Computation of amplification functions in the Wakkanai formation, Horonobe area (Document on Collaborative study)

Birkhaser, P.*; Lacave, C.*; Ohara, Hidefumi; Niizato, Tadafumi  

The goal of this study was to determine the frequency dependent amplification function by calculation of the ratio between the surface response spectrum and the response spectrum at depth. A 1D model (CyberQuake) was used for the computations, because it could be concluded from the local geology that there are no significant 2D effects to be expected. Acceleration time histories from a weak motion local earthquake at a depth of 13 and 16 km recorded on July 20th (M 0.7) and August 18th (M 0.8) 2003 and from the Tokachi off-shore earthquake to the south of Hokkaido located some 430 to 450 km away from the borehole, recorded on September 26th (M 8.0) plus an aftershock recorded on September 27th 2003 were available as input data. All events are characterised by rather small acceleration values. The amplifications seem to be magnitude dependent. With the data at hand, the observed amplification functions are however rather unstable, showing different shapes for the different earthquakes and for two components of a single event. The observed ground motion amplification was much higher than the one calculated. Possible explanations for the observed discrepancy are (1) an incorrectness in the derivation of the input S-wave velocity profile used for the calculations of this study, or (2) problems with the recording instruments It is therefore recommended to check the basis of the derivation of the S-wave velocity input data and, if possible, perform direct S-wave downhole logging or S-wave crosshole measurements. It is further recommended to plan for an installation of high quality accelerometers in the future JNC Horonobe URL shaft. Additional measurements of field data would allow to record a more reliable dataset for proving and characterising significant attenuation of earthquake accelerations with depth, which also will be an important argument for feasibility studies related to the site investigations for geological disposal elsewhere in Japan or worldwide.