South Fault Research Articles

Geomorphology, Three-Dimensional Geology, and Seismologic Hazards of the New Madrid Seismic Zone in Dyer County, Tennessee

ABSTRACTGeomorphic and three-dimensional geologic mapping reveals two major fault systems of the seismically active Reelfoot rift pass beneath Dyer County in northwestern Tennessee, the Reelfoot South fault, and the east-bounding faults of the Reelfoot rift. The Dyer County mapping also indicates that the two principal Reelfoot South fault hanging wall structures, the Lake County uplift and Tiptonville dome, pass beneath the county. Quaternary displacement was identified on southeastern Reelfoot rift margin faults in Dyer County, thus indicating that this rift margin has been active during the Quaternary from adjacent Obion County through Dyer County to Lauderdale County, Tennessee, for a distance of at least 60 km. The three-dimensional geologic mapping also provides stratigraphic thicknesses of surface sediment and underlying Paleogene and Cretaceous strata that significantly contribute to the estimation of ground motion in the event of a future large New Madrid seismic zone earthquake. The new ground motion maps using the three-dimensional geology of Dyer County are compared to the current U.S. Geological Survey earthquake hazard maps. This comparison reveals generally lower acceleration for buildings less than four stories high and greater acceleration for buildings greater than 10 stories high in the event of a large New Madrid seismic zone earthquake.

The Chemical Characteristics and Metallogenic Mechanism of Beryl from Cuonadong Sn-W-Be Rare Polymetallic Deposit in Southern Tibet, China

The Cuonadong deposit is the first large scale Sn-W-Be rare polymetallic deposit located in southern Tibet, China, where beryl is the main beryllium-bearing mineral. In this paper, the beryl crystals in the pegmatitic and hydrothermal vein orebody from the Xianglin area of the Cuonadong deposit are the research objects, marked as Beryl-I and Beryl-II, and they are investigated by EPMA, LA-ICP-MS and in situ micro-X-ray diffraction (XRD). Data by EPMA and LA-ICP-MS reveal that beryls from this area are alkaline beryls, among which Beryl-I is composed of Li-Cs beryl, and Beryl-II is composed of Na beryl and Na-Li beryl, indicating that beryls have undergone noticeable alkali metasomatism during formation. The Cs/Na ratio in Beryl-I ranges from 0.10 to 0.44, and the Mg/Fe ratio is almost 0, showing that Beryl-I is formed under high-differentiation evolution conditions and is rarely affected by hydrothermal transformation, whereas the Mg/Fe ratio in Beryl-II ranges from 2.73 to 17.31, and the Cs/Na ratio is nearly 0, indicating that Beryl-II has been obviously affected by late hydrothermal metasomatism. In situ XRD analysis shows that both Beryl-I and Beryl-II are t-beryl, and the c/a ratio of Beryl-I (1.0010–1.0012) is slightly higher than that of Beryl-II (1.0005–1.0008), which may also reflect the transition from magmatism to hydrothermal metasomatism in the late stage of pegmatitic magmatism. Based on comprehensive analysis, we believe that the precipitation of Beryl-I is mainly caused by the emplacement of highly fractionated magma containing Be to the top of the rock mass or surrounding rock, the melt-fluid undercooling, and the crystallization of volatile-bearing minerals (such as tourmaline and fluorite). Moreover, the Be-bearing ore-forming fluid has further migrated upward along the near north–south faults formed in the middle Miocene (16–15 Ma), during which Beryl-II precipitates owing to the hydrothermal water mixing, the ore-forming fluid cooling, and large amounts of crystallization of volatile-bearing minerals (mainly fluorite). Therefore, it can be concluded that beryl mineralization largely reflects the process of magmatic–hydrothermal mineralization. Because of a large number of mineralized areas with the similar metallogenic backgrounds to the Cuonadong deposit in the Himalayan region, it has great potential to be a new globally significant rare metal metallogenic belt.

