Analysis of debris flow control effect and hazard assessment in Xinqiao Gully,Wenchuan M_(s)8.0 earthquake area based on numerical simulation

Xinqiao Gully is located in the area of the 2008 Wenchuan M_(s)8.0 earthquake in Sichuan province,China.Based on the investigation of the 2023"6-26"Xinqiao Gully debris flow event,this study assessed the effectiveness of the debris flow control project and evaluated the debris flow hazards.Through field investigation and numerical simulation methods,the indicators of flow intensity reduction rate and storage capacity fullness were proposed to quantify the effectiveness of the engineering measures in the debris flow event.The simulation results show that the debris flow control project reduced the flow intensity by41.05%to 64.61%.The storage capacity of the dam decreases gradually from upstream to the mouth of the gully,thus effectively intercepting and controlling the debris flow.By evaluating the debris flow of different recurrence intervals,further measures are recommended for managing debris flow events.

Surface Rupture and Co-seismic Displacement Produced by the Ms 8.0 Wenchuan Earthquake of May ^(12)th,2008,Sichuan,China:Eastwards Growth of the Qinghai-Tibet Plateau

An earthquake of Ms 8 struck Wenchuan County, western Sichuan, China, on May 12^th, 2008 and resulted in long surface ruptures （〉300 km）. The first-hand observations about the surface ruptures produced by the earthquake in the worst-hit areas of Yingxiu, Beichuan and Qingchuan, ascertained that the causative structure of the earthquake was in the central fault zones of the Longmenshan tectonic belt. Average co-seismic vertical displacements along the individual fault of the Yingxiu-Beiehuan rupture zone reach 2.514 m and the cumulative vertical displacements across the central and frontal Longmenshan fault belt is about 5-6 m. The surface rupture strength was reduced from north of Beichuan to Qingchuan County and shows 2-3 m dextral strike-slip component. The Wenchuan thrust-faulting earthquake is a manifestation of eastward growth of the Tibetan Plateau under the action of continuous convergence of the Indian and Eurasian continents.

Preliminary Results of In-situ Stress Measurements along the Longmenshan Fault Zone after the Wenchuan M_s 8.0 Earthquake

Four months after the Wenchuan Ms 8 earthquake in western Sichuan, China, in situ stress measurements were carried out along the Longmenshan fault zone with the purpose of obtaining stress parameters for earthquake hazard assessment. In-situ stresses were measured in three new boreholes by using overcoring with the piezomagnetic stress gauges for shallow depths and hydraulic fracturing for lower depths. The maximum horizontal stress in shallow depths （-20 m） is about 4.3 MPa, oriented N19°E, in the epicenter area at Yingxiu Town, about 9.7 MPa, oriented N51°W, at Baoxing County in the southwestern Longmenshan range, and about 2.6 MPa, oriented N39°E, near Kangding in the southernmost zone of the Longmenshan range. Hydraulic fracturing at borehole depths from 100 to 400 m shows a tendency towards increasing stress with depth. A comparison with the results measured before the Wenchuan earthquake along the Longmenshan zone and in the Tibetan Plateau demonstrates that the stress level remains relatively high in the southwestern segment of the Longmenshan range, and is still moderate in the epicenter zone. These results provide a key appraisal for future assessment of earthquake hazards of the Longmenshan fault zone and the aftershock occurrences of the Wenchuan earthquake.

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.

Analysis of the Wenchuan Ms8.0 Earthquake's Co-seismic Stress and Displacement Change by Using the Finite Element Method

