Directional seismic response to the complex topography:A case study of 2013 Lushan Ms 7.0 earthquake

Azimuthal variations in site response can provide a good insight into the site amplification and seismic conditions of geohazard occurrences.In this study,multiple directional site response methods,including D-Arias(Directional-Arias),D-SER(Directional-Shaking energy ratio),D-HVSR(Directional-Horizontal to vertical spectral ratio)and D-SSR(Directional–Standard spectral ratio),are adopted to analyse seismic data of the 2013 Lushan Ms 7.0 earthquake captured by the self-established Lengzhuguan(LZG)station which consists of the complex topography of isolated ridge,large mountain and some typical micro-reliefs.The results show that the isolated ridge could cause stronger site responses than the large mountain,and whose pronounced response direction is roughly perpendicular to its ridgeline.With the growth of elevation,the siteresonant frequency decreases.The different microreliefs on the mountain cause different site responses,which present as protruding slope>linear slope.The site response mainly exists on the surficial layer of the mountain and shows that with the increase of the distance to mountain surface,the site response gets weaker,the site resonant frequency gets higher,and the pronounced response direction is perpendicular to its ridgeline.

Geohazards Induced by the Lushan Ms7.0 Earthquake in Sichuan Province, Southwest China:Typical Examples, Types and Distributional Characteristics

Geohazards induced by the Lushan Ms 7.0 earthquake on April 20, 2013 mainly have four types： collapse, landslide, slope debris flow, and sand-soil liquefaction. These geohazards mainly occurred near the epicenter, on steep slopes or below cliffs in high mountain and deep valley areas, and at or near fault ends. They have no obvious relationships to active faults, but their relationships to the weathering degree and structures of rock and rock mass are obvious. Compared with the Wenchuan Ms 8.0 earthquake on May 12, 2008, the Lnshan earthquake is relatively little in the impact force and the throwing amount. All of these should be related to the magnitude of this earthquake, not very large but not very little. This character of the Lushan earthquake would make some processes uncompleted so as to bring about some concealed geohazards. Finally, in order to deal with challenges presented by such conceal geohazards, some brief recommendations are put forward.

Near-field surface deformation during the April 20,2013,Ms7.0 Lushan earthquake measured by 1-Hz GNSS

The April 20, 2013, Ms7.0 Lushan earthquake was successfully recorded by closely spaced Continuous Global Positioning System （CGPS） stations owned by the Crustal Movement Observation Network of Chi- na （CMONC）. The 1-Hz GNSS data from eight CGPS stations, which are located between 30 km and 200 km from the hypocenter, were processed within quasi-real-time. The near-field surface deformation indicated the following characteristics ： the near-field movements were limited to several centimeters ; the peak of the deformation wave was significantly larger than the static permanent offset; at the beginning of the event, the north wall of the fault moved to the southeast as the south wall moved to the southwest ; station SCTQ, which was the closest station to the hypocenter at 30 km, had the largest static permanent displacement of 2 cm; the peaks of the deformation waves were 1.5 cm, 5 cm and 3 cm, to the east, the south and vertically upward, respectively ; and the peaks of velocity and acceleration, derived from the deformation, were 3.4 cm/s and 5.3 cm/s^2,respectively.

Co-and post-seismic surface deformation and gravity changes of MS 7.0 Lushan,earthquake

On April 20, 2013, an earthquake with mag- nitude 7.0 occurred in the southwest of the Longmenshan fault system in and around Lushan County, Sichuan Province, China. This devastating earthquake killed hun- dreds of people, injured 10 thousand others, and collapsed countless buildings. In order to analyze the potential risk of this big earthquake, we calculate the co- and post-seismic surface deformation and gravity changes of this event. In this work, a multilayered crustal model is designed, and the elastic dislocation theory is utilized to calculate the co- and post-seismic deformations and gravity changes. During the process, a rupture model obtained by seismic waveform inversion （Liu et al. Sci China Earth Sci 56（7）： 1187-1192, 2013） is applied. The time-dependent relaxation results show that the influences on Lushan and its surrounding areas caused by the Ms7.0 Lushan earthquake will last as long as 10 years. The maximum horizontal displacement, vertical uplift, and settlement are about 5 cm, 21.24 cm, and 0.16 m, respectively; the maximal positive and nega- tive values of gravity changes are 45 and -0.47 μGal, respectively. These results may be applied to evaluate the long-term potential risk caused by this earthquake and to provide necessary information for post-earthquake reconstruction.