New Insights into the Distribution and Evolution of WNW-Directed Faults in the Liaodong Bay Subbasin of the Bohai Bay Basin, Eastern China

WNW-directed faults are widespread in eastern China, but debates regarding their distributions and evolutionary processes remain unsettled. Based on the latest 3-D seismic data, a series of WNW-directed faults south of the Liaodong Bay subbasin was identified, for which the evolution and formation mechanisms were discussed. The results show that four WNW-directed faults are characterized by poor continuity and nearly parallel orientations. Vertically, they exhibit listric geometries and cut through Paleozoic and Mesozoic formations. Since the late Triassic, these faults began as reverse faults under nearly S-N horizontal compression. In the Jurassic, those faults maintained their reverse-faulting activities with dramatically decreased intensities. In the Early Cretaceous, the WNW-directed faults were changed into normal faults under regional extension and were influenced by the sinistral strike-slip movement along the Tan-Lu fault zone. In the Late Cretaceous, the WNW-directed normal faults probably stopped moving due to a regional compressional event. During the Paleogene, the WNW-directed faults were reactivated with decreased intensities and were cut by NNE-directed faults. Here, we emphasize that the evolution of the WNW-directed faults could shed light on the regional tectonics. The WNW-trending faults that developed in the Liaodong Bay subbasin are closely related to the faults in the Yanshan orogenic belt. Therefore, investigating the characteristics and origin of WNW-induced faults will provide evidence for the tectonic evolution of the North China Block. In addition, the development of WNW-directed faults in the southern Liaodong Bay subbasin was conducive to the formation of buried Mesozoic and Paleozoic hills and hydrocarbon accumulations. In addition, we suggest that the compressional segment of the conjugated strike-slip transition zone that was formed by the interaction of the WNW- and NNE-directed strike-slip faults was conducive to hydrocarbon accumulations.

Aeromagnetic-Imaged Basement Fault Structure of the Eastern Tarim Basin and Its Tectonic Implication

The property of the magnetic basement and the faults in the basement is significant for structural evolution, the Phanerozoic deposition, and oil resource exploration of the Tarim Basin. Based on the newly acquired aeromagnetic and industry seismic data, we mapped the distribution of basement faults by applying magnetic gradient-processing methods such as the horizontal gradient derivative, the first vertical derivative, the tilt derivative, and the upward continuation method. The dips of basement faults were confirmed and the susceptibilities of basement blocks were obtained by forward modeling of five profiles using the constraint of sedimentary strata depth and Moho topography. On the basis of comprehensive analysis of the magnetic anomalies, the distribution and inclination of basement faults, and susceptibilities differentiation obtained by forward modeling and field measurement, the property of the basement faults and their implication were discussed and interpreted. Our results show that the origin of the Central Highly Magnetic Anomaly Belt is highly magnetic Archean metamorphic rocks. The weakly magnetic Southeastern Domain and highly magnetic Central Tadong Domain assembled along the Tadong South Fault during the Paleoproterozoic. The Paleozoic Cherchen Fault is just an interior fault in the weakly magnetic Southeastern Domain although it presents a large vertical fault displacement. Considering the prominent variation of strikes of the Tadong North Fault system, and the moderately magnetic anomalies in the Northeastern Mangal Domain corresponding to the center of Neoproterozoic deposition, it is likely that the basement of the Northeastern Mangal Domain modified by the Neoproterozoic rifting could be originally the same as the basement of Central Highly Magnetic Anomaly Belt.

Feasibility Study for Well pattern Encryption in Complex Fractured Block Reservoirs--Taking the Liu 90 South Fault Block in the Jidong Oilfield as an example

Jidong oilfield belongs to complex fault block reservoir, and the development has now entered into high water-cut stage. when using Well pattern encryption technology to stabilize production, it is necessary to study the feasibility of Well pattern encryption to ensure a better economic benefit. In this paper, we use theoretical analysis method, consider the influence of oil price on the calculation, improve the economic limit Well pattern density calculation formula, calculate the economic limit Well pattern density under different recovery rates, make recovery rate prediction for different Well pattern density scenarios by combining with Sherkachov's formula, and use the curve intersection method to determine the Well pattern density at the break-even point, so as to determine the economic feasibility of Well pattern encryption. The example calculations show that the use of well network encryption alone does not meet the development requirements of the Liu 90 South block and has no significant economic benefits, so a combination of two-three methods is needed to further improve the recovery rate. Numerical simulation prediction results show that the two-three combination method can improve the recovery rate by 14.54%, accumulate 28.88×104t of oil increase and increase revenue by 678,245,000 yuan.

