TYPE OF FAULTING AND ORIENTATION OF STRESS AND STRAIN AS A FUNCTION OF SPACE AND TIME IN KILAUEAS SOUTH FLANK, HAWAII

Earthquake focal mechanisms of events occurring between 1972 and 1992 in the south flank of Kilauea volcano, Hawaii, are used to infer the s tate of stress and strain as a function of time and space. We have det ermined 870 fault plane solutions from P wave first motion polarities for events with magnitudes M(L) greater than or equal to 2.5 and depth ranging between 6 and 12 km. Faulting is characterized by a mixture o f decollement, reverse, and normal faults. Most large earthquakes with magnitude M < 7 slip on reverse faults striking NE at 40 degrees and dipping SE between 60 degrees and 70 degrees. In Hawaii, the earthquak es with M > 7 rupture the decollement plane, since it is the only surf ace large enough to generate magnitude 7 or larger earthquakes. The pe rcentage of reverse faulting events is high compared to the decollemen t and normal faulting mechanisms for the period 1972-1983. The percent age of decollement type focal mechanisms becomes dominant after 1983. This pattern of faulting activity suggests that pressure was building up within Kilauea's rift zone prior to the 1983 Puu'Oo eruption. Overa ll, a single stress orientation with the maximum compressive stress or iented SE perpendicular to the rift and dipping at 45 degrees is compa tible with the coeval existence of decollement, reverse, and normal fa ults. However, in a crustal volume east of longitude 155 degrees 10'W, we find a change of the orientation of ol from nearly horizontal to p lunging 45 degrees SE occurring in 1979. This stress rotation suggests magma movements within the aseismic part of Kilauea's east rift zone. The strain and stress orientations are coaxial in the south flank exc ept within the volume where the stress rotation is observed. We observ e a change in the relationship between stress and strain directions ca used either by the shifting of seismic activity from reverse faults to decollements, while stress stays constant, or by a rotation of stress , while strain remains constant. Assuming that the model of a noncohes ive Coulomb wedge is appropriate for Kilauea's south flank, we find th at high pore pressures are prevalent along the decollement and within the wedge for a coefficient of friction equal to 0.85.