Real-Time Estimation Of The Ground Motion Field Of Large Earthquakes

Earthquake early warning(EEW)system needs to accurately predict the distribution of the ground motion field during on an ongoing earthquake.Automatically determine the area that may be damaged by the earthquake,to publish timely and reliable early warning information for major projects and the public.It has always been a popular and difficult topic in the field of quickly and accurate estimation of ground motion field of larger earthquakes.At present,conventional EEWs firstly quickly determine the basic parameters of an earthquake,including the epicenter,focal depth,magnitude and origin time,after the occurrence of a destructive earthquake,and then to predict the ground motion field distribution of the target site or the entire near-field region on the base of the ground motion models.However,the conventional EEW is to estimate the ground motion field of the point-source model based on the basic parameters of an earthquake,without considering the information of the extent and directivity of the fault.But when a large earthquake which is usually caused by large scale fault rupture occurs,the estimation of the ground motion field will deviate greatly from the real distribution when the estimation of the ground motion field is still based on the point-source model.In addition,the universal phenomena of overestimating the magnitude of small events and underestimating the magnitude of large earthquake events generally exist when rapid estimate the magnitude in EEW.It still can lead to the inconsistency between the estimatied results and the actual situation,and severe causes the leakage and the false alarm in EEW.In view of the above technical challenges faced by EEW at the present stage,this paper systematically studies on the rapid estimation of magnitude,the determination of the dominant direction and model of fault rupture,and real-time estimation of the ground motion field.The aim is to quickly estimate the distribution of the ground motion field in real-time after the earthquake and provide the basis for EEWs to publish the EEW information quickly and accurately.The contents are summarized as follows:(1)A method of earthquake magnitude estimation based on acceleration amplitude of near-source S wave is proposed.Based on the observation of a large number of strong ground motion recordings,it is found that the acceleration peak of the nearsource S wave will appear about 4 seconds after the arrival of the first P wave with the increase of the seismic network density(e.g.network density of 20 km).Therefore,the statistical relationships between the peak acceleration of near-source S wave and the magnitude are established by using the peak acceleration of S-wave near the epicenter at 10 km,20 km,30 km,40 km and 50 km,respectively.The results show that it is less discrete than the estimation methods by Pd and ?pmax,especially when the magnitude is less than 7.2.In other words,the overestimation has improved a lot for small events and the estimation of large events is also improved compared with the estimation results based on Pd and ?pmax parameter.(2)To improve the method of determination the dominant direction of fault rupture and the percentage of unilateral rupture based on the directivity function of fault rupture.One is to modify the distance parameters considering the fault finiteness;the other is to analyze the characteristics of the real-time variation of the ground motion parameters(peak acceleration,velocity and displacement)with the distance,and determine the theoretical value of the ground motion parameters in real time.And the flow chart of real-time calculation is given.The feasibility and reliability of the improved technique is demonstrated by offline numerical tests using data from ten earthquakes occurring in different seismotectonic environments and seismic fault types include normal fault,reverse fault and strike-slip fault.The results show that the improved technique allows for quickly and accurately estimates of the direction and model of fault rupture and can be applied to real-time calculation with reliable results.(3)A real-time estimation method of based on simulation of ground motion envelope time series is proposed.In this method,the seismic fault of a large earthquake is equivalent to a finite number of small seismic sources based on the information of direction and extent of fault rupture,and the time-history envelope of large earthquake is simulated by the combination of time-history envelope of each subsource in real time when these sub-sources rupture in a certain model,and then to estimate the distribution of ground motion(intensity)field in real time according to the envelope peak.An example shows that this method can better reflect the directional effect of fault rupture than the method based on ground motion model.(4)A hybrid method for real-time estimation of ground motion field is proposed to solve the problem of generating false alarm using the estimation method of earthquake warning parameters based on single earthquake when multiple earthquakes occur simultaneously over the wide source region or the estimation of magnitude and epicenter is inaccurate or unknown.This method is based on the ground motion field estimation method considering the dominant direction and extent of fault rupture,and propagation of local undamped motion method(PLUM)is introduced to supplement the results of estimation of the ground motion field.PLUM method does not need the basic parameters of earthquake,and directly uses the real-time information of the observation station to estimate the ground motion field within the range of 30 km around,which can effectively solve the problem of missed warning of earthquake.The hybrid method not only meets the need of earthquake warning time,but also improves the accuracy of estimation of the ground motion field.