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本文主要讨论意大利国家地震台网1975年到1991年期间记录的地震序列的前震。我们认为前震是超过某一震级阈值的地震,它发生在一段平静期后和给定时距范围内的主震之前。该算法确定的潜在前震总数取决干预定的平静区的大小和持续时间,而由前震预测的主震数也取决于预报区的面积大小和预报期的持续时间。我们的目的是优选确定作为强震前兆的潜在前震的参数,以提高其真实性和可靠性。意大利全境的初步调查使我们能确定前震——主震序列可能发生的区域。其中之一是意大利中部的面积约32000 km~2的地区,该区有M≥2.5的地震2671次,并有在中等地震之后,近距离和短时间内发生较大地震的明显趋势。本文根据前震的优化定义——半径140km范围内经至少80天平静期后超过3级的地震,能提出63次预报。在圈定半径30km范围、在随后48小时内超过4级的主震发生比例为23次中有6次。相应的概率增量为150。作为对比,有M≥2.5地震453次的另一大地区(波河流域),比例为82次前兆中只有一次。该区大多数地震是孤立型主震。分析表明,可把前震活动当做明显的地震前兆,所以建议用地震活动性实时观测来提高预测系统的质量。
This paper mainly discusses the earthquake precursors recorded by Italy National Seismograph from 1975 to 1991. We consider foreshocks to be earthquakes that exceed a certain magnitude threshold before a mainshock that occurs after a quiet period and within a given time interval. The total number of potential forerunners determined by this algorithm depends on the size and duration of the calm zone to be predicted. The number of main shocks predicted by the foreshock also depends on the area of the forecast zone and the duration of the forecast period. Our aim is to optimize the parameters of potential foreshocks that are precursors to strong earthquakes to improve their authenticity and reliability. A preliminary survey across Italy allowed us to determine the areas where the foreshock - the mainshock sequence might occur. One of them is the area of central Italy with an area of about 32000 km ~ 2. There are 2671 earthquakes with M≥2.5 in this area, and there is a clear trend of large earthquakes occurring after medium earthquakes, at short distances and in short time. According to the optimization definition of foreshock - 63 earthquakes within a radius of 140km after at least 80 days of quiescence beyond Grade 3, 63 forecasts can be made. Within a radius of 30 km, the occurrence of mainshock exceeding Grade 4 in the next 48 hours was 6 out of 23 times. The corresponding probability increment is 150. In contrast, there is another large area (Po River basin) 453 times with M ≥ 2.5 earthquakes, only one of 82 precursors. Most of the earthquakes in this area are isolated main shocks. The analysis shows that foreshock activity can be regarded as a significant earthquake precursor, so it is suggested to use real-time seismicity observation to improve the quality of prediction system.