Eastern Margin Fault Research Articles

Estimation of the Slip Rate Along the Unruptured Fault Segment of the M7.2 1896 Rikuu Earthquake, Northeast Japan

AbstractThe Ou Backbone Range (OBR) is one of the most seismically active intraplate regions in northeast Japan. Over the past 200 years, it has been the locus of five large magnitude earthquakes, including the M7.2 1896 Rikuu earthquake, which produced coseismic surface deformation along the northern segment of the Eastern Margin Fault Zone of Yokote Basin (EFZYB), while the southern segment remained unruptured. Despite the extensive paleoseismic investigations along the ruptured segment, the slip rate estimates, and the recurrence interval remained largely unresolved along the southern unruptured segment. This study reports the long‐term slip rate, and recurrence interval along the Higashi Chokai san Fault and the signatures of active faulting along the Kanazawa and Omoriyama Faults. Based on the seismic reflection and borehole survey, we have estimated a vertical displacement of 29.3–36.1 m across the Higashi Chokai san Fault, a long‐term slip rate of 1.6–1.9 mm/yr over the past 35 ka, and a recurrence interval of about 3,800–4,600 years.Combining our results with the already published data along the OBR, revealed two seismic gaps, first along the western margin of the OBR, along the partial rupture zone of the 1970 Akita‐ken earthquake, and second, along the eastern margin of the OBR, north of the 2008 Iwate‐Miyagi Nairiku earthquake (IMEQ) rupture zone. Given most of the slip is released elastically, we propose that these seismic gaps have a potential to produce earthquakes with magnitudes Mw7.1 and Mw7.3, respectively if they were to rupture in a single event.

Geophysical evidence of the Eastern Marginal Fault of the Cheb Basin (Czech Republic)

The western part of the Bohemian Massif hosts an intersection of two regional fault zones, the SW-NE trending Ohře/Eger Graben and the NNW-SSE trending Marianske Lazně Fault, which has been reactivated several times in the geological history and controlled the formation of the Tertiary Cheb Basin. The broader area of the Cheb Basin is also related to permanent seismic activity of ML 3+ earthquake swarms. The Eastern Marginal Fault of the Cheb Basin (northern segment of the Marianske Lazně Fault) separates the basin sediments and underlying granites in the SW from the Krusne Hory/Erzgebirge Mts. crystalline unit in the NE. We describe a detailed geophysical survey targeted to locating the Eastern Marginal Fault and determining its geometry in the depth. The survey was conducted at the Kopanina site near the Nový Kostel focal zone, which shows the strongest seismic activity of the whole Western Bohemia earthquake swarm region. Complex geophysical survey included gravimetry, electrical resistivity tomography, audiomagnetotellurics and seismic refraction. We found that the rocks within the Eastern Marginal Fault show low resistivity, low seismic velocity and density, which indicates their deep fracturing, weathering and higher water content. The dip of the fault in shallow depths is about 60° towards SW. At greater depths, the slope turns to subvertical with dip angle of about 80°. Results of geoelectrical methods show blocky fabric of the Cheb Basin and deep weathering of the granite bedrock, which is consistent with geologic models based on borehole surveys.

Fore-arc basin deformation in the Andaman-Nicobar segment of the Sumatra-Andaman subduction zone: Insight from high-resolution seismic reflection data

The Andaman-Nicobar region is the northernmost segment of the Sumatra-Andaman subduction zone and marks the western boundary of the Andaman Sea, which is a complex active back-arc extensional basin. We present the interpretation of a new set of deep seismic reflection data acquired across the Andaman-Nicobar fore-arc basin, from 8°N to 11°N, in order to better understand its structure and evolution, focusing on (1) how obliquity of convergence affects deformation in the fore arc, (2) the nature and role of the Diligent Fault (DF), and (3) the Eastern Margin Fault (EMF). Despite the obliquity of convergence, back thrusting and compression seem to dominate the Andaman-Nicobar fore-arc basin deformation. The DF is primarily a back thrust and corresponds to the Mentawai and West Andaman Fault systems farther in the south, along Sumatra. The DF is expressed in the fore-arc basin as a series of mostly landward verging folds and faults, deforming the early to late Miocene sediments. The DF seems to root from the boundary between the accretionary complex and the continental backstop, where it meets the EMF. The EMF marks the western boundary of the fore-arc basin; it is associated with subsidence and is expressed as a deep piggyback basin, containing recent Pliocene to Pleistocene sediments. The eastern edge of the fore-arc basin is the Invisible Bank (IB), which is thought to be tilted and uplifted continental crust. Subsidence along the EMF and uplift and tilting of the IB seem to be related to different opening phases in the Andaman Sea.

