In South Iceland, populated and agricultural areas are at risk of earthquakes due to their location within the South Iceland Seismic Zone (SISZ). In 2008, two moderate-sized earthquakes (M5.8 and M5.9) occurred in Ölfus, the western end of this highly active transform zone. We analyze temporal seismic velocity variations (dv/v) related to the Ölfus earthquake doublet, using cross-correlations of ambient noise in the frequency range of 0.1–3.0 Hz. The two mainshocks decrease the average velocity by 0.8% at the nearest stations. The co-seismic changes are most noticeable from 0.7 to 1.7 Hz and affect a 40 km wide region. We present a first-time comparison of dv/v to crustal deformation, seismicity, co-seismic volumetric stress changes and reported PGA distribution for the Ölfus doublet. Ground accelerations caused by mainshocks at intermediate distances suggest that strong shaking-related damage may contribute to the co-seismic dv/v decrease. A rapid velocity increase (0.3%) in a month after the co-seismic drop indicates crustal rock healing. We find 3-months of post-seismic decorrelation, followed by a nearly permanent velocity decrease (0.2%) confined to a shallow layer (1 km) until the end of the observation period. Afterslip and pore fluid effects in the near-source region are likely to influence post-seismic dv/v. We demonstrate that seismic interferometry can contribute to future fault-zone monitoring operations in the SISZ by detecting small changes in velocity.

中文翻译：

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与 Mw 6.1 相关的时间地震速度变化，2008 年 5 月 Ölfus Doublet，冰岛南部：dv/v 和 GPS 的联合解释

在冰岛南部，人口稠密区和农业区由于位于冰岛南部地震带 (SISZ) 内，因此面临地震风险。 2008年，这个高度活跃转换带的西端奥尔福斯发生了两次中型地震（M5.8和M5.9）。我们利用 0.1–3.0 Hz 频率范围内环境噪声的互相关分析与 Ölfus 地震双峰相关的时间地震速度变化 (dv/v)。两次主震使最近台站的平均速度降低了 0.8%。同震变化在 0.7 至 1.7 Hz 范围内最为明显，影响范围达 40 公里。我们首次将 dv/v 与地壳变形、地震活动、同震体积应力变化以及报告的 Ölfus 双峰的 PGA 分布进行了比较。中距离主震引起的地面加速度表明，强烈的震动相关损坏可能导致同震 dv/v 降低。同震下降后一个月内速度快速增加（0.3％）表明地壳岩石正在愈合。我们发现震后 3 个月出现去相关，随后速度几乎永久下降 (0.2%)，且仅限于浅层 (1 公里)，直到观测期结束。近震源区的余滑和孔隙流体效应可能会影响震后 dv/v。我们证明，地震干涉测量可以通过检测速度的微小变化，为 SISZ 未来的断层带监测作业做出贡献。