1997年9月26日意大利中部Umbria-marche地震前的水文地质变化

分析了1997年9月26日意大利中部Umbria与marche交界地区的Umbria-marche地震序列引起的水文体系的变化。为了量化水文体系的变化,从地方当局和矿泉水公司收集了地下水位、泉水的流量以及河水流速等的资料,这些资料可以确定异常的影响面积,并且可以通过与从震前几年资料估算出的年平均体系的对比来量化异常的变化,试图对异常进行判定。分析结果表明,两个含水单元,即Umbria-marche山峰和Val-nerina单元,具有紧密的联系,它们都由于水文系统的改变而受到影响。地震前几个月两条河流测量站记录到了异常变化,认为是地震前兆异常。     

A site effect study in the Verchiano valley during the 1997 Umbria-Marche (Central Italy) earthquakes

Strong site effects were observed during the two M _W 5.7 and M _W 6.0 main shocks of the Colfiorito seismic crisis which occured on September 26, 1997 in Umbria-Marche (Central Italy).The most obvious indications of these effects are the dramatic differences in damage shown by buildings of similar construction in neighboring villages.Such observations were specifically made in the Verchiano valley in the fault area, 15 km south of Colfiorito where the Verchiano village and the Colle and Camino hamlets were heavily damaged (MCS intensity IX-X) since the first main shock of 1997/09/26,while in contrast, the Curasci village located 2 km eastwards remains almost intact.In order to study the anomalous ground motion amplifications in this area, an array of 11, 3-components seismo- and accelero-meters was set up during the 1997/10/20-24 period, extending from the western side of the valley, up to the top of Mount San Salvatore, going accross the Colle and Curasci hamlets.During the experiment, 67 aftershocks enlightened the valley from the Colfiorito (10 km north) and the Sellano (6 km south) active swarms.Seismic refraction experiments were conducted at the same time in the 500 m wide, 1500 m long Verchiano valley in order to determine the thickness and main characteristics of the alluvial infilling.The main results are: (i) compared to the valley side ground motion, and for all the events, recordings in the central part of the valley (piana di Verchiano) show relative amplification of 10 with a clear lengthening of the seismogram duration by a factor of 2 – (ii) broad band relative amplification of 6–8 is also clearly identified at the top of the Mount San Salvatore overhanging the valley – (iii) any of the site effect measurements done explains by itself the strongly contrasted damage observed at Colle and Curasci: i.e. the modification of the near-field radiation pattern by interaction with the free heterogeneous surface may have induced local shadow zones that saved Curasci.     

The 1997 Umbria-Marche (Italy) earthquake sequence: analysis of the data recorded by the local and temporary networks

We present some preliminary results obtained from thejoined analysis of the data collected by the permanentand the temporary networks operating in the area ofthe earthquake sequence that followed (andanticipated) the 26 September, Central Italy, mainshocks. In particular, these earthquake data haveallowed us to determine a well constrainedwave-velocity model (both P and S) with stationcorrections which demonstrated to produce robusthypocentral locations. These velocity modelswith station corrections have been used forre-locating the whole September 1997–July 1998subset of data of the permanent network, and theprevious background seismicity, starting from May1996. The focal mechanisms of the largest events werealso obtained from an analysis of the first-motionpolarities.Our results indicate that 1) the seismic activityaligns on a SE-NW trend for a total length of about50 km of extension; 2) the focal depth of theseevents is restricted to the range 0–9 km; 3) mostevents can be related to sub-parallel SW dipping faultplanes; 4) focal mechanisms of the largest shocks(M_L > 4) show a coherent behaviour, withnormal fault solution on SSE-NNW striking, SW dippingplanes; 5) the space-time evolution of the activitydisplays a discontinuous mode of energy release, withdifferent episodes of activation and an apparentclustering of aftershocks at the edges of the areaswhich presumably ruptured in the main shocks.     

