In a 30-day period beginning March 29, 1965, some 2026 microearthquakes were recorded at MBC and by the array NPNT. About 10 per cent of them were located. The extent of the epicentral region was 3 sq km and of the hypocentral region 8 cu km. The centroid of the hypocenters was 12.6 km from MBC on an azimuth of 121° and at a depth of 6.8 km, very nearly on the strike of an old fault. Although no foreshock-aftershock series could be distinguished, the events were not independent nor randomly distributed in time.
Particle motion diagrams were drawn for 10 events. These identified the longitudinal and transverse waves and confirmed the azimuths and angles of emergence obtained from amplitude data. The array data also confirmed the azimuths and yielded a P-wave velocity of 4.33 km/s near the surface.
The magnitudes ranged from − 1.1 to 2.9 and the frequency-magnitude relation was:
The coefficient 0.68 suggests tectonic activity. The partition of energy between P and S, and the observed directions of motion (all compressions at MBC) are compatible with the double couple model of focal mechanism and motion on the old fault. However, magma movement cannot be entirely ruled out as there exists geomagnetic evidence of abnormally high upper mantle temperatures.
The following energy-magnitude relation was determined from 100 events in the range 0.1 ≦ ML ≦ 1.6: