Microseisms Generated by the 2010 Typhoon Megi in the Western Pacific Ocean

Abstract:

The mechanisms controlling the source locations and initiation process of microseisms are still a subject of considerable debate. Here, we investigate the characteristics and evolution of microseisms generated by Typhoon Megi (Oct 13-23, 2010) using records on both land and offshore stations to investigate the ocean-land coupling process. The typhoon Megi was the strongest typhoon in 2010, being classified as Category 5 on the Saffir-Simpson hurricane wind scale. We tracked Typhoon Megi through spectrogram analysis of seismic records at 49 seismic stations in Southeastern China over spatial dimension of about 950 km. The evolution of the microseismic energy was found to be strongly correlated with the spatial proximity of Megi to the recording stations and coastlines. We further analyzed spatial and temporal variations of microseismic energy in three frequency bands of single frequency (SF, 0.05-0.1 Hz), long period double frequency (LPDF, 0.1-0.18 Hz), and short period double frequency (SPDF, 0.18-0.4 Hz), respectively, for which different physical mechanisms have been proposed. Our analysis reveals the following preliminary results: (1) Temporal variations in LPDF and SF are well correlated, implying that LPDF might be excited primarily by the interaction between incoming ocean swells directly induced by the typhoon and the reflected waves by the coastal topography in shallow waters. (2) When the typhoon changed track from the westward to northward direction between Oct 19 and 20, the SPDF signals strengthened while both the SF and LPDF signals weakened, suggesting that SPDF might be generated by the intensified wave-wave interaction around the typhoon “eyes” when the typhoon turned. We further obtained the directivity of the energy source and found two main source regions: one near the typhoon center and another near the coastal region with shallow water depth. The coupling between ocean wave and seafloor depends on bathymetry, coupling coefficient, and the recorded microseismic signals depend also on path effect, local noises, etc. We are currently comparing observations with numerical models that include the coupling coefficients and path attenuation of seismic waves in order to better understand the mechanisms controlling the spatial-temporal evolution of microseisms.