Tiltmeter observations at Cape Yakataga, Alaska, preceding the St. Elias earthquake, M = 7.8, of February 28, 1979

abstract On February 28, 1979 at 2127 UTC, a magnitude of 7.7 earthquake occurred near Mt. St. Elias in southeastern Alaska. The U.S. Geological Survey has been operating a shallow borehole tiltmeter at Cape Yakataga, Alaska, 80 km southeast of the epicenter since March 1977. This instrument has been telemetering data to Menlo Park, California, via Landsat satellite with an onsite strip chart recorder as backup. The strip chart record shows fluctuations due to meteorological effects. However, for 2 months preceding the earthquake, the east-west trace is steady at the 2-μrad level. Instrumental response at the time of the earthquake shows an offset of 4.5 μrad with an exponential decay over several hours to a new level. Five days after the earthquake, both north-south and east-west components returned to pre-earthquake levels. This postseismic tilting may be complicated by meteorological effects.