Abstract:Nowadays, the geological conditions in internal crust are complicated, but the intersection of epicenter migration could indicate regions where the velocity of tectonic force is high, which means the intersection of epicenter migration can contribute to the prediction of large earthquakes.

Abstract:We solved the five focal mechanisms of moderate and small swarm earthquakes occurring since 1999 in Yingkou and Haicheng using the P-wave first-motion method. We defined the sum of the inclinations of three orthogonal and principal stress axes of the focal mechanisms for the moderate and small earthquakes and that of the tectonic stress field in corresponding areas in the 3D coordinate system as parameter of the consistency of the mechanisms. It is a quantitative index used to measure the release stress field of an earthquake source and the conformance degree of the regional tectonic stress field. The conformance degree of the regional stress field and the focal mechanisms increase as the value decreases. The conformance degree of the regional stress field and the focal mechanisms of moderate and small earthquakes are higher because of the enhanced tendency of the regional tectonic stress field and higher possibility of occurrence of a stronger earthquake in the future. The result shows that the parameter of the consistency of the focal mechanisms was reduced to a minimum value of approximately 10~60 days before the occurrence of larger earthquakes for the earthquake swarms in Yingkou and Haicheng. Time and fluctuation range of the low value may be related to the characteristics of the earthquake swarm. The consistency parameter fluctuated significantly in common swarms such as the 2008 Haicheng earthquake swarm and the 2012 and 2013 Qingshiling earthquake swarms in Gaizhou City. All the mean values of the consistency parameter were greater than 125°. The value was low before the occurrence of the two stronger earthquakes (M5.4 on November 29, 1999 and M5.1 on January 12, 2000) in the foreshock sequence in Xiuyan. The value varied between 20° and 30°, and the minimum value was less than 65°. The consistency parameter did not vary significantly, and the mean value is not high (73°) for the 2014 earthquake swarms of Xihaiyu and Gaizhou, thus indicating some strengthening tendency of the stress field. However, because the consistency parameter is not consistently low, further research must be conducted in the future. Nevertheless, the parameter of consistency of focal mechanism is more useful for judging the stress condition in the focal region than using the P-axis trend of focal mechanism.

Abstract:After the completion of projects as part of the ninth and tenth five-year plans, the geomagnetic observatories networks of Gansu and Qinghai provinces have become densely distributed. The Gansu network has four geomagnetic stations and a geomagnetic array, while there are eight geomagnetic stations in the Qinghai network. The two networks have produced a large amount of geomagnetic data since 2008.
The Gansu-Qinghai region (33°~43° N, 94°~107° E) was identified as the study area for this study. The geomagnetic data in the study area was analyzed using the harmonic amplitude ratio method, and the relationships between the results and the earthquakes in the Gansu-Qinghai region were investigated. Feng Zhi-sheng et al. analyzed the geomagnetic humorous wave amplitude ratios of Chongming, Sheshan, Kashi, and other geomagnetic stations and reported the existence of down-turning-recovery characteristics before and after medium-to-strong earthquakes, which are similar to the earthquake precursor anomaly variations of earth resistivity. We analyzed the geomagnetic harmonic amplitude ratios of Jiayuguan, Shandan, Dulan, Huangyuan, and Yingge geomagnetic stations and reached the same conclusions. The amplitude ratio method has been widely employed for earthquake prediction. The duration of geomagnetic humorous wave amplitude ratio curve abnormalities of earthquakes with M≥5 is 1~3 years. Earthquakes typically occur at the abnormal twist or recovery points. The anomaly amplitude varies between 0.03 and 1.30. Feng Zhi-sheng et al. reported the occurrence of migration phenomenon among the curves of different periods. Further, there may be some correlation between the distance of the phenomenon and the earthquake epicenter. In this study, the migration phenomenon was observed only in the curves corresponding to the Shandan station, and no obvious correlation was observed between the migration phenomenon and the earthquake epicenter.
The judgment standard must be determined when analyzing abnormal variations and summarizing earthquake case studies. Researchers analyze abnormal variations in geomagnetic humorous wave amplitude ratio curves using the earth resistivity anomaly judgment standard and the characteristics of different items or different periods during the same item. The starting times of inconsistent variations in the geomagnetic humorous wave amplitude ratio curves of Jiayuguan and Shandan geomagnetic stations are the same. Normally, if the geomagnetic humorous amplitude ratio curves of different item or different periods during the same item show a poor synchronization, it indicates an abnormal body near the distance station. In the future, we will focus on the variations in harmonic amplitude ratios of the two abovementioned geomagnetic stations.

