2017 Vol. 39 No. 5
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2017, 39(5): 633-647.
DOI: 10.11939/jass.2017.05.001
Abstract:
We collected seismic ambient noise data recorded at 98 permanent seismic stations in Sichuan-Yunnan area to image crustal structure. The Love wave cross-correlation functions are obtained by cross-correlation of ambient noise. The group velocity dispersion curves for Love wave are measured by automated time-frequency analysis. Love wave group velocity is mapped in the periods between 8 s and 30 s. The tomographies show seismic images at short periods exhibit apparent horizontal heterogeneities, which is largely consistent with geological features and geophysical studies. The Sichuan basin appears as low group velocity anomaly, and the Chengdu plain shows lower group velocity relative to the hilly area in Sichuan basin, with the line connecting Suining with Emeishan being the boundary between higher velocity and lower one. The variation of group velocity anomalies in Sichuan basin reflects variable sedimentary thicknesses. The Panzhihua area appears as high velocity anomaly, which is associated with an ancient mantle plume. The materials from mantle is remained at the different depth in crust in the form of intrusive rocks and magmatic underplating, resulting in the characteristics of high velocity anomaly. These seismic images provide necessary information for the research on crustal radial anisotropy and understanding of tectonic process.
We collected seismic ambient noise data recorded at 98 permanent seismic stations in Sichuan-Yunnan area to image crustal structure. The Love wave cross-correlation functions are obtained by cross-correlation of ambient noise. The group velocity dispersion curves for Love wave are measured by automated time-frequency analysis. Love wave group velocity is mapped in the periods between 8 s and 30 s. The tomographies show seismic images at short periods exhibit apparent horizontal heterogeneities, which is largely consistent with geological features and geophysical studies. The Sichuan basin appears as low group velocity anomaly, and the Chengdu plain shows lower group velocity relative to the hilly area in Sichuan basin, with the line connecting Suining with Emeishan being the boundary between higher velocity and lower one. The variation of group velocity anomalies in Sichuan basin reflects variable sedimentary thicknesses. The Panzhihua area appears as high velocity anomaly, which is associated with an ancient mantle plume. The materials from mantle is remained at the different depth in crust in the form of intrusive rocks and magmatic underplating, resulting in the characteristics of high velocity anomaly. These seismic images provide necessary information for the research on crustal radial anisotropy and understanding of tectonic process.
Abstract:
We relocated 4184 earthquakes that occurred in Shanxi reservoir, Zhejiang Province from 12 September to 30 December, 2014 by using double-difference algorithm and determined the focal mechanism solutions of 11 ML≥3.0 earthquakes using the method of CAP in this paper. Based on the analysis of the high-resolution hypocenters and focal mechanisms, we investigated the activity features of the earthquake swarm and its relationship with the fault. On the mapping view, the epicenters of the 2014 earthquake swarm occurred in the extended northwest direction of 2006 seismic swarm belt, which forms a linear belt, running parallel to the south of the Shuangxi-Jiaoxiyang fault. The relocated hypocenters mainly distribute in layers from 0.7 to 6 km in depth. The cross section profile which is perpendicular to the seismic belt strike shows that the seismic plane dips to southwest with a high angle. Most focal mechanisms are strike-slip type. The nodal planes of the focal mechanism solutions that share the same orientation with the trend of the seismic belt distribution of NW show the feature of dextral strike-slip. Taking the fault location error into consideration, the earthquake swarm may align along the rupture plane of the Shuangxi-Jiaoxiyang fault. The high-resolution hypocenters and focal mechanisms delineate the structure and activity nature of the fault. Since there is no distinctive main shock in the earthquake swarm and most earthquakes locate above 6 km in depth, so we can conclude that the earthquake swarm was not caused by the activity of the Shuangxi-Jiaoxiyang fault, but induced by the reservoir water storage. However, it is possible that the Shuangxi-Jiaoxiyang fault may be activated by the water storage of reservoir to generate tectonic earthquake in the future. No evidence shows that there is obvious relationship between the level of the reservoir water storage and seismicity, and the swarm activity becomes greater with time, which may be caused by the pore pressure increasement due to the long term penetration of the reservoir water to the fault plane.
