Resistivity mapping using the VLF-MT method around surface fault ruptures of the 1995 Hyogo-ken Nanbu earthquake, Japan

Citation
S. Yamaguchi et al., Resistivity mapping using the VLF-MT method around surface fault ruptures of the 1995 Hyogo-ken Nanbu earthquake, Japan, ISL ARC, 10(3-4), 2001, pp. 296-305
Citations number
11
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
ISLAND ARC
ISSN journal
1038-4871 → ACNP
Volume
10
Issue
3-4
Year of publication
2001
Pages
296 - 305
Database
ISI
SICI code
1038-4871(200109/12)10:3-4<296:RMUTVM>2.0.ZU;2-8
Abstract
Distinctive fault ruptures, the Nojima Fault and Ogura Fault, appeared alon g the northwestern coast of Awaji Island at the time of the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). In order to delineate the shallow resis tivity structures around the faults just after they formed, Very Low Freque ncy Magnetotelluric (VLF-MT) surveys were made at five sites along the Noji ma Fault and at one site along the Ogura Fault. Fourteen transects were mad e at the one site on the Ogura Fault, and another transect covers the area between the two faults. Changes in apparent resistivity or phase, or both, commonly occur when crossing the surface location of one of the faults, exc ept for the northern transects at OGR-0 on the Ogura Fault. Apparent resist ivity values of less than 100 Omegam were observed for Tertiary and Quatern ary sediments and values larger than 200 Omegam for granitic rocks. The res istivity structures are related to the morphological characteristics of the fault ruptures. Remarkably conductive zones (less than 10 Omegam in appare nt resistivity and 30-40 m in width) were found where the surface displacem ent is distinct and prominent along a single fault plane. If remarkably con ductive zones were formed at the time of the 1995 Hyogo-ken Nanbu earthquak e, the results provide a good constraint on the dimensions of a conductive zone near the surface that was made by one earthquake. Alternatively, if ch aracteristic resistivity structures existed prior to the earthquake, the co nductive zone was probably formed by some tens of earthquakes in relatively modern times. In this case, this phenomenon is inferred to be a concentrat ion of fracturing in a narrow zone and is associated with the formation of clay minerals, which enhance rock conductivity.