Seismically Detected Ground Tilts Induced by Precipitation and Fluvial Processes: An Example From Taiwan

Shear strains, among other ground motions, can be induced by weather-related processes. As a result, broadband seismic data offer a unique tool for understanding these natural weather events. Here we used continuous seismic, meteorological, and stream data to analyze weather-related ground motions during typhoons and rainy seasons in Taiwan. In addition to high-frequency signals, we detected ultralong period seismic signals at the station Mahsi (MASB) during three meteorological events: Typhoon Kalmaegi in 2008, Typhoon Morakot in 2009, and the East Asian rainy season in 2012. Seismic velocity signals with frequencies lower than 0.3mHz correlate with precipitation and with the time derivative of the water level in a nearby river. We converted the seismic signals to ground tilt and found that the tilt correlates with the time history of river water level, which fluctuates with precipitation. The seismically derived tilt ranges from 10(-8) to 10(-7) radians, consistent with an analytic circular loading model, indicating water level fluctuations of higher than 1m in the small river 30m away from MASB. To quantify precipitation using seismic data, we successfully modeled seismic waveforms using an empirical Green's function method, which gives good fits to both main event seismic waveforms and precipitation time history. This work demonstrates that continuous recordings from broadband seismometers may help determine local fluvial water load and could be useful for decadal rainfall change research.