Research On The Seismicity In Yunnan, China

Yunnan(21-29°N,97-106°E) is located at the eastern edge of the collision zone between Indian plate and Chinese continent in Eurasian plate,with high frequency of seismic activity,large earthquake magnitudes,and a broad distribution area, belonging to a region with mixed types of interplate and intraplate earthquakes. Based on rapid accumulation and release of seismic energy,obvious classification of active blocks and coordination movements,the relatively fixed dynamic boundaries and source,and special need for earthquake forecast and prediction,seismicity has become a major concern.This dissertation focuses on seismic regularity and seismogenic environment. The researcher takes medium,structure,and dynamic source as the major aspects,as well as the seismic characteristics and parameters,which have been generally accepted based on the relevant research results by the previous investigators,to describe the background of regional seismicity.Through analytical methods, including the imaging of seismic activities,correlation of regional seismic activities, quality of crustal medium,evolution of geological tectonics,active blocks,as well as the major fault geometry,kinematics,dynamics,and so on,the researcher systematically studied the seismic activity level in Yunnan,spatio-temporary distribution of seismic activities,earthquake type classification,earthquake repetition interval,earthquake correlation,and so forth.Based on these studies,the researcher has obtained important under- standings in 7 aspects.The major dynamic source for Yunnan originates from the intensive extrusion and compression by Indian plate against Chinese continent in Eurasian plate,displaying a joint action by sources with different order of strengths.The major dynamic source for Yunnan comes from the collision between Indian plate and Chinese continent in Eurasian plate;yet,the strengths are different for eastern and western Yunnan,though the source is the same.The dynamic source for western Yunnan is from the direct action of Indian plate,showing intensive lateral extrusion and com- pression;the dynamic source for eastern Yunnan is the results from several transforms:Indian plate collides with the Qinghai-Tibet region in the northeast direction,leading the shortening and uprising of the Qinghai-Tibet plateau, accompanied with the Qiangtang block squeezed out eastward,which is blocked in the middle of Sichuan basin with strong crystalline basement,and then the Sichuan-Yunnan rhombic block(SYRB) is wedged in the south-south-east direction. In Yunnan,the dynamic source from west to east as well as that from north to south transmits though the related blocks,acts in the joint area,and even displays a pattern that one is weak and the other is strong,which is the important basis for studying and understanding the regularity and characteristics of earthquake activities in Yunnan.There is a serious omission in records for historic seismic events in Yunnan.It is about 300 events omitted in every century on average.This situation is the most obvious in western Yunnan,where the major reasons for the omission are as the following:the high earthquake frequency,strong intensity,weak ability for written records,crude residence of common people,and frequent transitions of the political and economic centers.We should carefully reexamine the research results based on the "historic felt-earthquakes",which may provide import clues for the omitted large earthquakes.The researcher suggested study the seismicity in Yunnan using catalogs determined by instruments,and check the completeness of the catalogs.Seismic events with magnitude equal to or greater than 5.0 since the early 20th century(using seismic events since the 1950s for southwestern Yunnan area) and seismic events with magnitude equal to or greater than 3.0 since 1965 may objectively reflect the states of seismicity in Yunnan.There exits strong seismogenic capacity in Yunnan.On average,the annual energy input from Indian plate to Yunnan is approximately equal to the strain energy release by an earthquake with magnitude 6.7.Based on analysis of the repetition intervals of earthquakes in different ranges in typical areas,it is estimated that Yunnan is able to generate an event with magnitude 7 in about 50 years,an event with mag- nitude 6 in about 15 years,an event with magnitude 5 in about 10 years, and the capacity shows a trend to increase from west to east.Yet,it should be noted that earthquakes do not always occur with the predetermined magnitudes and in the equal intervals,and they may also be affected by the different trigger factors,for the seismogenic environment may be changed with the occurrences of earthquakes.There are obvious characteristics of spatio-temporary clustering for strong seismic activities in Yunnan.Activities of strong earthquakes with magnitude greater than 6.7 may be divided into 4 active periods.They are 1913-1925,1941-1955, 1970-1979,and 1988-1996 respectively;the duration for an active period is 8-14 years,with the relative quiescence for 9-16 years.The number for large earthquakes during an active period is about 3-4;the major areas for seismic activities show a pattern of alternative changes in east and west.In the 20th century,the first and third active periods were located in eastern Yunnan,the second and fourth active periods were located in western Yunnan,and the leading earthquakes were all located on the same faulting systems.The seismogenic processes are relatively independent in eastern and western Yunnan,though the rate for seismic energy accumulation is faster in west than in east,and the quiescent period is shorter in west than in east. Therefore,there may be a