Focal mechanisms and locations of earthquakes in the vicinity of the 1975 Kalapana Earthquake aftershock zone 1970–1979: Implications for tectonics of the south flank of Kilauea Volcano, Island of Hawaii

Abstract

The November 29, 1975, Kalapana earthquake of magnitude 7.1 on the south flank of Kilauea Volcano, Hawaii, represents a major rift tectonic event. It is generally understood that the earthquake resulted from movement on a nearly horizontal fault plane, with the crustal block south of Kilauea's east rift zone moving south‐southeastward up to several meters. Well‐recorded earthquakes on the south flank of Kilauea from 1970 to 1979 were analyzed and focal mechanisms determined to obtain a better understanding of the details of south flank tectonics. Included are a large number of earthquakes from the aftershock sequence of the 1975 earthquake. Hypocenters are distributed in a subplanar zone that dips 2° to 3° to the west at a depth of about 8–9 km depth. We interpret this pattern to coincide with a portion of the slip plane of the Kalapana earthquake and to be consistent with the notion of the slip plane occurring in a weak layer at the top of the old oceanic crust. Focal mechanisms are remarkably constant throughout this time period, both before and after the 1975 event, and closely agree with the mainshock mechanism. P axis azimuths and slip vectors for the low‐angle fault planes are normal to the middle and lower east rift zones of Kilauea. There is suggestion of a change in the orientation of P axes from shallower to steeper plunge at the time of the Kalapana earthquake. Such a change is consistent with a general reduction in the horizontal compressional stress normal to the rift zone at the time of the large earthquake. No other evidence of precursory reorientation of the stress field was found and the precise predictability of this event remains in doubt. However, a general model of the tectonics of the volcanic rift system is now available that satisfies a number of geological and geophysical observations.