Guy Tytgat

Aftershocks of the 13 May 1993 Shumagin Alaska earthquake

The 13 May 1993 Ms 6.9 Shumagin earthquake had an aftershock sequence of 247 earthquakes with magnitudes greater than or equal to 1.5 by 1 June 1993. Of these aftershocks, 79 were located by using S-P travel times at the only two stations within 570 km of the mainshock epicenter. The rupture area inferred from the aftershocks is about 600 km2 and we estimate for the mainshock a mean fault displacement of 1.0 m and a 28 bar stress drop. The magnitude-frequency plots give a b-value for the aftershock sequence of about 0.4, which is low compared to the background value of approximately 0.8. The decay of the aftershock sequence followed the modified Omori law with a p-value of 0.79, which is also lower than the typical values of about 1.1 observed in Alaska. Both of these facts can be interpreted as indicating relatively high ambient stress in the Shumagin seismic gap and the possibility that the 13 May earthquake was a foreshock to a larger gap-filling event to occur within the next few years.

Stress directions along the Alaska Wadati-Benioff zone from inversion of focal mechanism data

Abstract The focal mechanisms of 137 intermediate depth (60–180 km) earthquakes in the Alaska Wadati-Benioff zone (WBZ) were derived from P-wave first-motions. The study focused on three earthquake clusters referred to as Iliamna, Talkeetna, and Denali. In all three clusters, the orientations of the principal stresses (pressure (P), null (B), and tension (T) axes) are similar. The P axes for most of the events show NNE-SSW trends with shallow plunge conforming to the geometry of the WBZ in the study area. The rest of the events yielded E-W oriented P axes. The T axis for most of the events plunge at a steeper angle than P axes, but parallel the dip of the WBZ. We interpret this feature as indicting the predominance of gravitational sinking of the associated underthrust lithospheric plate. The regional stress direction was estimated from the focal mechanisms using the inversion method of Gephart and Forsyth (1984). The results show that the maximum regional stress in the 60–90 km depth range plunges shallowly in the Iliamna (14°) and Talkeetna (5°) areas, and approximately in the direction of convergence (N163°E) of the Pacific and North American plates. In this depth range, similar results for the Denali area could not be obtained due to a lack of data. However, the inversion of the focal mechanism data for all three clusters for depths greater than 100 km yielded large values for both misfit and 95 percent confidence area compared with those in the depth range of 60–90 km. This may indicate the stress directions for depths greater than 100 km along the Alaska WBZ are more heterogeneous than in the depth range of 60–90 km.

Infrasonic wave observations at Fairbanks, AK, of Augustine volcano eruptions during January 2006

The recent Augustine eruptions, from the 11th to the 28th of January 2006, have produced a series of ten infrasonic signals observed at the I53US array. The eruption times for the signals were provided by the Alaska Volcanic Observatory at UAF using a Chaparral microphone present on Augustine Island a few kilometers from the crater. The bearing and distance of Augustine from I53US are respectively 207.8 deg and 675 km. The analysis of the signals is done with a least‐squares detector/estimator that calculates, among other things, the horizontal trace‐velocity and the azimuth of arrival of the signal. The average values of the trace‐velocity and azimuth for all ten Augustine signals are 0.336±0.0136 km/s and 208.7±1.5 deg. respectively. The celerity for each signal was calculated using the range 675 km and the individual travel times to I53US. The average celerity for all ten eruption signals was 0.287±0.018 km/s. Ray tracing studies, using wind speed and temperature profiles supplied by Dr. Doug Drob at N...