中国大陆强震发生前地震活动增强统计特征分析

Statistical characteristics of enhanced seismicity before strong earthquakes based on earthquake cases in Chinese mainland

依据《中国震例》(1966—2017年)对1966年以来中国大陆东部M_(S)>6.0、西部M_(S)>7.0强震前地震活动增强的异常现象特征进行了系统的梳理,并试图总结不同活动构造地块周缘强震前地震活动增强的异常时空演化特征。结果显示:①我国大陆大震前普遍存在地震活动增强现象,33个震例中有21个震例存在震前地震活动增强异常,占比达64%;②西部地区大多数震例的地震活动增强空间范围表现为中间尺度或构造尺度,且主震震级越大,越有可能出现大范围的地震活动增强;③西部地区出现中强地震活动增强的概率高于东部地区,前兆地震活动增强的平均震级较大,发生大震(M>7.0)的概率增大;④西部地区地震活动增强的时间尺度与震级成正比,而东部地区随着震级增大,更可能出现中期到中短期的地震活动增强异常;⑤多数西部地区震例在震前出现不同时间尺度的地震活动增强叠加的现象;⑥川滇菱形地块的强震前均出现了中长期尺度的地震条带交会现象、不同空间尺度的地震空区以及中短期尺度的中小震活动增强现象,并且对大震地点有较好的指示意义。巴颜喀拉东边界与北、西及南边界大震前存在显著的不同地震活动增强特征,震前未出现中强地震围空的现象。

Before strong earthquakes,enhanced seismicity manifested by increased magnitude,frequency,or accelerated strain release generally appears within a specific temporal and spatial range of the source area.This significant enhanced seismicity is often observed before moderate-strong earthquakes occurred in Chinese mainland or abroad.The seismogenic processes of large earthquakes are multiscale and diverse,involving localization of deformation,fault heterogeneities,and variable local loading rate effects.Enhanced seismicity prior to moderate-strong earthquakes is closely related to such processes and exhibits different characteristics.An in-depth study of enhanced seismicity will help us to understand the seismogenic process of strong-large shocks,which may bring positive effect on predicting strong shocks based on them.Many studies have been performed to analyze the characteristics of enhanced seismicity.A more thoughtful and systematic study is needed due to rapidly increased strong earthquake data in Chinese mainland and the urgent requirement for statistical predictive indicators.In this study,we intend to summarize the statistical characteristics of the prominent enhanced seismicities before moderate-strong earthquakes and attempt to seek the proper mechanism.Based on Earthquake Cases in China(1966−2017),the spatio-temporal characteristics of the seismicity before strong earthquakes with magnitude M_(S)≥6.0 in the eastern Chinese mainland and M_(S)≥7.0 in the western Chinese mainland are summarized statistically.In the meantime,the regional features of enhanced seismicity before the strong earthquakes within the Sichuan-Yunnan rhombic block,Bayan Har block,and North China block are also studied.The main contents and conclusions are as follows:Among the 33 earthquake cases studied in this paper,21 showed enhanced seismicity before the main shock,accounting for 64%,including five earthquakes of M_(S)6.0−6.9,14 earthquakes of M_(S)7.0−7.9,and two earthquakes of M_(S)8.0 or above.The percentage of enhanced seismicity is 42%for sub-grade class from M_(S)6.0 to M_(S)6.9,74%for for sub-grade class from M_(S)7.0 to M_(S)7.9 and 100%for sub-grade class from M_(S)8.0 or above.The possibility of the occurrence of enhanced seismic activity will increase with the magnitude of the main shock.The enhancement of seismicity appeared in 13 out of 18 cases in western Chinese mainland,accounting for 72%;8 out of 15 cases in eastern Chinese mainland,accounts for 53%.Secondly,in most cases for western Chinese mainland,the spatial extent of enhanced seismicities was observed within the intermediate or tectonic scale,and the probability of enhanced seismicity with a significant spatial scale increases with the magnitude of the main shock.Furthermore,the likelihood of enhanced seismicity with a large magnitude in western Chinese mainland is higher than in eastern Chinese mainland.The larger the average magnitude of enhanced premonitory seismicity,the more likely strong earthquakes with M_(S)>7.0 occur.The duration of seismic activity enhancement in the western Chinese mainland is directly proportion-al to magnitude of the main shock,while in the eastern Chinese mainland,the relatively more significant events tend to be associated with a mid-short-term enhanced seismicity.Thirdly,the strong earthquakes in Sichuan-Yunnan rhombic block were preceded by the medium-long term intersected seismic strips,the various spatial-scale seismic gaps,and the en-hancement of small-moderate earthquakes at medium-short-term scales.These features signific-antly indicate the location of further quakes,which deserves more attention.Different from the eastern border of the Bayan Har block,at the other three boundaries of the Bayan Har block,strong earthquakes are often attacked with seismic gaps encircled by premonitory medium-strong earthquakes.The seismic gap generally arises in medium-long-term time scales,and the mid-short-term scale enhanced seismicity is notable before strong earthquakes in the northern margin of North China block.In particular,the magnitude of the Haicheng earthquake is com-parable to that of the Tangshan earthquake.Still,the Haicheng earthquake was not preceded by a significant and long seismicity enhancement,which suggests that the secondary blocks or ad-jacent tectonic influences may also control the enhancement of seismicity before earthquake.Fourthly,enhanced seismicity prior to large earthquakes drives damage to the surrounding rocks.These enhanced seismicities are not limited to the faults that generate large earthquakes.Still,they drive distributed rupture and local rock mass deformation,ultimately resulting in major slip zones and large earthquakes.Laboratory studies of rocks and similar samples have shown that a relatively long period of distributed deformation precedes the onset of large rup-tures.The enhanced seismicity manifested in foreshocks is the most significant signal for the subsequent occurrence of a larger seismic event at a similar time and space.However,the en-hanced seismicity does not appear as a foreshock in every case related to the seismogenic mech-anism.The cascade-up framework and pre-slip model are generally used to account for the occurrence of a foreshock,whereas the progressive localization framework is suitable for ex-plaining the enhancement of seismicity without significant foreshocks.