We relocated 4184 earthquakes that occurred in Shanxi reservoir, Zhejiang Province from 12 September to 30 December, 2014 by using double-difference algorithm and determined the focal mechanism solutions of 11 ML≥3.0 earthquakes using the method of CAP in this paper. Based on the analysis of the high-resolution hypocenters and focal mechanisms, we investigated the activity features of the earthquake swarm and its relationship with the fault. On the mapping view, the epicenters of the 2014 earthquake swarm occurred in the extended northwest direction of 2006 seismic swarm belt, which forms a linear belt, running parallel to the south of the Shuangxi-Jiaoxiyang fault. The relocated hypocenters mainly distribute in layers from 0.7 to 6 km in depth. The cross section profile which is perpendicular to the seismic belt strike shows that the seismic plane dips to southwest with a high angle. Most focal mechanisms are strike-slip type. The nodal planes of the focal mechanism solutions that share the same orientation with the trend of the seismic belt distribution of NW show the feature of dextral strike-slip. Taking the fault location error into consideration, the earthquake swarm may align along the rupture plane of the Shuangxi-Jiaoxiyang fault. The high-resolution hypocenters and focal mechanisms delineate the structure and activity nature of the fault. Since there is no distinctive main shock in the earthquake swarm and most earthquakes locate above 6 km in depth, so we can conclude that the earthquake swarm was not caused by the activity of the Shuangxi-Jiaoxiyang fault, but induced by the reservoir water storage. However, it is possible that the Shuangxi-Jiaoxiyang fault may be activated by the water storage of reservoir to generate tectonic earthquake in the future. No evidence shows that there is obvious relationship between the level of the reservoir water storage and seismicity, and the swarm activity becomes greater with time, which may be caused by the pore pressure increasement due to the long term penetration of the reservoir water to the fault plane.
Abstract:
Based on the digital waveforms of Jiangsu seismic network, the focal mechanism of the Sheyang MS4.4 earthquake was determined by using the CAP (cut and paste) method, and the ML≥1.5 events of Sheyang earthquake sequence were relocated by using the HypoDD method. The result shows that the strike, dip and rake of the MS4.4 event are 304°, 53°, 0° for the nodal plane Ⅰ, and 214°, 90°, 143° for the nodal plane Ⅱ, respectively, and the centroid depth is about 14 km. The relocation for the Sheyang earthquake sequence shows that the earthquake sequence was located between Hongze-Goudun fault and Yancheng-Nanyang'an fault with the predominant direction NW60° in horizontal space, and the sequence propagated from southeast to northwest with the focal depth of 6--23 km. According to the focal mechanism solutions and the relocation results, we deduce that the Sheyang MS4.4 earthquake is a left-lateral slip event induced by shear rupture northwestwards under regional stress field, and its fault plane is nodal plane Ⅰ.
Based on the digital waveforms of Jiangsu seismic network, the focal mechanism of the Sheyang MS4.4 earthquake was determined by using the CAP (cut and paste) method, and the ML≥1.5 events of Sheyang earthquake sequence were relocated by using the HypoDD method. The result shows that the strike, dip and rake of the MS4.4 event are 304°, 53°, 0° for the nodal plane Ⅰ, and 214°, 90°, 143° for the nodal plane Ⅱ, respectively, and the centroid depth is about 14 km. The relocation for the Sheyang earthquake sequence shows that the earthquake sequence was located between Hongze-Goudun fault and Yancheng-Nanyang'an fault with the predominant direction NW60° in horizontal space, and the sequence propagated from southeast to northwest with the focal depth of 6--23 km. According to the focal mechanism solutions and the relocation results, we deduce that the Sheyang MS4.4 earthquake is a left-lateral slip event induced by shear rupture northwestwards under regional stress field, and its fault plane is nodal plane Ⅰ.
