Western Termination Research Articles

Capitalizing on the Tidelands: Chinese Migrants, Environmental Conflicts, and the Circuits of US Empire in Nineteenth-Century Oakland

Abstract In 1869, the Pacific seaport of Oakland, California, became the western terminus of the transcontinental railroad. The city's infilled tidelands bridged distant geographies of war and trade, from Nevada mines to Hawai‘i plantations to far-flung naval coaling stations. In Oakland, as in the continental and overseas outposts linked by the city's infrastructure, Chinese contract workers performed much of the dangerous, poorly compensated labor. Chinese fishing villages hugging the shores of Oakland and neighboring cities sustained this system of production. Their bountiful catches fed local Chinese migrants, as well as migrants scattered across the mines and plantations of the Pacific. Populated by former contract workers, the fishing villages offered alternatives to the racial hierarchies, brutal labor, and environmental ruin characterizing the political economy of the post-emancipation era. But Oakland's conversion into a launchpad for military and commercial ventures threatened their ecologies, a food base nourishing the workers who made that conversion possible. In the face of ecological enclosure and state-sanctioned repression, fishing villages managed to multiply. This article traces their footprints, showing how Chinese migrants made use of nature and capitalist markets to escape stifling conditions in the waged economy. Situated at the underexplored intersection of migration and environmental change, the article builds on a growing body of scholarship documenting the reconstruction of race, labor, and property in the post–Civil War American West. It argues that environmental conflicts in a rapidly industrializing Pacific Coast entrepôt reflected contradictions at the heart of the commercial and territorial expansion of the late nineteenth-century United States.

Spatiotemporal characteristics and earthquake statistics of the 2020 and 2022 adjacent earthquake sequences in North Aegean Sea (Greece)

The two moderate earthquakes that occurred close and to the north of the North Aegean Trough (NAT) on 26 September 2020 (Mw5.3) and 16 January 2022 (Mw5.4), both followed by aftershock activity, are examined. Seismic activity along the NAT and its parallel branches is continuous and remarkable, with numerous strong instrumental (M≥6.0) earthquakes. Yet, the frequency of moderate (5.0≤M<6.0) earthquakes outside these major fault branches is rather rare and therefore their investigation provides the optimal means to decipher the seismotectonic properties of the broader area. The temporal and spatial proximity of the two seismic excitations from late September of 2020 through early 2022, intrigues for exhaustive investigation of seismic activity with the employment of earthquake relocation techniques, moment tensor solutions and statistical analysis. Our research revealed that this seismic activity purely falls inside the Mainshock – Aftershock type, with fast aftershock decay rates and moderate productivity. According to our findings, the two seismic sequences, despite their close proximity, exhibit distinctive features as a result of the intricate stress field generated at the western termination of the NAF system in an extensional domain.

Post-collisional calc alkaline-adakites from a metasomatized subcontinental lithospheric mantle: Sr-Nd isotopes and bulk-rock geochemistry of Miocene volcanic rocks from the Varzaghan area, NW Iran

ABSTRACT The NW Iran is located at the western termination of the Alborz Mountain range and continuation of the Sanandaj-Sirjan magmatic-metamorphic zone at the NW. The volcanic rocks in the Varzaghan area are distributed over different compositional ranges from high-K basalt, andesite, dacite, and trachyandesite, dacite, to rhyolite with adakitic signatures. Chondrite-normalized REE diagrams show flat MREE and HREE patterns for calc-alkaline volcanic rocks and enrichment in LREE, indicating a garnet-free source. Adakitic rock sample chondrite-normalized REE patterns are relatively steep with significant enrichment in LREE and depletion in HREE, indicating a garnet-bearing source or garnet fractionation process. The arc trace element signatures, narrow range of initial87Sr/86Sr(i) = 0.70425–0.70487, different initial143Nd/144Nd(i) = 0.51264–0.51286 and the presence of adakitic rocks reveal that the volcanic rock sources were not the same and can be related to the partial melting of metasomatized mantle and lower continental crust sources. Slab break-off and upwelling of hot asthenosphere in a post-collisional tectonic regime resulted in the melting of the subcontinental lithosphere and lower continental crust which produced calc-alkaline-adakitic volcanic rocks in the Varzaghan area. The main petrological processes involved in the volcanic rocks petrogenesis were fractional crystallization and crustal assimilation.

