Investigation of non-linear site amplification at two downhole strong ground motion arrays in Taiwan

Non-linear seismic response of soil is studied by comparing the spectral ratios of surface to downhole horizontal accelerations on weak and strong motion. Data from two boreholes are analysed. One is drilled in the alluvial deposits in the south–west quadrant of the SMART 1 array. The second one penetrates Pleistocene terrace deposits in the northern part of the SMART2 array. Observed weak and strong motion spectral ratios are compared with the theoretical ones predicted by the geotechnical soil model which postulates a hysteretic constitutive law. A significant non-linear response is found at the first site for the events with surface peak acceleration exceeding roughly 0–15g. Deamplification of the strong motion occurred in the frequency range from approximately 1 to 10 Hz. The maximum observed difference between the average weak and strong motion amplification functions of an 11 m-thick near-surface stratum is a factor of 2–3. Nonlinear response characteristics are in qualitative agreement with the model. An additional corollary is that the amplification function calculated from the shear wave coda is equivalent to the average amplification calculated over the ensemble of small earthquakes. No statistically significant non-linear response is detected on the second array, that is tentatively accounted for by the stiffer soil conditions and weaker accelerations achieved at the SMART2 site. The results indicate that the non-linear amplification can be detectable at certain soil conditions above a threshold acceleration level.     

The strong ground motion in Mexico City: Analysis of data recorded by a 3D array

In 2001, we installed 11 new accelerometers in the lake-bed zone of Mexico City, which, together with the existing stations, form a 3D array of small aperture, with 15 surface and six borehole stations. This new array recorded the 10/08/2001 Guerrero event (M=6.1) and the 01/22/2003 Colima (M=7.6) event. The analysis of these two events showed that, for periods longer than 2.5 s, the dominant wavefield is composed of a fundamental mode of surface waves arriving from the epicenter and diffracted surface waves generated at the southern boundary of the Mexican Volcanic Belt. For periods shorter than 2.5 s, the resonance of the superficial clay layer becomes predominant. The wavefield consists in a superposition of diffracted waves propagating from the western part of the valley. Our results show that the ground motion is strongly amplified by the soft clay layer but its duration is controlled by the incident wavefield.     

Evaluation of blast-induced ground vibration effects in a new residential zone

The results from an experimental blasting program that was performed at the special explosives training field of the General Directorate of Security in Ankara, where new residential blocks are planned to be constructed nearby, are presented. With the objective of estimating the blast-induced ground vibration effects on the proposed structures, various blasting parameters of nineteen surface and underground explosions were recorded in two directions at three measurement stations. Site-specific empirical relationships between peak particle velocity, the amount of explosive and the distance were developed. These relationships were used to construct a practical blasting chart, which gives the maximum amount of explosive to be used as a function of distance, for future underground and surface blasting operations in the training field. Since the use of peak particle velocity in the field of civil engineering has been limited so far, site-specific parameters were also estimated to predict the blast-induced horizontal peak ground acceleration. Then, an attempt was made to investigate the dynamic responses of four and six storey reinforced concrete structures that consist of frame and shear wall type structural systems under the measured accelerations using finite element analysis. The limitations of this approach were discussed within the context of damage estimation.     

Dynamic soil parameters determination by geophysical prospecting in Mexico City: implication for site effect modeling

Seismic experiments were conducted in Mexico City in order to determine the dynamic characteristics of the soft clay layer present at the surface in the lake bed zone. In-situ measurements of V_p, V_s, Q_p and Q_s were performed in the Texcoco lake area. Q_s values ranging from 8 to 60 have been found from the surface down to 30 m depth, while V_s values are between 30 and 115 m/s for the same depths. These relatively low Q values support the idea that surface waves generated at the edges of the Mexico basin cannot propagate very far in the structure. During this study, the elastic characteristics of the surficial basalt layers outcropping in the hill zone have also been investigated. Very low P-wave velocity values (500–800 m/s) have been found in the basalt. In the hill zone where basalts are interbedded with soil, numerical modeling has shown that such low values can only be partially explained by the relation between wavelength and basalt thickness and that other mechanisms, like fracturation, must be involved.     

