Coda-Q before and after the 1986 Andreanof Islands earthquake

Coda-Q values before and after the May 7, 1986 Andreanof Islands earthquake in the central Aleutian subduction zone were calculated from microearthquake seismograms on the basis of Aki's single backscattering model. Digital waveforms from 329 microeathquakes in 5.4 years before the mainshock and 40 aftershocks recorded by 11 stations of the Central Aleutians Seismic Network were utilized. Before the mainshock, coda-Q in the rupture zone west of the mainshock epicenter was 15% higher than theQ outside the rupture zone to the west. A lowQ in the region east of the mainshock epicenter is inferred, but lacks solid evidence. The highQ region accords with the part of the rupture zone where most seismic moment was released. During approximately two months after the mainshock, coda-Q in the rupture zone decreased about 10%. PrecursoryQ changes were not found in the 5.4 years before the mainshock. It is deduced that a coda-Q precursor, if it existed, was of small magnitude (<10%), or its duration was either shorter than one year or longer than four years.