SEISMOLOGICAL ASPECTS OF THE 1988-1989 LONQUIMAY (CHILE) VOLCANIC-ERUPTION

More than 1600 earthquakes were recorded, in a four-Month period, on portable seismographic stations installed in the Lonquimay area, immediately after the initiation of the volcanic eruption of December 25, 1988. Hypocentral parameters were calculated on a subset of 150 events showing clear P and S arrivals. Seismic activity was restricted to a north-south elongated region that included the main crater of the Lonquimay volcano. The new Navidad crater lies along the eastern margin of the area of seismicity. Most of the hypocenters are located at depths shallower than 6 km and none have depths exceeding 10 km. The largest earthquakes of the sequence took place on Dec. 28 (M = 4.7), Jan. 12-13 (M = 4.8 and 5.0), and Feb. 24 (M = 5.3). The number of events per day varied; it peaked on Dec. 27 with more than 260 events, and more than 200 events were recorded on Jan. 13, both of which corresponded to the days of large earthquakes. The relation of large to small events given by the b value of the magnitude-frequency distribution Log N = a - bM is typical of volcanic sequences (b = 1.7) and much larger than the equivalent for tectonic environments. The rate of decay of the number of aftershocks of each large earthquake is proportional to t(-p) with p increasing from 0.63 to 1.08 as the eruption progressed in time. Initial low values of p indicate a slow stress release in the region. Larger absolute values of p are comparable with tectonic environments and might be indicators of the last stages of the eruption process. The total seismic energy release is 9.6 X 10(19) ergs, about five orders of magnitude less than the estimated thermal energy. The spatial distribution of earthquakes suggests that the sources of stress change associated with the eruption are located under, and possibly to the south of, the main crater of the Lonquimay volcano, and that the evacuation channels of magma occupied a zone of weakness in the region of the new crater.