Calibration of stochastic finite-fault ground motion simulations for the 1997 Umbria-Marche, Central Italy, earthquake sequence

The recent 1997 Umbria-Marche, Central Italy, earthquake sequence allowed us to model recorded ground motions using a method developed by Beresnev and Atkinson [Bull Seism Soc Am 87 (1997) 67–84; Seism Res Lett, 69 (1998) 27–32; Bull Seism Soc Am 88 (1998) 1392–1401]. The method generalizes the stochastic ground-motion simulation technique, developed for point sources, to the case of finite faults. It subdivides the fault plane into subfaults and assumes each subfault to be a point source with a ω² spectrum. Geometric spreading and regional anelastic attenuation are included in the model. The data include horizontal acceleration recordings from the SSN and ENEL databases of the 1997 Umbria-Marche events on 26 September, at 00:33 GMT, with M_w=5.7, and at 09:40 GMT, with M_w=6.0; and on 14 October at 15:23 GMT, with M_w=5.6. The strong motion simulations are performed using model parameters based on the results of previous studies, and adjusting the subfault size to calibrate the simulation model against recorded ground motions. Local site response is considered to account for observed amplification effects at specific recording sites (e.g. Nocera Umbra). A good agreement is found between the simulated response spectra and the recorded data, concluding that this method reproduces the salient ground-motion characteristics at different distances and azimuths.     

Study of site effects in the area of Nocera Umbra (Central Italy) during the 1997 Umbria-Marche seismic sequence

During the M_L = 5.6 and 5.8 earthquakes occurredin central Italy on 26 September 1997 the historiccentre of Nocera Umbra, lying on top of a 120 m highhill, was diffusely damaged (VII-VIII degrees of MCSintensity). Some recently built houses in the modernpart of the town suffered an even higher level ofdamage. A temporary seismic array was deployed toinvestigate a possible correlation between localamplifications of ground motion in this area and theobserved pattern of damage. After a geologic andmacroseismic survey, eight sites were selected asrepresentative of different local conditions, such astopographic irregularities, sharp hard-to-softlithology transitions, alluvium-filled valleys, andboth undisturbed and deformed rocks.Horizontal-to-vertical spectral ratios for bothmicrotremor and earthquake recordings, as well asspectral ratios referred to undisturbed rock sites,were used to quantify local variations of groundmotion. In spite of the diffuse damage in the historiccentre of Nocera Umbra, a small amplification isobserved at the stations on the hill's top. Thissuggests that the higher vulnerability of the ancientbuildings mainly accounts for the diffuse damage inthat part of the town. In the frequency band ofengineering interest (1 to 10 Hz) the largestamplifications of ground motion are found at softsites: in the Topino river valley, where many episodesof severe structural damage occurred, spectralamplification is significant over a broad frequencyband ranging from 2 Hz to more than 20 Hz. Inparticular, in the central part of the valley highamplification (> 4) is found from 3 to 10 Hz,reaching a maximum of 20 around 4 Hz. At the edge ofthe valley, close to the soil-to-rock transition,amplification is as large as 10 in a frequency bandranging from 4 to more than 20 Hz. A significantamplification (by a factor of 10 around 10 Hz) isobserved also at one of the rock sites, possibly dueto the presence of a cataclastic zone related to theactivity of a regional fault that altered themechanical properties of the rock.     

Monte Carlo Inversion of DInSAR Data for Dislocation Modeling: Application to the 1997 Umbria-Marche Seismic Sequence (Central Italy)

— The study of surface deformation due to seismic activity is often made using dislocations with uniform slip and simple geometries. A better modeling of coseismic and postseismic surface displacements can be obtained by using dislocations with variable slip and nonregular shapes. This is consistent with the asperity model of fault surfaces, assuming a friction distribution on faults made of locked zones with much higher friction than surrounding zones. In this paper we consider the 1997–1998 Colfiorito seismic sequence. The coseismic surface displacements in the Colfiorito zone are used in order to infer the slip distribution on the fault surface at different stages of the sequence. The displacement field has been modeled varying the slip distribution on the fault, and comparing the deformation observed by SAR and GPS techniques with model results. The slip distribution is calculated by Monte Carlo simulations on a normal fault with the dip angle equal to 40°. A good approximation is obtained by using square asperity units of 1.5×1.5 km². In the first stage, we employed a simplified model with uniform slip, in which each asperity unit is allowed to slip a constant amount or not to slip at all, and in the second stage, we evaluate the slip distribution in the dislocation area determined by the Monte Carlo inversion: in this case we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. The results show that the 1997 seismic events of the sequence can be modeled by irregular dislocations, obtaining a good fit to the DInSAR and GPS observations. The model also confirms the results of previous studies by a different methodology, defining the distribution of asperities on the fault plane using the fault geometry, the geodetic data and the seismic moment of the 1997–1998 Colfiorito seismic sequence. Furthermore, the analysis of 1997 aftershocks in the seismogenic region shows a strong correlation between most events and the asperity distribution, which can be considered as an independent test of the validity of the model.     