Interseismic and coseismic displacements of the Lushan Ms 7.0 earthquake inferred from leveling measurements

By using precise leveling data observed between 1985 and 2010 across the south section of the Longmenshan fault zone,and eliminating the coseismic displacements caused by the Wenchuan Ms 8.0 earthquake,the interseismic vertical deformation field was obtained.The result shows that the Lushan region,located between the Shuangshi–Dachuan fault(front range of the Longmenshan fault) and the Xinkaidian fault(south section of the Dayi fault),is situated in the intersection zone of positive and negative vertical deformation gradient zones,indicating that this zone was locked within 25 years before the Lushan earthquake.Based on leveling data across the rupture zone surveyed between 2010 and 2013,and by eliminating the vertical deformation within 3 years before the earthquake,the coseismic vertical displacement was derived.The coseismic vertical displacement for the benchmark DD35,which is closest to the epicenter,is up to198.4 mm(with respect to MY165A).The coseismic displacement field revealed that the northwest region(hanging wall) moved upwards in comparison with the southeastern region(foot wall),suggesting that the seismogenic fault mainly underwent thrust faulting.By comparing the coseismic and interseismic vertical deformation fields,it was found that the mechanisms of this earthquake are consistent with the elastic rebound theory; the elastic strain energy(displacement deficit) accumulated before the Lushan earthquake was released during this quake.

A rapid stability assessment of China's IGS sites after the Ms7.0 Lushan earthquake

A rapid and accurate assessment of the stability of surveying and mapping reference points is important for post - disaster rescue, disaster relief and reconstruction activities. Using Precise Point Positioning （PPP） technology, a rapid assessment of the stability of the IGS sites in China was performed after the Ms7.0 Lushan earthquake using rapid precise ephemeris and rapid precise satellite clock products. The results show that the earthquake had a very small impact and did not cause significant permanent deformation at the IGS sites. Most of the sites were unaffected and remained stable after the earthquake.

Did the MS 7.0 Lushan earthquake dynamically trigger earthquakes in the Datong volcanic region(Shanxi Province)?

Immediately following the Ms7.0 Lushan earthquake on April 20, 2013, using high-pass and low- pass filtering on the digital seismic stations in the Shanxi Province, located about 870-1,452 km from the earthquake epicenter, we detected some earthquakes at a time corre- sponding to the first arrival of surface waves in high-pass filtering waveform. The earthquakes were especially noticed at stations in Youyu （YUY）, Shanzizao （SZZ）, Shanghuangzhuang （SHZ）, and Zhenchuan （ZCH）, which are located in a volcanic region in the Shanxi Province,but they were not listed in the Shanxi seismic observation report. These earthquakes occurred 4-50 rain after the passage of the maximum amplitude Rayleigh wave, and the periods of the surface waves were mainly between 15 and 20 s following. The Coulomb stresses caused by the Ray- leigh waves that acted on the four stations was about 0.001 MPa, which is a little lower than the threshold value of dynamic triggering, therefore, we may conclude that the Datong volcanic region is more sensitive to the Coulomb stress change. To verify, if the similar phenomena are widespread, we used the same filtering to observecontrastively continuous waveform data before, and 5 h after, the Ms7.0 Lushan earthquake and Ms9.0 Tohoku earthquake in 2011. The results show that the similar phenomena occur before the earthquakes, but the seis- micity rates after the earthquakes are remarkably increased. Since these weak earthquakes are quite small, it is hard to get clear phase arrival time from three or more stations to locate them. In addition, the travel time differences between P waves and S waves （S-P） are all less than 4 s, that means the events should occur in 34 km around the stations in the volcanic region. The stress of initial dynamic triggering of the Ms9.0 Tohoku earthquake was about 0.09 MPa, which is much higher than the threshold value of dynamic triggering stress. The earthquakes after the Ms9.0 Tohoku earthquake are related to dynamic triggering stress, but the events before the earthquake cannot be linked to seismic events, but may be related to the back- ground seismicity or from other kinds of local sources, such as anthropogenic sources （i.e., explosions）. Using two teleseismic filtering, the small background earthquakes in the Datong volcanic region occur frequently, thus we postulate that previous catalog does not apply bandpass filter to pick out the weak earthquakes, and some of the observed weak events were not triggered by changes in the dynamic stress field.

