Field survey and analysis on near-fault severely damaged high-speed railway bridge in 2022 M6.9 Menyuan earthquake

The 2022 M6.9 Menyuan earthquake caused severe damage to a high-speed railway bridge,which was designed for high-speed trains running at speeds of above 250 km/h and is located right next to the fault.Bridges of this type have been widely used for rapidly constructing the high-speed railway network,but few bridges have been tested by near-fault devastating earthquakes.The potential severe impact of the earthquake on the high-speed railway is not only the safety of the infrastructure,trains and passengers,but also economic loss due to interrupted railway use.Therefore,a field survey was carried out immediately after the earthquake to collect time-sensitive data.The damage to the bridge was carefully investigated,and quantitative analyses were conducted to better understand the mechanism of the bridge failure.It was found that seismic action perpendicular to the bridge’s longitudinal direction caused severe damage to the girders and rails,while none of the piers showed obvious deformation or cracking.The maximum values of transverse displacement,out-of-plane rotation and twisting angle of girders reached 212.6 cm,3.1 degrees and 19.9 degrees,respectively,causing severe damage to the bearing supports and anti-seismic retaining blocks.These observations provide a basis for improving the seismic design of high-speed railway bridges located in near-fault areas.

GPS measured static and kinematic offsets at near and far field of the 2011 Mw 9.0 Tohoku-Oki earthquake

The Mw 9.0 Tohoku-Oki earthquake that hit the mainland Japan on 11 th March, 2011 had resulted a devastating Tsunami due to an active thrusting between the Pacific and the North American Plates. Static and kinematic offsets at the offshore epicentre of the Mw 9.0 event remain unanswered and being investigated along with their near and far field limiting distances from the epicentre. Accordingly, offset measurements from 60 continuously operating IGS and GEONET GNSS stations were radially classified from the epicentre and interpreted with analytical models to find their linear offset decay rates. Co-and post-seismic static positional anomaly offsets of sixty days show almost all near field stations had strong or appreciable eastward or south eastward static shifts. Near stations(<250 km) showed both kinematic and static offsets. GEONET station ’0175’ showed maximum resultant static offset of-4.5 m, which diminishes approximately 1-2 cm at far sites like SMST and AIRA. Characteristic decay duration(’b’) of the mean kinematic co-seismic shift(’a’)of near field stations was 17.28 s during earthquake hours with an EW component shift >1.5 m. Spatial models of projected N-S static and kinematic offsets show their asymmetrical distributions around the epicentre with maximum model offset of-1.84 m displaced towards south at-45 km north of the epicentre. The Tohoku-Oki earthquake produced a resultant kinematic offset of-10.2 m towards East at its offshore epicentre;while the estimated near field static offset is ~9.82 m. However, both estimates are bigger than double the resultant offset measured value(~4.3 m) in the Japanese mainland using GPS. The difference in the kinematic and static near field offsets highlight that the near surface had elastic or in-elastic kinematic strain dissipation as against the lithospheric level viscoelastic static response, which resulted rapid kinematic strain release(1.12 cm/km)within the limiting radius of ~220 km from the Tohoku-Oki epicentre.

Prediction of near-field strong ground motions for scenario earthquakes on active fault

A method to predict near-field strong ground motions for scenario earthquakes on active faults is proposed. First, macro-source parameters characterizing the entire source area, i.e., global source parameters, including fault length, fault width, rupture area, average slip on the fault plane, etc., are estimated by seismogeology survey, seismicity and seismic scaling laws. Second, slip distributions characterizing heterogeneity or roughness on the fault plane, i.e., local source parameters, are reproduced/evaluated by the hybrid slip model. Finally, the finite fault source model, developed from both the global and local source parameters, is combined with the stochastically synthetic technique of ground motion using the dynamic comer frequency based on seismology. The proposed method is applied to simulate the acceleration time histories on three base-rock stations during the 1994 Northridge earthquake. Comparisons between the predicted and recorded acceleration time histories show that the method is feasible and practicable.

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.

