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1.
In this study, we analyze the recent (1990–1997) seismicity that affected the northern sector (Sannio–Benevento area) of the Southern Apennines chain. We applied the Best Estimate Method (BEM), which collapses hypocentral clouds, to the events of low energy (Md max=4.1) seismic sequences in order to constrain the location and geometry of the seismogenetic structures. The results indicate that earthquakes aligned along three main structures: two sub-parallel structures striking NW–SE (1990–1992, Benevento sequence) and one structure striking NE–SW (1997, Sannio sequence). The southernmost NW–SE structure, which dips towards NE, overlies the fault that is likely to be responsible for a larger historical earthquake (Io max=XI MCS, 1688 earthquake). The northernmost NW–SE striking structure dips towards SW. The NE–SW striking structure is sub-vertical and it is located at the northern tip of the fault segment supposed to be responsible for the 1688 earthquake. The spatio-temporal evolution of the 1990–1997 seismicity indicates a progressive migration from SE (Benevento) to NW (Sannio) associated to a deepening of hypocenters (i.e., from about 5 to 12 km). Hypocenters cluster at the interface between the major structural discontinuities (e.g., pre-existing thrust surfaces) or within higher rigidity layers (e.g., the Apulia carbonates). Available focal mechanisms from earthquakes occurred on the recognized NW–SE and NE–SW faults are consistent with dip-slip normal solutions. This evidences the occurrence of coexisting NW–SE and NE–SW extensions in Southern Apennines.  相似文献   

2.
The horizontal-to-vertical spectral ratio technique has applied to detect the fundamental frequency at the sites of ambient noise recordings for New Domiat city. Noise measurements are acquired at 90 of sites for 1?h of continuous recording with a sampling rate of 100?Hz. Then, these data are processed following to SESAME project scheme. The presence of deep sedimentary basin in the Nile Delta suggests that the site response should be important. Consequently, the obtained fundamental frequency has lower values (0.2?C0.6?Hz). However, low-frequency ground motions attenuate more gradually with distance and can excite vibrations in large engineered structures, such as tall buildings and long bridges. There is hazardous threat even from the distant earthquakes originated from Mediterranean convergence zone for the structures in the city. It is recommended that the results of this study must be taken into consideration from civil engineering point of view before construction of civil engineering structures at this part.  相似文献   

3.
A “standard procedure” to characterize the seismic hazard of a given area was proposed. It is based on a multidisciplinary approach implying: (1) the knowledge of the seismic history of the area; (2) detailed geological surveys; (3)seismic noise measurements; (4) simulations of earthquake scenarios. The downtown of Acireale, a typical baroque town located on Eastern Sicily, was chosen as the “test area”. A catalog of the local seismogenic faults (able to generate earthquakes in historical times) has been compiled, as well as a seismic catalog for the effects of both local and regional earthquakes. The analysis of both catalogs allowed us to make the following conclusions: (1) the most important seismogenic faults affecting the Acireale municipality do not affect the downtown, while the related local earthquakes attenuate their energy (and intensity) in short (few km) distances; (2) the highest seismic intensity (degree X) experienced in Acireale downtown was caused by the 1693 regional earthquake; (3) over the last 140 years, the downtown has experienced the highest intensity value of VII only once, while six times the intensity was VI. On the whole, this implies a moderate seismic hazard. The estimation of the seismic hazard has been also approached by the experimental method of recording seismic noise. Measurements have been performed at seven different sites, where drills gave detailed information on the shallow subsurface geology to obtain HV (horizontal/vertical) spectral ratios. On the whole, the highest site amplification factor was moderate (about 7). A further investigation based on synthetic seismograms (and spectra) produced by simulating two given earthquake scenarios was also performed. The two scenarios are, respectively, representative of the largest expected earthquake in the area (the 1693 shock) and of a moderate (magnitude ca. 5.5) local earthquake (as the 1818 one). Moderate to strong locally expected accelerations were evidenced.  相似文献   

