Lg Body-Wave Magnitude Scaling for the Continental Margin around Korea and Japan

Abstract

Regional body-wave magnitude scalings are essential for quantification of small and moderate-size earthquakes that are observed only up to regional distances. Crustally-guided shear waves, Lg, develop stably at regional distances in continental crusts and are minimally influenced by the source radiation patterns. Lg body-wave magnitude scalings, \(m_b(Lg),\) are widely used for assessment of sizes of regional crustal events. The \(m_b(Lg)\) scaling has rarely been tested in continental margins where Lg waves are significantly attenuated due to abrupt lateral variation of crustal structures. We test the applicability of \(m_b(Lg)\) scaling to the eastern margin of the Eurasian plate around the Korean Peninsula and Japanese islands. Both third-peak and root-mean-square (rms) amplitudes of Lg vary significantly according to the crustal structures along raypaths, causing apparent underestimation of \(m_b(Lg).\) Implementation of raypath-dependent quality factors (Q) allows accurate estimation of \(m_b(Lg),\) retaining the transportability of \(m_b(Lg)\) in the continental margin around Korea and Japan. The calibration constants for an rms-amplitude-based \(m_b(Lg)\) scaling are not determined to vary by region in the continental margin due to complicated crustal structures. The calibration constants are determined to be distance-dependent. Both the third-peak-amplitude-based and rms-amplitude-based \(m_b(Lg)\) scalings yield accurate magnitude estimates when raypath-dependent quality factors are implemented.