Investigation on relationship between epicentral distance and growth curve of initial P-wave propagating in local heterogeneous media for earthquake early warning system

Abstract

In the earthquake early warning (EEW) system, the epicenter location and magnitude of earthquakes are estimated using the amplitude growth rate of initial P-waves. It has been empirically pointed out that the growth rate becomes smaller as epicentral distance becomes far regardless of the magnitude of earthquakes. So, the epicentral distance can be estimated from the growth rate using this empirical relationship. However, the growth rates calculated from different earthquakes at the same epicentral distance mark considerably different values from each other. Sometimes the growth rates of earthquakes having the same epicentral distance vary by 10 4 times. Qualitatively, it has been considered that the gap in the growth rates is due to differences in the local heterogeneities that the P-waves propagate through. In this study, we demonstrate theoretically how local heterogeneities in the subsurface disturb the relationship between the growth rate and the epicentral distance. Firstly, we calculate seismic scattered waves in a heterogeneous medium. First-ordered PP, PS, SP, and SS scatterings are considered. The correlation distance of the heterogeneities and fractional fluctuation of elastic parameters control the heterogeneous conditions for the calculation. From the synthesized waves, the growth rate of the initial P-wave is obtained. As a result, we find that a parameter (in this study, correlation distance) controlling heterogeneities plays a key role in the magnitude of the fluctuation of the growth rate. Then, we calculate the regional correlation distances in Japan that can account for the fluctuation of the growth rate of real earthquakes from 1997 to 2011 observed by K-NET and KiK-net. As a result, the spatial distribution of the correlation distance shows locality. So, it is revealed that the growth rates fluctuate according to the locality. When this local fluctuation is taken into account, the accuracy of the estimation of epicentral distances from initial P-waves can improve, which will in turn improve the accuracy of the EEW system. Findings

Triggering processes of microseismic events associated with water injection in Okuaizu Geothermal Field, Japan

“…P codas can be synthesized from the theoretical power spectral densities for PP, PS, and SP conversion. By fitting theoretical waves to the observed waves, the parameter ε can be estimated [details appear in Okamoto and Tsuno (2015)]. In this study, we assumed that ε = V P inc. /V P back , and we estimated the changes in V P inc. associated with the injection.…”

Prediction of maximum P- and S-wave amplitude distributions incorporating frequency- and distance-dependent characteristics of the observed apparent radiation patterns

“…Although the frequency-distance change model for S-wave radiation pattern proposed by Satoh (2002b) has been used in some applications, to achieve more accurate estimation and prediction of high-frequency seismic radiation, a precise frequency-and distance-dependent model for the apparent radiation pattern for both P and S waves is required. Relationship of the apparent radiation patterns between P and S waves has been important due to recent development real-time systems, such as urgent Open Access *Correspondence: shunsuke@bosai.go.jp 1 National Research Institute for Earth Science and Disaster Resilience, 3-1 Tennodai, Tsukuba 305-0006, Japan Full list of author information is available at the end of the article earthquake detection and earthquake early warning (e.g., Okamoto and Tsuno 2015).…”

“…The frequency-dependent characteristics of the observed apparent radiation patterns have been incorporated into various applications such as the predictions of strong ground motions (e.g., Pitarka et al 2000;Pulido and Kubo 2004), the estimation of high-frequency seismic energy radiation during large earthquakes (e.g., Nakahara 2013), nonvolcanic/volcanic tremors (e.g., Maeda and Obara 2009;Kumagai et al 2010;Cannata et al 2013;, and landslides (e.g., Ogiso and Yomogida 2015), and the earthquake early warning systems (e.g., Okamoto and Tsuno 2015). Although the frequency-distance change model for S-wave radiation pattern proposed by Satoh (2002b) has been used in some applications, to achieve more accurate estimation and prediction of high-frequency seismic radiation, a precise frequency-and distance-dependent model for the apparent radiation pattern for both P and S waves is required.…”

<b>Improved Epicentral Distance Estimations for Railway Earthquake Early Warning</b>

“…Since 2003, the JR EEW systems have estimated epicentral distance, Δ, using an empirical relationship, determined from recorded data, between the measured epicentral distance of earthquakes from single seismic stations and a parameter describing the amplitude growth rate of some period of the initial P-wave [3]. As epicentral distance increases, the amplitude growth rate decreases due to geometrical scattering and seismic wave attenuation [6]. Provided the calculation time period is short, the relationship appears to be independent of magnitude [7].…”

“…Provided the calculation time period is short, the relationship appears to be independent of magnitude [7]. The two main approaches are commonly referred to as the B-Δ [2,3,6,8,9] and C-Δ methods [10,11,12] where B and C refer to amplitude growth rate parameters. Poor Δ estimation may cause missed alarms that waste lead time and increase the risk of damage, or false alarms that unduly disrupt train operation.…”