time period when large earthquakes occur in eastern and western Yunnan in the same time period.In this dissertation,we established a relation between the change of block moving speed,the clustering of large earthquake preparation,occurrence,and adjustment process,by deepening studies of the model for strong earthquake activities in eastern Yunnan(SYRB).Meanwhile, we found that after the continuous occurrences of large earthquakes during an active period of strong seismic activities,there would occur the block slip rate much greater than the average one,then the active period ended;it was a risk signal when the duration of the seismic quiescence lasted for a relatively longer time and the block slip rate became smaller,which was a risk signal for the starting of a strong earthquake;if the block slip rate did not increase rapidly after a large earthquake,it indicated that the seismic level did not lower down,and we have to note the possibility of large earthquake occurrences.There are obvious characteristics for classification of earthquake types in Yunnan.Results from studies of seismic sequence and focal mechanism of strong earthquakes show that the major seismic faults in Yunnan as a whole are strike-slip types,and the major seismic sequences are main-aftershock types.Under these conditions,considering the proportion by all the other non-strike-slip faults,and non-main-aftershock sequences,the researcher divided Yunnan into 2 strike-slip-main- aftershock areas in eastern and southwestern Yunnan,1 oblique-slip-swarm area in northwestern Yunnan,and 1 quasi-strike-slip-double-mainshock area in western Yunnan.In eastern Yunnan,the strike-slip characteristics is obvious,the blocks are relatively complete,the deformation is not distinguishable before earthquakes,and the energy release by a mainshock is relatively complete;in northwestern Yunnan,the oblique slip component in northwestern Yunnan may be resulted from the uprising force of the SYRB climbing to south at a place near the Lijiang-Xiaojinhe fault with striking in northeast;in southwestern Yunnan,the coseismic activities developed well because of the inherited movements of the preexisted faults with striking in northeast,and the forming of the newly-developed faults with strike in north-north-west,under the strong lateral extrusion and compression.For the reason that the patterns of combined precursors and evolution processes are different for different earthquake types,classification of earthquake types may provide theoretical guidance for monitoring seismogenic processes at multiple sites in Yunnan.The Lijiang-Xiaojinhe fault zone and the Red River fault zone are the import fault boundaries dividing the SYRB area and Yunnan area.The Lijiang-Xiaojinhe fault zone demonstrated the crustal uprising,and the extension force derived from the block "climbing" may be manifested by large earthquakes;the historic role of the Red River fault zone as a boundary is gradually weakening,though the fault is still the level-Ⅰboundary for seismic activities in Yunnan,playing a role of a weak boundary as a whole.During the process of SYRB moving southward in modern time,accom- panied by an additional clockwise rotation,the dynamic and deformation may affect the southwestern Yunnan area through some sections or layers of the Red River fault zone.There are three levels of correlations for seismic activities in Yunnan and the relevant areas.From far to near,there are the correlations with the Yunnan-Burma Arc-Andaman sea tectonic belt,with the eastern area of the Everest,and with the north of the SYRB respectively.The initiation of seismic active periods in Yunnan is closely related with the Yunnan-Burma Arc-Andaman sea tectonic belt.Before the leading large earthquakes in Yunnan in three of the four seismic active periods,great earthquakes occurred in the Burma Arc-Andaman sea tectonic belt.Earthquakes in eastern Yunnan and western Sichuan are mainly resulted from activities of the SYRB. Results based on studies of historic earthquakes show that seismicity in Yunnan is usually occurred earlier than that in Sichuan,which may be resulted from the cause that earthquakes occurred in the back edge of SYRB may need space from earthquakes in the front edge of SYRB.Another noteworthy phenomenon is the weak correlation between the large earthquakes in northwestern Sichuan and the strong seismic activities in Yunnan,which effectively guided the judgment for seismic trend in Yunnan after the 12th May 2008 Wenchuan earthquake.In this dissertation,based on the research results from seismicity as well as the seismogenic environment,the researcher conducted the discussion and summary for the medium-long-term earthquake forecast in Yunnan,and the foundations and bases for forecasts of the 1995 Menglian M7.3 earthquake,the 2000 Yao'an M6.5 earthquake and the 2007 Ning'er M6.4 earthquake.This dissertation also proposed 5 important suggestions for further studies on seismicity in Yunnan:1) strengthening studies on the major controlling factors for earthquake preparation,occurrence and adjustment;2) conducting analyses of causes for alternative changes of the major areas for strong seismic activities in east and west;3) strengthening the macroscopic judgment for regional seismic risk;4) establishing the initiation model for large earthquakes in eastern Yunnan;and 5) starting the observations and researches for slow earthquakes.The researcher even believes that the slow earthquake may become an important factor that may affect the seismicity and in particular the establishment of seismic regularity;an earthquake catalog can not be considered as a complete one without the slow earthquakes, neither can the strong seismic regularity be established.