THE KRAFLA SPREADING SEGMENT, ICELAND .2. THE ACCRETIONARY STRESS CYCLE AND NONSHEAR EARTHQUAKE FOCAL MECHANISMS

The continuous geothermal seismicity of the Krafla volcanic system, NE Iceland, was monitored in 1985, immediately following a major spreadi ng episode (Foulger et al., 1989; Amott, 1990; Arnott and Foulger, thi s issue). Focal mechanisms of 153 well-located, shallow earthquakes we re determined using P wave polarity data. Errors resulting from poorly modeled crustal structure were reduced by calculating hypocentral loc ations using a three-dimensional crustal model derived from a simultan eous inversion and by deriving takeoff angles and azimuths using three -dimensional ray tracing, The final data are thus of exceptionally hig h quality. Most of the events display radiation patterns consistent wi th a double-couple source model. The source orientations exhibit no sy stematic pattern beneath the Bjarnarflag well field, a little coherenc e in the Krafla caldera, and a general tendency for the greatest princ ipal stress to be oriented normal to the rift zone in a zone of recent dike injection. Normal, thrust, and strike-slip shear events are mixe d together, and five events have radiation patterns inconsistent with a double-couple source model and are attributed to nonshear source pro cesses. Variable non-double-couple events were observed, with implosiv e and explosive/volume-conserving mechanisms and no clear systematic p attern of type or orientation for the system as a whole, though patter ns were discernable in some subsets of events. The nonshear suite, lik e the shear suite, was therefore in general heterogeneous. The results indicate that a systematic deviatoric stress field was absent in the Krafla spreading segment in 1985, in contrast with the Reykjanes segme nt in 1977 and the Hengill segment in 1981 (Klein et al., 1977; Foulge r, 1988b). The observations are consistent with a model where a strong , systematic deviatoric stress field characterizes the accretionary pl ate boundary during the interrifting and prerifting phases of the rift ing cycle. This systematic stress is partially or wholly released duri ng epistodic rifting and spreading episodes, such as that experienced by the KraAa system 1975-1984, and is absent in the immediate postrift ing phase of the rifting cycle. Nonshear geothermal earthquakes have b een reported from all of the three Icelandic spreading segments studie d in detail to date.