Determination of source parameters of mid-plate earthquakes from the waveforms of body waves

The source parameters and stress drops of five mid-plate earthquakes were determined by matching the synthetic and observed far-field P waveforms in the time domain. The stress drops estimated from these mid-plate events are on the order of a hundred to a few hundred bars. These values are significantly higher than those for interplate (30 bars) and intraplate (100 bars) earthquakes which occurred near the plate boundaries. The difference of the earthquake stress levels between mid-plate and plate boundary events may suggest a lateral variation of stress level in the lithosphere and provide important constraints on the driving mechanism of plate motion.