Stress Transfer at the Northeastern End of the Bayan Har Block and Its Implications for Seismic Hazards: Insights From Numerical Simulations

AbstractThe 2021Mw7.4 Maduo earthquake in Qinghai, China, filled the seismic gap in the eastern segment of the northern boundary of the Bayan Har Block, and four otherM > 7 historical strong earthquakes occurred at the northeastern end of the Bayan Har Block within a period of 80 years before the Maduo earthquake. This series of large seismic events provides an excellent example for investigating the stress transfer and fault interactions of strong earthquakes on the continent. In this study, we established a viscoelastic numerical model to calculate the stress transfer before and after the 2021 Maduo earthquake produced by historical earthquakes at the northeastern end of the Bayan Har Block. We found that the Maduo earthquake occurred in the Coulomb stress shadow zone caused by the historical earthquakes, and the 1947 Dari earthquake and the 1937 Tuosuo Lake earthquake jointly delayed the occurrence of the Maduo earthquake. The stress transfer of fiveM > 7 earthquakes to the surrounding faults after 1937 resulted in the segments at both ends of the Dari fault that were unruptured in the 1947 earthquake, the East Kunlun fault zone west of Alan Lake, the Maqin‐Maqu segment of the East Kunlun fault zone, the Longriba fault, the segment of the Ganzi‐Yushu fault south of Yushu, the Minjiang fault, and the northern segment of the Xianshuihe fault being located in the Coulomb stress loading zone. Sustained attention should be paid to the block boundary faults and intra‐block faults with increased seismic hazards.

Three‐Dimensional Electrical Resistivity Characterization of Mountain Pass, California and Surrounding Region

AbstractThe Sulphide Queen carbonatite deposit at Mountain Pass in southeast California is a world class rare earth element (REE) resource. This study images electrical resistivity structure of the REE deposit and surrounding area to characterize resources under cover. An east‐west elongated grid (35 × 15 km) of 65 wideband magnetotelluric stations spanning from eastern Shadow Valley to eastern Ivanpah Valley were collected and modeled in three‐dimensions (3‐D). Gravity, aeromagnetic, and geologic data are used to inform interpretation of structures in the resistivity model, including the following observations. Shadow Valley is filled with conductive sediment that locally dips southward to a depth of 1 km. The Kingston Range‐Halloran Hills detachment fault dips westward at ∼15 degrees. The REE deposit is a moderate low resistivity zone dipping southwest to a possible depth of ∼1 km, and is bounded by the North and South faults and bisected by the Middle fault. Ivanpah Dry Lake is underlain by a north striking southward dipping sedimentary basin. Two possible zones of mineralization are observed in Ivanpah Valley, one along the western edge of Ivanpah Dry Lake and one on the western edge of valley along a new inferred fault. The brittle‐ductile transition is imaged at ∼10 km below mean sea level. No deep electrically conductive structures are imaged to be related to the REE deposit likely due to the complex geologic history of the Mojave terrane. Future studies should regional target Proterozoic rocks and search within for geophysical signatures similar to Mountain Pass.

