T-wave observations on ocean-bottom seismometers offshore eastern Taiwan: effects of ocean sound speed perturbations and seafloor topography
Date Issued
2014
Date
2014
Author(s)
Lin, Chien-Wen
Abstract
T waves excited by earthquakes propagate along the SOFAR channel with low transmission loss, and therefore can be recorded on land-based seismic stations and hydrophones located thousands of kilometers away from earthquake epicenters. Early T-wave observations are mostly based on recordings by land-based stations due to the mechanics of the energy conversion of acoustic waves into seismic phases. Recently, T-wave signals have also been detected by ocean-bottom seismometers (OBS) at deep ocean basin off-shore eastern Taiwan, raising the question of how deep ocean environment and sound speed perturbation affects the generation and propagation of T-waves.
In this study, we examined the seismic waveform data recorded at 33 OBSs deployed in Okinawa Trough and Huatung Basin from 2006 to 2012. During this time period, there are 440 regional earthquakes with magnitude larger than 5 in the Western Pacific Ocean. A total of 88 T-wave events are identified using the criteria that significant energy in the dominant frequency of about 1 10 Hz and time duration longer than 100 seconds and spindle shape waveform. Most of these events were generated by shallow-depth (less than 50 km) earthquakes, with only one exception by deep source of 225km. Among these 88 events, 32 events were recorded on 3 OBSs located at 4500-m depth of Huatung Basin, where the depth of minimum sound speed is around 1100 m. The difference in seafloor topography around the OBS sites may influence the characteristics of T-wave signals. The seafloor topography in Okinawa Trough (S002) is relatively flat, with an average depth of 2000 m. In contrast, Huatung Basin (S004) has more complex topography between the OBS and the epicenter of earthquake.
To understand how acoustic energy scatters from the SOFAR channel into the ocean bottom, we apply the acoustic ray theory to simulate acoustic propagation in the presence of realistic ocean floor topography and sound speed profile. Our simulations indicate that seafloor topography indeed affects the acoustic propagation pattern, part of which may reach deep ocean regions. We further investigate potential conversion points of T-waves through 2-D grid-search technique. By minimizing the differences between predicted and observed arrival times of T-waves, we reveal possible conversion points around each OBS station. We also simulate seismic energy of T-waves by stacking energy coming from a series of potential conversion points within a specific time-window. The stacked energy distribution expresses a pattern similar to the envelope function of T-waves, indicating that the long-lasting waveform may result from a series of seismic-acoustic conversion processes.
Subjects
T波
水中聲學
聲發聲道
拋物型方程式法
聲線追蹤法
Type
thesis
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