Deformation field around a thrust fault:A comparison between laboratory results and GPS observations of the 2008 Wenchuan earthquake

On May 12,2008,an Mw7.9 earthquake occurred in Wenchuan County,Sichuan Province,China.Movement of Yingxiu–Beichuan Fault in the Longmenshan Fault Zone was considered to be the main cause of the earthquake.Earthquakes are closely related to fault activities.Therefore,studying the strain distribution and evolution process around active fault zones is important to the understanding of seismic activities.In this study,we conduct laboratory experiments with uniaxial compression applied to marble sheets with intentionally fabricated cracks.The speckle patterns of the rock samples under different loading conditions are recorded in real time by a digital camera.To calculate the deformation fields of the deliberately cracked marble sheets during different stages of the loading processes,the recorded images are processed by the digital image correlation method.The distribution and variation of the displacement and strain are further analyzed in order to understand the strain localization of and observed damage in the experimental fracture zones.Finally,we compare these laboratory results with the GPS-observed coseismic displacements during the 2008 Wenchuan earthquake,to assess the consistency between our laboratory observations and the field observations of the earthquake,but also to suggest how laboratory results can improve thinking about how earthquake patterns do and do not reflect fault patterns.

The seismicity and tectonic stress field characteristics of the Longmenshan fault zone before the Wenchuan M_S8.0 earthquake

The seismicity of Longrnenshan fault zone and its vicinities before the 12 May 2008 Wenchuan Ms8.0 earthquake is studied. Based on the digital seismic waveform data observed from regional seismic networks and mobile stations, the focal mechanism solutions are determined. Our analysis results show that the seismicities of Longmenshan fault zone before the 12 May 2008 Wenchuan earthquake were in stable state. No obvious phenomena of seismic activity intensifying appeared. According to focal mechanism solutions of some small earthquakes before the 12 May 2008 Wenchuan earthquake, the direction of principal compressive stress P-axis is WNW-ESE. The two hypocenter fault planes are NE-striking and NW-striking. The plane of NE direction is among N50°-70°E, the dip angles of fault planes are 60°-70° and it is very steep. The faultings of most earthquakes are dominantly characterized by dip-slip reverse and small part of faultings present strike-slip. The azimuths of principal compressive stress, the strikes of source fault planes and the dislocation types calculated from some small earthquakes before the 12 May 2008 Wenchuan earthquake are in accordance with that of the main shock. The average stress field of micro-rupture along the Longmenshan fault zone before the great earthquake is also consistent with that calculated from main shock. Zipingpu dam is located in the east side 20 km from the initial rupture area of the 12 May 2008 Wenchuan earthquake. The activity increment of small earthquakes in the Zipingpu dam is in the period of water discharging. The source parameter results of the small earthquakes which occurred near the initial rupture area of the 12 May 2008 Wenchuan earthquake indicate that the focal depths are 5 to 14 km and the source parameters are identical with that of earthquake.

3-D finite element modeling for evolution of stress field and interaction among strong earthquakes in Sichuan-Yunnan region

Based on the latest achievement about activities of geological structure, a 3-D finite-element model containing four layers of upper crust, lower crust (two layers) and upper mantle is established in the paper. By repeated tests and revisions, the boundary conditions of the model are determined. And then the background stress field, the stress field caused by fault creep and the stress field triggered by strong earthquake in Sichuan-Yunnan region, as well as their dynamic variations are calculated. The results indicate that the latter earthquake often occurs in the area with positive Coulomb rupture stress change associated with the former one, the former earthquake has a triggering effect on the latter one to a certain extent, and strong earthquake often occur in groups under the background of high stress, which is of great significance for distinguishing seismic anomalies, as well as for improving the level of earthquake prediction.

The stress field variation caused by faulting and the prediction for seismic risk

The stress field caused by faulting has an effect on the stability of the neighboring faults, and the study on the fault interaction has a close relation with the prediction of seismic risk. Stress field caused by the rectangle fault in the semi-infinite elastic medium is calculated on the basis of the elastic dislocation theory. The result shows that most of the successive large earthquakes, in the southwestern part of China and North China, occurred in the increasing area of shear stress S(xy) and the decreasing area of normal stress S(yy) The increasing of earthquake occurrence probability has a function relation with the increasing of stress. Earthquake triggering is resulted from the increasing of shear stress and the decreasing of normal stress. An activation coefficient A, of the earthquake is defined to express the change of seismic activity. The concrete risk region can be obtained through space scanning of At value. Finally, the fault interaction in a large scope is discussed in this paper.