Abstract:On June 30, 2012, an M_S6.6 earthquake occurred at the border of Xinyuan and Hejing in Xinjiang. This study inversed the spatial distribution of the consistency parameter Var (variance) and the azimuth of the mean maximum principal compressive stress P axis and found some changes in these parameters. First, on the basis of the waveform data of January 2009-May 2013 recorded by the Xinjiang Seismic Network Center, we used the cut-and-paste and P-wave first motion methods to inverse the focal mechanism solutions of 418 small-to-moderate earthquakes around the main shock area of the above mentioned M_S6.6 earthquake. Then, we inversed the consistency parameter and azimuth variation of the mean principal compressive stress P axis. From the study results, we derive three conclusions. First, prior to the main shock, around the seismogenic zone, the Var value was relatively low, which means that the earthquake action was affected by the stress field of the seismogenic zone. After the main shock, the Var value significantly increased, which means that the control of the background stress field began to strengthen. Second, prior to the main shock, the stress near the epicenter area began to concentrate, and the azimuth of the mean maximum principal compressive stress P axis was consistent with the main shock. However, after the main shock, this azimuth exhibited a certain angle from the main shock and was consistent with the azimuth of the regional stress field. Third, prior to the main shock, the Var value was relatively low, and the azimuth of the maximum principal compressive stress P axis was not consistent with the historical average value.

Abstract:During the Wenchuan M_S8.0 earthquake on May 12, 2008 and the Lushan M_S7.0 earthquake on April 20, 2013, surface rupture phenomena did not occur on the Yanjing-Wulong fault in the southern segment of the Longmenshan central fault zone. Moreover, shallow geophysical data for the Yanjing-Wulong fault are extremely scarce. The seismic hazard and the capacity for potential earthquakes were evaluated in the southern section of the Longmenshan fault zone to a certain extent. However, the major counties (towns) crossed by the Yanjing-Wulong fault are mostly located in valleys with widths less than 300 m. As a result, such detection areas are subject to inconvenient traffic conditions and narrow working spaces. Therefore, detection through shallow seismic reflection must be applied with small trail spacing, small displacement distance, reception of multiple short arrangements, multiple coverage observations of common reflection points, and data processing. In the present study, shallow seismic reflection is combined with high-density resistivity tomography, trench excavation, and drilling to reveal the size, spatial distribution, and near-surface structure of the NE-trending Yanjing-Wulong fault in the area of Dongfeng village, Wulong, Baoxing County. These detection results not only provide reliable seismological evidence for determining the near-surface activity of the Yanjing-Wulong fault but also offer scientific fundamental data for selecting sites for post-disaster reconstruction, earthquake risk assessment, and planning of earthquake resistance and hazardous prevention in the Baoxing County.

