Fault Scarps Research Articles

Collapse caldera walls: Mechanical controls on slope failure and morphology

Volcanic calderas are enclosed topographic depressions that are commonly delimited by steep annular cliffs (caldera walls). The walls represent degraded scarps of ring faults that accommodated caldera collapse. Here, we assess the mechanical controls on caldera wall morphology and failure by: (i) analysing slope properties of recently formed calderas; (ii) examining analytical solutions for slope failure; and (iii) simulating progressive slope failure via Distinct Element Method (DEM) numerical modelling. Average slope angles of the caldera walls in nature range from 20° to 65° and slope heights range from 99 m to 1080 m. Smaller slope heights are weakly tied to greater slope inclinations. Almost 60% of sampled caldera wall profiles show a steeper upper section and a gentler lower section, which in numerical models are reproduced as an upper slope comprising intact wall rock and a lower slope comprising failed material inclined at the angle of repose. Moreover, modelled material with a low angle of internal friction and low cohesion exhibits shear failure, whereas material with high friction and high cohesion shows tension cracking, buckling and toppling. The slope height and inclination data from the numerical models agree well with those in nature. The numerical model data correspond more closely to analytically-predicted failure planes than critical (i.e. limit equilibrium) slopes because they are short-lived and therefore unlikely to be sampled in either model or nature. If rock mass cohesions or friction angles constrained analytically from natural slopes are determined by assuming the slope is critical, they are likely to be underestimates. The homogeneous numerical models here give an estimated natural rock mass cohesion of 0.5 MPa or less. This is much lower than typical laboratory-scale cohesion values, and it highlights the importance of strength reduction when upscaling mechanical parameters from laboratory experiments for modelling slope stability and volcano deformation.

Rate of E–W extension in the Volcanic Tableland, California (USA): A comparison of strain rates on two different timescales

Abstract The Eastern California shear zone (USA) is a broad zone of transtensional deformation related to the relative motion between the Pacific and North American plates. Due to its active deformation and seismicity, the zone receives great attention, with specific focus on slip rates of major active faults. To contribute to a better understanding of the long-term strain accumulation in this zone, this study quantifies the long-term E–W-directed extensional strain rate based on the analysis of N–S-trending normal fault scarps in the 765-k.y.-old Bishop tuff (Volcanic Tableland). The average extensional strain rate determined over the past 765 k.y. is 0.29 ± 0.10 mm/yr per 10 km (29 ± 10 nanostrain/yr) and similar to the current rate of elastic strain accumulation rate in the Volcanic Tableland (0.30 ± 0.13 mm/yr per 10 km; 30 ± 13 nanostrain/yr) determined by Global Positioning System (GPS) data. The present-day E–W strain rate across the entire Eastern California shear zone at the latitude of the Volcanic Tableland is 0.36 ± 0.05 mm/yr per 10 km (36 ± 5 nanostrain/yr). This suggests that the local rate of E–W extension has not changed significantly since the mid-Pleistocene. Furthermore, if the Volcanic Tableland is representative of the greater region, as the GPS data suggest, this would also indicate a constant extension rate across the Eastern California shear zone at the latitude of ~37.5°N over the 765 k.y. time period. These results suggest that late Pleistocene and Holocene extension rates of major faults in this zone can be interpreted in light of a presumably unchanged far-field stress system since at least the mid-Pleistocene.

Late Quaternary Deformation of the Ambon fault, Indonesia: Preliminary Results

The Ambon fault in the northern Banda Arc has capability of generating Mw 6.9 earthquake. The Ambon fault poses a significant seismic hazard to Ambon Island because the fault is located near populous areas, the Ambon City. Recent seismicity in Ambon Island and surrounding areas indicates that this region is actively deforming. Ambon Island has experienced many damaging earthquakes since the 17th century. However, the potential seismic hazard on this island remains enigmatic due to the lack of detailed geological information, such as fault traces, slip rate, and paleoseismic history. This study reports the preliminary results of tectonic geomorphic investigations and ground-penetrating radar (GPR) surveys on the Ambon fault. Tectonic geomorphic features, such as fault scarps, warped surfaces, and sag ponds indicate the late Quaternary activity of the Ambon fault. The Ambon fault predominantly shows a normal sense of motion with a dip of ~60-70°. The Ambon fault is related to the extensional tectonics in the Banda Sea.

