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…     

Analysis on the master event method and precise location of 1997 Jiashi strong earthquake swarm in western China

IntroductionEarthquakelocationisoneoftheoldestinverseproblemsinseismology.Preciseearthquakelocationisthebasisoftheseismicresearch.Theprecisionoftheearthquakelocationdependsonmanyfactors,suchasthequalityofthestationnetwork,theprecisionoftheseismicwavearrivaltimemeasured,andtheknowledgeaboutthecrustalvelocitystructureandsoon.AccordingtotheresearchmadebyZhao(1983),thehorizontallocationerroroftheeanhquakelocatedbythetraditionalabsolutemethodinthecentralareaofNorthChinaisabouttZkin,intheedgeregio…     

Seismo-tectonic divisions of strong earthquakes with M S≥7.0 and their tectonic geomorphology along Xianshuihe-Xiaojiang fault zone

Seismo-tectonic areas of historical strong earthquakes with M _S≥7 along Xianshuihe-Xiaojiang fault zone are divided, and their individual fault-pattern and tectonic geomorphology are analyzed. Those strong-earthquake areas are located in some special parts of the fault zone, where the major branch-faults of the fault zone form left stepping, parallel, and fork-like patterns. In the strong-earthquake areas structurally complicated basins are developed, such as pull-apart basins in fork-like area, in double stepping area, and in stepping and fork-like areas. Foundation item: Chinese Joint Seismological Science Foundation (9507424). Contribution No. 2001A003, Institute of Crustal Dynamics, China Seismological Bureau.     

Research on the completeness of earthquake data in the Chinese mainland (I) — North China

In terms of the temporal-spatial distribution features of earthquakes, we study the completeness of historical data in North China where there is the most plenty historical data and with the longest record history by some methods of analysis and comparison. The results are obtained for events withM _s≥4 3/4 are largely complete since 1484 in North China (except Huanghai sea region and remote districts, such as Nei Mongol Autonomous region), but quakes withM _s≥6 are largely complete since 1291 in the middle and lower reaches of the Yellow River. Contribution No. 94A0049, Institute of Geophysics, SSB, China. This subject is one of the important projects of the State Seismological Bureau (No. 85-05-01-01-01).     

Response spectrum to strong aftershocks of the Lancang-Gengma earthquake

Near-field records of two strong aftershocks with magnitudeM _S=6.7 andM _S=6.3 in the Lancang-Gengma earthquakes sequence, Yunnan Province, November 1988, are used to calculate the response spectrum. The instruments, site conditions and the methods for computing are also illustrated in this paper. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 338–343, 1991. This project is supported by The Chinese Joint Seismological Science Foundation, SSB and the West Yunnan Earthquake Prediction Test Field, Yunnan Seismological Bureau.     

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.     

Relocation of earthquakes in northeastern region of Qinghai-Xizang plateau and characteristics of earthquake activity

IntroductionThenortheasternregionofQinghai-Xizangplateauisthejunctionregionofthethreeblocks,ie.,Qinghai-Xizang,AIxaandordosblock.TianandDing(l998)studiedtheclockwisetypequasi-trijunctionaroundHaiyuan-YinchuaninnortheasternregionofQinghai-Xizangplateau.Thethreet6ctonicbranchesofthequasi4rjunctionareQiIianshanfaultzone,Yinchuan-Jedai-Linhe(YJL)fractureddepressionbasinandLiupanshanfaultzone.TheQilianshanfaultzoneshowssin-istraIandcompressionalmovement,themovementofYJLbasinisofdextraland…     