The variation of in situ stress before and after earthquakes is an issue studied by geologists. In this paper, on the basis of the fault slip dislocation model of Wenchuan Ms8.0 earthquake, the changes of co-seismic displacement and the distribution functions of stress tensor around the Longmen Shan fault zone are calculated. The results show that the co-seismic maximum surface displacement is 4.9 m in the horizontal direction and 6.5 m in the vertical direction, which is almost consistent with the on-site survey and GPS observations. The co-seismic maximum horizontal stress in the hanging wall and footwall decreased sharply as the distance from the Longmen Shan fault zone increased. However, the vertical stress and minimum horizontal stress increased in the footwall and in some areas of the hanging wall. The study of the co-seismic displacement and stress was mainly focused on the long and narrow region along the Longmen Shan fault zone, which coincides with the distribution of the earthquake aftershocks. Therefore, the co-seismic stress only affects the aftershocks, and does not affect distant faults and seismic activities. The results are almost consistent with in situ stress measurements at the two sites before and after Wenchuan Ms8.0 earthquake. Along the fault plane, the co-seismic shear stress in the dip direction is larger than that in the strike direction, which indicates that the faulting mechanism of the Longmen Shan fault zone is a dominant thrust with minor strike-slipping. The results can be used as a reference value for future studies of earthquake mechanisms.

Tectonic Stress State Changes Before and After the Wenchuan M_s 8.0 Earthquake in the Eastern Margin of the Tibetan Plateau

Crustal tectonic activities are essentially the consequences of the accumulation and release of in situ stress. Therefore, studying the stress state near active faults is important for understanding crustal dynamics and earthquake occurrences. In this paper, using in situ stress measurement results obtained by hydraulic fracturing in the vicinity of the Longmenshan fault zone before and after the Wenchuan Ms 8.0 earthquake and finite element modeling, the variation of stress state before and after the Wenchuan M. 8.0 earthquake is investigated. The results show that the shear stress, which is proportional to the difference between principal stresses, increases with depth and distance from the active fault in the calm period or after the earthquakes, and tends to approach to the regional stress level outside the zone influenced by the fault. This distribution appears to gradually reverse with time and the change of fault properties such as frictional strength. With an increase in friction coefficient, low stress areas are reduced and areas with increased stress accumulation are more obvious near the fault. In sections of the fault with high frictional strengths, in situ stress clearly increases in the fault. Stress accumulates more rapidly in the fault zone relative to the surrounding areas, eventually leading to a stress field that peaks at the fault zone. Such a reversal in the stress field between the fault zone and surrounding areas in the magnitude of the stress field is a potential indicator for the occurrence of strong earthquakes.

Mercury indicating inflow zones and ruptures along the Wenchuan Ms 8.0 earthquake fault

During the Wenchuan Fault Scientific Drilling Project,we determined the values of total mercury(HgT)and gaseous elemental mercury(GEM) from drilled cores and drilling mud,respectively.Geochemical analysis shows HgT values ranging from 0.24 to 6.45 ng/g for the Penguan complex and from 2.90 to 137.54 ng/g for T3 sediment.The average levels of HgT for the Penguan complex and T3 sediments are 1.81 ± 0.26 ng/g and23.96 ± 4.80 ng/g,respectively.Major anomalous peaks of HgT appear at depth of 614,731,993 and 1,107 m,which correspond to the long-term high seismic activity during crustal deformation in response to tectonic stresses.Gaseous elemental mercury dissolved in drilling mud was also analyzed.We found fluid inflow zones with high GEM at depths of 590-750 m,suggesting that fluid-filled ruptures exist in the LMS fault zone.It indicates that mercury provides geochemical evidence for inflow zones and ruptures/fault zones in the Wenchuan Ms 8.0 earthquake fault.

Anomalous phenomena in DC–ULF geomagnetic daily variation registered three days before the 12 May 2008 Wenchuan M_S 8.0 earthquake

The hourly data of the vertical Z and the horizontal H components of 37 ground-based DC-ULF geomagnetic stations are examined during 20 April-12 May 2008. On 9 May 2008, three days before the Wenchuan MS 8.0 shock, anomalies-a double low-point and a decreased amplitude-are registered on the curves of the Z component at 25 stations in a large-scale area surrounding the Wenchuan epicentral area. The H component shows none of the double low-point phenomenon but does exhibit a reduced magnitude at the same time. The geomagnetic index Kp is also examined and indicates that the anomalies appear at a solar quiet period. The appearing time shift(Tzs) between the first low-point on May 9 and the minimum point occurring time of May 1-5, 2008 is also checked.The results show that Tzs is on the order of 1-2 hours earlier or later than usual and there is a 2-6 hours’ gap between these two lowpoints. However, there is still a transition area which includes the epicenter where Tzs=0. Variation amplitude examined on vertical Z increases as the distance from the epicenter decreases. An Earth-air-ionosphere model has been employed to investigate a possible mechanism of this phenomenon and positive results have been unexpectedly attained. All these above-related results tend to prove that the variations of the Z and H on May 9, 2008 during the solar quiet period are probably associated with the forthcoming Wenchuan MS 8.0 earthquake.

