Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

In 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time. Beneath the trench, rough oceanic crust is covered by ∼3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends >100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism. Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

Regional 3-D lithosphere structure of the northern half of Iran by local earthquake tomography

SUMMARY The 3-D P-wave velocity structure of the northern half of Iran crust has been determined from the local earthquake tomography using a high-quality data set of semi-automatically re-picked arrival times. The quality and quantity of these re-picked phase data allow the 3-D imaging of large parts of the northern half of Iran lithosphere between 0 and 60 km depth. Our new P-wave tomography model represents a major improvement over existing models in terms of reliability, resolution and consistency. First-order anomalies such as the crustal roots of the Zagros and Alborz Mountains are clearly resolved. In addition, several shallow smaller-scale features like the Central Iran sedimentary basin and volcanic and igneous rocks are visible in the tomographic image. Our results show deep Moho depressions beneath the Central Alborz and Zagros mountain ranges that are part of the Arabia–Iranian–Eurasia continental collision zone and locally this Moho topography agrees very well with existing models of other studies. The observed P-wave velocity structure suggests that compared to the Sanandaj-Sirjan and Zagros mountain ranges there is a minor crustal thickening beneath the Alborz mountain range and Kopeh Dagh region.