THE BAY OF PLENTY EARTHQUAKES OF JULY 1989 - COMPARISONS WITH FORESHOCK ACTIVITY OF THE 1987 EDGECUMBE EARTHQUAKE

Observations of the 1989 July Bay of Plenty earthquake sequence, using both portable and permanent seismograph stations, allow location of the aftershocks and comparison of the sequence with the foreshocks of the 1987 Edgecumbe earthquake. The two mainshocks (M(L) 4.6 and 4.7) and the aftershocks fall within a 6 x 2 km zone, elongated in the northeast-southwest direction, centered 20 km northwest of Whakatane. This is within the zone of foreshocks of the 1987 shock. Calculated depths ranged from 4.7 to 10.5 km. The time history of the sequence, which began abruptly, is typical of a mainshock-aftershock sequence, and the b-value (1.37 +/- 0.11) is similar to the regional value. In contrast, the 1987 foreshocks had a low b-value (0.86 +/- 0.15) and an irregular time history. Coda durations, for a given amplitude magnitude, were longer for the 1987 foreshocks than for the 1989 aftershocks. This can be explained by the backscattering theory of coda formation if the scattering coefficient of the crust at the time of the foreshocks was higher than in 1989. However, an explanation in terms of event depths is possible if, as suggested by felt reports, the 1987 foreshocks were very shallow.