Linking active faults and the earthquake cycle to Seismic Hazard Assessment: Onshore and Offshore Perspectives

Recent developments in understanding of faulting and the earthquake cycle are improving our approaches to seismic hazard assessment. These developments are due to the increasing availability of datasets, including remote sensing, geochemical, offshore geophysics, boreholes and numerical modelling, and have been applied to both onshore and offshore faults which contribute to seismic hazard. From these datasets and developments, it is clear that there is more variability in the earthquake cycle and the seismogenic potential of faults than previously accounted for in seismic hazard assessment. This session will explore a range of datasets and study locations and discuss how they can be applied to different aspect of seismic hazard assessment.

Co-organized by NH4
Convener: Zoe MildonECSECS | Co-conveners: Franz Livio, Sara Martínez-Loriente, Laura Gómez de la PeñaECSECS, Sambit NaikECSECS, Hector Perea, Pia Victor, Morelia UrlaubECSECS
vPICO presentations
| Fri, 30 Apr, 15:30–17:00 (CEST)

Session assets

Session materials

vPICO presentations: Fri, 30 Apr

Chairpersons: Sara Martínez-Loriente, Franz Livio
Submarine active faults: from regional observations to seismic hazard characterisation
Paola Vannucchi, David Iacopini, and Jason P. Morgan

According to Plate Tectonics, fracture zones (FZs) are born at Transform Faults (TFs), which leave behind "inactive" FZs traces as scars on the seafloor that reflect their initial use as one side of a strike-slip transform fault. FZs were originally thought to "heal" as the oceanic lithosphere cooled and strengthened with time. However, the occurrence of recent earthquakes reveals that FZs can be associated with significant seismic activity (for example during the recent Mw 8.6 2012 EQ offshore Sumatra and Mw 7.9 2018 EQ offshore SE Kodiak), and also with permanent deformation that occurs well after passage through the TF.

The TF at the spreading center is known to be accompanied by the formation of the transform valley which exposes serpentinized peridotite to the ocean floor. Valley relief itself can drive fluid flow that promotes continued serpentinization, and also cooling- and volume-change-linked stress variations. Off-axis seismicity suggests that FZs remain weaker that neighbouring oceanic lithosphere. The transform valley relief in general persists as a fracture zone valley that itself can continue to be a major drive of fluid flow even in the “healed” oceanic lithosphere. After reviewing evidence for FZ activity on (normal) ocean floor we will focus on the long-lived impact of FZs at continental margins. Offshore/onshore evidence of ongoing deformation at FZs is observed through seismic activity at both the western Brazilian and eastern Ghana-Côte d’Ivoire ends of the Romanche FZ. The western Brazil end is also characterized by recent folding and faulting, both offshore across the FZ, and onshore co-linearly with FZ extensions into the continent. Seismic activity in continental Brazil is focused where the FZ intersects the continental margin. This activity suggests that FZs remain as permanent weak lithospheric heterogeneities that are able to store elastic strain.

The reasons why FZs remain active are still poorly understood. Possible causes include i) effects of serpentinization that occurs both in the TF and in the FZ through hydrothermal fluid/mantle interaction, ii) thermal stress, iii) changing tectonic stresses related to plate driving forces.

How to cite: Vannucchi, P., Iacopini, D., and Morgan, J. P.: Transforms are forever, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12200, https://doi.org/10.5194/egusphere-egu21-12200, 2021.

Fabian Kutschera, Sara Aniko Wirp, Bo Li, Alice-Agnes Gabriel, Benedikt Halldórsson, Claudia Abril, and Leonhard Rannabauer

Earthquake generated tsunamis are generally associated with large submarine events on dip-slip faults, in particular on subduction zone megathrusts (Bilek and Lay, 2018). Submerged ruptures across strike-slip fault systems mostly produce minor vertical offset and hence no significant disturbance of the water column. For the 2018 Mw 7.5 Sulawesi earthquake in Indonesia, linked dynamic earthquake rupture and tsunami modeling implies that coseismic, mixed strike-slip and normal faulting induced seafloor displacements were a critical component generating an unexpected and devastating local tsunami in Palu Bay (Ulrich et al., 2019), with important implications for tsunami hazard assessment of submarine strike-slip fault systems in transtensional tectonic settings worldwide. 

We reassess the tsunami potential of the ~100 km Húsavík Flatey Fault (HFF) in North Iceland using physics-based, linked earthquake-tsunami modelling. The HFF consists of multiple fault segments that localise both strike-slip and normal movements, agreeing with a transtensional deformation pattern (Garcia and Dhont, 2005). The HFF hosted several historical earthquakes with M>6. It crosses from off-shore to on-shore in immediate proximity to the town of Húsavík. We analyse simple and complex fault geometries and varying hypocenter locations accounting for newly inferred fault geometries (Einarsson et al., 2019), 3-D subsurface structure (Abril et al., 2020), bathymetry and topography of the area, primary stress orientations and the stress shape ratio constrained by the inversion of earthquake focal mechanisms (Ziegler et al., 2016).

Dynamic rupture models are simulated with SeisSol (https://github.com/SeisSol/SeisSol), a scientific open-source software for 3D dynamic earthquake rupture simulation (www.seissol.org, Pelties et al., 2014). SeisSol, a flagship code of the ChEESE project (https://cheese-coe.eu), enables us to explore simple and complex fault and subsurface geometries by using unstructured tetrahedral meshes. The dynamically adaptive, parallel software sam(oa)²-flash (https://gitlab.lrz.de/samoa/samoa) is used for tsunami propagation and inundation simulations and solves the hydrostatic shallow water equations (Meister, 2016). We consider the contribution of the horizontal ground deformation of realistic bathymetry to the vertical displacement following Tanioka and Satake, 1996. The tsunami simulations use time-dependent seafloor displacements to initialise bathymetry perturbations. 

We show that up to 2 m of vertical coseismic offset can be generated during dynamic earthquake rupture scenarios across the HFF, which resemble historic magnitudes and are controlled by spontaneous fault interaction in terms of dynamic and static stress transfer and rupture jumping across the complex fault network. Our models reveal rake deviations from pure right-lateral strike-slip motion, indicating the presence of dip-slip components, in combination with large shallow fault slip (~8 m for a hypocenter in the East), which can cause a sizable tsunami affecting North Iceland. Sea surface height (ssh), which is defined as the deviation from the mean sea level, and inundation synthetics give an estimate about the impact of the tsunami along the coastline. We further investigate a physically plausible worst-case scenario of a tsunamigenic HFF event, accounting for tsunami sourcing mechanisms similar to the one causing the Sulawesi Tsunami in 2018.

