THREE DESTRUCTIVE MODERATE-SIZED EARTHQUAKES: INSTRUMENTAL AND FIELD OBSERVATIONS FROM THE EASTERN US, NEW ZEALAND AND CHINA

Several recent moderate-size earthquakes have generated anomalously large strong ground motions. We discuss instrumental measurements and seismic intensities of moderate earthquakes in three locations: (1) the 2011 Mw6.2 Christchurch, New Zealand; (2) the 2012 Mw5.8 Mineral, Virgina; and (3) the 2013 Mw6.6 Lushan, China. The Christchurch earthquake caused 185 deaths, and the Lushan earthquake caused 196 deaths; both events caused severe property damage within 15-25 km of the rupture. The high losses can in part be explained by the high values of peak ground acceleration (PGA) and peak ground velocity (PGV) measurements within 50 km of the epicenters. In Christchurch, several accelerometers recorded over 1g acceleration and 90-98 cm/s velocity. The highest reported PGA was 1.4 g at two free-field sites located within 2 km of the rupture on a high-angle reverse fault. In Lushan, the PGA also exceeded 1 g and PGV was 26 cm/s at 10 km from the rupture. A detailed field study after the Lushan earthquake revealed Modified Mercalli Intensities (MMI) of IX within 10 km of the epicenter, decreasing to MMI VI 40 km from the epicenter. Such a rapid decrease in MMI with distance is consistent with California earthquakes such as the 1994 Northridge earthquake. Although instrumental recordings are sparse within 100 km of the 2012 Virginia earthquake, closely spaced (~2 km) recordings of 40 aftershocks provide precise measurements of the attenuation of ground velocity with distance. We find that the ground velocity decreases as R (distance) with the exponent -1.2 in the first 45 km from the epicenter. Using this measurement of ground motion attenuation versus distance, Ground Motion Prediction Equations (GMPEs) allow us to forecast ground motions from a hypothetical, moderate-size (Mw=6.5) Eastern US event that generates near-rupture PGA and PGV values similar to those recorded in Christchurch and Lushan.