We use converted body‐wave phases from local earthquakes to constrain depth to basement and average attenuation relations for the Seattle basin in Washington and the Tualatin basin in Oregon. P‐, P‐to‐S‐(Ps), S‐to‐P‐(Sp), and S‐wave arrivals are present in three‐component recordings of magnitude 2.5–4.0 earthquakes at seismic stations located in these basins. Based on their relative travel times, these phases are attributed to body‐wave conversions at the basement‐to‐basin contact or to high‐impedance interfaces within the basins. Depth to basement values are calculated using the differential travel times between direct and converted phases, as well as average P‐ and S‐wave velocity values. We also identify a high‐impedance layer in the Tualatin basin that likely represents a laterally extensive deposit of volcanic materials embedded between the basement contact and the Columbia River Basalt Group. In addition, the average QP–QS attenuation relation is calculated for each station by taking the spectral ratio of converted phases to their parent body‐wave arrivals. For the Seattle basin, our analysis yields an average QP value of 73 and an average QS value of 60 for seismic waves with frequencies between 2 and 25 Hz. In the Tualatin basin, a much reduced QP–QS relation suggests that average body‐wave attenuation is likely higher than in the Seattle basin. The converted phase techniques presented here provide a reliable way to develop estimates of basin depth and attenuation structure for undercharacterized regions using simple passive source seismic records.

中文翻译：

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西雅图和图拉丁盆地的结构和 QP-QS 关系从转换的地震阶段

我们使用从本地地震转换而来的体波相位来约束深度到地下室，以及华盛顿的西雅图盆地和俄勒冈的图拉丁盆地的平均衰减关系。P-、P-to-S-(Ps)、S-to-P-(Sp) 和 S 波到达在位于这些盆地的地震台站的 2.5-4.0 级地震的三分量记录中存在。根据它们的相对传播时间，这些相位归因于基底到盆地接触处的体波转换或盆地内的高阻抗界面。使用直接相位和转换相位之间的差走时以及平均 P 波和 S 波速度值计算到基底的深度值。我们还发现了图拉丁盆地中的一个高阻抗层，这很可能代表了在基底接触层与哥伦比亚河玄武岩群之间埋藏的火山岩的横向广泛沉积。此外，通过转换相位与其母体波到达的频谱比计算每个站的平均 QP-QS 衰减关系。对于西雅图盆地，我们的分析得出频率在 2 到 25 Hz 之间的地震波的平均 QP 值为 73，平均 QS 值为 60。在图拉丁盆地，大大降低的 QP-QS 关系表明平均体波衰减可能高于西雅图盆地。