Shake table tests of suspended ceilings to simulate the observed damage in the M_s 7.0 Lushan earthquake, China

Severe damage to suspended ceilings of metal grids and lay-in panels was observed in public buildings during the 2013 M7.0 Lushan earthquake in China. Over the past several years, suspended ceilings have been widely used practice in public buildings throughout China, including government offices, schools and hospitals. To investigate the damage mechanism of suspended ceilings, a series of three-dimensional shake table tests was conducted to reproduce the observed damage. A full-scale reinforced concrete frame was constructed as the testing frame for the ceiling, which was single-story and infilled with brick masonry walls to represent the local construction of low-rise buildings. In general, the ceiling in the tests exhibited similar damage phenomena as the field observations, such as higher vulnerability of perimeter elements and extensive damage to the cross runners. However, it exhibited lower fragility in terms of peak ground/roof accelerations at the initiation of damage. Further investigations are needed to clarify the reasons for this behavior.

2013年岷县漳县M_(S)6.6地震和2017年九寨沟M_(S)7.0地震震前地球物理观测异常空间分布机理分析

对甘东南地区2013年岷县漳县M_(S)6.6地震和2017年九寨沟M_(S)7.0地震震前地球物理观测异常特征进行总结梳理。根据活动构造单元对地球物理观测台站进行区域划分,统计了相关构造单元上异常的数量和百分比,以及不同学科震前异常数量、百分比、异常持续时间等特征,并对异常的空间分布和机理进行分析,讨论了活动构造对异常分布的影响、异常强度与震源机制及断层应力之间的关系。结果表明:①2013年岷县漳县M_(S)6.6地震比2017年九寨沟M_(S)7.0地震震前地球物理观测异常百分比高,两次地震的震前电磁异常和跨断层水准测量异常均较为显著,而流体异常不明显;②震前地球物理观测异常分布与活动构造相关,2013年岷县漳县M_(S)6.6地震震前异常主要集中在东昆仑-西秦岭断裂带和六盘山-海原断裂带,2017年九寨沟M_(S)7.0地震震前异常则主要集中在龙门山断裂带和东昆仑-西秦岭断裂带;③两次地震震前地球物理观测异常分布均与GNSS速度场分布特征有较好的对应关系;④安德森断层应力模式解释了2013年岷县漳县M_(S)6.6地震(逆冲型)比2017年九寨沟M_(S)7.0地震(走滑型)的形成需要更多的应力积累,因此2013年岷县漳县M_(S)6.6地震虽然震级较小但震前异常更显著。

Characteristics of subsurface density variations before the 4.20 Lushan M_s7.0 earthquake in the Longmenshan area: inversion results

The 4.20 Lushan Ms7.0 earthquake occurred on the southwest segment of the Longmenshan fault on 20 April 2013. Some meaningful information on the prepa- ration and occurrence of this earthquake was found based on the dynamic variation of gravity （DVG）. To examine the great progress of the Lushan earthquake, we obtained the density variation （DENV） derived from the DVG using the compact gravity inversion method in this article. The inversion results reveal three main findings： （1） the DENV in the crust in the Jinshajiang fault area changed from positive in 2010-2011 to negative in 2011-2012. （2） The DENV in the Xianshuihe fault area decreased continuously from 2010 to 2012. （3） The DENV of the uppermost mantle of South China decreased in 2010-2011 and increased in 2011-2012. We propose that the flow/expansion of the middle-lower crust beneath the Bayan Har block and Moho subsidence on the southwest margin of the Chuan-Dian block may have been the major causes of the Lushan earthquake.

New characteristics of intensity assessment of Sichuan Lushan “4.20” M_s7.0 earthquake

The post-earthquake rapid accurate assessment of macro influence of seismic ground motion is of significance for earthquake emergency relief,post-earthquake reconstruction and scientific research. The seismic intensity distribution map released by the Lushan earthquake field team of the China Earthquake Administration(CEA) five days after the strong earthquake(M7.0) occurred in Lushan County of Sichuan Ya’an City at 8:02 on April 20,2013 provides a scientific basis for emergency relief,economic loss assessment and post-earthquake reconstruction. In this paper,the means for blind estimation of macroscopic intensity,field estimation of macro intensity,and review of intensity,as well as corresponding problems are discussed in detail,and the intensity distribution characteristics of the Lushan '4.20' M7.0 earthquake and its influential factors are analyzed,providing a reference for future seismic intensity assessments.