Spatial Slip Rate Distribution Along the SE Xianshuihe Fault, Eastern Tibet, and Earthquake Hazard Assessment

AbstractThe Xianshuihe (XSH) fault in eastern Tibet is one of the most active faults in China, with the next large earthquake most likely to occur along its SE part, where the fault splits into three parallel branches: Yalahe, Selaha and Zheduotang (ZDT). Precisely quantifying their slip rates at various timescales is essential to evaluate regional earthquake hazard. Here, we expand our previous work on the Selaha fault to the nearby ZDT and Moxi (MX) faults, and add observations on the Yalahe fault and on the newly discovered Mugecuo South fault zone. Using tectonic‐geomorphology approaches with10Be dating, we had previously determined average late Quaternary slip rates of 9.75 ± 0.15 and 4.4 ± 0.5 mm/yr along the NW and SE Selaha fault, respectively. Using the same methods here, we determine a slip rate of 3.4–4.8 mm/yr on the ZDT fault and of 9.6–13.4 mm/yr on the MX fault. This is consistent with the southeastward slip rate increase we had proposed along the XSH fault system from 6‐8 mm/yr (Ganzi fault) to ∼10 mm/yr (Selaha fault), and >9.6 mm/yr (MX fault). We propose a new model for the SE XSH fault, where the large‐scale Mugecuo pull‐apart basin lies within an even larger scale compressive uplift zone in a restraining bend of the XSH fault, where the highest peak in eastern Tibet is located (Gongga Shan, 7,556 m). Our slip rate determination helps to constrain a relatively high regional Mw ∼ 7 earthquake hazard at present on the SE XSH fault.

A case study of complex carbonate reservoir connectivity analysis, Tarim Basin, China

The connectivity of complex carbonate reservoirs has an essential impact on the exploration and development of these reservoirs. From geologic genesis, the connectivity of complex carbonate reservoirs is mainly controlled by faults and dissolution. Therefore, accurate identification of faults and karst caves is the key to studying reservoir connectivity. The Ordovician carbonate reservoir in the Hudson Oilfield of the Tarim Basin is used for our reservoir connectivity analysis study. First, we calculate the coherence and curvature attributes, respectively, and then merge the two attributes using a neural network algorithm. Finally, we use the ant-tracking method to track the faults for the merged data. The results show that the approach substantially enhances deterministic faults that can be seen directly on the seismic data, and subtle faults can also be identified. For reservoir identification, we use the diffraction imaging method to describe the karst reservoir in this study area. The results show that diffraction imaging can identify small-scale caves that cannot be well recognized on the seismic reflection data. Furthermore, the caves connected on the diffraction seismic data are isolated from each other on the seismic reflection data, making the connection between caves clearer. Based on the results of the fault and cave identification, we analyze the reservoir connectivity of the study area using the oil pressure and daily production data, which indicates that the north–northwest and near-north–south faults probably play a role in the connection of the reservoirs, whereas the northeast–east faults tend to block the connection of the reservoirs.

Crustal architecture and structural evolution of a Neoarchean sedimentary basin: geological and geophysical evidence from Metal Earth Chicobi transect in the Abitibi Subprovince, Superior Province, Quebec, Canada

Our understanding of crustal architecture and tectonic evolution has advanced due to integrated geological and geophysical research on Precambrian cratons. However, it is still a heated debate whether Archean crust structure developed by an allochthonous model similar to modern plate tectonics or autochthonous model emphasizing in-situ evolution of supracrustal belts. The < ca. 2696 Ma Chicobi belt is a sedimentary basin in the central Abitibi Subprovince of the Superior Province, and as such provides a unique opportunity to contribute to this debate. It comprises both Porcupine-type turbidite and Timiskaming-type polymictic conglomerate and cross-bedded sandstone, and unconformably overlies ca. 2728–2711 Ma metavolcanic rocks, including the Amos Group to the south. The metasedimentary rocks were deposited after an early (D1 > ca. 2696 Ma) deformation event, which is expressed by internally foliated clasts in conglomerate. The Chicobi belt was regionally folded and cut by shear zones during a second (D2) deformation event, and was re-folded by Z-shaped folds during a third (D3) dextral shearing event. The D2 event is constrained between ca. 2696 Ma, the pre-existing age of detrital zircon in the Scapa Group along strike of the Chicobi belt, and ca. 2681 Ma, the new age of the cross-cutting syenitic Gémini-Saint-Éloi Pluton.New high-resolution seismic reflection and magnetotelluric surveys were conducted along the same transect across the Chicobi belt. The seismic reflection survey imaged subhorizontal to shallowly north-dipping reflective bands at depths of 3–7 km. Regional-scale inversion of new and pre-existing gravity data indicates that these reflective bands are linked to the Beauchamp Fault south of the Amos Group. Magnetotelluric inversion model shows high resistivity (>1000 Ω·m) zones under metasedimentary rocks, as well as a synform defined by low resistivity (10–100 Ω·m) zones, which aligns with folded metavolcanic rocks south of the Chicobi belt.Integration of the geophysical data with surface mapping suggests that the upper-middle crustal architecture of the Chicobi belt is dominated by D2 upright folds overlying a fault zone. We suggest that deformation of the Chicobi belt is dominated by thrusting and folding, a deformation style compatible with allochthonous growth of the Abitibi Subprovince.