Abstract:
By using P-wave travel time data from three shots of the deep seismic sounding profiles passing through Yinchuan basin in 2014, based on travel time inversion method Rayinvr, we get the crustal and upper-mantle structure in the studied area. The results show that crustal thickness in the studied area varies from 42 km to 48 km, Moho depth is shallower in the east and west sides of the profile, and much deeper in the middle segment, the deepest Moho interface is beneath Helan mountain. P wave velocity increases with the increasing of depth in a positive gradient, but two distinct interfaces can be identified in the lithosphere mantle within the depths of 90-103 km. This layer does not exhibit the characteristic that the velocity increases obviously with the depth, suggesting the structure in the studied area present a characteristic that did not conform to the global average model, whose velocity increases according to the depth, therefore it is deduced that a velocity transition zone exists between lithosphere and asthenosphere beneath Yinchuan basin.
By using P-wave travel time data from three shots of the deep seismic sounding profiles passing through Yinchuan basin in 2014, based on travel time inversion method Rayinvr, we get the crustal and upper-mantle structure in the studied area. The results show that crustal thickness in the studied area varies from 42 km to 48 km, Moho depth is shallower in the east and west sides of the profile, and much deeper in the middle segment, the deepest Moho interface is beneath Helan mountain. P wave velocity increases with the increasing of depth in a positive gradient, but two distinct interfaces can be identified in the lithosphere mantle within the depths of 90-103 km. This layer does not exhibit the characteristic that the velocity increases obviously with the depth, suggesting the structure in the studied area present a characteristic that did not conform to the global average model, whose velocity increases according to the depth, therefore it is deduced that a velocity transition zone exists between lithosphere and asthenosphere beneath Yinchuan basin.
Abstract:
The campaigned gravity survey is a sort of time-lapse terrestrial gravity survey, which generally means the gravity observed repeatedly at a fixed station with the same routes and similar time schedules. The direct output of campaigned gravity survey is discrete time-variable gravity data. Each station in the network is visited with a fixed time interval (half or one years), in order to measure the time-variable gravity change, which is at a scale few tens of microgals, at each station during the period between these two observations. Because the shape of gravity network is very irregular and the gravity changes is not significant compared with the observation error, the general gridding method is not suitable to visualize this sort of gravity data. In this paper, we proposed a new approach for the visualization of time-variable gravity data according to the data measured by the campaigned gravity survey. According to this approach, the patterns of gravity changes and uncertainty associated with the measurement can be visualized in the same figure with the gravity difference between two adjacent stations together. Furthermore, for the purpose of evaluating the magnitude of regional gravity change, we also defined two indexes, G and C, to evaluate the significant levels of the regional gravity changes. On the basis of this approach, we analyzed the real campaigned gravity data in the capital area. The result show that the approaches are different from the classical contour map in expressing the regional gravity changes derived by the campaigned gravity survey, and can visualize and locate where the gravity changes happen, the reliable gravity changes and the level of significance. The approaches can provide more quantitative basis for the study of the gravity change potentially associated with the earthquake preparation and occurrence.
The campaigned gravity survey is a sort of time-lapse terrestrial gravity survey, which generally means the gravity observed repeatedly at a fixed station with the same routes and similar time schedules. The direct output of campaigned gravity survey is discrete time-variable gravity data. Each station in the network is visited with a fixed time interval (half or one years), in order to measure the time-variable gravity change, which is at a scale few tens of microgals, at each station during the period between these two observations. Because the shape of gravity network is very irregular and the gravity changes is not significant compared with the observation error, the general gridding method is not suitable to visualize this sort of gravity data. In this paper, we proposed a new approach for the visualization of time-variable gravity data according to the data measured by the campaigned gravity survey. According to this approach, the patterns of gravity changes and uncertainty associated with the measurement can be visualized in the same figure with the gravity difference between two adjacent stations together. Furthermore, for the purpose of evaluating the magnitude of regional gravity change, we also defined two indexes, G and C, to evaluate the significant levels of the regional gravity changes. On the basis of this approach, we analyzed the real campaigned gravity data in the capital area. The result show that the approaches are different from the classical contour map in expressing the regional gravity changes derived by the campaigned gravity survey, and can visualize and locate where the gravity changes happen, the reliable gravity changes and the level of significance. The approaches can provide more quantitative basis for the study of the gravity change potentially associated with the earthquake preparation and occurrence.
