Earthquake-induced burial of archaeological sites along the southern Washington coast about A.D. 1700

Although inhabited by thousands of people when first reached by Europeans, the Pacific coast of southern Washington has little recognized evidence of prehistoric human occupation. This apparent contradiction may be explained partly by geologic evidence for coastal submergence during prehistoric earthquakes on the Cascadia subduction zone. Recently discovered archaeological sites, exposed in the banks of two tidal streams, show evidence for earthquake-induced submergence and consequent burial by intertidal mud about A.D. 1700. We surmise that, because of prehistoric earthquakes, other archaeological sites may now lie hidden beneath the surfaces of modern tidelands. Such burial of archaeological sites raises questions about the estimation of prehistoric human population densities along coasts subject to earthquake-induced submergence. © 1996 John Wiley & Sons, Inc.     

The Portland Hills Fault: uncovering a hidden fault in Portland, Oregon using high-resolution geophysical methods

The Portland metropolitan area historically is the most seismically active region in Oregon. At least three potentially active faults are located in the immediate vicinity of downtown Portland, with the Portland Hills Fault (PHF) extending directly beneath downtown Portland. The faults are poorly understood, and the surface geologic record does not provide the information required to assess the seismic hazards associated with them. The limited geologic information stems from a surface topography that has not maintained a cumulative geologic record of faulting, in part, due to rapid erosion and deposition from late Pleistocene catastrophic flood events and a possible strike-slip component of the faults. We integrated multiple high-resolution geophysical techniques, including seismic reflection, ground penetrating radar (GPR), and magnetic methods, with regional geological and geophysical surveys to determine that the Portland Hills Fault is presently active with a zone of deformation that extends at least 400 m. The style of deformation is consistent with at least two major earthquakes in the last 12–15 ka, as confirmed by a sidehill excavation trench. High-resolution geophysical methods provide detailed images of the upper 100 m across the active fault zone. The geophysical images are critical to characterizing the structural style within the zone of deformation, and when integrated with a paleoseismic trench, can accurately record the seismic history of a region with little surface geologic exposure.     

根据地表破裂位移测量估计古地震的震级(摘要)

本文提出一种利用位移资料估计古地震 (史前地震 )震级的方法。现在估计古地震震级通常是根据地表破裂长度与矩震级的关系 ,需要确定同震地表破裂的总长度或者断层分段模型。但地表破裂总长度很少能够准确地测量 ,断层分段模型则有很大的不确定性而不可定量化。虽然在表示历史地震震级时用地表破裂长度比用位移好 ,但古地震研究能较好地提供断层某一位置的位移量。本文提出的方法的关键是考虑了对 1 4个现代地震观测得到的位移量的可变化性 ,这样可估计出古地震震级的不确定性。分析表明 ,这种不确定性以渐近线方式靠近破裂的自然可变化性 ,于是有 5至 1 0个点的位移测量值便足以判定古地震的特征。本文结论是由相当于原始破裂长度 1 0 %的断层崖采样就可提供合理估计地震震级所需要的数值。用1 992年ＬａｎｄｅｒｓＭｗ7 3地震和 1 95 4年ＤｉｘｉｅＶａｌｌｅｙＭＳ6 8地震的随机采样资料对该方法进行了检验 ,所得到的震级估计值与实际值很接近。根据地表破裂位移测量估计古地震的震级(摘要)@Mark A.Hemphill-Haley @Ray J.Welodn