| Nowadays,using moment magnitude to measure the size of an earthquake has become the mainstream trend in the world.The magnitude saturation cannot affect the moment magnitude,which makes the moment magnitude be a very reliable magnitude scale for evaluating earthquake disasters.However,the moment magnitude is based on the estimation of the seismic moment and provides only the fault mean and low frequency information about the source process,with relatively less information about the the details of the small wavelength high frequency ruptures.In contrast to the moment magnitude,the energy magnitude is based on the estimation of the energy radiated by the earthquake.It is related to the source dynamics and more sensitive to high-frequency details of the source.Therefore,the determination of radiated seismic energy and energy magnitude can complement moment magnitude to provide a complete description of ground motion over the whole frequency range of engineering interest.The more energy an earthquake radiates,the more damage it causes.From the point of view of preventing and reducing earthquake disasters,measuring the radiated seismic energy of earthquake has important and direct significance for earthquake prevention and disaster reduaction.However,at present,the research on the method of measuring radiated seismic energy is very lacking in our country.A systematic research system has not been formed.In conclution,this paper focuses on the measurement method of radiated seismic energy and carries out systematic research.The radiated seismic energy is the characteristic quantity of seismic source quantification.The determination of radiated seismic energy is an important basic problem in the study of seismic quantification.In this paper,the existing methods of measuring radiated seismic energy are improved and developed,and a new method of measuring radiated seismic energy is proposed based on the point source model.On this basis,the independent development of software system of measuring radiated seismic energy is carried out,and a software system of measuring radiated seismic energy by using far-field P-wave data is developed.The software system can not only rapidly measure the radiated seismic energy and energy magnitude of shallow earthquakes with magnitude 5.5 and above in the world,but also measure the seismic moment and moment magnitude,and then provide the dynamic source parameters such as energy to moment ratio,slowness coefficient and equivalent magnitude difference.The determination of these dynamic source parameters will greatly enrich the conventional output of the network and complement with other source parameters,providing a more comprehensive reference for earthquake prevention and mitigation agencies.The proposed new method and the development of software system make this paper form a systematic research system.The radiated seismic energy measurement method established in this paper has the following advantages:Firstly,we use the theoretical Green’s function method to eliminate the propagation effect experienced by seismic waves during the propagation process.The advantage of using theoretical Green’s function method to eliminate the path effect is that the propagation effect of different frequencies can be explained from a known point source.Secondly,we make source correction when calculating the radiated seismic energy.The calibration of the source can make the measurement results of radiated seismic energy and energy magnitude of each station less discrete,so as to make the final results more stable and reliable.Based on the above information,we independently developed a set of software system for the rapid determiantion of radiated seismic energy.Then,taking Wenchuan earthquake in 2008 as an example,the science and accuracy of measuring method and software system of radiated seismic energy are tested,and we get the following conclusions:(1)The radiated seismic energy of the Wenchuan earthquake is 2.84×1016J.The distribution of radiated seismic energy of each station has no obvious dependence on the epicenter distance and azimuth,indicating that the constraint is good when using theoretical Green’s function to eliminate the path effect.(2)The energy magnitude Me of the Wenchuan earthquake is 8.04.A correction for the source can make the standard deviation of Me smaller and more stable.(3)The inversion results show that the seismic moment of the Wenchuan earthquake is 8.97×1020N·m,which converts into the moment magnitude Mw of 7.9.Me is 0.14 higher than Mw,indicating that the radiated seismic energy of the Wenchuan earthquake is high.(4)The energy-moment ratio of the Wenchuan earthquake is 3.17×10-5,which is about 2.6 times the average of global shallow earthquakes and about 4 times the average of global thrust earthquakes,indicating that the release efficiency of the radiated seismic energy is high.The slowness coefficient is-4.50 when the logarithm of the energy-moment ratio is taken.The slowness coefficient can more intuitively show the release efficiency of the radiated seismic energy,which makes it more suitable for popularization.(5)The new seismic parameters,such as radiated seismic energy,energy magnitude,energy-moment ratio,slowness coefficient and equivalent magnitude difference,can quantitatively represent the dynamic characteristics of the source,which is of great significance for the rapid assessment of seismic disasters.Based on the broadband observation data of global seismic network,the distribution of radiated seismic energy of shallow earthquakes in the Qinghai-Tibet Plateau of China is studied for the first time.In this paper,we measured the radiated seismic energy and energy magnitude of 34 shallow earthquakes occurred in the Qinghai-Tibet Plateau region of China since 1990,and obtained stable and reliable results.The energy moment ratio,an important dynamic source parameter,is calculated based on the seismic moment to study the radiated seismic energy distribution characteristics of shallow earthquakes over the Qinghai-Tibet Plateau in China.After a detailed analysis of the results,we draw the following conclusions:(1)the average energy to moment ratio of shallow earthquakes in the Qinghai-Tibet Plateau region of China is 1.9× 10-5,which is 1.6 times the global of 1.2×10-5.The energy release level of shallow earthquakes in the Tibetan Plateau region of China is higher than the global average.Under the same condition,earthquakes will release more energy and cause greater damage.The energy to moment ratio is related to the focal mechanism.The average energy to moment ratio of strike-slip earthquakes is higher than that of dip-slip earthquakes.(2)The distribution of energy to moment ratio of shallow earthquakes in the Qinghai-Tibet Plateau of China has regional characteristic.The average energy to moment ratio in the eastern and western regions is 2.25×10-5 and 1.62×10-5,respectively,and it is significantly higher of estern than western.The regional difference of the energy moment ratio is related to the geological structure background.The radiated seismic energy can reflect the geological structure state in this area.(3)The energy to moment ratio of earthquakes in the same region is also different.The energy to moment ratios of shallow earthquakes in the Tibetan Plateau region of China range from 5.03×10-6 to 4.80×10-5,which reflects the different energy release processe of earthquakes occurring on different faults.(4)The energy magnitude of the Gaize earthquake and the Nima earthquake are 6.47 and 6.00,and the radiated seismic energy are 1.26×1014 J and 2.54×1013 J,respectively.The energy release of two earthquakes with similar source locations and the same moment magnitude can differ by as much as five times.In order to find the reasons for the difference in energy release between the Gaize earthquake and the Nima earthquake,time-frequency analysis of the waveforms of the two earthquakes was carried out by S-transform.The results of S transform show that the different dynamic processes of seismic source lead to the different process of radiated seismic energy release.Dynamic source parameters such as energy magnitude and radiated seismic energy are more suitable to describe the dynamic process of source and the potential damage of earthquake than moment magnitude.Therefore,it is necessary to measure the energy magnitude and moment magnitude together in order to reflect the source characteristics of the earthquake comprehensively. |
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