Right-stepping Faults Research Articles

Strain partitioning dominated growth of strike-slip fault: Insight from the Tan-Lu fault in Eastern China

The dynamic variations of the strain field and associated tectonic deformation during fault evolution can provide useful insights into the fault growth mechanisms. As one of the longest strike-slip fault system in the world, the ∼3000-km length Tan–Lu fault zone (TLFZ) in eastern China, is ideal for studying the mechanisms of fault propagation and linkage. This study investigated the Cenozoic Liaodong Bay subbasin, which hosts main structures of four faults in the central TLFZ, using structural analysis and strain calculations based on abundant seismic data. Our new results reveal three stages of fault growth (D1, D2, and D3) during the Cenozoic. Strain partitioning occurred during D1, with non-coaxial deformation within the high-strain regions leading to the growth of individual fault segments. D2 involved the redistribution of strain partitioning, with localized strain overlapping along fault tips to produce both left- and right-stepping fault sets. The locally increased simple-shear component favored the development of right-stepping fault arrays. Fault linkage appears to be achieved in two ways: 1) left-stepping, dextral faults were linked by folds dominated by pure shear along overlapping fault tips, and 2) fault segments were linked via fractures developed in simple-shear-dominated right-stepping overlap zones. D3 was characterized by the continued accumulation of simple shear, which resulted in fault linkage through fractures and formed continuous faults with high strain. Therefore, we propose a tectonic model in which strain partitioning results in linkage of segmented faults along fault tips. This model takes into account the spatial and temporal variations in fault geometry and strain type during the growth of the strike-slip fault, provides theoretical support for the structural patterns observed in natural faults worldwide.

The Isle of Wedmore relay ramp: how fault evolution created King Alfred's historic landmark

The Isle of Wedmore covers an area of ~19 km2, rises up to ~65 m above the surrounding lowlands of the Somerset Levels, and was an island until the Middle Ages. The topography is interpreted as having been formed by a relay ramp between two right-stepping faults (the Weare Fault to the west and the Mudgley Fault to the east) which have tens of metres of downthrow to the south, and which are probably normal faults. The relay ramp has a dip of about 3° to the SW and is breached by the NW-striking Wedmore Fault, which has up to ~23 m downthrow to the NE. Several NE-trending faults occur in the relay ramp, which are interpreted as having formed when the relay ramp became a contractional step when the Weare and Mudgley faults underwent sinistral reactivation, or as N–S contraction occurred during the Cenozoic. Analogues for this behaviour are presented from the Liassic rocks on the coast between Lilstock and East Quantoxhead.

3D Paleoseismology from iOS Lidar and Structure from Motion Photogrammetry: a case study on the Dog Valley fault, California

Measuring displacements of strike-slip paleoearthquakes from trenching excavations requires detailed 3D trenching excavations. Here a new methodology utilizing an iOS based laser scanner and structure-from-motion is used to reconstruct stratigraphy and trace a displaced fluvial channel sequence across the Dog Valley fault in Northeastern California. The Dog Valley fault is a left-lateral strike slip fault in the northern Walker Lane. The northern Walker Lane accommodates ~5-7 mm/yr of dextral shear; however, the relative rates of deformation and earthquake history of the fault have not been previously assessed. Here, we present geomorphic mapping observations and preliminary paleoseismic trenching results from the Dog Valley fault. Lidar data reveal a clear east-northeast striking fault trace that extends about ~25 km from the Prosser Creek drainage west of the Polaris Fault near Highway 89 to the northwest flank of Peavine Mountain. The main trace of the fault appears to project through Stampede dam. Youthful fault scarps are visible along much of the fault, with alternating northwest- and southeast-facing scarps. Clear lateral displacements are largely absent along the fault, however right-stepping fault strands, sidehill benches, linear valleys and ridges, and alternating scarp facing directions are all consistent with left-lateral strike slip displacement. Stratigraphic and structural relations exposed in the Dog Valley fault trench show clear truncations and tilting of bedded fluvial and peat deposits and provide evidence for the occurrence of two Holocene earthquakes: the most recent earthquake postdates ~8 ka, and an earlier earthquake is inferred to have occurred between 8491-8345 cal. ybp. Based on 3D excavations of a prominent channel margin, the most recent earthquake was associated with ~ 115 cm of left-lateral displacement, corresponding to an M6.7 earthquake.

