Seismicity at the volcanoes of Katmai National Park, Alaska; July 1995–December 1997

Upper-crustal seismicity located within Katmai National Park occurs mainly within four distinct clusters originating near Martin–Mageik (MM subnet) volcanoes, Trident volcano, Katmai caldera and Snowy volcano. Analyses of earthquake frequency–magnitude distributions reveal high b-values beneath MM subnet (∼1.5) normal b-values at Trident volcano (∼1.0) and intermediate b-values at Katmai caldera (∼1.3) for all seismicity occurring between July 1995 and December 1997. Detailed analyses of subsets of b-values and hypocenter locations at MM subnet reveal a temporal increase in b-value associated with an increase in the maximum depth of seismicity. The changes occurred during a shallow earthquake swarm beneath Mageik volcano between October 16 and 25, 1996 and again in November–December 1997. Before the swarm, the weighted least squares b-value was 1.01 at MM subnet, increased to 1.59 during the swarm and remained anomalously high until April 1997. The corresponding maximum depth of seismicity is generally less than 5 km for well located earthquakes occurring after September 18, 1996, but shifted to ∼10 km during both the October 1996 swarm and the November–December 1997 period. The November–December 1997 event is not associated with an increase in the rate of seismicity or the b-value. The October 1996 swarm had a cumulative moment release of 5.0×1020 dyn-cm, and decayed from a peak rate of 110 events per day with a modified Omori law p-value of 1.06±0.11. Modelling by the flow law with a 10 km depth limit for seismicity suggests that temperature gradients are on the order of 20–40°C km−1 in agreement with the p-value estimate. We infer that a simple pressurizing intrusion was not associated with the October swarm because higher stresses should increase seismicity and lower the b-value, the opposite of what we observed. Alternatively, an actively degassing intrusion would reduce the effective stress and increase the b-value at shallow depths while the increased stress would induce seismicity at depth. Surface temperature measurements taken at the Mageik crater lake in July 1995, and again in July 1997, revealed an increase of about 10°C in the lake water temperature, consistent with a degassing intrusive event