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• We investigated microseisms induced by Kalmaegi with multi-instrument observations. • A novel frequency-domain beamforming method was proposed to track microseism source. • Kalmaegi generated microseisms with source trailing behind only while moving fast. • Microseisms observed at the ocean bottom seismometers were dominated by Kalmaegi. • Coastal generated microseisms cannot propagate effectively across the ocean basin. The typhoon-generated microseisms, originating from the complex energy coupling and transferring among the Atmosphere-Ocean-Solid Earth spheres, can be detected remotely by seismometers as the strongest ambient seismic noise. The lack of in situ observations during the passage of typhoons has hampered numerical modeling of wind fields and ocean waves, and limited our understanding of the generation mechanisms of microseisms associated with typhoons. Here we present a comprehensive investigation of microseisms generated by typhoon Kalmaegi (September 2014) based on multiple-instrument constraints from observations including terrestrial and ocean-bottom seismic stations as well as ocean buoys. To understand the generation mechanisms, we apply an improved frequency-domain beamforming method to seismic array data leading to successful location of double-frequency (DF) microseism source regions. For comparison, we calculate the typhoon-induced ocean waves and theoretical source regions of the DF microseisms using the coupled ocean–atmosphere–wave–sediment transport modeling system with validation by ocean buoy observations. Both observations and numerical modeling results reveal two different generation mechanisms for typhoon-induced DF microseisms during the lifespan of Kalmaegi. When over the Philippine Sea, the DF microseisms were generated mainly by opposing ocean waves from two distinct storms. After Kalmaegi entered the South China Sea, the DF microseisms were generated mainly by the fast-moving typhoon with source regions just trailing behind, with the minimum frequencies determined by the typhoon translation speed. DF microseisms generated in coastal source regions were not detected by ocean bottom seismometers, suggesting that DF microseisms might not effectively propagate across the ocean-basin seafloor covered by thick sediments, owing to severe seismic attenuation and spreading losses. This information is crucial for the use of DF microseisms for future tracking and monitoring of typhoons. [ABSTRACT FROM AUTHOR]

• Magnitude of the 26 s and 28 s signals are like the Gutenberg-Richter relation. • Interval time distributions of the 26 s and 28 s signals conform to a Poisson process. • Swell transiently affects the 26 s signal; primary microseism modulates both signals. • Both signals are generated by resonance of underground fluid-filled conduits. • Reservoir networks compose of lots of tiny cracks without a characteristic scale. The earth is a dynamic planet with abundant vibrating processes. Besides the earthquakes, volcanoes and other activities, there is a special type of sources called persistent localized microseismic source (PL) with long-period almost harmonic signals and fixed location. The 26 s (0.038 Hz) and 28 s (0.036 Hz) tremors in the Gulf of Guinea are two typical PL sources in the world, but their generation mechanisms are still enigmatic. Moreover, understanding behaviors of these two sources helps to reduce their interference to ambient noise tomography. We implemented an algorithm to detect events in the PL signals for the past 30 years, and then performed statistical analysis of magnitude-cumulative number (M-N) and interval time-number (T-N). We found that the magnitude distribution is similar to the Gutenberg-Richter relation (G-R relation or power law) and the distribution of interval between events is consistent with a Poisson process. We propose that the two sources are probably related to underground complex crack networks featuring fractal characteristics, and are dominantly driven by temporally random dynamic processes. However, ocean swell might affect the 26 s source occasionally while the primary microseism seems to modulate the two PL sources. [ABSTRACT FROM AUTHOR]