How to cite: Kutschera, F., Wirp, S. A., Li, B., Gabriel, A.-A., Halldórsson, B., Abril, C., and Rannabauer, L.: Linking dynamic earthquake rupture to tsunami modeling for the Húsavík-Flatey transform fault system in North Iceland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15891, https://doi.org/10.5194/egusphere-egu21-15891, 2021.

Cecilia McHugh, Leonardo Seeber, Michael Steckler, Syed Humayun Akhter, and Nickolas Dubin

Incoming sediment thickness and composition are primary factors in the morphology and shallow structure of subduction boundaries. Sediment thickness in the Indian Ocean increases SE to NW along the Sunda arc. From <1km along Java to >15km where the boundary encounters the Ganges-Brahmaputra Delta (GBD). Here the accretionary prism broadens to the NW to >300 km wide. It is dominated by shallow-water to non-marine sediment. This segment also features a broad shallow megathrust overlain by linear anticlines rooted in splay faults. It is entirely above sea level and blind in its frontal part. This GBD segment transitions to a more familiar subduction structure and morphology along the submerged Arakan segment to the SE. The SE portion of this segment is characterized by larger splay faults that expose deep-water sediment with mud diapirism forming volcanoes and circular synclines. With increasing sediment input, the NW portion of the Arakan segment encroaches onto the GBD shelf. Both the SE and NW portions of the Arakan segment ruptured in the Mw>8.5 1762 tsunamigenic earthquake according to field and modeling evidence.

Uplifted coral reefs and marine terraces along the Myanmar and Bangladesh coasts document a >500 km rupture in 1762. The uplift, ranging from 6 m to 2 m from south to north, has been linked to rupture on the megathrust and on shallow splays. Tsunami deposits are traced for ~10 km along the St. Martin’s Island anticline and for >40 km along the Teknaf peninsula. Microfossils and mollusk assemblages in these deposits are consistently of shallow water affinity and date the tsunami to 1762. This deposit covers only a small fraction of the inferred megathrust rupture. If it is representative of the total tsunami distribution, a local anticline may have been the main source. Evidence from live coral microatolls show uplift on St. Martin’s Island continuing 250 years after the earthquake. This motion could stem from continued anelastic deformation of the anticline updip of the rupture. More widely distributed evidence from sediment and corals could address questions about megathrust and splay behavior in 1762 and after. Plans include multichannel seismic surveying, high resolution subbottom profiling and 40 m long piston coring to compare the SE to NW shelf portions to the Arakan segment along the Myanmar and Bangladesh coasts. More generally, we aim to better understand subduction and geohazards along thickly sedimented systems.

How to cite: McHugh, C., Seeber, L., Steckler, M., Akhter, S. H., and Dubin, N.: Deformation, earthquakes and tsunamis along thickly sedimented subduction: Arakan segment of the Sunda Arc, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-745, https://doi.org/10.5194/egusphere-egu21-745, 2021.

Yuri Panara, Francesco Emanuele Maesano, Chiara Amadori, Jakub Fedorik, Manlio Ghielmi, Giovanni Toscani, and Roberto Basili

The northern Apennines foredeep is characterized by a Plio-Pleistocene tectonic activity whose driving structures (thrust faults and related anticlines) are always buried by a high amount of syntectonic sediments, both onshore (Po Plain) and offshore (Adriatic Sea), thereby leaving subtle or none apparent signatures in the topo-bathymetry. Compared to the rest of Italy, this area has a relatively moderate earthquake hazard, but historical reports and instrumental recordings testify for significant seismicity. Seismological analyses of recent sizeable earthquakes (e.g., the Mw 6.1, Emilia earthquake in 2012) confirmed that the seismic activity is mainly due to the outermost northern Apennines buried thrusts. In this work, we reconstructed and parameterized the 3D geometry of such buried faults in an offshore sector just south of the Po River delta by interpreting a dense network of 2D seismic reflection profiles. The availability of two regional seismic reflection profiles, coupled with a detailed reconstruction of Plio-Pleistocene horizons, allowed us to restore the deformation cumulated by these thrusts. Our analysis was aimed at (1) establishing the age of inception of the main crustal thrusts, (2) reconstruct the main Plio-Pleistocene tectonic events affecting the study area, and (3) calculate the Plio-Pleistocene slip rates at different time slices and reporting them through probability distribution functions that take into account the uncertainties associated with horizon ages and restoration parameters. Our results show that the Plio-Pleistocene tectonic activity is variably distributed on different thrust faults and decreases exponentially over time after the Gelasian. The analysis performed on the most recent reflectors suggests that a low but not negligible tectonic activity characterizes the main thrusts in the studied region in the last 4-500 ka and hints at a residual activity that may last until the present.

How to cite: Panara, Y., Maesano, F. E., Amadori, C., Fedorik, J., Ghielmi, M., Toscani, G., and Basili, R.: Structural reconstruction, fault parameterization, and slip rate analysis in the northern Adriatic Sea (Italy): implications for the Plio-Pleistocene tectonic activity of long-lived offshore buried thrusts., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15554, https://doi.org/10.5194/egusphere-egu21-15554, 2021.

Ricardo Correia, Manel Prada, Valenti Sallares, Irene Merino, Alcinoe Calahorrano, Luis M. Pinheiro, and César R. Ranero

The SW Iberian Margin has a complex tectonic setting and crustal structure derived from a succession of rift events related to the opening of North Atlantic and Neotethys from the Mesozoic to the Lower Cretaceous, and to the subsequent convergence between Nubian and Eurasian plates from Lower Oligocene to present day. Neogene plate convergence led to the reactivation of pre-existing extensional faults originated during the Mesozoic rifting in a combination of thrust and strike-slip systems. Despite a slow convergence rate, these faults now represent a major regional seismological and tsunamigenic hazard, as demonstrated by the devastating 1755 Lisbon earthquake of M>8.5 and the ensuing tsunami. Thus, unveiling the lithospheric structure along the SW Iberian Margin is not only important to understand the different stages of rifting and compression, but also to characterize the distribution of major lithospheric-scale boundaries, currently active and potentially capable of generating great, destructive tsunamigenic earthquakes.

To this end, we use here a spatially coincident wide-angle seismic (WAS) and multichannel seismic (MCS) data set collected along a NW-SE ~320 km transect SW of São Vicente cape during the FRAME survey in 2018. WAS data were recorded by 24 ocean bottom seismometers and hydrophones (OBS/H) while the MCS data were recorded by a 6 km long, 480 channel streamer. From NW to SE, the transect runs from the Tagus Abyssal plain to the westernmost extension of the Gulf of Cadiz across four major thrust faults, namely, the Tagus Abyssal Plain fault, Marquês de Pombal fault, São Vicente fault, and Horseshoe fault.

