强震前地震时空相关特征研究

首先,基于强震前地震活动时间变异系数与b值之间具有相关性,对我国四十次6.5级以上强震震例的震前地震活动时空相关特征作讨论。其次,尝试利用相关过程中"第一阶段"的特性来预测未来强震三要素,给出了预测时间、震级的经验公式。最后,对相关过程"第一阶段"判定的可靠性进行了讨论。     

2001年昆仑山口西8.1级特大地震孕育过程及中强地震活动图像演化

通过在百年时间域、45°×35°空间域对2001年昆仑山口西8.1级特大地震的孕震过程和中强地震活动图像演化进行时空扫描研究,认为该地震存在清晰的长期、中期、短期和临震几个孕育阶段,给出了各阶段清晰有序的地震活动图像,并找出划分各孕育阶段的标志性地震.同时指出8.1级特大地震的特殊性和预测预报的困难性.     

Homogeneous Velocity Models of The West Bohemian Swarm Region Obtained By Grid Search

West Bohemian earthquake swarms are used to determine the parameters of simple homogeneous velocity models of the individual subregions of the given area, based on a group of earthquakes that occurred in these subregions. The grid search method is used for location. Models yielding the minimum sum of squares of the travel-time residua in locating the whole group of earthquakes in the given subregion are considered suitable. Relocation obtained by grid search is compared with that obtained by the FASTHYPO method. The computations indicate that the subregions under consideration can be, from the point of view of earthquake location, sufficiently represented by homogeneous models, but the models differ for the data from different subregions. The velocities of these models are given. The models under consideration are compared with some of the previously published 1-D models of the broader region of Western Bohemia.     

The August 27, 2008, <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> = 6.3, Kultuk earthquake effects in the Nearepicenter zone: Macroseismic survey results

In this work, the macroseismic effects of the Kultuk earthquake (M _W = 6.3), which took place on August 27, 2008 in the southwestern closure of the Baikal Lake, are under consideration. The intensity of shocks in inhabited localities located in the epicentral zone reached 7–8 points on the MSK-64 scale. The earthquake was named after the local settlement of Kultuk, which was the mostly damaged area by the earthquake. The considered seismic event caused significant material damage (about 250 million rubles according to preliminary estimates). In inhabited localities of Southern Pribaikalie thousands of cases of damaged ovens and chimneys were registered. Some buildings were highly damaged and, accordingly, they are unfit for further use. The earthquake was the cause of numerous rockslides, rockfalls, and landslides on steep natural and artificial slopes. A macroseismic survey allowed us to establish the asymmetrical distribution of the intensity of shocks relative to the epicenter.     

Characteristics of strong ground motion for typical Australian intra-plate earthquakes and their relationship with the recommended response spectra

Global epicentre maps show that the majority of earthquakes are inter-plate, although moderate to large earthquakes do occur intra-plate, i.e. within the plates. The seismicity of the Australian continent is typical of intra-plate environments and a magnitude M_L 6 earthquake has an average return period of about 5 years. Recordings of Australian intra-plate earthquakes are investigated here to characterise their frequency content, peak acceleration and duration.Due to lack of quality strong motion records of large intra-plate earthquakes at short distances, synthetic seismograms are commonly used for testing structural behaviour. An empirical Green's Function method (Geophys. Res. Lett., 5 (1978), 1–4; Proceedings of the Third International Microzonation Conference, Seattle, USA, vol. 1, (1982), pp. 447–458.) is chosen to simulate a large earthquake by summation in time of a number of smaller earthquakes or sub-events, each given a slightly different origin time to represent more realistically the propagation of a rupture along an assumed fault plane. In the first instance, recordings on rock of the magnitude M_L 2.3 aftershock of the 29 December 1989 Newcastle earthquake were used as sub-events to simulate the main shock of magnitude M_L 5.6. Validation studies for events recorded elsewhere in Australia are also considered.The response spectra of such synthetic events will be compared with the recommended spectra developed empirically from a statistical analysis of strong motion data for magnitude 5.4–6.5 intra-plate earthquakes recorded in other parts of the world and normalised to a peak ground velocity of 50 mm/s which is typical for a return period of 500 years in Australia (Australasian Structural Engineering Conference, Auckland, New Zealand, (1998), pp. 439–444.). Preliminary results from this comparison with the response spectra recommended for the Building Code of Australia show that the synthetic waveforms produced by this method are realistic and can be used to represent ground motion during typical Australian intra-plate earthquakes.     