Coupling rare earth element analyses and high-resolution topography along fault scarps to investigate past earthquakes: A case study from the Southern Apennines (Italy)

AbstractThe systematic study of faults that have released strong earthquakes in the past is a challenge for seismic hazard assessment. In carbonate landscapes, the use of rare earth element (REE) concentrations on slickensides may aid the reconstruction of fault slip history. We applied this methodology to the Caggiano normal fault (Southern Apennines, Italy), cropping out southeast of the Irpinia 1980 CE earthquake fault (Mw 6.9), which was responsible for both the 1561 CE and partly the 1857 CE Basilicata earthquakes (Mw 6.7 and 7.1). We integrated the REE analysis approach with a high-resolution topographic analysis along 98 serial topographic profiles to measure vertical separations attributable to faulting since the Last Glacial Maximum (LGM). The asymmetric scarp height profiles suggest fault-lateral propagation and along-strike variations in the fault evolution. Our results indicate the occurrence of 7 to 11 earthquakes with variable slip between ~40 cm and ~70 cm within post-LGM times. We estimated the magnitudes of the respective earthquakes, between 5.5 and 7.0, and most commonly between 6.3 and 6.5. The results suggest a recurrence time between 1.6 k.y. and 2.3 k.y. and a slip rate ranging between 0.6 mm/yr and 0.9 mm/yr. This approach may be useful for application to carbonate fault planes in similar tectonic contexts worldwide.

Branch fault discovered in Tangshan fault zone on the Kaiping-Guye boundary, North China

Abstract One outcrop of the NE–SW-trending fault with a steep occurrence and grown gouge was discovered in the north segment of the Tangshan fault zone during a field survey near Jujialing village on the Kaiping-Guye boundary of Hebei province, North China. This zone has received less attention in seismic geologic surveys. In addition, three neighboring and parallel NE trending scarps with heights in the range of 1–1.5 m were discovered on the generally flat terrain close to the fault outcrop. The geochemical and multi-electrode resistivity surveys revealed sudden huge fluctuations in the Hg concentration within short distances from the fault outcrop and the three surface scarps. The multi-electrode resistivity survey also revealed a resistivity structure with extreme horizontal discontinuity, indicating the presence of a subsurface fault zone with a width of 40–50 m and a SE dip. This finding confirmed that the three surface NE trending scarps were late Quaternary fault scarps. Following an examination of the focal mechanism solutions and survey findings, it is suggested that the newly detected fault trending toward the northeast could be an unexplored branch fault in the flower structure of the northern section of the Tangshan fault zone. It is marked by recent activity that is mainly controlled by the right-lateral strike-slip. The discovery of this active fault is critical for an accurate assessment of the seismic hazard of north segment of the Tangshan fault zone.

Forecasting Earthquake-Induced Floods

Surface-rupturing earthquakes can abruptly reroute rivers when fault scarps function like dams. Researchers have now successfully modeled such an event that occurred in New Zealand.

Late Medieval seismicity on the Himalayan Frontal Thrust Fault at Lal Dhang, Uttarakhand, India

The Himalayan Frontal Thrust Fault (HFT) lies at the active tectonic boundary between Eurasia and the Indian subcontinent. Past earthquakes on the HFT have resulted in significant ground displacement that is evidenced by sizable scarps along the fault. Here we present findings from an ~28-m-long x 5-m-wide x 8-m-deep paleoseismic trench excavated across an ~10-m-high fault scarp at the site of Lal Dhang in the Central Seismic Gap. Excavation exposed three recumbent fault-propagation folds that support occurrence of at least two earthquakes with >25 m of cumulative coseismic slip. Findings suggest that two great (Mw ≥ 8.0) earthquakes may have occurred in rapid succession at the site during the Late Medieval period. Radiocarbon dating of stratigraphic units that are modeled using the OxCal software program constrain timing of the events to 1317–1391 CE and 1447–1572 CE, which is coincident with historical earthquakes in 1344 CE and 1505 CE. With over 500 years of quiescence since the last great earthquake in the Central Seismic Gap, it is possible that another devastating rupture could be imminent.