Slumping as a record of regional tectonics and palaeoslope changes in the Satpura Basin, central India

Abstract Soft-sediment deformation structures play an important role in interpreting regional tectonics and basin evolution during slumping events. The Satpura Basin is interpreted as pull-apart with a monoclinal northerly palaeoslope throughout its evolution. The basin formed as a result of sinistral strike-slip faulting, induced by the ENE–WSW-trending Son-Narmada South fault in the north and the Tapti North fault in the south. We have analysed the slump folds within the basalmost Talchir Formation and related these to regional tectonics and palaeoslope changes in the Satpura Basin. The glaciofluvial strata of the Talchir Formation, exposed in the southern part of the Satpura Basin, record intricacies of folds created during slumping. Several fold styles can be distinguished, within alternations of competent sandstone and incompetent shale layers, some of which indicate buckling. Upright folds, resulting from pure shear, underwent rotation of their axial planes and fold axes during simple shear-dominated progressive deformation when the slump moved downslope. The soft-sediment deformation structures that we have studied show refolding patterns that closely resemble comparable folds known from lithified rocks. These layers with refolded structures are overlain by unde-formed sediments, which proves that they are the product of a single ongoing slumping process, rather than of successive deformation events. Our analysis of their fold axes and axial planes, together with fold vergences and thrust directions within the slumps, suggests a mean slumping direction towards the southwest. Analyses of slump folds and their relationship with regional tectonics have allowed us to reinterpret basin evolution history. The southwesterly trending palaeoslope of the basin suggest that the slope of the basin was not uniform throughout its evolution. At the opening, the oblique slip fault, which trended NE–SW, generated due to movement along the ENE–WSW basin bounding faults, was more active and triggered slumping event within the Talchir deposits in the basin. With progressive overlapping of the basin-bounding faults, the Satpura Basin gradually tilted towards the north.

Seismic Hazard Zonation in Gedebage Future Development in Bandung City Using HVSR Inversion

Greater Bandung, the largest economic growth corridor in Indonesia after Jabodetabek, is sitting on the Bandung basin. This deep sedimentary basin is situated just 12 km south of the active Lembang fault. Gedebage is an area intended to be the new economic and business center located in the easternmost of Bandung. The research aims to identify the vulnerability of Gedebage area against seismic ground motions. The area's morphology is dominated by a flat area with 0 - 8% of slope and predominantly composed of an old lake deposit. Topography, including basement morphology, sediment thickness, and physical properties, plays a great role in escalating/de-escalating seismic ground motions. A specific morphology may trap and prolong seismic shaking. Furthermore, stiffness and bedrock depth are instrumental in passing the spectral ground motions to the surface. The HVSR inversion method is applied to map subsurface conditions that successfully applied in Palu and its surrounding area. The research shows that Gedebage areas are vulnerable to the seismic hazard, referring to the shear wave velocity (Vs30) distribution and seismic hazard micro zonation maps. The discussion of the research findings is useful for future infrastructure development in the research area. The area is categorized as soft soil and medium soil classes, and it has a high vulnerability for destruction if there is an earthquake. The area should be cleared from vital infrastructures such as government buildings, schools, or hospitals.

