Deformation patterns associated with the M5.9 Athens (Greece) earthquake of 7 September 1999

The static displacement field of the Athens 1999 earthquake has been numerically modeled by a BEM method and analysed from SAR interferometry images with compatible results: (a) for a fault that reaches the surface the subsidence field coincides with the hangingwall domain of the Fili neotectonic normal fault with maximum amplitude, d _max, 5.5–7 cm, which is consistent with the possibly co-seismic displacement of 6–10 cm observed in the field, the average fault dislocation of 5–8 cm found by the application of circular source models, and the displacement up to 6 cm predicted by empirical relations between magnitude and displacement; the field of uplift covers the footwall domain of the fault with d _max1.5 cm;d gradually decreases with distance from the fault at a gradient of 0.4 cm/km, (b) for a blind fault d _max is only 1.8 and 0.3 cm in the hangingwall and footwall, respectively, and the decay gradient becomes 0.15 cm/km, (c) the total deformation area is 15 km × 15 km and the Fili fault, with a preferred mean dip of 60°, constitutes the natural boundary between the subsidence and uplift areas. The macroseismic field pattern is similar with that of the static ground deformation. The majority of intensity values VI (MM and EMS-98 scales), are distributed within the hangingwall of the Fili fault, while the highest intensities (VIII and IX) concentrate very close to the Fili fault within its hangingwall domain. A gradual decrease of the intensities with the distance from the Fili fault is evident. Because of the similarity between the intensity distribution pattern and the static ground deformation pattern, we make the hypothesis that the latter predicts well enough the main characteristics of the former although the ground displacement is dominated by relatively low frequency as compared to the ground acceleration.     

Total Atmospheric Transmittance in the UV and VIS Spectra in Athens,Greece

This work investigates the seasonal and diurnal variation of total spectral transmittance of solar radiation. Such a study dealing with ultraviolet and visible wavelengths (310–575 nm) is carried out in Athens for the first time. The spectral values presented are averages over a number of records in a period of 42 days and have been calculated using ground-based spectral measurements of direct-beam irradiance. The data were recorded on selected days throughout a year by a simple pyrheliometer capable of performing spectral measurements. All data were recorded under clear-sky conditions in the city center of Athens and the total spectral atmospheric transmittance was estimated in the direction of local zenith to become independent of any optical mass effects. The comparison between seasons reveals that the atmospheric transmittance is higher in the cold period of the year than in the warm, simply because the photochemical pollution in the summer is more severe. Various features of seasonal and diurnal variation are also discussed with respect to emission sources, topographic peculiarities and wind regime.     

Rupture Directivity During the September 7, 1999 (<Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript> 5.9) Athens (Greece) Earthquake Inferred from Forward Modeling of Strong Ground Motion

The empirical Green's functions technique is applied to simulate strong ground motion records from the September 7, 1999, Athens earthquake. Information on the fault parameters from previous independent studies has been used and several scenarios were examined, in regard to the location of the starting point of the rupture, by comparing the synthetic records with the corresponding observed ones, through a residual function and a correlation function. The results show that the rupture started at the deepest, 4–5 km, part of the fault from its western edge. This hypocenter was then used, in combination with the initial fault model, to stochastically simulate the strong ground motion during the Athens main shock, in terms of peak-ground acceleration at hard rock. The results show that directivity might have significantly contributed to the destructiveness of this earthquake at specific parts of the Athens Metropolitan area.     

1999年台湾集集7.6级地震的震源参数

利用广东省数字地震台网记录的１９９９年台湾集集７．６级地震的波形资料,通过频谱分析,得出此次地震的霜相动力学特征,计算出Ｐ波和Ｓ波的应力降、地震矩、震源尺度等震源参数和介质品质因子Ｑ值,并求出了它的距震级。     

Empirical Orthogonal Function (EOF) analysis of spatial random fields: Theory,accuracy of the numerical approximations and sampling effects

Empirical Orthogonal Function (EOF) analysis of spatial random fields involves calculation of the eigenfunctions of the covariance kernel of the field. For real-world applications, a numerical approximation is necessary because the process is spatially discretized. An approximation for two-dimensional fields is proposed and then, analytical solutions of the integral problem are derived and used to study the accuracy of the numerical approximations. Sampling effects are also considered.     

Observed Evolution of Linear and Nonlinear Effects at the Dahan Downhole Array,Taiwan: Analysis of the September 21, 1999 M 7.3 Chi-Chi Earthquake Sequence

— In this paper, the site characteristics of the Dahan downhole array are studied by analyzing the September 21, 1999 M 7.3 Chi-Chi earthquake sequence including the main shock and some aftershocks. The four-level array (0 m, 50 m, 100 m and 200 m) is located to the north of Hualien City in eastern Taiwan. Polarization analysis is used to check the orientation errors of the seismometers at different levels of depth. If the surface instrument is chosen as reference, the angle between the major polarization axes of the surface and any downhole records is the orientation error that must be corrected for the downhole accelerographs. The orientation errors at depths of 50 m, 100 m and 200 m are 32°, 120° and –84°. After the corrections, the coherency between the surface and downhole records is substantially improved. Spectral ratio analysis shows that the predominant frequency of the Chi-Chi main shock shifts to a lower frequency. We also simulate ground motions at different depths by using the Haskell method with a linear velocity structure model. The record at surface is chosen as the input motion. Compared with the observed data, ground acceleration can be well reproduced for the aftershocks (weak-motion events) of the September 21, 1999 M 7.3 Chi-Chi earthquake. However, for the Chi-Chi main shock, the synthetic waveform cannot match well with the observation neither in amplitude nor in phase. This indicates that large ground shaking probably induced the nonlinear site effect at that time, and the model used cannot support it.Acknowledgement. The authors would like to express their thanks to Dr. L.F. Bonilla and one anonymous reviewer for their valuable suggestions. This research was supported by the National Science Council under grant number NSC 89-2921-M-194-007. The Institute of Earth Sciences, Academia Sinica supplied the strong-motion data. The support of these organizations is gratefully acknowledged.     

汶川8.0级地震中汶川县城房屋震害及其加固情况分析

依据5.12汶川地震中汶川县城(威远镇)11个区域共522栋房屋的房屋安全鉴定报告,深入统计分析房屋的震害情况及特点。结果表明,县城房屋倒塌较少,但破坏程度较重。破坏多集中在低层房屋以及砌体结构和木结构,而钢筋混凝土框架及高层房屋破坏较轻。结合房屋的破坏情况,对县城房屋的维修和加固情况进行分类统计,分析维修加固中出现的问题,并提出了相应建议。