Study On Quickly Determining The Hypocenter Parameters In The Earthquake Early Warning

In order to reduce effectively the casualties and the losses which are caused by earthquake, the essential approach is to predict the earthquake accurately and adopt some measures before and after the event accordingly. Due to the premature status of earthquake predication and the vulnerable structures to earthquakes, it is important to look at the earthquake early warning system hoping to reduce the casualties and the losses from future earthquakes. With the rapid development of strong ground motion networks and the intelligent emergency response and controlling system, the government as well as the general public has paid special attention to earthquake early warning systems. The essential requirements of an earthquake early warning system include speed, relatively accurate estimation of earthquake source parameters and continuous evolution with time.'Speed'means getting the seismic data fast enough so that we can determine the location and the magnitude of the earthquake in little time after an earthquake happens.'Relative accuracy'means that the result is good enough to issue or not issue warning, with the understanding that the final answer might differ from this rapid estimation, but the difference should be acceptable.'Evolution with time'means that the result will be updated continuously until an acceptable result is obtained. This paper introduces the major components of an earthquake early warning system and the functions of each component based on the summarizing of the systems in the other countries. This paper also introduced different types of earthquake early warning system and their advantages and disadvantages. The contents of this paper are summarized as follows:1. Determination of the seismic wave arrival time. This paper determines the arrival time of the seismic waves by the method of STA/LTA. When the seismic wave arrives, variation of STA(short time average) is faster than LTA(long time average), therefore, the value of STA/LTA rises obviously. When the ratio is larger than a threshhold, that point becomes the start of the initial motion which is in turn the time of seismic wave arriving. This method is convenient for calculating that needs little time but suitable for real-time processing. Its disadvantage is that when the Signal-to-Noise ratio is low or when initial motion is not clear, the method does not produce good results. 2. Determination of the epicenter. This paper determines the location of epicenter by using iterative method to solve equation. This method selects three stations with seismic wave arrival time and sets up equation by the distance between every station and epicenter. After solving every equation , we can get the location of epicenter. This method's advantages are easy for calculate and speed of anlysis, but the disadvantage is that the result is not unique and could differ from the final result.3. Determination of the seismic magnitude. This paper uses the empirical formula developed for California to determine the magnitude. This method's advantage is that the record of vertical direction and horizontal direction uses different formula to calculate the magnitude, and the formula of P-wave and S-wave at each direction are calculated differently, so an average can be used to get the estimated value. Another advantage is that it can estimate the magnitude relatively exactly with the reduction of incidental errors. Its disadvantage is that this formular is good when the earthquake happened in California, but extra calibration is needed when applied in other areas.4. Application example. This paper selects another earthquake in California as an example to apply the methods. Based on the methods described in previous chapters, we can calculate the arrival time, the location of epicenter and the magnitude of an earthquake. Initial comparison of the estimated result and the actual value demonstrates that the proposed framework is a good one for earthquake early warning systems.