Fragmentary Evidence of Great-Earthquake Subsidence during Holocene Emergence, Valdivia Estuary, South Central Chile
Fragmentary Evidence of Great-Earthquake Subsidence during Holocene Emergence, Valdivia Estuary, South Central Chile
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智利中南部瓦尔迪维亚河口全新世出现期间大地震沉降的碎片证据
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
L. Bradley
中科院分区：
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作者：
A. Nelson;K. Kashima;L. Bradley
对瓦尔迪亚河口边缘沼泽下全新世地层的调查显示，史前同震下沉只有零星证据。1960年，9：5级地震在瓦尔迪亚河口造成了1-2米的区域性同震下沉。在所研究的150个手工岩心中，大多数全新世晚期潮滩泥沙、泥炭和砂岩与下伏的中全新世潮下泥沙和砂岩之间有一个明显的不整合面。在地层学保存最完好的拉斯科罗拉达斯遗址，两个A级的沼泽和草甸土壤突然被沙子和泥浆覆盖，很可能记录了海啸后不久的同震下沉。由于现代硅藻组合和化石硅藻组合的不同，用硅藻传递函数很难恢复地震期间的沉降量。来自拉斯科罗拉达斯遗址1.7-1.3ka和2.7-1.7ka两个A层的宏观化石的最大14C年龄允许将较年轻的地平线与南部200公里处的海啸沙或海啸后潮汐沙埋藏的六个14 C-A层中的两个相关联，并与东南100公里处的Lago Puyehue的一次湖泊范围的物质损耗事件相关联。瓦尔迪亚史前同震下沉的潮汐记录很少，因为在过去6000年里，海平面下降了3-8m，在下沉引发河口洪水期间，沼泽和草甸土壤受到了侵蚀，在同震下沉和震后抬升周期中，地面水平基本上完全恢复。在线材料：来自拉斯科罗拉达斯网站的硅藻物种数据，瓦尔迪亚河口。
A reconnaissance of Holocene stratigraphy beneath fringing marshes of the Valdivia estuary, where an M 9:5 earthquake caused 1-2 m of regional coseismic subsidence in 1960, shows only fragmentary evidence of prehistoric coseismic sub- sidence. In most of the 150 hand-driven cores that were examined, a distinct uncon- formity separates 0.5-1.5 m of late Holocene tidal and floodplain mud, peat, and sand from underlying middle Holocene subtidal mud and sand. At the Las Coloradas site, where stratigraphy is best preserved, two A horizons of marsh and meadow soils abruptly overlain by sand and mud probably record coseismic subsidence shortly fol- lowed by tsunamis. The amount of subsidence during the earthquakes proved difficult to reconstruct with a diatom transfer function because of differences between modern and fossil diatom assemblages. Maximum 14 C ages on macrofossils from the two A horizons at the Las Coloradas site of 1.7-1.3 ka and 2.7-1.7 ka allow correlation of the younger horizon with either of two of six 14 C-dated A horizons buried by tsunami sand or post-tsunami tidal sand 200 km to the south at Maullin, and with a lake-wide mass wasting event in Lago Puyehue, 100 km to the southeast. Tidal records of prehistoric coseismic subsidence at Valdivia are scarce because of a sea-level fall of 3-8 m over the past 6000 years, erosion of marsh and meadow soils during subsidence-induced flooding of the estuary, and largely complete land-level recovery during cycles of coseismic subsidence and postseismic uplift. Online Material: Diatom species data from the Las Coloradas site, Valdivia estuary.