Tectonic activity and structural features of active intracontinental normal faults in the Weihe Graben, central China
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Horst and graben
Half-graben
Normal fault
The Minou fault is an active fault system and runs through 25km long from Kurume City to Ukiha Town, North Kyushu, Southwest Japan. The fault is considered as the source fault of Tsukushi great earthquake that occurred at 679A.D. We carried out geophysical surveys such as gravity, controlled source magnetotelluric and γ-ray intensity survey, at the western part of the Minou fault to discuss the subsurface structure around the fault. Based on the gravity survey, the conspicuous graben structure was detected as the low Bouguer anomaly at the north side of the fault. The maximum depth of the graben was estimated at around 2km deep. A steep slope dipping to northward appeared at the south side of the graben and located at 0.5 to 1km north of the active fault line drawn by the topographic discontinuity. The difference of resistivity structure was also detected at the same place. These results show that a buried fault is located at 0.5 to 1km north of the active fault. A marked displacement of the basement was not detected by the gravity data nor the resistivity structure at the active fault. We interpreted that the fault seemed to have been active at the south wall of the graben before the Alluvium. The active place was moved to the south and the new activity has been started at the topographic discontinuity in recent time. These fault displacement suggests that extensional movement has been prevailing in this area during Quaternary.
Half-graben
Horst and graben
Magnetotellurics
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Horst and graben
Half-graben
Normal fault
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Fault scarps, if well-preserved, include precise evidence of past earthquakes. Using cosmogenic 36Cl dating, the timing and slip of paleoearthquakes are recoverable beyond the available earthquake archives. One of the appropriate seismically active regions to apply 36Cl dating is western Anatolia, where its deformation is influenced by an N-S extensional regime, where the horst-graben structures are characterized by normal faults. We have studied well-preserved (meta-) carbonates Kalafat and Yavansu fault scarps in the westernmost part of the Büyük Menderes Graben within western Anatolia. The distribution of cosmogenic 36Cl against height along the fault surfaces indicates that faults experienced minimum three high paleoseismically active phases. The recovered ages of seismic events are ca. 15, 8.4, and 3.6 ka, with vertical components of slip of ca. 0.7, 0.9 and 3.1 m, respectively, for the Kalafat Fault, and ca. 7.9, 3.4, and 2.0 ka with vertical components of slip of ca. 0.6, 3.5, and 2.6 m, respectively, for the Yavansu Fault. The recurrence interval of active periods is generally becoming shortened over time. The ruptures mostly occurred as clustered earthquakes close in time with magnitudes of 6.5–7.1. The vertical slip rates of >0.1, 0.1, and 1.5 mm/yr, and >0.1, 0.8, and 1.9 mm/yr were calculated for the Kalafat and Yavansu faults, respectively. Long-term slip rates were also estimated about 1.0 and 0.6 mm/yr for the Kalafat and Yavansu faults, respectively. Considering the fault lengths, they are capable of producing earthquakes with magnitudes larger than 6.5, and are seismogenic faults.
Horst
Paleoseismology
Half-graben
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A new reverse fault zone,60 km long,was found on the alluvial and pluvial fan of the northern bank of Bayinguole river, Delingha, Qinghai Province. The fault is a sub-extrusion tectonics belongs to the northern margin of NNW-NWW oriented active fault system in Qaidam basin. Its tectonic geomorphology is characterized by clear linear fault scarps, the average vertical slip rate of the fault is about 0.41±0.27 mm/a since late period of the late Pleistocene. About three palaeo-earthquake events are found by trenching along the active fault zone, their ages are 32.7±1.45 ka, 15.54±1.32 ka and 3.2±0.335 ka, respectively.
Pluvial
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In recent times,some moderate-large earthquakes occurred in active folds and thrusts,which seem not directly related with known active faults on the surface and did not form surface ruptures. Although such individual earthquakes might correspond to a known surface active fault,most of them occurred under active folds,formed by displacement of burial thrusts which are located at depth of tens kilometers beneath the folds. Stein named such earthquake asfolding earthquake. It is quite a challenging issue to study and assess the seismic hazards of folding earthquakes occurring in compressive tectonic regions with active folds and burial thrusts. Derived from active folding secondary faults such as flexural-slip faults,bend-moment faults,it is easier to identify that the fold itself. These secondary faults have coseismic slip at the surface and record the active history of seismogenic thrusts which provide an effective way to study the seismic activity of blind thrusts. Many flexural-slip fault scarps are developed on several terrace surfaces at the two limbs of Mingyaole anticline,located along the western margin of the Tarim Basin. These scarps mainly form on the limb of steep beds closest to active axial surfaces(dips of 74° ~ 89°,18° ~ 20° and 45° ~ 60°,separately),within a range of 50 ~ 1 200m from active axial surface,and most are 90 ~ 1 000m wide. Overall,the height of the flexuralslip fault scarps gradually deceases away from the active axial surface. These scarps occur at nearly equidistant or multiple distance spacing on the same terrace surface. The strike of the flexural-slip fault scarp is consistent with the strike of underlying bedrock,which is dominated by interbedded medium-thick layered sandstone or fine-grain sandstone with similar rock mechanical properties. Since the abandonment of terrace T3 at the south limb of the Mingyaole anticline,the shortening rate and uplift rate absorbed by flexural-slip faults are at least(1. 0 ± 0. 2)mm / a,(1. 2 ± 0. 1)mm / a, respectively. Movement of the flexural-slip faults is characterized by repeatability and neo-activity.
Anticline
Escarpment
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The linkage between the development of south‐facing Cretan graben and large‐scale detachment faulting in the southern Aegean is unknown. Widespread Serravallian deposits in the Ierapetra graben of Crete supply constraints to Middle Miocene graben development in the southern Aegean. The Ierapetra graben, and by inference the Cretan graben in general, were hitherto believed to have formed as a result of sinistral transpression during N–S shortening. We argue that the formation of the Cretan graben is due to N–S extension. The south‐dipping, N–S‐extending Kritsa normal fault served as the master fault controlling graben development in the Ierapetra graben. The Kritsa normal fault is either an antithetic fault related to the top‐N Cretan detachment or a synthetic fault associated with the top‐S Ios detachment. The overall geometry and timing relationships lead us to favour a connection with the Ios detachment, which in turn implies a large‐scale displacement on the Ios detachment.
Horst and graben
Detachment fault
Normal fault
Transpression
Half-graben
Echelon formation
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Horst and graben
Paleoseismology
Half-graben
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Thrust fault
Landform
Fault plane
Paleoseismology
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Reishui-Riyue Mt. active fault zone is a very important NNW oriented rightlateral strikeslip active fault zone which developed in the Chaidam-Qilian Mt. active crustal block,northeastern margin of Qinghai-Tibet Plateau.The activity of the fault causes a series of microgeomorphologies,such as ridge, valley and terrace,rightlateral offseted and forms fault cliff and fault scarp et al. vertical offset microgeomorphology.In this paper,the palaeoearthquake events on the fault are studied.Combining the trench profiles and fault scarp measurement,two palasoearthquake events are determined,which occured in 6 280±120 a B.P. and 2 220±360 a B.P.,the recurrence interval is about 4 000 a.
Terrace (agriculture)
Escarpment
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