The Development of Fault Networks at the Termination of Continental Transform Faults When Their Connecting Plate Boundary Is “Misaligned”

AbstractWe report the results of a series of scaled laboratory experiments that investigate the development of fault systems in plate boundary transition zones, where deformation is distributed between a continental transform fault (e.g., Alpine Fault, New Zealand; North Anatolian Fault (NAF), Turkey; San Andreas Fault (SAF), USA) and its connecting plate boundary. In these transition zones, continental transform faults are observed to branch into multiple subsidiary faults. Here we show that large‐scale transition zone fault networks comprise crustal‐scale Riedel shears that develop sequentially outwards and away from the parent transform fault. Such fault networks form within brittle upper crust that overlies a ductile lower crust that deforms by large‐scale distributed simple shear. We argue that large‐scale distributed deformation of the lower crust occurs when continental transform faults are “misaligned” relative to their connecting plate boundaries. This misalignment may occur: (a) where a transform fault does not directly connect with a convergent or divergent plate boundary, as in the northern termination of the Alpine Fault and the western termination of the NAF; and (b) where a significant bend in the transform fault occurs in the plate boundary transition zone, as in the southern termination of the SAF. The development of such plate boundary misalignments appears to occur when a plate boundary transition zone develops as a continental transform fault propagates toward a different type of “connecting” plate boundary.

Exhumation of ultrahigh-pressure eclogite and metarodingite within UHP serpentinites from the Chinese southwestern Tianshan

Extensive research over the past two decades suggests that serpentinite loses its ability to effectively facilitate the exhumation of high-density high-pressure (HP) eclogites at depths of 60–80 km under average subduction zone geotherms. However, at Changawuzi, the western termination of Chinese southwestern Tianshan, eclogite and eclogitic metarodingite are commonly enclosed within ultrahigh-pressure Ti-chondrodite serpentinites (510–530 °C, 37 ± 7 kbar). This metamorphic mafic–ultramafic association, which has undergone a complex evolution from oceanic alteration to deep subduction and subsequent exhumation, presents a unique opportunity to study the incompletely understood mechanisms leading to the exhumation of ultrahigh-pressure (UHP) eclogite in UHP serpentinite.The Changawuzi serpentinite wraps three different rock types: retrograded eclogite, Rodingite I and Rodingite II. Field observations and petrological analyses indicate that the precursor of Rodingite I intruded as vein into the serpentinite protolith prior to subduction, sharing the evolution towards ultrahigh-pressure conditions. According to its composition, the serpentinite originated from a depleted oceanic mantle peridotite, with only minor enrichment in fluid-mobile elements (FME) during oceanic serpentinization. The eclogites have N-MORB like bulk compositions, with lower Si but higher Mg content compared to other eclogites in the Tianshan area, and appear to have been relatively unaffected by oceanic alteration processes. The Changawuzi serpentinite and eclogites likely evolved within a slab setting with limited interaction with seawater. Petrological studies and phase equilibria modeling suggest that the eclogites experienced peak UHP metamorphism at 28–34 kbar and around 460–465 °C. Density measurements yielded relatively low densities of 2.66–2.97 g/cm3 for the host serpentinites, but 3.27–3.63 g/cm3 for Rodingite I. The similarity in the low temperature/ultrahigh pressure conditions, as well as the geochemical characteristics shared by the serpentinite and eclogites, further support the connection between the Changawuzi serpentinites and the interior of the subducting slab. According to our data, two types of UHP eclogitic mafic rocks exhumated with UHP serpentinite. (1) High density Rodingite I veins are embedded within serpentinite and share a common evolution and exhumation history with the host serpentinite. (2) The eclogite and serpentinite tectonically detached from the interior of the subducting slab at different depths and ascended to the plate interface above the slab (subduction channel). These two rock types experienced separate exhumation processes within the subduction channel during the early stages of exhumation. Subsequently, the buoyant serpentinite captured the denser retrograded eclogite at a depth of approximately 80 km, and together they underwent further retrograde metamorphism as they ascended towards the surface. During a later exhumation stage, external fluid induced a second serpentinization process, leading to the formation of Rodingite II within the subduction channel. This study presents a novel exhumation mechanism specific to the Tianshan eclogites and highlights the significant role of deeply subducted serpentinite in the exhumation of eclogites and eclogitic metarodingites from sub-arc depths within an oceanic subduction zone.

