Optimisation of amplitude curves of seismic P-, S- and L-waves in the homogeneous magnitude system of the Eurasian continent

Amplitude curves for seismic P-, S- and L-waves have been constructed on the basis of the homogeneous magnitude system of the Eurasian continent. The optimum amplitude curves are compared with the standard amplitude curves used in seismological practice.     

Great earthquakes of variable magnitude at the Cascadia subduction zone

Comparison of histories of great earthquakes and accompanying tsunamis at eight coastal sites suggests plate-boundary ruptures of varying length, implying great earthquakes of variable magnitude at the Cascadia subduction zone. Inference of rupture length relies on degree of overlap on radiocarbon age ranges for earthquakes and tsunamis, and relative amounts of coseismic subsidence and heights of tsunamis. Written records of a tsunami in Japan provide the most conclusive evidence for rupture of much of the plate boundary during the earthquake of 26 January 1700. Cascadia stratigraphic evidence dating from about 1600 cal yr B.P., similar to that for the 1700 earthquake, implies a similarly long rupture with substantial subsidence and a high tsunami. Correlations are consistent with other long ruptures about 1350 cal yr B.P., 2500 cal yr B.P., 3400 cal yr B.P., 3800 cal yr B.P., 4400 cal yr B.P., and 4900 cal yr B.P. A rupture about 700-1100 cal yr B.P. was limited to the northern and central parts of the subduction zone, and a northern rupture about 2900 cal yr B.P. may have been similarly limited. Times of probable short ruptures in southern Cascadia include about 1100 cal yr B.P., 1700 cal yr B.P., 3200 cal yr B.P., 4200 cal yr B.P., 4600 cal yr B.P., and 4700 cal yr B.P. Rupture patterns suggest that the plate boundary in northern Cascadia usually breaks in long ruptures during the greatest earthquakes. Ruptures in southernmost Cascadia vary in length and recurrence intervals more than ruptures in northern Cascadia.     

基于修正伪Helmholtz分解算子的横向各向同性介质纵横波波场分离

纵横波波场分离是弹性波逆时偏移成像中的关键步骤,可以有效消除纵横波串扰和成像畸变,提高成像精度。在各向异性介质中,常用的波场分离法有非平稳滤波器或者低秩近似的方法,但由于使用多次傅里叶变换,导致波场分离计算成本较高。借鉴基于Helmhotz算子构建各向同性介质纵横波解耦方程的思路,提出消除振幅畸变的修正伪Helmholtz分解算子,通过待定系数方法求解横向各向同性介质中P波和S波2种模式的表达式,并将其变换为一阶解耦的伪弹性波方程,实现时空域纵横波波场解耦。通过简单模型的波场分离测试,得到分离后的P波与S波波场,验证了该方法的波场分离的有效性。此外,将解耦得到的矢量P波与矢量S波应用到弹性逆时偏移之中,利用矢量点积互相关成像条件得到清晰的弹性波逆时偏移成像结果,说明本方法在复杂介质中有着较好的适用性,同时可以有效地应用到VTI介质弹性逆时偏移之中。     

福建数字地震台网测定台湾地震的震级问题

本文对 1 999年 1月至 2 0 0 0年 1 2月福建数字地震台网测定的台湾地区ML≥ 5 .0级地震 (9.2 1地震序列取ML≥ 5 .5级地震 )测定了Ms震级 ,并将福建台网测定的ML、Ms震级与中国地震局分析预报中心测定的大震速报Ms震级进行了比较。     

Factors affecting wave magnitude and period during the earthquakes of September, 1985

Summary The geology and structure of the Mexico Basin are discussed and hypotheses developed to explain the magnification of wave amplitudes during the 1985 earthquake.     

Amplitude-periodband diagrams and amplitude spectra of teleseismic body waves

The relation between amplitude-periodband (APB) diagrams and Fourier spectra of teleseismic P and S waves was analyzed using displacement-proportional signals derived from magnetic tape records of the feedback-controlled broadband velocigraph operating at the KHC seismic station. Most Fourier spectra and all the APB diagrams of the wave groups investigated exhibit a more or less pronounced maximum and do not correspond to the shape of theoretical far-field spectra for the Brune model. Contrary to Fourier spectra, APB diagrams have an unambiguously defined level and are not influenced by truncation, incorrect zero level and other disturbing effects.     

