Trench Study Research Articles

Holocene Activity of the Wudaoliang–Changshagongma Fault of the Eastern Tibetan Plateau

The Wudaoliang–Changshagongma fault is one of the NW-trending faults located within the southern Bayan Har Block of the Tibetan Plateau in China. In this paper, we used high-resolution imagery and digital elevation model data to study the geomorphological and geological characteristics of the fault. Furthermore, the result also determined the fault trace and estimated the average horizontal slip rate of the fault since the late Quaternary to have been 2.6 ± 0.6 mm/a. This slip rate is approximately equivalent to that of the Awancang, Madoi–Garde, and Dari faults, which are also located within the block. Furthermore, the slip rates of these faults obtained by remote sensing and geological methods are consistent with GPS observations. It indicates that tectonic deformation within the block is continuous and diffuse. Using trenching study results and sedimentary radiocarbon dating, we identified four paleoearthquake events that occurred at 42,378–32,975, 33,935–20,663, 5052–4862, and after 673–628 cal BP, respectively. The recurrence intervals of large earthquakes on the faults within the block are much longer than those of the boundary faults, and the slip rates are also smaller, indicating that faults within the block play a regulatory role in the tectonic deformation of the Bayan Har Block.

Hanging-wall deformation at the active Sierra Palomera extensional fault (Jiloca basin, Spain) from structural, morphotectonic, geophysical and trench study

The NNW-SSE trending Sierra Palomera fault is characterized as an active, nearly pure extensional fault with mean transport direction towards N230°E, consistent with the ENE-WSW extension trajectories of the recent to present-day regional stress field. Its macrostructure is described from surface geology and magnetometric and electromagnetic surveys, which have allowed identifying two subsidiary, nearly parallel normal faults (antithetic and synthetic, respectively). The structural contour map of an extensive planation surface, dated to 3.8 Ma, provides a maximum fault throw s.s. of 330 m for the main fault (480 m including bending), and a net slip rate of 0.09–0.10 mm/a (0.13–0.15 mm/a including bending). Trench study focussed on the subsidiary antithetic fault shows evidence of its activity during Middle-Late Pleistocene times, offsetting ca. 2.5 m the slope of a well-preserved alluvial fan. Detailed analysis and retrodeformation of the antithetic fault and other minor ruptures in the trench has allowed defining seven deformation events. The lack of a consistent age model for the involved sedimentary sequence makes them almost meaningless in terms of paleoseismic history. However, geometry and sequential development of meso-scale faults (intermediate between seismic-scale and analogue models) allows unravelling the extensional deformation history within the hanging-wall block of the Sierra Palomera fault. Progressive rupture patterns reveal shifting from dominantly synthetic to dominantly antithetic faulting, suggesting both kinematical control linked to rollover growth, and dynamical control by the regional stress field.

Reevaluation of the Seismogenic Fault of the 1654 M8.0 Tianshui Earthquake: Evidence from Geology, Geomorphology, and Chronology along the Lixian-Luojiapu Fault

AbstractA historically documented M8.0 earthquake occurred in Tianshui, Gansu, China, in 1654. Its epicenter was located near the Lixian-Luojiapu fault in the triangular area at the conjunction of the Tibetan Plateau, North China block and South China block. Due to the lack of geologic and geomorphologic evidence and precise chronologic constraints, the seismogenic fault of this large event remains controversial. This work attempted to further address this issue. Based on data from unmanned aerial vehicle measurements, field investigations, analysis of typical fault profiles, and 14C dating, we determined the age of the latest slip on the Lixian-Luojiapu fault and the corresponding surface rupture range. In combination with historical earthquake data for this region, our work suggests that two sections of the Lixian-Luojiapu fault, the Tanchang-Lixian subsegment and the eastern portion of the Lixian-Luojiapu subsegment, were indeed involved in the surface rupturing of the 1654 Tianshui M8.0 earthquake. The scratches on the fault plane indicate that this seismic rupturing exhibited a left-lateral slip with a normal component and the maximum vertical displacement by this shock on the Tanchang-Lixian subsegment was 1.7±0.2 m. We suggest that the Tanchang-Lixian subsegment and the eastern portion of the Lixian-Luojiapu subsegment of the Lixian-Luojiapu fault and the West Qinling fault in the north jointly constituted the seismogenic structure of the Tianshui M8.0 earthquake based on a previous trench study. Located in the triangular zone between the Tibetan Plateau and the North China and South China blocks, tectonic stress accumulates rapidly on the Lixian-Luojiapu fault, resulting in a high potential of moderate to strong earthquakes, and this issue should be given more attention in the future.

