Analysis of faulting destruction and water supply pipeline damage from the first mainshock of the February 6,2023 Türkiye earthquake doublet

In 2023,two consecutive earthquakes exceeding a magnitude of 7 occurred in Türkiye,causing severe casualties and economic losses.The damage to critical urban infrastructure and building structures,including highways,railroads,and water supply pipelines,was particularly severe in areas where these structures intersected the seismogenic fault.Critical infrastructure projects that traverse active faults are susceptible to the influence of fault movement,pulse velocity,and ground motions.In this study,we used a unique approach to analyze the acceleration records obtained from the seismic station array(9 strong ground motion stations)located along the East Anatolian Fault(the seismogenic fault of the MW7.8 mainshock of the 2023 Türkiye earthquake doublet).The acceleration records were filtered and integrated to obtain the velocity and displacement time histories.We used the results of an on-site investigation,jointly conducted by China Earthquake Administration and Türkiye’s AFAD,to analyze the distribution of PGA,PGV,and PGD recorded by the strong motion array of the East Anatolian Fault.We found that the maximum horizontal PGA in this earthquake was 3.0 g,and the maximum co-seismic surface displacement caused by the East Anatolian Fault rupture was 6.50 m.As the fault rupture propagated southwest,the velocity pulse caused by the directional effect of the rupture increased gradually,with the maximum PGA reaching 162.3 cm/s.We also discussed the seismic safety of critical infrastructure projects traversing active faults,using two case studies of water supply pipelines in Türkiye that were damaged by earthquakes.We used a three-dimensional finite element model of the PE(polyethylene)water pipeline at the Islahiye State Hospital and fault displacement observations obtained through on-site investigation to analyze pipeline failure mechanisms.We further investigated the effect of the fault-crossing angle on seismic safety of a pipeline,based on our analysis and the failure performance of the large-diameter Thames Water pipeline during the 1999 Kocaeli earthquake.The seismic method of buried pipelines crossing the fault was summarized.

Active Faulting Pattern,Present-day Tectonic Stress Field and Block Kinematics in the East Tibetan Plateau

This paper examines major active faults and the present-day tectonic stress field in the East Tibetan Plateau by integrating available data from published literature and proposes a block kinematics model of the region. It shows that the East Tibetan Plateau is dominated by strike-slip and reverse faulting stress regimes and that the maximum horizontal stress is roughly consistent with the contemporary velocity field, except for the west Qinling range where it parallels the striking of the major strike-slip faults. Active tectonics in the East Tibetan Plateau is characterized by three faulting systems. The left-slip Kunlun-Qinling faulting system combines the east Kunlun fault zone, sinistral oblique reverse faults along the Minshan range and two major NEE-striking faults cutting the west Qinling range, which accommodates eastward motion, at 10--14 mm/a, of the Chuan-Qing block. The left-slip Xianshuihe faulting system accommodated clockwise rotation of the Chuan-Dian block. The Longmenshan thrust faulting system forms the eastern margin of the East Tibetan Plateau and has been propagated to the SW of the Sichuan basin. Crustal shortening across the Longmenshan range seems low （2-4 mm/a） and absorbed only a small part of the eastward motion of the Chuan-Qing block. Most of this eastward motion has been transmitted to South China, which is moving SEE-ward at 7-9 mm/a. It is suggested from geophysical data interpretation that the crust and lithosphere of the East Tibetan Plateau is considerably thickened and theologically layered. The upper crust seems to be decoupled from the lower crust through a decollement zone at a depth of 15-20 kin, which involved the Longmenshan fault belt and propagated eastward to the SW of the Sichuan basin. The Wenchuan earthquake was just formed at the bifurcated point of this decollement system. A rheological boundary should exist beneath the Longmenshan fault belt where the lower crust of the East Tibetan Plateau and the lithospheric mantle of the Yangze block are juxtaposed.