Estimation of strong ground motions from hypothetical earthquakes on the Cascadia subduction zone,Pacific Northwest

Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes ofM _s7.0 are used to estimate the response spectra that may result from earthquakesM _w<81/4. Large variations in observed ground motion levels are noted for a given site distance and earthquake magnitude. When compared with motions that have been observed in the western United States, large subduction zone earthquakes produce relatively large ground motions at surprisingly large distances. An earthquake similar to the 22 May 1960 Chilean earthquake (M _w 9.5) is the largest event that is considered to be plausible for the Cascadia subduction zone. This event has a moment which is two orders of magnitude larger than the largest earthquake for which we have strong motion records. The empirical Green's function technique is used to synthesize strong ground motions for such giant earthquakes. Observed teleseismicP-waveforms from giant earthquakes are also modeled using the empirical Green's function technique in order to constrain model parameters. The teleseismic modeling in the period range of 1.0 to 50 sec strongly suggests that fewer Green's functions should be randomly summed than is required to match the long-period moments of giant earthquakes. It appears that a large portion of the moment associated with giant earthquakes occurs at very long periods that are outside the frequency band of interest for strong ground motions. Nevertheless, the occurrence of a giant earthquake in the Pacific Northwest may produce quite strong shaking over a very large region.     

Uncertainities in estimation of extrapolated annual occurrence rate of earthquakes using logical tree

The logical tree methods are used for evaluate quantitatively relationship between frequency and magnitude, and deduce uncertainties of annual occurrence rate of earthquakes in the periods of lower magnitude earthquake. The uncertainties include deviations from the self-similarity of frequency-magnitude relations, different fitting methods, different methods obtained the annual occurrence rate, magnitude step used in fitting, start magnitude, error of magnitude and so on. Taking Xianshuihe River source zone as an example, we analyze uncertainties of occurrence rate of earthquakes M ≥ 4, which is needed in risk evaluation extrapolating from frequency-magnitude relations of stronger earthquakes. The annual occurrence rate of M ≥ 4 is usually required for seismic hazard assessment. The sensitivity analysis and examinations indicate that, in the same frequency-magnitude relations fitting method, the most sensitive factor is annual occurrence rate, the second is magnitude step and the following is start magnitude. Effect of magnitude error is rather small. Procedure of estimating the uncertainties is as follows: (1) Establishing a logical tree described uncertainties in frequency-magnitude relations by available data and knowledge about studied region. (2) Calculating frequency-magnitude relations for each end branches. (3) Examining sensitivities of each uncertainty factors, amending structure of logical tree and adjusting original weights. (4) Recalculating frequency-magnitude relations of end branches and complementary cumulative distribution function (CCDF) in each magnitude intervals. (5) Obtaining an annual occurrence rate of M ≥ 4 earthquakes under given fractiles. Taking fractiles as 20% and 80%, annual occurrence rate of M ≥ 4 events in Xianshuihe seismic zone is 0.643 0. The annual occurrence rate is 0.631 8 under fractiles of 50%, which is very close to that under fractiles 20% and 80%.     

Interrelation among strong earthquakes and its application in seismic hazard analysis

The intterrelation among strong earthquakes and its application are emphatically studied in this paper. Taking North China seismic region as study area, we have investigated how a great earthquake influence other strong earthqukaes in neighbouring area? Does there exist earthqukae immunity phenomenon? If it exists, what distributional pattern did it has in space-time domain? The results show that occurrence of earthquakes withM⩾7 has cetain immunity phenomenon to earthquakes withM⩾6 in North China. Among others, the immunity area of earthquakes withM=8 is much larger than that ofM=7. For earthquakes withM⩾8, the immunity area to the earthquakes ofM=7 is larger than toM=6. Based on the above analysis, using some statistical methods, we gave the variational regularity of seismic immunity factor with space and time, and explored its concrete application in seismic hazard analysis. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 339–346, 1993.     

Earthquake Triggering along the Xianshuihe Fault Zone of Western Sichuan,China

Western Sichuan is among the most seismically active regions in southwestern China and is characterized by frequent strong (M 6.5) earthquakes, mainly along the Xianshuihe fault zone. Historical and instrumental seismicity show a temporal pattern of active periods separated by inactive ones, while in space a remarkable epicenter migration has been observed. During the last active period starting in 1893, the sinistral strike–slip Xianshuihe fault of 350 km total length, was entirely broken with the epicenters of successive strong earthquakes migrating along its strike. This pattern is investigated by resolving changes of Coulomb failure function (CFF) since 1893 and hence the evolution of the stress field in the area during the last 110 years. Coulomb stress changes were calculated assuming that earthquakes can be modeled as static dislocations in an elastic halfspace, and taking into account both the coseismic slip in strong (M 6.5) earthquakes and the slow tectonic stress buildup associated with major fault segments. The stress change calculations were performed for faults of strike, dip, and rake appropriate to the strong events. We evaluate whether these stress changes brought a given strong earthquake closer to, or sent it farther from, failure. It was found that all strong earthquakes, and moreover, the majority of smaller events for which reliable fault plane solutions are available, have occurred on stress–enhanced fault segments providing a convincing case in which Coulomb stress modeling gives insight into the temporal and spatial manifestation of seismic activity. We extend the stress calculations to the year 2025 and provide an assessment for future seismic hazard by identifying the fault segments that are possible sites of future strong earthquakes.     

