Variations of apparent stresses and stress drops prior to the earthquake of 6 May 1984 (m_b = 5.8) in the Adak Seismic Zone

On 6 May 1984, a m_b = 5.8 earthquake occurred within a subregion of the Adak Seismic Zone that had shown clustering of m_b>4.5 earthquakes during the last 3 yr. This region has been examined for premonitory variations in stress drop and apparent stresses, calculated from the S-wave spectra of 69 microearthquakes (M_d < 2.7) which occurred between January 1981 and May 1984. The influence of local site conditions seems to control the corner frequencies at certain stations. Both the distribution of apparent stresses and stress drops shows local concentrations of high values close to the hypocenter of the 6 May event and an increase with time over 3 yr for earthquakes closer than ≈ 7.5 km. The significance of the increase is greater than 99.5 percent for the apparent stresses and greater than 95 percent for the stress drops. The quantitative interpretation in terms of ambient stresses is aggravated since the stress drops with seismic moments less than about 1.0 × 10¹³ Nm decrease with decreasing moment.