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

The Shumagin seismic gap structure and associated tsunami hazards, Alaska convergent margin

The potential for a major earthquake in the Shumagin seismic gap, and the tsunami it could generate, was reported in 1971. However, while potentially tsunamigenic splay faults in the adjacent Unimak and Semidi earthquake segments are known, such features along the Shumagin segment were undocumented until recently. To investigate margin structure and search for splay faults, we reprocessed six legacy seismic records and also processed seismic data acquired by RV Langseth during the ALEUT project (cf. Bécel et al., 2017). All records show splay faults separating the frontal prism from the margin framework. A ridge uplifted by the splay fault hanging wall extends along the entire segment. At the plate interface, the splay fault cuts across subducted sediment strata in some images, whereas in others, the plate interface sediment cuts across the fault. Splay fault zones are commonly associated with subducting lower-plate relief. Along the upper slope, beneath a sediment cover, major normal faults dipping landward and seaward border a ridge of basement rock. The faults displace a regional unconformity that elsewhere received Oligocene–Miocene sediment. Low seafloor scarps above some normal faults indicate recent tectonism. The buried ridge is a continuation of the Unimak Ridge structure that extends NE of the Unimak/Shumagin segment boundary. Some geological characteristics of the Shumagin segment differ from those of other Alaskan earthquake segments, but a causal link to the proposed Shumagin creeping seismic behavior is equivocal

Offshore-aftershock sequence of the Mw 8.1 2014 Iquique earthquake

On 1 April 2014, a Mw 8.1 earthquake ruptured a portion of the subduction zone in northern Chile offshore Iquique between 19.5◦S to 21◦S. A large earthquake had been expected in the subduction zone off northern Chile, because it had not ruptured in a megathrust earthquake since a M∼8.8 event in 1877. The 2014 earthquake did only affect the northern region of the 1877 rupture and left an unbroken segment to the South. In December 2014 we deployed an offshore network of 15 ocean-bottom-seismometers (OBS) between 19◦S and 22◦S using the Chilean Navy ship OPV Toro, covering the aftershock zone of the 2014 Iquique event and the un-ruptured offshore domain in the South. The network was recovered in November 2015 with RV SONNE

Strike-slip 23 January 2018 MW 7.9 Gulf of Alaska rare intraplate earthquake: Complex rupture of a fracture zone system

Large intraplate earthquakes in oceanic lithosphere are rare and usually related to regions of diffuse deformation within the oceanic plate. The 23 January 2018 MW 7.9 strike-slip Gulf of Alaska earthquake ruptured an oceanic fracture zone system offshore Kodiak Island. Bathymetric compilations show a muted topographic expression of the fracture zone due to the thick sediment that covers oceanic basement but the fracture zone system can be identified by offset N-S magnetic anomalies and E-W linear zones in the vertical gravity gradient. Back-projection from global seismic stations reveals that the initial rupture at first propagated from the epicenter to the north, likely rupturing along a weak zone parallel to the ocean crustal fabric. The rupture then changed direction to eastward directed with most energy emitted on Aka fracture zone resulting in an unusual multi-fault earthquake. Similarly, the aftershocks show complex behavior and are related to two different tectonic structures: (1) events along N-S trending oceanic fabric, which ruptured mainly strike-slip and additionally, in normal and oblique slip mechanisms and (2) strike-slip events along E-W oriented fracture zones. To explain the complex faulting behavior we adopt the classical stress and strain partitioning concept and propose a generalized model for large intra-oceanic strike-slip earthquakes of trench-oblique oriented fracture zones/ocean plate fabric near subduction zones. Taking the Kodiak asperity position of 1964 maximum afterslip and outer-rise Coulomb stress distribution into account, we propose that the unusual 2018 Gulf of Alaska moment release was stress transferred to the incoming oceanic plate from co- and post-processes of the nearby great 1964 MW 9.2 megathrust earthquake

Deep structure of the Ionian Sea and Sicily Dionysus - Cruise No. M111, October 10 - November 1, 2014, Catania (Italy) – Catania (Italy)

Summary The origin of the Ionian Sea lithosphere and the deep structure of its margins remain a little investigated part of the Mediterranean Sea. To shed light on the plate tectonic setting in this central part of southern Europe, R/V METEOR cruise M111 set out to acquire deep penetrating seismic data in the Ionian Sea. M111 formed the core of an amphibious investigation covering the Ionian Sea and island of Sicily. A total of 153 OBS/OBH deployments using French and German instruments were successfully carried out, in addition to 12 land stations installed on Sicily, which recorded the offshore air gun shots. The aim of this onshore-offshore study is to quantify the deep geometry and architecture of the Calabria subduction zone and Ionian Sea lithosphere and to shed light on the nature of the Ionian Sea crust (oceanic crust vs. thinned continental crust). Investigating the structure of the Ionian crust and lithospheric mantle will contribute to unravel the unknown ocean-continent transition and Tethys margin. Analyzing the tectonic activity and active deformation zones is essential for understanding the subduction processes that underlie the neotectonics of the Calabrian subduction zone and earthquake hazard of the Calabria/Sicily region, especially in the vicinity of local decoupling zones

ADRIA LITHOSPHERE INVESTIGATION ALPHA - Cruise No. M86/3, January 20 - February 04, 2012, Brindisi (Italy) - Dubrovnik (Croatia)

The Adriatic Sea and underlying lithosphere remains the least investigated part of the Mediterranean Sea. To shed light on the plate tectonic setting in this central part of southern Europe, R/V METEOR cruise M86/3 set out to acquire deep penetrating seismic data in the Adriatic Sea. M86/3 formed the core of an amphibious investigation crossing Adria from the Italian Peninsula into Montenegro/Albania. A total of 111 OBS/OBH deployments were successfully carried out, in addition to 47 landstations both in Italy and Montenegro/Albania, which recorded the offshore airgun shots. In the scope of this shoreline-crossing study, the aim is to quantify the shallow geometry, deep boundaries and the architecture of the southern Adriatic crust and lithosphere and to provide insights on a possible decoupling zone between the northern and southern Adriatic domains. Investigating the structure of the Adriatic crust and lithospheric mantle and analyzing the tectonic activity are essential for understanding the mountain-building processes that underlie the neotectonics and earthquake hazard of the Periadriatic region, especially in the vicinity of local decoupling zones

Patterns of seafloor morphology as a response to tectonic- and sedimentary processes south of the Messina Strait, Italy

The Ionian Sea between Sicily and Calabria is known for its complex geological setting, as it is located at the convergence zone of the African and Eurasian plates. The seismogenic potential in this region is manifested by several high magnitude and disastrous earthquakes like the 1908 Messina Earthquake. Furthermore, the area is affected by intense volcanism like the Aeolian Island volcanos in the Tyrrhenian Sea and Europe’s largest active volcano, Mt Etna, sitting directly at the eastern coast of Sicily. During the last years, the possible presence of Subduction Tear Edge Propagator faults (STEP-faults) has been heavily debated. The main candidates for these faults are the Ionian Fault in the Northeast and the Alfeo-Etna Fault in the Southwest of the working area between Sicily and Calabria. Nevertheless, only little is known about near seafloor deformation zones and sedimentary processes in the Ionian Sea directly south of the Messina Strait.peer-reviewe