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Migrating earthquakesare earthquakes that occur sequentially from one end of the fault to the other The North Anatolian fault in Turkey runs east-west for 900 km (comparable to San Andreas fault) oEarthquakes have occurred from east to west, from 1939 to 1999oKnowledge of past earthquakes makes it possible to estimate the recurrence interval for earthquakes of various sizes We can statistically estimate the probability that an earthquake of a particular size will strike a particular region within a specified time period oSome faults exhibit regular patterns of activity, though often not too reliablyoThe Parkfield section of San Andreas fault experienced a moderate earthquake every 22 years, until a 38 year interval ended in 2004oTokyo, Japan experienced strong earthquakes about every 70 years, with the most recent expected in 1993 – which hasn’t occurred yet oTokyo was leveled in 1923
oA magnitude 8.2 earthquake in 1923 destroyed Tokyo from an epicenter 90 kilometers away. Earthquakes may occur in groups oThe North Anatolian fault in Turkey has periods of activity and inactivityoThe Coast of northern California, Oregon, Washington and southern British Columbia experiences major earthquakes at long intervals, with the last earthquake 300 years ago The earthquake epicenter maps for Canada and the United states from 1899 to 1990 is seen here. oA seismic gap remains along the west coast. oLarger symbols represent larger earthquakesoShallow epicenters are purple, green are deeper, and yellow are deepest, primarily along the Aleutian subduction zone of southwestern Alaska.Unit 7 – Minimizing Earthquake DamageThe primary cause of death and damage is from the collapse of buildings and other structures The aftermath includes fire, disease, and epidemics Types of structural damage include load-bearing masonry walls that can shake apart and collapse Bridge decks and parking garage floors can be shaken off unanchored supportsoThe precast floor of the Northridge Fashion Center’s three-year-old parking garage, which was supported on the ledge of a supporting
beam, failed during the 1994 Northridge, California, earthquakeExternal walls are loosely attached to framework and can collapse oThe exterior facade loosely hung on a building framework in the community of Reseda collapsed in the 1994 Northridge earthquake. Directional shaking detached one wall while leaving the perpendicularwall intact.Reinforced concrete can break, leaving steel unenclosed so it can buckle and fail
oUnsupported reinforcing bars on a concrete freeway support failed during the 1994 Northridge, California earthquakeWeak floors, unbraced windows can not resist lateral movements Shattering windows are major source of injuries – safety glass are becoming more common Older buildings have loosely-resting beams – that can pull out, leading to a collapse Older houses are not bolted down – they can be shaken off their foundations