Intraplate earthquakes in the Svalbard Archipelago

A small, seismically active region, designated the Heerland seismic zone, has been delineated in the Svalbard archipelago. Over 500 earthquakes in this region, of which nearly 50 were reliably located, were recorded during a 9‐week field season in 1977. This zone provides an example of intraplate earthquake activity in continental‐type crust not far from an active plate boundary. A large proportion of the located earthquakes suggests a trend with a dominant east‐west direction which is 25 km or less in length. The orientation of this grouping of epicenters is nearly transverse to all of the major mapped faults and structural trends in that region. A fault plane solution was obtained for one relatively large teleseismically recorded earthquake which was located near the Heerland zone. One of the derived nodal planes approximately corresponds with the trend of locally obtained epicenters. First‐motion data from the larger locally recorded earthquakes are consistent with the solution obtained for the teleseismically recorded earthquake. These data, if combined with the observed seismicity pattern, indicate sinistral strike‐slip motion along a vertical fault trending in an approximate eastsoutheast west‐northwest direction. Presentday seismicity along the major mapped north‐south faults of Spitsbergen is insignificant in comparison to that of the Heerland zone. Since movement along those faults has been documented at least into the Paleocene, the paucity of recent earthquakes along them and the unexpected pattern of seismicity in the Heerland zone suggest that the stress field acting in the crust of that region is quite different from that acting during the early Tertiary.