S370 Bulletin of the Seismological Society of America, Vol. 94, No. 6B, pp. S370–S383, December 2004 Intermediate-Term Declines in Seismicity at Mt. Wrangell and Mt. Veniaminof Volcanoes, Alaska, following the 3 November 2002 M w 7.9 Denali Fault Earthquake by John J. Sa ´nchez and Stephen R. McNutt Abstract The M w 7.9 Denali fault earthquake ruptured segments of the Susitna Glacier, Denali, and Totschunda faults in central Alaska, providing a unique oppor- tunity to look for intermediate-term (weeks to months) responses of active volcanoes to shaking from a large earthquake. The Alaska Volcano Observatory (AVO) moni- tors 24 volcanoes with seismograph networks. We examined one station per volcano. Digitally-filtered data for the period four weeks before to four weeks after the main- shock were plotted at a standard scale. Mt. Wrangell, the closest volcano to the epicenter (247 km), had a background rate of 16 events/day. For the following 30 days, however, its seismicity rate dropped by 50%. Mt. Veniaminof (1400 km from the epicenter) had a rate of 8 seismic events/day, but suffered a drop in seismicity by 80% after the maishock; this may have lasted for 15 days. The seismicity at both volcanoes is dominated by long-period seismic events. With the exception of Martin and Novarupta volcanoes, the other 20 volcanoes showed no changes in seismicity attributable to the Denali fault earthquake. We conclude that the changes in seismic- ity observed are real, and are related to the Denali fault earthquake. These seismicity drops are in strong contrast to cases of short-term triggered seismicity increases observed at other volcanic systems such as Martin-Novarupta, Mt. Rainier, Yellow- stone, Mammoth Mountain, and The Geysers, Coso and Cerro Prieto (Mexico) geo- thermal fields. This suggests that fundamentally different mechanisms may be acting to modify seismicity at volcanoes. Introduction Earthquake–volcano interactions have been suggested because of reports of eruptions and increases in volcanic seismicity following large regional earthquakes (Linde and Sacks, 1998; Hill et al., 2002). In terms of triggering of seismicity in both volcanic and nonvolcanic areas after dis- tant earthquakes, the M w 7.3 Landers and the M w 7.1 Hector Mine, California, earthquakes settled the question when dis- tinct increases in earthquake rates were reported at locations as far as 1200 km from the epicenter of the Landers earth- quake (Hill et al., 1993), and roughly 800 km in the case of the Hector Mine shock (Gomberg et al., 2001). On 3 No- vember 2002 the M w 7.9 Denali fault earthquake (DFE) rup- tured along the Susitna Glacier, Denali, and Totshunda faults in Alaska and again, reports of triggered seismicity, this time at distances of at least 3800 km from the mainshock, were documented (Eberhart-Phillips et al., 2003; Hill et al., 2003; Hough et al., 2003; Pankow et al., 2003; Husen and Wiemer, 2005; Moran et al., 2005). Thus, at present there is no ques- tion that tectonic earthquakes do affect the activity at some volcanoes, and so far, the reported observations have been of short-lived triggering of earthquakes that start during or immediately following the passage of seismic waves. In broad terms, however, it is of interest to document both decreases and increases to provide constraints on source processes. In this paper we document intermediate-term (weeks to months) decreases in seismicity at Mt. Wrangell (MW) and Mt. Veniaminof (MV) following the DFE se- quence (Fig. 1a). We make detailed comparisons of the rates of seismicity from four weeks before to four weeks after the DFE and perform statistical tests for the significance of the changes. The seismicity rates are estimated by counting the numbers of local earthquakes on filtered and unfiltered seismograms from stations located close to the vents of the volcanoes. The catalog of located earthquakes for MW was also analyzed over a longer time scale. We then discuss the seismicity declines in terms of possible mechanisms, while recognizing that the study of these volcanoes is preliminary.