JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B4, PAGES 4769-4790, APRIL 10, 1992 Comparison of Orientations of Stress andStrain Tensors Based on Fault Plane Solutions in Kaoiki, Hawaii MAX WYSS 1AND BE1YUAN LIANG Cooperative Institute for Research in Environmental Sciences andDepartment of Geological Sciences University of Colorado, BouMer W. R. TANIGAWA U.S. Geological Survey, Hawaii Volcano Observatory, Hawaii NationalPark XIAOPING WU Joint Institute of Laboratory for Astrophysics, University of Colorado, Boulder The stress tensor orientation was estimated based on inversion from 238 firstmotion faultplane solutions of earthquakes with mostlyM = 3.5 + 0.6 located in the 10-km radius Kaoiki cmstalvolume.Separate inversions for subvolumes containing 20-50 events yieldedthe sameresults in several adjacent volumes, suggesting that thestress tensor is homogeneous in those parts of theKaoiki area and thattheinversion results are stable and meaningful. Five spatial subsets of thedata were found for which the orientation of at least one of the principal axes was different from that in the other sets by 200-80 ø and at confidence levelsexceeding 95%. The volcano summits of Kilaueaand Mauna Loa, andtheir rift systems, are identified as the source of stress in theKaoikicmst, because thegreatest principal stress points to Kilauea and Mauna Loa. In addition, the strain tensor due to energy released by these 238 earthquakes was computed for the Kaoiki area, and several subvolumes of it, by summing the momenttensors. The momenttensorof each earthquake was constmcted fromtheindividual faultplane solutions and from an estimate of the scalar moment derived from the moment-magnitude relationship. A comparison of the directions of strain andstress tensors showed close agreement for subvolumes with predominantly strike-slip faulting. In these volumes the inversion process for stress directions led to misfits of approximately equal size for the conjugate near vertical nodal planes. These observations are interpreted to show thatin the strike-slip regime of the upper part of the cmst, neither of the nodal planes is preferred for faulting. Rupture probably occurs along the NW andalong the NE striking nodal planes in separate earthquakes. Subvolumes with moredecollement faulting showed significant differences of 300-40 ø between the principal strain and stress directions. In these volumes the near-horizontal nodal planes showed noticeably smaller misfits in the inversion for the stress directions. Thesefacts are interpreted to indicate thatthe decollement plane is weak, allowing slipon it evenif the principal stresses are inclined at a large angle to it. It is proposed that comparison of strain and stress tensor calculations may be able to differentiate between tectonic regimes uniform in strength (no well developed fault plane)and regimes in which a fault with low frictionalstrength dominates. As a function of time, significant rotations of the strain tensor by approximately 45 ø can be observed, which seemto be relatedto the occurrences of Kaoiki mainshocks. During threebackground periods of about7 yearseach,the average straintensor showed an orientation typical for predominant decollement faulting, while two premainshock periods of 2.5 years each showed an orientation closer to strike-slip faulting. It is proposed thatthispattern may be repeated before the next Kaoiki mainshock. The strain released seismically is less than the geodetically observed strainby approximately an order of magnitude. (Stress, strain, faultplane solutions, Hawaii.) INTRODUCTION The Kaoiki area is located between the two active volcanos, Mauna Loa and Kilauea on theisland of Hawaii (Figure 1). Fault plane solutions of earthquakes in this area show a great deal of variations (Figure 2). The two dominant types of solutions indicate either strike-slip faultingon nearly verticalplanes or low angle faulting [Endo,1985].The firstpurpose of this paper is to test the hypothesis that all of the different fault plane solutions throughout the Kaoiki source volume could be due to 1Now at Geophysical Institute, University ofAlaska, Fairbanks. Copyright 1992by the American Geophysical Union. Papernumber 91IB02968. 0148-0227/92/9 HB-02968505.00 a single stress tensor orientation, with the difference in solutions causedby the availability of different planes of weakness for faulting. For example, if a setof near-vertical and another set of near-horizontal planesof weakness are available (Figure 3), a single orientation of the stress tensorcan cause slip on both of theseplanes in separate earthquakes. In this casea heterogeneous set of fault plane solutions may actually reflect a homogeneous stress field [e.g., Gephart and Forsyth, 1984]. Alternatively, it may be that the differencesin fault plane solutions can define spatial differences in the stress tensor orientation in the Kaoiki area. We will invert for the stress tensor orientation, usingthe method of Gephart [1990a, b] to test this hypothesis. The second purpose of this paperis to explorewhether it is possible to identify the plane of faulting for some of the focal mechanisms. In the stress tensor inversion the misfits for the 4769