Recent earthquakes in Svalbard sourced from transform-parallel thrust systems: implications for linear earthquake swarms and earthquake risk

<ns3:p> Background Thus far, most earthquakes around the Svalbard Archipelago in the past 75 years, some reaching up to moment magnitude (M <ns3:sub>W</ns3:sub> ) 6.1, have been attributed to movements along N–S- to NE–SW-trending fault surfaces, possibly including inherited Caledonian (ca. 460–425 Ma) and late Paleozoic–Cenozoic structures. Recent geophysical studies in the Barents Sea and Svalbard have revealed that active transform faults in the Fram Strait parallel a newly identified structural trend comprising WNW–ESE-striking thrust systems of the Timanian Orogeny (ca. 650–550 Ma) thousands of km long. Methods We used seismicity data from the International Seismological Centre and seismic reflection data from the DISKOS database to correlate recent earthquakes and discrete slip surfaces in the sub-surface. Results Analysis of seismic reflection data in Storfjorden reveals a set of km-scale normal faults, which truncate the seafloor and are located above and parallel to a WNW–ESE-striking Timanian thrust system. Another Timanian thrust system, the Kinnhøgda–Daudbjørnpynten fault zone, coincides with the location of the 2008–2016 Storfjorden seismic sequence. The southwards migration and shallowing of the aftershocks over time is explained by rupture along a deep, gently NNE-dipping slip surface within the basal part of the thrust in the north and ensuing collapse of overlying rocks and thrust sheets along the shallow portion of nearby slip surfaces. Conclusions Seismicity and seismic reflection data in Svalbard indicate that most earthquakes in the last 75 years may be associated with movement along WNW–ESE-striking Timanian thrust systems. The regional stress field causing movement along the inherited Timanian thrusts is likely related to ongoing rifting and transform faulting in the Fram Strait. The present study suggests that linear earthquake swarms may be generated by and occur orthogonal to inherited brittle–ductile thrust systems. The structures responsible for the seismicity around Svalbard may generate earthquakes up to magnitude 8.9. </ns3:p>