[S07-7] Site effects II

[S07-7-04] DETERMINATION OF DEEP SUBSURFACE SHAREWAVE VELOCITY STRUCTURE IN THE CENTRAL PART OF THE KATHMANDU BASIN, NEPAL USING BROAD BAND SEISMOGRAPH ARRAYS FOR LONG PERIOD MICROTREMOR

^○Mukunda Bhattarai¹, Dinesh Nepali¹, Santosh Dhakal¹, Suresh Shrestha¹, Toshiaki Yokoi², Takumi Hayashida² (1.Department of Mines and Geology, Kathmandu, Nepal, 2.International Institute of Seismology and Earthquake Engineering, Building Research Institute, Tsukuba, Japan)

We conduct microtremor array exploration in the central part of the Kathmandu Valley, Nepal assuming the deepest part of the basin in order to determine the shear wave velocity structure of deep sedimentary layers for strong ground motion simulation within the frame work of SATREPS (JST - JICA) project for 5 years. The four seismographs used are consist of the broadband seismometer CMG40T (Gularp) and the digital data logger LS8800 (Hakusan) for exploration as deep as 500 – 600 m using long period microtremor (ambient noise), the power of which is weak due to a distance as long as 700 Km from the nearest ocean Bay of Bengal. Up to Feb. 18 2017, the following three arrays have been performed. First an irregular shape array consist of three deployments was performed at and around the strong ground motion observation site in the office building of Department of Mines and Geology, Lainchaur (DMG). A seismograph was installed and worked for more than three nights at DMG and other three were set for overnight measurement, at different sites every night, within a distance of approximately 1.2 Km situated at Narayanhiti Durbar Museum, Durbar Marg and Nepal Police Head quarter, Naxal in the single storey buildings or on the open ground during December 2016. Second an array consists of two almost equilateral triangular deployments and an irregular quadrilateral one has been deployed inside Singhadurbar an administrative center of Nepal government and also the third at and around the gas well point of DMG in Teku during February 2017. Preliminary analysis showed a clear peak of the power spectra of the horizontal components at a little longer period than 3 seconds that has not been detected by the microtremor observation using accelerometer due to the low power of long period microtremor.