Abstract
Abstract The velocity structure and accurate aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake (thrust type) are obtained by inverting the arrival times from 848 aftershocks observed by a dense seismic network deployed immediately after the mainshock (8 h later). Both the detailed velocity structure and the accurate aftershock distribution show lateral heterogeneity along the fault strike. In the northeast area, aftershocks are aligned along both the NW- and SE-dipping planes. These planes are conjugate to each other. The mainshock hypocenter is located close to the bottom of an approximately 50° NW-dipping plane, which indicates that the mainshock rupture could have initiated on the NW-dipping plane. The high-V p body beneath this aftershock alignment shows a convex upward shape. In contrast, from the center to the southwest area, most of the aftershocks are aligned along SE-dipping planes. The high-V p body beneath this aftershock alignment shows a convex downward shape. Based on these results, we suggest that the crustal structure in the source region is divided into two segments by a boundary zone situated between the northeast and southwest areas. It should be noted that this segment boundary zone is coincident with the complex aftershock zone where numerous conjugate fault planes exist. We propose that the mainshock rupture initiated near the bottom of the NW-dipping fault plane and ran to the southwest, then transferred at the segment boundary zone which has numerous conjugate fault planes to the SE-dipping plane.
Highlights
IntroductionIn order to accurately assess the velocity structure and aftershock distributions in and around the source region, we deployed an inland dense network of temporary seismic stations immediately after the 2007 Niigataken Chuetsu-oki Earthquake (8 h later)
On July 16, 2007, a shallow Mw 6.6 earthquake occurred off-shore of the Chuetsu region in Niigata at 10:13 (Local time: JST = UT + 9 h)
This situation is significantly different from that identified in the source region of the 2004 mid-Niigata Prefecture Earthquake, where most of the aftershocks associated with the mainshock rupture are aligned along the NW-dipping boundary between the sediments and the basement (e.g., Kato et al, 2006)
Summary
In order to accurately assess the velocity structure and aftershock distributions in and around the source region, we deployed an inland dense network of temporary seismic stations immediately after the 2007 Niigataken Chuetsu-oki Earthquake (8 h later). The double-difference tomography method (Zhang and Thurber, 2003) was applied to the arrival time data, which consisted of both absolute and differential arrival times This method simultaneously solves for the three-dimensional Pand S-wave velocity structure (Vp, Vs) and seismic event locations. On the cross sections of Y ≤ 0 km, a high-Vp body greater than 6.25 km/s shows a convex downward shape It appears that some aftershocks in the southwest areas are aligned roughly along a velocity boundary dipping to the SE (from Y = −10 to 0 km). The off-shore velocities near the coastline have lower values from the center to the southwest
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