4.
Several source parameters (source dimensions, slip, particle velocity, static and dynamic stress drop) are determined for the moderate-size October 27th, 2004 (MW = 5.8), and the large August 30th, 1986 (MW = 7.1) and March 4th, 1977 (MW = 7.4) Vrancea (Romania) intermediate-depth earthquakes. For this purpose, the empirical Green's functions method of Irikura [e.g. Irikura, K. (1983). Semi-Empirical Estimation of Strong Ground Motions during Large Earthquakes. Bull. Dis. Prev. Res. Inst., Kyoto Univ., 33, Part 2, No. 298, 63–104., Irikura, K. (1986). Prediction of strong acceleration motions using empirical Green's function, in Proceedings of the 7th Japan earthquake engineering symposium, 151–156., Irikura, K. (1999). Techniques for the simulation of strong ground motion and deterministic seismic hazard analysis, in Proceedings of the advanced study course seismotectonic and microzonation techniques in earthquake engineering: integrated training in earthquake risk reduction practices, Kefallinia, 453–554.] is used to generate synthetic time series from recordings of smaller events (with 4 ≤ MW ≤ 5) in order to estimate several parameters characterizing the so-called strong motion generation area, which is defined as an extended area with homogeneous slip and rise time and, for crustal earthquakes, corresponds to an asperity of about 100 bar stress release [Miyake, H., T. Iwata and K. Irikura (2003). Source characterization for broadband ground-motion simulation: Kinematic heterogeneous source model and strong motion generation area. Bull. Seism. Soc. Am., 93, 2531–2545.] The parameters are obtained by acceleration envelope and displacement waveform inversion for the 2004 and 1986 events and MSK intensity pattern inversion for the 1977 event using a genetic algorithm. The strong motion recordings of the analyzed Vrancea earthquakes as well as the MSK intensity pattern of the 1977 earthquake can be well reproduced using relatively small strong motion generation areas, which corresponds to small asperities with high stress drops (300–1200 bar) and high particle velocities (3–5 m/s). These results imply a very efficient high-frequency radiation, which has to be taken into account for strong ground motion prediction, and indicate that the intermediate-depth Vrancea earthquakes are inherently different from crustal events.  相似文献   

5.
The evaluation of seismic site response in the urban area of Catania was tackled by selecting test areas having peculiar lithological and structural features, potentially favourable to large local amplifications of ground motion. The two selected areas are located in the historical downtown and in the northern part of Catania where the presence of a fault is evident. Site response was evaluated using spectral ratio technique taking the horizontal- to-vertical component ratio of ambient noise. Inferences from microtremor measurements are compared with results from synthetic accelerograms and response spectra computed at all drillings available for this area. Such method is particularly suitable in urban areas where the nature of the outcropping geological units is masked by city growth and anthropic intervention on the surface geology. The microtremor H/V spectral ratios evaluated at soft sites located within the downtown profile tend to be smaller than that usually reported in the literature for such soils. A tendency for amplifications to peaks near 2 Hz is observed only in some sites located on recent alluvial deposits. Evidences for amplifications of site effects (frequency range 4–8 Hz) were observed in the sampling sites located on the fault, with a rapid decrease of spectral amplitude just a few tenths of metres away from the discontinuity. Numerical simulations evidenced the importance of geolithological features at depth levels even greater than 20–30 m. Besides this, the results strongly confirm the importance of the subsurface geological conditions, in the estimate of seismic hazard at urban scale.  相似文献   

6.
The systematic analysis of seismograms recorded on the Romanian territory using Vrancea intermediate-depth earthquakes shows a strong asymmetric pattern relative to the epicentral area: on one side, in the Transylvanian Basin and the Eastern Carpathians (approximately along the inner volcanic chain), the amplitudes are reduced by a factor of 20 on average and the high frequencies are attenuated, in contrast with the other side, in the foreland platform. This pattern is explained by a significant attenuation increase caused by a strong lateral variation of the structure in the upper mantle, immediately towards NW of the Vrancea seismic active volume. This region corresponds to the most recent volcanic activity in the Persani Mountains and with the low-velocity body adjacent toward NW to the high-velocity body subducted beneath Vrancea area as indicated by seismic tomography and heat flow results. The CALIXTO'99 tomography experiment, deployed for 6 months in 1999, provides the largest number of observations for Vrancea earthquakes ever recorded on the Romanian territory. We select data from 8 earthquakes generated in this time interval in the Vrancea nest, which were recorded with signal / noise ratio greater than 5 by at least 25 stations. All of them are small- to moderate-magnitude events (3.6 ≤ Mw ≤ 4.2). The attenuation is much more important in the high-frequency range (> 1 Hz), than at low frequencies. Since the large Vrancea earthquakes can radiate significant energy in the low-frequency range (< 1 Hz), our results show that the seismic hazard level is much more uniform all over the Romanian territory in the low-frequency range than in the high-frequency range.  相似文献   