Reassessing Eastern Mediterranean Tectonics and Earthquake Hazard From the 365 CE Earthquake

AbstractThe hallmark of great earthquakes in the Mediterranean is the 21 July 365 CE earthquake and tsunami that destroyed cities and killed thousands of people throughout the Eastern Mediterranean. This event is intriguing because most Mediterranean subduction forearcs exhibit pervasive crustal extension and minimal definitive evidence exists for great subduction megathrust earthquakes, consistent with weak seismic coupling. This conundrum has led many to favor rupture of a previously unrecognized upper plate splay fault south of Crete in an M w 8.3–8.5 earthquake, uplifting a Cretan Holocene paleoshoreline by up to 9 m. Similar source mechanisms have been adapted for the region, which are commonly used for seismic and tsunami hazard estimation. We present an alternative model for Holocene paleoshoreline uplift and the 365 CE tsunami that centers on known active normal fault systems offshore of western and southwestern Crete. We use new and published radiocarbon dates and historical records to show that uplift of the Cretan paleoshoreline likely occurred during two or more earthquakes within 2–3 centuries. Visco‐elastic dislocation modeling demonstrates that the rupture of these normal faults fits observed data as well as reverse fault models but requires reduced slip and lower cumulative earthquake energy release (∼M w 7.9). Tsunami modeling shows that normal‐fault ruptures produce strong tsunamis that better match historical reports than a hypothetical reverse fault. Our findings collectively favor the interpretation that damaging earthquakes and tsunamis in the Eastern Mediterranean can originate on normal faults, highlighting the potential hazard from tsunamigenic upper plate normal fault earthquakes.

Origin of synchronous extension and inversion in a rift basin: The Phitsanulok Basin, central Thailand

Abstract The Phitsanulok Basin is a Cenozoic rift basin in central Thailand. The basin opened in the Oligocene to Early Miocene with the development of N-S and NNW-SSE trending normal faults. In the Middle Miocene the northern part of the basin continued to experience extension, while the southern part was affected by compressive inversion. Extension resumed in the Late Miocene to Pliocene, followed by post-rift subsidence in the Quaternary. The Middle Miocene anomalous deformation is attributed to activities of the NE-SW Uttaradit Fault and NW-SE Mae Ping Fault, respectively at the northern and southern basin margins. Both faults had limited activities during the Oligocene to Early Miocene rifting, where E-W extension was probably accompanied by vertical maximum principal stress. In the Middle Miocene, the maximum principal stress rotated to a N-S direction and produced sinistral motion on the Uttaradit Fault and dextral motion on the Mae Ping Fault. The strike-slip motions contributed to a counter-clockwise rotation of the continental block west of the Phitsanulok Basin, creating the synchronous extension and contraction. The kinematic model introduces the possibility of local inversion in a rift basin being originated from short-term switch between the maximum and intermediate principal stress axes and reactivation of long-lived strike-slip faults, as opposed to major change in regional stress related to plate movements. The model implies that the segment of the Mae Ping Fault south of the Phitsanulok Basin was inactive during the Oligocene to Early Miocene and was reactivated as a dextral fault in the Middle Miocene.

Geochemical characteristics, origin, and mechanism of differential accumulation of natural gas in the carboniferous kelameili gas field in Junggar basin, China

