Rupture directivity of the 3 November 2002 Denali Fault earthquake determined from surface waves

The M (sub w) 7.9 earthquake that struck central Alaska on 3 November 2002 was preceded 11 days earlier by an M (sub w) 6.7 strike-slip foreshock on 23 October 2002. Both events were predominantly strike-slip and ruptured structures associated with the Denali fault system. Previous studies have shown that the mainshock began with failure on a relatively small northeast-striking reverse fault, before breaking out for 300 km of right-lateral strike-slip rupture. Aftershock patterns suggest that the foreshock ruptured a region west of the mainshock, which began near the eastern extent of the foreshock sequence and proceeded east-southeast. To constrain and to quantify source duration and directivity effects, we examine surface-wave displacement seismograms and use an empirical Green's function (EGF) to isolate and explore mainshock rupture kinematics. Our particular interest lies in large-amplitude focussing caused by directivity. We observe Love and Rayleigh wave amplification of two orders of magnitude in the period range from 10 to 33 sec. These remarkable directivity-enhanced surface waves triggered small earthquakes more than 3000 km from the mainshock rupture.