Preparatory mechanism of Ms8.0 Wenchuan earthquake evidenced by crust-deformation data

Some crustal-deformation data related to the Ms8.0 Wenchuan in 2008, was described and a model that is capable of explaining the observed deformation features is presented. The data include ： pre-earthquake uplift in an area south of the epicenter obtained by repeated-leveling measurements ; pre-earthquake horizontal deformation by GPS observation during two periods in Sichuan-Yunnan area;vertical deformation along a short cross-fault leveling line in the epicenter area; and co-seismic near-field vertical and horizontal crustal-move- ment data by GPS. The model is basically ＂elastic-rebound＂, but involves a zone between two local faults that was squeezed out at the time of earthquake. ：

Co- and post-seismic vertical displacements of Wenchuan Ms8.0 earthquake near Beichuan

Co- and post-seismic vertical displacements of the Wenchuan earthquake derived from two measurements in 2008 and 2010 along two partly-damaged leveling lines near the epicenter show the following features： Co-seismic displacement at Beichuan-Yingxiu fault was as large as 4. 711 m near Beichuan, where the maximum observed fault offset was 5.1 m. In contrast, the observed co-seismic offset of the Qingchuan fault in Pingwu County was only 0. 064 m. During 2008 - 2010, the post-seismic displacement rate was 5 - 27 mm/a near Beichuan-Yingxiu fault in Beichuan area, 20.6 mrn/a at Jiangyou-Guangyuan fault near Dakang, and only 0.2 - 1.3 mm/a at Qingehuan fault near Gucheng.

Anomalous tremor before 2008 Ms8.0 Wenchuan earthquake: a review

In this paper we give a review of several previously published papers on anomalous tremors observed before the 2008 Ms8.0 Weuchuan earthquake. Based on the observed time and frequency shifts between coastal and inland stations, we discussed some methods to distinguish different kinds of microseisms, and speculated that a pre-earthquake typhoon might have caused a ＂mainland-originated microseism＂ which in turn trig- gered the earthquake.

Temporal and spatial characteristics of VTEC anomalies before Wenchuan Ms8.0 earthquake

GPS and the COD VTEC data were studied in search of ionospheric VTEC changes in space and time that might be associated with the Wenchuan Ms8.0 earthquake on 12 May,2008. The result shows several significant anomalous decreases at 12：00 UT- 16：00 UT on April 29 and an anomalously increase at 14：00 UT - 18：00 UT on May 9. The anomalies had two humps, that were located on both sides of the geomagnetic equator and had a tendency of drifting towards the equator. Since the observed anomalies cannot be attributed to any other causes and since they occurred close to the time of the earthquake, we consider them to be possibly premonitory to the earthquake.

Improvement of instantaneous positioning by spatial stacking and modified sidereal filtering methods with application to the 2008 Ms8.0 Wenchuan earthquake

We obtained several displacement time series from the Sichuan permanent GPS net and processed the 1-Hz data observed during a few days before the 2008 Ms8.0 Wenchuan earthquake by double-difference instantaneous positioning technique. We filtered the data by the spatial stacking and the modified sidereal filte- ring methods to reduce correlation bias in space and time. The results indicate that these methods can improve the precision significantly.