2017, 39(5): 694-707.
DOI: 10.11939/jass.2017.05.006
Abstract:
Luxi uplift has complex geological conditions and well-developed faults. By analyzing and calculating the gravity and magnetic data in this area, we studied the characteristic of gravity-magnetic anomalies, Moho discontinuity and Curie isothermal surface on the Yishu fault zone, Qihe-Guangrao fault zone, Liaocheng-Lankao fault zone, Fengpei fault zone and the uplift block, as well as the geological tectonic features and tectonic activity of this area. The results show that basement of Luxi uplift is widely exposed, and sediments are mainly distributed in the sags which are controlled by the down-thrown side of faults. Faults depth of this area can reach up to more than 20 km, and one of them is a regional upper mantle deep structure named Mengshan fault. Meng-shan fault controls output of the kimberlite-type diamond. The depth of Moho discontinuity in this area varies from 30 km to 35 km, and it features a dustpan shape which is open to the west. The central block is thick and the surrounding areas are gradually thinning except for the west part which is stepped thickening. The Curie isothermal surface depth ranges from 20 km to 33 km with an average of 26 km. The Curie depth of the central area is deeper than its surrounding areas, and it is a stable block. The fault zones correspond to the Curie surface gradient zones. The earthquake activities mainly focus on the areas where the fault zones meet with Moho discontinuity gradient zones, as well as the fault zone where Curie surface changes abruptly.
Luxi uplift has complex geological conditions and well-developed faults. By analyzing and calculating the gravity and magnetic data in this area, we studied the characteristic of gravity-magnetic anomalies, Moho discontinuity and Curie isothermal surface on the Yishu fault zone, Qihe-Guangrao fault zone, Liaocheng-Lankao fault zone, Fengpei fault zone and the uplift block, as well as the geological tectonic features and tectonic activity of this area. The results show that basement of Luxi uplift is widely exposed, and sediments are mainly distributed in the sags which are controlled by the down-thrown side of faults. Faults depth of this area can reach up to more than 20 km, and one of them is a regional upper mantle deep structure named Mengshan fault. Meng-shan fault controls output of the kimberlite-type diamond. The depth of Moho discontinuity in this area varies from 30 km to 35 km, and it features a dustpan shape which is open to the west. The central block is thick and the surrounding areas are gradually thinning except for the west part which is stepped thickening. The Curie isothermal surface depth ranges from 20 km to 33 km with an average of 26 km. The Curie depth of the central area is deeper than its surrounding areas, and it is a stable block. The fault zones correspond to the Curie surface gradient zones. The earthquake activities mainly focus on the areas where the fault zones meet with Moho discontinuity gradient zones, as well as the fault zone where Curie surface changes abruptly.
Abstract:
This paper collected remote sensing image, digital elevation model and Bouguer gravity data of the south segment of Yishu fault zone (namely the segment between Yishui county and Tancheng county) and its surrounding areas, and studied the regional tectonic landform and deep crustal structural characteristics so as to analyze the intersection relationships between south segment of Yishu fault zone and its surrounding faults. The research results indicate that Mengshan piedmont fault and Cangni fault exhibit obvious tectonic characteristics consisting of river bends, fault steep, gully leaps, fault scarps and fault facets in the remote sensing images. In detail, Mengshan piedmont fault extends eastward to Ju'nan county, while Cangni fault extends eastward to Tancheng. Both the faults crosscut Yishu fault zone on the geomorphology; in the gravity detail field, the two faults form a gravitational gradient zone on different scales, which extend down to the lower crust and appear as deep faults. Moreover, the lineaments intersect with Yishu fault zone and contribute to the distorted, broken, beaded gravity anomalies, confirming that Yishu fault zone has been cut through in the deep crust. Therefore, a great uniformity between the interpretation results of two faults in remote sensing and gravity field suggests that the two faults all crosscut south segment of Yishu fault zone in landform and deep crust, so that the segmentation characteristics occur. In addition, there is a new fault developed near Linshu county, namely Xiangzhuang-Shaling fault. The fault is higher in north and lower in south, along which the rivers left-hand bend synchronously, and in the 1-to-3-order gravity field there are linear gradient zones, so it is speculated that the fault extends down to the middle crust, and crosscuts the eastern graben of Yishu fault zone near Linshu county, but not extends to the western graben.