7.
朱新运 《地球科学》2016,41(12):2109-2117
华北盆地是中国大陆地震活跃区之一,通过地震波衰减及场地响应参数研究该区构造介质属性及台基属性对地震预测预报、灾害评估具有重要意义.基于Lg波谱比的联合反演方法是获得地震波衰减参数及场地响应的有效方法,通过随机重采样方法可以检验解的稳定性.使用华北盆地68个台站记录的2004—2008年的149次地震,震级ML为1.7~5.3的震中距为100~600 km,按信噪比大于2的标准挑选有效垂向记录1 000多条,地震射线较好地覆盖了华北盆地38°N~41°N、114°E~120°E区域.采用2.60~3.65 km/s的速度窗截取Lg波形并转化为频谱,研究频率范围为1~7 Hz,频率间隔0.2 Hz.计算得到的地震波衰减品质因子Q(f)与频率f的关系可表示为Q(f)=125±4.4f0.86±0.03,研究区为低Q0(对应频率1 Hz),高频率依赖性的构造活跃区.基岩台站对地震波没有表现出明显放大作用,黄土沉积台站低频端比高频端明显放大;场地响应波动较大台站其解的标准偏差也大,说明场地响应的不稳定性体现了台基属性的非稳定性特征.   相似文献   

8.
The prediction of the variability of the seismic ground motion in a given built-up area is considered an effective tool to plan appropriate urban development, to undertake actions on seismic risk mitigation and to understand the damage pattern caused by a strong-motion event. The procedures for studying the seismic response and the seismic microzonation of an urban area are well established; nevertheless, some controversial points still exists and are discussed here. In this paper, the selection of a reference input motion, the construction of a subsoil model and the seismic response analysis procedures are discussed in detail, based on the authors’ experience in two Italian case histories: the seismic microzonation of the city of Benevento, which was a predictive study, and the simulation of seismic response and damage distribution in the village of San Giuliano di Puglia, which was a retrospective analysis.  相似文献   

9.
I. Kawasaki  Y. Asai  Y. Tamura 《Tectonophysics》2001,330(3-4):267-283
Along the Japan trench where some Mw8 class interplate earthquakes occurred in the past century such as the 1896 Sanriku tsunami earthquake (M6.8, Mt8.6, 12×1020 N m) and the 1968 Tokachi-oki earthquake (Mw8.2, 28×1020 N m), the Pacific plate is subducting under northeast Japan at a rate of around 8 cm/year. The seismic coupling coefficient in this region has been estimated to be 20–40%. In the past decade, three ultra-slow earthquakes have occurred in the Sanriku-oki region (39°N–42°N): the 1989 Sanriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki (Mw7.7) earthquakes. Integrating their interplate moments released both seismically and aseismically, we have the following conclusions. (1) The sum of the seismic moments of the three ultra-slow earthquakes was (4.8–6.6)×1020 N m, which was 20–35% of the accumulated moment (18.6–23.0)×1020 N m, in the region (39°N–40.6°N, 142°E–144°E) for the 21–26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consistent with the previous estimates of the seismic coupling coefficient of 20–40%. On the other hand, the sum of the interplate moments including aseismic faulting is (11–16)×1020 N m, leading to a “seismo-geodetic coupling coefficient” of 50–85%, which is an extension of the seismic coupling coefficient to include slow events. (2) The time constants showed a large range from 1 min (102 s) for the 1968 Tokachi-oki earthquake to 10–20 min (103 s) for the 1896 Sanriku tsunami earthquake, to one day (105 s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (107 s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space–time distribution, three “gaps of moment release,” (40.6°N–42°N, 142°E–144°E) 39°N–40°N, 142°E–143°E) and (39°N–40°N, 142°E–144°E), are identified, instead of the gaps of seismicity.  相似文献   

10.
Situated in an active tectonic region the highly industrialised Cologne area (Germany) suffers from moderate-sized earthquakes. Our contribution to the mitigation of earthquake risk includes a microzonation study using ambient seismic noise and earthquake recordings from two field campaigns. An analysis of these data using the horizontal-to-vertical (H/V) as well as the classical spectral ratio (CSR) technique led to the following observations and conclusions: (1) The spatial variation in the thickness of the sedimentary cover is reliably retrieved using the fundamental resonance frequency estimated from the peak in the (H/V) ratio of ambient noise. (2) This fundamental resonance frequency is stable in time, but the amplitude of the peak (the amplification factor) is not. (3) The relative amplification variation of the H/V ratio in the area should therefore be checked systematically with repeated measurements before drawing conclusions about its significance. (4) The analysis of the H/V ratio of ambient noise provides the lower frequency bound, above which amplification may occur. (5) The shapes of the spectral ratios obtained by the different methods are generally in good agreement. However, the analysis of earthquake data shows that significant amplification of ground motion may also occur at frequencies higher than the fundamental one even when thick sediments are present.  相似文献   