The Dynamic Characteristics of Strain Fields and Crustal Movement before the Wenchuan Earthquake (M_S=8.0)

In this paper, we analyze the crustal movements, strain field changes and large scale dynamic characteristics of horizontal deformation before the Wenchuan earthquake (M_S=8.0) using GPS data obtained from the Crustal Movement Observation Network of China. The following issues are discussed. First, the strain fields of the Longmenshan fault zone located at the epicenter show slow accumulation, because of the tectonic dynamics process subjected to the eastward movement of the Bayan Har block. Second, the different movements between the Longmenshan fault and South China block are smaller than the errors of GPS observation. Third, the high value of compressive strain (2004～2007) is located at the epicenter, which shows that the local squeezing action is stronger than before. Fourth, the data from GPS reference stations in the Chinese Mainland show that crustal shortening is faster than before in the north-eastern direction, which is part of the background of the local tectonic dynamics increase in the Longmenshan fault zone.

Characteristics of coseismic water level changes at Tangshan well for the Wenchuan MS_8.0 earthquake and its larger aftershocks

Coseismic water level changes which may have been induced by the Wenchuan Ms8.0 earthquake and its 15 larger aftershocks （Ms〉5.4） have been observed at Tangshan well. We analyze the correlation between coseismic parameters （maximum amplitude, duration, coseismic step and the time when the coseismic reach its maximum amplitude） and earthquake parameters （magnitude, well-epicenter distance and depth）, and then compare the time when the coseismic oscillation reaches its maximum amplitude with the seismogram from Douhe seismic station which is about 16.3 km away from Tangshan well. The analysis indicates that magnitude is the main factor influencing the induced coseismic water level changes, and that the well-epicenter distance and depth have less influence. Ms magnitude has the strongest correlation with the coseismic water level changes comparing to Mw and ML magnitudes. There exists strong correlation between the maximum amplitude, step size and the oscillation duration. The water level oscillation and step are both caused by dynamic strain sourcing from seismic waves. Most of the times when the oscillations reach their maximum amplitudes are between S and Rayleigh waves. The coseismic water level changes are due to the co-effect of seismic waves and hydro-geological environments.

Temporal and Spatial Evolution of Precursory Anomalies of the Wenchuan MS8.0 Earthquake and Their Mechanical Analysis

In order to study the spatiotemporal evolution of the precursory anomalies 10 years before the Wenchuan M_S8. 0 earthquake in 2008, the epicentral distance of the precursory anomalies is calculated by using the geometric center of the rupture region and the elliptical centerline of the aftershock region. The result shows, precursor anomalies gradually increased about 2 years before the Wenchuan earthquake. The ratio of abnormal items is greater than 25% in the near source area (about twice the source scale) and 17%-24% in the remote area (about 3-5 times the source scale). There are three different stages of spatiotemporal evolution of precursory anomalies. During the α stage (including α_1 and α_2,between 700 to 3000 days before the main earthquake),the anomalies are mainly distributed in the southwest and northwest area of the Wenchuan aftershocks area. It is shown that the precursors of the far source region and the near source area have the characteristics of outward expansion. During the β stage (between 300 to 700 days before the main earthquake), the anomalies are distributed in the southwest and northern region of the aftershock region, showing a large range of anomalies. During the γ stage (including γ_1 and γ_2, 300 days before the main earthquake),the range of anomaly distribution is wide,and the anomalies are distributed in the southwest and northeast of the aftershock area. The anomalies converged to epicenter (γ_1) in the far source region and expand outwards (γ_2) in the near source region. Results of the experimental study and mechanical analysis of earthquake preparation process indicate that the three-stage characteristics of precursory anomalies in the process of earthquake preparation may be controlled by the seismogenic body,which is a form of expression in the process of earthquake preparation and a universal featureduring the earthquake preparation process,which has a certain guiding role in earthquake prediction.

The Energy Transformation and Efficiency of the May 12,2008 Ms 8.0 Wenchuan Earthquake

The energy transformation and efficiency is now a hot topic among researches of scientific drilling into fault zones （Tanaka et al., 2006; Ma et al., 2006）. This study conducted temperature measurements and fault gouge particle analysis of borehole WFSD-1 from the Wenchuan Earthquake Fault Science Drilling Project （WFSD）, and discussed the earthquake energy budget. The research progress is illuminated as follows.

