lineations exhibiting significant branching and segmentation. Seismicity below 8 km depth forms highly localized fault structures subparallel to the mainshock strike. Shallow seismicity near the main rupture plane forms more diffuse clusters and lineations that often are at a high angle (in map view) to the mainshock strike. An empirical Green's function technique is used to derive apparent source time functions for the mainshock, which show a large amplitude pulse 2–4 s long. We invert the apparent source time functions for a slip distribution and observe a ~16 km2 patch with average slip ~3.2 m. 93% of the seismic moment is below 8 km depth, which is approximately the depth below which the seismicity becomes very localized. These observations suggest that the mainshock rupture area was entirely within the lower half of the seismogenic zone. The radiated seismic energy is estimated to be 5.7 × 1013 J, while the static stress drop is estimated to be 18–27 MPa. These values yield a radiation efficiency of 5–7%, which indicates that the Tottori mainshock was extremely dissipative. We conclude that this inefficiency in energy radiation is likely a product of the immature intraplate environment and the underlying geometric complexity.

Going to Any Lengths: Solving for Fault Size and Fractal Slip for the 2016, Mw 6.2 Central Tottori Earthquake, Japan, Using a Transdimensional Inversion Scheme

“…This is a similar result to the recently accepted geodetic study by Meneses‐Gutierrez et al () and is in contrast to the seismological study by Ross et al (), which suggests that significant slip occurred only at depths of ∼10–12 km (supporting information Figure S6). Ross et al () find a concentrated slip patch of ∼5‐m slip with 93% of the seismic moment released below 8 km, which would leave the surface at risk from further faulting and considerable seismic hazard. Ross et al () also estimate a much higher peak slip (maximum of 5 m of slip), whereas we estimate a peak slip of 1.8 m. Kubo et al (), however, find two distinct slipping patches of 0.6‐m slip from 3‐km depth down to 12‐km depth, supporting our view that most of the seismogenic zone ruptured, though the peak slip magnitude is only 0.6 m. The InSAR data also encompass several days to weeks of activity after the earthquake (up to 4 days for the four ALOS‐2 scenes and 15 days for Sentinel‐1), which means there could be postseismic and aftershock signal included in our data.…”

“…The tectonics in Southwest Japan is dominated by the Philippine Sea plate subducting below the Eurasian plate at the Nankai trough. Two hundred kilometers north of the Nankai trough, the Median Tectonic line is a mature shear zone (Ross et al, ) as is evident from the topography and orientation of faults (Figure ). But at 350 km north of the Nankai trough, the San‐in Shear Zone (Nishimura & Takada, ; within the right‐lateral Northern Chugoku shear zone ; Gutscher and Lallemand, ) in which these earthquakes occurred is less developed, without visible surface expressions of active faulting.…”

The 2018 Hokkaido Eastern Iburi earthquake (MJMA = 6.7) was triggered by a strike-slip faulting in a stepover segment: insights from the aftershock distribution and the focal mechanism solution of the main shock

“…This range is extraordinarily deeper than other intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Table S1 in Additional file 3 shows that in the case of the 1995 Hyogoken-Nanbu earthquake (M JMA 7.2), the depth range of the aftershock area is from 0 to 20 km (Hirata et al 1996); the 2000 Western Tottori (M JMA 7.3), 2-13 km (Shibutani et al 2005); the 2004 mid-Niigata (M JMA 6.8), 2-13 km (Okada et al 2005); the 2005 West Off Fukuoka (M JMA 7.0), 2-16 km (Shimizu et al 2006); the 2007 Chuetsu-oki (M JMA 6.8), 5-22 km (Nakahigashi et al 2012); the 2007 Noto Hanto (M JMA 6.9), 1-13 km (Sakai et al 2008); the 2008 Iwate-Miyagi Nairiku (M JMA 7.2), 2-8 km (Okada et al 2012); the 2011 Iwaki (M JMA 7.0), 2-15 km (Kato et al 2013); the 2016 Kumamoto (M JMA 7.3), 2-18 km (Shito et al 2017); and the 2016 Tottori (M JMA 6.6), 5-15 km (Ross et al 2018). On the other hand, the aftershock area is deep in and around the HCZ, Hokkaido.…”

Coseismic and Postseismic Deformation of the 2016 Central Tottori Earthquake and its Slip Model

“…In this study, we analyze coseismic and postseismic deformation of the 2016 Central Tottori earthquake. To date, two coseismic slip distributions based on seismic data have been published for this event (Kubo et al, ; Ross et al, ). Kubo et al () analyzed near‐field strong motion data using a multiple time window linear kinematic waveform inversion.…”

“…Kubo et al () analyzed near‐field strong motion data using a multiple time window linear kinematic waveform inversion. They calculated Green's functions with the discrete wavenumber method in a 1‐D layered velocity structure model and delimited two areas of concentrated slip on the fault plane and maximum slip of 0.6 m. On the other hand, Ross et al () inverted near‐ and far‐field strong motion data using an empirical Green's function method. Their results indicate slip at depths, with large slip (~3.2 m) concentrated within a 3‐km × 3‐km area below 8‐km depth.…”