IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS008 Poster presentation 2017
Resistivity structure at damage area of the 2006 Mid Java earthquake
Prof. Toru Mogi
Institute of Seismology and Volcanology Hokkaido University IAGA
Yasuo Ogawa, Nurhasan, Djedi S. Widarto
We carried out the AMT and wide band MT survey at the damage area of the Mid Java Earthquake
(M6.3) that occurred at 27th May, 2006. The damage area was distributing in the plain extending to the
south of Yogyakarta district where thick soft sediment were deposited. Generally, an extent of damage
due to shaking of a ground depends on the velocity contrast of S wave between basement rocks and
sediments distributing in the surface, and thickness of the sediments as well. Our purpose of the survey
was to clarify what subsurface structure was cause of the severe damage. We set 20 AMT sites along
the line of WNW-ESE direction (A line) and 14 sites on the line of NNE-SSW direction (B line). These
lines were through the damage zone in the center part and crossed each other. The wide band MT sites
set at every other site and 11 sites were on the A bline and 7 sites were on the B line. The AMT, using
the Phoenix MTU5A system, covers frequency range at 10kHz to 10 Hz and measured natural
electromagnetic field over 1.5 to 2 hours at each site. The wide band MT covers at 320Hz to 0.01Hz and
measured at overnight of 12 to 15 hours in this survey. We performed data process using the MTU
standard data process to obtain apparent resistivity and phase, and using the Groomy-Berly
decomposition to find strike direction. The apparent resistivity and phase at 1 to 0.1 of dead band
showed almost large error, but another frequency ranges were relatively better quality. The strike
directions in higher frequencies were not clear in all sites. This means that the resitivity structure is
almost layer form at a shallower depth. We derived resitivity section image across the damage area
using the 2D inversion code developed by Ogawa and Uchida (1996). The resistivity image showed
conductive layer, less than 10 ohm-m, is distributing in the plain and its thickness increase at the center
where the conductive layer was 1.5km thick and severe damage was happened. The conductive layer
was not found beyond the east side of the plain and entering mountainous area where the resistive
layer was distributing to a shallower depth. We interpret that the shallower conductive layer found
under the plain is correspond to the soft sediment layer and the thick sediment caused to the sever
damage. The resistivity contrast between the basement layer and the sediment was about 10 times.
Probably contrast of S wave velocity between these layer was estimated to about 30% based on the
relation between resitivity and S wave velocity change. As a conclusion of this survey, the MT study has
possibility of providing us the important information of damage estimation factor through the resitivity
image covered over the damage area.
Keywords: magnetotelluric, damage area