We applied joint refraction and reflection travel-time tomography (TTT) using a combination of arrival times identified at both WAS and MCS recordings to invert for the 2D P-wave velocity (Vp) structure of the crust and uppermost mantle, as well as the geometry of the main structural boundaries identified as seismic reflectors: the top of the acoustic basement and the Moho. Combining WAS and MCS travel-times brings a remarkable increase in the resolution and accuracy of the structure of the upper layers (i.e. top of the basement) thanks to the huge increase of spatial sampling in the shallow parts of the crust provided  by MCS data as compared to WAS data alone.

The inverted model shows a Vp structure with abrupt lateral velocity and structural variations marked by a rough Top of Basement topography and sharp changes in crustal thickness. In the northernmost part of the model there is evidence of mantle exhumation. The Moho shallows beneath the NE continuation of the Horseshoe Basin and the Gorringe Bank, coinciding with the location of the Marquês de Pombal, São Vicente, and Horseshoe thrust faults. The inversion of deep seismic phases reveals the presence of four southwest dipping reflectors that sheds new light into the deep geometry these major regional thrust faults.

How to cite: Correia, R., Prada, M., Sallares, V., Merino, I., Calahorrano, A., M. Pinheiro, L., and R. Ranero, C.: Structure of the SW Iberian Margin from Combined Wide-angle and Multichannel Seismic Reflection Data (FRAME Project), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6715, https://doi.org/10.5194/egusphere-egu21-6715, 2021.

Paraskevi Nomikou, Dimitris Evangelidis, Dimitris Papanikolaou, Danai Lampridou, Dimitris Litsas, Giannis Tsaparas, and Ilias Koliopanos

On October 30th 2020 a strong earthquake of magnitude 7.0 occurred north of Samos Island at the Eastern Aegean Sea. This seismic event was another destructive active deformation in the long seismic history of Samos since the ancient times. Preliminary reports focused the seismic epicenter at about 10 km north of Karlovassi, situated at the western part of the Samos E-W trending coastline. The earthquake mechanism corresponds to an E-W normal fault dipping to the north. The activated fault was assumed to be running along the northern margin of Samos Island, which bounds from the south the Samos basin.

Immediately after the seismic activity and during the aftershock period in December 2020 an hydrographic survey off the northern coastal margin of Samos Island was conducted with R/V NAUTILOS of the Hellenic Navy Hydrographic Service, using the multibeam SeaBat 7160 RESON. The result of the hydrographic survey was a detailed bathymetric map with 15m grid interval and 50m isobaths.  The main morphological aspects of Samos Basin are a 14 km long, 6 km wide and 690 m deep elongated E-W basin developed north of Samos Island.

The southern margin of the basin is abrupt with morphological slopes of more than 10o, following the major E-W normal fault surface, running along the coastal zone, with an overall throw of more than 500m. In contrast, the northern margin of the basin shows a gradual slope increase towards the south from 1o to 5o. Numerous small canyons trending N-S transversal to the main direction of the Samos coastline are observed along the southern margin, between 600 and 100 m water depth.  These canyons have a length around 2,7 km and width between 100-300 m. Two large submarine landslides with a canyon width of 1,3 km and 0,8 Km, are located north of Karlovasi. The creation of the canyons is probably due to the uplift of Northern Samos Island and their 500 m vertical height difference corresponds to the average fault throw that has controlled the steep slopes of the margin. The orientation of the fault scarp changes at the western Samos coastline from E-W to ENE-WSW facing the neighboring Ikaria Basin, which is developed to the west of Samos Basin. The division line between the Ikaria and Samos basins runs N-S from the northern slopes and coast of the Kerketeas mountain (1443m). The aftershocks of the 30th October main shock are limited east of the N-S division line with only a minor activity 15 km to the west within the eastern margin of the Ikaria Basin.

How to cite: Nomikou, P., Evangelidis, D., Papanikolaou, D., Lampridou, D., Litsas, D., Tsaparas, G., and Koliopanos, I.: Morphotectonic analysis along the northern margin of Samos Island, related to the seismic activity of October 2020, Aegean Sea, Greece, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3849, https://doi.org/10.5194/egusphere-egu21-3849, 2021.

Javier Escartín, Alex Hughes, Jean-Emmanuel Martelat, Valentine Puzenat, Thibaut Barreyre, Paraskevi Nomikou, Guillaume Jouvé, Alban Bouchard, Remi Stephan, Alain Philippe, and Mathias Delescluse

The Milos hydrothermal field is one of the largest known shallow water hydrothermal systems, and shows both fluid and gas outflow through the seafloor. Recent studies based on imagery acquired by both aerial and submarine drones (Puzenat et al., submitted) reveal several types of fluid outflow associated with bacterial mats along the SE coast of the island (Paleochori, Spathi, and Agia Kyriaki bays). From these observations? include: a) zones of polygonal hydrothermal outflow and associated bacterial mats, b) extended white (bacterial) patches, and c) isolated ones. Subseafloor hydrothermal circulation is hosted in sediments with subseafloor temperatures >50°C, and there is a clear association between hydrothermal circulation and active degassing.

To understand the controls on and relationships between fluid and gas outflow in the area, we need to characterise: a) the nature of the subseafloor (sediment thickness, composition & permeability); b) the distribution of gas and subseafloor fluids, and c) the distribution of gas flares emanating from the seafloor. In November 2020, we conducted a short pilot geophysical study at Paleochori Bay, deploying a towed catamaran with a multibeam echo sounder (iXblue Seapix) to obtain seafloor bathymetry, acoustic backscatter and water column detection of gas flares. We also deployed a sub-bottom profiler (iXblue Echoes 3500 T1) to image sediment architecture and gas/fluid diffusion within the sediment. Our survey focused on Paleochori Bay, surveing areas from ~5 m (nearshore) to ~100 m waterdepth (offshore).

Preliminary results of this geophysical survey suggest that subseafloor gas accumulations play a major role on the nature and structure of hydrothermal activity at Milos. These gas accumulations within the sediments develop along an onshore/offshore fault system, and likely control the shallow subseafloor thermal structure, establishing a thin thermal conductive layer between the roof of gas pockets and the seafloor.[GJ1] [je2]   We will report on the link between the distribution and geometry (extent, depth, acoustic nature of the accumulations) of gas pockets, fluid outflows, and gas outflows, all of which will be characterised from both seafloor imagery and subsurface geophysical surveys. We will also discuss how gas pocket geometry may be linked to both fluid flow and subseafloor temperature structure. [HA3] 


How to cite: Escartín, J., Hughes, A., Martelat, J.-E., Puzenat, V., Barreyre, T., Nomikou, P., Jouvé, G., Bouchard, A., Stephan, R., Philippe, A., and Delescluse, M.: Tectonic constraints on submarine hydrothermal activity, degassing, and subseafloor gas storage (Milos Shallow Water Hydrothermal System, Greece), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12597, https://doi.org/10.5194/egusphere-egu21-12597, 2021.