Observations of vertical ground accelerations exceeding gravity during the 1997 Umbria-Marche (central Italy) earthquakes

We found extensive evidence that the vertical ground accelerations produced during the largest shock (M = 6.0) of the 1997 Umbria-Marche earthquake sequence exceeded 1g in two areas close to the heavily-damaged villages of Annifo and Colle Croce. This evidence comes from the striking observation of thousands of freshly fractured and broken rocks and stones in these areas. Some of the broken stones lie isolated on soft detritic soil while others had been previously piled up, probably a long time agoto clear the fields for farming. The freshness of the cuts and fractures and the consistency of the observations for thousands of rocks and stones in these areas indicate that these rocks were thrown upwards during the earthquake, with breakage occurring at the time of impact. Ground motion calculations consistent with the static deformation inferred from GPS and interferometry data, show that the broken stones and rocks are found in the zone where the strongest shaking took place during the earthquake and that most of the shaking there was vertical.     

地震活动性与日月引潮力相关性统计分析

基于ISC地震记录,利用天体动力学中二体问题的轨道方程和摄动理论,确定发震时刻太阳在地面的投影点位置,并进一步计算出太阳相对震中的地心天顶距,将地震按天顶距的大小进行统计,得到地震的太阳天顶距地震频度分布。同样的方法,可得地震的月球天顶距地震频度分布。统计发现:地震的太阳天顶距地震频度和月球天顶距地震频度分布表现出一致的规律性,且较大地震和小地震的活动规律不同,较大地震丛集发生在太阳(月球)天顶距0°和180°附近,与地面引潮力的绝对值分布有较好的一致性;而小地震多丛集在太阳(月球)天顶距60°和120°附近。在此基础上,我们对地震的日月天顶距同时进行统计,建立了日月天顶距地震概率密度分布,结果表明,日月投影点及其对蹠点周围,较大地震发生的概率较高。     

The 1997 Umbria-Marche earthquakes (Italy): relation between the surface tectonic breaks and the area of deformation

We classified the most outstanding rupturesof the 1997 Umbria-Marche seismic sequence assecondary tectonic effects that occur within the zoneof deformation induced by the deep displacement on theseismogenic structure. The trend of the surfacedeformation is homogeneous within the entire area ofinterest and consistent with NE-oriented extensionevidenced by CMT focal solutions of the three mainshocks. We extrapolate the discontinuous sites ofbreak measurements and suggest that the localdeformation concentrates along four narrow bands.Location and direction of these bands are locallycontrolled by pre-existing structures. The comparisonbetween our data with the seismological data – such asmain rupture planes and spatial aftershockdistribution – highlights that three bands mark partof the boundaries of the NW-SE elongated aftershocksarea and the fourth occurs where this area is widest.Moreover, the analysis of the structural setting ofthe area suggests that N-S shear zones have stronglycontrolled the extension of the main rupture segmentsand the aftershock distribution. The surface rupturesare located within the area of coseismic deformationresulting from DInSAR data; we propose that theyrepresent the localized response to the verticalground deformation of the area. Finally, we discussthe contribution of the pattern of the 1997 surfacebreaks to the characterization of the seismogenicsource.     

Simulation of Seismicity in the Block-structure Model of Italy and its Surroundings

