Harmonic seabed tremors recorded by ocean bottom seismometers offshore Taiwan

Thursday, 18 December 2014
Shiou-Ya Wang, Shu-Kun Hsu and Chin-Wei Liang, Institute of Geophysics National Central University, Jhongli, Taiwan
Several temporary networks of Ocean Bottom Seismometers (OBSs) have been deployed offshore Taiwan between 2011 and 2013. The deployed water depths range from 100 m to 3000 m. For the sites at water depth shallower than 1000 m, harmonic tremors with monochromatic frequency can be observed. Harmonic tremors appearing as intense monochromatic signals last for several hours. The dominant frequency is different for each site and ranges from 6 to 12 Hz. It is noted that the harmonic tremor presents a systematic delay every day. In order to understand the mechanism of this atypical signal, we first summed the amplitude of each components every 6 minutes and calculate the spectrum of the summed amplitude respectively. The spectrum displays several picks that are corresponding to major tidal constituents such as K1, O1 and M2. This atypical signal is especially clear in shallow water, indicating that they are highly associated with the tidal variation. We suggest the harmonic tremors would be the results of seabed resonance caused by fluid (gas) beneath the seabed. The seabed harmonic tremors are related to the stress variations due to the tidal change of the oceanic load. One problem aroused by seabed tremors is that the existence of tremors would degrade the earthquake detection. In case of an earthquake occurrence, the amplitude of the harmonic tremor is too large in shallow waters, so it becomes difficult to identify the seismic phase arrivals.