Abstract:In this study, we analyzed the characteristics of different types of earthquakes occurring in the north rim of the Tibetan Plateau, Tianshan, and Altai in view of regional geodynamic background, focal mechanisms, and research results of seismic sequences of the Xinjiang area considering the effect of earthquake rupture and sequences.
West Kunlun is located in the eastern wing of Pamir arc, with a tectonic style of extrusion conversion system consisting of both strike-slip and thrust structures, thus indicating upliftment of West Kunlun in the Cenozoic.In the area, earthquakes have a combination of characteristics of strike-slip and major-after earthquakes, with the dominant strike-slip characteristics accounting for 60% of the sequences, and major-after characteristics accounting for 63% of sequences. Earthquakes with magnitudes greater than 6 are primarily major-after and individual multiple-shock type earthquakes. Seismic activity characteristics of major-after earthquakes indicate that the stress state of West Kunlun is strong and the rupture strength is medium.
The Arkin fault has been pushed to the north and west by the Qingzang Block. The escape wedge formed by the Arkin and Karakoram faults causes the thrust of northern West Kunlun and North Qilian faults to gradually transit through the Arkin fault zone. On the other hand, the sinistral displacement of the Arkin fault transforms through both-end thrust extrusion, resulting in an arc bending at the eastern and western end of the West Kunlun and North Qilian. Therefore, the northern border of the whole plateau extends outward. Similarly, earthquake sequence types in the Arkin fault are characterized by strike-slip, major-after, and isolated earthquakes. Strike-slip ruptures are dominant in the area, and major-after and isolated earthquakes each account for 44% of the seismic sequence. Isolated earthquakes typically have a magnitude below 6.5. The 7.3 magnitude Yutian earthquake that occurred in 2014 conformed to the characteristics of this type of earthquake.
Owing to the effect of India-Asia continental collision and its effect on the later continental convergence distance, Tianshan shows complete vertical crust shortening under the horizontal compressive, oblique shear transform deformation, and lateral deformation expanding on both sides of the basin. Active faults include reverse and strike-slip faults with rotating characteristics. Seismic rupture mode of Tianshan is complex, with strike-slip and thrust earthquakes being dominant (46% and 38%, respectively). Contact ways of tectonics in different directions and movement characteristics add to the complexity and diversity of seismic sequence types in the Tianshan region. Major-after earthquake sequences account for 50% of seismic sequences, whereas isolated earthquake sequences account for approximately 40% of sequences. In individual regions such as the western part of south Tianshan, tectonics crisscross with strong movement, and the earthquake sequence shows multiple aftershocks.
The Altai fault exhibits dextral shear dislocation under NNE and SW horizontal extrusion, and some of the deep fractures experienced significant thrust movement with obvious fracture topography. Strike-slip, thrust and major-after, and isolated combined earthquake seismic types are formed under different conditions such as regional tectonic stress and fault movement. Isolated earthquakes are primarily medium earthquakes with a magnitude of approximately 5.

Abstract:Focal depth, the vertical distance from an earthquake source to the ground (the epicenter), is one of the basic earthquake parameters and is one of the most difficult of the seismic parameters to accurately determine. The accurate determination of the focal depth involves a series of important problems including a correct understanding of the earthquake preparation process, the earthquake mechanism, its geological structure, and the stress field. Since the fifteen stations of the Ningxia digital seismic observation network were officially established in 2009, a large volume of digital seismic waveform data has accumulated, and much research work has been possible. On the basis of previous studies, using the lag of the sPn and Pn phases, in this study, we calculated the focal depth of nine earthquakes with M≥3.0, which occurred in the area surrounding Ningxia in recent years. The results of the sPn method are mainly dependent on the velocity model selection and precision of the seismic phase. We selected the approved and widely used Ningxia local velocity model. The Pn and sPn phases and reading accuracy are particularly important, because the initial amplitude of this type of wave phase is generally weak. As such, if there is an arrival time difference of 0.5~1 s, it may result in a calculation error of 1.5~3 km in the focal depth. With the availability of multiple records of the sPn phase, using the averages of each calculation depth, it is possible reduce the error caused by the reading. Research shows that it is a relatively simple matter to accurately calculate the near-earthquake depth within the crust using the lag of the sPn and Pn phases. Determining the exact value of the focal depth can provide an important criterion in earthquake research with respect to the causal factors of the formation and structure of deep crustal activities.