The numerical block-model of the lithosphere dynamics is used to simulate seismicity in Italy and its surroundings, based on the available structural and geodynamics information. The purpose of the study is to understand which are the tectonic processes that control the main features of the observed seismicity and the kinematics of the region. The influence of the rheology of the fault systems is studied as well. The model we use differs from other modeling approaches in that it simulates earthquakes and hence it possibly relates to seismicity and geodynamics. The model provides an effective capability to include the set of documented constraints supplied by widely available earthquake catalogs. This is done by means of the comparison of the GR relation, of the focal mechanisms and of the space distribution for observed and computed seismicity. The region is modeled as a system of perfectly rigid blocks, separated by infinitely thin fault planes, in viscoelastic interaction between themselves and with the underlying medium. The movement of the boundary blocks and of the underlying medium determines the motion of the blocks. The synthetic seismicity obtained with the defined block-model is similar to the observed one for the most seismically active areas. A linear frequency-magnitude (FM) relation (Gutenberg-Richter law) is obtained for synthetic earthquakes; the slope (b-value) of the FM plot appears larger for the synthetic seismicity than for the observed one. Nevertheless, the b-value is essentially larger in northern and central Italy than that in southern Italy, both in the model and in the observations. The analysis of the source mechanisms of the synthetic earthquakes shows a good agreement with the observations. In the model normal faulting is typical for the Apennines, the eastern edge of Sicily and the Calabrian arc, while reverse faulting takes place at the northwestern boundary of the Adriatic Sea, in the southern Alps and along the eastern edge of the Adria, along the Dinarides. The model correctly reproduces the extension zone along the Apennines and the contraction zone along the northwestern boundary of the Adriatic Sea; the counter-clockwise rotation of the Adria is mimed. The resulting movements of the blocks are in overall agreement with GPS (Global Positioning System) observations. The results of the modeling experiments suggest that the main features of dynamics and seismicity in the central Mediterranean region cannot be satisfactorily explained as a consequence of Africa and Eurasia convergence only; the passive subduction in the Calabrian arc and the different rheology of faults are essential as well.     

Analysis and modelling of rainfall fields at different resolutions in southern Italy

Abstract

The spatial and temporal variability of the scaling properties and correlation structure of a data set of rainfall time series, aggregated over different temporal resolutions, and observed in 70 raingauges across the Basilicata and Calabria regions of southern Italy, is investigated. Two types of random cascade model, namely canonical and microcanonical models, were used for each raingauge and selected season. For both models, different hypotheses concerning dependency of parameters on time scale and rainfall height can be adopted. In particular, a new approach is proposed which consists of several combinations of models with a different scale dependence of parameters for different temporal resolutions. The goal is to improve the modelling of the main features of rainfall time series, especially for cases where the variability of rainfall changes irregularly with temporal aggregation. The results obtained with the new methodology showed good agreement with the observed data, in particular, for the summer months. In fact, during this season, rainfall heights aggregated at fine temporal resolutions (from 5 to 20 min) are more similar (relative to the winter season) to the values cumulated on 1 or 3 h (due to convective phenomena) and, consequently, the process of rainfall breakdown is nearly stationary for a range of finer temporal resolutions.

Editor D. Koutsoyiannis; Associate editor A. Montanari     

中亚地震目录震级转换及其完整性分析

为编制能够应用于地震危险性分析的中亚地区统一震级标度为矩震级的地震目录,从国际地震中心（ISC）下载得到该地区1907-2012年的地震数据,该数据包含众多机构不同震级标度的地震记录.以MW,GCMT为参考机构震级标度,并用最小二乘法拟合了其他机构震级标度与MW,GCMT之间的转换关系.在挑选某次地震事件的唯一震级记录时,以对应转换关系的相关系数大且剩余标准差小为准则进行筛选.对于少量没有震级转换关系或者拟合优度过差的地震记录,使用间接转换关系或者全球转换关系予以补充转换.震级转换后,用时空窗法删除前余震,并考虑构造环境和地震活动水平的空间差异性将研究区划分成5个子区域.采用地震记录时间累积曲线法、最大曲率法（MAXC）和拟合优度检验法（GFT）综合分析各个分区的最小完整震级（MC）,并在此基础上用极大似然法拟合相应的地震活动性参数.结果表明,每种完整性分析方法各具一定的优缺点,但采用综合分析的方法能够得出最佳的MC.地震记录时间累积曲线法能分析出高质量地震目录的起始时间,以作为后两种方法的基础,但容易受到地震活动水平随时间波动的影响.由于研究区目录质量较差,最大曲率法误差过大以至于只能作为其他方法的补充,拟合优度检验法的GFT参数也普遍只能达到80％左右.GFT最大值点可能并不对应MC,但是MC通常都在GFT极大值点取得.5个分区1964-2012年的MC普遍在Mw4.8左右,b值在1.136-1.514之间波动.     