Identification and differentiation of vertical movement through morphological changes and stratigraphic imprint: Two distinct uplifting mechanisms in the upper Calabrian accretionary wedge, western Ionian Sea

AbstractThe seafloor morphology reflects both past and on‐going sedimentary, oceanographic and tectonic processes. Vertical movement is one of the drivers responsible for reshaping the seafloor through forming steep flanks that decrease slope stability, favour landslides, change current paths, form minibasins and control the sediment deposition, distribution and geometry. Here, we make use of these interactions to derive vertical movements and constrain the active tectonic processes at the western termination of the upper Calabrian accretionary wedge from the integrated analysis of bathymetric, backscatter, surface attributes and high‐resolution reflection seismic data. Within this area, we identify two types of deformational features and mechanisms that affect the depositional, erosional and tectonic processes at different scales. These include the deviation of channels, landslide scars, mass transport deposits (MTDs), separated drifts, sediment waves, lineaments and offset seafloor structures. The first type (long‐wavelength uplift) is an uplifted 22‐km‐wide region, in which seismic onlap relationships and the dip of deep reflectors suggest long‐lasting but slow tectonic uplift affecting sedimentation, and the second type (short‐wavelength uplift) includes three narrow elongated structures and one circular dome encircling the first region of uplift. We interpret that the first type of uplift feature was caused by tectonic deformation, while the second type is interpreted as formed by the fast uplift, tilting and faulting of modern sediments caused by diapirism due to rapid sedimentation in response to the first tectonically driven uplift. The study provides insight into the complex interaction of tectonic and sedimentary processes in the upper Calabrian accretionary wedge.

Conrad Seabaugh & Company: An Inside Look at an Illinois Hinterland Merchant

Conrad Seabaugh & Company: An Inside Look at an Illinois Hinterland Merchant

Characteristics of the North Anatolian Fault at the eastern end of Marmara seismic gap based on multidisciplinary field evidence

SUMMARYThe northern branch of the North Anatolian Fault (NAF) is the primary source of the greatest natural hazard for the Marmara Megapolis and specifically the İstanbul Metropolitan area. The Sea of Marmara section of the NAF is a ∼125-km-long seismic gap bound by the M7.4 1912 Mürefte earthquake rupture in the west and M7.4 1999 İzmit earthquake rupture in the east. Terminations of these ruptures define the length of the seismic gap in the Marmara Sea and consequently the magnitude and rupture extent of the expected large magnitude earthquake. The Hersek Peninsula is the last location where the NAF can be studied on land before it enters the Sea of Marmara. In addition, it is a geologically key location for the western termination of the M7.4 İzmit surface rupture. This study integrates high-resolution onshore and offshore data from Hersek Peninsula and its vicinity in İzmit Bay to assess the seismic hazard for the Marmara region. Detailed geological and geomorphic mapping, palaeoseismic trenching, geophysical and geotechnical subsurface data and archeoseismologic investigations demonstrate that the 1999 İzmit earthquake surface rupture died off east of the Hersek Peninsula due to structural complexity of the NAF at this location and the most recent earthquake to rupture the surface at Hersek Peninsula was the historical 1509 CE event. A restraining bend and bifurcation of the NAF at Hersek Peninsula suggests that this location might be a persistent asperity forming a surface rupture segmentation point at least since Pleistocene. Surface ruptures documented in palaeoseismic trenches north of the Hersek Lagoon and an offset 6th century East Roman aqueduct; however, suggests that this section of the NAF has ruptured the surface multiple times in the past and is highly likely to rupture again during the expected Marmara earthquake. The aqueduct; which crosses the surface trace of the NAF, is offset 14 ± 1 m and yields 13.6 ± 1 mm yr–1 dextral slip rate indicating that the northern branch of the NAF accrues and releases the majority of the stress between Eurasia and Anatolian block in the Marmara region. If the ∼125-km-long seismic gap between the 1912 and 1999 ruptures fail during a single event the expected Marmara earthquake can reach M7.4.