Simulating pumping-induced regional land subsidence with the use of InSAR and field data in the Toluca Valley, Mexico

A multidisciplinary approach is presented here for quantifying land subsidence in a heavily pumped aquifer system with complex stratigraphy. The methodology consists in incorporating Terzaghi’s 1D instantaneous compaction principle into a 3D groundwater flow model that is then applied and calibrated to reproduce observed hydraulic heads and compaction for the Toluca Valley, Mexico. Differential Interferometric Synthetic Aperture Radar (D-InSAR), a generated 3D-geological model, extensometers, monitoring wells, and available literature are used to constrain the model. The D-InSAR measured subsidence, extensometers, and numerical simulations of subsidence agree relatively well. Simulations show that since regional subsidence began in the mid 1960s there has been up to 2 m of subsidence in the industrial corridor, where heavy pumping and thick clay layers are found. This study shows that an approach using various sources of data is useful in estimating and constraining the vertical component of the inelastic skeletal specific storage.     

淮南采煤塌陷湖泊浮游植物优势种的营养动力学

在淮南潘谢矿区选取3个营养盐结构差异较大的塌陷湖泊,于2014—2015年4个季度分别对浮游植物群落结构组成进行调查,选取3个湖泊中的优势种(属)具尾蓝隐藻(Chroomonas caudata)、链形小环藻(Cyclotella catenata)和伪鱼腥藻(Pseudanabaena sp.)作为研究对象,设置不同的氮(N)、磷(P)浓度梯度进行营养动力学培养实验,并结合Monod方程,获得3个藻种在不同营养盐限制下的营养动力学参数.N限制下具尾蓝隐藻、小环藻和伪鱼腥藻的最大生长速率(μmax)和半饱和常数(Ks)分别为:0.66 d~(-1)、1.66 mg/L;0.37 d~(-1)、1.06 mg/L;0.71 d~(-1)、2.26 mg/L;P限制下它们的μmax和Ks则分别为:0.51 d~(-1)、0.023 mg/L;0.31 d~(-1)、0.035 mg/L;0.90 d~(-1)、0.015 mg/L.综上所述,在N充足时,伪鱼腥藻能够在竞争中形成优势,同时在P限制情况下易成为优势种,从营养动力学的角度揭示了其在塌陷湖泊中占据优势的营养盐动力学机制.研究结果可以为沉陷区水域开发利用和营养盐管理提供科学依据.     

Earthquake ground motion in Mexico City: An analysis of data recorded at Roma array

We analysed in detail three earthquakes recorded in a small-aperture accelerometric array in Mexico City, using the correlation of the records as a function of time along the accelerogram and frequency. Ground response is strongly conditioned by the fundamental period of the soft soils at the site of the array (T₀). Energy at periods longer than 2T₀ is guided by the crustal structure (with a thickness of 45 km). The wave field at periods between T₀ and 2T₀ also consists of surface waves but guided by the upper 2–3 km of volcanic sediments in central Mexico. For periods smaller than T₀, ground motion is uncorrelated among the stations. Our results indicate that seismic response of Mexico City, including its very long duration, results from deeply guided surface waves (between 2 and 45 km depth) interacting with the very local response of the soft surficial clay layer.     

The contribution of the built environment to the ‘free-field’ ground motion in Mexico City

Soil–structure interaction (SSI) effects on building dynamic behaviour have been studied extensively. In comparison, the radiation of waves away from the soil–foundation interface has received little attention. Recent studies point out that SSI in an urban environment can modify the ground motion recorded in the free-field. These modifications will be important when two conditions are met: structures founded on soft soils and coincidence between the vibration periods of the structure and those of the superficial layers. Both conditions are met in Mexico City lake zone. In this study, we investigate SSI effects on ‘free-field’ motion. The data we use consist of microtremors recorded on soft soils in Mexico City, a densely built environment. Our objective was to identify the modifications to free-field ground motion caused by neighbouring structures. Data were analysed using H/V spectral ratios. Large variations in the level of amplification and resonant frequency were determined from microtremors in very closely spaced stations. Our results suggest consistently that free-field ground motion is significantly affected by the presence of neighbouring structures.     

