汶川地震区青川县乔庄镇地震断裂问题

汶川特大地震导致了青川县乔庄镇(主城区)遭遇了较为惨重的特大灾难.县城的恢复重建工作受到地震断裂、山体变形、强烈的余震活动等严重制约.尤其是地震断裂问题,更是对能否原址重建起决定性作用.在参考以往研究成果的基础上,结合实地调查,对该镇的地震断裂问题进行了评估,确定了青川-平武断裂(龙门山后山断裂主断裂之一)分三条分支断裂(北支断裂、中间断裂、南支断裂)穿过乔庄镇主城区的大致方位和走势,并且现场发现该三条分支断裂均发生了不同程度的地震破裂(如地表破裂,山体震裂等),已逐渐发展成为中强震的孕震构造.     

Seismic image of a CO2 reservoir beneath a seismically active volcano

Mammoth Mountain is a seismically active volcano 200 000 to 50 000 years old, situated on the southwestern rim of Long Valley caldera, California. Since 1989 it has shown evidence of unrest in the form of earthquake swarms (Hill et al. 1990), volcanic 'long-period' earthquakes (Pitt & Hill 1994), increased output of magmatic ³He (Sorey et al. 1993) and the emission of about 500 tonnes day ⁻¹ of CO₂ (Farrar et al. 1995; Hill 1996; M. Sorey, personal communication, 1997), which has killed trees and poses a threat to human safety. Local-earthquake tomography shows that in mid-1989 areas of subsequent tree-kill were underlain by extensive regions where the ratio of the compressional and shear elastic-wave speeds V_P/V_S was about 9 per cent lower than in the surrounding rocks. Theory (Mavko & Mukerji 1995), experiment (Ito, DeVilbiss & Nur 1979), and experience at other geothermal/volcanic areas (Julian et al. 1996) and at petroleum reservoirs (Harris et al. 1996) indicate that V_P/V_S is sensitive to pore-fluid compressibility, through its effect on V_P . The observed V_P/V_S anomaly is probably caused directly by CO₂, and seismic V_P/V_S tomography is thus a promising tool for monitoring gas concentration and movement in volcanoes, which may in turn be related to volcanic activity.     

Fractal structure and characteristic scale in the distributions of earthquake epicentres, active faults and rivers in Japan

Using a box-counting method, we examined the fractal structures of the spatial distributions of three geological systems in Japan, namely those of earthquake epicentres, active faults, and rivers. Our results show that, in the scale range from 1 to 100  km, all geological systems have a common characteristic scale of ~13  km, which divides the spatial distribution into two bands: a smaller scale r < 13  km and larger scale r > 13  km (where r is the box size). In both bands, the three systems obey a power law distribution, and therefore it is proposed that all geological systems have a band-limited fractal structure. Since the characteristic scale of ~13  km is in agreement with the depth of the brittle–ductile transition zone of the crust, we suggest that it is a common feature of the heterogeneity of the crust.     

A note on the surface volume change of shallow earthquakes

Summary. This note presents an exact analytical formula for determining the magnitude of coseismic surface volume change (δ V ) of earthquake faults in a half-space. For a Poisson solid, the formula is remarkably simple; δ V = M _zz |8μ, where M _zz is one of the moment tensor elements of the source. Maximum δ V values derive from dip slip on faults plunging 45°. For these events, surface volume changes of 0.0001 and 4.3 km³ can be expected for magnitude 5 and 8 earthquakes respectively. All of the coseismic surface volume change is recovered in the interseismic period through relaxation of the Earth and rebound of the surface. A useful rule of thumb for estimating the magnitude of vertical rebound in 45° dip slip events is δ h _p=Δ s /24, where Δ s is the coseismic slip on the fault.     

The IASPEI procedure for the evaluation of earthquake precursors

Full scientific evaluation of proposed earthquake precursors for earthquake prediction is a problem because independent testing is difficult or impossible. To approach this difficulty, and to assess the current state of the art of earthquake prediction research, IASPEI has devised a peer-review procedure for precursor evaluation. The procedure does not consider predictions of impending earthquakes, but evaluates case histories of proposed precursors for past events according to stated validation criteria, which are specified in terms of guidelines concerning the hypothesized physical model, data quality, anomaly definition, the rules of association of precursor with earthquake, and statistical significance. So far, five precursors have been placed on a preliminary list of significant earthquake precursors, although none has satisfied the validation criteria well enough to ensure that their placement is permanent. Exclusion of a precursor from the list does not mean it is useless, but further work is required if it is to become convincing. The main objectives in producing the list are to establish a consensus on the criteria which a precursor must satisfy to be recognized as validated, and to find case histories which satisfy these criteria. Further nominations of precursor candidates are requested for evaluation by the IASPEI procedure.     