Basin-scale interaction between post-LGM faulting and morpho-sedimentary processes in the S. Eufemia Gulf (Southern Tyrrhenian Sea)

The integrated interpretation of high-resolution multibeam bathymetry, seismic profiles and backscatter data in the S. Eufemia Gulf (SEG; Calabro-Tyrrhenian continental margin, south-eastern Tyrrhenian Sea) documents the relationship between postglacial fault activity and morpho-sedimentary processes. Three systems of active normal faults that affect the seafloor or the shallow subsurface, have been identified: 1) the S. Eufemia fault system located on the continental shelf with fault planes mainly oriented N26E-N40E; 2) the offshore fault system that lies on the continental slope off Capo Suvero with fault planes mainly oriented N28E-N60E; 3) the Angitola Canyon fault system located on the seafloor adjacent to the canyon having fault planes oriented N60E-N85E. The faults produce a belt of linear escarpments with vertical displacement varying from a few decimeters to about 12 m. One of the most prominent active structures is the fault F1 with the highest fault length (about 9.5 km). Two main segments of this fault are identified: a segment characterised by seafloor deformation with metric slip affecting Holocene deposits; a segment characterised by folding of the seafloor. A combined tectono-stratigraphic model of an extensional fault propagation fold is proposed here to explain such different deformation.In addition to the seabed escarpments produced by fault deformation, in the SEG, a strong control of fault activity on recent sedimentary processes is clearly observed. For example, canyons and channels frequently change their course in response to their interaction with main tectonic structures. Moreover, the upper branch of the Angitola Canyon shows straight flanks determined by fault scarps. Tectonics also determined different sediment accumulation rates and types of sedimentation (e.g., the accumulation of hanging wall turbidite deposits and the development of contourite deposits around the Maida Ridge). Furthermore, the distribution of landslides is often connected to main fault scarps and fluids are locally confined in the hanging wall side of faults and can escape at the seabed, generating pockmarks aligned along their footwall.

The mechanism of REE-Y impregnation on active carbonate normal fault scarps

The fluctuations of the Rare Earth Elements and Yttrium (REE-Y) concentrations on exhumed carbonate normal fault scarps may reveal the number and size of paleoearthquakes that exposed the scarp subaerially. This is because, prior to each large-magnitude earthquake, narrow (<50 cm) sections of the fault plane which are in direct contact with the soil become enriched in REE-Y before they are exhumed co-seismically, together with deeper, non-enriched, scarp sections. Following exhumation, depletion in REE-Y commences on both the enriched (i.e. ‘soil rupture zone’) and non-enriched (i.e. ‘rock rupture zone’) scarp sections. Although these processes are commonly described to occur on carbonate scarps, the mechanisms through which they operate remains poorly understood. Here, we present a series of laboratory tests that mimic the natural process of REE-Y enrichment/depletion to elucidate the mechanism of REE-Y impregnation. Our results indicate a fast uptake of REE-Y by the carbonate plane, when in contact with soil, either as (REE, Y)2(CO3)3 precipitate or by adsorption on calcite surfaces. The source of REE-Y in soil solution is released in a “pulses” due to alternations of dry and wet periods, characteristic of Mediterranean climatic conditions. Organic matter oxidation during the first rain events, triggers the Mn reductive dissolution and the release of REE-Y into the soil solution. The pH decrease due to organic matter dissolution is buffered by calcite, especially in the vicinity of the scarp, where calcite dissolution and re-precipitation occurs with a marked pH oscillation between 9.3 and 7.7. Further, comparison of these results with empirical data from three co-seismically exhumed fault scarps in Greece and Italy places quantitative constraints on the timing of these processes: the REE-Y enrichment within the ‘soil rupture zone’ may reach a maximum of ∼50% in about 500 years (+0.53 μg/kg/year), while the REE-Y depletion from the scarp is slow (−0.021 μg/kg/year), with a maximum recorded retention time of ∼16 ka. These enrichment and depletion characteristics work together to preserve paleoearthquake signal on carbonate scarps. Thus, this methodology is a valuable tool for quantifying the number of past earthquakes on carbonate fault scarps and allows more targeted use of expensive dating techniques (i.e. with cosmogenic nuclides) in order to derive the precise timing of these paleoearthquakes.