Large amplification of ground motion at rock sites within a fault zone in Nocera Umbra (central Italy)

During the two mainshocks of September 26, 1997 inthe Umbria-Marche border a strong-motion accelerographrecorded peak ground accelerations as large as 0.6 g,approximately, in the town of Nocera Umbra, atdistances of 10 to 15 km from the epicentres. Thisvalue is significantly larger than expected on thebasis of the usual regressions with magnitude anddistance. A broad-band amplification up to a factor of10 was consistently estimated in previous papers,using both weak and strong motion data recorded at theaccelerograph site during local moderate earthquakes.To study the cause of this amplification we deployedsix seismologic stations across the tectonic contactbetween the Ceno-Mesozoic limestone and the Mesozoicmarly sandstone where the accelerograph is installed.Seismograms of 21 shallow aftershocks in the magnituderange from 2.2 to 4.0 and a subcrustal M_w = 5.3event are analysed. Regardless of epicentre location,waveforms show a large complexity in an approximately200 m wide band adjacent to the tectonic contact. Thisis interpreted as the effect of trapped waves in thehighly fractured, lower velocity materials within thefault zone.     

Duration of strong ground motion during Mexican earthquakes in terms of magnitude,distance to the rupture area and dominant site period

A study of the duration of strong ground motion using accelerometric data of subduction and normal‐faulting Mexican earthquakes is presented. Duration is obtained based on the time between 2.5 and 97.5 per cent of the Arias intensity. An expression to predict this duration in terms of the magnitude, distance to the rupture area and site period is proposed and compared with predictions available in the literature. The effect of large duration for very distant sites and the contribution of soft soils to the duration of strong ground motion are widely discussed. We have found that large magnitude not only yields long duration at the source, but also proportionally longer duration with distance and with dominant site period compared to small magnitude. The duration obtained from the regression is used as a parameter to obtain input and hysteretic energy and on the use of damage models available in the literature. Finally, duration is used together with the random vibration theory to predict response spectra. Copyright © 2001 John Wiley & Sons, Ltd.     

Evaluation of building fundamental periods and effects of local geology on ground motion parameters in the Siracusa area,Italy

The Siracusa area, located in the southeastern coast of Sicily (Italy), is mainly characterized by the outcropping of a limestone formation. This lithotype, which is overlain by soft sediments such as sandy clays and detritus, can be considered as the local bedrock. Records of ambient noise, processed through spectral ratio techniques, were used to assess the dynamic properties of a sample survey of both reinforced concrete and masonry buildings. The results show that experimental periods of existing buildings are always lower than those proposed by the European seismic code. This disagreement could be related to the role played by stiff masonry infills, as well as the influence of adjacent buildings, especially in downtown Siracusa. Numerical modeling was also used to study the effect of local geology on the seismic site response of the Siracusa area. Seismic urban scenarios were simulated considering a moderate magnitude earthquake (December 13th, 1990) to assess the shaking level of the different outcropping formations. Spectral acceleration at different periods, peak ground acceleration, and velocity were obtained through a stochastic approach adopting an extended source model code. Seismic ground motion scenario highlighted that amplification mainly occurs in the sedimentary deposits that are widespread to the south of the study area as well as on some spot areas where coarse detritus and sandy clay outcrop. On the other hand, the level of shaking appears moderate in all zones with outcropping limestone and volcanics.     

淮北临涣矿采煤沉陷区不同水体水化学特征及其影响因素

为研究淮北临涣矿采煤沉陷区不同水体的补给水源及溶质来源,在现场调查的基础上,系统采集丰水期、平水期、枯水期沉陷区积水、地表河水和浅层地下水样进行测试分析,采用Piper三线图、Gibbs图和因子分析方法,对不同水体水化学特征及其影响因素进行讨论.结果表明:地表水水体总溶解性固体(TDS)质量浓度表现为枯水期丰水期平水期,浅层地下水表现为枯水期平水期丰水期,地表水TDS质量浓度明显高于浅层地下水.地表水中主要阴阳离子为Na~+、Cl~-和SO_4~(2-),水化学类型主要为SO_4~(2-)-Cl~--Na~+型;浅层地下水离子以HCO_3~-、Ca~(2+)和Mg~(2+)为主,表现为HCO_3~--Ca~(2+)-Mg~(2+)型.结合Gibbs图和因子分析可知,地表水受蒸发作用、地表径流以及采煤活动等因素影响,浅层地下水在一定程度上体现出大气降水和地表水补给的特点,受岩石风化作用影响较为明显.     

