Characteristics of Source Spectra of Moderate Earthquakes in a Subduction Zone along the Pacific Coast of the Southern Tohoku District, Japan

Abstract

Strong motion records for 22 earthquakes from 1982 to 1987 in the subduction zone off Fukushima Prefecture are analyzed. Magnitude M of the earthquakes ranges from 4.0 to 6.7. The source spectrum of each earthquake is obtained from S-waves in horizontal components. Seismic moment Mo of each event is evaluated from spectral amplitudes in a low frequency range. Two corner frequencies are identified on the source spectra for 7 events of M>6, while source spectra of 15 events of M<6 are characterized by one corner frequency. The first corner frequency fc of the M>6 events satisfies a relation of Mo∝fc-3, which is consistent with the data of seismic moment and fault area for the M=7 to 8 class events in the 1938 large earthquake sequence off Fukushima Prefecture. Complicated S-wave forms for M>6 events suggest that the second corner frequency fc* is related to the complex faulting process with small-scale fault heterogeneities. This means that fc* corresponds to the patch corner frequency. It is found that the seismic moment density Mo* at a frequency of fc* is correlative to the excitation of high frequency waves, while the values of fc* are nearly constant, about 1 Hz, for all the M>6 events. We found two types of earthquakes with M>6 off Fukushima Prefecture: high frequency events and low frequency events. High frequency events show larger peak accelerations than a value expected from the attenuation curve, and show larger Mo* values compared with low frequency events with the same seismic moment. High frequency events mostly occurred in the northwestern part, while low frequency events occurred in the southeastern part off Fukushima Prefecture. This indicates that the faulting process is more heterogeneous in the northwestern part than the southeastern part. Concerning the events of M<6 which occurred mostly in the northwestern part, simple S-wave forms suggest that those earthquakes are due to a single fault patch rupture process. These events satisfy another relation of Mo∝fc-3 on the average, which is different from one for larger events. Stress drop deduced from this relation is about ten times as high as the average stress drop from the Mo-fc relation for the event of M>6. Higher stress drop of these events may be due to fault patches with large intensity.