Subaqueous fault scarps of the North Anatolian Fault in the Gulf of Saros (NE Aegean); where is the western limit of the 1912 Mürefte-Şarköy earthquake rupture?

SUMMARYThe westernmost segment of the North Anatolian fault in NW Turkey lies mostly offshore, in the Sea of Marmara and the Gulf of Saros (NE Aegean), respectively to the E and W of a 45 km inland central portion. The 9 August 1912 Mürefte-Şarköy (Ms 7.4) and 13 September 1912 (Ms 6.8) earthquakes occurred along this segment. To date, the segment was studied mostly onshore although estimated magnitude and location suggest an offshore extension. Recent studies show the eastern rupture extension in the Sea of Marmara, while its western counterpart in the Gulf of Saros remains less documented. Here we use new observations from high-resolution marine geophysical data (multibeam bathymetry, side-scan-sonar, and seismic reflection profiles), to constrain the offshore 1912 ruptures in the Gulf of Saros. Detailed mapping of the subaqueous fine-scale morphology and structure of the fault provides a new insight for the western limit of the two 1912 surface ruptures. Distribution of fresh scarps, 3-D structural reconstructions, the complexity of fault segments, and the recent seismicity, altogether suggest that the western termination of the 1912 rupture(s) ends 37 km offshore in the Gulf of Saros. Following the 1999 Kocaeli earthquake, in the eastern Sea of Marmara, the unruptured segment length between the 1999 and 1912 ruptures became a critical issue, because of its implication for future earthquakes in the so-called Marmara seismic gap. If a 150–160 km total rupture length for the two 1912 earthquakes is assumed, a western rupture termination point at the inner Saros basin margin means that the eastern extension of the 9 August earthquake rupture reached the Central Marmara Basin. This outcome necessarily has implications for the seismic hazard in the Marmara coastal area that includes the Istanbul metropolitan area.

Seven Decades of Coastal Change at Barter Island, Alaska: Exploring the Importance of Waves and Temperature on Erosion of Coastal Permafrost Bluffs

Observational data of coastal change over much of the Arctic are limited largely due to its immensity, remoteness, harsh environment, and restricted periods of sunlight and ice-free conditions. Barter Island, Alaska, is one of the few locations where an extensive, observational dataset exists, which enables a detailed assessment of the trends and patterns of coastal change over decadal to annual time scales. Coastal bluff and shoreline positions were delineated from maps, aerial photographs, and satellite imagery acquired between 1947 and 2020, and at a nearly annual rate since 2004. Rates and patterns of shoreline and bluff change varied widely over the observational period. Shorelines showed a consistent trend of southerly erosion and westerly extension of the western termini of Barter Island and Bernard Spit, which has accelerated since at least 2000. The 3.2 km long stretch of ocean-exposed coastal permafrost bluffs retreated on average 114 m and at a maximum of 163 m at an average long-term rate (70 year) of 1.6 ± 0.1 m/yr. The long-term retreat rate was punctuated by individual years with retreat rates up to four times higher (6.6 ± 1.9 m/yr; 2012–2013) and both long-term (multidecadal) and short-term (annual to semiannual) rates showed a steady increase in retreat rates through time, with consistently high rates since 2015. A best-fit polynomial trend indicated acceleration in retreat rates that was independent of the large spatial and temporal variations observed on an annual basis. Rates and patterns of bluff retreat were correlated to incident wave energy and air and water temperatures. Wave energy was found to be the dominant driver of bluff retreat, followed by sea surface temperatures and warming air temperatures that are considered proxies for evaluating thermo-erosion and denudation. Normalized anomalies of cumulative wave energy, duration of open water, and air and sea temperature showed at least three distinct phases since 1979: a negative phase prior to 1987, a mixed phase between 1987 and the early to late 2000s, followed by a positive phase extending to 2020. The duration of the open-water season has tripled since 1979, increasing from approximately 40 to 140 days. Acceleration in retreat rates at Barter Island may be related to increases in both thermodenudation, associated with increasing air temperature, and the number of niche-forming and block-collapsing episodes associated with higher air and water temperature, more frequent storms, and longer ice-free conditions in the Beaufort Sea.

