Accuracy of hypocenter location, in particular focal depth, is a precondition for high-resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub-kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter-velocity inversion procedure restricted to direct, first-arriving, mainly upper-crustal Pg and Sg phases. On the basis of synthetic tests and selected ground-truth events we demonstrate that a sub-kilometer hypocenter accuracy can be achieved by regional-scale, three-dimensional Pg and Sg velocity models combined with dynamic phase selection and a non-linear location algorithm. The tomographic inversion uses about 60,000 Pg and 30,000 Sg quality-checked phases of local earthquakes in the Central Alps region. The derived models image the V_P and V_S structure of the Central Alps upper crust at unprecedented resolution, including small-scale anomalies such as those caused by Subalpine Molasse units below the Alpine front. The relocation procedure is applied to more than 18,000 earthquakes and the relocated hypocenters reveal previously unrecognized seismogenic structures, for instance in the Swiss Molasse basin south of Bern. The M_L 4.6 Urnerboden earthquake of 2017 is used as an example to demonstrate how the derived 3D velocity structure and relocated hypocenters can be jointly interpreted to constrain the lithology hosting upper-crustal seismicity in the Central Alps.

中文翻译：

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利用 3D Pg 和 Sg 体波反演程序提高绝对震源精度并在中部阿尔卑斯地区地震中的应用

震源位置的准确性，特别是震源深度，是对自然和诱发地震活动进行高分辨率地震构造分析的先决条件。例如，将地震活动与绘制的断层段联系起来需要亚公里级的震源精度。在这项研究中，我们证明了不准确的速度模型和不正确的相位选择会使绝对震中位置和位置不确定性产生偏差，从而导致误差大于目标精度。为了避免密集仪器地震网络中的这种偏差，我们提出了一种耦合震中速度反演程序，仅限于直接、先到、主要是上地壳Pg和Sg阶段。在综合测试和选定的地面实况事件的基础上，我们证明了通过区域尺度的三维Pg和Sg速度模型结合动态相位选择和非线性定位算法可以实现亚公里震源精度。层析反演使用了中阿尔卑斯地区局部地震的约 60,000 Pg和 30,000 Sg质量检查相位。派生模型对V _P和V _S成像以前所未有的分辨率了解中阿尔卑斯山上地壳的结构，包括小规模异常，例如由阿尔卑斯山锋下方的亚高山磨拉石单元引起的异常。重新定位程序应用于超过 18,000 次地震，重新定位的震源揭示了以前未被识别的地震结构，例如在伯尔尼以南的瑞士莫拉塞盆地。的中号_大号的2017 4.6 Urnerboden地震被用作一个例子，以说明如何导出三维速度结构和重新定位震源可以共同解释为限制承载在中央阿尔卑斯上部地壳地震岩性。