Determining the time and space domains for estimating b value in seismic zoning

To take the seismic zone that includes the great shock with M _s≥8.2 as the statistical unit of estimating b value can often lead to more large variance, because the seismogenic zone of the great shock with M _s≥8.2 are larger than that delineated in general seismic zone. Two-level statistical units are considered in this paper. The seismic province is the first level unit that is suitable for group of earthquakes including the great shock of M _s≥8.5. A seismic province can be divided into several seismic zones. They can be taken as the second level unit for group of quakes in which the super magnitude of the greatest shock do not exceed 8. Because of the nonstationarity in time of seismic activity, the unbalancedness of data and differential of seismic temporal series feature in different areas need to be considered when we select the time period for estimating b value. According to local conditions, the time period is selected at one’s discretion in order to reflect seismicity level of this statistical unit in future 100 years. Contribution No. 98A02039, Institute of Geophysics, China Seismological Bureau, China.     

Probabilistic Assessment of Earthquake Recurrence in Northeast India and Adjoining Regions

Northeast India and adjoining regions (20°–32° N and 87°–100° E) are highly vulnerable to earthquake hazard in the Indian sub-continent, which fall under seismic zones V, IV and III in the seismic zoning map of India with magnitudes M exceeding 8, 7 and 6, respectively. It has experienced two devastating earthquakes, namely, the Shillong Plateau earthquake of June 12, 1897 (M _w 8.1) and the Assam earthquake of August 15, 1950 (M _w 8.5) that caused huge loss of lives and property in the Indian sub-continent. In the present study, the probabilities of the occurrences of earthquakes with magnitude M ≥ 7.0 during a specified interval of time has been estimated on the basis of three probabilistic models, namely, Weibull, Gamma and Lognormal, with the help of the earthquake catalogue spanning the period 1846 to 1995. The method of maximum likelihood has been used to estimate the earthquake hazard parameters. The logarithmic probability of likelihood function (ln L) is estimated and used to compare the suitability of models and it was found that the Gamma model fits best with the actual data. The sample mean interval of occurrence of such earthquakes is estimated as 7.82 years in the northeast India region and the expected mean values for Weibull, Gamma and Lognormal distributions are estimated as 7.837, 7.820 and 8.269 years, respectively. The estimated cumulative probability for an earthquake M ≥ 7.0 reaches 0.8 after about 15–16 (2010–2011) years and 0.9 after about 18–20 (2013–2015) years from the occurrence of the last earthquake (1995) in the region. The estimated conditional probability also reaches 0.8 to 0.9 after about 13–17 (2008–2012) years in the considered region for an earthquake M ≥ 7.0 when the elapsed time is zero years. However, the conditional probability reaches 0.8 to 0.9 after about 9–13 (2018–2022) years for earthquake M ≥ 7.0 when the elapsed time is 14 years (i.e. 2009).     

Inversion of source process and related studies of the Taiwan Strait earthquake using genetic algorithm

Applying genetic algorithm to inversion of seismic moment tensor solution and using the data of P waveform from digital network and initial motion directions of P waves of Taiwan network stations, we studied the moment tensor solutions and focal parameters of the earthquake of M=7.3 on 16 September of 1994 in Taiwan Strait and other four quakes of M _L≥5.8 in the near region (21°–26°N, 115°–120°E). Among the five earthquakes, the quake of M=7.3 on September 16, 1994 in Taiwan Strait is the strongest one in the southeastern coast area since Nan’ao earthquake of M=7.3 in 1918. The results show that moment tensor solution of M=7.3 earthquake is mainly double-couple component, and is normal fault whose fault plane is near NW. The strike of the fault plane resembles that of the distributive bands of earthquakes before the main event and fracture pattern shown by aftershocks. The tension stress axis of focal mechanism is about horizontal, near in NE strike, the compressive stress axis is approximately vertical, near in NWW strike. It seems that this quake is controlled by the force of Philippine plate’s pressing Eurasian plate in NW direction. But from the viewpoint of P axis of near vertical and T axis of near horizontal, it is a normal fault of strong tensibility. There are relatively big difference between focal mechanism solution of this quake and those of the four other strong quakes. The complexity of source mechanism solution of these quakes represents the complexity of the process of the strait earthquake sequences. Contribution No. 98A01001, Institute of Geophysics, State Seismological Bureau, China. The subject is supported and helped by Academician Yun-Tai CHEN, Profs. Qing-Yao HONG, Zhen-Xing YAO, Tian-Yu ZHENG, Yao-Lin SHI, Ji-An XU, Bo-Shou HUANG and colleague Mei-Jian AN, Xue-Reng DING, Rui-Feng LIU. De-Chong ZHANG and Ming Li provided the digital data warm-heartedly. Lin-Ying WANG offered us the catalogue of earthquakes in southeastern coastal area in China. Xi-Li WANG and Tong-Xia BAI provided us the issued annual reports data. The authors would like to express their gratitude to all of these people. This paper is sponsored by the National Natural Science Foundation of China and Scientific and Technological Commission of Shantou, Guangdong Province.     