This paper collected remote sensing image, digital elevation model and Bouguer gravity data of the south segment of Yishu fault zone (namely the segment between Yishui county and Tancheng county) and its surrounding areas, and studied the regional tectonic landform and deep crustal structural characteristics so as to analyze the intersection relationships between south segment of Yishu fault zone and its surrounding faults. The research results indicate that Mengshan piedmont fault and Cangni fault exhibit obvious tectonic characteristics consisting of river bends, fault steep, gully leaps, fault scarps and fault facets in the remote sensing images. In detail, Mengshan piedmont fault extends eastward to Ju'nan county, while Cangni fault extends eastward to Tancheng. Both the faults crosscut Yishu fault zone on the geomorphology; in the gravity detail field, the two faults form a gravitational gradient zone on different scales, which extend down to the lower crust and appear as deep faults. Moreover, the lineaments intersect with Yishu fault zone and contribute to the distorted, broken, beaded gravity anomalies, confirming that Yishu fault zone has been cut through in the deep crust. Therefore, a great uniformity between the interpretation results of two faults in remote sensing and gravity field suggests that the two faults all crosscut south segment of Yishu fault zone in landform and deep crust, so that the segmentation characteristics occur. In addition, there is a new fault developed near Linshu county, namely Xiangzhuang-Shaling fault. The fault is higher in north and lower in south, along which the rivers left-hand bend synchronously, and in the 1-to-3-order gravity field there are linear gradient zones, so it is speculated that the fault extends down to the middle crust, and crosscuts the eastern graben of Yishu fault zone near Linshu county, but not extends to the western graben.
Abstract:
In order to analyze the temporal and spatial signatures of typhoons on strain records, we processed the strain data of four-component borehole strain meter (FCBS for short) at Sheshan station during the impacting periods of typhoons "Neoguri" and "Haikui". We integrated methods of wavelet decomposition, continuous spectral analysis and predominant polarization direction to investigate the influence of ocean storms on low-frequency noise. The results show that the process of typhoon development had significantly influenced spectrum amplitudes, which showed an "increase--peak--decrease" pattern, especially over the frequency band 2--4 min. This pattern was highly correlated with the distance between the station and the center of typhoon. And with the increases of the periods, the amplitude of the spectra decreased. When the storm center was approaching to the station, the predominant polarization directions of the strain records were concentrated on the southeast of 160°, which indicates that local coastlines may play an important role in the noise excitation mechanism. The comparison of the mean spectra amplitudes with the wind speed showed that the wind was not the major excitation factor. Finally we come to the conclusion that during the typhoon development, the energy of ocean waves greatly increases, and the waves with higher energy are continuously lapping at the shore, which results in low-frequency disturbances on continents.
In order to analyze the temporal and spatial signatures of typhoons on strain records, we processed the strain data of four-component borehole strain meter (FCBS for short) at Sheshan station during the impacting periods of typhoons "Neoguri" and "Haikui". We integrated methods of wavelet decomposition, continuous spectral analysis and predominant polarization direction to investigate the influence of ocean storms on low-frequency noise. The results show that the process of typhoon development had significantly influenced spectrum amplitudes, which showed an "increase--peak--decrease" pattern, especially over the frequency band 2--4 min. This pattern was highly correlated with the distance between the station and the center of typhoon. And with the increases of the periods, the amplitude of the spectra decreased. When the storm center was approaching to the station, the predominant polarization directions of the strain records were concentrated on the southeast of 160°, which indicates that local coastlines may play an important role in the noise excitation mechanism. The comparison of the mean spectra amplitudes with the wind speed showed that the wind was not the major excitation factor. Finally we come to the conclusion that during the typhoon development, the energy of ocean waves greatly increases, and the waves with higher energy are continuously lapping at the shore, which results in low-frequency disturbances on continents.
