Earthquake Magnitude Estimation Prior to Rupture Termination

Erik L Olson and Richard M Allen
University of Wisconsin-Madison

AGU Fall Meeting 2004

An estimate of earthquake moment magnitude is possible within 5 seconds of earthquake initiation for events of every magnitude, including those events which rupture for several 10’s of seconds. The magnitude estimate is calculated based on the empirical relationship between the predominant period of the P-wave and moment magnitude. The predominant period is calculated from the frequency content of the data on a moving average basis for the first five seconds after the trigger. The empirically observed relationship is based upon a data set consisting of more than one hundred events with moment magnitude 3.5 – 8.3, thirty events have a moment magnitude of 6.0 or greater. The magnitude estimates for several well known large magnitude events, including Chi Chi (Taiwan, 1999), Hokkaido (Japan, 2003) and Denali (Alaska, 2002), are all calculated within 5 seconds of the P-wave trigger. These events lasted for tens of seconds and the fault rupture extended 10’s – 100’s of kilometers, yet in each case, the magnitude can be calculated long before the rupture has propagated over the entire surface of the faults. Additionally, for large magnitude events, such as the Chi Chi earthquake mentioned above, less than 5% of the seismic moment is released within the 5 seconds required to measure the predominant period.