Evidence for Liquefaction Identified in Peeled Slices of Holocene Deposits along the Lower Columbia River, Washington
Evidence for Liquefaction Identified in Peeled Slices of Holocene Deposits along the Lower Columbia River, Washington
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在华盛顿州哥伦比亚河下游的全新世沉积物剥落切片中发现液化的证据
DOI：
10.1785/0120020152
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发表时间：
2004
影响因子：
3
通讯作者：
B. F. Atwater
中科院分区：
文献类型：
--
作者：
K. Takada;B. F. Atwater
在距离太平洋海岸45公里的华盛顿狩猎岛，从河岸下的10个地质切片中提取的剥皮有助于识别在卡斯卡迪亚俯冲带估计为8-9级的史前地震中液化的沙子。每片都是通过将薄桩和百叶窗板打入6-8米的深度来获得的。得到的样品长8米，具有42-55厘米× 8厘米的梯形截面。切片除了在切片边缘产生弯曲和涂污外，几乎没有产生人工制品。每一片都是由哥伦比亚河潮汐沉积而成的分层良好的沙和泥。近90%的沙子呈明显的层状。沙子中含有0.5米厚、至少20米长的泥层，上面覆盖着一层泥层，其中含有被掩埋的土壤，这是1700年卡斯卡迪亚地震的标志，估计震级为9级。每片流态化砂都有岩脉相交，许多片也有褶皱和断层。比堤防还多的基岩，大多是沿着泥床的底部，局部侵入。泥浆层可能阻碍了从液化砂中喷出的水向上扩散。这些水被困在泥层下面，横向流动，通过夹带（流化）沙子破坏了层理，并局部冲刷了上覆的泥浆。褶皱砂水平带至少延伸10 ~ 20 m，部分发育低角度断裂。许多褶皱可能是在沙子因液化而变弱时形成的。低角度断层可能标志着河底滑坡或横向扩展的底部。在过去的2000年里，多达四次的卡斯卡迪亚大地震造成了这些侵入、褶皱和断层。这种流体逃逸和变形的地下证据使人们对以前根据哥伦比亚河沿岸地表局部缺乏液化特征推断出的最大加速度产生了怀疑。地质切片的液化证据不仅存在于布满堤防的河岸之下，也存在于没有堤防的河岸之下。如果不研究其下方沉积物的沉积后结构，就解释哥伦比亚河这种无堤防的河岸，就不能为确定卡斯卡迪亚大地震的震动强度上限提供充分的依据。在线资料：来自露头测量、振动孔和穿透仪测试的数据；地片沉积和后沉积特征的表格总结。
Peels made from 10 geoslices beneath a riverbank at Washington9s Hunting Island, 45 km inland from the Pacific coast, aid in identifying sand that liquefied during prehistoric earthquakes of estimated magnitude 8-9 at the Cascadia subduction zone. Each slice was obtained by driving sheetpile and a shutter plate to depths of 6-8 m. The resulting sample, as long as 8 m, had a trapezoidal cross section 42-55 cm by 8 cm. The slicing created few artifacts other than bending and smearing at slice edges. Each slice is dominated by well-stratified sand and mud deposited by the tidal Columbia River. Nearly 90% of the sand is distinctly laminated. The sand contains mud beds as thick as 0.5 m and at least 20 m long, and it is capped by a mud bed that contains a buried soil that marks the 1700 Cascadia earthquake of estimated magnitude 9. Every slice intersected sills and dikes of fluidized sand, and many slices show folds and faults as well. Sills, which outnumber dikes, mostly follow and locally invade the undersides of mud beds. The mud beds probably impeded diffuse upward flow of water expelled from liquefied sand. Trapped beneath mud beds, this water flowed laterally, destroyed bedding by entraining (fluidizing) sand, and locally scoured the overlying mud. Horizontal zones of folded sand extend at least 10 or 20 m, and some contain low-angle faults. Many of the folds probably formed while sand was weakened by liquefaction. The low-angle faults may mark the soles of river-bottom slumps or lateral spreads. As many as four great Cascadia earthquakes in the past 2000 yr contributed to the intrusions, folds, and faults. This subsurface evidence for fluid escape and deformation casts doubt on maximum accelerations that were previously inferred from local absence of liquefaction features at the ground surface along the Columbia River. The geosliced evidence for liquefaction abounds not only beneath banks riddled with dikes but also beneath banks in which dikes are absent. Such dike-free banks of the Columbia River, if interpreted without study of postdepositional structures in deposits beneath them, provide insufficient basis for setting upper bounds on the strength of shaking from great Cascadia earthquakes. Online material: Data from outcrop surveys, vibracores, and penetrometer tests; tabular summary of depositional and postdepositional features in geoslices.