Deep Seismogenic Environment in the Southern Section of the Longmenshan Fault Zone on the Eastern Margin of the Tibetan Plateau and Lushan M_s 7.0 Earthquake

The 2,026 earthquake events registered by the Sichuan regional digital seismic network and mobile seismic array after the April 20th, 2013 Lushan earthquake and 28,188 pieces of data were selected to determine direct P waves arrival times. We applied the tomographic method to inverse the characteristics of the velocity structure for the three-dimensional （3D） P wave in the mid-upper crust of the seismic source region of the Lushan earthquake. The imaging results were combined with the apparent magnetization inversion and magnetotelluric （MT） sounding retest data to comprehensively study the causes of the deep seismogenic environment in the southern section of the Longmenshan fault zone and explore the formation of the Lushan earthquake. Research has shown that there are obvious differences in velocity structure and magnetic distribution between the southern and northern sections of the Longmenshan fault zone. The epicenter of the Lushan earthquake is located near the boundary of the high and low-velocity anomalies and favorable for a high-velocity section. Moreover, at the epicenter of the Lushan earthquake located on the magnetic dome boundary of Ya＇an, the development of high velocity and magnetic solid medium favors the accumulation and release of strain energy. Low- velocity anomalies are distributed underneath the are of seismogenic origin, The inversion results of the MT retest data after the April 20th Lushan earthquake also indicate that there a high-conductor anomaly occurs under the area of seismogenic origin of the Lushan earthquake, Therefore, we speculated that the presence of a high-conductivity anomaly and low-velocity anomaly underneath the seismogenic body of the Lushan earthquake could be related to the existence of fluids. The role of fluids caused the weakening of the seismogenic layer inside the mid-upper crust and resulted in a seismogenic fault that was prone to rupture and pIayed a triggering role in the Lushan earthquake.

Abnormal strain changes observed by a borehole strainmeter at Guza Station before the Ms7. 0 Lushan earthquake

Several days before the MsT. 0 Lushan earthquake, the YRY-4 borehole Strainmeter at Guza Station recorded prominent abnormal changes. The strain anomalies are very striking on the smooth background of several years＇ recording after the Wenchuan earthquake. However, because construction in the town of Guza has been undergoing rapid development in recent years, many factors have interfered with observations at the station. Whether or not the observed strain changes before the Lushan earthquake were affected by any of the sources of interference becomes a question that must be answered. Among the likely sources of interference, apartment construction, sportsground reconstruction, and tunnel cutting can be excluded by analyzing the morphological characteristic of the anomalies. The two remaining most possible sources are road construction in front of the station and the water level change of the nearby Dadu River caused by water filling into and discharging from an upstream reservoir. Through field investigation, comparison of the correlation between the strain and the seismographic recordings, comparison of the correlation between the strain and the Dadu River flow recordings, and analysis of the strain anomaly characteristics, we conclude that the abnormal changes observed at Guza Station cannot be attributed to either of these two sources but should be related to the Lushan earthquake.

Detection of a half-microgal coseismic gravity change after the Ms7. 0 Lushan earthquake

Because only a small near-field coseismie gravity change signal remains after removal of noise from the accuracy of observations and the time and spatial resolution of the earth＇s surface gravity observation system, it is difficult to verify simulations of dislocation theory. In this study, it is shown that the GS15 gravimeter, located 99.5 km from the epicenter of the Ms7.0 Lushan earthquake on April 20, 2013 at 08 ： 04 UTC ＋ 8, showed the influence of the earthquake from 2013-04-16 to 2013-04-26 after a time calibration, tide correc- tions, drift correction, period correction and relaxation correction were applied to its data. The post-seismic relaxation process of the spring in the gravimeter took approximately 430 minutes and showed a 2. 5 ×10^-8 ms^-2 gravity change. After correcting for the relaxation process, it is shown that a coseismic gravity change of approximately ＋0.59 ＋-0. 4 ~ 10-Sms-2 was observed by the GS15 gravimeter; this agrees with the simulated gravity change of approximately 0.31 ~ 10 -8 ms-2. The rate of the coseismie gravity change and the coseismic vertical displacement, as measured by one-second and one-day sampling interval GPS units, is also consistent with the theoretical rate of change. Therefore, the GS15 gravimeter at the Pixian Station observed a coseismic gravity change after the Ms7.0 Lushan earthquake. This and similar measurements could be applied to test and confirm the theory used for these simulations.