Abstract:
Based on the shallow-focus earthquakes lunar catalog in Chinese mainland from 1900 to 2015, modulated lunar date and threshold of modulation ratio are defined respectively according to seismic active characteristics. The modulation ratio method is employed to carry out spatio-temporal scanning for three moderate earthquakes in northwest China, and the spatio-temporal characteristics of modulation ratio of regional small earthquakes are analyzed. Results indicate that there are modulation ratio anomalies around epicentral area before Minxian-Zhangxian MS6.6 earthquake and Alxa MS5.8 earthquake, evolution time of modulation ratio anomaly is relatively long for Menyuan MS6.4 earthquake. All three moderate earthquakes happened around the area with higher earth tide modulation ratio according to the spatio-temporal scanning results, and the length of time window and the abnormal area size of the earth tide modu-lation ratio may have correlations with the magnitude of moderate earthquakes.
Based on the shallow-focus earthquakes lunar catalog in Chinese mainland from 1900 to 2015, modulated lunar date and threshold of modulation ratio are defined respectively according to seismic active characteristics. The modulation ratio method is employed to carry out spatio-temporal scanning for three moderate earthquakes in northwest China, and the spatio-temporal characteristics of modulation ratio of regional small earthquakes are analyzed. Results indicate that there are modulation ratio anomalies around epicentral area before Minxian-Zhangxian MS6.6 earthquake and Alxa MS5.8 earthquake, evolution time of modulation ratio anomaly is relatively long for Menyuan MS6.4 earthquake. All three moderate earthquakes happened around the area with higher earth tide modulation ratio according to the spatio-temporal scanning results, and the length of time window and the abnormal area size of the earth tide modu-lation ratio may have correlations with the magnitude of moderate earthquakes.
2017, 39(5): 751-763.
DOI: 10.11939/jass.2017.05.010
Abstract:
In this study, we simulated strong ground motion of two earthquakes with MS5.6 and MS5.1 occurred in Xinjiang Vrümqi in 2013 based on modified stochastic finite fault modeling with dynamic corner frequency. 45 strong ground motion records from 23 stations were chosen to simulate by different source parameters, and response spectrum and accelerograms were compared with the observed ones. The results show that simulated strong ground motion have some differences with observed data in duration and shape, and the value of simulated peak ground acceleration (PGA) is less than observed in near-source. For acceleration response spectrum, the simulated result is consistent with observed. The bias of stochastic finite fault is between ±0.5 suggesting a good agreement for high frequency. For different source parameters model, the distribution characteristic of simulated PGAs are consistent with observed, but the simulated PGAs are less than observed ones.
In this study, we simulated strong ground motion of two earthquakes with MS5.6 and MS5.1 occurred in Xinjiang Vrümqi in 2013 based on modified stochastic finite fault modeling with dynamic corner frequency. 45 strong ground motion records from 23 stations were chosen to simulate by different source parameters, and response spectrum and accelerograms were compared with the observed ones. The results show that simulated strong ground motion have some differences with observed data in duration and shape, and the value of simulated peak ground acceleration (PGA) is less than observed in near-source. For acceleration response spectrum, the simulated result is consistent with observed. The bias of stochastic finite fault is between ±0.5 suggesting a good agreement for high frequency. For different source parameters model, the distribution characteristic of simulated PGAs are consistent with observed, but the simulated PGAs are less than observed ones.