Abstract:In this study, we analyze the distribution characteristics of Q value of seismic coda in Ningxia and neighboring areas by considering 665 digital wave data points of earthquakes with M_L≥2.0 from January, 2008 to December, 2013 recorded by the Ningxia Earthquake Network using the Aki model. The selected waveform records are clear, and the noise levels satisfy the requirements. By comparing the Q values of seismic coda in each area of the Ningxia Province and its adjacent region with existing results, the distribution characteristics of Q value of seismic coda are obtained. The results show that the Q value of seismic coda is higher in the north than in the south. The Q values of seismic coda in Wuzhong and Lingwu areas are relatively high, corresponding to the high-speed zone. The Q values of seismic coda in the region southwest of Alashan Zuoqi and those in Zhongwei, Haiyuan, and their adjacent regions are relatively low. These results may be associated with the high temperatures and heat-flow values in the deep crust of those areas.

Abstract:In this study, remotely sensed infrared brightness temperature data from the China Geostationary Meteorological Satellite FY-2C/E from 2006 to 2013 are full time-space analyzed to determine the dominant frequency and amplitude using the relative power spectrum method. The variation in time and space in the relative power spectrum of each frequency was analyzed and compared with the three elements of 17 earthquakes having magnitudes greater than 5.0 in Zhejiang Province and its neighboring area. The following results were obtained: the relative power spectrum anomaly of infrared brightness temperature appears before nine earthquakes. The corresponding ratio of the first three dominant frequencies is higher than that of last three ratios. The anomalies occurred several days or months before earthquakes, and most earthquakes occurred after the anomalies reached the maximum value or were recovering. A few earthquakes occurred before the anomalies reached the maximum value. The anomalies increase in strength and then disappear, in a process from sporadic to conglomerate or banded along the fault zone. Anomaly time and amplitude and the time period from the appearance of the anomaly until the earthquakes occurred have no obvious relation with the three elements of the earthquake.

Abstract:The load/unload response ratio (LURR) aims to identify a physical parameter, which reflects the damaging process in the seismogenic zone, and to use this parameter to predict an earthquake. In the seismically quiet period, the LURR value fluctuates around 1. Nevertheless, when the rock is close to failure or just before a strong earthquake, the value rises significantly, and it is much larger than 1. The LURR probably decreases sharply and quickly before the main shock. Therefore, the anomalous increase in the LURR time series covering a time interval of months to years may be efficiently used in earthquake prediction. In the present study, the spatio-temporal evolution of LURR is analyzed before moderate earthquakes from 1990 to 1999. Based on the LURR theory, the seismogenic integral is deduced, which is applied to predict the earthquakes during the studies interval. The results show that high Y values are observed with an elliptical or circular spatial distribution around the epicenter before the main earthquake. In addition, earthquakes usually occurred during periods of abnormally high fluctuations. In retrospective historical earthquake studies, the future earthquakes are usually located in the abnormal zones or at the edge abnormal areas. However, there is a significant difference in terms of the starting time, and the amplitude and area of abnormity are different because of the different magnitude and seismogenic zone. An obvious anomaly, with a probability of approximately 78.8%, is observed within 200 km from the epicenter before moderately strong events. In addition, compared with the theoretical time, the statistical time from the peak to the earthquake is shorter, which may indicate that the tectonic activities in Xinjiang are intensive, the pregnant period is short, and the rate of the shear strain is large.

Abstract:In this study, we developed a program to analyze the time-frequency spectrum of seismology events using MATLAB software. Among all the time-frequency analysis tools, we selected short-time Fourier transform (STFT) owing to its computational simplicity, adequate time and frequency resolution, and absence of undesirable cross terms. One disadvantage of STFT is the tradeoff between window length and time and frequency resolution. In order to determine the optimal window length, we wrote the code according to the sample rate of data records. We collected 82 broadband records, including data of 27 stations, of 5 natural earthquakes and 5 explosion events that occurred in the capital area between 2006 and 2008 having a magnitude in the range of 2.4 to 2.7. For the sake of convenience, we selected five pairs of events, each pair with one natural earthquake and one explosion event at similar epicenter distance, and analyzed the time-frequency spectrum. We plotted the time-frequency spectrum for each record with its original waveform and Fourier spectrum including the three components and their combination. Different time-frequency patterns exist for natural earthquakes and explosion events. In the first place, more peaks can be seen in the natural earthquake time-frequency spectrum, while the explosion events spectrum has fewer peaks. In the second place, the peaks of natural earthquakes are located in a broad frequency band, whereas the peaks of explosion time-frequency spectrum are concentrated in a narrow area. We analyzed all the data, not restricted to the five pairs of events, and the time-frequency spectrum pattern was found to be consistent. However, this pattern may be applicable only in the capital area because the explosion techniques are fairly complicated. The operator can use completely different explosion plans, and different local seismological wave propagation models produce diverse wave fields. When compared with Fourier analysis, the time-frequency spectrum, which is the time domain expansion of Fourier Spectrum, provides more information.