Aaron Micallef, Daniele Spatola, Antonio Caracausi, Francesco Italiano, Giovanni Barreca, Sebastiano D'Amico, Lorenzo Petronio, Franco Coren, Lorenzo Facchin, Rita Blanos, Alessandro Pavan, Paolo Paganini, Marco Taviani, Luca Baradello, and Emiliano Gordini

The Maltese Islands (central Mediterranean Sea) are intersected by two normal fault systems associated with continental rifting to the south. Because of a lack of evidence for offshore displacement and insignificant historical seismicity, the systems have been considered to be inactive. Here we integrate aerial and marine geological, geophysical and geochemical data to demonstrate that: (i) the majority of faults offshore the Maltese Islands underwent extensional to transtensional deformation during the last 20 ka, (ii) active degassing of CH4 and CO2 occurs via these faults. The gases migrate through Miocene carbonate bedrock and the overlying Plio-Pleistocene sedimentary layers to generate pockmarks at the muddy seafloor and rise through the water column into the atmosphere. We infer that the offshore faults systems are permeable and that they were active recently and simultaneously. The latter can be explained by a transtensional system involving two right-stepping, right-lateral NW-SE trending faults, either binding a pull-apart basin between the islands of Malta and Gozo or associated with minor connecting antitethic structures. Such a configuration may be responsible for the generation or reactivation of faults onshore and offshore the Maltese Islands, and fits into the modern divergent strain-stress regime inferred from geodetic data.

How to cite: Micallef, A., Spatola, D., Caracausi, A., Italiano, F., Barreca, G., D'Amico, S., Petronio, L., Coren, F., Facchin, L., Blanos, R., Pavan, A., Paganini, P., Taviani, M., Baradello, L., and Gordini, E.: Active faulting offshore the Maltese Islands revealed by geophysical and geochemical observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6570, https://doi.org/10.5194/egusphere-egu21-6570, 2021.

Patrick Oswald, Jasper Moernaut, Stefano Fabbri, Marc De Batist, Irka Hajdas, Ortner Hugo, and Michael Strasser

Intraplate tectonic regimes such as the European Alps are characterized by low crustal deformation rates and thus long recurrence rates of severe earthquakes. High-quality paleoseismic archives are required to overcome our limited perspective of earthquake recurrence and maximum magnitude. However, especially on-fault paleoseismic evidence is scarcely found because of high erosion rates, gravitational slope processes and penetrative anthropogenic landscape modification, which often obscure geomorphic features related to surface ruptures.

Here, we present the inneralpine lake archive of Achensee in the Northern Calcareous Alps (6.8km² area; 133m water depth) cross-cut by a major fault and potentially holding a continuous paleoseismic archive since the last deglaciation at ~18 ka BP. This major fault is a Cretaceous-Paleogene relatively steep-dipping thrust, with at least 15km length and several hundreds of meters geological offset, located within the current area of enhanced seismicity and oriented to be preferentially re-activated in the current stress field. We used a high-resolution multi-beam bathymetry, a combination of a very dense grid of 3.5kHz “pinger” subbottom profiler and single-channel high-frequency (~0.8-2.0kHz) “sparker” reflection seismics to investigate the postglacial infill with high-resolution and image the deeper structures (e.g. the glacially scoured valley). The seismo-stratigraphic interpretation was ground-truthed and 14C-dated by five, up to 11m long sediment cores from the two main subbasins.

We discovered at least eight strong earthquakes hitting the region in the past 11,000 years by off-fault paleoseismic evidence expressed by coeval, multiple mass-transport deposits (MTDs) and co-genetic turbidites. These earthquakes must have reached seismic intensity of >VI (EMS-98) at the lake site calibrated with the strongest known historical earthquake of the region (ML 5.2 in Hall CE1670). MTD size and extent corresponding to the CE1670 earthquake compared to the other earthquake imprints let us infer that at least four of the paleo-earthquakes reached higher intensities at Achensee.

Strikingly, Achensee has also recorded on-fault evidence expressed by steeply-dipping to vertical faults offsetting the lacustrine stratigraphy. These stratigraphic offsets can be traced downwards to the acoustic basement, which hints at faulting originating in the bedrock. For at least two stratigraphic levels, these faults are directly overlain by multiple MTDs indicating that fault activity and slope failures have occurred quasi-simultaneously. The faults observed on the seismic data, affecting the sedimentary infill of the lake, are located above the inferred trace of the major fault where it crosses the lake. Based on this rather unique combined on-fault and off-fault evidence we propose strong paleo-earthquakes documenting activity of this major thrust at ~8.5 ka BP and in the Late Glacial period (below reach of sediment cores). We suggest that these earthquakes have reached ML~5.5-6, which is within the magnitude capability of this thrust and at the lower limit of generating surface ruptures according to worldwide magnitude-surface rupture relationships. The other six event horizons lacking in on-fault evidence either represent earthquakes sourced from another fault in the region, earthquakes with a smaller magnitude not capable of surface rupturing like the ML5.2 earthquake in Hall CE1670 or on-fault evidence is blurred in seismic data by subsequent stacking of MTDs.

How to cite: Oswald, P., Moernaut, J., Fabbri, S., De Batist, M., Hajdas, I., Hugo, O., and Strasser, M.: Combined on-fault and off-fault paleoseismic evidence in the postglacial lacustrine sediments of Achensee (Austria, Eastern Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-555, https://doi.org/10.5194/egusphere-egu21-555, 2021.