2008年汶川M_(S)8.0地震地表破裂带沙坝村吴家院一带几何结构变化特征研究

以往学者通常侧重于大尺度(数公里级别)的地表形态对大地震同震地表破裂的影响研究,而鲜有涉及短距离尺度(10~100 m级)的相关报道。本文基于2008年汶川M_(S)8.0地震地表破裂带沙坝村吴家院一带的几何展布样式与微地貌测绘,为回答短距离尺度是否存在显著的几何结构变化和探究其影响因素提供一个典型案例。具体通过无人机摄影测量技术及激光雷达技术精确的获取了该段落断层展布特征及近断层地形地貌数据,发现吴家院一带地表破裂带在西南侧地形相对稳定无起伏时,破裂带集中为一支断层,而往北东向延伸遇到地形起伏如负地形时,断层分叉为三支分支断层。上述研究结果表明局部范围内地形起伏可能会导致同震地表破裂发生分叉现象,单一断层可能弥散为多支断层。

Relation between the characteristics of strong earthquake activities in Chinese mainland and the Wenchuan earthquake

This paper studies the relations between the great Wenchuan earthquake and the active-quiet periodic characteristics of strong earthquakes, the rhythmic feature of great earthquakes, and the grouped spatial distribution of Ms8.0 earthquakes in Chinese mainland. We also studied the relation between the Wenchuan earthquake and the stepwise migration characteristics of Ms≥7.0 earthquakes on the North-South seismic belt, the features of the energy releasing acceleration in the active crustal blocks related to the Wenchuan earthquake and the relation between the Wenchuan earthquake and the so called second-arc fault zone. The results can be summarized as follows： ① the occurrence of the Wenchuan earthquake was consistent with the activequiet periodic characteristics of strong earthquakes; ② its occurrence is consistent with the features of grouped occurrence of Ms8.0 earthquakes and follows the 25 years rhythm （each circulation experiences the same time） of great earthquakes; ③ the Wenchuan Ms8.0 earthquake follows the well known stepwise migration feature of strong earthquakes on the North-South seismic belt; ④ the location where the Wenchuan Ms8.0 earthquake took place has an obvious consistency with the temporal and spatial characteristic of grouped activity of Ms≥7.0 strong earthquakes on the second-arc fault zone; ⑤ the second-arc fault zone is not only the lower boundary for earthquakes with more than 30 km focal depth, but also looks like a lower boundary for deep substance movement; and ⑥ there are obvious seismic accelerations nearby the Qaidam and Qiangtang active crustal blocks （the northern and southern neighbors of the Bayan Har active block, respectively）, which agrees with the GPS observation data.

A Modified Certainty Coefficient Method(M-CF) for Debris Flow Susceptibility Assessment:A Case Study for the Wenchuan Earthquake Meizoseismal Areas

In the meizoseismal areas hit by the China Wenchuan earthquake on May 12, 2008, the disasterprone environment has changed dramatically, making the susceptibility assessment of debris flow more complex and uncertain. After the earthquake, debris flow hazards occurred frequently and effective susceptibility assessment of debris flow has become extremely important. Shenxi gully in Du Jiangyan city, located in the meizoseismal areas, was selected as the study area. Based on the research of disaster-prone environment and the main factors controlling debris flow, the susceptibility zonations of debris flow were mapped using factor weight method(FW), certainty coefficient method(CF) and geomorphic information entropy method(GI). Through comparative analysis, the study showed that these three methods underestimated susceptible degree of debris flow when used in the meizoseismal areas of Wenchuan earthquake. In order to solve this problem, this paper developed a modified certainty coefficient method(M-CF) to reflect the impact of rich loose materials on the susceptible degree of debris flow. In the modified method, the distribution and area of loose materials were obtained by field investigations and postearthquake remote sensing image, and four data sets, namely, lithology, elevation, slop and aspect, wereused to calculate the CF values. The result of M-CF method is in agreement with field investigations and the accuracy of the method is satisfied. The method has a wide application to the susceptibility assessment of debris flow in the earthquake stricken areas.