11.
The magnitudes of early instrumentally recorded earthquakes in the Iberian region (1912–1962) have been studied through processing of digitized seismograms of Wiechert seismograph and analysis of macroseismic information. A magnitude system based on instrumental registrations and macroseismic observations has been proposed. It consists of two compatible magnitude formulae depending on the total duration of seismic oscillations and on the maximum ground amplitude/period ratio of surface waves and includes correspondent intensity–magnitude relationships.  相似文献   

12.
分析了烟台市区的地震环境特点和地震活动态势 ,研究了烟台市区抗震防灾的现状与问题 ,并提出抗震防灾的对策与建议。  相似文献   

13.
A short period three component seismic system was operated for a period of more than 1000 hrs (distributed over five intervals) at the Badra area, east central Iraq, during the period November 1975–March 1976.

A short review is given of the analyses of the seismograms, such as rate of occurrence of microearthquakes, statistical analysis and spectral analysis of selected events.  相似文献   


14.
Traditional methods of imaging the Earth's subsurface using seismic waves require an identifiable, impulsive source of seismic energy, for example an earthquake or explosive source. Naturally occurring, ambient seismic waves form an ever-present source of energy that is conventionally regarded as unusable since it is not impulsive. As such it is generally removed from seismic data and subsequent analysis. A new method known as seismic interferometry can be used to extract useful information about the Earth's subsurface from the ambient noise wavefield. Consequently, seismic interferometry is an important new tool for exploring areas which are otherwise seismically quiescent, such as the British Isles in which there are relatively few strong earthquakes. One of the possible applications of seismic interferometry is ambient noise tomography (ANT). ANT is a way of using interferometry to image subsurface seismic velocity variations using seismic (surface) waves extracted from the background ambient vibrations of the Earth. To date, ANT has been used successfully to image the Earth's crust and upper-mantle on regional and continental scales in many locations and has the power to resolve major geological features such as sedimentary basins and igneous and metamorphic cores. Here we provide a review of seismic interferometry and ANT, and show that the seismic interferometry method works well within the British Isles. We illustrate the usefulness of the method in seismically quiescent areas by presenting the first surface wave group velocity maps of the Scottish Highlands using only ambient seismic noise. These maps show low velocity anomalies in sedimentary basins such as the Moray Firth, and high velocity anomalies in igneous and metamorphic centres such as the Lewisian complex. They also suggest that the Moho shallows from south to north across Scotland which agrees with previous geophysical studies in the region.  相似文献   

15.
Site response measurements provide information on the amplification of ground motions generated by local conditions. Recent studies of large destructive earthquakes have shown that damage during the earthquakes are often caused by the amplification of seismic waves in near-surface sedimentary layers. The estimation of site response is therefore critical, in order to evaluate the true seismic hazard potential of a given area. We investigated local site amplifications in the South Iceland Seismic Zone (SISZ). Nine digital seismographs were deployed, temporarily, in an area of approximately 400 km2, in the westernmost part of the SISZ. Among the 90 events recorded, 15 were used in this study, including a magnitude 3.1 (ML) event and selected aftershocks, which occurred in the northern outskirts of the village Hveragerdi. Single Station Spectral Ratios (SSSR) of the recorded earthquakes revealed some of the effects of local site conditions. Spectral amplification factors of 2–5 on average, can be expected in the SIL area, depending on the sediment type and thickness. Higher site amplifications occur in the southern part of the study region, where the thickest sedimentary cover is found. Spectral amplification, related to topographical effects, is observed at the bedrock reference station, Bjarnastadir. Standard Spectral Ratios (SSR), with respect to the bedrock reference station, Bjarnastadir, were also calculated for some stations, in order to compare the two spectral ratio results. The two methods show a good correlation at some stations, whereas at others they vary considerably. The comparison between the earthquake and ambient noise data, on the other hand, gave better correlation when the SSSR method is used.  相似文献   

16.
The archaeological site of Sagalassos (SW Turkey) is located in a region characterized by the absence of any significant recent seismic activity, contrary to adjacent regions. However, the assessment of earthquake-related damage at the site suggests that the earthquakes that have been demonstrated to have struck this Pisidian city in ca. AD 500 and in the middle or second half of the 7th century AD are characterized by an MSK intensity of at least VIII and occurred on a fault very close to the city. Different investigation techniques (archaeoseismology, remote sensing and geomorphology, surface geology and structural data, 2D resistivity imaging and palaeoseismological trenching) have been applied at the archaeological site and its direct surroundings in search for the causative fault of these earthquakes. This multidisciplinary approach shows that each of the different approaches independently provides only partial, non-conclusive information with respect to the fault identification. Integration is imperative to give a conclusive answer in the search for the causative fault. This study has, indeed, revealed the existence of a to date unknown active normal fault system passing underneath ancient Sagalassos, i.e. the Sagalassos fault. A historical coseismic surface rupture event on this fault could be identified. This event possibly corresponds to the devastating Sagalassos earthquakes of ca. AD 500 and the middle or second half of the 7th century AD. Finally, this study demonstrates that in the particular geodynamic setting of SW Turkey archaeological sites with extensive earthquake-related damage form an important tool in any attempt to asses the seismic hazard.  相似文献   