The September 16, 2015 Mw 8.3 Illapel, Chile Earthquake： characteristics of tsunami wave from near-field to far-field

On September 16, 2015, an earthquake with magnitude ofMw 8.3 occurred 46 km offshore from Illapel, Chile, generating a 4.4-m local tsunami measured at Coquimbo. In this study, the characteristics of tsunami are presented by a combination of analysis of observations and numerical simulation based on sources of USGS and NOAA. The records of 16 DART buoys in deep water, ten tidal gauges along coasts of near-field, and ten coastal gauges in the far-field are studied by applying Fourier analyses. The numerical simulation based on nonlinear shallow water equations and nested grids is carried out to provide overall tsunami propagation scenarios, and the results match well with the observations in deep water and but not well in coasts closed to the epicenter. Due to the short distance to the epicenter and the shelf resonance of southern Peru and Chile, the maximum amplitude ranged from 0.1 m to 2 m, except for Coquimbo. In deep water, the maximum amplitude of buoys decayed from 9.8 cm to 0.8 cm, suggesting a centimeter-scale Pacific-wide tsunami, while the governing period was 13-17 min and 32 min. Whereas in the far-field coastal region, the tsunami wave amplified to be around 0.2 m to 0.8 m, mostly as a result of run-up effect and resonance from coast reflection. Although the tsunami was relatively moderate in deep water, it still produced non-negligible tsunami hazards in local region and the coasts of farfield.

Focal Fault and Stress Field of the Yao'an M 6.5 Earthquake on January 15, 2001

Using the 78 focal mechanism solutions of the fore shocks, main shock and after shocks of the earthquake sequence for the Yao’an earthquake, the characteristics of the focal faults and stress field for the earthquake sequence are analyzed. The results show that the main rupture plane of the Yao’an earthquake sequence is a tectonic fault with N50°W strike and steep dip and all the main shock, the fore shocks and the vast majority of after shocks occurred on the main rupture plane. A tectonic fracture with NNE-NE strike also participated in development process of the sequence. The focal stress field of the sequence dominated by principal compressional stress with nearly horizontal orientation SSE is consistent with the regional tectonic stress field. In the sequence development, the stress field in the focal region was complex with multi-azimuths and multi-action models and the focal rupture showed complex features with multi-directions and multi-patterns.

The 2018 M_S 5.9 Mojiang Earthquake:Source model and intensity based on near-field seismic recordings

On September 8, 2018, an M_S 5.9 earthquake struck Mojiang, a county in Yunnan Province, China. We collect near-field seismic recordings(epicentral distances less than 200 km) to relocate the mainshock and the aftershocks within the first 60 hours to determine the focal mechanism solutions of the mainshock and some of the aftershocks and to invert for the finite-fault model of the mainshock.The focal mechanism solution of the mainshock and the relocation results of the aftershocks constrain the mainshock on a nearly vertical fault plane striking northeast and dipping to the southeast. The inversion of the finite-fault model reveals only a single slip asperity on the fault plane. The major slip is distributed above the initiation point, ～14 km wide along the down-dip direction and ～14 km long along the strike direction, with a maximal slip of ～22 cm at a depth of ～6 km. The focal mechanism solutions of the aftershocks show that most of the aftershocks are of the strike-slip type, a number of them are of the normal-slip type, and only a few of them are of the thrust-slip type.On average, strike-slip is dominant on the fault plane of the mainshock, as the focal mechanism solution of the mainshock suggests, but when examined in detail, slight thrust-slip appears on the southwest of the fault plane while an obvious part of normal-slip appears on the northeast, which is consistent with what the focal mechanism solutions of the aftershocks display. The multiple types of aftershock focal mechanism solutions and the slip details of the mainshock both suggest a complex tectonic setting, stress setting, or both. The intensity contours predicted exhibit a longer axis trending from northeast to southwest and a maximal intensity of Ⅷ around the epicenter and in the northwest.