Simulation of coseismic effects of the Ms7. 0 Lushan earthquake

Using plane dislocation theory and the seismic-wave inversion results from the Institute of Geophysics, China Earthquake Administration and the Institute of Geodesy and Geophysics, Chinese Academy of Sciences models, the surface coseismic deformation and gravity changes caused by the 2013 Ms7.0 Lushan earthquake are simulated. The simulations of coseismic gravity change and deformation indicate that the dislocation has dip-slip characteristics. The results also show that the coseismic deformation exhibits a symmetrical, positive-and-negative distribution, with the deformation usually being less than 10 mm in the far- field but up to 140 mm in the near-field. The gravity changes are concentrated on the fault-projection area, which is greatly affected by the vertical surface deformation. The gravity change and vertical deformation in the far field are usually less than and 5 mm, respectively, but reach and 330 mm, respectively, in the near field. The simulated results agree well with the measured resuhs, which suggests a theoretical basis for the observed change in gravity before and after this earthquake.

Factor analysis of earthquake-induced geological disasters of the M7.0 Lushan earthquake in China

The seismic intensities, lithologic characteristics and terrain features from a 3000 km2-region near the epicenter of the Lushan earthquake are used to analyze earthquake-induced geological disaster. The preliminary results indicate that secondary effects of the earthquake will affect specific areas, including those with glutenite and carbonate bedrock, a seismic intensity of IX, slopes between 40° and 50°, elevations of less than 2500 m, slope change rates between 20° and 30°, slope curvatures from - 1 to -0.5 and 0. 5 to 1, and relief between 50 and 100 m. Regions with susceptibility indices greater than 0.71 are prone to landslides and collapses. The secondary features are mainly distributed on both sides of the ridges that extend from Baosheng to Shuangshi and from Baosheng to Longxing. Other features are scattered on both sides of the ridges that extend from Qishuping to Baosheng and from Masangping to Lingguan. The distribution of the earthquake-related features trends in the NE direction, and the area that was most affected by the Lushan earthquake covers approximately 52.4 km^2.

Upper crustal anisotropy observed around the Longmenshan fault in the 2013 M_S7.0 Lushan earthquake region

Based on the shear wave splitting analysis of the seismic recordings at 17 temporary stations and three permanent stations, we measured the shear wave splitting parameters(i.e., the polarization direction of fast shear wave and the time delay of slow wave) to perform a systematic analysis of the crustal seismic anisotropy around the Longmenshan fault in the 2013 M7.0 Lushan earthquake region. We observed apparent spatio-temporal characteristics in the shear wave splitting parameters. The spatial distribution of fast polarization directions showed a clear partitioning in the characteristics from northwest to southeast in the focal region,which changed from NW-SE to NE-SW. In the northwest of the focal region, the fast polarization direction was oriented to NW-SE, which was parallel to the maximum horizontal compressive stress direction. However, the NE-SW fast polarization direction in the southeast of the focal region was parallel to the Longmenshan fault strike. For station BAX on the Central fault in the middle of the focal region, the distribution of fast polarization directions showed a bimodal pattern, with one dominant in the NE-SW direction and the other in the NW-SE direction. With regard to the temporal variation, the time delays were large in the initial stage after the mainshock but then gradually decreased over time and tended to be stable in the later period. This indicated that stress in the focal region increased to a maximum when the main shock occurred, with the stress release caused by the mainshock and aftershock activity, and the stress gradually decreased after a period of time. The scatter of fast polarization directions was large after the main shock, but over time the scatter gradually decreased, indicating that the Lushan earthquake caused a large perturbation in the local stress field. As the stress gradually decreased and was adjusted by the aftershock activity, the perturbation gradually weakened.

