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 spreading episode (Foulger et al., 1989; Amott, 1990; Arnott and Foulger, this issue). Focal mechanisms of 153 well-located, shallow earthquakes were determined using P wave polarity data. Errors resulting from poorly modeled crustal structure were reduced by calculating hypocentral locations using a three-dimensional crustal model derived from a simultaneous inversion and by deriving takeoff angles and azimuths using three-dimensional ray tracing, The final data are thus of exceptionally high quality. Most of the events display radiation patterns consistent with a double-couple source model. The source orientations exhibit no systematic pattern beneath the Bjarnarflag well field, a little coherence in the Krafla caldera, and a general tendency for the greatest principal stress to be oriented normal to the rift zone in a zone of recent dike injection. Normal, thrust, and strike-slip shear events are mixed together, and five events have radiation patterns inconsistent with a double-couple source model and are attributed to nonshear source processes. Variable non-double-couple events were observed, with implosive and explosive/volume-conserving mechanisms and no clear systematic pattern of type or orientation for the system as a whole, though patterns were discernable in some subsets of events. The nonshear suite, like 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 segment in 1977 and the Hengill segment in 1981 (Klein et al., 1977; Foulger, 1988b). The observations are consistent with a model where a strong, systematic deviatoric stress field characterizes the accretionary plate boundary during the interrifting and prerifting phases of the rifting cycle. This systematic stress is partially or wholly released during epistodic rifting and spreading episodes, such as that experienced by the KraAa system 1975-1984, and is absent in the immediate postrifting phase of the rifting cycle. Nonshear geothermal earthquakes have been reported from all of the three Icelandic spreading segments studied in detail to date.