Coseismic slip gradient at the western terminus of the 1920 Haiyuan Mw 7.9 earthquake

Stepovers in strike-slip fault systems play important roles in controlling the propagation of earthquake ruptures, depending partially on the competition between the stepover width and the coseismic slip gradient approaching the step. The 1920 Mw 7.9 Haiyuan earthquake is the most recent major earthquake that has occurred along the Haiyuan fault. The earthquake rupture broke through multiple stepovers along the fault and finally ended at the 4 km-wide Jingtai pull-apart basin (releasing stepover) at the western end. To understand the process of this termination of the Haiyuan earthquake rupture, we conducted detailed mapping of the surface rupture geometry, coseismic slip measurement and slip gradient calculation in the vicinity of the endpoint based on the 0.2 m-resolution SfM-derived DEM along the 20 km section east of the Jingtai pull-apart basin. Combining coseismic slip measurements from this study and published slip data from fieldwork in the 1980s, we calculated slip gradients of 84–160 cm/km as the rupture approached the Jingtai releasing stepover. These values are high for the 4 km-wide Jingtai releasing stepover compared to those from a dataset of worldwide historical earthquakes compiled by Elliott et al. (2009). The high values imply that the rupture could have a relatively high likelihood of breaching through the Jingtai releasing stepover. Alternatively, the slip gradient may be overestimated. Detailed mapping and field investigation, however, show that the rupture may extend ~1.86 km farther west of the end location indicated in previous studies. We acquired the new slip gradient with lower value of 50–82 cm/km, which is still considerably high compared to dataset of Elliott et al. (2009). We speculate that the slip gradient could be larger variation along the stepover boundary to stop fault slip in different geologic setting. Another factor control slip termination is related to fault properties. Creep along the southern boundary fault of the Jingtai stepover, a velocity-strengthening region, may also have played a role in stopping the rupture at this location. Our observations further indicate that the increasing LOS velocity of InSAR data within the Jingtai stepover is probably related to fault behavior rather than a nontectonic signal of subsidence.

Timing of Alpine Orogeny and Postorogenic Extension in the Alboran Domain, Inner Rif Chain, Morocco

AbstractThe variety of temporal and structural constraints on the Alpine tectonometamorphic signature of the metamorphic core of the Betic‐Rif orogen (Alboran Domain) has supported a long‐lasting debate regarding the Alpine tectonic and geodynamic evolution of the Western Mediterranean region. Uncertainty still exists on the timing and tectonic significance of (a) the Alpine orogenic construction; (b) exhumation of the deep roots of the orogen; and (c) transition from orogenic shortening to crustal extension. In this study, we address these major geological issues by focusing on the lower‐grade units of the Alboran Domain (Upper Sebtides and Ghomarides) exposed in the Rif belt of northern Morocco. Through a multidisciplinary approach that integrates mesostructural and microstructural investigations with X‐ray diffraction, quantitative mineral chemistry, and 40Ar/39Ar geochronology, a 20 Ma long tectonic history is reconstructed, which involves burial of the tectonic units at depth (late Eocene) and postorogenic exhumation under brittle conditions in the upper crust (early Miocene). We document a Priabonian (∼37‐34 Ma) D1/M1 progressive compressional deformation, during the formation of a SW‐verging orogenic wedge (present coordinates), accreted toward the Africa plate. Brittle extensional detachment tectonics operated during the Burdigalian (∼18‐17 Ma), controlling the thinning of the previously structured Alboran Domain nappe stack and the final exhumation of the Alpine orogenic units. We propose that transition from orogenic build‐up to collapse in the hinterland of the Betic‐Rif orogen occurred when the retreat of the Apennine‐Maghrebian subduction was efficient to drive transition from shortening to extension in the back‐arc domain of the western termination of the Apennine‐Maghrebian subduction zone.

