| dc.description.abstract |
Seismic ambient noise technology has many advantages over the traditional two-station method, the most important reason is that noise is happening all the time, so the signal can be widely and evenly distributed. Also, data resources will not be limited by earthquake events or artificial source. Since Green’s Function of any two stations can be obtained easily through cross-correlation method, related studies become more and more popular in the recent ten years. There are many studies using this concept to obtain the underground velocity structures based on onshore seismic stations. Only a few studies were reported for the marine area due to the relatively shorter recording time of ocean bottom seismometers (OBS) deployment and the high cost of the marine experiment. However, the understanding about the shear-wave velocity (Vs) of the marine sediments is very crucial for the hazard assessment related to submarine landslides, particularly with the growing of submarine resources exploration.
In this study, we applied the ambient noise technique to the four OBS seismic networks located offshore Taiwan in the aim of getting more information about the noise sources and having the preliminary estimation for the Vs of the marine sediments. Two of the seismic networks were deployed in the NE part of Taiwan, near the Ryukyu subduction system, whereas the others were in the SW area, near the Kao-Ping Canyon and Palm Ridge areas. Generally, ambient seismic noise could be associated with wind, ocean waves, rock fracturing and anthropogenic activity. In the southwestern Taiwan, the cross-correlation function obtained from two seismic networks indicate similar direction, suggestion that the source from the south of the network could be the origin of the noise. However, the two networks in the northeastern Taiwan show various source directions, which could be caused by the abrupt change of bathymetry. The Vs determined from the dispersion curve shows a relatively higher value for the networks in the Okinawa Trough (OT) off NE Taiwan than that in the continental margin offshore SW Taiwan. This observation could be linked to the presence of numerous volcanic outcrops in the shallow marine sediments is the OT area. By comparing the 1-D velocity shear-wave profile with the previous studies, we found that the low Vs area could be associated with a sedimentary layer filled with gas in the OT and the creeping or slumping area along the continental margin. Relatively high shear wave velocity structures near the Kao-Ping Canyon could be linked to the presence of mud diapir or submarine ridge. The whole area of Palm Ridge is characterized by a relatively lower shear wave velocity, which may be due to the entire area is covered by a thick distribution of submarine material. The Vs range between 0.38 to 0.62 km/s was estimated from our study also shows a good agreement with the velocity profile obtained based on the OBS seismic refraction experiment, suggesting that this method could be a more economical and effective way for the acquisition of the Vs parameters. | en_US |