GRACE detection of the medium-to far-field coseismic gravity changes caused by the 2004 Mw9.3 Sumatra-Andaman earthquake

Large earthquakes cause observable changes in the Earth’s gravity field, which have been detected by the Gravity Recovery and Climate Experiment (GRACE). Since most previous studies focus on the detection of near-field gravity effects, this study provides the results from the medium- to far-field gravity changes caused by the 2004 Sumatra-Andaman earthquake that are recorded within GRACE monthly solutions. Utilizing a spherical-earth dislocation model we documented that large-scale signals predominate in the global field of the coseismic gravity changes caused by the earthquake. After removing the near-field effects, the coseismic gravity changes show a negative anomaly feature with an average magnitude of -0.18×10-8 m·s-2 in the region ranging ～40° from the epicenter, which is considered as the 'medium ffield' in this study. From the GRACE data released by Center for Space Research from August 2002 to December 2008, we retrieved the large-scale gravity changes smoothed with 3 000 km Gaussian ffilter. The results show that the coseismic gravity changes detected by GRACE in the medium field have an average of (-0.20±0.06)×10-8 m·s-2, which agrees with the model prediction. The detection confirms that GRACE is sensitive to large-scale medium-field coseismic gravitational effects of mega earthquakes, and also validates the spherical-earth dislocation model in the medium field from the perspective of satellite gravimetry.

Characteristics of the seismic source stress field in the joint region of Xianshuihe, Longmenshan and An(ninghe faults

Twenty-two earthquakes (ML=2.2-3.7) in the joint region of Xianshuihe, Longmenshan and An'ninghe faults are studied in this paper. The source mechanism solutions of these events are obtained using P-wave first motion method, and the characteristics of the source stress field and rupture in the joint region are summarized preliminarily with some results of other researchers. Being strongly extruded by the approximately horizontal regional stress with the direction from north-west to south-east and the effect of the complex tectonics in the region, the source stress field has complex and variable characteristics. The earthquakes mainly show normal or strike-slip faults in Yajiang, North-triangle and west of Chengdu-block areas, indicating that the vertical forces have been playing an important role in the source stress fields, while the earthquakes mainly show reverse or strike-slip faults in Baoxing-Tianquan area, with the horizontal components of the principal pressure stress axes identical to the south-west direction to which the shallow mass is moving. We think that the manifold combinations of earthquake faults are the micro-mechanism based upon which the large regional shallow crust mass has been moving continually.

Far-field analysis of effects of unilateral rupturing fault on the space correlation of strong ground motion

On the assumption that seismic source is simplified as linear rupture fault with finite length, this paper qualitatively studies the seismic source effects on space correlation of strong ground motion. Based on expanding expression of Fourier spectrum of strong ground motion with space coordinate variables, this paper also gives a expression of describing correlation of strong ground motion field. According to far-field condition, the theoretical formula of the expression can be obtained. Furthermore, this paper presents a theoretical formula of estimation the radius of strong ground motion field, which depends on expansion condition of Fourier spectrum of strong ground motion, with space variables. At last, taking one earthquake as an example, this paper gives three-dimension patterns of radius of the field with epicenter distance and azimuth as well as frequency.

Fault plane parameters of Sanhe-Pinggu M8 earthquake in 1679 determined using present-day small earthquakes

The great Sanhe-Pinggu M8 earthquake occurred in 1679 was the largest surface rupture event recorded in history in the northern part of North China plain. This study determines the fault geometry of this earthquake by inverting seismological data of present-day moderate-small earthquakes in the focal area. We relocated those earthquakes with the double-difference method. Based on the assumption that clustered small earthquakes often occur in the vicinity of fault plane of large earthquake, and referring to the morphology of the long axis of the isoseismal line obtained by the predecessors, we selected a strip-shaped zone from the relocated earthquake catalog in the period from 1980 to 2009 to invert fault plane parameters of this earthquake. The inversion results are as follows： the strike is 38.23°, the dip angle is 82.54°, the slip angle is -156.08°, the fault length is about 80 km, the lower-boundary depth is about 23 km and the buried depth of upper boundary is about 3 kin. This shows that the seismogenic fault is a NNE-trending normal dip-slip fault, southeast wall downward and northwest wall uplift, with the right-lateral strike-slip component. Moreover, the surface rupture zone, intensity distribution of the earth-quake and seismic-wave velocity profile in the focal area all verified our study result.