Geometric and kinematic characteristics of the Khazar and North Alborz Faults: Links to the structural evolution of the North Alborz-South Caspian boundary, Northern Iran

Geometric-kinematic characteristics of the Khazar fault, roughly 600 km long along the North Alborz- South Caspian boundary, are investigated using field evidence and satellite images. The fault trace along the strike and the fault zone architecture has been determined. This study indicates the northern margin of the Alborz mountains is complicated by the presence of four main fault systems, the Khazar, Sari, North Alborz, and Langrud faults from the east to the west, respectively. The results of this study show the western termination of the Khazar fault in the eastern Alborz lies somewhere in the east of the Neka city (Longitude: ~53°20′E), and where laterally juxtaposes to the North Alborz fault through Sari fault. According to the recent movement of the Khazar and North Alborz faults, an extensional area has been created along the Sari fault zone in the middle part of the northern Albors mountains. Moreover in the western Alborz, the Langrud fault and the western segment of the North Alborz fault have created a transpressional deformation zone. The collision of Iranian plateau with the South Caspian basin, in the late Miocene, has made curved shape of the eastern and western parts of the Alborz mountains that progressively wrapped it with anticlockwise and clockwise rotations, respectively. Geodetic rotation rates indicate the Alborz mountains currently have anticlockwise rotation since its amount decreases from east to west.

Late Quaternary sea level, isostatic response, and sediment dispersal along the Queen Charlotte fault

Abstract The active Pacific margin of the Haida Gwaii and southeast Alaska has been subject to vigorous storm activity, dramatic sea-level change, and active tectonism since glacial times. Glaciation was minimal along the western shelf margin, except for large ice streams that formed glacial valleys to the shelf break between the major islands of southeast Alaska and Haida Gwaii. Upon deglaciation, sediment discharge was extensive, but it terminated quickly due to rapid glacial retreat and sea-level lowering with the development of a glacio-isostatic forebulge, coupled with eustatic lowering. Glacial sedimentation offshore ended soon after 15.0 ka. The shelf became emergent, with sea level lowering by, and possibly greater than, 175 m. The rapid transgression that followed began sometime before 12.7 ka off Haida Gwaii and 12.0 ka off southeast Alaska, and with the extreme wave-dominated environment, the unconsolidated sediment that was left on the shelf was effectively removed. Temperate carbonate sands make up the few sediment deposits presently found on the shelf. The Queen Charlotte fault, which lies just below the shelf break for most of its length, was extensively gullied during this short period of significant sediment discharge, when sediment was transported though the glacial valleys and across the narrow shelf through fluvial and submarine channels and was deposited offshore as sea level dropped. The Queen Charlotte fault became the western terminus of the glacio-isostatic forebulge, with the fault acting as a hinged flap taking up the uplift and collapse along the fault of 70+ m. This may have resulted in the development of the distinctive fault valley that presently acts as a very linear channel pathway for sediment throughout the fault system.

Role of multiple inherited basement structures on orogen geometry and evolution: Insights from analogue modelling

The role of inherited weaknesses in metamorphic basements with respect to the geometry and tectonic evolution of subsequent orogens formation forms a cornerstone of structural reconstructions, especially in non-inverted orogens with old Variscan basements. In this study, we analyse the geometry, kinematics, and tectonic evolution of the complex western termination of the Alpine Pyrenean–Cantabrian orogen using an integrated approach based on analogue modelling, constrained by structural mapping and geochronology, to provide new insights into the role of pre-existing structures and the shift in the orientation of the stress field. Our results demonstrate that pre-existing structures are necessary to explain the complex western termination, which does not follow the same tectonic architecture as the rest of the orogen. The reason for this is the different orientation of the pre-existing Variscan structures in this area, which do not follow the general E–W trend. These western structures (including NNE–SSW and NW–SE strike-slip fault systems and a greater number of E–W thrusts) accommodate the shortening, but have a less favourable orientation. The reactivation of the NNE–SSW strike-slip faults and the southern E–W thrusts in the recent stages indicates a late intraplate tectonic regime, mainly following the shift of the Shmax (from N–S to NNW–SSE) which occurred when the convergence between the Iberian, Eurasian, and African plates changed.

