Realtime determination of earthquake parameters for earthquake early warning

Richard M Allen
University of Wisconsin-Madison

International Workshop on seismic early warning for European cities, held in Napoli from September 23 to 25, 2004

The Elarms methodology for earthquake early warning uses P-arrivals recorded by broadband velocity and strong motion sensors close to the epicenter to both locate the earthquake and estimate its magnitude. Given these parameters, attenuation relations can be used to map the predicted distribution of peak ground shaking across the affected region and traveltime curves provide the onset time. The utility of the warnings are dependent on the accuracy and speed with which the information can be provided.

Earthquake magnitude can be estimated from the predominant period of the first few seconds of the P-wave as was first described by Nakamura [1988]. Using data from over one hundred earthquakes with moment magnitudes ranging from 3.5 to 8.3 and gathered from around the world we find that only 4 seconds of data are required to estimate the magnitude. The dataset includes many events with rupture durations of tens of seconds including the M8.3 Hokkaido, M7.9 Denali and M7.6 Chi-Chi earthquakes, but we still find that we can determine the magnitude within 4 seconds. Using a dataset from southern California we find that the average error in the magnitude estimate is 0.70 magnitude units when the predominant period from one station is used but this error drops to 0.48 and 0.45 when three and four stations provide data. Although it is possible to issue a warning based on a single P-arrival, it is clearly preferable to have several recoded P-arrivals before a warning is issued to both reduce the error in the magnitude and also prevent warnings based on noise spikes. The arrival times of the same P-waves provide for accurate locations and origin times. Given earthquake magnitude and location, attenuation relations can be used to estimate the distribution of various ground shaking parameters including PGA and PGV across a region.