Subducting fracture zones control earthquake distribution and upper plate properties: examples from Sumatra and Kamchatka

With newly acquired marine geophysical data from the oceanic crust off Sumatra and Kamchatka (SO186 and SO201) we investigate the influence of the relief of the downgoing plate on seismicity and fore arc structure, architecture and properties along two different active margins, namely the Sumatra and the Kamchatka subduction zones. Off northern Sumatra two mega-thrust events occurred on 26.12.2004 (Mw=9.1-9.3) and on 28.03.2005 (Mw=8.6). Seismological investigations, GPS measurements and in-situ and remote observation of vertical motion on fore arc islands show both, an abrupt southern termination of the large 12/2004 rupture and a sharp northern termination of the rupture zone of the 03/2005 mega-thrust. Wide-angle/refraction seismic and MCS data show an abrupt arc parallel depth change of 3 km within 40 km in the oceanic crust beneath the fore arc SW of Simeulue Island. We interpret the abrupt depth change originates from a ramp or tear in the subducted oceanic crust. The discontinuity in the oceanic crust likely trends NNE and is located east of a continuation of an extinct FZ on the subducting Indo-Australian plate. This indicates a pervasive lower plate control on margin structure, particularly its segmentation. The tear might be the reason for rupture propagation termination of the great Sumatra-Andaman earthquakes. During RV Sonne cruise SO201 we collected geophysical profiles in the NW Pacific off Kamchatka and the Aleutian arc crossing the Emperor Seamount Chain and the Krusenstern FZ. The Krusenstern FZ is being subducted at the Kamchatka margin. It comprises a maximum vertical offset of about 1080 m. From our data we suggest that the Krusenstern FZ is reactivated in the vicinity of the Kamchatka margin due to the load of the subducting/colliding Meiji Guyot. It enters the subduction zone right off Kronotsky Peninsula, where a major segment boundary separates domains of different properties of the fore arc: It differs in terms of exhumation, uplift and the style of deformation. Seismicity changes significantly north and south of the subducting Krusenstern FZ, in addition the magmatic arc shows a sharp bend right at the subducting fracture. The Sumatra and Kamchatka examples show that preexisting structures and morphology of the oceanic crust significantly influences earthquake occurrence and extension of earthquake ruptures as well as the properties of the fore arc and the magmatic arc.