Manfred Lafosse, Elia d'Acremont, Sara Lafuerza, Alain Rabaute, Martin Jollivet-Castelot, Belén Alonso, Gemma Ercilla, Juan-Tomas Vazquez, and Christian Gorini

The tectonics of the Alboran Sea control the distribution of topographic highs and depressions, influencing the water masses' paths controlling the deep basins sedimentation rates. Altogether tectonics and deep currents shaped the seafloor on which we map active faults, contourites, pockmarks, and submarine landslides.  Recent numerical models highlight that some of those landslides can generate tsunamis waves on nearby coastal areas, creating the need to describe better and understand those seabed features. Consequently, we put together bathymetric and seismic data to measure Pleistocene slides affecting the deep Alboran basin in an unprecedented collective effort.
We mapped and relatively dated 66 mass transport deposits (MTDs) in the West Alboran and Pytheas fields on the north and south flank of the Alboran Ridge.  We measured their surfaces, decompacted volumes, slopes, run out, scars heights, scars elevations, and described their type (debris flow or slide) qualitatively from their echo facies.  When possible, we also measured the run-off from the scar. We investigated the factor of safety (FOS) and computed based seismic moments based on empirical relationship and faults geometry to characterize the preconditioning factors and triggering mechanisms. The first important result is that post-1.12 Ma MTDs mobilized the most important volumes, in line with the Alboran Sea's topographic highs. Second, seismic lines and the bathymetric images evidence blind reverse faults related to fluid escapes that could contribute to local overpressures in shallow contouritic sediments. Third, we show that local slopes are too flat to allow slopes to destabilize under gravity force only, suggesting that other causal factors need to be considered. Fourth, known seismicity on strike-slip faults in the Alboran Sea is unlikely to trigger MTDs in most investigated areas, suggesting that a combination of preconditioning factors, such as local overpressures and/or reduced strength properties with reverse faults activity, seems the most plausible explanation to trigger the observed MTDs.
The overall results highlight that the Alboran tectonics explain the slope destabilization, with varying local sediment properties. However, those properties and past overpressures, deformations, and fluid flows remain to be locally detailed. Future works involving sediments characterizations and dating will follow after the ALBACORE survey scheduled in 2021.

How to cite: Lafosse, M., d'Acremont, E., Lafuerza, S., Rabaute, A., Jollivet-Castelot, M., Alonso, B., Ercilla, G., Vazquez, J.-T., and Gorini, C.: Did the Pleistocene tectonics in the Alboran Sea change the distribution and origin of submarine landslides? , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12955, https://doi.org/10.5194/egusphere-egu21-12955, 2021.

From Earthquake DeformatIon To seismic Hazard assessment (EDITH) – finding novel approaches to combining observations and modelling to understand the seismic cycle
Kirsty McKenzie and Kevin Furlong

Upper plate faults along the Cascadia subduction margin of North America go through a 3 stage evolution over millions of years as a consequence of the migrating Mendocino Triple Junction (MTJ). Initially, NE-directed cyclic shortening produced by the Cascadia subduction earthquake cycle drives reverse dip-slip motion on trench-parallel faults. As the triple junction moves north, NNW-shortening associated with the Mendocino Crustal Conveyor (MCC, Furlong & Govers, 1999) is superimposed on the cyclic subduction-earthquake-cycle regional stress field. As the triple junction migrates further north, and these faults transfer from the subduction to transform plate boundary, they become part of the San Andreas system and are loaded by right-lateral shear. In this work we investigate how the faulting behavior in northern California evolves through time from first being driven by cyclic subduction zone stresses (superimposed on a NNW-oriented shortening field) to eventually forming the primary structures within a dominantly strike-slip stress regime.

We decompose the observed horizontal GPS velocity field in southern Cascadia to determine a subduction coupling component and a NNW-directed displacement component to separate the subduction cycle effects from other tectonic effects on the behavior of upper plate faulting and its evolution through time. Since the MCC processes acts over millions of years, we assume that the effects associated with the NNW-directed signal can be represented by a constant stress field over subduction earthquake cycle timescales. Early in the subduction earthquake cycle, the principal stresses north of the MTJ are oriented in this NNW-SSE direction and rotate clockwise as the subduction component increases. This stress cycle then resets following each large megathrust event. Coulomb stress analyses indicate that the cyclic nature of the regional stress field, changes the likelihood of faulting and slip behavior on faults in southern Cascadia over time intervals of 100s of years. Trench-parallel faults are most likely to exhibit right-lateral or oblique motion early in the seismic cycle, however by ~100-200 years following a megathrust event, they are more likely to exhibit reverse dip-slip motion as the stress effects from the subduction component increase.

Though the NNW-oriented displacement field is assumed to be temporally constant over subduction earthquake cycle timescales, the spatial extent of this deformation field constrains strain localization within the upper plate. For example, a steep decrease in GPS velocities from SW to NE in southernmost Cascadia indicates right-lateral strain is accumulating adjacent to the relatively rigid Klamath Mountain Province. This region of localized right-lateral shear coincides with the location of the development of several regional-scale right-lateral strike slip faults. We hypothesize these faults, formed within the subduction regime, evolve to become regional-scale 'San Andreas-type' plate boundary faults. Understanding the implications of such time- and space-variable stress regimes provides insight into interpreting geologic estimates of the slip history of faults along the Cascadia and northern San Andreas margins of North America, and also a framework for understanding how a new plate boundary develops following a major change in plate interaction.

How to cite: McKenzie, K. and Furlong, K.: Stress-cycle driven evolution of faulting in a plate boundary transition zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-941, https://doi.org/10.5194/egusphere-egu21-941, 2021.

Irina Vladimirova, Yurii Gabsatarov, Grigory Steblov, and Leopold Lobkovsky

The subduction zone is a natural laboratory for studying the seismic cycle. On March 11, 2011, in the central part of the Japan subduction zone, the strongest Mw=9.0 Tohoku earthquake occurred, terminating a seismic cycle that lasted about 1200 years. We analyzed two decades of GNSS observations at 1400 GEONET stations to reveal the peculiarities of the tectonic and rheological structure of the Japan subduction zone which driven such a long-term seismic cycle. We consider GNSS data within the framework of a generalized approach, including the assessment of the coupling of the interplate interface before the earthquake, the construction of a model of the distributed displacement in the source zone, and the study of postseismic processes characterizing the relaxation of elastic stresses in the vicinity of the source.

As a result, we found that in the last year before the earthquake, there was an increase in the rates of elastic deformation of the continental margin and a corresponding increase in the interplate coupling. To study the process of the release of elastic energy during the Tohoku earthquake, we built a model of the distributed slip in the source. We used different earth models during inversion of GNSS data to study the impact of the regional tectonic and rheological structure and confirm the resilience of our inversion technique. We used GNSS data to build a model of pure afterslip in the first six months after the Tohoku earthquake and a model of afterslip combined with the short-term viscoelastic relaxation to estimate the relative contributions of these postseismic processes to the observed displacement field. Long-term postseismic time series of GNSS displacements were used to build the model of viscoelastic relaxation in the asthenosphere following the Tohoku earthquake. To estimate the transition time of the subduction zone to the steady-state of elastic stress accumulation we constructed a forecast of attenuation of viscoelastic stresses in the asthenosphere on the basis of our viscoelastic relaxation model.

We also studied the possible block structure of the Japanese Islands and its impact on the seismic cycle performing cluster analysis of GNSS displacement data at different stages of the seismic cycle.

