LOW-FREQUENCY MAGNETIC-FIELD MEASUREMENTS NEAR THE EPICENTER OF THE MS-7.1 LOMA-PRIETA EARTHQUAKE
LOW-FREQUENCY MAGNETIC-FIELD MEASUREMENTS NEAR THE EPICENTER OF THE MS-7.1 LOMA-PRIETA EARTHQUAKE
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DOI：
10.1029/gl017i009p01465
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发表时间：
1990-08-01
影响因子：
5.2
通讯作者：
VILLARD, OG
中科院分区：
文献类型：
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作者：
FRASERSMITH, AC;BERNARDI, A;VILLARD, OG
本文报道了1989年10月17日洛马普列塔MS7.1地震前两个独立监测系统对低频磁噪声的测量结果。我们的测量覆盖了60多个频率范围0.01 Hz-32 kHz的25个窄频带，时间分辨率从ULF范围（0.01-10 Hz）的半小时到ELF/VLF范围（10 Hz-32 kHz）的一秒不等。ULF系统位于Corralitos附近，距离震中约7公里。ELF/VLF系统位于斯坦福大学校园内，距离震中约52公里。对极低频/甚低频数据的分析表明，我们目前没有发现任何前兆活动。然而，ULF数据具有一些独特的和异常的特征。首先，窄带信号出现在0.05-0.2的范围内。在9月12日前后，超低频系统的频率出现了一个100 Hz的峰值，并一直持续到第二个异常特征的出现。第二个异常特征是从10月5日开始，背景噪音大幅增加，几乎覆盖了整个超低频系统的频率范围。第三，在0.2-5 Hz范围内的噪声背景中有一个异常下降，从地震前一天开始。最后，也许也是最引人注目的，从地震前大约三小时开始，在0.01-0.5赫兹范围内的活动增加到异常高的水平。似乎没有任何起源于高层大气的磁场波动可以解释这种增加。此外，虽然我们的系统对运动很敏感，但地震测量表明，地震前没有明显的震动。因此，我们数据中的各种异常特征，特别是地震前三小时开始的活动大幅增加，可能是磁前兆。
We report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the MS7.1 Loma Prieta earthquake of 17 October 1989. Our measurements cover 25 narrow frequency bands in the more than six‐decade frequency range 0.01 Hz–32 kHz, with a time resolution varying from a half hour in the ULF range (0.01–10 Hz) to one second in the ELF/VLF range (10 Hz–32 kHz). The ULF system is located near Corralitos, about 7 km from the epicenter. The ELF/VLF system is located on the Stanford campus, about 52 km from the epicenter. Analysis of the ELF/VLF data has revealed no precursor activity that we can identify at this time. However, the ULF data have some distinctive and anomalous features. First, a narrow‐band signal appeared in the range 0.05–0.2. Hz around September 12 and persisted until the appearance of the second anomalous feature, which consisted of a substantial increase in the noise background starting on 5 October and covering almost the entire frequency range of the ULF system. Third, there was an anomalous dip in the noise background in the range 0.2–5 Hz, starting one day ahead of the earthquake. Finally, and perhaps most compelling, there was an increase to an exceptionally high level of activity in the range 0.01–0.5 Hz starting approximately three hours before the earthquake. There do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase. Further, while our systems are sensitive to motion, seismic measurements indicate that there were no significant shocks preceding the quake. Thus, the various anomalous features in our data, and in particular the large‐amplitude increase in activity starting three hours before the quake, may have been magnetic precursors.