Earthquake Damage The Cape Mendocino Earthquakes April 25 And 26 1992

Author : Ross S. Stein
Publisher :
ISBN 13 :
Total Pages : 54 pages
Book Rating : 4.F/5 ( download)

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Download or read book Permanent Ground Movement Associated with the 1992 M written by Ross S. Stein and published by . This book was released on 1994 with total page 54 pages. Available in PDF, EPUB and Kindle. Book excerpt: A critical element of the Humboldt County infrastructure is its vertical control network. The positions of bench marks (BMs) in the network are used for cadastral, roadways, engineering, and flood plain surveys. At the request of the Department of Public Works of Humboldt County, 333 km of the network nearest the 1992 Cape Mendocino earthquake was resurveyed by the National Geodetic Survey in late 1992, with 100 new BMs emplaced. Comparing the new heights with those measured before 1992 gives the vertical deformation associated with the earthquake, which includes motion caused by slip on the earthquake fault, earthquake-triggered surficial disturbances such as landslides and liquefaction, and settling of engineered structures due to earthquake shaking. We find that 37 BMs in Humboldt County display height changes that can not be accounted for by the earthquake fault slip, and thus indicate monument, soil or structural instability: 18 BMs show subsidence caused by groundwatet withdrawal in the Eel River basin; 16 BMs show residual movement of less than a few centimeters, and 4 BMs show major disturbance of up to 4 m. One of the disturbed BMs lies near an earthquake-induced tension crack, and another is near the site of liquefaction in the Salt river bed. Nine of the disturbed BMs are set in bridge abutments or highway overpasses, and 6 are set in retaining walls, and thus might be indicators of structural weakness or incipient failure. In addition, the distribution of landslides, liquefaction, and road cracking reveals other sites of potential ground failure along highways, seawalls, and foundations. Some 1992 BMs could be resurveyed to monitor subsequent movement. Using a model for the earthquake fault slip that best fits the geodetic and seismic observations of the earthquake, we infer the uplift of shoals and rocks offshore Humboldt County. We estimate mat 13 shoals lying in less than 3 fathoms (5.5 m) of water were uplifted by more than 0.45 fathoms (0.83 m), presenting increased hazards to coastal navigation. In addition, a 4 x 8 nautical-mile wide (7 x 14 km) zone is identified as the site of potential seafloor faulting with displacement of up to 3 m, and the head of Mattole Canyon is suggested as the site of possible seafloor landslides. These findings have been furnished to the U.S. Coast Guard for issuance as a Notice to Mariners, and to the National Ocean Survey of NOAA for bathymetric survey planning. We also find that the flood plain boundary of the Mattole river at Petrolia will be shifted by as much as 62 m (200f) as a result of the permanent change in elevation caused by the earthquake.