Morphology and tectonics of the Andaman Forearc, northeastern Indian Ocean

SUMMARY The Andaman Sea has developed as the result of highly oblique subduction at the western Sunda Trench, leading to partitioning of convergence into trench-perpendicular and trench-parallel components and the formation of a northward-moving sliver plate to accommodate the trench parallel motion. The Andaman forearc contains structures resulting from both components of motion. The main elements of the forearc are the accretionary prism and outerarc ridge, a series of forearc basins and major N–S faults. The accretionary prism is an imbricate stack of fault slices and folds consisting of ophiolites and sediments scrapped off the subducting Indian Plate. The western, outer slope of the accretionary prism is very steep, rising to depths of 1500–2000 m within a distance of 30 km. There is a difference in the short wavelength morphology between the western and eastern portions of the accretionary prism. The outer portion consists of a series of faulted anticlines and synclines with amplitudes of a few 100 to ∼1000 m and widths of 5–15 km resulting from ongoing deformation of the sediments. The inner portion is smoother with lower slopes and forms a strong backstop. The width of the deforming portion of the accretionary prism narrows from 80 to 100 km in the south to about 40 km between 10 ◦ N and 11 ◦ 30 � N. It remains at about 40 km to ∼14 ◦ 40 � N. North of there, the inner trench wall becomes a single steep slope up to the Myanmar shelf. The eastern edge of the outerarc ridge is fault bounded and, north of the Nicobar Islands, a forearc basin is located immediately to the east. A deep gravity low with very steep gradients lies directly over the forearc basin. The West Andaman Fault (WAF) and/or the Seulimeum strand of the Sumatra Fault System form the boundary between the Burma and Sunda plates south of the Andaman spreading centre. The WAF is the most prominent morphologic feature of the Andaman Sea and divides the sea into a shallow forearc and a deeper backarc region. The Diligent Fault runs through the forearc basin east of Little Andaman Island. Although it has the general appearance of a normal fault, multichannel seismic data show that it is a compressional feature that probably resulted from deformation of the hanging wall of the Eastern Margin Fault. This could occur if the forearc basins were formed by subduction erosion of the underlying crust rather than by east–west extension.

Lake-mire deposition, earthquakes and wildfires along a basin margin fault; Rønne Graben, Middle Jurassic, Denmark

Regional uplift of structural highs took place in the greater North Sea region in early Middle Jurassic times, resulting in a change from marine shelf deposition in the Early Jurassic to widespread emergence, erosion and localized deposition in grabens. The Middle Jurassic Bagå Formation exposed on the island of Bornholm in the Baltic Sea was deposited in the Rønne Graben, which forms a dog-leg pull-apart basin linking the northern and southern branches of the Tornquist Zone. The studied succession is situated on the hanging wall immediately adjacent to the main eastern graben margin fault and the footwall consists of deeply weathered granite. It is the only locality along the northern branch of the Tornquist Zone where active tectonic influence on Middle Jurassic deposition can be demonstrated in outcrop. Small alluvial fans composed of poorly sorted washout sands and muddy debris flows with weathered granite boulders fringed the footwall scarp. Syn-depositional soft-sediment deformation and chaotic bedding structures interpreted as seismites caused by earthquake shocks are characteristic at several levels. On the hanging wall further away from the active fault the depositional environment was dominated by shallow freshwater lakes with deposition of clay. Lake deposition outpaced subsidence and open, oxygen-deficient freshwater mires were formed during periods of no or only incremental fault-movement. The mires were only occasionally isolated from clastic input due to doming and dense vegetation, resulting in accumulation of pure peat beds. The content of spores and pollen shows that the mires were covered by a vegetation of arborescent and herbaceous Filicopsida (ferns), Lycopsida (club mosses), Cycadales/Bennettitales, Ginkgoales, Caytoniales, Taxodiaceae or Cupressaceae, and Araucariaceae. Wildfires were common in the mires and also on the footwall probably followed by episodes of increased run-off and erosion of the upland. The presence of common washout sands, debrites, seismites and abundant lacustrine flooding surfaces indicating sudden drowning of the mires was caused by abrupt movements on the fault with associated earthquakes. The succession thus provides an example of temporal changes in deposition in response to changes in tectonic conditions. The study illustrates how regional uplift of structural highs caused a restriction of deposition to narrow grabens like the Rønne Graben. Facies patterns changed dramatically from laterally extensive uniform packages controlled by sea-level changes in the Early Jurassic to strongly localized deposition with marked facies changes controlled by faulting and earthquakes in the Middle Jurassic. The succession thus serves as an excellent example of tectonically controlled deposition under humid conditions in a continental graben.