Assessing current faulting behaviors and seismic risk of the Anninghe-Zemuhe fault zone from seismicity parameters

Using the data of regional seismic network, this paper analyzes the current faulting behaviors of different segments of the Anninghe-Zemuhe fault zone, western Sichuan, and identifies the likely risky segments for potential large earthquakes. The authors map the probable asperities from the abnormally low b-value distribution, develop and employ a method for identifying current faulting behaviors of individual fault segment from the combinations of multiple seismicity parameter values, and make an effort to estimate the average recurrence intervals of character-istic earthquakes by using the parameters of magnitude-frequency relationship of the asperity segment. The result suggests that the studied fault zone contains 5 segments of different current faulting behaviors. Among them, the Mianning-Xichang segment of the Anninghe fault has been locked under high stress, its central part is probably an asperity with a relatively large scale. The Xichang-Puge segment of the Zemuhe fault displays very low seismicity under low stress. Both the locked segment and the low-seismicity segment can be outlined on the across-profile of relocated hypocenter depths. The Mianning-Xichang segment is identified to be the one with potential large earth-quake risk, for which the average recurrence interval between the latest M = 6.7 earthquake in 1952 and the next characteristic event is estimated to be 55 to 67 years, and the magnitude of the potential earthquake between 7.0 and 7.5. Also, it has been preliminarily suggested that for a certain fault segment, its faulting behaviors may change and evolve with time gradually.

Centrifuge modeling of buried continuous pipelines subjected to normal faulting

Seismic ground faulting is the greatest hazard for continuous buried pipelines.Over the years,researchers have attempted to understand pipeline behavior mostly via numerical modeling such as the finite element method.The lack of well-documented field case histories of pipeline failure from seismic ground faulting and the cost and complicated facilities needed for full-scale experimental simulation mean that a centrifuge-based method to determine the behavior of pipelines subjected to faulting is best to verify numerical approaches.This paper presents results from three centrifuge tests designed to investigate continuous buried steel pipeline behavior subjected to normal faulting.The experimental setup and procedure are described and the recorded axial and bending strains induced in a pipeline are presented and compared to those obtained via analytical methods.The influence of factors such as faulting offset,burial depth and pipe diameter on the axial and bending strains of pipes and on ground soil failure and pipeline deformation patterns are also investigated.Finally,the tensile rupture of a pipeline due to normal faulting is investigated.

Neotectonics of the Eastern Margin of the Tibetan Plateau: New Geological Evidence for the Change from Early Pleistocene Transpression to Late Pleistocene-Holocene Strike-slip Faulting

We present in this paper some new evidence for the change during the Quaternary in kinematics of faults cutting the eastern margin of the Tibetan Plateau. It shows that significant shortening deformation occurred during the Early Pleistocene, evidenced by eastward thrusting of Mesozoic carbonates on the Pliocene lacustrine deposits along the Minjiang upstream fault zone and by development of the transpressional ridges of basement rocks along the Anninghe river valley. The Middle Pleistocene seems to be a relaxant stage with local development of the intra-mountain basins particularly prominent along the Minjiang Upstream and along the southern segment of the Anninghe River Valley. This relaxation may have been duo to a local collapse of the thickened crust attained during the late Neogene to early Pleistocene across this marginal zone. Fault kinematics has been changed since the late Pleistocene, and was predominated by reverse sinistral strike-slip along the Minshan Uplift, reverse dextral strike-slip on the Longmenshan fault zone and pure sinistral strike-slip on the Anninghe fault. This change in fault kinematics during the Quaternary allows a better understanding of the mechanism by which the marginal ranges of the plateau has been built through episodic activities.

Influences of pre-existing fracture on ground deformation induced by normal faulting in mixed ground conditions

Physical model tests have been conducted by various researchers to investigate fault rupture propagation and ground deformation induced by bedrock faulting. However, the effects of pre-existing fracture on ground deformation are not fully understood. In this work, six centrifuge tests are reported to investigate the influence of pre-existing fracture on ground deformation induced by normal faulting in sand, clay and nine-layered soil with interbedded sand and clay layers. Shear box tests were conducted to develop a filter paper technique, which was adopted in soil model preparation to simulate the effects of pre-existing fracture in centrifuge tests. Centrifuge test results show that ground deformation mechanism in clay, sand and nine-layered soil strata is classified as a stationary zone, a shearing zone and a rigid body zone. Inclination of the strain localization is governed by the dilatancy of soil material. Moreover, the pre-existing fracture provides a preferential path for ground deformation and results in a scarp at the ground surface in sand. On the contrary, fault ruptures are observed at the ground surface in clay and nine-layered soil strata.