The contribution of organic acid on heterotrophic CO2 flux from tropical peat: a trenching study

Quantification of CO2 flux from peat has been studied with various methods of measurement and data analysis. Several studies have applied regression analysis to assess carbon flux from tropical peatland as a function of groundwater level. Such an analysis simplified the complex nature of peat decomposition, which involved microbial activities. The study was conducted at Buatan Village, Siak Indrapura Regency, Riau Province, Indonesia. Soil sampling was done every month for a year observation, from July 2018 to June 2019. This study aimed to comprehend CO2 production from the respiration of heterotrophic components (Rh-CO2) as a function of soil properties determined by soil pH, N-NH4, N-NO3, available P, exchangeable-K, C-organic acids, and environmental factors that are determined by soil water content, and groundwater level. The study applied trenching experimentation to quantify Rh-CO2 flux by first removing plant roots from the trenching plot. The CO2 flux and groundwater level were measured for five consecutive days each month for a one-year period. Multiple regression analysis was performed to determine the main determinant for the Rh-CO2 flux. The results showed that seasonal fluctuation of Rh-CO2 flux, negatively correlated with available P (p = 0.037), and positively (p = 0.018) with C-substrate as C-organic acids but not with either of the speciated ones as acetic, lactic, citric, malic, nor oxalic acids. More specifically, the C-organic acids were found as the main determinant factor (p = 0.039) affecting the Rh-CO2 flux.

Active tectonics along the Khazar fault (Alborz, Iran)

The Alborz Mountain accommodates some of the convergence between Central Iran and Eurasia, is characterized by active range-parallel fold and thrust structures. At present, the kinematics of the range involves a strain partitioning mechanism and clockwise rotation of the South Caspian Basin. Range-parallel, left-lateral strikeslip faulting dominates the central part of the mountain range, while reverse faulting affects its northern and southern borders. Several slip-rate studies have been carried out along active faults in the internal and southern parts of the range. However, the characteristics of the main northern bounding fault (the Khazar [Persian: Caspian] Fault) remain poorly known. Our analysis provides new constraints on the activity of this fault. We first show that the fault generally is a hidden thrust fault, often associated with fault-bend and fault-propagation folds (forebergs). In the central part of the fault, the radiocarbon dating of an uplifted terrace allows estimating minimum vertical and average horizontal slip rates of 2.0 ± 0.5 mm/yr and 3 mm/yr respectively. hence, minimum slip rate along the fault reach to 3.6 mm/yr. About 150 km further east, near the city of Behshahr, within the archeological site of Gohar-Tappe, a paleoseismological trench study on a young detachment fold suggests that at least 5 events occurred in the past 5,300 years, 3 of them with surface-rupturing between 5300- and 3900-years cal BP, our results confirm that the Khazar Fault is a major active structure in northern Iran, and represents a significant seismic hazard for the entire Central Alborz region.

Palaeoseismic behaviour of strike-slip faults in slowly deforming regions: palaeoearthquakes and long-term slip history of the Ovacık Fault (eastern Turkey)

The Ovacik Fault (OF) is one of the internal structures of the Anatolian Block, located close to its eastern boundary. Although it shows a very clear surface trace, there are no instrumentally recorded surface rupturing earthquakes on this fault. This study concludes results of the first palaeoseismological trench study on the OF with the evidence for three surface rupturing earthquakes for the last 6 kyr. Structural, stratigraphical and geochronological constraints show that the oldest event happened between 4700 and 3022 BCE, whereas the penultimate event was between 2262 and 1643 BCE. The youngest event happened after 56 BCE, which we could only delimit from below because of the absence of any dateable material from the upper layers of the relevant horizons. The derived mean interevent time of these three palaeoevents is 2400 ± 765 years. We could not correlate any of these palaeoevents with historical earthquakes due to the absence of records in catalogues for the region of interest. Moreover, we used cumulative offsets of the major drainages to calculate the long-term slip rate of 2.7 mm/year since the Pliocene, which exceeds slip rates of other internal faults of Anatolia almost by the factor of two. Our results do not only show significant deformation for the internal parts of Anatolia but also point out a seismic potential of the OF.