Possible thermal brightness temperature anomalies associated with the Lushan(China) M_s7.0 earthquake on 20 April 2013

Lushan Ms7.0 earthquake occurred in Lushan county, Ya＇an city, Sichuan province of China, on 20 April 2013, and caused 196 deaths, 23 people of missing and more than 12 thousand of people injured. In order to analyze the possible seismic brightness temperature anomalies which may be associated with Lushan earthquake, daily brightness temperature data for the period from 1 June 2011 to 31 May 2013 and the geographical extent of 25°E-35°N latitude and 98°E-108°E longitude are collected from Chinese geostationary meteorological satellite FY-2E. Continuous wavelet transform method which has good resolution both in time and frequency domains is used to analyze power spectrum of brightness temperature data. The results show that the rela- tive wavelet power spectrum （RWPS） anomalies appeared since 24 January 2013 and still lasted on 19 April. Anomalies firstly appeared at the middle part of Longmenshan fault zone and gradually spread toward the southwestern part of Longmenshan fault. Anomalies also appeared along the Xianshuihe fault since about 1 March. Eventually, anomalies gathered at the intersection zone of Longmenshan and Xianshuihe faults. The anomalous areas and RWPS ampli- tude increased since the appearance of anomalies and reached maximum in late March. Anomalies attenuated with the earthquake approaching. And eventually the earthquake occurred at the southeastern edge of anomalous areas. Lushah earthquake was the only obvious geological event within the anomalous area during the time period, so the anomalous changes of RWPS are possibly associated with the earthquake.

青藏高原东南缘三维S波速度和径向各向异性及泸定M_(S)6.8和芦山M_(S)7.0地震孕震环境探讨

利用青藏高原东南缘布设的684个流动地震台和150个固定地震台的连续背景噪声波形数据,提取了周期5~50 s的Rayleigh波和Love波相速度频散,并反演获得了该区域下方0~70 km三维S波速度和径向各向异性结构.结果表明:(1)受川滇地块南部高速体的阻挡,东南缘中下地壳内存在两条空间分布独立的低速带.西侧(L1)从川滇地块北部向南延伸至滇西南地块,其中下地壳平均V_(S)小于3.4 km·s^(-1)并表现为正径向各向异性结构,反映了高原中下地壳可能存在部分熔融和韧性变形;东侧低速带(L2)沿着小江断层南北分布,受到红河断层的阻挡,弱物质不太可能进入滇西南地块,该区域下地壳和上地幔顶部均显示较强的正各向异性,推测其更可能是沿小江断层相互驱动的块体在横向挤压过程中引起的地壳部分熔融而导致.(2)芦山M_(S)7.0地震位于龙门山逆冲断层南段,泸定M_(S)6.8地震位于鲜水河走滑断层东南段,两者都发生在地壳浅层高低速异常过渡带上,但其深部孕震环境有所不同.芦山震区西北部中下地壳低速体为负各向异性,推测韧性物质沿着龙门山下方陡倾断层向上运移,上地壳积累应力并通过薄弱区域释放导致了芦山地震的发生;泸定震区为正各向异性的中下地壳韧性变形促进了川滇地块SE向的水平运动,受刚性的四川盆地阻挡,加剧了上地壳发震断层的滑动变形和应力积累,脆性上地壳突然破裂导致了泸定地震的发生.

Rapid report of June 1,2022 M_(W) 5.9 Lushan earthquake,China with geodetic and teleseismic data

Timely response to earthquake characterization can facilitate earthquake emergency rescue and further scientific investigations.On June 1,2022,M_(W) 5.9 earthquake occurred in the southern area of the Longmenshan fault zone.This event also happened at the south end of the Dayi seismic gap and is the largest earthquake that has occurred in this seismic gap since the 1970 M 6.2 event.The slip-distribution model constrained by the seismic waveforms suggests a thrust-dominated faulting mechanism.The main slip occurs at a depth of~14 km,and the cumulative energy is released in the first 6 s.The variations of Coulomb stress caused by the mainshock show a positive change in the southwest area of the Dayi seismic gap,indicating possible activation of future earthquakes.In addition,we emphasize the importance of rapid estimation of deformation for near-field hazard delineation,especially when interferometric radar fails to image coseismic deformation in a high relief terrain.