Abstract:Because of environmental pollution, the frequency of acid rain is increasing. In rural areas and small towns in China, many ancient buildings are masonry structures that are easily corroded by acid rain. Because of the poor seismic performance of masonry structures subjected to acid rain, research on these structures corroded by acid rain has become increasingly important. In this study, we designed four brick walls with common characteristics and tested them with a series of 0, 100, 200, and 300 corrosion cycles of acid rain. Then, using a low reversed cyclic loading test, we obtained the hysteresis curve of the four brick walls under these corrosion cycles. Based on the three-spring element model combined with the Lu Xinzheng-Qu Zhe restoring force model, we used the finite element software Marc to simulate the response of the brick walls. Our experimental results proved that the three-spring element model can accurately predict the hysteretic properties of masonry structures. To verify the rationality of using the three-spring element model to simulate the whole structure, we used it to construct a finite element model of a teaching building for modal analysis and compared it with the ABAQUS modal analysis results of previous research. Our results show that the three-spring element model can better reflect basic dynamic structural characteristics and satisfy requirements for nonlinear earthquake analysis. We also summarize domestic and overseas drift allowance angles of different damage states combined with our experimental results and China's national code to demarcate appropriate drift allowance angles. We used 15 seismic waves from the ATC-63 project to perform an incremental dynamic analysis of structures and to obtain seismic vulnerability curves under different corrosion cycles. Based on our analysis of the failure probability for different structural damage states from small, medium, and large earthquakes, acid rain can seriously affect structural mechanical properties and significantly reduce seismic performance.

Abstract:Using a GDS hollow cylindrical torsional shear apparatus to model the stress path of strain-induced vibration, we investigated the dynamic strength of Nanjing fine sand under different loading conditions. To simplify the loading method, we modeled the relationship of the normal and shear stresses with an oval, rather than heart-shaped, stress path. Our results show that when the confining pressure and the amplitude of the train-induced vibration are small, the dynamic strength of the Nanjing fine sand first increases and then stays stable over time. However, when the confining pressure increases, the increasing stage of the dynamic strength of the Nanjing fine sand is very short and the dynamic strength values are also much smaller. After some vibration time, the dynamic strength of the Nanjing fine sand decreases almost linearly with increasing vibration. Also, as the amplitude of the vibration increases, the dynamic strength weakens more quickly. This also proves that the drain condition mainly affects the increasing stage of the dynamic strength and has very little influence on its weakening stage.

