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The current eruption of Augustine Volcano, which forms the bulk of Augustine Island, began in early December 2005. Observers saw vigorous steaming from the volcano's summit, and residents of coastal communities 80 to 120 km (50-75 mi) away reported strong sulfurous odors. High-intensity, high-frequency seismic signals recorded December 1-17 are now interpreted as signs of forceful emissions of steam and other gases from the volcano, which is commonly obscured from view by darkness and cloudy weather. The difficulty of seeing Augustine Volcano means that monitoring with seismometers, which sense earthquakes caused by magma and other fluids moving beneath and within the volcano, is sometimes the only way to detect and record eruptive activity. In early February, we assembled in Homer to deploy ocean-bottom seismometers as supplements to AVO's seismic network on the island.
Nature makes its presence known in Alaska. On our first morning in Homer, on February 5, we were woken up by what felt like a violent explosion. At first we thought the volcano had exploded, but the cause turned out to be a magnitude 5.3 earthquake located almost directly under the town and unrelated to the current volcanic activity on Augustine.
The weather along the south coast of Alaska is stormy this time of the year and is constantly changing. The deployment was postponed until a relatively calm weather window presented itself on February 8. Visibility was unlimited when we left port at 9 a.m., and snow-covered mountains glistened in the rising sun. A following sea with waves of only a few feet made our 3-hour transit to Augustine Volcano at a speed of 26 knots quite enjoyable. Augustine was in full view, with steam rising from its peak, and flanks covered with ash and fresh pyroclastic flows.
We deployed the ocean-bottom seismometers in a counterclockwise direction, beginning in the lee of the weather. At the first site, in a channel northwest of the island, we lowered a bucket attached to an anchor to test bottom conditions. Overly soft bottom materials, such as deposits of volcanic ash, can complicate the release of the instrument from its anchor. To our relief, no sediment was recovered by the bucket.
After deploying the first instrument, we moved in a counterclockwise direction around the island to deploy the other four instruments. Each site required about 15 minutes to carry out final instrument prep, deploy the seismometer overboard, and test acoustic communication with the device. Transit times between sites ranged from 20 to 70 minutes, depending on sea conditions. Ice floes were encountered southwest of Augustine and slowed the transit, because the cutter's skin can easily be dented by ice. As the afternoon wore on, clouds began to cover the volcano above 450-m (1,500 ft) elevation, and waves built up. Freezing sea spray started accumulating on deck by the time we deployed the last instrument at 5 p.m. The cutter could no longer keep station long enough for us to lower a transducer to test acoustic communication with the seismometer on the bottom, and so we omitted this step for the last instrument. The trip back to Homer took 4.5 hours against rising seas. A snow blizzard started after we arrived safely in port.
Augustine Volcano is part of the Aleutian Arc, a curving line of volcanoes that extends from the Alaska Range westward to Russia's Kamchatka Peninsula. Like many Aleutian Arc volcanoes, Augustine is on a small island (approx 13 km east-west by 11 km north-south) that greatly restricts placement of AVO's onland seismometers. The constricted geometry of the onland seismic network limits the accuracy with which it can be used to locate the hypocenters, or points of origin, of volcano-induced earthquakes. Each seismometer records the time when energy from an earthquake reaches it, and the hypocenter is located through an iterative process of modeling a likely location, comparing the arrival times predicted by the model with the arrival times recorded at each station, and refining the estimated location until one is found that provides the best match between predicted and recorded arrival times. For greatest accuracy, the seismic network needs to have a much larger spread, or aperture, than the depth of the earthquake. (The aperture is the distance across the network, similar to the aperture of a telescope, which is the diameter of its lens.) The small ring of seismic stations that can be set up on Augustine Island works well for locating shallow (less than 5 km deep) earthquake hypocenters but is not accurate for deeper earthquakes, thus precluding the detection and tracking of volcano-induced seismicity in the middle to lower crust. Seismometers set up across the water from the island, on the shores of Cook Inlet, would be in a good position for locating deeper earthquake sources but too far away to sense many of the small earthquakes that accompany eruptive activity. The siting of seismic stations on Alaska's volcanic islands is further restricted by strong noise generated by ocean waves, which masks seismic signals. This noise is often amplified by the unconsolidated pyroclastic deposits that make up the flanks of many Aleutian arc volcanoes. Seismic stations on the volcanic islands are also vulnerable to damage by eruptive activity, such as pyroclastic flows; several of AVO's permanent seismic stations on Augustine have been knocked out by the current eruption.
Ocean-bottom seismometers help to solve these problems. Because they rest on the ocean floor, ocean-bottom seismometers are beyond the range of noise generated by ocean waves and are unlikely to be hit by material ejected from the volcano. They can be placed far enough from the volcano to allow accurate determination of deeper earthquake sources but still close enough to detect small earthquakes. The deployment of ocean-bottom seismometers around Augustine Island will improve AVO's ability to accurately determine the location of volcano-related seismicity and the nature of the volcano's internal structure. These improvements will further our understanding of the subsurface components of the magmatic system and the processes that precede and lead to eruptions.
An ocean-bottom seismometer, or OBS, is a self-contained data-acquisition system that free falls to the ocean floor and records seismic data generated by earthquakes and manmade sound sources (see U.S. Geological Survey Ocean Bottom Seismometer Facility). The OBS is designed to be naturally buoyant and is held at the bottom by a small weight. It can be placed on an ocean floor as deep as 6,000 m (18,000 ft). When the time comes to recover the OBS, an acoustic signal is sent from the surface ship to the OBS to release its weight. The OBS then rises to the surface and is picked up by the ship, and the data are downloaded onto a computer. At a weight of about 100 lb each, OBSes are easy to ship and can be deployed and retrieved from a wide range of vessels. The USGS has an agreement with the national OBS facility at the Woods Hole Oceanographic Institution (see Woods Hole Oceanograhpic Institution Marine Seismology Group) to build and maintain 16 OBSes within the facility. Five of these OBSes and modest amounts of funding are available for both USGS and non-USGS investigators to conduct rapid deployment in response to an earthquake or volcanic eruption in the coastal ocean and its vicinity (see A Policy for Rapid Mobilization of USGS OBS (RMOBS)). The recent operation off Augustine Island was the first OBS rapid response to a potential natural disaster in U.S. waters.
The Alaska Volcano Observatorya joint program of the USGS; the Geophysical Institute of the University of Alaska, Fairbanks; and the State of Alaska Division of Geological and Geophysical Surveysmonitors Aleutian Arc volcanoes and provides warnings to local communities and affected industries.
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