Since the discovery of the Kelameili gas field, the exploration of volcanic gas reservoirs in the Junggar Basin has attracted considerable attention. However, owing to the limitations involved in studying the characteristics of the origin and distribution of the volcanic gas reservoirs in this area, the study of gas reservoirs in the South fault terrace area of the Kelameili gas field as a typical peripheral area is greatly restricted, and the exploration process is relatively slow. Therefore, it is necessary to analyse and compare the differences in the geochemical characteristics of the natural gas between the Kelameili gas field and the fault terrace area of the south belt by analysing the natural gas geochemistry, hydrocarbon generation evolution, and structural evolution. In addition, geochemical experiments involving the natural gas components, carbon isotopes, light hydrocarbons, hydrocarbon generation, and evolutionary history of the source rocks and the interpretation of seismic profiles reflecting the structural evolution, help clarifying the genetic type of the volcanic gas reservoirs and the source of the natural gas in this area. The results indicate the following. (1) The natural gas in the study area has undergone thermal genesis and is generated by kerogen cracking. It is a typical humic gas with high maturity. (2) The origin of the natural gas is the Carboniferous source rock in the depression, which is significantly different from the natural gas from the typical Permian and Jurassic. (3) The primary reason for the difference in natural gas accumulation is that the Carboniferous source rock entered the high-maturity stage in the Early Jurassic. The formation time of the trap is earlier than that of the high-maturity stage of the source rock evolution, which can capture multi-stage source rock products. (4) Based on the hydrocarbon source conditions that the hydrocarbon supply horizon of the Kelameili gas field has a high maturity of organic matter, and insufficient hydrocarbon supply, it is considered that traps capturing the multi-stage source rock products represent a larger scale of natural gas accumulation. The results of this study can deepen the understanding of volcanic gas reservoirs in the Junggar Basin and guidethe exploration and development of global volcanic gas reservoirs and the surrounding large-scale gas fields.

Spatial variations of coda-wave attenuation in the Khurgu region, south-east of the Zagros from analysis of local earthquakes

In this study, we estimate the characteristics of coda-wave attenuation in the Khurgu region, southeast of Zagros, with the use of single-back scattering procedure. The selected database comprises of 4870 seismograms of 259 local earthquakes recorded from March to June 2006, at 25 seismic stations within 2–100 km of epicentral distance, depth range 4–25 km, and 2–4.7 local magnitude range. oda attenuation analyzed on narrow bandpass-filtered seismograms from the time-domain decay of coda-wave amplitudes. The intended frequency-band ranged from 0.5 to 24.0 Hz.Moreover, coda Q values have been estimated by using different window length, from 5 to 50 s. The coda Q value shows frequency decency in the form of Qof n.The mean-frequency relation for the above region has been found to be Qc = 29 ± 1.0f 1.13±0.007 for the 5-sec lapse-time window length as well as Qc = 147 ± 1.4f 0.99 ±0.045 for the 50 s lapse time window length.The Qc value has been estimated at 25 stations and its lateral variation has been analyzed in this study. The distinct Coda Q values were observed at separate stations between the southern and northern regions. Our observations showed major differences in the coda depth. In addition, Coda Q values in the northern part are lower than southern part at all depths.Beneath south and north of High Zagros Fault (HZF), the increase rate of Qc has been higher at the depth 70 and 80 km, respectively. This may indicate the presence of the increased attenuation anomaly beneath the area lithosphere.

Sedimentology and provenance of the Lower Old Red Sandstone Grampian outliers: implications for Caledonian orogenic basin development and the northward extension of the Midland Valley Basin

Reconstructing regional geological histories is challenging where basins have limited and/or fragmentary preservation. Several isolated outlier basins of Lower Old Red Sandstone (LORS) occur on the Scottish Grampian terrane; however, their sedimentology and relationship to other similar-aged LORS deposits has been poorly constrained, as has their significance in the wider Caledonian orogenic framework. Here we present a combination of new sedimentological and multidisciplinary provenance analyses for the LORS outliers at Aberdeen, Cabrach, New Aberdour, Rhynie and Tomintoul. Three facies associations are identified, the deposits of locally derived conglomeratic alluvial fans, which pass upward into fluvial channel and floodplain facies associations. Provenance data from conglomerate clast populations, sandstone petrography, heavy mineral assemblages, and U–Pb detrital zircon geochronology indicate derivation from the surrounding Dalradian Supergroup, with local influence from contemporaneous volcanic or plutonic lithologies. These data suggest a similar provenance to that of the northern Midland Valley Basin LORS, and a direct relationship between the LORS north and south of the Highland Boundary Fault. This indicates that the preserved outliers represent fragments of wider, unpreserved LORS cover that accumulated between the late phases of the Caledonian Orogeny and onset of post-orogenic collapse in the Mid-Devonian.

