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U.S. Geological Survey volcanologists and geophysicists began to conduct the first comprehensive, high-resolution airborne magnetic survey of the rock layers under Mono Basin and Long Valley this week.
When the analysis of the data is complete, the resulting state-of-the-art 3D subsurface geologic map will improve assessment of both volcanic and earthquake hazards in the Mono Basin-Long Valley region.
The map will be published by the USGS and made available to the public via the USGS California Volcano Observatory website.
The USGS warned residents and visitors to not be alarmed if they witness a low-flying helicopter over the Mono Basin and Long Valley areas.
An experienced pilot—approved for low-level flying—is operating the helicopter. All flights have been coordinated with the Federal Aviation Administration to ensure they are in accordance with U.S. law, a USGS spokesperson said.
“Volcanic rocks are rich in iron-bearing minerals whose magnetic signatures can be detected at aircraft altitudes in an efficient manner,” said USGS Director Marcia McNutt.
“The amount and direction of motion along faults may be revealed by dislocations of magnetic signatures across linear features, even if buried beneath soils and streams.”
The aeromagnetic survey is intended to cover all of Mono Basin and Long Valley from about 9 miles north of Lee Vining to 15 miles south of Mammoth Lakes. Flight lines will form a grid pattern, flown a quarter of a mile apart at an elevation of 500 feet above the ground in a northeasterly direction, and four miles apart in a northwesterly direction.
The entire survey will be flown during daylight hours and take about two weeks to complete, McNutt said.
About 26 eruptions have occurred in the Long Valley area in the last 8,000 years. The pattern of past activity suggests that in any given year, the likelihood of eruption is about one in 200, which is roughly comparable to the chances of repeating the Great Earthquake of 1906 in San Francisco.
The area is also prone to earthquakes as demonstrated by frequent small tremors and by the series of magnitude 6 earthquakes that occurred in May 1980.
Unlike earthquakes, volcanic eruptions are preceded by measurable precursors, including surface ground movement, volcanic-type earthquakes, and gas emissions.
Since the 1980s, the USGS CalVO (formerly known as the USGS Long Valley Observatory) has employed sensors to detect such disturbances and, when necessary, issued hazard information to the public and the appropriate emergency response officials within greater Mono County.