Time-Domain Calculation Method Of Source Parameters For Earthquake Early Warning

Based on a dense real-time seismic monitoring station network,the earthquake early warning system uses limited waveform records from a limited number of observation sites to calculate,analyze and research seismic events in real time and issue early earthquake warning information to users who may cause damage and impact.The earthquake early warning system extends the earthquake prevention and mitigation system engineering to the real time stage,providing earthquake information to the public and various industries within the second-level time scale.However,the scientific issues involved in earthquake early warning technology are still not absolutely solved,such as whether and when there is a significant difference in the initial rupture of earthquakes of different magnitudes in terms of the source mechanism is not uniformly understood.Due to this limitation,mostly based on the point source assumption,earthquake early warning systems employ continuously increasing observation records to real-time estimate and update the magnitude through empirical regression relationships,and then bring in the ground motion prediction equation to predict the ground motion parameters.For large earthquakes with certain rupture scales,such simplified methods ignore the source parameters that control the spatial distribution of ground motion,such as s rupture length and direction,which affect the prediction and estimation of ground motion in the near-fault region,and thus cause large deviations in the scope of the warning area and warning level.In this paper,we focus on the near-field P wave records to explore the basic laws of source rupture evolution.We develop a series of time-domain methods to rapidly calculate the source parameters(moment magnitude,stress drop,corner frequency and rupture scale(area))by combining the source rupture characteristics of different magnitude ranges.We propose a real-time estimation method of moderate-and large-earthquake rupture scale,orientation and mode according to the modulation mechanism of ground motion by source effects,which provides a new method and strategy for earthquake early warning system.The main results of the paper are as follows:1.The weak deterministic evolution characteristics of early rupture of earthquakes of different magnitudes is revealed from near-field P wave.The velocity amplitude growth rate and frequency components of P waves are systematically analyzed by source distance segmentation with shallow-source moderate and large earthquakes in Japan and southwest China.The results show that the initial growth of P-wave velocity amplitude is similar and cannot indicate the earthquake magnitude.With the rupture development,the P-wave velocity amplitude and growth trends of earthquakes of different magnitudes gradually show differences.At this time,the characteristic period of P waves shows regular ’bell-shaped’ fluctuations with the increase of time window,and the fluctuation amplitude and duration are related to the earthquake magnitude.The distinguishable characteristics of P waves is before the rupture half-duration,which supports the weak determinism of earthquake rupture.2.Based on the source parameter calculation method proposed by Zollo et al.(2021),a time-domain method for real-time estimation of source parameters such as moment magnitude,stress drop and rupture radius from P wave records is modified based on the P-wave displacement growth characteristics in conjunction with the circle model.The correctness and reliability of this time-domain method are confirmed by a large number of Japanese earthquake cases,and it is also shown that it is realistic and feasible to determine the final magnitude before one-half of the rupture duration,which verifies the weak deterministic evolution of earthquake rupture.3.Based on the difference between the long and short axes of the sources of moderate and large earthquakes,a time-domain method for calculating the source parameters of moderate and large earthquakes(referred to as the root-mean-square method of ground motion)is developed by considering statistical methods.In view of the influence of rupture shape on the measured results of source parameters,this paper derives an approximate constraint relationship between source parameters and root mean square of ground motion in the elliptical model based on the ω2-type source spectra,to establish a bridge for the real-time calculation of the source parameters of moderate and large earthquakes from the time-domain waveform records.The validation results of KiK-net data show that the method can reliably obtain the source parameters of moderate and large earthquakes from the time-domain P-wave records and quantify the source energy release process.Although the method is complicated by the source effect and site effect,it can still obtain relatively reliable source parameters for large earthquake events directly using the uniformly distributed ground P wave records.4.Based on the root-mean-square method of ground motion,a real-time estimation method for the rupture length,direction and pattern of the moderate and large earthquake is developed.Based on the modulation mechanism of rupture direction and radiation pattern of source effects on the moment magnitude results obtained from P-wave records of different azimuths,the moment magnitude results of P-wave within long-and short-time windows are differenced to remove the radiation pattern factor and retain the rupture directionality effect.And then the rupture length,direction and pattern are then real-time inverted by nonlinear least squares inversion method.Finally,combined with the rupture length obtained from the moment magnitude growth duration,a complete real-time estimation method for the parameters of moderate and large earthquake line source faults is constructed.The results of a series of earthquake cases show that the real-time algorithm can quickly and reliably obtain the key source parameters such as rupture length,direction and rupture mode and their evolution process by P-wave records.The method breaks through the point source assumption of earthquake early warning and provides a new method and strategy for the earthquake early warning system to cope with large earthquakes and catastrophes.