P-Wave Microseism Source Areas: Tracking Tropical Cyclones

8 Nonlinear wave-wave interactions generate double-frequency (DF) microseisms, 9 which include both surface waves (mainly Rayleigh-type) and compressional (P) waves. 10 Although it is unclear whether DF surface waves generated in deep oceans are observed 11 on land, we show that DF P waves generated under Super Typhoon Ioke far offshore are 12 detected by beamforming of land-based seismic array data, allowing tropical cyclones to 13 be well-tracked seismologically. Two distinct spectral bands associated with different 14 microseismic P-wave source locations are observed. The short-period DF band (~ 0.16– 15 0.35 Hz) is dominated by P waves generated in the deep ocean by local wind seas near 16 the storm. In contrast, P waves in the long-period DF band (~ 0.1–0.15 Hz) are weaker 17 and generated closer to the coast of Japan from swell interactions. The accurate 18 identification of DF P-wave microseism source areas is necessary to image Earth 19 structure using ambient noise.