3:30 PM - 4:30 PM
[S01-P-01] Fast hypocenter determination with a 3D velocity model and its implication for seismicity monitoring
Hypocenter location still has important role in any kinds of seismic analysis. Clear image of aftershock distribution helps us to understand the fault geometry of big earthquakes and tectonic meaning of the event. A precise velocity model is indispensable to get good image of hypocenter distribution. Velocity models have been estimated with seismic surveys and tomographic analyses of natural earthquakes. A crustal structure model of the Japanese model was obtained with the tomographic method (Katsumata, 2010), in which the Moho discontinuity depth distribution was estimated as well as the velocity distribution in the layers. The velocity model indicated various characteristics of the Japanese Islands, which reflected the history of the islands formation.
It usually takes a long time to estimate a hypocenter location with a 3D complex velocity structure. It sometimes takes more than one hour to locate a single hypocenter using a 3D velocity model because of the very time-consuming calculation in ray-tracing. The time had been reduced by introducing 3D travel time table for each station (Katsumata, 2015). Almost any events can be located within one seconds. It was confirmed that calculation time was reduced by about 1,800 times.
The effect of the inhomogeneous velocity structure on the event location is often observed in determination of shallow inland events. Events under a thick sediment layer tend to be located at the deeper with 1D velocity model than that with 3D model. Aftershock distribution of the 2004 Mid-Niigata Earthquake was one of such cases. Similar depth difference was recognized in the activity of the 2017 Kumamoto Earthquake. However, precision improvement in offshore areas seems to be limited. Noticeable improvement is sometimes seen, for example, events off Ibaraki Prefecture. It is considered that local velocity structure difference makes the difference in the effects for offshore event locations.
It usually takes a long time to estimate a hypocenter location with a 3D complex velocity structure. It sometimes takes more than one hour to locate a single hypocenter using a 3D velocity model because of the very time-consuming calculation in ray-tracing. The time had been reduced by introducing 3D travel time table for each station (Katsumata, 2015). Almost any events can be located within one seconds. It was confirmed that calculation time was reduced by about 1,800 times.
The effect of the inhomogeneous velocity structure on the event location is often observed in determination of shallow inland events. Events under a thick sediment layer tend to be located at the deeper with 1D velocity model than that with 3D model. Aftershock distribution of the 2004 Mid-Niigata Earthquake was one of such cases. Similar depth difference was recognized in the activity of the 2017 Kumamoto Earthquake. However, precision improvement in offshore areas seems to be limited. Noticeable improvement is sometimes seen, for example, events off Ibaraki Prefecture. It is considered that local velocity structure difference makes the difference in the effects for offshore event locations.