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首页> 外文期刊>Journal of Volcanology and Geothermal Research >The role of hydrothermal fluids in triggering the July-August 2000 seismic swarm at Campi Flegrei, Italy: evidence from seismological and mesostructural data
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The role of hydrothermal fluids in triggering the July-August 2000 seismic swarm at Campi Flegrei, Italy: evidence from seismological and mesostructural data

机译：热液在触发意大利Campi Flegrei 2000年7月至8月地震群中的作用：来自地震学和细观结构数据的证据

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摘要

This study presents a detailed analysis and interpretation of the seismicity that occurred on July 2-7 and August 22, 2000, during a ground uplift episode which started on March 2000 at Solfatara crater, Campi Flegrei. Earthquakes are located using a probabilistic grid-search procedure acting on a 3-D heterogeneous earth structure. The mainshock of the July swarm depicts a spectrum characterized by a few narrow peaks spanning the 1-5-Hz frequency band. For this event, we hypothesize a direct involvement of magmatic fluids in the source process. Conversely, the spectra of the August events are typical of shear failure. For these latter events, we evaluate the source properties from P- and S-wave displacement spectra. Results for the most energetic shocks (M_d around 2) yield a source radius in the order of 100 m and stress drop around 10 bars, in agreement with most of the earthquakes that occurred during the 1982-1984 bradyseismic crises. For the August swarm we identify two clusters of similar earthquakes. Application of high-resolution relative location techniques to these events allows for the recognition of two parallel alignments trending NE-SW. The relationship among source dimension and relative location evidences overlapping of sources. This may be interpreted in terms of either a heterogeneous stress field or a lubrication process acting over the fault surface. For a selected subset of the August events, we also analyze the splitting of the shear waves: results are indicative of wave propagation through a densely fractured medium characterized by a distribution of cracks oriented NE-SW. The pattern of faulting suggested by relative locations and shear-wave splitting is not consistent with the surface trace of NW-SE striking faults. However, a detailed mesostructural analysis carried out over the Solfatara area indicated the occurrence of two main crack systems striking NW-SE and NE-SW. This latter system shows a strike consistent with that derived from seismic evidence. Results from a stress analysis of the crack systems indicate that a fluid overpressure within the NW-SE-striking faults is able to form NE-SW cracks. We found that the pressure of fluids P_f required to activate the NW-SE faults is less than σ_(Hmin), while the P_f value required to open the NE-SW cracks is higher than σ_(Hmax). Our main conclusions are: (a) the Solfatara area is affected by two orthogonal fracture systems, and the fluid pathway during the 2000 crisis mainly occurred along the NNE-SSW/NE-SW-striking crack system; (b) the July seismicity is associated to the upward migration of a pressure front triggered by an excess of fluid pressure from a small-size magmatic intrusion; conversely, the August events are associated to the brittle readjustment of the inflated system occurring along some lubricated structures.
机译：这项研究对2000年7月2日至7日和2000年8月22日发生在2000年3月在Campi Flegrei的Solfatara火山口开始的地面隆起事件进行了详细的分析和解释。使用作用于3D异质地球结构的概率网格搜索程序来定位地震。七月群的主震波描绘了一个频谱，其特征是跨过1-5-Hz频带的几个窄峰。对于此事件，我们假设岩浆流体直接参与了源过程。相反，八月事件的频谱是剪切破坏的典型特征。对于后面这些事件，我们从P波和S波位移谱评估源特性。与1982年至1984年的缓震危机中发生的大多数地震一致，最剧烈的冲击（M_d约为2）产生的震源半径约为100 m，应力下降约10 bar。对于八月群，我们确定了两个相似地震的集群。高分辨率相对定位技术在这些事件上的应用允许识别趋势NE-SW的两个平行比对。源尺寸和相对位置之间的关系证明了源的重叠。这可以用非均匀应力场或作用在断层表面的润滑过程来解释。对于8月事件的选定子集，我们还分析了剪切波的分裂：结果表明波通过密集裂缝性介质的传播，其特征是面向裂纹的NE-SW的分布。相对位置和剪切波分裂表明的断层模式与NW-SE走向断层的表面轨迹不一致。然而，对索尔法塔拉地区进行的详细的细观结构分析表明，发生了两个主要的裂缝系统，分别是西北-东南和东北-西南。后一个系统显示出与地震证据一致的走向。裂缝系统应力分析的结果表明，西北-东南走向断裂中的流体超压能够形成NE-SW裂缝。我们发现激活NW-SE断层所需的流体压力P_f小于σ_（Hmin），而打开NE-SW裂缝所需的流体P_f值大于σ_（Hmax）。我们的主要结论是：（a）Solfatara地区受到两个正交裂缝系统的影响，2000年危机期间的流体通道主要发生在NNE-SSW / NE-SW撞击裂缝系统上； （b）7月地震活动与小型岩浆侵入引起的流体压力过高触发的压力前缘向上移动有关；相反，八月事件与沿某些润滑结构发生的充气系统的脆性重新调节有关。

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