Abstract:
The actual geotechnical property parameters have obvious randomness. The two-dimensional scattering of the P and SV waves in the sedimentary valleys is studied based on the indirect boundary element method (IBEM) in order to explore the effect of the randomness of the medium velocity and material damping on seismic ground amplification effect of the valley. The Monte Carlo method is used to simulate 30 groups of wave velocity ratio samples and 30 groups of damping ratio samples. Finally the present paper gives the single frequency and spectral results of random response of the valley under P wave and SV wave incidence with different incident angles and frequencies. The results show that the variation coefficient extremum of surface displacement response amplitude is significantly larger than the variation coefficient of velocity ratio, and the effect of velocity ratio randomness on surface displacement is the most significant at system resonance frequency, and the maximum standard deviation of surface displacement can reach 7.168. The variation of the ground motion response in the sedimentary center is more significant at low frequency, but it is the largest at the edge of the sedimentary valley at high frequency. On the other hand, the incident angle has little effect on the variation coefficient extremum of surface displacement response amplitude, but it has a significant effect on the spatial variation of the variation coefficient. Meanwhile, the damping randomness has little effect on the surface displacement at low frequencies, but the effect is obvious at high frequencies, especially at the natural vibration frequency of the sedimentary valley, the effect is most significant.
The actual geotechnical property parameters have obvious randomness. The two-dimensional scattering of the P and SV waves in the sedimentary valleys is studied based on the indirect boundary element method (IBEM) in order to explore the effect of the randomness of the medium velocity and material damping on seismic ground amplification effect of the valley. The Monte Carlo method is used to simulate 30 groups of wave velocity ratio samples and 30 groups of damping ratio samples. Finally the present paper gives the single frequency and spectral results of random response of the valley under P wave and SV wave incidence with different incident angles and frequencies. The results show that the variation coefficient extremum of surface displacement response amplitude is significantly larger than the variation coefficient of velocity ratio, and the effect of velocity ratio randomness on surface displacement is the most significant at system resonance frequency, and the maximum standard deviation of surface displacement can reach 7.168. The variation of the ground motion response in the sedimentary center is more significant at low frequency, but it is the largest at the edge of the sedimentary valley at high frequency. On the other hand, the incident angle has little effect on the variation coefficient extremum of surface displacement response amplitude, but it has a significant effect on the spatial variation of the variation coefficient. Meanwhile, the damping randomness has little effect on the surface displacement at low frequencies, but the effect is obvious at high frequencies, especially at the natural vibration frequency of the sedimentary valley, the effect is most significant.
Abstract:
Site amplification consists of two-dimensional (2D) soil layer amplification effects and topographic amplification effects. The objective of this study is to investigate the relative contribution of soil amplification and topographic amplification. To that end, the wavefunction series solution for the scattering of plane SH waves by a partially filled circular-arc alluvial valley is proposed. The wavefunction series solution is obtained by a novel method of over-determined system of equations in the framework of the wavefunction expansion technique with the aid of a region-matching strategy. The convergence tests are conducted to reveal the necessity of the proposed over-determined system of equations method. The validity of the proposed solution is verified by comparison with previous results. By adjusting the material parameters of the two sub-regions in the analytical model, both of the surface motions of the alluvial valley and the empty canyon are calculated. The site amplification patterns of the alluvial valley are compared with the topographic amplification patterns of the empty canyon with the same geometry, the results show that the soil amplification effects are usually larger than the topographic amplification effects within the alluvial valley, while the topographic effects dominate the amplification pattern of ground motions outside the alluvial valley. Afterwards, a parametric study in terms of the maximum surface motion is carried out to determine the relative importance of soil and topographic contributions in a more comprehensive manner and to further characterize the 2D soil layer amplification effects. It is evident that the maximum soil layer amplification generally far outweighs the maximum topographic amplification. The 2D soil amplification increases with the impedance contrast between the soil layer and the underlying bedrock but is not a simple linear superposition of 1D soil amplification and 2D topographic amplification.
