Stratigraphic, lithologic, foraminiferal, and radiocarbon analyses indicate that at least four abrupt mud-over-peat contacts are recorded across three sites (Jacoby Creek, McDaniel Creek, and Mad River Slough) in northern Humboldt Bay, California, USA (∼44.8°N, −124.2°W). The stratigraphy records subsidence during past megathrust earthquakes at the southern Cascadia subduction zone ∼40 km north of the Mendocino Triple Junction. Maximum and minimum radiocarbon ages on plant macrofossils from above and below laterally extensive (>6 km) contacts suggest regional synchroneity of subsidence. The shallowest contact has radiocarbon ages that are consistent with the most recent great earthquake at Cascadia, which occurred at 250 cal yr B.P. (1700 CE). Using Bchron and OxCal software, we model ages for the three older contacts of ca. 875 cal yr B.P., ca. 1120 cal yr B.P., and ca. 1620 cal yr B.P.For each of the four earthquakes, we analyze foraminifera across representative mud-over-peat contacts selected from McDaniel Creek. Changes in fossil foraminiferal assemblages across all four contacts reveal sudden relative sea-level (RSL) rise (land subsidence) with submergence lasting from decades to centuries. To estimate subsidence during each earthquake, we reconstructed RSL rise across the contacts using the fossil foraminiferal assemblages in a Bayesian transfer function. The coseismic subsidence estimates are 0.85 ± 0.46 m for the 1700 CE earthquake, 0.42 ± 0.37 m for the ca. 875 cal yr B.P. earthquake, 0.79 ± 0.47 m for the ca. 1120 cal yr B.P. earthquake, and ≥0.93 m for the ca. 1620 cal yr B.P. earthquake. The subsidence estimate for the ca. 1620 cal yr B.P. earthquake is a minimum because the pre-subsidence paleoenvironment likely was above the upper limit of foraminiferal habitation. The subsidence estimate for the ca. 875 cal yr B.P. earthquake is less than (<50%) the subsidence estimates for other contacts and suggests that subsidence magnitude varied over the past four earthquake cycles in southern Cascadia.

中文翻译：

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美国加利福尼亚州洪堡湾北部过去 1700 年来南卡斯卡迪亚地震沉降的时间和数量

地层学、岩性、有孔虫和放射性碳分析表明，在美国加利福尼亚州洪堡湾北部（~44.8°北，-124.2°W）。地层记录了在门多西诺三重交汇点以北约 40 公里的卡斯卡迪亚南部俯冲带过去发生的大地震期间的沉降。来自横向广泛（> 6 km）接触上方和下方的植物大型化石的最大和最小放射性碳年龄表明沉降的区域同步性。最浅接触层的放射性碳年龄与卡斯卡迪亚最近发生在 250 cal yr BP (1700 CE) 的大地震一致。使用 Bchron 和 OxCal 软件，我们为大约三个较老的联系人的年龄建模。875 cal yr BP，ca。1120 cal yr BP，和ca。1620 cal yr BP 对于四次地震中的每一次，我们分析了从 McDaniel Creek 中选择的代表性泥炭-泥炭接触处的有孔虫。所有四个接触点的化石有孔虫组合的变化揭示了相对海平面 (RSL) 突然上升（陆地下沉），下沉持续了数十年到数百年。为了估计每次地震期间的沉降，我们使用贝叶斯传递函数中的化石有孔虫组合重建了接触面的 RSL 上升。1700 CE 地震的同震沉降估计值为 0.85 ± 0.46 m，约 0.42 ± 0.37 m。875 cal yr BP 地震，约 0.79 ± 0.47 m。1120 cal yr BP 地震，并且 ≥ 0.93 m 为 ca。1620 cal yr BP 地震。大约的沉降估计。1620 卡年 BP 地震是最小的，因为沉降前的古环境可能高于有孔虫栖息地的上限。大约的沉降估计。875 cal yr BP 地震小于 (<50%) 其他接触的沉降估计值，并表明在卡斯卡迪亚南部的过去四个地震周期中沉降幅度发生了变化。