An integrated structural and GPS study of the Jalpatagua fault, southeastern Guatemala

Abstract The Jalpatagua fault in Guatemala accommodates dextral movement of the Central America forearc. We present new global positioning system (GPS) data, minor fault analysis, geochronological analyses, and analysis of lineaments to characterize deformation along the fault and near its terminations. Our data indicate that the Jalpatagua fault terminates at both ends into extensional regions. The western termination occurs near the Amatitlan caldera and the southern extension of the Guatemala City graben, as no through-going structures were observed to continue west into the active volcanic arc. Along the Jalpatagua fault, new and updated GPS site velocities are consistent with a slip rate of 7.1 ± 1.8 mm yr−1. Minor faulting along the central section of the fault includes: (1) N-S–striking normal faults accommodating E-W elongation; and (2) four sets of strike-slip faults (oriented 330°, 020°, 055°, and 295°, parallel to the Jalpatagua fault trace). Minor fault arrays support dextral movement along a major fault in the orientation of the Jalpatagua fault. GPS and fault data indicate that the Jalpatagua fault terminates to the east near the Guatemala–El Salvador border. Data delineate a pull-apart basin southeast of the fault termination, which is undergoing transtension as the Jalpatagua fault transitions into the El Salvador fault system to the east. Within the basin, minor faulting and lineations trend to the NW and accommodate NE-directed elongation. This faulting differs from E-W elongation observed along the Jalpatagua fault and is more similar to minor faults within the El Salvador fault system.

Earthquake Swarms, Slow Slip and Fault Interactions at the Western‐End of the Hellenic Subduction System Precede the Mw 6.9 Zakynthos Earthquake, Greece

AbstractThe month‐to‐year‐long deformation of the Earth's crust where active subduction zones terminate is poorly explored. Here we report on a multidisciplinary data set that captures the synergy of slow‐slip events, earthquake swarms and fault interactions during the ∼5 years leading up to the 2018 Mw 6.9 Zakynthos Earthquake at the western termination of the Hellenic Subduction System (HSS). It appears that this long‐lasting preparatory phase initiated due to a slow‐slip event that lasted ∼4 months and released strain equivalent to a ∼Mw 6.3 earthquake. We propose that the slow‐slip event, which is the first to be reported in the HSS, tectonically destabilized the upper 20–40 km of the crust, producing alternating phases of seismic and aseismic deformation, including intense microseismicity (Mw < 4) on neighboring faults, earthquake swarms in the epicentral area of the Mw 6.9 earthquake ∼1.5 years before the main event, another episode of slow slip immediately preceding the mainshock and, eventually, the large (Mw 6.9) Zakynthos Earthquake. Tectonic instability in the area is evidenced by a prolonged (∼4 years) period of overall suppressed b‐values (<1) and strong earthquake interactions on discrete strike‐slip, thrust and normal faults. We propose that composite faulting patterns accompanied by alternating (seismic/aseismic) deformation styles may characterize multifault subduction‐termination zones and may operate over a range of timescales (from individual earthquakes to millions of years).

Clusty, the waveform-based network similarity clustering toolbox: concept and application to image complex faulting offshore Zakynthos (Greece)

SUMMARYClusty is a new open source toolbox dedicated to earthquake clustering based on waveforms recorded across a network of seismic stations. Its main application is the study of active faults and the detection and characterization of faults and fault networks. By using a density-based clustering approach, earthquakes pertaining to a common fault can be recognized even over long fault segments, and the first-order geometry and extent of active faults can be inferred. Clusty implements multiple techniques to compute a waveform based network similarity from maximum cross-correlation coefficients at multiple stations. The clustering procedure is designed to be transparent and parameters can be easily tuned. It is supported by a number of analysis visualization tools which help to assess the homogeneity within each cluster and the differences among distinct clusters. The toolbox returns graphical representations of the results. A list of representative events and stacked waveforms facilitate further analyses like moment tensor inversion. Results obtained in various frequency bands can be combined to account for large magnitude ranges. Thanks to the simple configuration, the toolbox is easily adaptable to new data sets and to large magnitude ranges. To show the potential of our new toolbox, we apply Clusty to the aftershock sequence of the Mw 6.9 25 October 2018 Zakynthos (Greece) Earthquake. Thanks to the complex tectonic setting at the western termination of the Hellenic Subduction System where multiple faults and faulting styles operate simultaneously, the Zakynthos data set provides an ideal case-study for our clustering analysis toolbox. Our results support the activation of several faults and provide insight into the geometry of faults or fault segments. We identify two large thrust faulting clusters in the vicinity of the main shock and multiple strike-slip clusters to the east, west and south of these clusters. Despite its location within the largest thrust cluster, the main shock does not show a high waveform similarity to any of the clusters. This is consistent with the results of other studies suggesting a complex failure mechanism for the main shock. We propose the existence of conjugated strike-slip faults in the south of the study area. Our waveform similarity based clustering toolbox is able to reveal distinct event clusters which cannot be discriminated based on locations and/or timing only. Additionally, the clustering results allows distinction between fault and auxiliary planes of focal mechanisms and to associate them to known active faults.