General order kinetics model for OSL rock surface exposure dating

Dating of rock surface using optically stimulated luminescence (OSL) is an emerging tool to estimate exposure age and post exposure erosion rates. The present study points out the lacuna in the mathematical model that is used for this method. So far, all existing research, use first order kinetics (FOK) model. The FOK model predicts an exponential decay of OSL signal of polymineral rock with sunlight exposure, which is contrary to the observation. The non-exponential decay of OSL signal, more precisely infrared stimulated luminescence (IRSL) of feldspar, can be better explained by general order kinetics (GOK). Thus, we propose a new GOK model for OSL surface exposure dating. Present theoretical study shows, for FOK model the OSL (IRSL) depth profile propagates faster with time of light exposure than actual. Consequently, the FOK model predicts lower exposure age. The propagation of OSL profile with time depends on order of kinetics of the GOK model. We demonstrate how the order of kinetics can be constrained through laboratory bleaching experiment. The impact of the GOK model on exposure age and upper age limit, for different order of kinetics are presented. We apply this new GOK model on experimental data, available in the literature (Luo et al. in Geophysical Research Letter 49: e2022GL099526, 2022), of OSL depth profile on exposed fault scarp and reevaluate the exposure ages for different order of kinetics to validate the GOK model. We conclude that the GOK model better explains the IRSL of feldspar than the FOK model and should be preferred for OSL rock surface dating.

Lichenometric Analysis Applied to Bedrock Fault Scarps: The Sencelles Fault and the 1851 CE Mallorca Earthquake (Balearic Islands, Spain)

The Sencelles Fault constitutes the main extensional structure of Mallorca Island (Spain), holds a NE-SE orientation, and has been identified as the possible seismic source of the 1851 CE Palma earthquake (VII EMS.) The SE termination of the fault (Sta. Eugenia Segment) features a linear bedrock fault scarp of a maximum of 3.15 m height. The last 840 m of this rocky scarp display a significant horizontal banding, with up to five differentially weathered ribbons colonized by lichens. The lichenometric analysis is based on the measurement of 155 specimens of Aspicilia calcarea (Ac) and Aspicilia radiosa (Ar) in tombstones and funerary monuments (with inscribed dates) from the nearby cemeteries of Sta. María del Camí, Sta. Eugenia and Sencelles, to obtain the local lichen growth rates (LGR), with the two last graveyards being directly located in the fault zone. Lichens were measured on variously oriented (N, S, NE, SW, etc…) horizontal and vertical surfaces, generating differentially oriented lichen populations (DOLPs) to be compared with the Ac and Ar specimens colonizing the studied fault scarp (38 measured individual specimens). After successive trial and error regression tests, vertical DOLPs resulted in the best appropriate groups for the analysis, with LGR of 0.23–0.31 mm/yr. Horizontal ones reached widths of up to 20 cm, with LGR up to 0.84 mm/yr, which were clearly oversized. The application of the selected LGR points to a human-induced origin for the thin basal lichen ribbon of the scarp (10–13 cm), which should have developed during the middle 20th century (c. 1950–1966) because of documented ground leveling works. However, the second ribbon of the scarp (23–47 cm) shows exposure dates of 1852 ± 40 (Ar) and 1841 ± 59 (Ac), overlapping the date of the 1851 CE earthquake. The study is complemented with data from a fault trench excavated in the year 2002 at the toe of the scarp. The combined data of lichenometry, fault trenching, and the length of the analyzed fault scarp (c. 840 m) indicate that the studied segment of the fault cannot be considered a co-seismic surface faulting related to the 1851 CE event as a whole, but a relevant secondary earthquake effect on a pre-existing fault scarp (e.g., sympathetic ground ruptures).

