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Earth Planets Space, Vol. 60 (No. 2), pp. 111-115, 2008

LETTER

Gravity anomalies and the geologic block structures in and around the aftershock area of the 2007 Noto Hanto Earthquake

Ryo Honda1, Yoshihiro Hiramatsu2, Yoshiteru Kono3, and Hideki Katagawa4

1Institute of Seismology and Volcanology, Hokkaido University, N10W8 Kita-ku, Sapporo, Japan
2Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Japan
3Emeritus Professor of Kanazawa University, Kanazawa University, Kakuma, Kanazawa, Japan
4Department of Civil Engineering, Hokuriku Electric Power Co., Ltd., 15-1 Ushijima-Cho, Toyama, Japan

(Received June 29, 2007; Revised September 17, 2007; Accepted October 4, 2007; Online published February 19, 2008)

Abstract: The 2007 Noto Hanto Earthquake occurred on 25 March. We have constructed a detailed gravity anomaly map over the Noto peninsula. Four block boundaries which are identified by morphological/geological studies are also recognized on the gravity anomaly map. Various other observations indicate that the mainshock ruptured only one of the blocks, the Kuwatsuka block, although aftershocks are distributed over two blocks, the Kuwatsuka and the Saruyama blocks. The Bouguer anomalies over the Kuwatsuka block are relatively high compared to those of the Saruyama block, i.e. the basement depth of the Kuwatsuka block is shallower than that of the Saruyama block. This difference in the basement depth is consistent with the deformation pattern observed by SAR interferometry. At the block boundary, an offset is recognized in the aftershock distribution. Based on the relationships among the gravity anomalies, the geologic structures, the aftershock distribution and the source fault, we propose that the rupture size of the earthquake was constrained by the block structure in this region.
Key words: Gravity anomaly, the 2007 Noto Hanto Earthquake, geologic block structure.


Corresponding author E-mail: honda05@mail.sci.hokudai.ac.jp


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