Abstract
We present a detailed study of T-waves originating from earthquakes in the South China Sea region, near the Indochina Peninsula and Luzon islands which were recorded by a broadband seismic station at Nansha Island. Most of these T-waves appear to have been the source originating from earthquakes with epicentral distances greater than 600 km from this station. The T-waves in this region were identified via their apparent stable measured velocities of about 1.45 km s^(-1), and represent the first reported T-waves and the first T-waves observed from an island station in the South China Sea. However, during the period of analysis (November 2004 to December 2005) additional earthquakes also occurred beyond the South China Sea region, but in these instances, any associated T-waves were not picked up by the station at Nansha Island. An analysis of Twave travel times reveals the possible locations of the P-wave to T-wave transitions at the ocean to crust interface were presumably situated near the earthquake source side. Our results indicate that the Sound Fixing and Ranging (SOFAR) channel is well developed in the South China Sea region. Ultimately, developing a solid understanding of the effective transmission of T-waves through the ocean may provide new opportunities for detecting and locating small earthquakes which would be useful for both seismic monitoring and in helping to predict and reduce the damaging effects of earthquakes and tsunamis in the South China Sea region.
Highlights
The acoustic phase related to earthquake energy propagating through seawater is called the T-phase, which was first analyzed by Linehan (1940) for Caribbean earthquakes recorded at Harvard Observatory
Amongst 33 selected seismic events documented in the South China Sea (SCS) by the United States Geological Survey (USGS) between November 2004 and December 2005 (Table 1), T-phases were positively identified in the data for 28 events that had epicentral distances ranging from 667 to 1404 km
The role of sloping bathymetry in trapping acoustic energy inside the low-velocity waveguide (SOFAR channel) is a fundamental aspect of the conversion of seismic energy into acoustic waves at the event source location, the T-waves themselves, are considered to represent acoustic energy that was converted from the P- or Lg-phases originating from shallow seismic events (Koyanagi et al 1995; Talandier and Okal 1998), or alternatively, from the S-waves originating from deep earthquakes (Okal and Talandier 1997; Lin 2001)
Summary
The acoustic phase related to earthquake energy propagating through seawater is called the T-phase (or Twave), which was first analyzed by Linehan (1940) for Caribbean earthquakes recorded at Harvard Observatory. Several investigations of earthquake-generated T-waves have been carried out - most of which have benefited, incidentally, from the enforcement of the Comprehensive Nuclear - Test Ban Treaty (CTBT), which requires the constant monitoring of acoustic disturbances in the world’s oceans in order to detect and guard against possible nuclear testing. These new studies have recorded important data regarding natural earthquakegenerated T-waves, which has led to a wide variety of novel seismic applications for these acoustic phases propagating through seawater. Recently T-waves have been shown to provide important information in detecting and monitoring a range of physical disturbances in global ocean environments, including earthquakes, submarine landslides, and volcanic activity, as well as providing clues to global climate change and aiding in the development of tsunami warning systems (Okal 2001; Graeber and Piserchia 2004)
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