利用电磁观测粗略判定地震及台湾地震震级公式校正

通过对点频及5KC电磁波传播路径的理论特征分析并结合99年度发生在闽台地区的几个地震震前所记录到的电磁波异常持续时间进行分析、研究,总结出如何利用电磁观测粗略判定闽台地震的方法,如何利用公式来估算震级,以及台湾地震震级估算公式校正。     

Empirical relationships between aftershock area dimensions and magnitude for earthquakes in the Mediterranean Sea region

Fault dimension estimates derived from the aftershock area extent of 36 shallow depth (≤ 31 km) earthquakes that occurred in the Mediterranean Sea region have been used in order to establish empirical relationships between length, width, area and surface-wave/moment magnitude. This dataset consists of events whose aftershock sequence was recorded by a dense local or regional network and the reported location errors did not exceed on average 3–5 km. Surface-wave magnitudes for these events were obtained from the NEIC database and/or published reports, while moment magnitudes as well as focal mechanisms were available from the Harvard/USGS catalogues. Contrary to the results of some previously published studies we found no evidence in our dataset that faulting type may have an effect on the fault dimension estimates and therefore we derived relationships for the whole of the dataset. Comparisons, by means of statistical F-tests, of our relationships with other previously published regional and global relationships were performed in order to check possible similarities or differences. Most such comparisons showed relatively low significance levels (< 95%), since the differences in source dimension estimates were large mainly for magnitudes lower than 6.5, becoming smaller with increasing magnitude. Some degree of similarity, however, could be observed between our fault length relationship and the one derived from aftershock area lengths of events in Greece, while a difference was found between our regional and global fault length relationships. A calculation of the ratio defined as the fault length, derived from our relationships, to the length estimated from regional empirical relationships involving surface ruptures showed that it can take a maximum value of about 7 for small magnitudes while it approaches unity at Ms 7.2. When calculating the same ratio using instead global empirical relationships we see the maximum value not exceeding 1.8, while unity is reached at Mw 7.8, indicating the existence of a strong regional variation in the fault lengths of earthquakes occurring in the Mediterranean Sea region. Also, a relationship between the logarithms of the rupture area and seismic moment is established and it is inferred that there is some variation of stress drop as a function of seismic moment. In particular, it is observed that for magnitudes lower than 6.6 the stress drop fluctuates around 10 bar, while for larger magnitudes the stress drop reaches a value as high as 60 bar.     

全球5.0级以上地震活动与地球自转变化的Granger因果关系检验

采用国际地球自转服务中心(IERS)发布的日长变化(Length of the Day,LOD)数据和美国国家地震信息中心(NEIC)公布的地震目录中的震级在5.0级(含5.0级)以上的地震数据,对1973年以来的每日地球自转变化时间序列与地震时间序列之间的Granger因果关系进行检验,结果表明:地球自转变化与地震之间存在着双向Granger因果关系,即地球自转变化可能导致地震发生,地震也会对地球自转变化产生影响,检验结果暗示了两者之间可能存在着复杂的动力机制。     

Acceleration response spectra for the earthquakes of the southwestern flank of the Baikal Rift Zone

The acceleration response spectra of earthquakes with M = 4–6.5 in the southwestern part of the Baikal Rift Zone have been studied. The absorption properties of the medium and the attenuation of seismic signals in the study area were determined. Average acceleration response spectra were obtained for regional earthquakes. A comparative analysis of the acceleration response spectra was made for earthquake focal mechanisms with different senses of motion: reverse fault, reverse slip, strike slip, and oblique slip. The effect of the sense of fault motion in the seismic source on acceleration response spectra was determined.     

The basic parameters of acceleration spectra for M > 5 earthquakes in the Baikal Rift Zone

We consider the acceleration spectra of S waves from M = 5-6.5 earthquakes that occurred in three regions of the Baikal Rift Zone. The main characteristics of the shape and level of the spectra are given, and the difference between the spectra in the relative positions of the focus and seismic station is shown. Average acceleration spectra have been obtained for all three regions with regard to shape and the magnitude of the earthquakes. The spectra have been transformed so that they correspond to the accelerations of M = 6.5 and M = 7.5 earthquakes. The conclusion is made that the spectra not necessarily retain their shape even with the same movements in the focus.     