トレンチ調査からみた十日町盆地東縁断層の変位様式と活動履歴

The Eastern Marginal Fault of the Tokamachi Basin is located on the right bank of the Shinano River, the longest river in Japan, in the Niigata area, within the Neogene fold and thrust belt of central Japan. The activity of this fault is partly responsible for the formation of the Tokamachi tectonic basin. The fault zone is composed of several subparallel fault branches that strike N-S to NNW-SSE and deformed late Pleistocene to Holocene fluvial terraces. These fault branches are characterized by west-facing scarps with some subsidiary east-facing scarps to the east of the basin. Although the destructive 2004 and 2007 earthquakes occurred in the northern part of this fold and thrust belt, no historical rupture has been recorded in the Tokamachi Basin. To obtain paleoseismic records in this tectonically formed basin, we excavated four trenches across different fault branches. The Banba South, Banba North, and Shinmiya trenches are located across the west-facing scarps, and the Miyakuri trench is on an east-facing scarp. Fault exposures and deformed terrace deposits are present in these four trenches. These results confirm that these geomorphic scarps are indeed produced by faulting. At the Banba S trench, three fault traces that have nearly horizontal to very shallow dipping fault planes with upthrown side on the east are present, and show clear evidence of the latest event at ca. 3,500-3,100 yrs BP (BC 1,965-1,630 to BC 1,505-1,145). A penultimate event possibly occurred at after ca.11,000 yrs BP. (BC 11,810-9,800), although its exact timing is not determined. At the Banba N trench, the terrace deposits of ca. 5,300 yrs BP (BC 4,225-3,965) are deformed. This suggests the age of deformation is younger, and probably coincides with the latest event at the Banba S trench. At the Shinmiya trench, the late Pleistocene terrace is divided into two parts by a flexural scarp, probably produced by a blind reverse fault. The scarp is formed prior to 14,000 yrs BP (BC 14,710-13,700). In addition, a younger event is recognized to have occurred between 9,400 and 8,900 yrs BP (BC 8,555-8385 to BC 8,000-7,910). In contrast to those trenches on the west-facing scarps, trench logs at Miyakuri, located on an east-facing scarp, show possibly two events with relatively steep fault planes during the last 32,000 yrs. The latest activity at Miyakuri occurred between 9,500 and 7,400 yrs BP (BC 8,955 to BC 6,365-6,045), but the age of the penultimate event cannot be precisely determined. However, we suggest that the faulting interval at Miyakuri is longer than that of other fault branches. In summary, from this study, we can identify at least three paleoseismic events, namely, I, II, and III, and a possible event IV for the Eastern Marginal Fault of the Tokamachi Basin. Event I is clearly identified at Banba S and N trench. Three faults probably moved simultaneously by Event II. Two other events are only recognized locally. Faulting on the west-facing scarp is more frequent and has a high slip rate, reaching nearly 1 m/ky.

Subsurface geometry and structural evolution of the eastern margin fault zone of the Yokote basin based on seismic reflection data, northeast Japan

High-resolution seismic reflection profiles across coseismic fault scarps reactivated during the A.D. 1896 Rikuu earthquake along the eastern margin fault zone of the Yokote basin (EFZYB) in northeast Japan, correlated with borehole stratigraphy and geologic mapping, provide insights into its detailed kinematic history and structural evolution. In spite of along-strike variations of thrust geometries both at ground surface and at shallow depth, the EFZYB has commonly formed as forward breaking imbricate thrust systems. Near surface complexity of thrust geometries appears strongly affected by mechanical decoupling between layers within middle to late Miocene mudstone. Cross-section balancing across the Mahiru Mountains shows strong correlation of the millennial uplift rates with mountain topography but its weak correlation with the late Pleistocene uplift rates. Considering the thrust trajectories estimated by the balanced cross sections, the millennial dip-slip rate are consistent with the late Pleistocene dip-slip rate, suggesting that the EFZYB has accumulated strains at a constant rate since the onset of its fault activity.

Feature of fault treace and subsurface structure along the Eastern marginal fault zone of Yokote basin NE Japan

Feature of fault treace and subsurface structure along the Eastern marginal fault zone of Yokote basin NE Japan

Lithostratigraphy and depositional history of the Late Cenozoic hominid-bearing successions in the Yuanmou Basin, southwest China

Late Cenozoic strata bearing hominids and hominoids are distributed in the Yuanmou Basin, Yunnan, southwest China. Incisors of Homo erectus discovered in these strata in 1965 reportedly show primitive features known in Africa (Hu, Acta geologica sinica 1 (1973, p. 65). However, recent paleomagnetic study has revealed that the age of the hominid correlates to the early Brunhes chron (Hyodo et al., Journal of human evolution (2001), submitted). The lithostratigraphy of the successions that yield many mammalian fossils including those of the hominid, and the record of Equus is re-examined. The successions are divided in ascending order into the Longchuan, Shagou, Gantang (newly proposed Yangliuchun and Daipojing Members) and Yuanmou (newly proposed Dainawu and Niujianbao Members) Formations, based on different lithology and sedimentary facies. The newly proposed depositional systems of the formations are in ascending order, alluvial fan, ephemeral braided river, sandy braided river, ephemeral gravelly braided river and alluvial fan systems. The change in facies and paleocurrents revealed in the successions suggest that the basin was initiated as a syncline basin at ca 3.5 Ma and was completed as an asymmetric half graben by active movement of the eastern marginal fault in the early Middle Pleistocene.