地震非均匀度在华北地震中短期预报中的应用

利用空间扫描方法研究了华北地区中强以上地震前地震非均匀度GL值的中短期异常变化特征，结果表明在中强地震前1年左右，未来震中周围的GL值中期异常区通常出现明显的收缩或消失，一些地震临震前数月还再次出现GL值异常区，讨论了使用GL值进行地震中短期预报的有关问题。     

Grouping occurrences being the fundamental feature of the strong earthquakes in Chinese mainland

ＧｒｏｕｐｉｎｇｏｃｃｕｒｒｅｎｃｅｓｂｅｉｎｇｔｈｅｆｕｎｄａｍｅｎｔａｌｆｅａｔｕｒｅｏｆｔｈｅｓｔｒｏｎｇｅａｒｔｈｑｕａｋｅｓｉｎＣｈｉｎｅｓｅｍａｉｎｌａｎｄＱｉｎ－ＺｕＬＩ（李钦祖）；Ｌｉ－ＭｉｎＹＵ（于利民）；Ｊｉ－ＹｉＷＡＮＧ（王吉易...     

Multifractal analysis of earthquakes

Multifractal properties of the epicenter and hypocenter distribution and also of the energy distribution of earthquakes are studied for California, Japan, and Greece. The calculatedD _q-q curves (the generalized dimension) indicate that the earthquake process is multifractal or heterogeneous in the fractal dimension. Japanese earthquakes are the most heterogeneous and Californian earthquakes are the least. Since the earthquake process is multifractal, a single value of the so-called fractal dimension is not sufficient to characterize the earthquake process. Studies of multifractal models of earthquakes are recommended. Temporal changes of theD _q-q curve are also obtained for Californian and Japanese earthquakes. TheD _q-q curve shows two distinctly different types in each region; the gentle type and the steep type. The steeptype corresponds to a strongly heterogeneous multifractal, which appears during seismically active periods when large earthquakes occur.D _q for smallq or negativeq is considerably more sensitive to the change in fractal structure of earthquakes thanD _q forq2. We recommend use ofD _q at smallq to detect the seismicity change in a local area.     

Frequency characteristics of coda and determination of medium parameters in Lingwu region, China

In this paper, considering the influences of source spectrum, the scattering property of medium and instrument response on the dominant frequency of coda, a method of using the coda of local earthquake to determine the correlation length of medium andQ-value is given. We find the following formula as: {fx719-1} wheret* =t/Q, f is the dominant frequency of coda,u ₁ andu ₂ are the parameters depend on the correlation length and the corner frequency of the source spectrum respectively,I(f) is a function of instrument response. If the source parameter is given, we can obtain the correlation length andQ-value by means of the inversion of observed curves off-t of coda. We processed the data of coda wave of more than 40 earthquakes from 1982 to 1989 in Lingwu region, China, determined the correlation length andQ-value, and preliminarily studied the temporal change of correlation length before and after moderately strong earthquakes. The result suggests that there are indications that the correlation length of medium decreases before the moderate earthquake. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 62–70, 1992.     

地震活动因子A值及其在华北地震中短期预报中的应用

综合考虑了地震活动时空强３ 个方面的震兆特征,定义了地震活动因子 Ａ值．使用１９７２ ～１９９６ 年华北地区地震资料进行空间扫描,研究中强以上地震前 Ａ 值的中短期异常变化特征．结果表明,在中短期阶段大多数地震前出现的 Ａ 值中期异常区面积明显缩小或消失,也有一些地震前１ ～３ 个月 Ａ 值异常区面积再次增大．还就用 Ａ 值进行地震中短期预报的有关问题进行了讨论     

Fracture characteristics of the 1997 Jiashi, Xinjiang, China, earthquake swarm inferred from source spectra

Broadband P and S waves source spectra of 12 MS5.0 earthquakes of the 1997 Jiashi, Xinjiang, China, earthquake swarm recorded at 13 GDSN stations have been analyzed. Rupture size and static stress drop of these earthquakes have been estimated through measuring the corner frequency of the source spectra. Direction of rupture propagation of the earthquake faulting has also been inferred from the azimuthal variation of the corner frequency. The main results are as follows: ①The rupture size of MS6.0 strong earthquakes is in the range of 10～20 km, while that of MS=5.0～5.5 earthquakes is 6～10 km.② The static stress drop of the swarm earthquakes is rather low, being of the order of 0.1 MPa. This implies that the deformation release rate in the source region may be low. ③ Stress drop of the earthquakes appears to be proportional to their seismic moment, and also to be dependent on their focal mechanism. The stress drop of normal faulting earthquakes is usually lower than that of strike-slip type earthquakes. ④ For each MS6.0 earthquake there exists an apparent azimuthal variation of the corner frequencies. Azimuthally variation pattern of corner frequencies of different earthquakes shows that the source rupture pattern of the Jiashi earthquake swarm is complex and no uniform rupture expanding direction exists.     