Bacteria are important dimethylsulfoniopropionate producers in marine aphotic and high-pressure environments

Dimethylsulfoniopropionate (DMSP) is an important marine osmolyte. Aphotic environments are only recently being considered as potential contributors to global DMSP production. Here, our Mariana Trench study reveals a typical seawater DMSP/dimethylsulfide (DMS) profile, with highest concentrations in the euphotic zone and decreased but consistent levels below. The genetic potential for bacterial DMSP synthesis via the dsyB gene and its transcription is greater in the deep ocean, and is highest in the sediment.s DMSP catabolic potential is present throughout the trench waters, but is less prominent below 8000 m, perhaps indicating a preference to store DMSP in the deep for stress protection. Deep ocean bacterial isolates show enhanced DMSP production under increased hydrostatic pressure. Furthermore, bacterial dsyB mutants are less tolerant of deep ocean pressures than wild-type strains. Thus, we propose a physiological function for DMSP in hydrostatic pressure protection, and that bacteria are key DMSP producers in deep seawater and sediment.

3D geometry and architecture of a normal fault zone in poorly lithified sediments: A trench study on a strand of the Baza Fault, central Betic Cordillera, south Spain

Successive excavation of 13 trenches of different orientations reveals the complexity of a normal fault zone in Pliocene-Pleistocene unconsolidated sediments on a strand of the Baza Fault, central Betic Cordillera, south Spain. These trenches and the excavation floor are interpreted and integrated to reconstruct the 3D geometry and internal architecture of the fault zone. The structure consists of two main fault strands: an eastern one with a few hundred metres throw and a western one with at least 15 m throw. These strands interact and gradually merge to the south, bounding a main deformation zone narrowing from ∼7 to 1 m along strike. Fault-bounded rock bodies, clay and sand smears, and clay injections define the structure. These features are highly variable in 3D. In the northern part of the outcrop, deformation is localized around the main strands, brittle in the west and more ductile to the east. As the strands and their fault zones increasingly interact, fault throw, rock deformation and maturity of the structure increase. Mechanical stratigraphy also controls the style of deformation. A realistic representation of this 4D picture of fault deformation is critical for modelling fluid flow in shallow to possibly deep, faulted sedimentary reservoirs.

Paleoseismological implications of liquefaction-induced structures caused by the 2017 Pohang Earthquake

During and shortly after the 2017 Pohang Earthquake (Mw 5.4), sand blows were observed around the epicenter for the first time since the beginning of instrumental seismic recording in South Korea. We carried out field surveys plus satellite and drone imagery analyses, resulting in observation of approximately 600 sand blows on Quaternary sediment cover in this area. Most were observed within 3 km of the epicenter, with the farthest being 15 km away. In order to investigate the ground’s susceptibility to liquefaction, we conducted a trench study of a 30 m-long sand blow in a rice field 1 km from the earthquake epicenter. The physical characteristics of the liquified sediments (grain size, impermeable barriers, saturation, and low overburden pressure) closely matched the optimum ground conditions for liquefaction. Additionally, we found a series of soft sediment deformation structures (SSDSs) within the trench walls, such as load structures and water-escaped structures. The latter were vertically connected to sand blows on the surface, reflecting seismogenic liquefaction involving subsurface deformation during sand blow formation. This genetic linkage suggests that SSDS research would be useful for identifying prehistoric damage-inducing earthquakes (Mw > 5.0) in South Korea because SSDSs have a lower formation threshold and higher preservational potential than geomorphic markers formed by surface ruptures. Thus, future combined studies of Quaternary surface faults and SSDSs are required to provide reliable paleoseismological information in Korea.

