3. The ElarmS Earthquake Early Warning Methodology and Application across California

Earthquake Alarms Systems, ElarmS, is a methodology for providing warning of forthcoming ground shaking during earthquakes. The approach uses a network of seismic instruments to detect the first-arriving energy at the surface, the P-waves, and translate the information contained in these low amplitude waves into a prediction of the peak ground shaking that follows. The instruments closest to the epicenter are the first to detect the seismic energy, and by using a seismic network this information can be integrated to produce a map of future ground shaking everywhere. The ElarmS methodology uses the frequency content of the P-wave arrival to estimate earthquake magnitude, arrival times to determine location, and then predicts the ground shaking using a radial attenuation relation. All data is gathered continuously and the hazard map updated every second. As observations of peak ground shaking are also made close to the epicenter they are integrated into the hazard assessment. Here, the methodology is applied to a set of 32 earthquakes in southern California to assess the accuracy and timeliness of warning if such a system was implemented using the existing seismic network. If there was no data telemetry delays the first warning would be available before the S-wave arrival at the epicenter for 56% of the earthquakes. The average absolute magnitude error at this time is 0.44 units and the error in the average absolute peak ground acceleration [ln(PGApredicted) – ln(PGAobserved)] is 1.08. Within 5 sec warning are available for 97% of the events, the average magnitude error is 0.33 units, and the average PGA error is 1.00. To further assess the utility of ElarmS implementation in California, probabilistic warning time distribution functions are determined for cities in northern California. Using the set of future likely earthquakes provided by the Working Group on California Earthquake Probabilities (2003) the warning times that the ElarmS methodology could provide (if implemented) can be estimated, and a probability of occurrence associated. The alarm time is defined as the time when 4 sec of P-wave data is available at 4 seismic stations. At this point in time the average magnitude error is 0.5 units. The warning times range from zero seconds to over a minute, the most likely warning times range from seconds to a few tens of seconds depending on location. The largest magnitude earthquakes are also associated with the greatest warning times and it is more likely than not, that San Francisco would receive more than 20 sec warning for earthquakes generating the most damaging ground shaking. 3 The ElarmS Earthquake Early Warning Methodology and Application across California