Nature 337, 640 - 642 (16 February 1989); doi:10.1038/337640a0

Implosive earthquakes at the active accretionary plate boundary in northern Iceland

G. R. Foulger^*, R. E. Long^*, P. Einarsson^† & A. Bjornsson^‡

^*Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK
^†Science Institute, University of Iceland, Dunhaga 3, Reykjavik, Iceland
^‡National Energy Authority, Grensasvegi 9, Reykjavik, Iceland

Non-double-couple earthquake focal mechanisms imply a mode of seismic failure other than simple shear, and movements perpendicular to the fault plane. Reports of such events are rare and it is controversial whether such failure can occur at depth in the Earth. Following reports of such earthquakes from the Hengill and Reykjanes volcanic systems on the accretionary plate boundary in Iceland^1,3, a seismological survey was conducted in the Krafla system, Iceland, to explore whether such earthquakes occur elsewhere along the boundary. Here we report the observation of a mixed suite of focal mechanisms at Krafla, including shear events, explosive tensile-crack events such as were observed at Hengill and Reykjanes, and implosive events. These last represent cavity collapse and constitute an entirely new class of earthquake. The Krafla system had undergone a crustal spreading episode^4–10 during the decade before the observations were made, and our results indicate that immediately after such an episode the accretionary plate boundary is characterized by a heterogeneous stress regime, with shear, extensional and compressional sources in close juxtaposition.

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