5．Method using P waves for accurate location of epicentre and efficient emission of S wave warnings

A proposal was made for a new method to estimate the epicentre location of an earthquake using single P wave observation point.
Another method has been put forward for effective simultaneous emission over a large geographical area of S wave warnings based on the monitoring of multiple observation points to detect when prescribed S wave amplitudes are exceeded.

A method has been developed to accurately estimate earthquake epicentres (distance and azimuth of epicentre) within a shorter time. The new method for estimating distance to an epicentre reduces the analysed data length span of an initial P wave from the current 2 seconds to 0.5 seconds. After approximation of the growth of acceleration amplitude with a linear equation, the resulting slope is used to estimate the epicentral distance (Fig. 1). Verification using past earthquake data for 10 365 waveforms demonstrated that this method improved accuracy of estimation of the epicentral distance by 12% (Table 1).

The new method for estimating epicentral azimuth does not employ the present method’s fixed length data span (1.1 seconds) but rather automatically extracts the half wave length of the initial P wave displacement from the observed waveform. This variable length data is then analyzed using principle component analysis (Fig.2).

In addition to the above, another method was developed to produce highly efficient S wave warnings by monitoring multiple observation points which detect S wave amplitudes exceeding the prescribed values (Fig.3). In the case of a very large earthquake, S wave warnings under the existing system were emitted in succession as the S wave progressed through each area covered by an observation point. The new method broadcasts S wave warnings simultaneously across a wide area. This means that the S wave warning can be effectively emitted earlier.

- Fig.1 Approximation function and analyzed data span used for epicentral distance estimation
- Fig.2 variable length data spans (half-wave lengths) used for estimation of epicentral azimuth
- Fig.3 Warning method based on simultaneous monitoring of S wave
  amplitudes in multiple observation points exceeding the prescribed value
- Table 1 Comparative table of accuracy of epicentral distance estimation
- Table 2 Comparative table of accuracy of epicentral azimuth estimation