Episodic activity of a dormant fault in tectonically stable Europe: The Rauw fault (NE Belgium)

Our knowledge about large earthquakes in stable continental regions comes from studies of faults that generated historical surface rupturing earthquakes or were identified by their recent imprint in the morphology. Here, we evaluate the co-seismic character and movement history of the Rauw fault in Belgium, which lacks geomorphological expression and historical/present seismicity. This 55-km-long normal fault, with known Neogene and possibly Early Pleistocene activity, is the largest offset fault west of the active Roer Valley Graben. Its trace was identified in the shallow subsurface based on high resolution geophysics. All the layers within the Late Pliocene Mol Formation (3.6 to 2.59Ma) are displaced 7m vertically, without growth faulting, but deeper deposits show increasing offset. A paleoseismic trench study revealed cryoturbated, but unfaulted, late glacial coversands overlying faulted layers of Mol Formation. In-between those deposits, the fault tip was eroded, along with evidence for individual displacement events. Fragmented clay gouge observed in a micromorphology sample of the main fault evidences co-seismic faulting, as opposed to fault creep. Based on optical and electron spin resonance dating and trench stratigraphy, the 7m combined displacement is bracketed to have occurred between 2.59Ma and 45ka. The regional presence of the Sterksel Formation alluvial terrace deposits, limited to the hanging wall of the Rauw fault, indicates a deflection of the Meuse/Rhine confluence (1.0 to 0.5Ma) by the fault's activity, suggesting that most of the offset occurred prior to/at this time interval. In the trench, Sterksel Formation is eroded but reworked gravel testifies for its former presence. Hence, the Rauw fault appears as typical of plate interior context, with an episodic seismic activity concentrated between 1.0 and 0.5Ma or at least between 2.59Ma to 45ka, possibly related to activity variations in the adjacent, continuously active Roer Valley Graben.

The Manyas fault zone (southern Marmara region, NW Turkey): active tectonics and paleoseismology

The Manyas fault zone (MFZ) is a splay fault of the Yenice Gönen Fault, which is located on the southern branch of the North Anatolian Fault System. The MFZ is a 38 km long, WNW–ESE-trending and normal fault zone comprised of three en-echelon segments. On 6 October 1964, an earthquake (Ms = 6.9) occurred on the Salur segment. In this study, paleoseismic trench studies were performed along the Salur segment. Based on these paleoseismic trench studies, at least three earthquakes resulting in a surface rupture within the last 4000 years, including the 1964 earthquake have been identified and dated. The penultimate event can be correlated with the AD 1323 earthquake. There is no archaeological and/or historical record that can be associated with the oldest earthquake dated between BP 3800 ± 600 and BP 2300 ± 200 years. Additionally, the trench study performed to the north of the Salur segment demonstrates paleoliquefaction structures crossing each other. The surface deformation that occurred during the 1964 earthquake is determined primarily to be the consequence of liquefaction. According to the fault plane slip data, the MFZ is a purely normal fault demonstrating a listric geometry with a dip of 64°–74° to the NNE.

A discrepancy between observed and predicted NATM tunnel behaviors and updating: a case study of the Sabzkuh tunnel

The 11 km long Sabzkuh water conveyance tunnel is under construction using conventional and tunnel boring machine tunneling techniques in the Zagros Mountains of south west Iran. During excavation of the conventional section (i.e., 350 m), three daylighting collapses occurred in the tunnel, and large sinkholes developed at the ground surface, due to both poor ground conditions and inappropriate selection of excavation and support class (ESC). In this study, the influence of ESC on tunnel response and stability was investigated through three dimensional (3D) finite difference and finite element methods. The analysis results of numerical modeling indicated that the tunnel face would be stable if a single stage (full face) excavation and an advance step of 1.5 m (defined as ESC#1) were applied. However, the reality was different, and the tunnel collapsed soon after the face was only 35 m away from the portal. Having changed the ESC by taking into account new geological and geotechnical data obtained from face mapping, a field survey of surface cavity, and a trench study, the tunneling advance rate was satisfactory (1.5 m/day) despite the many challenges encountered. This paper presents a brief review of some of the key geological challenges faced in the tunnel design, including characterization of the ground conditions, selection of appropriate design parameters, and evaluation of the excavation and support installation sequence based on monitoring and analyzing ground behavior during construction. The discrepancy between the predicted and observed behavior of the tunnel, and some practical considerations, are also discussed.

A trench study to assess transfer of pesticides in subsurface lateral flow for a soil with contrasting texture on a sloping vineyard in Beaujolais.

