Ching-Ren Lin

Detection of landslides and submarine slumps using broadband seismic networks

[1] We detected 52 seismic events associated with landslides and submarine slumps on 8 August 2009 when Typhoon Morakot hit Taiwan. These events were neither felt nor reported because their seismic energy was primarily in the long period band (20-50 s). Most of these events were located in mountain areas where the accumulated rainfall was extremely high, though some were found offshore southern Taiwan. Among all events, the fatal landslide earthquake was located at Hsiaolin Village, where 474 people were buried. Inversion modelling of the seismic waveforms generated by the Hsiaolin landslide shows that the seismic source was represented by single force, consistent with downhill sliding. Some offshore seismic events indicate that submarine slumps probably occurred along submarine canyons and steep slopes. Our study shows that broadband seismic monitoring can be used to issue early warnings for mitigating disasters generated by inland and offshore landslides.

Marine Cable Hosted Observatory (MACHO) Project in Taiwan

Taiwan is located in a junction corner between the Philippine sea plate and Eurasian plate. Because of active convergence, numerous earthquakes have taken place in and around Taiwan. On average, there are about two earthquakes greater than magnitude 6 each year and over 70% of earthquakes occurred in the offshore area. Because of the subduction of Philippine Sea Plate beneath the western end of the Ryukyu Arc and northern Taiwan, both the tectonics and seismic activity are intensive. The 2004 Sumatra earthquake has induced giant tsunami attacking coastal countries of South Asia. In a similar geodynamic context, the Sumatra event has aroused the attention of Taiwan government. Specialists from Taiwan earth scientists and ocean engineers have quickly teamed up to discuss the potential and mitigation of natural hazards from the western end of the Ryukyu subduction zone. To construct a submarine cable observatory off eastern Taiwan (MACHO project) was proposed. MACHO means a sea goddess who protects people at sea. The purpose of MACHO project has several folds. Firstly, the extension of seismic stations on land to offshore area can increase the resolution of earthquake relocating. Secondly, the extension of seismic stations may obtain tens of second before the destructing seismic waves arrive on land or tens of minute before the arrival of giant tsunami, which is helpful for earthquake or tsunami warning. Thirdly, the seafloor scientific station can monitor the active volcanoes in the Okinawa Trough, which is directly adjacent to the Ilan plain in northeastern Taiwan. Fourthly, the seafloor observatory can be used to continuously study the Kurosho current, off eastern Taiwan. The MACHO project has been granted for the fiscal year of 2007. The MACHO project is expected to be fulfilled in 2009.

Faulting and hydration of the upper crust of the SW Okinawa Trough during continental rifting: Evidence from seafloor compliance inversion

The elastic response of seafloor to ocean gravity wave loading, or seafloor compliance, provides a constraint on the elastic properties of the crust. We measured seafloor compliance at three ocean bottom seismometer (OBS) sites around Taiwan—two in the southwestern (SW) Okinawa Trough and one on the Ryukyu arc—and performed inversion for crustal structures beneath them. Models best fitting the data demonstrate a decrease in upper crustal shear velocity and an increase in the compressional/shear velocity ratio from the arc site to the trough sites with increasing amount of back-arc extension. This variation suggests that the upper continental crust is highly faulted and hydrated during rifting of the Eurasian lithosphere.