Earthquakes and associated crustal movement provide major clues for evaluating occurrences in the Earth's crust. Seismologists have tried to monitor earthquakes using integrated measurements taken by seismometers, tiltmeters, gravimeters, and Global Positioning System (GPS) receivers. For major earthquakes on land, hypocenter locations or mechanisms of earthquakes have been well analyzed using dense land data. Recently fault lubrication and detailed rupture mechanisms for the 1999 Chichi earthquake in Taiwan were studied using acquired waveforms from dense observations surrounding the faults that caused the earthquake. On the other hand, major subduction-related earthquakes suffer in general from a lack of constraint especially around subduction faults.
There are two major shortcomings in studying subduction-related earthquakes. One is the absence of monitoring facilities in the offshore area. The other is the distance from land to the real source of the earthquakes. Therefore, observation instruments must be located offshore to record earthquake signals and associated phenomena in near field if we are to better understand the nature of plate boundary earthquakes.