Joint Inversion of Rupture across a Fault Stepover during the 8 August 2017 Mw 6.5 Jiuzhaigou, China, Earthquake

Abstract The 8 August 2017 Mw 6.5 Jiuzhaigou earthquake occurred in a tectonically fractured region in southwest China. We investigate the multifault coseismic rupture process by jointly analyzing teleseismic, strong-motion, high-rate Global Positioning System, and Interferometric Synthetic Aperture Radar (InSAR) datasets. We clearly identify two right-stepping fault segments and a compressional stepover based on variations in focal mechanisms constrained by coseismic InSAR deformation data. The average geometric parameters of the northwest and southeast segments are strike = 130°/dip = 57° and strike = 151°/dip = 70°, respectively. The rupture model estimated from a joint inversion of the seismic and geodetic datasets indicates that the rupture initiated on the southeastern segment and jumped to the northwestern segment, resulting in distinctive slip patches on the two segments. A 4-km-long coseismic slip gap was identified around the stepover, consistent with the aftershock locations and mechanisms. The right-stepping segmentation and coseismic rupture across the compressional stepover exhibited by the 2017 Jiuzhaigou earthquake are reminiscent of the multifault rupture pattern during the 1976 Songpan earthquake sequence farther south along the Huya fault system in three successive Ms∼7 events. Although the common features of fault geometry and stepover may control the similarity in event locations and focal mechanisms of the 2017 and 1976 sequences, the significantly wider (~15 km) stepover in the 1976 sequence likely prohibited coseismic rupture jumping and hence reduced seismic hazard.

Trichonis basin, western central Greece: is it an immature basin in the Corinth Rift or a pull-apart in a sinistral rift–trench link?

The 35 km long east–west-trending Trichonis basin in western central Greece is located between the Kephalonia Transform Fault and the east–west-trending Corinth Rift, exhibiting analogous, but smaller scale, morphotectonic features to the Corinth Rift. The final cooling and exhumation of the area started after at c. 10 Ma, after the burial owing to the emplacement of the Pindos thrust ( c. 30–20 Ma). The deformation includes (1) an Early–Middle Miocene transpression (T 1 ) during the waning stage of collision between Apulia and Eurasia, and (2) a Late Miocene–Pliocene radial extension (T 2 ), which relates to the separation of the Hellenic subduction zone from the Apulia (Adria)–Eurasia collision plate boundary through the Kephalonia Transform Fault. Two extensional stress tensors, T 3 and T 4 , with σ 3 in NNE (023°), and NNW (157°) directions, respectively, describe the modern stress regime in the Trichonis basin. This spatial stress partitioning is due to competition between back-arc stresses associated with Hellenic subduction-zone retreat and stresses of the post-collisional collapse of the Hellenic orogen. The Trichonis basin is not a pull-apart basin in a sinistral rift–trench link, but is an immature basin in the Corinth Rift along the Trichonis Fault. The later stands as a right-stepping fault branch to the main faults of the Corinth Rift.

Geologic framework of the Fang Hot Springs area with emphasis on structure, hydrology, and geothermal development, Chiang Mai Province, northern Thailand

