Microseisms observed during typhoon passage

Abstract

In microtremor exploration for deep structures, it is desirable that the amplitude of long-period microtremors is large. Long-period microtremors (microseisms) are known to originate from the ocean waves and are expected to be large amplitude during typhoons. Therefore, we investigated the spectral characteristics of the microseisms obtained at the two power plants facing the Pacific Ocean during the passage of Typhoon Hagibis in 2019. The microseisms caused by this typhoon had large amplitude in both horizontal and vertical components, and the amplitude of microseisms exceeded the trigger level of the velocity-type strong-motion seismometers many times. Although the two stations are about 150 km apart, the Fourier spectra of both sites’ microseisms show almost the same single peak type feature. The predominant microseisms period was 5 seconds in the horizontal components and 4 seconds in the vertical component when the typhoon stayed in the land area. The predominant microseisms period was 6 seconds in horizontal components and 5 seconds in vertical components after the typhoon left offshore. When the typhoon was offshore, the horizontal amplitude of microseisms was larger when the typhoon was on the land. When the typhoon was offshore, microseisms’ vertical amplitude was about the same or smaller when the typhoon was on the land. The peak period of HV spectral ratios was different, but HV spectral ratios’ shape stability was almost the same at both sites. The HV spectral ratios during the typhoon show almost the same amplitude and shape as those without typhoons. It seems that microseisms during the typhoon can also be used for microtremor exploration, although the period zone with large amplitude was narrow.