Normal Faulting Type Earthquake Activities in the Tibetan Plateau and Its Tectonic Implication

This paper analyzes various earthquake fault types, mechanism solutions, stress field as well as other geophysical data to study the crust movement in the Tibetan plateau and its tectonic implications. The results show that a lot of normal faulting type earthquakes concentrate in the central Tibetan plateau. Many of them are nearly perfect normal fault events. The strikes of the fault planes of the normal faulting earthquakes are almost in the N-S direction based on the analyses of the equal area projection diagrams of fault plane solutions. It implies that the dislocation slip vectors of the normal faulting type events have quite great components in the E-W direction. The extension is probably an eastward extensional motion, mainly a tectonic active regime in the altitudes of the plateau. The tensional stress in the E-W or WNW-ESE direction predominates the earthquake occurrence in the normal event region of the central plateau. A number of thrust fault and strike-slip fault type earthquakes with strong compressive stress nearly in the NNE-SSW direction occurred on the edges of the plateau. The eastward extensional motion in the Tibetan plateau is attributable to the eastward movement of materials in the upper mantle based on_seismo-tomographic results. The eastward extensional motion in the Tibetan plateau may be related to the eastward extrusion of hotter mantle materials beneath the east boundary of the plateau. The northward motion of the Tibetan plateau shortened in the N-S direction probably encounters strong obstructions at the western and northern margins. Extensional motions from the relaxation of the topography and/or gravitational collapse in the altitudes of the plateau occur hardly in the N-S direction. The obstruction for the plateau to move eastward is rather weak.

Study on anti-faulting design process of Urumqi subway line 2 tunnel crossing reverse fault

For the tunnel crossing active fault,the damage induced by fault movement is always serious.To solve such a problem,a detailed anti-faulting tunnel design process for Urumqi subway line 2 was introduced,and seven three-dimensional elastic-plastic finite element models were established.The anti-faulting design process included three steps.First,the damage of tunnel lining from different locations of fault rupture surfaces was analyzed.Then,the analysis of the effect on tunnel buried depth was given.Finally,the effect of the disaster mitigation method on the flexible joint was verified and the location of the flexible joint was discussed.The results show that when the properties of surrounding rock at the tunnel bottom grows soft,the tunnel deformation curve is smoother and tunnel damage induced by fault movement is less serious.The vertical displacement change ratio of secondary linings along the tunnel axis may be the main factor to cause shear damage to the tunnel.The interface between the hanging wall and fracture zone is defined as the most adverse fault rupture surface.The tunnel damage was reduced with the decrease in the tunnel buried depth as more energy was dissipated by overburden soil and the differential uplift zone of soil became more diffuse.The method of the flexible joint can reduce the tunnel damage significantly and the disaster mitigation effect of different locations on the flexible joint is different.The tunnel damage is reduced by the greatest degree when the flexible joint is located on the fault rupture surface.

Sequence palaeogeography and coal accumulation of inland faulting basin： an example from late Triassic Yangtze platform of China

To unveil formation mechanism of key sequence boundaries of inland faulting basin and coal accumulation charac- teristics of coal seams in isochrohal stratigraphic framework, sequence stratigraphy, palaeogeographic recovery and other methods were used to research the sequence stratigraphy and coal accumulation in the example of Banding Basn in the west margin of Yangtze Platform, and the authors advanced a coal accumulation model of Faulting basin on the basis of accommo- dation space changes in the background of palaeogeography of sequence framework. The results show that： normal lacustrine regression and forced lacustrine regression are the main driving forces for the formation of sequence boundaries of Faulting ba- sin; basement subsidence is the main source of accommodation space of Faulting basin; and subsidence disequilibrium is the main cause for the difference in generation rate of accommodation space of Faulting basin. Coal accumulation in Faulting basin is obviously controlled by accommodation space changes in sequence framework and basin evolution. As Faulting basin evolves into depression basin, both subsidence rate of basin basement and generation rate of accommodation space decrease vertically, it appears as a progradational reverse-graded sedimentary sequence, coal accumulation in secondary sequence framework intensifies first and then weakens, and coal accumulation at the middle stage of highstand system tract is the best. During steady rifting period, minable coal seams were mostly developed in initial lacustrine flooding surface of fourth-order sequence and at the middle-late period of highstand system tract, and coal accumulation center lay in palaeogeographic unit of delta plain with moderate available accommodation space. During shrinking period, minable coal seams gradually migrated to the maximum lacustrine flooding surface and coal accumulation center lay in palaeographical unit of lacustrine with large available accommodation space.