Site amplification consists of two-dimensional (2D) soil layer amplification effects and topographic amplification effects. The objective of this study is to investigate the relative contribution of soil amplification and topographic amplification. To that end, the wavefunction series solution for the scattering of plane SH waves by a partially filled circular-arc alluvial valley is proposed. The wavefunction series solution is obtained by a novel method of over-determined system of equations in the framework of the wavefunction expansion technique with the aid of a region-matching strategy. The convergence tests are conducted to reveal the necessity of the proposed over-determined system of equations method. The validity of the proposed solution is verified by comparison with previous results. By adjusting the material parameters of the two sub-regions in the analytical model, both of the surface motions of the alluvial valley and the empty canyon are calculated. The site amplification patterns of the alluvial valley are compared with the topographic amplification patterns of the empty canyon with the same geometry, the results show that the soil amplification effects are usually larger than the topographic amplification effects within the alluvial valley, while the topographic effects dominate the amplification pattern of ground motions outside the alluvial valley. Afterwards, a parametric study in terms of the maximum surface motion is carried out to determine the relative importance of soil and topographic contributions in a more comprehensive manner and to further characterize the 2D soil layer amplification effects. It is evident that the maximum soil layer amplification generally far outweighs the maximum topographic amplification. The 2D soil amplification increases with the impedance contrast between the soil layer and the underlying bedrock but is not a simple linear superposition of 1D soil amplification and 2D topographic amplification.
2017, 39(5): 798-805.
DOI: 10.11939/jass.2017.05.013
Abstract:
Six groups of strong ground motion records with different characteristic parameters were selected as seismic input, a two-dimensional slope model with homogeneous soil was established based on the finite element numerical simulation method, and then the acceleration and displacement responses of the slope model were calculated and analyzed. Finally the influence degree and laws of the PGA, frequency spectrum and duration on the slope responses in earthquakes were revealed. The results showed that the PGA, characteristic period and duration all had an significant effect on slope seismic response. It was also indicated that the displacement of the slope foot and shoulder evidently increased with increase of the PGA, characteristic period and duration of earthquakes, and the amplification coefficients of PGA along the slope free surface also increased with increase of characteristic period while decreased with increase of the PGA. The result can provide reference for the design and prevention of slope stability under earthquake effect.
Six groups of strong ground motion records with different characteristic parameters were selected as seismic input, a two-dimensional slope model with homogeneous soil was established based on the finite element numerical simulation method, and then the acceleration and displacement responses of the slope model were calculated and analyzed. Finally the influence degree and laws of the PGA, frequency spectrum and duration on the slope responses in earthquakes were revealed. The results showed that the PGA, characteristic period and duration all had an significant effect on slope seismic response. It was also indicated that the displacement of the slope foot and shoulder evidently increased with increase of the PGA, characteristic period and duration of earthquakes, and the amplification coefficients of PGA along the slope free surface also increased with increase of characteristic period while decreased with increase of the PGA. The result can provide reference for the design and prevention of slope stability under earthquake effect.
2017, 39(5): 806-813.
DOI: 10.11939/jass.2017.05.014
Abstract:
The self-noises of the seismic dataloggers have significant meaning for self-noise measurement of seismometers and dynamic range evaluation of broadband seimographs. In this study, we measured and analyzed the self-noises of six types of imported seismic dataloggers widely used in Chinese seismic observations based on multi-channel correlation analysis method. The results showed that self-noises of all six types of dataloggers are well below NLNM model when frequency domain is below 1 Hz in both high-gain and unity-gain modes by sensitivity conversion factor 750 V·s/m; however, for the frequency domain higher than 1 Hz, self-noises of the six types of dataloggers interlace with NLNM model, hence much care should be taken according to the goals of seismic observation. Reftek-130/130S and Q330HRS with high-gain sampling mode might be the preferable choice for the seismometer's self-noise measurements.
The self-noises of the seismic dataloggers have significant meaning for self-noise measurement of seismometers and dynamic range evaluation of broadband seimographs. In this study, we measured and analyzed the self-noises of six types of imported seismic dataloggers widely used in Chinese seismic observations based on multi-channel correlation analysis method. The results showed that self-noises of all six types of dataloggers are well below NLNM model when frequency domain is below 1 Hz in both high-gain and unity-gain modes by sensitivity conversion factor 750 V·s/m; however, for the frequency domain higher than 1 Hz, self-noises of the six types of dataloggers interlace with NLNM model, hence much care should be taken according to the goals of seismic observation. Reftek-130/130S and Q330HRS with high-gain sampling mode might be the preferable choice for the seismometer's self-noise measurements.