Hypocenter location accuracy and seismicity distribution in the Central Apennines (Italy)

In this paper, we analyse the seismicity distribution in the CentralApennines (Italy) using the recordings of the Rete SismometricaMarchigiana (RSM). In particular, the selected events are relocated usinga 1-D model calculated by means of an inversion procedure. The robustnessof the 1-D model and the location accuracy are tested. The capability ofthe RSM to well constrain crustal and subcrustal events in the studied areais discussed. We find that in the inner side of the chain the seismicity liesin the upper crustal layers, following the structural trend of the Apenninicbelt. A W-deepening of the events is observed in the Apenninic foredeep,where the seismicity is mainly confined in the lower crust. This evidenceimplies the deepening of the brittle to ductile transition. Some well-locatedsubcrustal events are found. Their locations seem to confirm the W-dippingsubduction of the Adriatic lithosphere beneath the Apennines.     

Earthquake focal mechanisms and stress inversion in the Irpinia Region (southern Italy)

The goal of this study was to estimate the stress field acting in the Irpinia Region, an area of southern Italy that has been struck in the past by destructive earthquakes and that is now characterized by low to moderate seismicity. The dataset are records of 2,352 aftershocks following the last strong event: the 23 November 1980 earthquake (M 6.9). The earthquakes were recorded at seven seismic stations, on average, and have been located using a three-dimensional (3D) P-wave velocity model and a probabilistic, non-linear, global search technique. The use of a 3D velocity model yielded a more stable estimation of take-off angles, a crucial parameter for focal mechanism computation. The earthquake focal mechanisms were computed from the P-wave first-motion polarity data using the FPFIT algorithm. Fault plane solutions show mostly normal component faulting (pure normal fault and normal fault with a strike-slip component). Only some fault plane solutions show strike-slip and reverse faulting. The stress field is estimated using the method proposed by Michael (J Geophys Res 92:357–368, 1987a) by inverting selected focal mechanisms, and the results show that the Irpinia Region is subjected to a NE–SW extension with horizontal σ ₃ (plunge 0°, trend 230°) and subvertical σ ₁ (plunge 80°, trend 320°), in agreement with the results derived from other stress indicators.     

藏、青、川、滇交界地区地震活动性分析

1955年以来藏、青、川、滇交界地区5级以上地震活动处于平静~活跃的韵律状态,本文详细讨论了这期间出现的10次完整的平静~活跃过程,讨论了过程中所表现出的若干现象,及其与青藏块体大范围地震活动存在的呼应关系。     

Possible creep-related tilt precursors obtained in the Central Apennines (Italy) and in the Southern Caucasus (Georgia)

Daily averaged tilt component data from two sites of the Central Apennines (Italy) and of the Southern Caucasus (Georgia), respectively, revealed intermediate-term tilts as possible precursors to earthquakes (M=3.0÷4.7) which occurred in the above-mentioned seismic areas within a distance of 50 km from the sites. A good temporal correlation as well as a fair spatial correspondence between these residual tilts (with amplitude and duration of some microradians and months, respectively) and main shocks were pointed out, by removing both secular trends and seasonal thermoelastic effects from the raw tilts. An attempt was made to justify the above-mentioned results, based on the assumption that the observed intermediate-term preseismic tilts are the manifestation of aseismic creep episodes of comparable duration in the fault materials of thrust faults close to the tilt sites. The mechanism refers to a strain field slowly propagating from the preparation focal area to the tilt site, through crustal blocks separated by weak transition zones. This propagation is thought to be the cause of the local aseismic fault slip recorded by the tiltmeters. Previously, both discrete structures and strain propagation effects were revealed in the Central Apennines and are thought also to exist in the Southern Caucasus. As in the past, the rheological properties of fault materials are revealed as viscoelastic ones. In fact, creep equations obtained by applying several viscoelastic models on our data, proved to fit quite well some of the observed tilt precursors, producing viscosity and rigidity values very similar to those reported in literature.Professor Petr Viktorovich Manjgaladze died during the writing of this paper     

Interpretation of Low Seismicity in the Eastern Anatolian Collisional Zone Using Geophysical (Seismicity and Aeromagnetic) and Geological Data