Neogene faulting and volcanism in the Victoria Land Basin of the Ross Sea, Antarctica

The Neogene Terror Rift in the Antarctic Victoria Land Basin(VLB)of the Ross Sea,Antarctica,is composed of the Discovery Graben and the Lee Arch.Many Neogene volcanoes are aligned in the north-south direction in the southern VLB,belonging to the McMurdo Volcanic Group.However,due to multiple glaciations and limited seismic data,the volcanic processes are still unclear in the northern VLB,especially in the Terror Rift.Multichannel seismic profiles were collected at the VLB from the 32nd Chinese National Antarctic Research Expedition(CHINARE).We utilized four seismic profiles from the CHINARE and additional historical profiles,along with gravity and magnetic anomalies,to analyze faults and stratigraphic characteristics in the northern Terror Rift and volcanism in the VLB.Negative flower structures found in the northern Terror Rift suggest that the Terror Rift was affected by dextral strike-slip faults extending from the northern Victoria Land(NVL).After the initial orthogonal tension,the rift transited into an oblique extension,forming a set of downward concaving normal faults and accommodation zones in the Terror Rift.On the Lee Arch,several imbricated normal faults formed and converged into a detachment fault.Under gravitational forces,the strata bent upward and formed a rollover anticline.Many deep faults and thin strata subjected to erosion facilitated volcanic activity.A brittle volcanic region in the VLB was affected by dextral strike-slip movements and east-west extension,resulting in two Neogene volcanic chains that connect three igneous provinces in the VLB:the Hallett,Melbourne,and Erebus Provinces.These two chains contain mud volcanoes with magnetic nuclei,volcanic intrusions,and late-stage volcanic eruptions.Volcanisms have brought about opposite polarities of magnetic anomalies in Antarctica,indicating the occurrence of multiple volcanic activities.

COMPLEXITY AND CHAOTIC DYNAMICS OF ROCK FAULTING

Fault is a complex dynamic system controlled by the coupling of rock texture, reaction, fluid flow,stress, and rock deformation mechanism. A coupled reaction-transport- mechanical dynamic model for fault system is established and described in this paper. An example is presented for the Shuikoushan deposit, Hunan. The results of dynamic simulation indicate that the evolution and magnitude of fracture permeability of different rocks are different, and that faulting can enhance the spatial heterogeneity of rock permeability and facilitate fluid flow and mineralization in local fault zone. The pressure for a fault usually shows a variation mode of aperiodic oscillation with time, which reflects the chaotic behavior of the evolution of a fault.

LATE QUATERNARY FAULTING OF JIALI FAULT

Jiali fault is one of the major faults in southeastern Tibetan plateau. From Naqu to Jiali along which the field investigation has been done, the fault roughly extends in N60W direction and consists of three segments arranged in en echelon. From Luoermano to Esukongma (about 40km) where the fault is the northern boundary of the Sangdi basin that extend north\|south, late Quaternary surface ruptures have been found. Within this segment the creeks and gullies that cross the fault were offset and the displacements range from several meters to about 5km. The average slip rate during late Quaternary is about 10mm/a for this segment. An interesting phenomena is that the large displacement can only be found at those places where the fault is related to the basins that extend north\|south. Outside the basins, no convincing evidence has been found for late Quaternary surface ruptures and average slip rate for the whole fault is only about 3mm/a middle Pleistocene. It seems that these strike\|slip faults behave like a transform fault and the strike\|slip motion along them were a consequence of east\|west extension that creates the north\|south graben systems rather than the vice versa.

CENOZOIC FAULTING ALONG THE SOUTHEASTERN EDGE OF THE TIBETAN PLATEAU IN THE YANYUAN AREA AND ITS TECTONIC IMPLICATIONS

The southeastern edge of the Tibetan plateau is marked by several thrust sheets trending roughly in E\|W direction. The Yanyuan thrust sheet is bounded by three arcuate thrust belts, marked by high mountain ranges with the Jinhe belt on the north, the Qianhe belt on the south and the Ninglang belt on the west. Within the Yanyuan thrust belt are sedimentary cover rocks of the Yangtze platform, with ages ranging from Sinian to Triassic. In the north, the thrust sheet is overlain by the Muli thrust sheet along the Jinhe belt, while in the south, it is underlain by the Kangdian paleoland along the Qianhe belt. The youngest rocks on the foot wall are late Eocene to Oligocene in age, indicating that the thrusting occurred in the late Tertiary. The top of the Yanyuan thrust belt is truncated by a flat erosion surface similar to that on the plateau to the north. Along a north\|dipping normal fault bordering the Yanyuan basin on the south, the erosion surface is tilted to the south against Triassic rocks. The basin is filled with coal\|bearing clastic sediments of Pliocene and early Pleistocene age, which gives the timing of the normal faulting. Based on the faulting pattern, we propose that the southeastern edge of the Tibetan plateau underwent a large amount of N\|S shortening and uplift along the Yanyuan thrust sheet in the late Tertiary, while the subsequent normal faulting that had occurred along the Yanyuan basin during the Pliocene and Pliocene can be interpreted to have accommodated gravitational collapse of the crust.

