| The collision between the Indian plate and the Eurasia plate plays an important role on the active tectonics and earthquake activities on China.In this tectonic environment,China has nurtured many famous seismic zones with devastating earthquakes,including the Tibetan plateau and surrounding area,the Tianshan seismic zone,the Shanxi seismic zone,and the North China seismic zone,and becomes the place with the most frequently intraplate earthquakes and disasters.Many catastrophic earthquakes in China has caused hundreds of thousands casualties,such as the M8 Hongdong earthquake in 1303,the M8.3 Huaxian earthquake in 1556,the MW 7.6 Tangshan earthquake in 1976.Recently a flock of big earthquakes stroke China and caused heavy disasters,including the MW 7.9 Wenchuan earthquake in 2008,the MW 6.9 Yushu earthquake in 2010,the MW 6.6 Lushan earthquake in 2013,and the MW 6.2 Ludian earthquake in 2014.It is desirable to create new reasonable seismic hazard models to provide the scientific supports for the earthquake prevention and disaster mitigation.The key point in the establishment of the seismic hazard model is how to establish a scientific one using the different kinds of data harmoniously,such as historical earthquake catalogs,active tectonics,paleoearthquakes,and geodetic data.After the concept that faults cause earthquakes,the prediction of the earthquake developed rapidly,from the deterministic earthquake prediction model in the early stage to probabilistic ones,and finally the seismic hazard models in present.Generally,these seismic hazard models could be divided into 4 steps,including the seismic zone division,the studies of the magnitude-frequency relationships,the analysis of the ground motion attenuation relationships,and the calculation of the site response of the seismic hazards.Time-dependent and time-independent probabilistic models are both used in the seismic hazard models.In recent years,the number of catastrophic earthquakes occurred beyond the results of the seismic hazard maps,such as the Wenchuan earthquake in 2008 and the Tohoku earthquake in 2011.These unexpected earthquakes enhanced the suspect of the rationality of the seismic gap theory.The new seismic hazard maps should also pay more attention to the potential strong earthquakes on the low slip rate faults and the earthquakes larger than the maximum magnitude of the historical records.The seismic ground motion parameter zonation map of China released in 2016 also has many obvious problems in each step.The new map is still using the surface magnitude MS as the standard magnitude scale rather than the moment magnitude MW,however the MWis more related to the rupture scales than the MS.The scaling relationships of the magnitude and rupture parameters are also using the MW scale.The publicizing material of the new released Chinese seismic zonation map has no clear information of the scaling relationship between the rupture length and magnitude,which could be used to determine the distance between the sites and the earthquakes.In the calculation of the seismic rate,the new Chinese seismic zonation map divide the seismic source zones into three levels,the seismic parameter statistical zone,the background seismic zone,and the tectonic seismic zone?potential seismic sources?.The new zonation map thinks highly of the mediate and strong earthquakes on the high slip rate faults,and allocates nearly all the mediate and strong earthquakes on the potential seismic sources.But to other regions outside the potential seismic sources,the allocation of the seismicity rate is based on the arbitrary experience,and with many uncertainties.The other issue should be considered in the seismic hazard model is the occurrence of the earthquakes with magnitude larger than the maximum ones from historical catalog,or ruptured longer than the existed potential seismic source.It is desirable to create a more scientific,comprehensive and transparent seismic hazard model with reasonable calculation steps,rather than the empirical treatments in each step.This model also could be revised easily for the further studies.According to the above problems in the seismic hazard models,this doctorate thesis studies the historical earthquake catalogs,the rupture parameters of the historical earthquakes,the division of the seismic source zones,and the procession of the data to create the new seismic hazard model.I will use the Sichuan-Yunnan region as an example to show the steps in the new seismic hazard model.The open source software Open Quake and the toolkits provided by Global Earthquake Model?GEM?are used to create the seismic hazard model for mainland China,and analyse the calculated process and results.1.Compilation of a new MW-based historical earthquake catalog for mainland ChinaChina has a nearly 3,000-year-long earthquake record,listed in various Chinese earthquake catalogs.The magnitudes in those catalogs are expressed in surface wave magnitude,MS.However,modern seismic hazard analysis requires earthquake catalogs in moment magnitude,MW.Here,I compile a homogeneous MW-based historical earthquake catalog based on three Chinese earthquake catalogs and two global MW catalogs.For post-1900 earthquakes,I compare the common events with MS in Chinese catalogs and with MW in global catalogs,and derive MS-MW regression relationships using a constrained general orthogonal regression method.The regressions are performed over two magnitude ranges?5.5?MS<7.0 and MS?7.0?and three