The eastern Anatolia is a continental collisional zone between the Arabia-Eurasia plates and is currently being accompanied by the westward escape of the Anatolian crustal block to the west-southwest along two major strike-slip fault zones, the NATF and the EATF. Although these major fault zones have experienced historical earthquakes with moderate to high magnitude, notably there is a low seismicity zone on the active EATF between the Bitlis and Pötürge massifs. The low seismicity zone is characterized by thinner crustal structure relative to its environs, the shallow Curie Point Depth (SCPD, ca 12–14 km in between 39–40°E and 38.5–39°N in the easternmost part of the Anatolian plate) and moderate to high b values (more than 0.7). We consider that the shallow CPD and moderate to high b values in the low seismicity zone characterized by thinner crustal area are closely related with the higher thermal structure of the crust, which most probably resulted from crust-hot asthenospheric mantle interactions.     

Simulation of syn-eruptive floods in the circumvesuvian plain (southern Italy)

During explosive eruptions the deposition of fine-grained volcanic ash fallout reduces soil permeability, favouring runoff of meteoric water and thus increasing the occurrence of catastrophic floods. A fully dynamic, two-dimensional model was used to simulate flooding scenarios in the Vesuvian area following an explosive volcanic eruption. The highest risk occurs in the catchment area of the Acerra-Nola Plain N and NE of Vesuvius. This plain has a population of 70,000 living in low-lying areas. This catchment area is vulnerable to ash fall because it lies downwind of the dominant synoptic circulation and it lacks a natural outflow toward the sea. Our numerical simulations predict dangerous scenarios, even in quiescent periods, during extreme rain events (return periods of 200 years have been considered), and a significant increase in the extent of the flooded areas due to renewed volcanic activity. Based on these simulations a hazard zonation has been proposed. Editorial responsibility: A Woods     

Rupture zones in the area of the 17.08.99 Izmit (NW Turkey) large earthquake (Mw 7.4) and stress changes caused by its generation

The M_w 7.4 Izmit earthquake of 17 August 1999 struck a part ofthe North Anatolian fault in the area of Izmit Bay (NW Turkey). Historicalinformation shows that the fault which moved during the generation of thisearthquake consists of two fault segments moved during the generation oflarge (M 7) earthquakes in 1719 and 1754, respectively. Since then onlythe central part (between Izmit and Lake Sapanca) of this fault ruptured bythe generation of a smaller shock (M = 6.6) in 1878.The spatial stress variations based on the calculation of changes in theCoulomb Failure Function (CFF) associated with this earthquake aresupported by the distribution of strong aftershock foci. Large positive valuesof CFF to the east and west of the mainshock epicenter are inagreement with the notion that secondary faults were triggered there by thegeneration of the main event. Large positive values of CFF are alsoobserved in the adjacent western fault segment where the 1766 event wasgenerated, evidencing the occurrence of the next strong earthquake in thissegment.     

Characteristics of strong ground motion data recorded in the Gubbio sedimentary basin (Central Italy)

Various authors, analysing the set of accelerograms recorded at Gubbio Piana (GBP) (central Italy), have demonstrated that strong amplification occurs at this accelerometric station, which is installed within an alluvial basin. In particular, Ambraseys et al. [(2005a), Bull Earthq Eng 3:1–53; (2005b), Bull Earth Eng 3:55–73] observed that the strong motion peaks at GBP greatly exceed the median values predicted by the attenuation relationships they derived for Europe. In this work, we analyse and discuss some characteristics of the ground motion recorded at the GBP station. We show that the ground motion parameters, such as peak-ground acceleration and peak-ground velocity, are strongly influenced by the presence of locally induced surface waves that produce both a lengthening of the significant shaking duration and an increase in the peak values with respect to a nearby bedrock site. The basin-induced surface waves are observed in the three components of motion and their effects on the peak values are particularly evident in the vertical component. In the frequency domain, the energy of the surface waves is mostly restricted to the frequency band 0.4–0.8 Hz for both the horizontal and vertical components. The horizontal and vertical Fourier amplitudes are also very similar, and this indicates that the H/V spectral ratio technique is not applicable to describing the site response due to the propagation of seismic wave in a complex 2D/3D geological structure. Finally, a preliminary polarization analysis shows that the directions of polarization, as well as the degree of elliptical polarization, exhibit a strong variability with time, that may be related to a complex propagation of Love and Rayleigh waves within the basin.