Fault structure and segmentation of the Punta Montalva Fault in southwestern Puerto Rico using high-resolution digital elevation models

The Punta Montalva Fault is an active fault that slipped during the 2019–2023 Puerto Rico seismic sequence. It lies in southwestern Puerto Rico within the ∼250-km-wide deformation zone of the North American-Caribbean plate boundary. The fault cuts Miocene limestone and contains surface features that suggest left-lateral displacement. Previous work suggested that the fault connects with the North Boquerón Bay Fault for a length of 33 km and poses a large seismic hazard. However, not much is known about the structure, segmentation, and spatial extent of the fault as it is obscured under thick vegetation. This study used high-resolution topographic data to map the structure, segmentation, and spatial extent of the fault. The results would allow us to better assess the seismic potential of the Punta Montalva Fault by constraining its spatial extent and how strain has been accommodated. A 0.5 m resolution bare-earth digital elevation model (DEM) derived from a 2018 airborne LiDAR survey and another 1 m resolution 2018 DEM showing shallow bathymetry were used to map the Punta Montalva Fault. High-resolution mapping revealed that the Punta Montalva Fault is comprised of three main fault segments on land, a possible additional scarp, and three fault scarps on the seafloor to the SE. No evidence suggests the Punta Montalva Fault extends further NW of Punta Montalva into La Parguera in Lajas. The DEM revealed that the fault cuts a shallowly dipping sedimentary sequence of the Ponce Limestone that has karst topography in places. On land, the fault is conspicuous where it forms scarps, drags and cuts sedimentary layers, and forms a duplex structure with a predominantly left-lateral offset. A left-lateral offset of an intermittent stream has been refined to 133.6 ± 7.3 m. The fault scarps along a marine shelf strike E-W and show a maximum normal offset of 4.8 m. Another sub-parallel fault scarp in the seafloor strikes NW-SE and displays a left-lateral offset of 98.3 m. The total length of the fault segments is ∼2.67 km long. The surface segmentation suggests that the fault would have a lower seismic hazard than if it were continuous and connected to the North Boquerón Bay Fault. However, if the Punta Montalva Fault is continuous beneath the surface, its total fault length would be approximately 7.97 km long and would pose a larger seismic hazard than that suggested by the surface segmentation. Strike-slip, folding, and normal offset suggest that the area is being deformed by transtentional deformation, which supports seismic analyses and tectonic models.

Active faults studies in Delhi and national capital region (NCR): Inferences from satellite data and field investigations

In recent years, the National Capital Region (NCR) of Delhi has experienced several earthquakes ranging in magnitude from 1.0 to 6.7. According to the last 50 years of earthquake data, the majority of earthquakes in the NCR have occurred near the Mahendragarh Dehradun Fault (MDF) and the Sohna Fault (SF). The region is bounded by a number of subsurface Ridges, Faults, and Lineaments, which are also influenced by the active plate boundary of the Indian and Eurasian plates. Active fault mapping is critical for the precise identification and marking of active fault traces in the NCR area for a precise seismic hazard assessment. We used high resolution Cartosat-1 stereopair data obtained from NRSC, Hyderabad, and Anaglyph (A 3D representation of the surface) and DEM prepared with ENVI software to map the active faults. We identified 12 sites in the NCR region based on satellite data interpretation, primarily along the MDF and Sohna Fault and their extensions. The presence of tectono-geomorphic markers along the MDF and Sohna Fault, such as warped surfaces indicative of fault scarps, stream offsets, gully erosion, and sag ponds, suggests active tectonic movement along these faults, most likely in the recent geological past. We believe the MDF is a right-lateral strike-slip fault with a compressional component on the western side and an extensional component on the eastern side. It acts as a segment boundary between compressional and extensional boundaries. We also identified the right lateral Nuh-Jhirka fault (NJF), which can be the Sohna Fault’s southern extension from Nuh to Jhirka. The western limb of the Delhi Mega fold has also seen a few right-lateral strike-slip movements that have extended up to the eastern bank of the Yamuna River, where the river reflects the base-level change and tight meandering on its upward side and a straight pattern on its downward side. This fault is known as the Delhi Fault (DF). The findings are preliminary, and further research would be required to create a detailed active fault map of the Delhi-NCR region to conduct a precise Seismic Hazard Assessment (SHA) of the region.