time periods?1900 to 1965,1966 to 1975,and 1976 to 2015?because earthquake monitoring capabilities in China varied over those magnitudes and time ranges.I use the regression results to convert MS magnitudes in the Chinese catalog to MW magnitudes.For pre-1900 events,I apply the regression results of the 1900-1965 period because the MS values of the pre-1900 events were converted from shaking intensity,and the MS-intensity relationships were derived from the post-1900 earthquakes.I also add earthquakes from other catalogs to obtain a more complete MW-based catalog for China.The final catalog has more than 4,000 earthquakes than the Chinese MS catalog,and includes about 15,700earthquakes of MW?4.0.I also analyse the completeness magnitude and time of the earthquakes in eastern and western China?divided by the longitude line of 105°E?,repectively.The catalog is more complete for eastern China than western China because eastern China has a much longer history of human settlement.In the calculated magnitude-frequency relationship,the b value is 0.88 in eastern China,while the one in western China is 0.96,and the seismicity rate in eastern China is significantly lower than that in western China.The magnitude-frequency relationship and the completeness magnitude can be used in the new seismic hazard model.2.New scaling relationships of the magnitude and rupture parameters in mainland ChinaIn the simulation of the ground motion in the seismic hazard models,it is necessary to obtain the fault rupture parameters according to the scaling relationships of the magnitude and the rupture parameters.Whether the global scaling relations are suitable for the intraplate environment in China is unknown and should be tested statistically.Meanwhile,the present scaling relationships based on the rupture parameters in mainland China are focused on the surface magnitude MS and the surface rupture parameters,and it is necessary to establish the scaling relationships between MWand the subsurface rupture parameters in mainland China.This thesis collects the rupture parameters of 90 earthquakes in mainland China and nearby regions from different types of sources,to study the relations among these parameters in eastern China and western China.The results show that the statistical results of the relationships between the subsurface rupture length and MWin eastern China and western China are different.The statistical result in western China has no obvious statistical difference from the global statistical results.For the statistical result in eastern China,the strike slip is the dominant earthquake type,and the slope of the regression line is obviously smaller than that of the results in western China.This difference is mainly caused by different tectonic environments.The earthquakes in western China are in the plate boundary-related region with lower stress drops,while the earthquakes in eastern China are in the intraplate tectonic environment with higher stress drops.The statistical results of the scaling relationship for the rupture area and MW also show the similar results,and the result for earthquakes with MW?6.70 in western China is also suitable for the small earthquakes in eastern China.3.Establishment of the earthquake source model for the Sichuan-Yunnan regionDuring the establishment of the earthquake source model for the Sichuan-Yunnan region in western China,I first divide the region and surrounding areas into 11 seismic source zones according to the regional tectonic activities,slip character of the fault,earthquake distribution and the focal mechanisms of the mediate and large earthquakes.The source zones includes the Myanmar subduction source zone,the southwestern Yunnan source zone,the Sichuan-Yunnan source zone,the South China Sea and East China Sea source zone,the South China source zone,the Longmenshan source zone,the Himalaya source zone,the Bayan Har source zone,the Qiangtang source zone,the West Qinling source zone,the Fenwei source zone.After the declustering of the earthquake catalog,I get the Gutenberg-Richter?G-R?relationship of each source zone,and calculate the corner magnitude in the Taper G-R relationships using strain rate model.The b value in the Sichuan-Yunnan source zone,the southwestern Yunnan source zone,and the Longmenshan source zone are 0.76,0.87,and 0.83,respectively,showing the difference among the seismicity rate in each source zone.According to the parameters in these G-R relations,the strain accumulation rates of the seismic source zones could be transferred to the seismicity rates accumulation for the seismic zones using the tapered G-R relationship.The slip rates of the active faults could also be transferred to the seismicity rates accumulation on the faults using the truncated G-R relationship.After comparison of the transferred seismicity rates accumulation for the seismic zones and the faults in the seismic zone,the background seismicity rates could be obtained from the difference of them,and will be allocated to the seismic zones according to the results of the smoothed seismicity.Comparing to the procedure in other models,the seismicity rate in my model could be allocated on the faults or to the background ones reasonablely.The new model also allows the occurrence of the great earthquakes larger than the maximum historical ones,or rupture through the potential seismic zones,although with small probabilities.That is also the reason why I do not divide the faults into small segments.4.Results