Accuracy improvement in the estimation of epicentral distance inferred from initial P-waves by considering local heterogeneity

ABSTRACT

Epicentral distance can be estimated from growth of the initial P-waves in real time, and this estimation is used in the earthquake early-warning system in Japan, especially for railways. This estimation is based on the relationship between the growing initial P-waves and epicentral distance. The initial P-waves grow gradually with increasing distance from the epicentre because geometrical spreading, scattering and intrinsic attenuation blunt their shape. Therefore, the growing form of the initial P-waves can be used to estimate the epicentral distance. However, in practice, the relationship is not uniform and the growth curves (i.e. gradient) are not a perfect function of epicentral distance. Therefore, the epicentral distance estimated from the initial P-waves involves significant uncertainty, which decreases the accuracy of the early-warning system for strong motions. In this study, we examined the causes of error in this relationship using theoretical calculations based on the Born approximation, which indicate that the growth curves of the P-waves fluctuate due to regionally different heterogeneous conditions in the subsurface medium. Therefore, we propose a robust relationship between the initial P-waves and epicentral distance considering regional heterogeneous conditions by analysing real earthquakes (magnitude 4–5) in Japan.

KEYWORDS:

• Seismology
• heterogeneous
• earthquake
• scattering
• wave propagation

Acknowledgements

We thank the National Research Institute for Earth Science and Disaster Prevention (NIED) for providing K-NET and KiK-net data and the Japan Meteorological Agency (JMA) for providing the JMA unified earthquake catalogue. We thank Dr. Hiroshi Asanuma for valuable discussions which improved our study and manuscript. Some of the figures were drawn using the GMT software.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Kyosuke Okamoto http://orcid.org/0000-0002-0400-4194