Present‐Day Kinematics in Southeastern Tibet Inferred From GPS Measurements

AbstractQuantifying the regional kinematics is critical for understanding the deformation mechanisms of the Tibetan Plateau. In this study, we construct an elastic block model using a dense GPS velocity field to derive the fault slip rates in SE Tibet. We uncover good agreement between GPS and late Quaternary slip estimates, suggesting that the GPS slip rates estimated from our model can represent the long‐term fault motions in SE Tibet. Specifically, our model shows that the slip along the Xianshuihe fault increases from west to east, which may result from the initiation of the Longriba fault. The Litang, Zhongdian, and Xiaojiang faults and the faults bounding the Lijiang block exhibit extensional motions, in agreement with the regional tectonics and earthquake focal mechanisms. In the regions north of the Red River fault whose kinematics can be described through the rotations of subblocks with small residuals, the fault motions agree with focal mechanism patterns and tectonic block models. In regions south of the Red River fault, the right‐lateral shearing among the Red River, Lancang and Sagaing faults appears to explain the first‐order deformation features. However, the relatively large residuals and complex tectonic setting suggest that various other factors, such as asthenospheric flow and the subduction of the Burma plate may also influence the deformation. Thus, a single model extended to interpret all the geodynamic features in SE Tibet, especially south of the Red River fault, may be overly simple, and the extent to which each factor contributes to this deformation requires further study.

Seismicity in the upper plate of the Northern Chilean offshore forearc: Evidence of splay fault south of the Mejillones Peninsula

The region around the Mejillones Peninsula is characterized by major onshore and offshore fault systems. The existence of major splay faults in these fault systems has been debated. To improve our undestanding of seismotectonic processes affecting the MP, we built a new catalog of aftershocks of the 1995 Antofagasta earthquake (Mw 8.1) recorded by the CINCA seismic network. In addition, we build a three-dimensional tomographic model to determine the physical properties of the upper plate using arrival times from our new catalog combined with other earthquakes catalogs derived from several networks deployments across the MP. We detected seismicity in the outer wedge of the upper plate. The seismicity is partially defined by a clear lineament between 22 and 5 km depth that we defined as a splay fault. The focal mechanisms indicate a rupture plane with NE orientation and SE dip. The seismicity in the upper plate and splay fault are located in the areas in which the Vp and Vs velocities are decreased and the Vp/Vs ratio is increased, suggesting a fractured fault zone within the outer wedge of the upper plate. We propose the presence of fluids in the fracture fault zone within the outer wedge because the decrease in Vs larger than that of Vp and repeating earthquakes clusters were detected. Our observations present clear evidence of the activation of the splay fault in the outer wedge of the North Chilean offshore forearc.

Space Imaging Geodesy Reveals Near Circular, Coseismic Block Rotation During the 2016 Mw 7.8 Kaikōura Earthquake, New Zealand

AbstractLarge earthquakes usually rupture plate boundary faults, releasing the accumulated stress as displacements localized along smooth, narrow faults. However, certain earthquakes initiate off main faults, rupturing adjacent, secondary faults. The mechanisms of such atypical stress release remain enigmatic, partly due to a lack of detailed geodetic evidence. Here using the 3D coseismic displacement field derived from space imaging geodesy, we detect 10‐km‐scale, nearly‐circular coseismic block rotation during the 2016 Mw 7.8 Kaikōura earthquake in New Zealand. Together, geodetic observations, longer term local paleomagnetic data, analytical, and discrete element modeling imply that localized block rotation occurred south of the Hope fault along weak, steep, bedding‐parallel boundaries within a narrow, ~20‐km‐wide dextral shear zone. That stress near plate boundary faults can be partially released in zones of distributed ruptures absorbing coseismic rotation may retard rupture along main faults. Our observations also suggest that coseismic rotation may help accomodate plate boundary propagation.