Houston16:A stable geodetic reference frame for subsidence and faulting study in the Houston metropolitan area,Texas,U.S.

Houston,Texas,is one of the earliest regions to employ continuous GPS technology for land subsidence monitoring.Currently,there are over 200 permanent GPS stations located in the greater Houston area.The need for a consistent and stable reference frame to precisely interpret local ground deformation has become critically important since the 1990s,because of the significant spreading of subsidence.We have established a stable Houston reference frame,designated Houston16,using publicly available GPS observations(>5 years)from 15 Continuously Operating Reference Stations(CORS)located outside of the greater Houston area.Applications of the reference frame in subsidence and faulting studies are demonstrated in this article.Houston16 is aligned in origin and scale with the International GNSS Reference Frame of 2008(IGS08).A 7-parameter method was employed to transform the GPS-derived positional time series from IGS08 to Houston16.The primary product of this study is the seven parameters for transforming Earth-Centered-Earth-Fixed XYZ coordinates from IGS08 to Houston16.The frame stability of Houston16 is approximately 1 mm/year.Houston16 will be updated every few years to mitigate degradation of the frame’s stability with time and to synchronize with future updates of the IGS reference frame.

GRAIN REDUCTION AND GROWTH IN FAULTINGAND NEW CLASSIFICATION OF FAULTED ROCKS

New classification scheme about faulted rocks is proposed, according to the extent of grain reduction and growth and their sequence, and faulted rocks are classified as follows: (1) faulted rocks formed by the reduction action mainly include breccia series, cataclasite series, tectonobutchite series, mylonite series; (2) faulted rocks formed by growth action is mainly tectonoschist (gneiss) series; (3) blastomylonite series formed by grain reduction first and then growth; (4) mylonitic schist (gneiss) series formed by crystal growth first and then grain reduction. All series can be further classified according to matrix contents.

Cenozoic Denudation and Vertical Faulting History of the Central Segment of the Longmenshan Thrust Belt

Objective The uplift process and uplift mechanism of the Tibetan Plateau has been a research focus among geologists in recent years. This work put emphasis on the Cenozoic exhumation histories of the blocks bounded by the major faults at the central segment of the Longmenshan thrust belt, and the vertical faulting history, including the starting time and the total vertical displacement, of the major faults. Then we quantitatively established a complete active process for the central segment of the Longmenshan thrust belt, combining with the previous geophysical data in the deep and geologcial data. This study is critical for deeply and completely understanding the Cenozoic uplift history of the Longmenshan, and also provides thermochronology constraints to the different models for the uplift of the eastern margin of the Tibetan Plateau.

Quaternary Geology and Faulting in the Damxung-Yangbajain Basin

The detailed geological mapping, conducted in the Damxung-Yangbajain basin, shows that there are many types of deposits formed since the Pliocene. The oldest sediments are formed during the Pliocene. The most prominent sediments are three sets of moraines and fluvioglacial deposits. The ESR, U-series and OSL dates indicate they are formed about 700-500 ka B.P., 250-125 ka B.P. and 75-12 ka B.P. respectively and indicate that there are three glacial periods since the mid-Pleistocene in the Nyainqentanglha Range. Along the southeast side of the Nyainqentanglha Range, the main southeast dipping fault zone which bounds the Damxung-Yangbajain Graben on its western edge was mapped. The fault zone consists of three secondary fault zones and their initiation ages that the fault zones became active gradually decrease southeastward. Prominent faulting occurred in about 700-500 ka B.P., 350-220 ka B.P., -140 ka B.P. and 70-50 ka B.P. since the mid-Pleistocene. The height of fault scarps which offset the sediments formed since the mid-Pleistocene suggest that the vertical slip rates change between 0.4 -2 mm/a and the cumulative average vertical movement at rates of 1.1±0.3 mm/a during the Quaternary period and the Holocene vertical throw rate is 1.4±0.6 mm/a along the fault zones on the western side of the Damxung-Yangbajain Graben.