Physical properties of the variations of the electric field of the earth preceding earthquakes. II. determination of epicenter and magnitude

As reported in the preceding paper, a transient change of the electric field of the earth (seismic electric signal), hereafter called SES, appears many hours before an earthquake (EQ). By measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction. Measurements in two directions (E-W and N-S) give the relative signal intensity J_rel at the station under consideration. By measuring J_rel at a number of stations and considering that it attenuates according to a 1/r-law, the epicenter can be determined with an accuracy usually around 100 km. Once the epicenter has been determined, the product J_rel · r can be evaluated so that the magnitude M can be estimated by resorting to an empirical log(J_rel · r) versus M plot. The uncertainty of M is around 0.5 units. Following Sobolev (1975) and for the statistics to be beyond any doubt, predictions were officially documented before the EQ-occurrence. For 23 earthquakes with a magnitude equal or greater than M_s = 5.0 two events were missed.The present method is compared to other electrical methods used in China, Japan and Soviet Union. A number of problems concerning the origin of the effect, its directivity and the attenuation with distance remain open for further studies.     

孕震空区和逼近地震在台湾地震中的应用

本文应用陆远忠等人提出的孕震空区和逼近地震方法 ,分析了台湾及其东部海域所有资料较完整的 6 0级以上的地震。结果表明该区普遍存在孕震空区和逼近地震。在判定是否为孕震空区时 ,采用同样的三个标志。用所得到的数据计算经验公式。最后得出 5点初步结论     

Estimation of maximum magnitude (M max ): Impending large earthquakes in northeast region,India

In the present study, the cumulative seismic energy released by earthquakes (M _w ≥ 5) for a period of 1897 to 2009 is analyzed for northeast (NE) India. For this purpose, a homogenized earthquake catalogue in moment magnitude (M _w ) has been prepared. Based on the geology, tectonics and seismicity, the study region is divided into three source zones namely, 1: Arakan-Yoma Zone (AYZ), 2: Himalayan Zone (HZ) and 3: Shillong Plateau Zone (SPZ). The maximum magnitude (M _max ) for each source zone is estimated using Tsuboi’s energy blocked model. As per the energy blocked model, the supply of energy for potential earthquakes in an area is remarkably uniform with respect to time and the difference between the supply energy and cumulative energy released for a span of time, is a good indicator of energy blocked and can be utilized for the estimation of maximum magnitude (M _max ) earthquakes. The proposed process provides a more consistent model of gradual accumulation of strain and non-uniform release through large earthquakes can be applied in the assessment of seismic hazard. Energy blocked for source zone 1, zone 2 and zone 3 regions is 1.35×10¹⁷ Joules, 4.25×10¹⁷ Joules and 7.25×10¹⁷ Joules respectively and will act as a supply for potential earthquakes in due course of time. The estimated M _max for each source zone AYZ, HZ, and SPZ are 8.2, 8.6, and 8.7 respectively. M _max obtained from this model is well comparable with the results of previous workers from NE region.     

Estimates of large magnitude Late Cambrian earthquakes from seismogenic soft‐sediment deformation structures: Central Rocky Mountains

A variety of unusual early post‐depositional deformation structures exist in grainstone and flat‐pebble conglomerate beds of Upper Cambrian strata, western Colorado, including slide scarps, thrusted beds, irregular blocks and internally deformed beds. Thrusted beds up to tens of centimetres thick record thrust movement of a part of a bed onto itself along a moderate to steeply inclined (15° to 40°) ramp, locally producing hanging wall lenses with fault‐bend geometries. Thrust plane orientations are widely distributed, and in some cases nearly oppositely oriented in close proximity, indicating that they did not form as failures acted upon by gravity forces. Irregular bedded to internally deformed blocks are isolated on generally flat upper bedding surfaces. These features represent parts of beds that detached, moved up onto and some distances across, the laterally adjacent undisturbed bed surfaces. Deformation of thin intervals of mud on the ocean floor by moving blocks rules out the possibility of storm‐induced deformation, because the mud was not eroded by high shear stresses that would accompany the extremely large forces required to produce and move the blocks. Finally, internally deformed beds are characterized by large blocks, fitted fabrics of highly irregular fragments and contorted lamination, which represent heterogeneous deformation, such as brecciation and liquefaction. The deformation structures were produced by earthquakes linked to the reactivation of Mesoproterozoic, crustal‐scale shear zones in the central Rockies during the Late Cambrian. Analysis of the deformation structures indicates very large body forces and calculated earthquake‐generated ground motion velocities of ca 1·6 m sec^?1. These correspond to moment magnitudes of ca 7·0 or more and a Mercalli Intensity of X+. These are the only known magnitude estimates of Phanerozoic (other than Quaternary) large‐intensity earthquakes for the Rocky Mountain region, and they are as large as, or larger than, previous estimates of Proterozoic earthquakes along these major shear zones of the central Rockies.