A REINTERPRETATION OF SEISMICITY ASSOCIATED WITH THE JANUARY 1983 DIKE INTRUSION AT KILAUEA VOLCANO, HAWAII

In January 1983, a dike intrusion/fissure eruption generated a swarm o f 375 magnitude 1 to 3 earthquakes along a 16-km segment of Kilauea's Middle East Rift Zone. We searched the Hawaiian Volcano Observatory ca talog for multiplets of similar events from this region from 1980 thro ugh 1985 and obtained precise relative locations by waveform cross cor relation. Over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for ac curate relocation. Some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm, consistent w ith generation near the propagating dike tip, while others were active for several days. The two multiplets nearest the origin of the seismi c swarm include events from the preceding days and months. Most multip lets span only 50 to 100 m following relocation, are located at about 3 to 4 km depth, and appear to deepen downrift. The catalog depths of those earthquakes in multiplets and those not in multiplets are simila r, suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface. By analogy with a mechanical model used to explain a simi lar clustering of background seismicity in the Upper East Rift in 1991 , we infer that the earthquakes are generated in regions of high stres s concentration immediately above Kilauea's deforming deep rift body. This conclusion is consistent with the depth of the top of the deep ri ft body inferred from geodetic data and with numerical calculations su ggesting that a significant ambient differential stress is required fo r dikes to produce earthquakes larger than magnitude 1.