Structural and Geophysical Characterization of the Western Termination of the South Pyrenean Triangle Zone

AbstractThe South Pyrenean triangle zone represents the southernmost front of the Pyrenees at its central portion deforming the Upper Eocene‐Miocene Ebro Basin deposits. Two main structures characterize its western termination, the Barbastro anticline and the San Román backthrust, which detached on the Barbastro Formation (and lateral equivalents), an Upper Eocene‐Lower Oligocene syntectonic evaporite‐rich formation that acted as a multidetachment unit. Northward, the south directed Pyrenean thrust unit (i.e., Gavarnie‐Sierras thrust sheet) detached along the Middle‐Upper Triassic evaporitic rocks to finally ramp up and glide along the same Upper Eocene‐Lower Oligocene multidetachment unit. A multidisciplinary approach allowed constructing a detailed structural and stratigraphic model of the study area. The workflow consisted of (1) constraining the geometry and structural architecture based on surface geology, interpretation of seismic lines (>900 km), and wells and (2) obtaining the 3‐D density distribution of the multidetachment unit using gravity stochastic inversion (more than 7,000 gravity stations and 1,500 density data). The geometry of the sole thrust of the Gavarnie‐Sierras thrust sheet was controlled by the distribution of the evaporite‐rich units of the Barbastro Fm. Weak detachments promoted thrust salient formation and thrust flat geometries. The western termination of the South Pyrenean triangle zone is defined as a westward transition from a ramp‐dominated and multiple triangle zone to a detachment‐dominated one. Its geometry, kinematics, and location were controlled by the heterogeneous lithology of the Barbastro Fm. and its basal, halite‐based detachment southern pinch‐out.

A 3D geological model for a potential CO2 reservoir in the El Bierzo Basin (Carboniferous, NW Spain)

ABSTRACTA 3D geological model of the Torre‐Bembibre Carboniferous sub‐basin, a potential CO2 storage reservoir, is proposed here. The Torre‐Bembibre sub‐basin is part of the El Bierzo coal‐bearing Carboniferous basin located in the westernmost part of León province (NW Spain). It covers an area of about 150 km2. Carboniferous deposits (up to 1500 m of continental siliciclastics) are divided into nine intervals, defined by earlier coal exploration work. They were deposited as intramontane basin infill within a transpressional regime during the last stages of the Variscan Orogeny and are partially covered by Neogene and Quaternary continental siliciclastic sediments, formed by alluvial plains, fans and terraces. To resolve a 3D geological model of the Torre‐Bembibre sub‐basin, including outcropping layers and their prolongation at depth, a 3D dip‐domain method was applied, where data from all stratigraphic positions were used to compose a coherent geological reconstruction of the stratigraphic surfaces. Subsurface information was available from previous and newly acquired seismic reflection profiles and boreholes. The 3D model helped resolving some uncertainties, specifically: (i) the western termination of the Carboniferous basin under the Tertiary sediments, corresponding to an NNE–SSW normal fault, (ii) the depth and morphology of the base of the Carboniferous strata and (iii) the morphology of the intra‐Carboniferous layers. The syncline/anticline shape of the Torre‐Bembibre sub‐basin suggests a reservoir which may be capable of storing CO2 under the Tertiary cover. The total estimated volume is 6.4 km3, while the total capacity for CO2 storage could be as much as 7 Mt. The reservoir is most likely located within the major massive sandstone bodies in the Chuchu interval, in the syncline–anticline structure north of the town of Bembibre. The potential capacity of the reservoir calls for undertaking further studies using the proposed 3D geological model in order to determine more precisely the petrophysics and geometry near Bembibre.