Pd is the peak amplitude of displacement in the first three seconds after the arrival of the P wave. We investigated the attenuation of Pd with the hypocentral distance R in southern California as a function of magnitude M, and obtained the following relationship: log (P_d) = −3.463 + 0.729 × M − 1.374 × log (R) ± 0.305. Given an earthquake location determined by the P‐wave arrival times at stations close to the epicenter, this relationship can be used to define a so‐called “Pd magnitude” of earthquakes. Our result shows that for earthquakes in southern California the Pd magnitudes agree with the catalog magnitudes with a standard deviation of 0.18 for events less than magnitude 6.5. Therefore, Pd is a robust measurement for estimating the magnitudes of earthquakes and has practical application in earthquake early warning systems.

To rapidly assess the potential for damage of an earthquake for purposes of earthquake early warning in Taiwan, we used the peak displacement and velocity amplitudes of the first 3 sec of the P wave. The vertical-component records, high- pass filtered at 0.075 Hz, are used. We found that the peak initial-displacement amplitude (Pd) correlates well with the peak ground-motion displacement (PGD) and the peak ground-motion velocity (PGV) at the same site. When Pd 0.5 cm, the event is most likely damaging. If Pd is combined with the period parameter sc, determined in an earlier study, then sc Pd provides an even more robust parameter for assessing the potential for damage.

Summary. Most earthquake early warning systems (EEWS) developed so far are conceived as either ‘regionaL‘ (network-based) or ‘on-site’ (stand-alone) systems. T