- Akutan Swarm Info
- AVO Updates and Information Releases
- Prognosis and Summary of Volcanic Hazards
- Special thanks to the Akutan School
At about 7:30 pm Alaska Standard Time (AST) on Sunday March 10, residents of Akutan began feeling earthquakes. A very high frequency of earthquakes continued through the night and the following day. Students of the Akutan school carefully counted the frequency and intensity of the earthquakes during the day on Monday. The resulting rate information was the first quantitative data about the earthquakes available and suggested that this was an earthquake swarm rather than a classic mainshock - aftershock sequence. The strongest shocks rattled small objects on tables and caused some cabinet doors to open - ground shaking was continuous. The largest of the earthquakes had a magnitude of about 5.1, and there were several with magnitudes between 4 and 5 -- most of them were probably in the M2.5 to M4 range. AVO learned of the seismic unrest monday morning and began organizing a response. There were no operating seismometers on Akutan Island or in the immediate vicinity at the onset of seismic unrest -- the nearest seismometer was at Sand Point, about 380 kilometers to the northeast. The intense seismic activity began subsiding at about 5 pm AST on Monday (3/11) and remained at a level which, although considerably less than that of Sunday night and Monday, was substantially above normal. Late Tuesday evening an AVO seismologist reached Akutan with a seismometer and a portable recording system. He determined that all earthquakes were of a type termed "volcano-tectonic" which indicates that they originated by rock breakage (faulting) rather than surging of gas or magma in open cracks. At about 5 pm AST on Wednesday, March 13, earthquake activity rapidly increased again and continued through the night. This earthquake swarm was more intense than the previous one. Felt earthquakes occurred more than once per minute and the strongest knocked small items off tables. Ground shaking was again continuous. There were a small handful of earthquakes with magnitude greater than or equal to 5, and more between 4 and 5. By this time the seismic station at Unalaska (45 km distant) was partially working (the GPS clock, which allows precise timing, and thus use of data in locating earthquakes, was not working). This swarm began subsiding about 18 hours after onset. On Friday, March 15, the clock on the Unalaska station was repaired and additional personnel and seismic monitoring equipment were able to reach Akutan. By 2:30 pm Saturday a dedicated telephone line was installed, and by about 4:45 pm seismic data was being transmitted to Fairbanks and Anchorage offices. Additional seismic stations were installed on Sunday and some additional data was beginning to be brought back Monday morning. By Friday, March 22nd a total of 5 seismic stations in 4 locations had been installed and all data were being brought into the Fairbanks and Anchorage laboratories in real time. Maximum separation of the stations is about 9 km. Four of the stations are located around Akutan Harbor, the fifth is on the eastern slopes of the volcano about midway between the city of Akutan and the summit of the volcano. The intention is to leave this seismic array in its present geometry until additional stations can be placed by helicopter this summer. By Sunday, March 24, all AVO personnel had left Akutan. Around-the-clock monitoring using the new seismic stations is being conducted from AVO offices.
