Rapid Magnitude Determination Using P-wave Arrivals: Magnitude-Period Relations for Japan and the Pacific Northwest

Andrew B. Lockman and Richard M. Allen
University of Wisconsin-Madison

AGU 2003 fall meeting

Recent efforts to mitigate earthquake hazards in Southern California using earthquake early warning demonstrate the utility of using the frequency content of P-wave arrivals to rapidly estimate earthquake magnitude. Studies have shown the P-wave period increases with magnitude, a relationship observed by applying a 10.0 Hz low-pass filter for events with magnitudes between 3.0 and 5.0, and applying a 3.0 Hz low-pass filter for events greater than magnitude 4.5. These findings indicate that P-wave detection and analysis could be used to issue a warning before ground motion begins in areas where population centers and active faults are co-located.

Although the magnitude-period relationship for Southern California has been identified, the limited availability of large magnitude events necessitates analysis of earthquakes in other regions to determine if the relationship is consistent in different geologic settings. Here we present the findings of a study performed using earthquakes in the Pacific Northwest and Japan. Our results show a similar behavior when the same 3.0 Hz and 10.0 Hz filters are applied, suggesting the attenuation characteristics of different regions do not significantly affect the magnitude-period relations. Although the trend observed in Southern California earthquakes remains for the Japan and Pacific Northwest earthquakes, we find that optimal sensitivity to magnitude is achieved by applying different filters for each region. Earthquakes in Japan show the greatest magnitude sensitivity by applying a 3 Hz low-pass filter for low magnitude events, and a 0.3 Hz low-pass filter for large magnitude events. Earthquakes in the Pacific Northwest show the greatest sensitivity by applying a 3.0 Hz low-pass filter for high magnitude events, but relationships that distinguish between events with magnitudes 3.5-5.0 are weak, at best.

Using the P-wave period vs. magnitude relationship that is observed in studying Japan earthquakes, the magnitude of events can be estimated to within 0.44 units using only four seismic stations. This demonstrates the ability to use P-wave detection as a means of mitigating the effects of earthquakes in regions without dense seismic networks.