Array observation of strong ground motion for estimating current wavefield in real time

Abstract:

 We aim at the construction of next generation of earthquake early warning (EEW) system without any information of hypocentral parameters. In this scheme, we use the current wavefield as the initial condition for real time simulation of wave propagation.

 Backazimuth and apparent velocity are important parameters for the estimation of current wavefield as well as amplitude distribution. Array observation is one of the powerful method for estimating incident angle and apparent velocity. We have constructed array observation network with six CV-374 type strong motion seismometers (Tokyo Sokushin co.) in our institute and have worked on the improvement of array analysis technique in real-time.

 Efficient calculation of array data is the key issue as well as the evaluation of analyzed result when we use array data in EEW system. We adopt semblance analysis and oct-tree search algorithm for estimating incident angle and apparent velocity of wavefield. As a result, we have been able to calculate semblance value with time window of 4 s within every 1 s.

 Semblance value tends to be high when direct P or S wave arrives, while they become gradually lower at the time of P or S coda. Absolute value of semblance is higher with direct P waves than those of direct S waves. Residuals for incident angles of P waves of 103 earthquakes show some azimuthal dependency. In addition, calculation of cross-spectrum with a reference station indicates that each station have its site effect in phase domain corresponding to the condition of installed place. Causes of these results and correction for them are still under consideration and we will further evaluate efficacy of array analysis to the next EEW system.