Shallow geological structures revealed by applying microtremor analysis in volcanic area in northern Taiwan

The Taiwan Island is the product of the orogeny: the Philippine Sea Plate has subducted the Eurasia Continent Plate and formed Ryukyu Volcanic Arc in northern and northeastern Taiwan. The Datun Volcano Group (DVG) located in northern Taiwan is the westernmost member of the Ryukyu Volcanic Arc and has the widest extent and largest eruption among the volcanic rock areas. About 1 Ma, compressional stress transformed into extensional stress in northern Taiwan, and magma from the depth erupted to form about 20 younger volcanoes in the same area. During this period, the Taipei Basin gradually formed as a half graben on a normal fault, namely the Shanjiao Fault.

The DVG and the Shanjiao Fault have been identified to be active for micro-earthquake activities and topographical features, respectively, revealed by dense and high-resolution surficial monitoring systems in the DVG area. However, owing to rugged landscape and dense vegetations, geological boreholes are few and shallow (10 to 20 meters) so that the underground structure of the Shanjiao Fault in the DVG area are still unclear. In this study, broad-band seismic sensors cross the presumed fault trace of the Shanjiao Fault were set to collect natural microtremor (0.02–50 Hz) in order to acquire S-wave velocity structure near the potential positions of the Shanjiao Fault. The horizontal-to-vertical spectral ratio (i.e., HVSR) for single-station analysis is applied to reveal different dominant frequency for different volcanic products and the high-resolution frequency wavenumber method (i.e., F-K method) for array-station analysis is applied to reveal boundaries of geological structures.

The resultant dispersion curves derived from the F-K method show that the phase velocity decreases at the frequence of 1.5 Hz from the southern array data, while it increases at 2–3 Hz from the northern array data within the hanging wall of the Shanjiao Fault. In addition, the results of this study also indicate that the stations on thin loose deposits (pyroclastic debris) underlying by lava flow (andesite) show the higher dominant frequency, and these stations are near crater, while the stations farther from the craters have lower dominant frequency with thick loose deposits. And these results are also consistent with the topography revealed by high-resolution digital terrain model of the Datun Mountain area.

Based on the results, the future work of this study will be describing spatial geometry of the Shanjiao Fault by inversion method for propose s-wave velocity structures in the study area.