Abstract
SUMMARY In short period seismograms of microearthquakes, S-wave envelopes are found to broaden with hypocentral distance increasing. For crustal events, envelope broadening is caused not only by multiple scattering due to velocity inhomogeneities but also by the contribution of guided waves and head waves. Intrinsic absorption is also an important parameter to truncate the time duration. Microearthquakes of focal depth less than 40 km and magnitude range from 1.5 to 4.5, which took place in northeastern Honshu, Japan, are used for the study of envelope broadening. Digital data acquired by Hi-net are used for the analysis. We estimated the 90 per cent time duration (T90) and the root mean square (rms) time duration (Trms) from the rms traces of horizontal component seismograms for frequency bands 1–2, 2–4, 4–8, 8–16 and 16–32 Hz. The estimated time duration increases with hypocentral distance increasing in both sides of the volcanic front, which runs from south to north in the study area. At reference distance of 100 km, the time duration in the backarc side decreases with frequency; however, that in the forearc side has the minimum at 4 Hz. The time duration at a reference distance in the backarc side is shorter than that in the forearc side especially for frequencies higher than 4 Hz, which can be interpreted by strong intrinsic absorption in the backarc side.
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