S U M M A R YWe use seismic reflection data to map the geometry and character of the subduction interface in the Gisborne area of the Hikurangi subduction margin, New Zealand, which experiences repeated shallow (<15 km) slow slip events. The reflection character and geometry in this area is highly variable, which we interpret to be related to the subduction of seamounts and underthrust sediments. Three zones of high-amplitude interface reflectivity (HRZ-1, 2 and 3) are interpreted to be the result of fluid-rich sediments that have been entrained with subducting seamounts. The interface above the HRZ zones is shallower than the surrounding areas by 2-4 km, due to the warping of the interface to accommodate seamount subduction. These zones of high-amplitude reflectivity and shallower interface geometry correlate broadly with locations of recorded slow slip events from 2002 to 2008. We hypothesize that effective stresses on the interface may be lower along the northeast margin in areas of high-amplitude reflectivity due to; (1) the enhanced underthrusting of fluid-rich sediment, (2) reduced overburden stresses where the interface has been warped to shallower depths to accommodate seamount subduction and (3) potential fluid flow concentration effects leading to overpressure along these shallower interface corrugations. From our observations we propose localized reductions in effective stress caused by interface structural relief may be a potential factor in promoting shallow slow slip events.

The Corinth Rift, central Greece, enables analysis of early rift development as it is young (<5 Ma) and highly active and its full history is recorded at high resolution by sedimentary systems. A complete compilation of marine geophysical data, complemented by onshore data, is used to develop a high-resolution chronostratigraphy and detailed fault history for the offshore Corinth Rift, integrating interpretations and reconciling previous discrepancies. Rift migration and localization of deformation have been significant within the rift since inception. Over the last circa 2 Myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift, but this transition did not occur synchronously along strike. Isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620-340 ka, characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at~30 m/kyr. Since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin, demonstrated by lateral growth of discrete depocenters at~40 m/kyr. A single central depocenter formed by circa 130 ka, indicating full fault linkage. These results indicate that rift localization is progressive (not instantaneous) and can be synchronous once a rift border fault system is established. This study illustrates that development processes within young rifts occur at 100 kyr timescales, including rapid changes in rift symmetry and growth and linkage of major rift faults.

[1] The Hikurangi subduction margin, New Zealand, has not experienced any significant (>M w 7.2) subduction interface earthquakes since historical records began $170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrust's seismogenic zone.Components: 20,324 words, 15 figures.

The style of extension and strain distribution during the early stages of intra-continental rifting is important for understanding rift-margin development and can provide constraints for lithospheric deformation mechanisms.The Corinth rift in central Greece is one of the few rifts to have experienced a short extensional history without subsequent overprinting.We synthesise existing seismic re£ection data throughout the active o¡shore Gulf of Corinth Basin to investigate fault activity history and the spatio -temporal evolution of the basin, producing for the ¢rst time basement depth and syn-rift sediment isopachs throughout the o¡shore rift. A major basin-wide unconformity surface with an age estimated from sea-level cycles at ca. 0.4 Ma separates distinct seismic stratigraphic units. Assuming that sedimentation rates are on average consistent, the present rift formed at 1^2 Ma, with no clear evidence for along-strike propagation of the rift axis.The rift has undergone major changes in relative fault activity and basin geometry during its short history. The basement depth is greatest in the central rift (maximum $3 km) and decreases to the east and west. In detail however, two separated depocentres 20^50 km long were created controlled by N-and S-dipping faults before 0.4 Ma, while since ca. 0.4 Ma a single depocentre (80 km long) has been controlled by several connected N-dipping faults, with maximum subsidence focused between the two older depocentres.Thus isolated but nearby faults can persist for timescales ca. 1 Ma and form major basins before becoming linked.There is a general evolution towards a dominance of N-dipping faults; however, in the western Gulf strain is distributed across several active N-and S-dipping faults throughout rift history, producing a more complex basin geometry.