Subsurface lateral flow in both texture-contrast soils and catchments with shallow bedrock is suspected to be a non-point source of contamination of watercourses by pesticides used in agriculture. As a case study, the north of the Beaujolais region (eastern France) provides a favorable environment for such contamination due to its agro-pedo-climatic conditions. Environments seen in the Beaujolais region include intense viticulture, permeable and shallow soils, steep hillslopes, and storms that occur during the periods of pesticide application. Watercourse contamination by pesticides has been widely observed in this region, and offsite pesticide transport by subsurface lateral flow is suspected to be involved in diffuse and chronic presence of pesticides in surface water. In order to confirm and quantify the potential role of such processes in pesticide transfer, an automated trench system has been designed. The trench was set up on a steep farmed hillslope in a texture-contrast soil. It was equipped with a tipping bucket flow meter and an automatic sampler to monitor pesticide concentrations in lateral flow at fine resolution, by means of a flow-dependent sampling strategy. Four pesticides currently used in vine growing were studied to provide a range of mobility properties: one insecticide (chlorpyrifos-methyl) and three fungicides (spiroxamine, tebuconazole, and dimethomorph). With this system, it was possible to study pesticide concentration dynamics in the subsurface lateral flow, generated by substantial rainfall events following pesticide applications. The experimental design ascertained to be a suitable method in which to monitor subsurface lateral flow and related transfer of pesticides.

Rigid dynamic performance simulation of an offshore pipeline plough

Abstract Offshore pipeline ploughs are used for laying pipelines in seabed trenches; these devices primarily consist of a plough body and skid and steering systems. Research on the virtual dynamic analysis of this system is essential for predicting the dynamic performance of the plough during trenching and steering. In this work, a 3-D rigid body dynamic simulation model is developed to investigate the performance of the simulated plough under different operating conditions. To verify this simulation model, an onshore field trenching study with a prototype plough was carried out. In terms of the tow force, the simulation results of the plough performance agree with the onshore field results and laboratory data within a certain range of simulation tolerance. We also present the effects of sandwaves and steering adjustment on the plough tow force. In addition, this paper addresses the development process and the dynamic results of the simulation model with a flexible towline.

Electrical resistivity tomography investigations on a paleoseismological trenching study

Two-dimensional electrical resistivity tomography (ERT) investigation was performed in a paleoseismological trenching study. Data acquisition strategies such as the selection of electrode configuration and electrode intervals of ERT application were investigated in this paper. The ERT results showed that the Wenner and Wenner–Schlumberger arrays yielded similar results for subsurface characteristics whereas the DD array provided slightly different results. The combined usage of these arrays produced satisfactory images of the subsurface resistivity distribution. In addition, the electrode spacing tests revealed that a suitable interpretation of subsurface geology can be obtained from a 5 m electrode interval. However, a suitable trenching location defined by successful 2D resistivity models was obtained for 1 m electrode spacing. Therefore, the comparison of the trench and ERT results was also possible. The results of trenching and ERT studies substantially support each other.

Late Holocene activity of the Gosukebashi fault revealed by trench study at the Nishitakigatani valley in the eastern Rokko Mountains, Japan

Late Holocene activity of the Gosukebashi fault revealed by trench study at the Nishitakigatani valley in the eastern Rokko Mountains, Japan

28 March 1970 Gediz earthquake fault, western Turkey: palaeoseismology and tectonic significance

On 28 March 1970, an unexpected and destructive earthquake (Ms = 7.2) originated along the Erdoğmuş fault (EF), which forms the southern margin of the modern Erdoğmuş–Yenigediz graben in the central part of the Akşehir–Simav fault system. The EF is a N-dipping normal fault, ∼12 km long, generally E–W-trending, and characterized by a minor right-lateral strike–slip component. To determine its past activity, a palaeoseismological exploratory trenching study was conducted. Two trenches (EFT-1 and EFT-2) were excavated on the ground surface rupture of the 1970 Gediz earthquake near Erdoğmuş village. Based on the relative displacement between units observed and mapped in EFT-1, at least three events were identified. Two events were also identified in EFT-2. Only one of the events in EFT-1 can be dated via 14C. The estimated recurrence interval on the EF is ∼910 ± 40 years.