This study was supported by the Russian Science Foundation (project 20–17-00140).

How to cite: Vladimirova, I., Gabsatarov, Y., Steblov, G., and Lobkovsky, L.: Influence of Regional Tectonic and Rheological Structure on the Seismic Cycle for Northeast Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-304, https://doi.org/10.5194/egusphere-egu21-304, 2020.

Shaogang Wei, Xiwei Xu, Tuo Shen, and Xiaoqiong Lei

The Capital Circle (CC) is a region with high risk of great damaging earthquake hazards. In our present study, by using a subset of rigorously GPS data around the North China Plain (NCP), med-small recent earthquakes data and focal mechanism of high earthquakes data covering its surrounding regions, the following major conclusions have been reached: (a) Driven by the deformation force associated with both eastward and westward motion, with respect to the NCP, of the rigid South China and the rigid Amurian block, widespread sinistral shear appear over the NCP, which results in clusters of parallel NNE-trending faults with predominant right-lateral strike-slips via bookshelf faulting within the interior of the NCP. (b) Fault plane solutions of recent earthquakes show that tectonic stress field in the NCP demonstrate overwhelming NE-ENE direction of the maximum horizontal principal stress, and that almost all great historical earthquakes in the NCP occurred along the NWW-trending Zhangjiakou-Bohai seismic belt and the NNE-trending Tangshan-Hejian-Cixian seismic belt. Additionally, We propose a simple conceptual model for inter-seismic deformation associated with the Capital Circle, which might suggest that two seismic gaps are located on the middle part of Tangshan-Hejian-Cixian fault seismic belt (Tianjin-Hejian segment) and the northeast part of Tanlu seismic belt (Anqiu segment), and constitute as, in our opinion, high risk areas prone to great earthquakes.

How to cite: Wei, S., Xu, X., Shen, T., and Lei, X.: Contemporary deformation and earthquake hazard of the Capital Circle of China inferred from GPS Measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4164, https://doi.org/10.5194/egusphere-egu21-4164, 2021.

Qi Ou, Simon Daout, Chris Rollins, Jonathan Weiss, and Barry Parsons

Seismic hazard assessment for the NE Tibetan Plateau is of paramount importance because of the growing population density and the accelerated communication and trade activities along the rejuvenated Ancient Silk Road, following the Belt and Road Initiative, and the opening of the high speed railways. Previous-generation seismic hazard assessments were largely based on earthquake catalogues which are shorter than typical earthquake cycles and are temporally and spatially incomplete. This is exacerbated by the fact that magnitudes of many historical Chinese earthquakes are overestimated. In this study, we present new earthquake rate estimates for the NE Tibetan Plateau derived from both an InSAR strain rate map and a re-estimated magnitude of the 1920 Haiyuan Earthquake. First, we obtain a ~100 m resolution strain rate map from five years of Sentinel-1 InSAR data covering an area of 439254 km2 which shows strain concentrated along the Haiyuan and East Kunlun Faults and distributed across the Qilian thrusts and the West Qingling Fault. Second, the magnitude of the Haiyuan Earthquake has been re-estimated to Mw 7.9 ± 0.2 using both historical seismograms and offset measurements. Taking the total moment release rate given by the strain rate map and the magnitude of the 1920 Haiyuan Earthquake as the largest magnitude in the Gutenberg-Richter relationship, we generate rate-balancing frequency-magnitude models with different b values and percentages of seismic moment release. Comparing our models against four earthquake catalogues covering different periods and magnitude ranges suggests the following: (1) With a b value of 1 and 75% seismic moment release, the calculated relationship fits well the International Seismological Centre - Global Earthquakes Catalogue (ISC-GEM, 97 years) catalogue in the range Mw>6.5, but overestimates all other catalogues not containing the Haiyuan Earthquake; (2) keeping a b value of 1 and in order to fit the Global Centroid Moment Tensor Catalogue (GCMT, 34 years), the China Earthquake Networks Center Catalogue (CENC, 12 years) and the China Historical Strong Earthquakes Catalogue (CHSEC, 411 years), a low seismic release rate of 30% would be required; the resultant relationship also fits the ISC-GEM catalogue excluding the Haiyuan Earthquake and its aftershocks; (3) to fit all of the catalogues, it is necessary to reduce the b value to 0.7, in which case only 25% aseismic moment release would be required, giving confidence that Mw 7.9 ± 0.2 is likely the largest magnitude required to balance the tectonic strain in the NE Tibetan Plateau. This study highlights the dominating strain release by, and the effect on the b value of, the largest earthquake and demonstrates the advantage of combining tectonic strain and earthquake catalogues for seismic hazard assessment.

How to cite: Ou, Q., Daout, S., Rollins, C., Weiss, J., and Parsons, B.: Crustal strain and seismic hazard of the NE Tibetan Plateau, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6588, https://doi.org/10.5194/egusphere-egu21-6588, 2021.

Elenora van Rijsingen, Eric Calais, Romain Jolivet, Jean-Bernard de Chabalier, Richard Robertson, Graham Ryan, and Steeve Symithe

Horizontal GPS velocities show that the Lesser Antilles subduction zone is currently experiencing low interseismic coupling, meaning that little to no elastic strain is building up as the North- and South American plates subduct beneath the Caribbean plate. However, geological data on Quaternary coral terraces and active micro-atolls in the central part of the arc reveal slow subsidence over the past 125,000 to 100 years, likely tectonic in origin. It has been proposed that coupling along the subduction interface could be responsible for this geological subsidence. We use forward elastic models with a realistic slab geometry to show that a locked subduction interface would actually produce uplift of the island arc, which contradicts these geological observations. We also show that vertical GPS data in the Lesser Antilles indicates a subsidence of 1-2 mm/yr of the entire arc. This short-term subsidence is in agreement with the ~100-year trend of 1.1 mm/yr subsidence derived from coral micro-atolls in eastern Martinique. Since locking of the subduction interface is inconsistent with this observed subsidence of the arc, we explore other mechanisms that could this observation, such as postseismic effects of historical earthquakes, slab retreat, tectonic erosion, accretionary wedge collapse or extension in the overriding plate. 

How to cite: van Rijsingen, E., Calais, E., Jolivet, R., de Chabalier, J.-B., Robertson, R., Ryan, G., and Symithe, S.: Vertical tectonic motions in the Lesser Antilles: linking short- and long-term observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-934, https://doi.org/10.5194/egusphere-egu21-934, 2021.

Yurii Gabsatarov, Irina Vladimirova, Dmitry Alexeev, and Leopold Lobkovsky

The strongest subduction earthquakes (M≥8) lead to the release of the huge amount of elastic stresses accumulated over hundreds or even thousands of years. Prediction of such earthquakes, causing significant socio-economic and environmental damage, is one of the most important and urgent tasks of geophysics.

