The Kodiak Island, Alaska Mw 7 Earthquake of 6 December 1999

Abstract

A Mw 7 earthquake occurred on 6 December 1999 at 2:12 pm AKST in the Kodiak Island region of Alaska. This event was felt strongly in the towns of Kodiak and Old Harbor as well as surrounding communities. It caused some minor damage, including power and phone outages in Kodiak. Felt reports were received from as far as Fairbanks, 900 km away. The earthquake was located by the Alaska Earthquake Information Center (AEIC) at 57.51°N and 154.67°W at 45.8 km depth. Aftershocks of ML 5.4 and Mw 6.4 followed the main shock by ten minutes and one hour, respectively. Aftershock activity decreased rapidly from about twenty-five events per hour in the first few hours after the main shock to five events a day ten days later. It appears that the main shock triggered a swarm of earthquakes in the Katmai volcano field which subsided in a few hours (Power et al. , 2000). Figure 1. Map of the study area and surrounding regions. Rupture zones of the 1964 ( Mw 9.2) and 1938 ( Mw 8.2) earthquakes, major faults, and the Aleutian trench are shown. Open triangles are seismic stations used in the earthquake relocations. Solid diamonds are active volcanoes. Convergence of the Pacific and North American plates dominates the tectonic framework of the Kodiak region (Figure 1). The plate boundary lies along the Aleutian trench about 100 km seaward of Kodiak Island. A typical volcanic arc accompanying subduction is located on the Alaska Peninsula and the west coast of Cook Inlet. Most of the seismic energy in southern Alaska is released in major earthquakes that rupture the shallow part of the megathrust. The Great 1964 Prince William Sound earthquake ( Mw 9.2) ruptured an 800-km-long portion of the megathrust including the Kodiak Island segment (Christensen and Beck, …