Application of GPR to normal faults in the Büyük Menderes Graben, western Turkey

Paleoseismology documents past surface-rupturing earthquakes that occurred on faults. This study is limited by the scarcity data on geomorphic and sedimentary environments that may preserve adequate records of deposition, erosion, and fault slip markers. Identifying relevant trenching sites can be difficult when a fault is buried or its surface expression has been eroded since the last tectonic motion. Ground penetrating radar (GPR) is an effective tool for locating suitable sites for trenching. Characteristic reflections are produced by boundaries between elements with contrasting electrical properties, such as grain size distribution (sorting, clay content, etc.), porosity, and water content. GPR is capable of resolving faults by imaging offset stratigraphic reflectors or reflections from the fault plane. GPR surveys were performed at two sites along the Büyük Menderes Graben (western Turkey) to precisely locate the normal fault zone; there is no clear evidence of surface rupture at these sites. We used 250 and 500MHz antennas for receiving the GPR data. From the GPR measurements, we determined locations suitable for paleoseismic investigations and performed a trenching study across the fault plane. The comparison of the GPR results and the trenching study indicates a good correlation between these methods.

Constraining regional paleo peak ground acceleration from back analysis of prehistoric landslides: Example from Sea of Galilee, Dead Sea transform

Accurate estimation of expected peak ground acceleration (PGA) in seismically active regions is a challenging task. The best way to estimate, quantitatively, expected PGA is by investigating instrumental data of past strong earthquakes in a given area. In some regions of the world however recorded data are scarce, and if they exist, they are typically available only since the late 19th century. As such they are hardly representative of the true seismicity in the studied region. We propose here an analytical approach to constrain the lower threshold of paleoseismic PGA on the basis of back analysis of old landslides. To perform the analysis we need a mapped landslide with geomorphic features that have been preserved in the field, the slip surface, a good reconstruction of the slope geometry and ground water level prior to failure, and the mechanical properties of the sheared material. We perform static and pseudo-static limit equilibrium analyses using standard solution procedures to obtain lower bounds of paleoseismic PGA. Back analyses of three different landslides around the Sea of Galilee (SOG) return similar results that range between 0.15 and 0.5 g, thus constraining the threshold paleoseismic PGA range for this region. The analytically inferred regional PGA is supported by results of an independent numerical analysis of toppled columns in a nearby Byzantine church. Using results from a recent paleoseismic trenching study performed on one of the studied landslides and a modified attenuation relationship for the study area we localize the loci of moment magnitude M w = 7.0 earthquakes that can explain the studied failures along the boundaries of the SOG, and find that they coincide with traces of the Eastern and Western Margin faults of the Dead Sea transform. The temporal relationships between the observed failures are discussed on the basis of dated colluvial sediments, geomorphologic constraints, and archeological evidence.

Late Quaternary and coseismic crustal deformation across the focal area of the 2008 Iwate–Miyagi Nairiku earthquake

We investigated late Quaternary and coseismic crustal deformation rates in the 2008 Iwate–Miyagi Nairiku earthquake (2008 IMEQ) focal area. We identified five fluvial terraces and used tephrochronology and 14C dating to correlate them with marine isotope stages (MISs) 6–5, slightly younger than 5.3–5.2, 4–5, 2–1, and 1. The late Quaternary uplift rates calculated from the relative heights of the MIS 6–5 (120–140 ka) and 2–1 (10 ka) terrace surfaces were 0.18–0.21 ± 0.13 m/ky and 0.62–0.74 ± 0.13 m/ky on the downthrown and upthrown sides of the 2008 IMEQ fault, respectively. We developed a fault model that reflected the coseismic vertical displacements and surface ruptures. This model had a fault plane dipping to the west at 40° with slip of 4.4 m on the deep (10–3 km depth) part of the fault plane and of 0.3 m on the shallow (3–0 km depth) part. The late Quaternary and coseismic deformation pattern of the ground surface roughly agree. Furthermore, the recurrence interval (2.9–4.2 ky) calculated from the late Quaternary vertical offset and coseismic vertical deformation roughly agrees with the findings of a previous trenching study. Thus, the slip on the fault of the 2008 IMEQ was a characteristic earthquake event.