Abstract:Due to reduce the damaging effect of superstructures subjected to strong earthquakes, isolation technology is widely used in infrastructure construction, such as in hospitals, teaching buildings, and so on. Many significant research achievements have been achieved with respect to base-isolated structures (BISs). Although isolation technology has been maturing over several decades of development, a number of questions remain. The influence on BISs subjected to strong ground motion, and environmental and other loads, the rationality of structure design, and the performance of BISs buildings that have experienced earthquakes must still be verified by structural health monitoring operation. A BIS structural health monitoring system consists of sensory system, data acquisition and transmission system, data processing and control system, structural health data management system, structural health evaluation system, and inspection and maintenance system. Based on site inspections, prior monitoring of dynamic parameters based on BIS characteristics has been proposed. The monitored data should include seismic ground motion, temperature and humidity of the isolation layer, the foundation settlement and wind load (high-rise BISs), horizontal and vertical static displacement of the isolation bearings, vertical strain on the isolation bearings and strain of the isolation layer girder, and horizontal and vertical dynamic displacement of the isolation bearings and acceleration response of the superstructure. Considering the BIS characteristics, in this paper, we propose the primary subjects to be monitored, and consider the general overall design requirements of a BIS health monitoring system. According to different monitoring variables (global and local), we propose basic principles for the sensors selection and layout, and the hardware and software designs of the data acquisition and transmission system. We also present approaches to the design verification and safety performance evaluation. Finally, we address the problems which need to be further studied with respect further study with respect to the influence rule for isolation bearing in the construction process, concrete shrinkage of the superstructure, and setting the site of the post-poured strip; temperature load spectrum and a correlation model of BISs with temperature; how to assess the performance of a BIS while isolation bearing in parallel caused an initial displacement; questions about the fatigue of BISs subjected to earthquakes and wind load; and questions about the rule of progressive and anti-progressive collapses of BISs.

Abstract:With the continuous development of structural seismic isolation technology, the use of seismic-isolation-device bridge designs is growing. The combination of isolation and additional damping devices is a commonly used method for controlling curved-beam bridges. Analyses of vibration control for research in the evaluation of seismic dynamic response have primarily focused on deterministic excitation, but deterministic earthquake excitation is not representative. In this study, we consider ground motion to be a uniformly modulated nonstationary random process and investigate long periods of low-frequency characteristics. Moreover, we select the Clough-Pension steady vibration power spectral model as a random vibration input for isolated curved bridges. To address the limitations of the classical optimal control algorithm, we derive vibration control equations using a sequential optimal control (SOC) algorithm. We then analyze the random responses of a curved bridge under three conditions: noncontrol, classical linear optimal control, and SOC algorithm. By establishing a curved beam bridge vibration equation of motion for random actions, we determine the displacement vibration control structure of the spectral density, acceleration spectral density response, and time variance. Analysis results show that the SOC algorithm can reduce the displacement of the isolation layer and more effectively control the seismic response of the upper structure, thus yielding a better control effect. The SOC algorithm has higher control performance and achieves better damping control.

Abstract:This paper proposes a new probability model for the fully nonstationary ground motion acceleration process, and provides a kind of seismic input for structures for use in seismic research. First, based on the Clough-Penzien power spectrum of the stationary ground motion process, we establish the evolutionary power spectrum of the fully nonstationary ground motion process. Most importantly, the evolutionary power spectrum considers not only the nonstationary intensity, but also the nonstationary frequency. Then, based on the seismic design building code (in China), we identify these parameters in the evolutionary power spectrum model for different site conditions. Meanwhile, we applied the spectral representation-random functions method to generate an ensemble of 152 representative samples, and assigned each representative sample a given probability. This method uses a few basic random variables to express the original seismic ground motion process. Next, we obtain second-order statistics and the seismic response spectrum of the representative sample ensemble, and compare them with the target values. The results show that the general characteristics of all 152 representative samples coincide well with the target values. In this study, to verify the superiority and effectiveness of the proposed method, we used ANASYS software to conduct a dynamic time-history analysis of a concrete framework. Finally, by combining the recent probability density evolution method with the control criteria for the displacement angle, we performed a stochastic dynamic response analysis and reliability calculation for the concrete framework.

Abstract:There are many deep excavations located in major cities because of the rapid development of underground engineering. The existence of deep excavations will affect the seismic response of the ground. However, there is great uncertainty concerning the dynamic performance of the adjacent ground and the current anti-earthquake design codes do not provide enough consideration to the problem. In this paper, a dynamic numerical simulation was performed to analyze the influence of deep excavations on the peak acceleration of the adjacent ground. In addition, the variation in peak acceleration along the depth was studied. The results show that the peak acceleration of the adjacent ground will significantly increase within a distance of two times the excavation depth.