Improvements of corner frequency and scaling factor for stochastic finite-fault modeling

In this paper, three existing source spectral models for stochastic finite-fault modeling of ground motion were reviewed. These three models were used to calculate the far-field received energy at a site from a vertical fault and the mean spectral ratio over 15 stations of the Northridge earthquake, and then compared. From the comparison, a necessary measure was observed to maintain the far-field received energy independent of subfault size and avoid overestimation of the long- period spectra/level. Two improvements were made to one of the three models （i.e., the model based on dynamic comer frequency） as follows： （i） a new method to compute the subfault comer frequency was proposed, where the subfault comer frequency is determined based on a basic value calculated from the total seismic moment of the entire fault and an increment depending on the seismic moment assigned to the subfault; and （ii） the difference of the radiation energy from each suhfault was considered into the scaling factor. The improved model was also compared with the unimproved model through the far-field received energy and the mean spectral ratio. The comparison proves that the improved model allows the received energy to be more independent of subfault size than the unimproved model, and decreases the overestimation degree of the long-period spectral amplitude.

Far-field deformations caused by the 2004 Sumatra earthquake

Theoretical horizontal displacements caused by the 2004 Sumatra earthquake in the Sichuan-Yunnan area have been calculated according to a spherical dislocation theory and an earthquake-fault model. The results show that the theoretical displacements are basically consistent with the GPS observations in situ. On this basis,we have calculated the co-seismic displacements, strains, changes of gravity and geoid of the whole Earth, including China mainland and vicinity, caused by this earthquake. Key wards：

Characteristics of block strain and fault movement in the Sichuan-Yunnan region before and after Wenchuan earthquake

Deformation characteristics of the Sichuan-Yunnan region during the two periods 1999--2007 and 2007--2009 are analyzed with a block deformation model and GPS velocity profiles. The results show that the direction of the principal compressive strain rate of the Northwest-Sichuan block - the Mid-Yunnan block - the Southwest-Yunnan block was characterized by a clockwise rotation from north to south. The Anninghe and the Zemuhe faults had some shear-strain accumulation. The southern segment of the Xiaojiang fault had mainly strike-slip movement, while the northern segment was mainly accumulating strain. The 2008 Ms8.0 Wenchuan earthquake had some influence on the mid-southern segment of the Lijiang-Xiaojinhe fault, the Anninghe fault and the Jinshajiang fault, but not the Zemuhe fault, the Xiaojiang fault and the Red River fault as much.

Seismic Responses of Asymmetric Base-Isolated Structures under Near-Fault Ground Motion

An inter-story shear model of asymmetric base-isolated structures incorporating deformation of each isolation bearing was built, and a method to simultaneously simulate bi-directional near-fault and far-field ground motions was proposed. A comparative study on the dynamic responses of asymmetric base-isolated structures under near-fault and far-field ground motions were conducted to investigate the effects of eccentricity in the isolation system and in the superstructures, the ratio of the uncoupled torsional to lateral frequency of the superstructure and the pulse period of near-fault ground motions on the nonlinear seismic response of asymmetric base-isolated structures. Numerical results show that eccentricity in the isolation system makes asymmetric base-isolated structure more sensitive to near-fault ground motions, and the pulse period of near-fault ground motions plays an import role in governing the seismic responses of asymmetric base-isolated structures.

Fracture Characters and Seismogenic Fault of the Yajiang Earthquake Sequence with M_S6.0 in Sichuan in 2001

The Yajiang earthquake sequence in 2001, with the major events of M S 5.1 on Feb. 14 and of M S 6.0 on Feb.23, are significant events in the Sichuan region during the last 13 years. Eighty-eight earthquakes in the sequence with at least 5 distinct onset parameters for each recorded by the Sichuan Seismic Network in the period of Jan. 1 through June 30, 2001 were chosen for this study. The events are relocated and the focal mechanism is derived from P-wave onsets for 13 events with relatively larger magnitudes. The focal depth of all earthquakes fall between a range of 2km to 16km, with dominant distribution between 9km to 11km. The foreshocks, the M S5.1 earthquake and the M S6.0 earthquake and their aftershocks are all located close to the Zihe fault and the dominant epicentral distribution is in NW direction, identical to that of the fault. The fracture surface of the focal mechanism is determined in accordance to the mass transfer orientation in the recent earth deformation field in the Yajiang region. The P axes of the principal compressive stress in focal mechanism solutions of the 13 events show bigger vertical components, and the horizontal projection trending SE. The earthquakes are of left-lateral, strike-slip normal, and normal strike-slip types. The rupture surface of most earthquakes strike NW-SE, dipping SW. Based on the above information, we conclude that the Zihe fault that crosses the earthquake area, striking NW and dipping SW, is the seismogenic fault for the Yajiang earthquake sequence.