Investigation of rupture process of the 1999 M =5.4 Xiuyan, Liaoning, earthquake sequence

Conclusions The sequence of the November 29, 1999 Xiuyan, Liaoning, earthquake withM _S=5.4 is relocated, and its rupture process is analyzed. Results are as follows: The rupture extended mainly before the January 12, 2000,M _S=5.1 earthquake. There are two phases of rupture extending: The first phase was before the November 29, 1999,M _S=5.4 earthquake, epicenters were situated within a small region with a dimension of about 5 km, and the focal depth increased. It shows that the rupture mainly extended from shallow part to deep in the vertical direction. The second phase was between theM _S=5.4 earthquake and theM _S=5.1 earthquake, earthquakes migrated along southeast, the focal depth decreased. It indicates that the rupture extended along southeast and from deep to shallow part. Foundation item: The Project of “Mechanism and Prediction of the Strong Continental Earthquake” (95-13-05-04). Contribution No. 01FE2017, Institute of Geophysics, China Seismological Bureau.     

Earthquake probabilities and magnitude distribution ( M ≥6.7) along the Haiyuan fault, northwestern China

In recent years, some researchers have studied the paleoearthquake along the Haiyuan fault and revealed a lot of paleoearthquake events. All available information allows more reliable analysis of earthquake recurrence interval and earthquake rupture patterns along the Haiyuan fault. Based on this paleoseismological information, the recurrence probability and magnitude distribution for M≥6.7 earthquakes in future 100 years along the Haiyuan fault can be obtained through weighted computation by using Poisson and Brownian passage time models and considering different rupture patterns. The result shows that the recurrence probability of M _S≥6.7 earthquakes is about 0.035 in future 100 years along the Haiyuan fault. Foundation item: Joint Seismological Science Foundation of China (103034) and Major Research “Research on Assessment of Seismic Safety” from China Earthquake Administration during the tenth Five-year Plan.     

A kind of information on short-term and imminent earthquake precursors—research on atmospheric electric field anomalies before earthquakes

The long-time practice of observational research on earthquake prediction has shown that the information on short-term and imminent earthquake precursors can hardly be detected, but it is very important for practical and effective earthquake prediction. The result of analysis and study in this paper has shown that the anomaly of quasi-static atmospheric electric field may be a kind of reliable information on short-term and imminent earthquake precursors. On such a basis, the 20 years’ continuous and reliable data of atmospheric electric field observed at the Baijiatuan seismic station are used to study the correlation between the anomalies in seismic activity and relative quiet periods bear on the occurrence of near earthquakes within 200 km range around Beijing after the Tangshan earthquake. The observational results recently reported before hand in written form and earthquakes that actually occurred in near field in corresponding time periods are compared and analyzed. The efficacy of these written prediction opinions about near earthquakes in the recent 10 years is tested. From the test results, the brilliant prospect that the anomaly of quasi-static atmospheric electric field may really become a reliable mark for making short-term and imminent earthquake predictions is discussed. Besides, as a preliminary step, some judgment indexes for predicting earthquakes by use of the observational data of atmospheric electric field before earthquakes are put forward. In the last part, it is pointed out that it would be possible to obtain more believable judgment indexes for determining the three elements of near earthquakes before greater earthquakes (M _S≥5) only if a relatively reasonable station network (2–4 stations every 10 000 km²) is deployed and further investigation is made. Contribution No. 97A0040, Institute of Geophysics, State Seismological Bureau, China. This subject is sponsored by Program No. 95-04-05-01-04, State Seismological Bureau, China.     