Northern continuation of Caledonian high-pressure metamorphic rocks in central-western Spitsbergen

Caledonian low-temperature, high-pressure metamorphic rocks, as first recognised at Motalafjella, central western Spitsbergen, include characteristic brown dolostone and serpentinite. Similar rocks are scattered in the strandflats, along the eastern marginal fault of the Tertiary Forlandsundet Graben, and along thrust faults to the east on both sides of St. Jonsfjorden. The mineral chemistries of the constituent carbonates and oxides are diagnostic of high-pressure metamorphic rocks, containing high contents of MgO in the carbonates and Cr2O3 in the oxides. Based on a surface magnetic-anomaly survey and fault-plane observations, some of these rocks are considered to have been pressed up along faults produced by strike-slip faulting during an early stage of the Forlandsundet Graben formation. The distribution of these rocks indicates that the high-pressure metamorphic rocks extend as much as 50 km to the NNW from Motalafjella to Sarsoyra.

Surface magnetic anomaly study on the eastern part of the Forlandsundet Graben

A surface magnetic survey was carried out by use of a proton magnetometer over wide strandflats along the eastern coast of Forlandsundet, western Spitsbergen, to decipher subsurface structures and lithologies. Distinctive linear high-anomaly segments and zones were recognised on the magnetic anomaly maps. These zones coincide well with the eastern marginal fault of the Tertiary Forlandsundet Graben and associated faults north of St Jonsfjorden, while they reflect bedrock lithologies in the south. The high-anomaly segments, which constitute the zones, are locally aligned in a left-stepping, en echelon arrangement within the zones, indicating a dextral transpressional stress regime on the eastern marginal fault of the graben during a certain time. Sudden termination and bends of the segments define a later transverse fault system.

Surface magnetic anomaly study on the eastern part of the Forlandsundet Graben

A surface magnetic survey was carried out by use of a proton magnetometer over wide strandflats along the eastern coast of Forlandsundet, western Spitsbergen, to decipher subsurface structures and lithologies. Distinctive linear high-anomaly segments and zones were recognised on the magnetic anomaly maps. These zones coincide well with the eastern marginal fault of the Tertiary Forlandsundet Graben and associated faults north of St Jonsfjorden, while they reflect bedrock lithologies in the south. The high-anomaly segments, which constitute the zones, are locally aligned in a left-stepping, en echelon arrangement within the zones, indicating a dextral transpressional stress regime on the eastern marginal fault of the graben during a certain time. Sudden termination and bends of the segments define a later transverse fault system.

Regional tectonic framework, structure and evolution of the western marginal basins of India

The Kutch-Saurashtra, Cambay and Narmada basins are pericontinental rift basins in the western margin of the Indian craton. These basins were formed by rifting along Precambrian tectonic trends. Interplay of three major Precambrian tectonic trends of western India, Dharwar (NNW-SSE), Aravalli-Delhi (NE-SW) and Satpura (ENE-WSW), controlled the tectonic style of the basins. The geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults. The Kutch basin opened up first in the Early Jurassic (rifting was initiated in Late Triassic) along the Delhi trend followed by the Cambay basin in the Early Cretaceous along the Dharwar trend and the Narmada basin in Late Cretaceous time along the Satpura trend. The evolution of the basins took place in four stages. These stages are synchronous with the important events in the evolution of the Indian sub-continent—its breakup from Gondwanaland in the Late Triassic-Early Jurassic, its northward drifting during the Jurassic-Cretaceous and collision with the Asian continent in the Early Tertiary. The most important tectonic events occurred in Late Cretaceous time. The present style of the continental margins of India evolved during Early Tertiary time. The Saurashtra arch, the extension of the Aravalli Range across the western continental shelf, subsided along the eastern margin fault of the Cambay basin during the Early Cretaceous. It formed an extensive depositional platform continuous with the Kutch shelf, for the accumulation of thick deltaic sediments. A part of the Saurashtra arch was uplifted as a horst during the main tectonic phase in the Late Cretaceous. The present high thermal regime of the Cambay-Bombay High region is suggestive of a renewed rifting phase.