The seismicity in the Azores plateau can be of volcanic or tectonic origin, the latter usually more destructive, especially in the central group of islands. In this study we aimed to obtain crustal stress indicators from seismic data collected in July 1998, following the July 9th M5.8 earthquake that struck near Faial and Pico islands. The maximum horizontal stress (SHmax) azimuth was obtained from two methods. For the seismogenic volume, the SHmax was obtained from single and composite focal mechanisms, whereas the state of stress beneath the seismic network was derived from shear-wave splitting analysis of local earthquakes. The composite focal mechanisms were obtained from a waveform clustering method applied to identify “identical” earthquakes. The seismic anisotropy analysis could be applied to only six stations, the polarization of the first S arrival being stable, and with a secondary perpendicular pulse arriving 0.6–0.24 s later. The focal mechanisms provided a NW–SE SHmax with azimuth range of 96°–114°, and an associated transition of normal-fault to strike-slip dominant regime consistent with the estimated regional field of 110°–120°, and coherent with movement along three fault systems. Inland the SHmax azimuths present greater variability. In Faial, the stations located north of the Caldeira present a stable azimuth of 151°–153° and are consistent with the effect of a NNW–SSE fault system present in the islands, rotating to 50°–77° in the eastern sector of the Pedro Miguel Graben, probably due to a complex interaction of the WSW–ENE dominant fault system with old eroded volcanic complexes that segmented the island into several blocks. In Pico island, the SHmax direction azimuth ranges from 93° to 135°, in a radial pattern around the island's main stratovolcano, overlapping the regional stress field.

中文翻译：

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从 1998 年地震的余震分析中洞察 Faial 和 Pico 群岛（亚速尔群岛）的地壳应力状态

亚速尔高原的地震活动可能源于火山或构造，后者通常更具破坏性，尤其是在中央岛屿群中。在这项研究中，我们的目的是从 1998 年 7 月收集的地震数据中获得地壳应力指标，这是在 7 月 9 日发生在 Faial 和 Pico 岛附近的 M5.8 地震之后。最大水平应力 (SHmax) 方位角通过两种方法获得。对于发震体积，SHmax 来自单一和复合震源机制，而地震台网下方的应力状态来自局部地震的剪切波分裂分析。复合震源机制是从用于识别“相同”地震的波形聚类方法中获得的。地震各向异性分析只能应用于 6 个台站，第一个 S 到达的极化是稳定的，第二个垂直脉冲在 0.6-0.24 秒后到达。震源机制提供了方位角范围为 96°–114° 的 NW–SE SHmax，以及与估计的 110°–120° 区域场一致并与运动相一致的正常断层向走滑主导状态的相关转变沿三个断层系统。内陆的 SHmax 方位角呈现出更大的可变性。在 Faial，位于 Caldeira 以北的台站呈现 151°–153° 的稳定方位角，并且与岛屿中存在的 NNW-SSE 断层系统的影响一致，在东部地区旋转至 50°–77° Pedro Miguel Graben，可能是由于 WSW-ENE 主导断层系统与古老的侵蚀火山复合体的复杂相互作用，将岛屿分割成几个区块。