龙陵地区强震活动的深部背景

对研究区（23.9°N～25.1°N,97.8°E～99.0°E）内地壳和上地幔S波速度结构与强震的关系和强震活动的深部背景进行了探讨。活动断裂的运动是孕育和发生强震的重要诱因,而有利于高应变积聚的深部地质构造则是产生强震的关键条件。研究区内,以瑞丽—龙陵断裂为界,西北侧地壳和上地幔存在大范围低速区,不利于高应变的积聚,不容易孕育和发生强震;东南侧上地幔无明显低速层,地壳内有较小范围低速层,介质偏于刚性,易于积累高应变,孕育和发生强震的可能性较大。     

中国农村地区住房结构的区域差异及其影响因素

农村住房结构是决定地震灾害脆弱性的重要因素。对全国农村住房结构的差异性及其影响因素进行分析,是科学评估农村房屋脆弱性的基础。首先,根据2000年全国第五次人口普查（五普）结果,将农村房屋分为“钢混”、“砖混”和“传统结构”（包括五普中住房墙体材料的“木、草、竹”和“其他”2个类型）3种结构类型,建立了全国县级尺度农村住房数据库,共2699个区县。探讨农村住房结构的空间差异,遴选可能与之相关的因素进行相关分析,然后通过主成分分析和因子分析归纳出影响各类房屋空间分布的主要因素。结果表明,砖混结构是全国大部分农村地区最主要的住房结构类型,尤其在中东部地区的比例很高,而传统结构住房在西部地区所占比例较高（接近50％）,影响这两种结构住房空间分布的主要因素是气候、地域文化和农村经济发展水平,农民收入水平越高,砖混结构房屋比例越高。钢混结构在农村房屋中所占比例普遍很低,东中部地区比西部、东北略高,气候、非农就业和镇域经济发展水平是影响其空间分布的主要因素。为了降低农村地区地震脆弱性,建议中国农村政策应以提高农民收入水平为主,并积极推动农村人口向小城镇聚集,还需要在农村地区逐步推广房屋抗震规范。     

Far-field simulation of the 1946 Aleutian tsunami

We present hydrodynamic far-field simulations of the Aleutian tsunami of 1946 April 1, using both a dislocation source representing a slow earthquake and a dipolar one modelling a large landslide. The earthquake source is derived from the recent seismological study by López and Okal, while the landslide source was previously used to explain the exceptional run-up at Scotch Cap in the near field. The simulations are compared to a field data set previously compiled from testimonies of elderly witnesses at 27 far-field locations principally in the Austral and Marquesas Islands, with additional sites at Pitcairn, Easter and Juan Fernández. We find that the data set is modelled satisfactorily by the dislocation source, while the landslide fails to match the measured amplitudes, and to give a proper rendition of the physical interaction of the wavefield with the shore, in particular at Nuku Hiva, Marquesas. The emerging picture is that the event involved both a very slow earthquake, responsible for the far-field tsunami, and a major landslide explaining the near-field run-up, but with a negligible contribution in the far field.     

Determination of the three-dimensional seismic structure of the crust and upper mantle in the Central Midlands of England

Summary. Teleseismic P -wave residuals relative to CWF, a permanent shortperiod seismic station on Charnwood Forest in the Central Midlands of England, have been determined for two small aperture arrays deployed over the Precambrian block of Charnwood and its surrounding Phanerozoic sediments. The data have been inverted to produce a block model of the P -wave velocity variations in the crust and upper mantle beneath the study region. The results are consistent with significant variations penetrating to a depth of at least 50 km. Low velocities are associated with two upper crustal intrusive bodies, the Caledonian Mountsorrel granodiorite and the South Leicestershire diorites. A longer-wavelength variation at lower crustal/upper mantle depths could arise from the Moho dipping to the south-west beneath the study region, and whose strike sub-parallels the dominant Charnian trend of the major basement structures in this part of Central England.