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

Abstract

The Hokkaido Eastern Iburi earthquake (M JMA = 6.7) occurred on September 6, 2018, in the Hokkaido corner region where the Kurile and northeastern Japan island arcs meet. We relocated aftershocks of this intraplate earthquake immediately after the main shock by using data from a permanent local seismic network and found that aftershock depths were concentrated from 20 to 40 km, which is extraordinarily deep compared with other shallow intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Further, we found that the aftershock area consists of three segments. The first segment is located in the northern part of the aftershock area, the second segment lies in the southern part, and the third segment forms a stepover between the other two segments. The hypocenter of the main shock, from which the rupture initiated, is located on the stepover segment. The centroid moment tensor solution for the main shock indicates a reverse faulting, whereas the focal mechanism solution determined by using the first-motion polarity of the P wave indicates strike-slip faulting. To explain this discrepancy qualitatively, we present a model in which the rupture started as a small strike-slip fault in the stepover segment of the aftershock area, followed by two large reverse faulting ruptures in the northern and southern segments.

Fault Slip Model of the 2018 Mw 6.6 Hokkaido Eastern Iburi, Japan, Earthquake Estimated from Satellite Radar and GPS Measurements

“…Additionally, according to Omuralieva et al [27], the average focal depth of inland earthquakes recorded throughout the Japanese islands is approximately 15 km. The 2018 Mw 6.6 inland-type earthquake, which occurred along a blind fault, was also reported by Katsumata et al [2] to have a relatively deep focal depth of approximately 40 km; similarly, the average depth of the aftershocks was reported to be approximately 20 to 40 km. Katsumata et al [2] proposed a model in which the 2018 Hokkaido eastern Iburi reverse rupture was triggered by a small strike-slip faulting in a stepover segment, according to their relocated aftershocks.…”

“…In contrast, the JMA solution suggests that this rupture occurred on a northwest-or southeast-striking fault, mainly with a strike-slip component ( Figure 1 and Table 1). Katsumata et al [2] also argued that this 2018 event The mapped coverage areas of ascending (T068A and P112A) and descending (T046D and P018D) track interferometric synthetic aperture radar (InSAR) frames are shown in black dashed boxes. The dark green pentagrams represent M > 6.5 earthquakes from 1922.…”

“…The U.S. Geological Survey (USGS) reported coordinates of 42.686 • N and 141.930 • E for the epicenter and a focal depth of approximately 35 km ( Figure 1) [1]. According to relocated aftershocks distribution of Katsumata et al [2], in total, 459 earthquakes occurred within~2 days, in which 160 M > 3 aftershocks. The largest aftershock's magnitude was M 5.9, which occurred only 3 h after the main shock.…”

“…In contrast, the JMA solution suggests that this rupture occurred on a northwest-or southeast-striking fault, mainly with a strike-slip component ( Figure 1 and Table 1). Katsumata et al [2] also argued that this 2018 event was triggered by a strike-slip faulting in a stepover segment, which was derived from aftershocks' distribution and the focal mechanism solution of the main shock. Nevertheless, robust constraints on the source parameters and slip distribution inversion Remote Sens.…”

“…Such high-precision coseismic surface observations can provide a valuable opportunity to determine the orientations of seismogenic faults and to develop a deeper understanding of the focal mechanism of this event. Several researchers have studied this earthquake in many aspects, for example, the focal mechanism by Katsumata et al [2], the seismic potential around this event by Ohtani and Imanishi [10], peak ground motions and characteristics by Dhakal et al [13], and strong motion and geodetic data by Kobayashi et al [14]. GSI has also investigated the fault geometry for the 2018 event with ALOS-2 and GNSS data, but the former did not completely cover the area of deformation compared with Sentinel-1 data [9]; consequently, the slip distribution by InSAR and GPS data is still unknown.…”