The τc-Pd Onsite algorithm belongs to the group of single-sensor approaches to EEW (Kanamori, 2005). As a matter of principle, this type of warning approach is quicker in detecting and processing of earthquakes, but is expected to be less reliable compared to regional warning algorithms that are based on observations at multiple seismic sensors.
The τc-Pd Onsite algorithm uses the period τc and amplitude Pd of initial shaking to estimate the size and forthcoming shaking in an earthquake (Kanamori, 2005). Wu et al.(2007) systematically studied the off-line records from earthquakes in southern California to determine a log(τc)-Mw scaling relation for EEW. Along with the Virtual Seismologist (Cua and Heaton, 2007) and ElarmS algorithms (Allen, 2007), τc-Pd Onsite is one of the three EEW algorithms contributing to CISN ShakeAlert (Böse et al., in prep.).
Our real-time tests of the τc-Pd Onsite algorithm in California over the past five years (Böse et al., 2009a) have shown that some modifications are necessary to decrease the number of false triggers and thus to increase the robustness of the algorithm, in particular in real-time mode during small to moderate-sized earthquakes (Böse et al., 2009b; Böse et al., in prep.). We are also developing alternative algorithms for more rapid and more robust onsite warning (Böse et al., 2012a).
Figure 1. Distribution of observed and predicted values of peak ground velocity (PGV) at 60 CISN stations during the July 29 2008 Mw5.4 Chino Hills earthquake (Böse et al., 2009a).
Maren Böse, Egill Hauksson, Tom Heaton