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| Soft sediment can amplify earthquake shaking, and the severity of shaking in Napa Valley is expected to differ from place to place because of differences in the sediment that forms the valley floor. This sediment has been deposited over the past 1-2 million years by streams flowing out of the hills and along the valley axis, and differ both in age and in texture. The younger, softer sediments, particularly the Bay mud (Qhbm) and alluvium along the rivers (Qhay, Qha), should shake more strongly, relative to nearby bedrock (br), than the older and coarser sediment, including the Late Pleistocene to Holocene alluvium (Qf) and particularly the older alluvium (Qoa). The Holocene deposits were formed within the past 10,000 years, whereas Late Pleistocene deposits are 10,000 to 30,000 years old. The beginning of the Pleistocene was 2 million years ago. |
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