Source,path and site effects on dominant frequency and spatial variation of strong ground motion recorded by smart1 and smart2 arrays in Taiwan

The reduction in spatial variance of strong ground motion with increasing earthquake magnitude has been reported recently. However, we show that the observed dependence of spatial variance on magnitude is its implicit dependence on the frequency content (dominant frequency) of the wave field. Time-domain cross-correlations of pairs of accelerograms are used to quantify the spatial variations in this paper. Magnitude is one of the factors contributing to the dominant frequency. We attempt to study separately the effects of magnitude, hypocentral distance, peak ground acceleration and focal depth on the dominant frequency in order to find the most significant one. The data base consists of 1965 records of horizontal acceleration from 148 local earthquakes in Taiwan. The analysis shows the overwhelming effect of the source magnitude on the formation of the dominant frequency with an empirical relationship: No significant effect of hypocentral distance, local acceleration amplitude or depth is detected for all their values available (up to 170 km, 250 cm/s², and 100 km, respectively). The prevailing effect of magnitude on the dominant frequency is a real cause of the consistently observed reduction of spatial variance of ground motion with increasing magnitude of earthquakes.     

Evaluation of seismic site factors in the Mississippi Embayment. II. Probabilistic seismic hazard analysis with nonlinear site effects

An integrated probabilistic seismic hazard analysis procedure that incorporates nonlinear site effects, PSHA-NL, is developed and used to characterize the influence of thick deposits of the upper Mississippi Embayment (ME) on seismic site coefficients. PSHA-NL follows the methodology of the 2002 USGS hazard maps and generates a compatible set of ground motion records. The motions are propagated using nonlinear and equivalent linear site response analyses and ME properties developed in a companion paper and used to derive surface uniform hazard response spectra. A set of generic site coefficients are derived and summarized in a format similar to NEHRP site coefficients, with an added dimension of ME deposits thickness to the Paleozoic rock, a physically meaningful impedance boundary. These coefficients compare well with NEHRP site coefficients for 30 m profiles. For thicker soil profiles, developed site coefficients are lower at short periods and higher at long periods than NEHRP site coefficients.     

1976年唐山MS7.8地震乐亭—滦南地区NW向地裂缝带赵滩点位调查

1976年唐山M_S7.8地震的早期野外调查结果显示, 在乐亭—滦南地区存在一条NW走向的地裂缝带, 自滦南县申营村往南断续延伸至乐亭县王滩镇王各庄村, 长度大于30 km, 以右旋走滑为主． 该地裂缝是1976年唐山M_S7.8地震形成的, 但其地理位置接近1976年唐山地震序列中滦县M_S7.1地震形成的NW向地表破裂带延伸方向上． 为进一步研究唐山地震的发震构造, 本文拟基于前人资料, 对乐亭—滦南地区NW向地裂缝带赵滩点位展开调查, 并实施探槽开挖和钻孔探测． 探槽揭示出小型砂脉和宽1.0—1.2 m的大型砂脉分别为1976年唐山M_S7.8地震和该地震之前另一次强震事件的产物; 钻孔探测结果表明主砂脉两侧地层不存在明显的垂直落差, 这与1976年唐山M_S7.8地震时该点位出现的地裂缝现象是右旋剪切以及赵滩探槽揭示的两侧垂直位移不明显的现象相吻合．      

Evaluation of seismic site factors in the Mississippi Embayment. I. Estimation of dynamic properties

Deep deposits of the Mississippi Embayment, overlying the New Madrid Seismic Zone, present unique challenges for the estimation of local site effects on propagated ground motion. Limited information is available on the dynamic properties of these deposits. This paper develops generalized depth dependent modulus degradation and damping curves specific to the embayment deposits. Depth dependent small strain damping is estimated using weak motion recordings during the Enola earthquake, 2001. Modulus degradation and damping curves are further constrained using limited laboratory test data of embayment soils at low confining pressure. At high confining pressures embayment specific data is unavailable and test data from outside the embayment is used in developing the dynamic properties. The representative modulus degradation and damping curves are used in non-linear and equivalent linear one-dimensional site response analyses. A companion paper describes a large-scale probabilistic seismic hazard analysis study in the Mississippi Embayment that integrates non-linear site effects.     