Geologic investigation of lobate scarps in the vicinity of Chandrayaan-3 landing site in the southern high latitudes of the moon

The Chandrayaan-3 primary landing site (PLS) is located in the southern high latitude region between 68–70° S and 31–33° E of the Moon. We have found two scarps (scarp #1 and #2) in the surrounding area of PLS. Since lobate scarps are one of the youngest tectonic structures on the Moon and they are seismically active recently, we have conducted a detailed morphological and topographic investigation of the scarps to examine their origin. Subsequently, chronological and seismic investigation of the inferred lobate thrust fault scarps in the PLS region was carried out to assess the safety and the possible hazards to the lander/rover of the Chandrayaan-3 mission. Our analysis reveals that out of the two scarps located in the PLS region, scarp #2 is a typical lobate thrust fault scarp, which is generally comparable in vergence direction, maximum relief, and maximum slope on the scarp face to the scarps found elsewhere on the Moon. Scarp #1 could be potentially a non-fault related scarp. The crater size-frequency distribution (CSFD) based best-fit ages derived for the scarp face, and hanging and footwalls of the scarp reveal that the scarp is a younger generation scarp (∼20–30 Ma). We have found evidence of fresh and faded boulder-fall trails on the rims and walls of multiple craters located around the scarp #2. The seismic measurements reveal potential shallow moonquakes induced by the lobate scarp #2 could have generated a moonquake of a cumulative or maximum possible moment magnitude of Mw 6.3. Based on our analysis of the potential ground shaking caused by the shallow moonquake, it appears that the landing site is safe from seismic hazards. Nevertheless, we propose that the Instrument for Lunar Seismic Activity (ILSA) onboard the Chandrayaan-3 lander could possibly detect recent Mw 4 shallow moonquakes, provided that they occur currently at a focal depth of <1 km.

Emplacement of monogenetic lava flows on eroded terrain, Part II: The case of the Artière valley (Grave Noire, France)

At the Grave Noire on the eastern edge of the Chaîne des Puys (Central France) we have a well-exposed case of lava erupted directly onto steep slopes to pass, over an abrupt break-in-slope, onto into a flat-floored basin. Erupted 60 ka ago at the head of a fault scarp, flows descended the 25° slopes of the scarp to spread into the basin at its foot. We carried out mapping and facies analysis down the main (Artière-Cézeaux) lava flow, collecting 29 samples down its 6-km length and on which we completed textural, geochemical, and Anisotropy of Magnetic Susceptibility (AMS) analyses. Results reveal four lava flow facies:i)a near-vent zone of thin (≤ 0.6 m) ʻaʻā sheet flow on the steep (25°) slopes of the fault scarp;ii)a > 26 m thick lobate body of outgassed lava at the foot of the fault scarp;iii)a zone of lava-scoria breccia and marl intruded by dense, outgassed lava; and,iv)a 20–30 m thick distal flow of outgassed lava on slopes of 0.5–0.8°.Down flow trends in vesicularity and crystal content, as well as the AMS data, mass balance calculations and analogy with systems displaying similar facies, reveal high effusion rate (40–60 m3/s) fountain-fed flows fed an outlet channel on the steep slopes of the fault scarp. At the break-in-slope at the foot of the scarp, flows stalled, spread and dug into the country rock (marl) to become ponded behind a self-constructed barrier of lava breccia and cone material. Lava resident in this perched pond outgassed, intruded the base of the barrier and surrounding marl, and fed seeps in the breccia barrier. Seeps fed a slow moving (0.001 m/s) plug-dominated flow of dense lava that moved a further 3.6 km, digging down into the marl substrate by at-least 15 m and carrying a mixture of lava breccia, air fall scoria, debris flow material and cone rafts all the way to the flow front. As at the Tiretaine lava flow (Part 1, Latutrie et al., 2023) we find that breaks-in-slope aid in creating a system of reservoirs and transfers. For Grave Noire, rapidly emplaced flows on steep slopes fed a zone of storage (a perched pond) at the break-in-slope at the base of a fault scarp. The pond, in turn, fed slow moving flow across the flat floor of a basin. Interaction with an easily eroded substrate led to mechanical erosion, as well as 30–40 m of inversion of topography in the 60 ky since the emplacement of the flow system. Thus, the slope and type basement on which a monogenetic lava flow field is emplaced plays a fundamental role in lava flow dynamics, emplacement style and system architecture, as well as subsequent evolution of the topography.

