A Fast, Non-saturating Magnitude Estimator for Great Earthquakes

Abstract

The concept of a body-wave magnitude was developed by Gutenberg (1945a, 1945b) using intermediate to long-period displacement recordings of earthquakes with magnitudes 6 and above. In contrast to the commonly determined short-period P -wave magnitude m b that already saturates at values between about 6 and 7, m B yields reliable magnitude estimates up to magnitudes of about 8 (Abe 1981; Kanamori 1983; Bormann and Saul 2008). Since Gutenberg proposed to measure the maximum ratio of displacement amplitude A divided by its period T, m B is related to ground motion velocity and thus energy. This allowed Gutenberg and Richter (1956) to propose a semi-empirical relationship between m B and the released seismic energy ES based on the equation for a wave group from a point source. IASPEI (2005) recommended reintroducing m B into global seismological practice and proposed measurement standards. m B should preferably be determined by directly measuring the maximum P -wave ground-motion velocity amplitudes on modern digital broadband records and calibrating them with the QPV (Δ, h ) values published by Gutenberg and Richter (1956) for vertical component P waves (where Δ is the epicentral distance and h the source depth). This new type of standard m B is termed m B(BB). Meanwhile, an automatic online procedure for m B(BB) determination has been developed, tested, and implemented in the operational prototype of the German Indonesian Tsunami Early Warning System (GITEWS; http://www.gitews.org/; Bormann and Saul 2009). When applied only to records at Δ > 20°, as recommended by the IASPEI standard procedure, it yields stable m B(BB) estimates within eight to 15 minutes after origin time (OT), depending on station availability (Bormann and Saul 2008). However, alarm times can be reduced to about five minutes by making use of a …