17.
The assessment of seismic hazard at five selected sites in the Sannio-Matese region is based on the computer program SRAMSC. Owing to the extensive historical data base for the output parameter, the MSK intensiy is chosen. The seismicity model is made up of five narrow area seismic sources. Circular or elliptical macroseismic fields are assigned to individual sources. A generalized Kövesligethy equation is used for this purpose as the attenuation relationship. The study reveals similar and a rather high hazard at the sites at Benevento, Boiano, and Melfi, which are located in the zone of highest seismic activity. At the Pomigliano and Lucera sites, the assessed hazard is much lower.  相似文献   

18.
利用2011年1—6月云南地区的连续波形资料,采用背景噪声和波形互相关方法分别反演该地区的速度结构以及2011年3月24日缅甸7.2级地震前后60d的速度变化图像。同时,根据云南地区中小地震计算缅甸地震前后应变能释放响应比空间分布,并利用缅甸地震的震源参数,计算了缅甸地震对云南地区主要断裂产生的库仑破裂应力影响。结果显示:(1)禄劝至华坪一带、永定至泸水区域和通海至建水地区震后波速增加,同时该地区地震活动增强,相应断裂上库仑破裂应力增加,说明缅甸地震对这些区域具有加速构造活动的正影响;(2)小江断裂带以东马龙至宣威地区和南汀河断裂带以南临沧至景洪地区震后波速降低,地震活动减弱,断裂上库仑破裂应力降低,说明缅甸地震对该区域具有减缓构造活动的负影响。  相似文献   

19.
Modeling of seismic hazard for Turkey using the recent neotectonic data   总被引：1，自引：0，他引：1  
Kamil Kayabali   《Engineering Geology》2002,63(3-4):221-232
Recent developments in the neotectonic framework of Turkey introduced new tectonic elements necessitating the reconstruction of Turkey's seismic hazard map. In this regard, 14 seismic source zones were delineated. Maximum earthquake magnitudes for each seismic zones were determined using the fault rupture length approximation. Regression coefficients of the earthquake magnitude–frequency relationships for the seismic zones were compiled mostly from earlier works. Along with these data, a strong ground motion attenuation relationship developed by Joyner and Boore [Joyner, W.B., Boore, D.M., 1988. Measurement, characterization, and prediction of strong ground motion. Earthquake Engineering and Soil Dynamics, 2. Recent Advances Ground Motion Evaluation, pp. 43–102.] was utilized to model the seismic hazard for Turkey using the probabilistic approach. For the modeling, the “earthquake location uncertainty” concept was employed. A grid of 5106 points with 0.2° intervals was constituted for the area encompassed by the 25–46°E longitudes and 35–43°N latitudes. For the return periods of 100 and 475 years, the peak horizontal ground acceleration (pga) in bedrock was computed for each grid point. Isoacceleration maps for the return periods of 100 and 475 years were constructed by contouring the pga values at each node.  相似文献   

20.
This study uses the seismic refraction and noise measurements to investigate the velocity structure of the subsurface and emphasize the advantage of ambient vibration over the conventional seismic refraction technique. Field measurements were carried out at nine sites in and around Zagazig city. Shallow seismic refraction data were interpreted using the delay time method to obtain the two-dimension ground model at each site. Ambient vibration arrays are used to infer the one-dimensional compressional and shear wave velocity profiles through two main steps. The first step is to derive the dispersion curve from the recorded signals using the frequency-wavenumber method. The second is to invert the dispersion curve to obtain the site velocity profiles. The results of the compressional wave velocities obtained from seismic refraction technique showed that the subsurface consists of a number of layers ranging from two to four layers and give a good agreement with the results of the seismic wave velocities obtained from the ambient vibration arrays. The ambient vibration arrays gave a deeper depth of penetration than the other method, providing more information on the subsurface structure without any disturbance to the environment. This work provides reliable estimates of the seismic velocity structures of both shallow and deep sedimentary layers within the area of interest.  相似文献   

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