[S-SS10] Strong Ground Motion and Earthquake Disaster

[SSS10-06] Observation of the inconsistent site responses between microtremor HVSR and array measurement in an active mud diatreme region in southwest Taiwan

*JyunYan Huang¹, CheMin Lin¹, KuoEn Ching², ChunHsiang Kuo³ (1.National Center for Research on Earthquake Engineering, Taiwan, 2.National Cheng Kung University, Taiwan, 3.National Central University, Taiwan)

Keywords:site response, mud diatreme, Southwest Taiwan

Long-term sustained geological disasters were frequently found inland within an active mud-diapir region. Identifying the spatial influence range of an active mud-diapir was difficult because its geophysical characteristics were similar to surrounding thick mudstone layers; in particular, it was difficult to identify a specific range in a small-scaled evaluation within several hundred-meter intervals. The resolution scales were usually at a kilometer level or larger for most geophysical or geological surveys, such as gravity anomalies or seismic exploration methods. We perform highly dense microtremor measurements with the smallest spatial interval of approximately 25 meters in a well-known Gunshuiping mud-volcano region in Taiwan. The resultant frequency-dependent amplification maps of the horizontal to vertical spectral ratio of microtremor (MHVR) indicate that Northwest-dipping oblique underground layer structures dominate site responses in this region, and a special rock-like (without a clear dominant frequency) feature unexpectedly appears and concentrates in the middle of the region within a 400-meter range closest to the volcano vent of mud-diatreme. However, the focusing region of significant spectral differences in MHVR does not show a substantial change in shallow shear-wave velocity (Vs) structures from the surrounding region resolved by microtremor array measurements (MAM). The fundamental difference in physical causes between MHVR and MAM methods is that the former focuses on the resonance of underground structures induced by layers with high Vs impedance, and the latter was derived from surface wave dispersion. The main findings of the rock-like spectral shape are speculated to be caused by the uplift intrusion motion of high-density and high-pressured mud that fills underground cracks and reduces Vs impedance in a relatively small zone but does not influence surface wave through entire layered features for a larger range. The special distribution of mud-diatreme resulted in a sudden change in MHVR but no significant change in inverted Vs from MAM and could be used as a convenient tool to identify specific influence regions within an active mud-diapir zone. Finally, evidence of fault invasion is found from a NE-SW striking linear feature by a joint consideration of the distribution map of the rock-like MHVRs, contour maps of frequency-dependent MHVR, and dense Vs structures, which might correspond to the Chekualin fault in Taiwan.