To date, significant advances have been made in the field of earthquake prediction using models based on the concept of a continuous geophysical medium that ruptured coseismically along the main fault. As an alternative, models are proposed that take into account the fault-block structure of the continental margin, confirmed by seismological and oceanographic studies. In our study, we consider one of such models - a keyboard-block model (single-element) which combines the ideas of possible synchronous destruction of several adjacent asperities, mutual slip along a plane with variable friction depending on velocity, and subsequent healing of destructed portions of the medium under high-pressure conditions. This concept made it possible to simulate the displacement of surface points of frontal seismogenic blocks at all stages of the seismic cycle.

GNSS observations in subduction regions are carried out mostly on islands situated on the rear massif far from the seismogenic blocks. Strong multidirectional motion registered on GNSS stations during the seismic cycle, as well as seismological and geological data, clearly indicate that the rear part of the arc also has a complex structure and is divided into separate segments by large faults rooted into the contact zone of interacting lithospheric plates. We made a generalization (double-element) of the original model to consider the discontinuity of not only the frontal but also the rear part of the island arc.

We compared the earth's surface displacements during the seismic cycle in the Central Kurils, obtained within the framework of the continuous model, as well as the single-element and two-element keyboard models, to establish the influence of various configurations of the fault-block structure of the continental margin on the seismic cycle. We constructed the continuous model on the basis of our slip distribution model for the 2006 Simushir earthquake which indicates the interplate coupling patches prior to this earthquake.

This study was supported by the Russian Science Foundation (project 20–17-00140).

How to cite: Gabsatarov, Y., Vladimirova, I., Alexeev, D., and Lobkovsky, L.: Two-Element Keyboard-Block Model Of Megathrust Earthquakes Generation For Central Kurils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-305, https://doi.org/10.5194/egusphere-egu21-305, 2020.

Jochen Woessner, Jessica Velasquez, Marleen Nyst, Delphine Fitzenz, and Laura Eads

Megathrust earthquakes along the South American subduction zone where the Nazca plate slips below the South American plate rapidly subducts below the South American plate contribute significantly to the seismic hazard in Chile, Peru, Ecuador and Colombia. Estimating recurrence of the megathrust events is of prime interest not only for securing effective counter measures for engineering purposes, but also for assessing seismic hazard and risk for appropriate disaster risk management solutions in the insurance sector.

We present an evaluation and interpretation of recent research on the recurrence of megathrust earthquakes along the South America subduction zone. The modelling approach is conceptually founded in the asperity model and in this spirit evidence for documented earthquakes is assembled. We utilize time-independent and time-dependent recurrence models to understand the range and likelihood of recurrence times given the incomplete picture of the seismic history and the impact from uncertain event dates based on paleo-seismic / paleo-tsunami studies. In addition, we illustrate the sensitivity of recurrence rates for the largest earthquakes due to assumptions on seismic coupling and the size of potential ruptures.

Downstream from the recurrence rate analysis, the results are used to estimate the impact of the subduction interface model seismicity on a select set of exposure subject to earthquake shaking due to those events. These examples highlight the potential range of seismic hazard and risk and set the basis to further constrain disaster risk management solutions. 

How to cite: Woessner, J., Velasquez, J., Nyst, M., Fitzenz, D., and Eads, L.: Recurrence of megathrust events: Impact on hazard and risk in South America, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7112, https://doi.org/10.5194/egusphere-egu21-7112, 2021.

Sylvain Michel, Romain Jolivet, Chris Rollins, Jorge Jara, and Luca Dal Zilio

Recent studies have shown that the Himalayan region is under the threat of earthquakes of magnitude 9 or larger. These estimates are based on comparisons of the geodetically inferred moment deficit rate with the seismicity of the region. However, these studies do not account for the physics of fault slip, specifically the influence of frictional barriers on earthquake rupture dynamics, which controls the extent and therefore the magnitude of large earthquakes. Here, we propose a methodology for incorporating outcomes of physics-based earthquake cycle models into hazard estimates. The methodology takes also into account the moment deficit rate, the magnitude-frequency of the current and historical catalogs, and the moment-area earthquake scaling law.

For the Himalaya setting, we estimate an improved probabilistic estimate moment deficit rate using coupling estimates inferred using a Bayesian framework. The locking distribution of the fault suggests an along-strike segmentation of the MHT with three segments that may act as aseismic barriers. The effect of the barriers on rupture propagation is assessed using results from dynamic models of the earthquake cycle. We show that, accounting for measurement and methodological uncertainties, the MHT is prone to rupturing in M8.7 earthquakes every T>200 yr, with M>9.5 events being greatly improbable. The methodology also allows to estimate the probability of the position of earthquakes on the fault based on the effect of the seismic barriers and their magnitude. This study provides a straightforward and computationally efficient method for estimating regional seismic hazard accounting for the full physics of fault slip.

How to cite: Michel, S., Jolivet, R., Rollins, C., Jara, J., and Dal Zilio, L.: Seismogenic potential of the Main Himalayan Thrust constrained by coupling segmentation and earthquake scaling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8649, https://doi.org/10.5194/egusphere-egu21-8649, 2021.

Jill Peikert, Andrea Hampel, and Meike Bagge

The analysis of the Coulomb stress changes has become an important tool for seismic hazard evaluation because such stress changes may trigger or delay next earthquakes. Processes that can cause significant Coulomb stress changes include coseismic slip, earthquake-induced poroelastic effects as well as transient postseismic processes such as viscoelastic relaxation. In this study, we investigate the spatial and temporal evolution of pore fluid pressure changes and fluid flow during the seismic cycle, their dependency on the permeability in the crust and the interaction with postseismic viscoelastic relaxation. To achieve this, we use 2D finite-element models for intra-continental normal and thrust faults, which include coseismic slip, poroelastic effects, postseismic viscoelastic relaxation and interseismic stress accumulation. In different experiments, we vary (1) the permeability of the upper and lower crust while keeping the viscosity structure constant and (2) the viscosity of the lower crust and lithospheric mantle, while we keep the permeabilities constant. (1) The modelling results show that the highest changes in pore fluid pressure during and after the earthquake occur within a distance of ~ 1 km around the lower fault tip at the transition between upper and lower crust. The evolution of pore pressure and fluid flow depends primarily on the permeability in the upper crust. With decreasing permeability, the possibility of the pore fluids to flow decreases and thus, in the postseismic phase, the duration of the poroelastic relaxation increases, from a few days to several years, until the pore pressure reaches the initial pressure of the preseismic phase. In contrast, the influence of variations of the permeability in the lower crust on the pore pressure changes is negligible. For high upper-crustal permeabilities, postseismic vertical velocities are high and decreases rapidly with time, from around 120 mm/a after the first year by two orders of magnitude after 10 years, whereas for low permeabilities they remain consistently low over the years after the earthquake. (2) Models with low viscosity of the lower crust show that the timescales of poroelastic effects and viscoelastic relaxation overlap and affect the postseismic velocity already in the early postseismic phase and that both processes decay within a few years after the earthquake. For higher viscosities, the velocity is initially dominated by pore pressure changes during the first few years, whereas viscoelastic relaxation lasts for decades. Both processes also show differences in their spatial scale. Poroelastic effects occur within a few kilometers around the fault, whereas viscoelastic relaxation acts on tens to hundreds of kilometers. As both processes can cause Coulomb stress changes on faults in the vicinity of the earthquake source fault, it is important to understand the spatial and temporal evolution, the effects on the individual faults and the interaction of both processes during the earthquake cycle. Future work will therefore include the calculation and examination of Coulomb stress changes on intra-continental normal and thrust faults using 3D models that include poroelastic effects and viscoelastic relaxation.