Geologic mapping, a magnetotelluric survey, well data, and earlier reports are integrated to guide further development of the Fang geothermal system. The Fang Hot Springs originally flowed ~ 20 l s−1 of 90–99 °C water from a 10-hectare area of crystalline rocks presumed to be of Triassic age. Four wells 92–500 m deep now flow ~ 20 l s−1 of 110–115 °C water and generate 115–250 kWe from the 1989 Ormat binary power plant. Wells are not pumped nor is the spent water re-injected. Temperatures of 130 °C occur in some wells and water chemistry indicates reservoir temperatures of 150 °C. The springs now flow ~ 10 l s−1. The Fang geothermal area is at the west end of the active left-lateral strike-slip Mae Chan fault (MCF). MCF transitions to extensional faulting along the western boundary of the Cenozoic Fang basin. The hot waters emanate from crystalline rocks 0.7 km north of the MCF. Permeable fractures may be tensile fractures at the right-stepping fault tip. The less permeable MCF fault core and Cenozoic sediments of the Fang basin to the SW are not considered to be drilling targets. Unrelated to the fracture system is the Doi Kia detachment fault which places Paleozoic sediments over crystalline rock with a low-angle contact. Electrical resistivity surveys detect low resistivity (< 60 Ωm) only within the upper 50–100 m of the hot springs area. Deeper crystalline rock is > 100 Ωm. Low resistivity is caused mostly by conductive minerals of hydrothermal alteration, and not by the geothermal water of resistivity 5.6 Ωm. No deep resistivity anomaly is detected beneath the seeps or producing wells, although resolution of past surveys would not have imaged narrow zones of alteration. High-resolution resistivity surveys focused on detecting the deeper fracture system are recommended over the hot well area and south over the area underlain by crystalline rocks. Future development should focus on drilling wells (≤ 500 m) with diameters large enough to install submersible pumps to increase flows. Development of several MWe may be possible and should include a designed re-injection well system to sustain pump levels.

Field evidences of secondary surface ruptures occurred during the 20 February 1956 Eskişehir earthquake in the NW Anatolia

Surface rupture and source fault of the 20 February 1956 Eskişehir earthquake have been a matter of debate that potentially contributes towards the understanding of the active deformation and seismic risk in the highly populated NW Anatolia. Field observations on the two fault segments (namely Kavacık and Uludere faults) in the north of the Eskişehir graben revealed evidences of co-seismic surface rupture and mass movements during the Eskişehir earthquake. Surface rupture was observed as a 2.5m wide, a 100m long and ca. 20 cm deep asymmetric depression in the Gümüşlü valley, 2 km east of the Uludere village. A trench dug on this depression confirms a prominent high-angle south dipping fault (dominantly left lateral strike slip) and two faint north-dipping antithetics as clear noticeable shear zones in organic-rich thick soil. Mass failures are particularly observed in spring depressions filled with loose torrent and carbonaceous material in front of the fault scarp. Some rock slides of several tens of meters in size that obviously require significantly high ground shaking were also developed on steep fault scarps. The orientation of the principal stress tensor as deduced from the surface rupture of the Eskişehir earthquake displays clear inconsistency with the geometry of prominent faults in the area. We concluded that this disagreement may be explained by a curved surface rupture. The western and eastern tips of this rupture are EW trending and the probable NW-running part in the middle would correspond to the bounding zone between two right-stepping faults.

P-velocity models and earthquake locations in the Livermore Valley Region, California

P-wave travel times from 223 well-recorded earthquakes and five timed explosions recorded by the US Geological Survey-Lawrence Livermore National Laboratory seismic network are inverted simultaneously for one- and three-dimensional velocity structures and hypocenter locations. The stability of earthquake locations between the one- and three-dimensional inversions demonstrates the adequacy of a layered earth model combined with station corrections for routine hypocenter locations in the Livermore Valley. Initial one-dimensional velocity inversions indicate a fairly strong positive velocity gradient in the upper crust of about 6 km. Three-dimensional models show that the strongest velocity contrasts occur along the eastern and southern edges of the valley where lower velocity down-dropped Tertiary sediments and basin-full alluvium are found in structural contact with higher velocity Late Mesozoic and Tertiary sediments of the Altamont anticline and the Diablo Range. The low-velocity basin structure is truncated along the western margin of the Livermore Valley by the Calaveras fault across which higher velocity aftershocks from the Livermore are exposed. Some of the relocated Cretaceous and Tertiary formations earthquake sequence show a good correlation with surface trace of the northwest-trending Greenville fault. In the southeast corner of the Livermore Valley, a number of short (5 km or less), linear, andmore » discontinuous epicentral zones are observed. These seismic lineations appear to define a conjugate set of right-stepping faults in a right-lateral shear zone. Focal mechanisms for these aftershocks show many complexities, but are generally consistent with this interpretation.« less