Precursors to the Kalapana M = 7. 2 earthquake
Precursors to the Kalapana M = 7. 2 earthquake
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卡拉帕纳 M = 7. 2 地震的前兆
DOI：
10.1029/jb086ib05p03881
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发表时间：
1981
影响因子：
--
通讯作者：
A. Johnston
中科院分区：
文献类型：
--
作者：
M. Wyss;F. Klein;A. Johnston
1975年11月的夏威夷卡拉帕纳地震的破裂长度为40至50公里，位于活火山基拉韦厄的南侧。震源机制为向东南倾斜约20度的平面上的倾滑正断层，最大主应力指向该方向，并由火山侵入基拉韦厄裂谷积累而成。1975年地震的震源区由于靠近火山，在地震发生前的许多年里，人们一直在对其进行深入的地质和地球物理研究。我们研究了1962 ~ 1975年小地震的震中分布。在主震发生前，位于余震3公里范围内的7台地震仪已经运行了3 ~ 10年，自1914年以来，在震源地区多次进行了大地三角测量和三边测量。我们发现，前兆变化发生在大部分破裂区域，但有两种明显不同的模式。在较大的外异常区，主震前3.8年的地震活动率下降了50%；此外，几条大地测量线表明在此期间有异常应变释放。在两个内部区域内，地震活动性仍然很高，然后在主震前不久增加。在其中一个内部区域，可以检测到P波传播时间延迟0.2秒，这是在主震发生前3.5年左右开始的。在另一个内部异常区内，大地应变一直在积累，直到1975年上半年，3.5×10−4应变（35 bar）被抗震释放。相比之下，外部异常体在1970/1971年以后经历了应变软化。我们认为，这些观测结果表明，外部异常区断层蠕变造成的应变软化将应力转移到两个主要凸起（断层的锁定部分）。在一个粗糙体中观察到速度下降和前震，而在另一个粗糙体中观察到高应力积累，这表明地壳可能发生了膨胀。我们的模型在定性上得到了强运动记录的独立证据的支持，这些证据表明卡拉帕纳地震是一次复杂的多次破裂。我们认为，卡拉帕纳地震发生前有一个持续3.8±0.3年的准备过程，其规模为45 × 10 km，大约覆盖了余震区。
The Kalapana, Hawaii earthquake of November 1975 had a rupture length of 40 to 50 km and was located on the south flank of the active volcano Kilauea. The source mechanism was dip slip normal faulting on a plane dipping ∼20 degrees to the SE, with the greatest principal stress oriented in that direction and accumulated by volcanic intrusions into Kilauea's rifts. The source area of the 1975 earthquake was subject to intensive geological and geophysical research for many years before this earthquake because of its proximity to the volcano. We studied the distribution of epicenters for small earthquakes from 1962 to 1975. Seven seismographs located within 3 km of the aftershock area had been in operation for 3 to 10 years before the mainshock, and geodetic triangulations and trilaterations in the source area had been carried out repeatedly since 1914. We found that precursory changes occurred throughout most of the rupture area but in two distinctly different patterns. In the larger outer anomalous area the seismicity rate was decreased by 50% during the 3.8 years before the mainshock; in addition, several geodetic lines indicated anomalous strain release during this time. Within two inner areas the seismicity remained high, then increased shortly before the mainshock. In one of the inner areas a P wave travel time delay of 0.2 s could be detected, which began about 3.5 years before the mainshock. Within the other inner anomalous area geodetic strain was accumulating until the first half of 1975 when 3.5×10−4 strain (35 bars), was released aseismically. By contrast, the outer anomalous volume was experiencing strain softening from 1970/1971 on. We interpret these observations as indicating that strain softening by fault creep in the outer anomalous area transferred stress into two major asperities (locked portions of the fault). A velocity decrease and foreshocks were observed in one asperity and high stress accumulation in the other, implying that dilatancy of the crust probably occurred. Our model is qualitatively supported by the independent evidence of strong motion records which show that the Kalapana earthquake was a complex multiple rupture. We conclude that the Kalapana earthquake was preceded by a preparatory process which lasted 3.8 ± 0.3 years and which had dimensions of 45 × 10 km covering approximately the aftershock area.