Abstract:Local strong ground motion can be significantly affected by site conditions. The characteristics of low-velocity surficial sediments play an important role in local site effects. Previous numerical studies and analyses of observation data have suggested that the amplification of strong ground motion is related to thickness and the properties of low-velocity sediments. However, these analyses have mainly been based on one-dimensional-layered sedimentary models. In this study, we use a two-dimensional lateral heterogeneous model to study the effects of low-velocity surficial sediments on strong ground motion. The constructed numerical model includes the seismic source, an elliptical sedimentary basin, and a low-velocity surface sedimentary layer above the basin. We simulated seismic wave propagation in this model using a hybrid finite difference and pseudospectral method on staggered grids. The results show that multiple reflections and conversions occurring within the low-velocity sedimentary layer enhance the duration of ground motion, and their constructive interference significantly enlarges the peak ground amplitude. The most significant amplification of the peak amplitude occurs near the edges of the basin. The horizontal peak ground amplitude shows a stronger amplification than the vertical component inside the basin. With increasing wave velocity in the surficial layer, the energy of multiple reflections and conversions decreases within the sedimentary layer. The duration of the ground motion decreases as well. However, the amplification of strong ground motion shows similar patterns for models with different thicknesses and seismic velocities. The study results show the mechanism of ground motion amplification by the low-velocity surficial layer as a process of seismic wave propagation. This suggests that the amplification mechanism of the surface sedimentary layer on the seismic ground motion for models that include the source, sedimentary basin, and surface layer is more complicated than that from a one-dimensional-layered model. The ground motion amplification is a joint effect of the sedimentary properties (thickness and seismic wave velocity), lateral heterogeneity of the sediments, seismic source, and seismic wave propagation path. Therefore, the studies of strong ground motion must consider the comprehensive effect of these factors.

Abstract:In this study, we chose high-performance shaking-table tests of a full-scale seven-story reinforced-concrete shear wall structure at the University of California, San Diego to test this method with respect to damage identification. We alternately tested the structure under the excitations of white noise, the environment, and nine earthquakes. For each case, we scaled the amplitudes of the input ground motions to various levels. We recorded the acceleration responses before and after the earthquake excitations with seismometers located on the seven floors. We determined the vibration characteristics for each earthquake excitation by analyzing the acceleration responses mentioned above. These characteristics include the modal information, the shear-wave propagation characteristics, and the inter-story drift ratio. We estimated the modal frequencies of the first three modes from the recordings when white noise was first applied to the building, and considered these as the criterion. The subsequent modal frequencies were then normalized and compared with this criterion. The normalized frequencies diminished gradually with the load case tests and the normalized frequency reduced by 51 percent for the first mode. The reduction in the modal characteristics indicates that crevices develop as the amplitude of the input ground motions increase, and thereby decrease the rigidity. Lower rigidity suggests that damage throughout the building has been aggravated. However, changes in modal frequencies cannot be used to locate damage. Mode shape curvatures of the building were similarly applied to identify the building damage. Test results demonstrate that the mode shape curvatures increase significantly with the test process and the main changes were concentrated on the second floor. After the excitations of earthquakes 1, 2, 3, and 4, the curvature values were 0.214, 1.214, 7.101, and 9.641, respectively. Therefore, we conclude that the damage on the second floor was more severe. Subsequently, we used a one-dimensional shear-wave propagation model to form the virtual waveform by deconvolving the recordings on each floor with the signal on the seventh floor. This waveform has a wave equation that is identical with that of a physical waveform and reflects the propagation characteristics of the shear wave in the building. Upward traveling and downward traveling waves are recognized in the virtual waveform. The travel time of the shear waves is inferred from the upward and downward traveling waves. At the same time, we obtained the changes in the travel time. The travel time and its changes both increase with the amplitude of the input ground motions. The travel change after the earthquake-4 excitation rose by 44.5 percent on the first floor. The travel time and its changes suggest that the lower two floors were more damaged than the upper floors, and are appropriate for damage identification as well. Finally, we computed the inter-story drift ratio and compared the results with the response after the excitations of the four earthquakes. The inter-story drift ratio increases after the input ground motions and breaks through the limit values of immediate occupancy of 0.5 percent and life safety of 1.0 percent for a reinforced concrete building. After excitation by earthquake 4, the drift ratio approaches the limit value of collapse prevention of 2.0 percent. Our analysis indicates that the parameters described above are sufficient to identify the damage.

