Characteristics of long period microtremor and validation of microtremor array measurements in inland areas of China

To study the characteristics of long period microtremor and applicability of microtremor survey,we have made microtremor observations using long period seismometers of the China's mainland from coastal cities like Shanghai and Tianjin through Beijing,Xi'an,to the far inland cities of Lanzhou and Tianshui.The observation shows that the level of power spectrum of long period microtremors reduced rapidly from the coast to the inland area.However,the energy of long period microtremors in Beijing,Xi'an,Lanzhou and Tianshui area are only approximately ten-thousandth to thousandth of that in Shanghai.Aiming at the complexity of the underground structure in the far inland,a series of underground structure models with different distributions were assessed using three-dimensional,dynamic finite element method(FEM) analyses.The results were used to evaluate microtremor survey methods and their limitations with regard to aggregate variability and thickness determinations.Multiple-wave reflections between layers with the change of underground structure distribution occurred,which have significant effect on the performance of the different field approaches.Information over a broad spectrum from which velocity-depth profiles were produced via inversion approaches.Neither the thickness nor the shear waveVelocity V of the subsurface layer inversion results appeared over a large evaluation with increasing slope angle.In particular,when the angle of the model reached 45°,it could not be accurately evaluated using one-dimensional inversion methods.Finally,the array microtremor survey(AMS) was carried out in Shanghai City,China.Although AMS techniques do not have the layer sensitivity or accuracy(velocity and layer thickness) of borehole techniques,the obtained shear wave velocity structure is especially useful for earthquake disaster prevention and seismic analysis.

Estimation of Shallow S-Wave Velocity Structure of Two Practical Sites from Microtremors Array Observation in Tangshan Area

Microtremors array observation for estimating S-wave velocity structure from phase velocities of Rayleigh and Love wave on two practical sites in Tangshan area by a China-US joint group are researched.The phase velocities of Rayleigh wave are estimated from vertical component records and those of Love wave are estimated from three-component records of microtremors array using modified spatial auto-correlation method.Haskell matrix method is used in calculating Rayleigh and Love wave phase velocities,and the shallow S-wave velocity structure of two practical sites are estimated by means of a hybrid approach of Genetic Algorithm and Simplex.The results are compared with the PS logging data of the two sites,showing it is feasible to estimate the shallow S-wave velocity structure of practical site from the observation of microtremor array.

Application of geophysical methods in fine detection of urban concealed karst:A case study of Wuhan City,China

The construction of modern livable cities faces challenges in karst areas,including ground collapse and engineering problems.Wuhan,with a population of 13.74×10^(6) and approximately 1161 km^(2)of soluble rocks in the urban area of 8569.15 km^(2),predominantly consists of concealed karst areas where occasional ground collapse events occur,posing significant threats to underground engineering projects.To address these challenges,a comprehensive geological survey was conducted in Wuhan,focusing on major karstrelated issues.Geophysical methods offer advantages over drilling in detecting concealed karst areas due to their efficiency,non-destructiveness,and flexibility.This paper reviewed the karst geological characteristics in Wuhan and the geophysical exploration methods for karst,selected eight effective geophysical methods for field experimentation,evaluated their suitability,and proposed method combinations for different karst scenarios.The results show that different geophysical methods have varying applicability for karst detection in Wuhan,and combining multiple methods enhances detection effectiveness.The specific recommendations for method combinations provided in this study serve as a valuable reference for karst detection in Wuhan.

Estimation of Deep Soil Profiles in Lima Peru

Deep shear-wave velocity profiles at eight places in Lima Peru were estimated based on the inversion of dispersion curves. The dispersion curves were calculated from small and large microtremor arrays using two methods： the F-k proposed by Capon （1969） and the CCA proposed by Cho et al. （2004）. For the purpose of large array measurement we introduced a new type of sensor. Important results are the relative shallow depths to the basement rock in the area classified as alluvial gravel that covers most of the area of Lima city; and the relative large depth to the bedrock in places identified as VSV and CMA. It is recommended that this study be complemented with PS loggings in order to verify the estimated profiles.

Preliminary Analysis for Ground Structure around Source Region of 2008 Iwate-Miyagi Nairiku Earthquake

To obtain a detailed model of the three-dimensional ground structure, the microtoremor and gravity surveys were carried out around Kurikoma area, Japan, where is the source reagion of 2008 Iwate-Miyagi nairiku earthquake and is located about 90 km north of Sendai. Using the microtremor data, velocity structures of shallow sediments are estimated and the results at some of the sites are reported preliminarily. We could not find the detailed velocity structure to bedrock, because small arrays for the microtremor observation were applied, However, significant structures are observed for shallow velocity structure at some sites. Furthermore, gravity data provided Bouguer anomaly, which is one of gravity anomalies, around the target area. From the Bouguer anomaly, area with low anomaly is tbund around the south-eastern area of the fault plane.

Identification of hidden faults using determining velocity structure profile by spatial autocorrelation method in the west of Mashhad plain(Northeast of Iran)

Characterizing the subsurface structure is an important parameter for the improvement of seismic hazard assessment.Due to the tectonic complexity of the earth,some deep fractures do not reach the earth's surface and are not detectable with visual analysis.Therefore,the lack of knowledge of faults and fractures can result in disasters,especially in urban planning.Many geophysical methods can be used to estimate subsurface structure characterization.However,a more reliable method is required to assess seismic hazards and reduce potential damage in metropolitan areas without destroying buildings and structures.This paper aims to identify hidden faults and structures using shear wave velocity sections.To do this,surface wave dispersion curve was extracted from the vertical component of microtremor array recording using the spatial autocorrelation(SPAC)method in two profiles and 13 array stations(perpendicular to the altitudes)to obtain shear wave velocity structure(Vs)in the west of Mashhad,northeast of Iran.The results of shear wave velocity profiles(Vs)indicate sudden changes in the thickness of sediments.This can be related to the displacement of a normal fault in this area causing the bottom rock to fall and an increase in the alluvial thickness in the central part of the plain.The velocity in the floor rock is 2000 meters per second in this area.According to the surface outcrops and water wells data,its material is slate and Phyllite metamorphic rocks that are exposed in the adjacent heights.Besides,the seismic profile results were well consistent with electrical resistance data and well logs indicating that the tool array method is flexible,non-invasive,relatively fast,and effective for urban areas with satisfactory accuracy.