abstract

The intraplate Meløy earthquake sequence in northern Norway began in November 1978 and lasted a few months, during which time more than 10,000 tremors were recorded with magnitudes (ML) up to 3.2. Three-component field systems with flat velocity response in the range 5 to 80 Hz were used for closein (∼10 km) recordings of events in the magnitude range −0.4 to 2.2. Through spectral analysis of S-wave data, we find that corner frequencies range from 30 to 16 Hz, with seismic moments (Mo) between 1017 and 3 × 1019 dyne-cm, and a moment/magnitude relationship of log Mo = 0.9 ML + 17.5. Source parameters have been computed for a simple circular source model, giving stress drop values between 0.6 and 22 bars, source radius between 84 and 45 m, and average displacement between 0.1 and 4.1mm. Corner frequencies increase only very slowly with decreasing seismic moment, and with the source model used, this causes stress drop to decrease rapidly with decreasing moment, while the source dimension decreases very slowly. This is commonly observed for small inter- and intraplate earthquakes. It is hypothesized that for weak events, the relationships between corner frequency and other source parameters as predicted by theory may not hold.

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