Controls on the morphology of closely spaced submarine canyons incising the continental slope of the northern South China Sea

Submarine canyons are key elements in source-to-sink systems that are commonly developed along continental margins. They act as major conduits transferring sediment and pollutants from continental shelves to deep-water basins, and control the general morphology and evolution of continental margins. This work uses multibeam bathymetric and high-resolution (two- and three-dimensional) seismic data to investigate the main factors controlling the morphology of the Shenhu Canyon System in the northern South China Sea, as well as its detailed morphological character. The Shenhu Canyon System consists of nineteen (19) submarine canyons whose morphologies vary from southwest to northeast along the continental slope. Canyons (C1–C10) in the southwest show greater incision depths, and steeper thalwegs and walls, when compared to their counterparts to the northeast (C11–C17). The southwest canyons are located close to the shelf edge, where the upper continental slope is relatively steep and multiple landslides are imaged. We show that the thalwegs and walls of the southwest canyons were more actively eroded by sediment flows, with respect to the northeast canyons, making them deeper and steeper. Hence, submarine canyons in the southwest, with a more linear geometry, are now directly connected to the Pearl River Canyon. In parallel, seafloor fault scarps act as barriers for sediment transported to the heads of the northeast canyons. This research highlights how sediment supply, sediment pathways, and seafloor scarps can influence submarine canyon morphology along continental slopes. It contributes to a better understanding of the factors controlling canyon morphology worldwide.

Geomechanical Modeling of Ground Surface Deformation Associated with Thrust and Reverse-Fault Earthquakes: A Distinct Element Approach

ABSTRACTWe seek to improve our understanding of the physical processes that control the style, distribution, and intensity of ground surface ruptures on thrust and reverse faults during large earthquakes. Our study combines insights from coseismic ground surface ruptures in historic earthquakes and patterns of deformation in analog sandbox fault experiments to inform the development of a suite of geomechanical models based on the distinct element method (DEM). We explore how model parameters related to fault geometry and sediment properties control ground deformation characteristics such as scarp height, width, dip, and patterns of secondary folding and fracturing. DEM is well suited to this investigation because it can effectively model the geologic processes of faulting at depth in cohesive rocks, as well as the granular mechanics of soil and sediment deformation in the shallow subsurface. Our results show that localized fault scarps are most prominent in cases with strong sediment on steeply dipping faults, whereas broader deformation is prominent in weaker sediment on shallowly dipping faults. Based on insights from 45 experiments, the key parameters that influence scarp morphology include the amount of accumulated slip on a fault, the fault dip, and the sediment strength. We propose a fault scarp classification system that describes the general patterns of surface deformation observed in natural settings and reproduced in our models, including monoclinal, pressure ridge, and simple scarps. Each fault scarp type is often modified by hanging-wall collapse. These results can help to guide both deterministic and probabilistic assessment in fault displacement hazard analysis.

A multi-scale approach to the recent activity of the Stradella thrust in the seismotectonic context of the Emilia Arc (northwestern Italy)

The frontal thrusts and folds of the northern Apennines - Italy - are mainly covered under the alluvial deposits of the Po Plain. Some of these structures show geological evidence of Late Quaternary activity, thus posing the need for an accurate seismic hazard assessment due to widespread housing settlements, industries, lifeline infrastructures, and large towns. We present new morphostructural, geophysical, and seismological data to discuss the recent activity of the Broni-Sarmato fault, an 18 km-long outcropping section of the north-verging Stradella thrust, located 50 km south of Milan, along the Pede-Apennine thrust front (PTF) in the rear of the Emilia Arc thrust system. The new geoelectrical surveys across the fault scarp show deformation of the shallow deposits. The outcropping deformations, with a fault scarp ranging up to 25.8 m, are investigated within the seismotectonic framework of the PTF and the Emilia Arc. The analysis of the associated seismicity and new focal mechanisms highlight two seismogenic contractional volumes dipping at low-angle southwest-ward, at upper (<12 km) and lower crustal depths (~20–30 km). The shallow seismicity partially illuminates the Stradella thrust and its along-strike southeastward prosecution along the extent of the Stradella-Salsomaggiore Arc. Subordinately, it also illuminates some of the Emilia Arc thrust planes. The deeper seismogenic volume shows large patches of the basal thrust of the Emilia Arc fault system. We interpret the above multi-scale data as evidence of ongoing tectonic activity of the outer fronts of the Emilia Arc under a regional NNE-oriented compressional stress field, with some evidence of thrust involvement along the Pede-Apennine front. In our 3D fault-model reconstruction, all the analyzed thrust structures appear as expressions of a thick-skinned deformation that controls earthquake release at different structural levels.