Faulting, magmatism and crustal oceanization of the Okinawa Trough

The paper focuses on the characteristics of faulting and magmatism of the Okinawa Trough and the relation between them. En-echelon grabens are ranked oblique to the continental shelf edge uplift, and the Longwang uplift, the rifting block ridge in the northern segment and the ＂Mianhua uplift＂ in the southern segment have possibly preserved characteristics of volcanism and magmatism occurring with those rifting phases. The clockwise rotation of the southern Ryukyu Islands, driven by collision between Luzon and Taiwan, has played a key role in the crustal oceanization, enhancing the crustal extension of the southern segment and inducing volcanic magmatism in those grabens, among which the Yaeyama graben is a typical example of the presence of oceanic crust. Faulting and magmatism were mainly migrating towards the island arc asymmetrically. The crustal oceanization of the Okinawa Trough is difficultly interpreted by the linear magnetic anomaly model, which is fit for the symmetric spreading of the mid-oceanic ridges.

Dextral-Slip Thrust Faulting and Seismic Events of the Ms 8.0 Wenchuan Earthquake,Longmenshan Mountains,Eastern Margin of the Tibetan Plateau

Dextral-slip thrust movement of the Songpan-Garze terrain over the Sichuan block caused the Ms 8.0 Wenchuan earthquake of May 12, 2008 and offset the Central Longmenshan Fault （CLF） along a distance of -250 km. Displacement along the CLF changes from Yingxiu to Qingchuan. The total oblique slip of up to 7.6 m in Yingxiu near the epicenter of the earthquake, decreases northeastward to 5.3 m, 6.6 m, 4.4 m, 2.5 m and 1.1 m in Hongkou, Beichuan, Pingtong, Nanba and Qingchuan, respectively. This offset apparently occurred during a sequence of four reported seismic events, EQ1-EQ4, which were identified by seismic inversion of the source mechanism. These events occurred in rapid succession as the fault break propagated northeastward during the earthquake. Variations in the plunge of slickensides along the CLF appear to match these events. The Mw 7.5 EQ1 event occurred during the first 0-10 s along the Yingxiu-Hongkou section of the CLF and is characterized by 1.7 m vertical slip and vertical slickensides. The Mw 8.0 EQ2 event, which occurred during the next 10-42 s along the Yingxiu-Yanziyan section of the CLF, is marked by major dextralslip with minor thrust and slickensides plunging 25°-35° southwestward. The Mw 7.5 EQ3 event occurred during the following 42-60 s and resulted in dextral-slip and slickensides plunging 10° southwestward in Beichuan and plunging 73° southwestward in Hongkou. The Mw 7.7 EQ4 event, which occurred during the final 60-95 s along the Beichuan-Qingchuan section of the CLF, is characterized by nearly equal values of dextral and vertical slips with slickensides plunging 45°-50° southwestward. These seismic events match and evidently controlled the concentrations of landslide dams caused by the Wenchuan earthquake in Longmenshan Mountains.

Modeling of normal faulting in the subducting plates of the Tonga,Japan,Izu-Bonin and Mariana Trenches:implications for near-trench plate weakening

The plate flexure and normal faulting characteristics along the Tonga, Japan, Izu-Bonin and Mariana Trenches are investigated by combining observations and modeling of elastoplastic deformation of the subducting plate. The observed average trench relief is found to be the smallest at the Japan Trench(3 km) and the largest at the Mariana Trench(4.9 km), and the average fault throw is the smallest at the Japan Trench(113 m) and the largest at the Tonga Trench(284 m). A subducting plate is modeled to bend and generate normal faults subjected to three types of tectonic loading at the trench axis: vertical loading, bending moment, and horizontal tensional force. It is inverted for the solutions of tectonic loading that best fit the observed plate flexure and normal faulting characteristics of the four trenches. The results reveal that a horizontal tensional force(HTF) for the Japan Trench is 33%, 50% and 60% smaller than those of the Mariana, Tonga and Izu-Bonin Trenches, respectively. The normal faults are modeled to penetrate to a maximum depth of 29, 23, 32 and 32 km below the sea floor for the Tonga,Japan, Izu-Bonin and Mariana Trenches, respectively, which is consistent with the depths of relocated normal faulting earthquakes in the Japan and Izu-Bonin Trenches. Moreover, it is argued that the calculated horizontal tensional force is generally positively correlated with the observed mean fault throw, while the integrated area of the reduction in the effective elastic thickness is correlated with the trench relief. These results imply that the HTF plays a key role in controlling the normal faulting pattern and that plate weakening can lead to significant increase in the trench relief.