Microearthquake study of Mount Katmai and vicinity, Alaska

A seismic investigation was made during the summer of 1965 in Katmai National Monument, Alaska. During 39 days of nearly continuous recording with a high-gain high-frequency tripartite array of seismometers, 1800 events were recorded. The majority of events were shallower than 10 km, although some occurred at depths up to 150 km. Most of the events were associated with the volcanic axis that trends northeast-southwest along the Alaska Peninsula. Although there are fifteen volcanic vents in the Katmai area, historic volcanic activity, ranging from minor steaming to major explosive eruptions has been limited to seven peaks. Seismic activity was generally greatest near the more recently active volcanos and was quite low near the volcanos with no record of historic eruptions. Two exceptions to this general tendency, however, were near Snowy and Fourpeaked Mountains. Most of the events in these two areas were attributed to short-term increases of seismicity in aftershock or swarm-type sequences. The b value in a log-log plot of frequency versus amplitude for all shocks was 1.4, which is intermediate between that of tectonic earthquakes (b = 0.5 to 1.0) and that of volcanic earthquakes (b = 2 to 3). This b value plus the nature of the earthquakes observed imply a mixture of tectonic and volcanic shocks in a highly heterogeneous structure with locally concentrated stresses. A comparison of seismograms recorded at Brooks, Overlook, and Katmai Canyon shows a disappearance of the S phase along a path crossing the volcanic range. A study of specific events suggests this is due to a shadowing effect by a magma chamber rather than to a source mechanism. A slight increase of seismicity was observed in early August near Mounts Trident and Martin, coinciding approximately with evidence for a minor eruption in that region. Detailed and confirming evidence of an eruption was limited because of remoteness and unfavorable weather conditions. A normalized count of seismic events in early August indicates that an increase of seismicity associated with a major eruption could be comparable to that observed at other volcanos of the explosive type and suggests that continuous seismic monitoring of the Katmai area would be of considerable value.