Further studies of the atmospheric temperature change produced by the Mt. Agung volcanic eruption in 1963

Monthly mean geopotential data at five tropical stations for the period 1959–1969 have been used to estimate the vertical distribution of tropospheric temperature change associated with the Mt. Agung volcanic eruption in March 1963. The estimate is that tropospheric temperature decreased by up to 1K with the maximum cooling occurring between August 1964 and August 1965, it is complicated by sea surface temperature changes. It is suggested that the change occurs either directly, by absorption of nearinfrared radiation by the stratospheric aerosol leading to less gaseous absorption and less heating in the troposphere, or indirectly by absorption of solar radiation and a concomitant decrease in evaporation and latent heat release in the troposphere.     

Lithological classification assisted by the joint inversion of electrical and seismic data at a control site in northeast Mexico

In this paper, evidence is presented that the combination of geospectral images and geophysical signatures (resistivity–velocity cross-plots) is a good tool to provide a natural visualization of the distribution and variations of lithological features in a test site. This was confirmed by the correlation between the electrical resistivity and seismic velocity values obtained after cross-gradient joint inversion at two profiles and geotechnical information provided by shallow boreholes in a site located in the Earth Sciences School grounds in Linares, Northeastern Mexico. The results obtained from this study show how the cross-gradient joint inversion facilitates the analysis of hydrological estimates and assists in lithological classification of subsurface materials.     

Estimation of site amplification,attenuation and source spectra of S-waves in the East-Central Iran

Generalized inversion of the S-wave amplitude spectra from the strong-motion network data in the East-Central Iran has been used to estimate simultaneously source parameters, site response and the S-wave attenuation (Q_s). In this regard, 190 three-component records were used corresponded to 40 earthquakes with the magnitudes M3.5–M7.3. These earthquakes were recorded at 42 stations in the hypocentral distance range from 9 to 200 km. The inverse problem was solved in 20 logarithmically equally spaced points in the frequency band from 0.4 to 15 Hz. The frequency-dependent site amplification was imposed, as a constraint, on two reference site responses in order to remove the undetermined degree of freedom in the inversion and obtain a unique inverse solution. Also, a geometrical spreading factor was assumed for removing the trade-off between geometrical spreading and anelastic attenuation. Different source parameters, such as seismic moment (M₀), seismic energy (E_s), corner frequency (f_c) and Brune stress drop (Δσ), were estimated for each event by fitting an ω² model to the spectra obtained from the inversion. The stress drop values of earthquakes, obtained in this research, are in good agreement with those of other studies. Also average site response values were correlated to the average shear wave velocities in the uppermost 30 m, in high and low frequency bands. The peak frequencies of site amplifications, estimated by the generalized inversion method, where in good agreement with those of horizontal to vertical (H/V) spectral ratios for the S-wave portion of records. However, no perfect matching in amplitude was obtained due to the deficiencies of the H/V ratio technique. By supposing a free shape for Q factor, a frequency dependent function was found, the logarithm of which could be approximated by a linear function, Q(f)=151f^0.75. The uncertainties of model parameters have been evaluated by covariance matrix of least-square fit. The residuals were also analyzed in order to assess the validity of the model. The analysis of residuals with respect to magnitude and distance indicates that they are distributed normally with approximately zero mean. The robustness of the results has been studied concerning their sensitivities to the omission of different datasets, selected randomly from original database. The results obtained here can be used in predicting ground-motion parameters applying stochastic methods.     

Signatures of thunderstorms in the variations of the secondary cosmic rays registered in Mexico City

We studied the effect of thunderstorms (TS) in the intensity variations of the electromagnetic and muon components of the Cosmic Rays during the year 2004, a year of minimum solar activity. We analyzed the variations in the counting rates of the upper and lower scintillators of the muon telescope installed in Mexico City at times of reported TS and compared those with variations during quiet times. The data were filtered to eliminate long trends, then a wavelet spectrum was calculated, searching for the temporal evolution of diverse periods of high significance; recurrent periodicities and total power distributions were obtained. The results show variations of short period whose main periodicities are arranged in a distribution where the most important are the shortest periodicities. These may be associated to the electric fields of the TS. Significant long period variations were found too, these could be due to other processes linked to rainstorms. No systematic effect on the power of variations due to TS was found.