Scott D. Stihler

Earthquake Triggering at Alaskan Volcanoes Following the 3 November 2002 Denali Fault Earthquake

The 3 November 2002 M W 7.9 Denali fault earthquake provided an excellent opportunity to investigate triggered earthquakes at Alaskan volcanoes. The Alaska Volcano Observatory operates short-period seismic networks on 24 historically active volcanoes in Alaska, 247–2159 km distant from the mainshock epicenter. We searched for evidence of triggered seismicity by examining the unfiltered waveforms for all stations in each volcano network for ∼1 hr after the M W 7.9 arrival time at each network and for significant increases in located earthquakes in the hours after the mainshock. We found compelling evidence for triggering only at the Katmai volcanic cluster (kvc, 720–755 km southwest of the epicenter), where small earthquakes with distinct P and S arrivals appeared within the mainshock coda at one station and a small increase in located earthquakes occurred for several hours after the mainshock. Peak dynamic stresses of ∼0.1 MPa at Augustine Volcano (560 km southwest of the epicenter) are significantly lower than those recorded in Yellowstone and Utah (>3000 km southeast of the epicenter), suggesting that strong directivity effects were at least partly responsible for the lack of triggering at Alaskan volcanoes. We describe other incidents of earthquake-induced triggering in the kvc, and outline a qualitative magnitude/distance-dependent triggering threshold. We argue that triggering results from the perturbation of magmatic-hydrothermal systems in the kvc and suggest that the comparative lack of triggering at other Alaskan volcanoes could be a result of differences in the nature of magmatic-hydrothermal systems.

Varve counting vs. tephrochronology and 137Cs and 210Pb dating: A comparative test at Skilak Lake, Alaska

The age of recently deposited sediments in Skilak Lake has previously been estimated only by counting [open quotes]varves[close quotes]. We measured the sedimentation rate of Skilak Lake using two radionuclides, [sup 137]Cs and [sup 210]Pb, and found it to be about an order of magnitude lower than previous estimates based on varve counts. We also identified several tephras through a combination of visual inspection, core X-radiographs, observation of variations of the magnetic susceptibility, intensity of magnetization of the unconsolidated sediments, and microprobe analyses of volcanic glass shards. Tephrochronologic dates using matches with the Katmai 1912 tephra and an Augustine tephra from 500-550 yr B.P. are in good agreement with the sedimentation rate estimates based on the radioisotope data, and indicate that the rhythmite layers at Skilak Lake are not annual varves. These new estimates of sedimentation rate reaffirm that care is needed in varve dating and require that earlier work on sunspot and climate changes thought to have been recorded in the Skilak Lake sediments be reevaluated. 16 refs., 6 figs., 1 tab.

Seismic Response of the Katmai Volcanoes to the 6 December 1999 Magnitude 7.0 Karluk Lake Earthquake, Alaska

A sudden increase in earthquake activity was observed beneath volcanoes in the Katmai area on the Alaska Peninsula immediately following the 6 December 1999 magnitude ( M W) 7.0 Karluk Lake earthquake beneath southern Kodiak Island, Alaska. The observed increase in earthquake activity consisted of small ( M L < 1.3), shallow ( Z < 5.0 km) events. These earthquakes were located beneath Mount Martin, Mount Mageik, Trident Volcano, and the Katmai caldera and began within the coda of the Karluk Lake mainshock. All of these earthquakes occurred in areas and magnitude ranges that are typical for the background seismicity observed in the Katmai area. Seismicity rates returned to background levels 8 to 13 hours after the Karluk Lake mainshock. The close temporal relationship with the Karluk Lake mainshock, the onset of activity within the mainshock coda, and the simultaneous increase beneath four separate volcanic centers all suggest these earthquakes were remotely triggered. Modeling of the Coulomb stress changes from the mainshock for optimally oriented faults suggests negligible change in static stress beneath the Katmai volcanoes. This result favors models that involve dynamic stresses as the mechanism for triggered seismicity at Katmai.

A 500-year-long record of tephra falls from Redoubt Volcano and other volcanoes in upper Cook Inlet, Alaska

Abstract Volcanic ash layers preserved in glacial-lacustrine sediments at Skilak Lake on the Kenai Peninsula of southcentral Alaska constitute a record of eruptions at Redoubt Volcano and other Alaskan volcanoes which affected the upper Cook Inlet area during the last 500 years. High-resolution magnetic susceptibility profiling delineates similar sequences of tephra layers in several 1-m-long lake sediment cores. Electron microprobe analyses of glass shards from the tephras indicate correlation of some ash layers with known reference tephras from the source volcanoes, while other ash layers record previously unknown prehistoric eruptions. Skilak Lake cores contain ash from the historic 1912 Katmai eruption, the 1902 Redoubt eruption, and the 1883 Mount St. Augustine eruption as well as prehistoric ash layers erupted from Crater Peak at Mt. Spurr ca. 250–350 years ago, from Redoubt Volcano at ca. 300–400 years ago and again at ca. 350–450 years ago, and a 500-year-old ash from Mount St. Augustine. Still older tephras from Redoubt Volcano and Crater Peak at Mt. Spurr are found lower in the cores. The cores indicate that volcanoes in the Cook Inlet area have erupted every 10–35 years during the 20th century, and ash falls have occurred at Skilak Lake at least once every 50–100 years for the last 500 years, with Redoubt, Spurr, and Augustine Volcanoes being the most important sources of tephra.