Emplacement of lava flows on eroded terrain, part I: The case of the Tiretaine valley (Chaîne des Puys, France)

Lava flows often erupt onto eroded landscapes, flowing across plateaus, into valleys and basins, and down escarpments. With the Tiretaine lava flow system in the Chaîne des Puys (Central France), lava flowed over the complex topography of an ancient rift margin. Erupted 41.3 ± 1.3 ka ago onto the granite plateau of the Massif Central, flow initially advanced over a surface that sloped 2°E. On reaching a fault scarp that delimits the plateau, flows descended the 6° slopes of a ravine that cuts the scarp E-W. In part 1 of this work we thus consider the case where lava is emplaced onto relatively flat terrain, then passes over a break-in-slope onto steeper slopes. To do this, we carried out mapping and facies analysis down this ∼6-km-long system, collecting 24 samples on which we completed textural and geochemical analyses. Results reveal three lava facies:(i)a 3 × 2 km proximal zone of inflated pāheohoe with a thickness of 25–50 m;(ii)a channelized medial zone with simple rubble levees nested in the ravine; and(iii)a valley-ponded distal zone with tubes and well-formed columnar jointing.Through simple mass balance calculations, and analogy with systems displaying similar facies, we find that the proximal inflated zone was fed at relatively low (1–5 m3/s) effusion rates. This built the proximal portion of the flow field over a period of 1–6 years, forming a volume of flow-field stored, degassed lava. Failure of the eastern edge of the proximal flow field at the head of the Tiretaine ravine fed high (215–315 m3/s) effusion rate flow down the ravine, to build a channelized flow system in just a few hours. However, the presence of a lava dam at the mouth of the ravine impeded further flow and resulted in ponding. This ponded volume then cooled in-place over a period of 10–11 years. Breaks-in-slope, and the resulting lava–topography interaction, thus created a system of reservoirs and transfers, with different flow rates, dynamics and emplacement time scales. For our case, where the lava flow system is mostly in a UNESCO World Heritage site, the study provides scientific support for Geoheritage efforts, outreach, and initiatives to support protection of geologically important sites.

Exceptional Scarp Preservation in SW Namibia Reveals Geological Controls on Large Magnitude Intraplate Seismicity in Southern Africa

AbstractFour previously unrecognized neotectonic fault scarps in southwest Namibia are described. These relatively straight, simple but segmented structures are 16–80 km long and have measured vertical separations of 0.7–10.2 m. We estimate that each is capable of producing earthquakes ofMw6.4 or greater, indicating that large earthquakes may occur despite limited cumulative displacement. There is strong evidence that some of these scarps were formed by repeated earthquakes. Comparison with aeromagnetic and geological maps reveal that the normal faults reactivate major crustal weaknesses that are orientated north‐south and northwest‐southeast and perpendicular to the local gravitational potential energy gradient. The presence of these structures in an area with a limited record of instrumental seismicity suggests that theMmaxof this region may be much larger than generally assumed. They highlight the necessity of incorporating information from fault studies into probabilistic seismic hazard assessments in this region, in a similar way to other stable continental regions such as Australia. The fact that such major structures have gone hitherto unrecorded suggests significant further research is needed to characterize these sources of hazard. The identification of an apparent cluster of large magnitude neotectonic earthquakes in the area may be related to the exceptional preservation potential of scarps rather than indicating an area of comparatively rapid deformation. If this interpretation is correct, then these scarps represent an important indication of the potential seismic hazard across the region, and the occurrence of infrequent large‐magnitude seismicity on similar structures should be considered throughout southwestern Africa.