How to cite: Peikert, J., Hampel, A., and Bagge, M.: 2D finite-element modelling of the interaction between poroelastic effects and viscoelastic relaxation during the seismic cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2599, https://doi.org/10.5194/egusphere-egu21-2599, 2021.

Eric Geist and Tom Parsons

A critical component of seismic hazard analysis is understanding the frequency and spatial distribution of earthquakes with different magnitudes on nearby faults.  A framework for determining the optimal spatial distribution of earthquakes on a complex fault system is developed using combinatorial optimization methods. Input to the framework is a millennia-scale sample of earthquakes taken from a regional Gutenberg-Richter (G-R) relation.  We then determine the optimal spatial arrangement of each earthquake in the fault system according to an objective function and constraints.  Our previously published results focus on minimizing the total misfit in slip rates as the objective function; constraints were maximum and minimum slip rate values that incorporate uncertainty in slip-rate values for each fault.  Both global and local combinatorial optimization methods have been developed to solve these problems: integer programming and the greedy sequential algorithm, respectively. Resulting on-fault magnitude distributions cannot be simply classified as being either purely characteristic or G-R. For example, faults may exhibit multiple “characteristic” magnitudes or a power-law distribution of magnitudes over a restricted range. Current research involves adapting the general combinatorial framework to include other and multiple objective functions, including minimizing the variation in accumulated stress over millennia.  The framework can also accommodate branching and step-over connections for the slip-rate objective, while current research is underway to include interaction stress loading among the different faults in the fault system for stress-based objectives.  Results from these methods are valuable for verifying the assumed magnitude-frequency distributions for faults in probabilistic seismic and tsunami hazard analyses.

How to cite: Geist, E. and Parsons, T.: Combinatorial Framework for Obtaining the Optimal Spatial Distribution of Earthquakes on Complex Fault Systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6723, https://doi.org/10.5194/egusphere-egu21-6723, 2021.

Remi Matrau, Yann Klinger, Jonathan Harrington, Ulas Avsar, Esther R. Gudmundsdottir, Thorvaldur Thordarson, Armann Hoskuldsson, and Sigurjon Jonsson

Paleoseismology is key to study earthquake recurrence and fault slip rates during the Late Pleistocene-Holocene. The Húsavík-Flatey Fault (HFF) in northern Iceland is a 100 km-long right-lateral transform fault connecting the onshore Northern Volcanic Zone to the offshore Kolbeinsey Ridge and accommodating, together with the Grímsey Oblique Rift (GOR), ~18 mm/yr of relative motion between the Eurasian and North American plates. Significant earthquakes occurred on the HFF in 1755, 1838 and 1872 with estimated magnitudes of 6.5-7. However, historical information on past earthquakes prior to 1755 is very limited in both timing and size.

We excavated five trenches in a small basin (Vestari Krubbsskál) located 5.5 km southeast of the town of Húsavík and at 300 m.a.s.l. and one trench in an alluvial fan (Traðargerði) located 0.5 km north of Húsavík and at 50 m.a.s.l. In a cold and wet environment, such as in coastal parts of Iceland, one has to take into account periglacial processes affecting the topsoil to discriminate tectonic from non-tectonic deformation. We used tephra layers in the Vestari Krubbsskál and Traðargerði trenches as well as birch wood samples in Traðargerði to constrain the timing of past earthquakes. Tephra layers Hekla-3 (2971 BP) and Hekla-4 (4331±20 BP) are visible in the top half of all the trenches. In addition, a few younger tephra layers are visible in the top part of the trenches. In Traðargerði several dark layers rich in organic matter are found, including birch wood-rich layers from the Earlier Birch Period (9000-7000 BP) and the Later Birch Period (5000-2500 BP). In Vestari Krubbsskál the lower halves of the trenches display mostly lacustrine deposits whereas in Traðargerði the lower half of the trench shows alluvial deposits overlaying coarser deposits (gravels/pebbles) most likely of late-glacial or early post-glacial origins. In addition, early Holocene tephra layers are observed in some of the trenches at both sites and may correspond to Askja-S (10800 BP), Saksunarvatn (10300 BP) and Vedde (12100 BP). These observations provide good age constraints and suggest that both the Vestari Krubbsskál and Traðargerði trenches cover the entire Holocene.

Trenches at both sites show significant normal deformation in addition to strike-slip, well correlated with their larger scale topographies (pull-apart basin in Vestari Krubbsskál and 45 m-high fault scarp in Traðargerði). We mapped layers, cracks and faults on all trench walls to build a catalogue of Holocene earthquakes. We identified events based on the upward terminations of the cracks and retrodeformation. Our results yield fewer major earthquakes than expected, suggesting that large earthquakes (around magnitude 7) are probably rare and the more typical HFF earthquakes of magnitude 6-6.5 likely produce limited topsoil deformation.[yk1]  Our interpretation also suggests that the Holocene slip rate [yk2] for the fault section we are studying may be slower than the estimated geodetic slip rate (6 to 9 mm/yr)[yk3]  for the entire onshore HFF, although secondary onshore sub-parallel fault strands could accommodate part of the deformation.

How to cite: Matrau, R., Klinger, Y., Harrington, J., Avsar, U., Gudmundsdottir, E. R., Thordarson, T., Hoskuldsson, A., and Jonsson, S.: Paleoearthquakes on the Húsavík-Flatey Fault in northern Iceland: Where are the large earthquakes?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13998, https://doi.org/10.5194/egusphere-egu21-13998, 2021.