Characteristics of foreshock and its identi-fication

Introduction The characteristics of generalized foreshock and direct foreshock and their identification,as well as their application to medium and short-term prediction of strong earthquake is a major study objective in seismometry both in China and abroad.China has made many short-term and imminent earthquake predictions.Among the ones with clear hazard-mitigating effect and social manifestation,direct foreshock has made an obvious contribution,for example,the MS=7.2Menglian earthquake occu…     

Statistical testing of the relation between burst of aftershocks and strong earthquakes

According to geological tectonics and seismic activites this paper devided North China (30°–45°N, 105°–130°E) into four areas. We analyzed the North China earthquake catalogue from 1970 to 1986 (from 1965 to 1986 for Huabei, the North China, plain region) and identified forty-two bursts of aftershock. Seven of them occurred in aftershock regions of strong earthquakes and seventeen of them in the seismic swarm regions. The relation between strong earthquakes with the remaining eighteen bursts of aftershocks has been studied and tested statistically in this paper. The result of statistical testing show that the random probabilityp of coincidence of bursts of aftershock with subsequent strong earthquakes is less than six percent. By Xu’sR scoring method the efficacy of predicting strong earthquake from bursts of aftershock is estimated greater than 39 percent. Following the method proposed in the paper we analyzed the earthquake catalogue of China from 1987 to June, 1988. The results show that there was only one burst of aftershock occurred on Jan. 6, 1988 withM=3.6 in Xiuyan of Northeast China. It implicates that a potential earthquake withM _S⩽5 might occur in one year afterwards in the region of Northeast China. Actually on Feb. 25, 1988 an earthquake withM _S=5.3 occurred in Zhangwu of Northeast China. Another example is Datong-Yanggao shock on October 18, 1989 which is a burst of aftershock. Three hours after an expected shock withM =6.1 took place in the same area. Two examples above have been tested in practical prediction and this shows that bursts of aftershocks are significant in predicting strong earthquakes. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 273–280, 1991. Part of earthquake catalogue is from Jinbiao Chen, Peiyan Chen and Quanlin Li.     

Differences between magnitudes of Taiwan earthquakes from catalogs of Taiwan and Beijing

IntroductionTaiwanlocatedinthecollisionboundalbetweentheEurasianandthePhilippineSeaplatesisoneofhighseismicityregionsintheworld.HundredsofearthquakeswithM25occurredperyearandmorethan40withM27since1900.Amongtheseevents,shalloweventswithdepthofseveraltensofkilometersandintermediate-deepeventswithdepthof100-200kinexistwhichrepresentsacharacterofthesubductionzone.ThemagnitudesofTaiwaneventslistedinthecatalogofChineseearthquakesaretakenfromsomehistoricaldocumentsandGutenbergandRichter'sworks(19…     

Response of vertical deformation on faults to remote strong earthquakes

Vertical coseismic deformation on non-causative fault caused by remote strong earthquakes(epicentral distance≥1500 km,MS≥7.0)are observed by fault-monitoring instruments of new type during recent two years.The monitor-ing result shows,delay time,maximum amplitude and duration of vertical deformation on the non-causative faulthave remarkable close relationship with earthquakes magnitude and epicentral distance.The delay time of verticalcoseismic deformation have positive linear relationship with epicentral distance.The velocity of coseismic defor-mation is 5.5 km/s,close to the velocity of surface wave in granite.The logarithms of maximum amplitude of co-seismic deformation and epicentral distance have remarkable linear relationship with magnitude.The greater themagnitude and the closer the epicentral distance are,the bigger the maximum amplitude of coseismic deformationon non-causative fault will be.Relative to the epicentral distance,the magnitude is the most important factor to theduration of coseismic vertical deformation on the non-causative fault.Stronger earthquake causes longer vibrationduration of coseismic deformation.The experiential equation of co-seismic deformation faults obtained by thiswork is significant on the coseismic deformation research.     

