Why do aftershocks occur? Relationship between mainshock rupture and aftershock sequence based on highly resolved hypocenter and focal mechanism distributions

Abstract

In order to clarify the origin of aftershocks, we precisely analyze the hypocenters and focal mechanisms of the aftershocks following the 2000 Western Tottori Earthquake, which occurred in the western part of Japan, using data from dense seismic observations. We investigate whether aftershocks occur on the mainshock fault plane on which coseismic slip occurred or they represent the rupture of fractures surrounding the mainshock fault plane. Based on the hypocenter distribution of the aftershocks, the subsurface fault structure of the mainshock is estimated using principal component analysis. As a result, we can obtain the detail fault structure composed of 8 best-fit planes. We demonstrate that the aftershocks around the mainshock fault are distributed within zones of 1.0-1.5 km in thicknesses, and their focal mechanisms are significantly diverse. This result suggests that most of the aftershocks represent the rupture of fractures surrounding the mainshock fault rather than the rerupture of the mainshock fault. The aftershocks have a much wider zone compared with the exhumed fault zone in field observations, suggesting that many aftershocks occur outside the fault damage zone. We find that most aftershocks except in and around the large-slip region are well explained by coseismic stress changes. These results suggest that the thickness of the aftershock distribution may be controlled by the stress changes caused by the heterogeneous slip distribution during the mainshock. The aftershock is also distributed within a much wider zone than the hypocenter distribution observed in swarm activity in the geothermal region, which is thought to be caused by the migration of hydrothermal fluid. This result implies a difference in generation processes: Stress changes due to the mainshock contribute primarily to the occurrence of aftershocks, whereas earthquake swarms in the geothermal region are caused by fluid migration within the localized zone.

Coseismic Displacements from Moderate-Size Earthquakes Mapped by Sentinel-1 Differential Interferometry: The Case of February 2017 Gulpinar Earthquake Sequence (Biga Peninsula, Turkey)

“…The spatial distribution of relocated events indicates an E-W orientation of the rupture zone of this sequence, in agreement with the focal mechanisms, with a total length of about 10 km (Figure 6b; along strike section D1-D2). The size (length) of the rupture zone is expected to be larger than the mainshock 6-km rupture extent (Table 6) because of off-fault aftershocks that immediately follow the mainshock, due to static stress transfer effects on elastic Earth ( [57][58][59][60][61]). In normal-slip events a large amount of static stress is transferred to optimally-oriented (receiver) faults near the mainshock, that are located along strike the source fault ( [60,62]) and host the off-fault aftershocks.…”

The Decollement Depth of Active Thrust Faults in Italy: Implications on Potential Earthquake Magnitude

“…The seismogenic volume can be constrained by results of geodetic interferometry (e.g., Cheloni et al, ; Feng et al, ; Simons et al, ) and by aftershocks distribution (e.g., Bath & Duda, ; Chiaraluce et al, ; Yukutake & Iio, ) possibly quantified by numerical models (Marc et al, ). Earthquakes magnitude increases proportionally to the logarithm of the involved volume (Bath & Duda, ; Doglioni, Carminati, et al, ; Petricca et al, ).…”

Crustal dynamics: unified understanding of geodynamic processes at different time and length scales

“…Yukutake and Iio (2017) conducted a precise analysis of hypocenters and focal mechanisms of upper crustal aftershocks from the 2000 Mw 6.6 Western Tottori, Japan, earthquake which involved predominantly sinistral strike-slip along a NNW-SSE fault structure disrupted by a conjugate set of dextral cross-faults. Aftershocks around the mainshock rupture plane occur within a tabular zone 1.0-1.5 km thick, significantly broader than the likely damage zone, with diverse mechanisms.…”