Continuity, segmentation and faulting type of active fault zones of the 2016 Kumamoto earthquake inferred from analyses of a gravity gradient tensor

Abstract

We analyze Bouguer anomalies in/around the focal region of the 2016 Kumamoto earthquake to examine features, such as continuity, segmentation and faulting type, of the active fault zones related to the earthquake. Several derivatives and structural parameters calculated from a gravity gradient tensor are applied to highlight the features. First horizontal and vertical derivatives, as well as a normalized total horizontal derivative, characterize well the continuous subsurface fault structure along the Futagawa fault zone. On the other hand, the Hinagu fault zone is not clearly detected by these derivatives, especially in the case of the Takano-Shirahata segment, suggesting a difference of cumulative vertical displacement between the two fault zones. The normalized total horizontal derivative and the dimensionality index indicate a discontinuity of the subsurface structure of the Hinagu fault zone, that is, a segment boundary between the Takano-Shirahata and the Hinagu segments. The aftershock distribution does not extend beyond this segment boundary. In other words, this segment boundary controls the southern end of the rupture area of the foreshock. We also recognize normal fault structures dipping to the northwest in some areas of the fault zones from estimations of dip angles.

Geomorphic features of surface ruptures associated with the 2016 Kumamoto earthquake in and around the downtown of Kumamoto City, and implications on triggered slip along active faults

“…In addition to the geomorphic and surface stratigraphic analyses, one of the distinctive subsurface structural boundaries estimated from Bouguer anomalies (Matsumoto et al 2016) is consistent with the Suizenji fault zone and the Akitsugawa flexure zone.…”

“…8c) if stress at the seismogenic depth is indeed accumulated. Furthermore, the elongated shallow seismic cluster implies that the subsurface fault structure might be further extended to the northwest as inferred from the Bouguer anomalies (Matsumoto et al 2016), which may have a capability to produce an earthquake as large as M ~ 6.5. It may be a part of the post-Kumamoto earthquake hazard as well as the widespread off-fault aftershocks beneath the entire Kumamoto Plain.…”

Effect of newly refined hypocenter locations on the seismic activity recorded during the 2016 Kumamoto Earthquake sequence

“…The source inversion result shows that the maximum slip area of the M7.3 event was close to this location (Kubo et al 2016). Moreover, there are segmentations found at this low-seismicity region by the analysis of gravity gradient tensors (Matsumoto et al 2016). This is interesting if this segmentation is responsible for this low-seismicity phenomenon during the Kumamoto Earthquake sequence.…”

“…P-wave velocity structure beneath the location between lines G-G′ and K-K′ has thick high-velocity layer (Matsubara and Obara 2011). In fact, lower Bouguer anomaly distribution was found in this range, which is responsible to the Aso volcano being filled with relatively light Quaternary volcanic and pyroclastic rocks (Matsumoto et al 2016). Seismic activity at the intracaldera faults of the Long Valley presents similar futures (Prejean 2002) such as they include a series of strike-slip faults beneath the caldera's moat and near vertical dipping angles.…”

Special issue “2016 Kumamoto earthquake sequence and its impact on earthquake science and hazard assessment”

“…Aizawa et al (2017) analyze broadband magnetotelluric data and reveal a three-dimensional distribution of resistivity to discuss its relationship to the seismicity. Kusumoto (2016) and Matsumoto N et al (2016) analyze the gravity gradient tensor and examine its relationship to the geometry of source faults. Mochizuki and Mitsui (2016) discuss the relationship between heterogeneous structure and interseismic crustal deformation.…”