Relation between the characteristics of strong earthquake activities in Chinese mainland and the Wenchuan earthquake

This paper studies the relations between the great Wenchuan earthquake and the active-quiet periodic characteristics of strong earthquakes, the rhythmic feature of great earthquakes, and the grouped spatial distribution of M_S8.0 earthquakes in Chinese mainland. We also studied the relation between the Wenchuan earthquake and the stepwise migration characteristics of M_S?≥7.0 earthquakes on the North-South seismic belt, the features of the energy releasing acceleration in the active crustal blocks related to the Wenchuan earthquake and the relation between the Wenchuan earthquake and the so called second-arc fault zone. The results can be summarized as follows: ① the occurrence of the Wenchuan earthquake was consistent with the activequiet periodic characteristics of strong earthquakes; ② its occurrence is consistent with the features of grouped occurrence of M_S8.0 earthquakes and follows the 25 years rhythm (each circulation experiences the same time) of great earthquakes; ③ the Wenchuan M_S8.0 earthquake follows the well known stepwise migration feature of strong earthquakes on the North-South seismic belt; ④ the location where the Wenchuan M_S8.0 earthquake took place has an obvious consistency with the temporal and spatial characteristic of grouped activity of M_S≥?7.0 strong earthquakes on the second-arc fault zone; ⑤ the second-arc fault zone is not only the lower boundary for earthquakes with more than 30 km focal depth, but also looks like a lower boundary for deep substance movement; and ⑥ there are obvious seismic accelerations nearby the Qaidam and Qiangtang active crustal blocks (the northern and southern neighbors of the Bayan Har active block, respectively), which agrees with the GPS observation data.     

On the relation of moderate-short term anomaly of earth resistivity to earthquake

IntroductionMany anomalies due to earthquake have been recorded in observation of earth-resistivity for30 years and over, which showed that there objectively existed the anomalies of each-resistivity.The crustal strUcture and medium conditions are quite complex, so the complexity of the temporal,spatial and intensive development of the anomalies is inevitable. Both of time and amplitUde ofanomalies among some stations near an epicenter are different (even among different observational directi…     

Apparent stress and stress drop for intraplate earthquakes and tectonic stress in the plates

The magnitude of shear stress in the lithosphere is bounded from below by the apparent stress and stress drop during intraplate earthquakes. Apparent stresses and stress drops for a number of mid-plate earthquakes are calculated from the earthquake magnitude, SH wave amplitude spectra, and estimates of the length of the fault zone. Apparent stresses vary between 0.1 and 2 bars, ifm _b is used as a measure of seismic energy, and stress drops lie between 2 and 70 bars. There is no systematic difference in either apparent stress or stress drop between these intraplate events and typical plate boundary earthquakes. These bounds on intraplate shear stresses are consistent with the inference from current models of plate tectonic driving forces that regional stress differences in the plates are typically on the order of 100 bars. The highest stress drops measured for midplate earthquakes under this model represent nearly total release of local tectonic stress.     

Normal faulting mechanisms in the Western Desert of Egypt

Egypt is recognized as a moderate seismicity region where earthquakes are distributed within several active regions. Owing to sparse distribution of both seismicity and seismic stations, mostly moderate-size Egyptian earthquakes were recorded by regional stations. One of such cases is the moderate-size earthquakes of moment magnitudes greater than 4.0 which struck the Western Desert of Egypt in 1998 and 1999. These events are the first instrumentally recorded earthquakes occurring in the area. In the present study, the source mechanism for these earthquakes was estimated using the waveform data recorded from one of the very broadband MedNet seismograph stations and polarities from the national short-period seismographs. An iterative technique was applied to find the best-fit double-couple mechanism by a grid search over strike, dip and rake. Regional synthetic seismograms were calculated by using f–k integration in the frequency range of 0.03–0.1 Hz. A crustal structure fitted to surface wave dispersion curves was used to compute Green’s function. Focal depths were determined through the grid search method for a range of source depths. Our results show a normal faulting mechanism with minor strike-slip component. The NNW trend has been chosen as a preferred rupture plane in consistence with surface and subsurface faults and microearthquake seismicity in the epicenteral area as well.