Evolution of cumulative Coulomb failure stress in northeastern Qinghai-Xizang （Tibetan） Plateau and its effect on large earthquake occurrence

We simulate accumulative Coulomb failure stress change in a layered Maxwell viscoelastic media in the north-eastern Qinghai-Xizang(Tibetan)Plateau since 1920.Lithospheric stress/strain evolution is assumed to be drivenby dislocations of large earthquakes(M≥7.0)and secular tectonic loading.The earthquake rupture parameters suchas the fault rupture length,width,and slip are either adopted from field investigations or estimated from their sta-tistic relationships with the earthquake magnitudes and seismic moments.Our study shows that among 20 largeearthquakes(M≥7.0)investigated,17 occurred in areas where the Coulomb failure stress change is positive,with atriggering rate of 85%.This study provides essential data for the intermediate to long-term likelihood estimation oflarge earthquakes in the northeastern Tibetan Plateau.     

Regional characteristics of coda attenuation after the lancang-Gengma earthquake

In the light of the single scattering model of coda originating from local earthquakes, and based on the aftershock coda registered respectively at the 4 short period stations installed near the foci shortly after theM7.6 Lancang andM7.2 Gengma earthquakes, this paper has tentatively calculated the rate of amplitude attenuation and theQ _c-value of the coda in the Lancang and Gengma areas using a newly-founded synthetic determination method. Result of the study shows the rate of coda amplitude attenuation demonstrates remarkable regional differences respectively in the southern and northern areas. The southern area presents a faster attenuation (Q _c=114), whereas the northern area shows a slower attenuation (Q _c=231). The paper also discusses the reasons causing such differences. Result of the study also suggests a fairly good linear relation between the coda source factorA _o(f) and the seismic moment and the magnitude. Using the earthquake scaling law, the following formulas can be derived: lgM ₀=lgA ₀(f)+17.6,M _D=0.67lgA ₀(f)+1.21 and logM ₀=1.5M _D+15.79. In addition, the rates of amplitude attenuationβ _s andβ _m are respectively calculated using the single scattering and multiple scattering models, and the ratioβ _s/β_m=1.20−1.50 is found for the results respectively from the two models. Finally, the mean free pathL of the S-wave scattering in the southern and northern areas are determined to be 54 km and 122 km respectively by the relations which can distinguish between the inherentQ _i and scatteringQ _s, testify to this areas having lowQ-values correspond to stronger scatterings. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 71–82, 1992. This study is partly supported by the Seismological Science Foundation of the State Seismological Bureau of China, and the present English version of the paper is translated from its Chinese original by Wenyi Xia, Seismological Bureau of Yunnan Province.     

Periodic spectral characteristics of seismicity before strong earthquakes and their application

IntroductionMaximumentropyspectralmethod(MEM)(Burg,1972)hadbeenamethodusuallyusedinstudyingtheseismicityanditsmainpurposeistofindthedominantspectrainthelong-termseismicityprocessesinthepastyears(Zhu,1985).Inthispaper,themethodisappliedtostudywhethertherearesomespecialspectraofseismicityinsomespecificstagesinearthquake-generatingprocesses.Sowestudyseparatelythenormalandabnormalstageofearthquakeactivity,whoseactiveprocessisregardedasstablestochasticprocess,inordertofindtheirspectracharactersan…