and discussion of the Seismic hazard modeling of the Sichuan-Yunnan regionBased on the established earthquake source model,I calculate the results of the predicted PGA distribution from the fault-based seismicity rate and the background one for the Sichuan-Yunnan region using the OpenQuake engine.In the calculation,I apply the time-independent conditional probability from the Poisson distribution,and the logic tree model for the ground motion prediction equations.The predicted PGA distribution with recurrence period of 475 years from the fault-based seismicity rate shows that the high values are concentrated near the large-scale high slip rate faults,such as the areas near to the Xianshuihe-Xiaojiang fault with PGA values larger than 0.4 g.These large-scale high slip rate faults are not subdivided into small segments,and the predicted great earthquakes with small probabilities are allocated on these faults,which caused the high PGA values.The calculated PGA distribution from the fault-based seismicity rate shows that the effects from the predicted large earthquakes on the faults,due to the long-term tectonic stress loading.These large earthquakes are rare,with one for thousands of years or even longer.For example,the modeled occurrence rate of the MW8.0 earthquake in the Longmenshan seismic source zone is only one in 17,000 years,and one in 5000 years for the MW 7.9 earthquakes corresponding to the Wenchuan earthquake in 2008.The predicted PGA distribution with recurrence period of 475 years from the background seismicity is the reflection of the historical earthquakes,and closely related to the complex structure in the small areas.From this result,the PGA distribution in the Sichuan-Yunnan region is similar to that from the fault-based one.The high values are in the areas with small scale complex structures,such as the ones near to the middle part of the Longmenshan fault,at the northern end of the Red River fault,and in the Yingjiang area in the southwest of Yunnan.But to the areas near to the large-scale faults with simple and clear slip characters,the results from the background seismicity rate are low,relatively.Adding the results of the fault-based and background seismicity rate together,we can get the PGA distribution map with recurrence period of 475 years.The shape of the colored map is similar to the result form the background seismicity rate,and with high values near to the faults.My results are higher than the ones from the Chinese seismic ground motion parameter zonation map on average.The probable reasons are as follows:?1?The unexpected great earthquakes are considered in this thesis,and the occurrence rates of these earthquakes are calculated using the Tapered G-R relationships with the strain rate model.These earthquakes are allocated on the large-scale faults.While the result of the Chinese seismic ground motion parameter zonation map do not considered the earthquakes larger than the maximum historical earthquake or with rupture through than the divided segments,and their result are relatively small as a consequence.?2?According to the calculated seismicity rates for the strong earthquakes in the three seismic source zones in the Sichuan-Yunan region,my results in the Sichuan-Yunnan source zone and in the Southwestern Yunnan source zone are higher than those from the Chinese seismic ground motion parameter zonation map,while my result in the Longmenshan source zone is smaller than the one in the Chinese seismic ground motion parameter zonation map.Futhermore,the average magnitude for each maigntude bin is used in the Chinese seismic ground motion parameter zonation map,which also reduces the seismic moment in total.?3?As to the shape of the PGA distribution map,my results are integrated by the fault-based results and the background seismicity rate.While the Chinese seismic ground motion parameter zonation map address more attention to the earthquakes on the faults.Nearly all the mediate or large earthquakes are allocated in the potential seismic zones,which causes the PGA distribution more consistent to the fault distributions.Comparing with the results from Global Seismic Hazard Assessment Program?GSHAP?,the results in this thesis are similar,but still with large differences.GSHAP overestimated the effects from the strong earthquakes on the big faults,and underestimated the effects from the faults with low slip rates,or with few historical earthquakes,such as the Longmenshan fault,the Red River fault,and the Mabian fault.Using the similar methodology based on the potential source zones,the results from the Chinese seismic ground motion parameter zonation map and GSHAP are both more coincident with the fault distributions than my result,but with less concerned on the dispersion of the earthquakes.While my results in this article both concerned the importance of the fault slip in the seismic hazard modeling,but also pay much attenttaion to the complexity of the earthquake distribution.To sum up,this work gives a new method to create the seismic hazard map for the Sichuan-Yunnan region,and there are still many uncertainties and errors in the procedures.This work provides the scientific steps for data preparation,standard procedures in the establishment of the earthquake source model,and the calculation process.The results are more reasonable compared to the ones from other models,and will provides a basis for the future studies. |
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