Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms
Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms
复制标题
根据模拟地震图波形匹配得出 1956 年阿莫尔戈斯双地震的震源机制和震源深度
DOI：
10.5194/se-5-1027-2014
复制
发表时间：
2013
期刊：
影响因子：
3.4
通讯作者：
C. Groß
中科院分区：
文献类型：
--
作者：
A. Brüstle;W. Friederich;T. Meier;C. Groß
抽象的。收集、数字化和重新分析了发生在希腊俯冲带(HSZ)火山弧中的1956年阿莫尔果斯双生大地震的历史模拟地震图，以获得对其深度和震源机制的精细估计。总共收集了来自29个欧洲电视台的80份事件记录，如果可能的话，将其数字化。此外，还在公告中搜索了计算仪器校正传递函数所需的仪器参数。然后进行网格搜索，将数字化的历史波形与完整的合成地震记录进行匹配，以推断深度和震源机制的最佳估计。由于仪器参数信息不完整或不可靠，以及在记录过程中经常出现的技术问题，如记录针从机械记录系统中跳出，证明适合于波形匹配的地震记录比收集的记录要少得多。对于第一次地震，只能使用斯图加特(STU)、哥廷根(GTT)和哥本哈根(COP)三个不同台站的七张地震图。然而，波形匹配网格搜索对于25公里和50公里的震源深度产生了两个稳定的失配最小值。相容断层面解为正断层或逆冲型。来自《国际地震学公报》的42个脉冲初动极性的单独分析排除了逆冲机制，显然倾向于至少有一个潜在断裂面沿西南-北东方向走向的正常断层解决方案。这一发现与圣托里尼-阿莫尔戈斯地块的局部构造和微震活动相一致。由于阿莫尔戈斯地区的地壳厚度一般不到30公里，震源深度为25公里似乎更现实。第二次地震的体波与面波振幅的比值非常高，表明这是一次位于希腊瓦达蒂-贝尼奥夫带的中等深度地震。这一假说得到了基于第一运动极性的震源机制分析的支持，这表明了一种与第一次事件非常不同的机制。为了支持中深度假设而进行的波形匹配网格搜索被证明是无效的，因为体波相位被抑制波形匹配的第一个事件的强烈面波尾波覆盖。然而，在德国区域地震台网(GRSN)上观测到的震中靠近米洛斯岛的现代中深度地震的体波与面波振幅比呈现出与第二次地震相似的模式，在0.05赫兹至0.3赫兹的频带内具有较高的值。相比之下，震中在克里特岛西部、震源机制和震级几乎相同的浅层地震，体波和面波的振幅比很低，最高可达0.17赫兹，而高于该频率的体波和面波幅度比则更高。根据与现代地震的对比，我们估计第二次地震的震源深度大于100公里。这两个事件在时间和空间上的接近表明，浅的第一个事件触发了第二个潜在的深部事件。
Abstract. Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic subduction zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording, such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only seven seismograms from three different stations at Stuttgart (STU), Gottingen (GTT) and Copenhagen (COP) could be used. Nevertheless, the waveform matching grid search yields two stable misfit minima for source depths of 25 and 50 km. Compatible fault plane solutions are either of normal faulting or thrusting type. A separate analysis of 42 impulsive first-motion polarities taken from the International Seismological Summary (ISS bulletin) excludes the thrusting mechanism and clearly favors a normal faulting solution with at least one of the potential fault planes striking in SW–NE direction. This finding is consistent with the local structure and microseismic activity of the Santorini–Amorgos graben. Since crustal thickness in the Amorgos area is generally less than 30 km, a source depth of 25 km appears to be more realistic. The second earthquake exhibits a conspicuously high ratio of body wave to surface wave amplitudes suggesting an intermediate-depth event located in the Hellenic Wadati–Benioff zone. This hypothesis is supported by a focal mechanism analysis based on first-motion polarities, which indicates a mechanism very different from that of the first event. A waveform matching grid search done to support the intermediate-depth hypothesis proved not to be fruitful because the body wave phases are overlain by strong surface wave coda of the first event inhibiting a waveform match. However, body to surface wave amplitude ratios of a modern intermediate-depth event with an epicenter close to the island of Milos observed at stations of the German Regional Seismic Network (GRSN) exhibit a pattern similar to the one observed for the second event with high values in a frequency band between 0.05 Hz and 0.3 Hz. In contrast, a shallow event with an epicenter in western Crete and nearly identical source mechanism and magnitude, shows very low ratios of body and surface wave amplitude up to 0.17 Hz and higher ratios only beyond that frequency. Based on this comparison with a modern event, we estimate the source depth of the second event to be greater than 100 km. The proximity in time and space of the two events suggests a triggering of the second, potentially deep event by the shallow first one.