Abstract:Polarization filtering methods based on a covariance matrix play an important role in the processing of multicomponent seismograms due to their explicit physical meaning, ease of implementation, and high efficiency. Conventional polarization filtering methods that are realized in a time domain have major limitations in resolving seismic signals in which waveforms or frequencies overlap. Time-frequency analysis methods are especially suitable for resolving separate seismic signals that overlap in time but have different spectra for instantaneous signal analysis. These methods can describe frequency components of a signal that change over time. Owing to the advantages of the time-frequency analysis method, it can be used in polarization analysis. This study presents a polarization filtering method based on the generalized S-transform to suppress surface waves in a time-frequency domain. On one hand, we remold the window function of the S-transform and improve the frequency resolution of seismic signals by increasing regulatory factors to create a nonlinear change in the window function with the signal frequency. On the other, we structure the cross-energy matrix in the time-frequency domain using the generalized S-transform, compute instantaneous polarization attributes by eigenanalysis, and design a filtering algorithm in the time-frequency domain to achieve polarization filtering of multicomponent seismic signals. The specialties of this method are that the length of the time window of the covariance matrix is determined by the instantaneous frequency of the multicomponent seismic data and it can adapt to the dominant period of the desired signal. Moreover, it calculates polarization parameters at each time-frequency point and no longer needs to perform interpolation. It is particularly accurate in processing signals with overlapping waveforms or frequencies in the time or frequency domain. The results of processing data from models and real three-component seismograms show that this method has very high clarity, high resolution, and practicability in the data analysis and processing of seismograms. This representation enables the detection of dispersion in polarization attributes, which can be further exploited to infer some physical characteristics of the medium under investigation. Moreover, this representation offers the ability to distinguish between attributes that belong to different coherent events that may overlap in time but with different frequency contents separated by time-dependent frequency cutoffs. Identifying and separating different wave types are made possible by designing filters that operate in the time-frequency domain. Attributes such as azimuth, dip, and signed ellipticity can also be used to improve the filtering algorithms.

Abstract:On August 30, 2013, a M_S5.1 earthquake occurred in Urumqi (43.8° N, 87.6° E), Xinjiang, China. This was the largest earthquake to have occurred in this area for decades, and the first earthquake with a magnitude equal to or greater than 5.0 to have occurred since establishment of the intensity fast report stations network in Urumqi. The microscopic epicenter of this earthquake was located in Hongshan. Although the earthquake was relatively small, most of the intensity fast report stations were triggered near the epicenter, and 32 main shock acceleration records were obtained. In this study, 23 typical strong motion records within different epicenter distances were selected to conduct a conventional analysis. By comparing the peak ground accelerations (PGA) of this earthquake with the acceleration attenuation relation of the soil layer in Xinjiang, observed values were found to be in good agreement with predicted values at a depth of 0~50 km. An earthquake location analysis was then conducted, and results were found to be influenced by several factors: the rationality of station distribution, accuracy of the regional velocity model, reliability of seismic phase discrimination, and the proper location method. We used the strong motion data to locate the M_S5.1 earthquake, and results showed a good linear relationship between P-wave travel time and epicentral distance. It is considered that in-depth research of motion characteristics could be conducted in other large cities, and results could be combined with records presented here.

Abstract:A M_S6.4 earthquake occurred at Menyuan county, Qinghai province, on 21 January 2016. It is the largest earthquake occurred in the border region of Gansu and Qinghai after the Minxian-Zhangxian earthquake on July 22, 2013. In this study, the related parameters and basic situation of the earthquake were introduced.