| Earthquake early warning (EEW) is a new technology and new measure for seismic hazard mitigation in last twenty years and has the potential to reduce fatalities, casualties and costs. Its core technology is to shorten the data processing time in all aspects, and combine with the principles that the P-wave (traveling at about6-7km/s) is faster than S-and surface waves (traveling at about3.5km/s or less) and information (which travels at the speed of an electromagnetic signal-about300,000km/s) is much faster than seismic waves (which travel at speeds of the order of a few km/s) so that it can provide few seconds to tens of seconds of advanced warning time for impending ground motions, allowing for mitigation measures to be taken in the short time. In order to obtain high timeliness for earthquake information, we need to break from every aspect for shortening data processing time of the system, increasing lead-time and reducing the range of "blind zone". From the point of view of earthquake instruments for real-time data acquisition, there is plenty of room for improvement, such as decreasing network latency of real-time data transimission, shortening the data transmission interval, providing network support for the vast majority of existing strong-motion observation stations, introducing digital correction, resolving "jumping seconds" phenomenon and problems of inconveniently installing GPS antenna in some places, etc. In this paper, these aspects are studied and the results are applied to a real-time embedded Linux system in the newly developed strong-motion seismograph in which an onsite-warning EEWs based on a single instrument is developed. The contents are summarized as follows:1. Digital correction techniques have been introduced into the integrated strong-motion seismograph. Fifteen different position data measured by Gravimetric Method are used to calculate sensitivity and installation azimuth deviation of the accelerometer and the results are applied to the embedded software system for real-time data correction, effectively improving the precision and accuracy of real-time data. On the basis of previous work, a new temperature compensation algorithm for accelerometer has been proposed. This algorithm uses the low frequency band (10-4Hz) of the thermostatic MEMS accelerometer data to compensate the force-balance accelerometer in real-time and MEMS thermostat point migration is adopted to dynamically adjust the thermostatic MEMS temperature, which can expand application range of the instrument and reduce system power. The test results show that the zero-drift level of the compensated force-balanced accelerometer reduces to the level of the thermostatic MEMS accelerometer, approximately one or two orders of the results without compensation, and the compensation results of the two horizontal components are better than the vertical component’s. In addition, the low-band noise of the force-balanced accelerometer is also improved through the use of low-pass filter and the goal of temperature compensation is achieved.2. An algorithm combining GPS and NTP for system timing service has been proposed. The timing accuracy can reach±10μs when existing GPS information, while using only NTP the timing accuracy can reach±2ms. The results obtained from long-term test show that this algorithm can effectively resolve the "jump second" problem when using only GPS for timing service and the inconvenience settiing up GPS antenna in some places, while timing accuracy can meet the requirements of the strong motion observation stations on time service.3. A new real-time data processing system in embedded Linux kernel has been developed, in which the caching mechanism and the organization and management structure have been newly designed and the mode for accessing real-time data stream has been changed from by a packet per second into by the user-specified data frame length, effectively increasing lead time to a maximum of0.9s. By porting the real-time FIR filtering and decimation operations originally realized in a separate data processing chip to the ARM9CPU which is shared with other system functions, and realizing these operations in optimized embedded ARM assembly language, the cost of the whole system and the CPU load have been effectively reduced.4. A new method for real-time network data service, namely "rapid data service", has been developed. According to this method, the real-time data can be transferred by the order of100ms. In order to solve the problem of more amount of data transmitted which is caused by the low-latency data transmission, a new algorithm for real-time data compression and decompression has been proposed. By comparing with the Steim2compression algorithm which is most wided used and has the best compression ratio recognized by the world seismic industry now, the newly designed algorithm has advantages in various types of32bit data compression. Especially in the code length of more than20bits, the compression algorithm will not appear the data expansion phenomenon existed in the Steim2algorithm and still has a certain degree of compression ratio.5. Based on the strong motion data of Wenchuan earthquake and its aftershocks, we have statistically analyzed the relationship between characteristic period (τc) and magnitude, the relationship between the displacement amplitude of the first3s of P wave (Pd) and the peak ground velocity (PGV), the relationship between τc×Pd and magnitude. The results show that these two early warning parameters can not only be applied independently, but also be joined together to discriminate damaging earthquakes from non-damaging earthquakes. In practical applications, taking into account buildings in China, the threshold of τc×Pd should be appropriately reduced in order to be applied to China’s earthquake early warning systems better. Finally, these results have been applied to the integrated strong motion seismograph and a real-time EEW data processing system has been developed which can be directly used for "On-site" EEW. |
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