Lecture 11 18 Oakridge Fault Only major fault in L A Region that dips to the south The chi chi Taiwan earthquake mostly ruptures horizontally to the north although it does rupture a little horizontally towards the south The slip is oblique going upwards then turning sideways The rupture plane was 90 km No directivity pulse on the Chi Chi Taiwan Eq Since fault slip and rupture propagation direction are not parallel we do not have any directivity pulse 1971 Mw 6 7 San Fernando Earthquake occurred on the north dipping Sierra Madre Fault while the 1994 Mw 6 7 Earthquake Northridge Earthquake occurred on a south dipping blind thrust fault to the west 2011 6 0 Earthquake in Virginia Area of shaking of different earthquakes can vary by a large amount Energy does not propagate in California as well as it does in Virginia Earthquakes waves propagate much further in the eastern two thirds of the United States Energy in Cali does not propagate far because the large number of faults and folds cause the waves to attenuate much faster The California lithosphere is filled with transform faults and is hence much more complex From this earthquake it is illustrated that seismic waves travel through much more efficiently through simple lithosphere than complex earthquake People looked at the MMI of the New Madrid Missouri Earthquakes and suggested that the magnitude of this subduction mega thrust earthquake might be as big as 8 4 However in reality the earthquake was Mw 7 5 It was felt across East seaboard On the other hand the 1906 San Francisco Earthquake was Mw 7 9 but had a lower total rupture plane The fault in the Southern San Andreas use the San Gabriel Mountains as a guide to turn towards Los Angeles to deliver large shear and surface waves The SF earthquake was given an Ms of 8 3 San Francisco to Europe is a very efficient pathway for surface wave propagation This led us to believing that the SF Earthquake had an Ms of 8 3 This is because in 1906 a vast majority of earthquake seismograms were situated in Europe Site Effects How the geology directly beneath your site affects intensity and shaking 1989 Mw 6 9 Loma Preita Earthquake Was on a fault just next to the San Andreas on a right lateral strike slip fault parallel to the San Andreas Bedrock strongly de amplifies ground motion Bedrock is mostly solidified mud which is generally found in mountains Most landslides in California are triggered due to earthquakes Downtown Santa Cruz was built on very weak unconsolidated river sand Therefore tremendously amplifies shaking The Loma Preita Earthquake damage was focused in places around San Francisco This place is known as the Marina District Bay mud is mostly situated around SF Average depth of the SF bay is about 4 feet This bay mud is the worst kind of ground to build buildings upon This causes a lot of ground shaking and hence there was a lot of damage from the Loma Preita Earthquake in SF Path effects Built on filled land more damage than built on bed rock Lecture 11 20 14 Chapter 1 6 on Peace of Mind in Earthquake Country Reading Assignment Double decker freeway Upper layer pancaked on the bottom one because of shaking This happened only along part which was built atop bay filled water saturated mud Hayward Earthquake Collapsed buildings were also because of bay mud The shoreline receded as the mud from the bay was used to engineer buildings Sediments are pieces or rock that get washed out Most of Kobe is filled with sediments Sediments are water saturated rocks Loma Preita Earthquake Weak sediment with some cohesion Caused the double decker bridge to pancake over one another except one part that was tightly embedded in bedrock Santa Cruz Mountains are made of bedrock In the silicon valley San Jose and Santa Clara there is a sedimentary basin right above the bedrock that continues down from the Santa Cruz Mountains As seismometers at these different grounds were measured ground measures from bedrocks to sediments steeply increased showing difference is site effects in different types of grounds Bedrock vs Sediments up to 11x larger amplitude Duration up to 4x larger Sediment velocity is much slower than bedrock velocity Sediment is squishy mud and seismic waves travel slower in squishy mud Duration is important since each time sheer waves hits buildings it loses more joints Waves pile up because soft sticky mid reduced its speed Los Angeles basin is 10 km deep Hydrolic mining causes loss of water and an increase in sediment Sacramento Bay Area filled up with sediment due to gold mining Buildings built at the old shoreline in San Francisco The foundation is water saturated mud sediments Sedimentary Basins at Los Angeles San Fernando and Ventura Ventura basin is the deepest basin in the world About 13 km deep These basins amplify magnitude of shaking and trap seismic energy Worst place to start cities but are flat so cities are situated at these basins A lot of basin amplification waves in Los Angeles Ground shaking persists at basins much more than places built on bedrock 1906 Mw San Francisco Earthquake The Santa Rose sedimentary Basin causes shaking much more than any place around it Earthquake persists only at the basin for a time much more than the surrounding area This is also the case of the Santa Rosa Sedimentary Basin 1985 Mw 8 5 Mexico Earthquake The city is surrounded by a lake hence causing the city build on sedimentary basin Liquefaction At the beach sinking of feet at the junction of water coming in and going out Pressurizing the water causing the sand grains to pull apart not being a solid material all together For example Sand Volcano 1979 Earthquake Imperial Valley CA Sand is squirted out at low pressure Even with good foundation buildings on liquefiable land will fall down Liquefaction causes the water table inside the ground to rise Liquefaction in 1906 San Francisco Earthquake Slumping of the land that disrupted the city s water system Hence the city burnt down Mountains are characterized by bedrock Ridge Top Shattering Seismic Waves travel up on a mountain and is focused at the ridge top This causes ridge top shattering and a subsequent landslide Fault Fling The phenomenon when the force exceeds the force of gravity vertically These are local increases in force that leas to heavy things being flung into the air Lecture 11 25 The most important part of building construction is that all the panels everything should be properly bolted and connected together 1 URM POR Un