Course Reader Notes and Review Session California s San Andreas Fault is one segment of the line of intersection between the North Pacific and the North American tectonic plates Both plates are moving slowly north and west at different rates Today these plates have changed direction The Pacific plate is moving northwestward in relation to North America while the North American plate pushed by the seafloor spreading at the Mid Atlantic Ridge moves west at a slower rate The newest and thinnest of the tectonic plates are the ocean floors which are still being formed from molten materials flowing from the earth s interior Shallow Earthquakes are more destructive but are experienced on a shorter radius Deep Earthquakes are less destructive as the waves dissipate as they reach the surface however they affect a larger region Hayward Fault near the SF bay is a creeping fault After the 1971 San Fernando Earthquake 33 aftershocks were experienced within a span of 1 hour The 1989 Loma Preita Earthquake had 4760 aftershocks within the span of 20 days The 1994 Northridge Earthquake gave more than 20 000 aftershocks San Jose is bounded on the west by the San Andreas Fault and on the east by both the Hayward and the Calaveras Fault The Hayward fault could produce the most destructive earthquake in the Bay Area because it runs through the most densely populated and oldest cities of the East Bay and is astride Silicon Valley An earthquake on the Cascadia Subduction Zone would likely produce the United States largest and most devastating tsunami Soils Quake effects are sharper in alluvial and landfill locations because the intensity of vibrations increases as the earthquake waves enter a thick layer of soft soil or the less dense mixtures of soil found in most landfills During an Earthquake firm ground vibrates at a frequency that is close to that of short to midrise buildings Soft ground on the other hand vibrates at a lower frequency and thereby has a greater effect on tall or long structures The compaction of sandy soil by earthquake vibrations causes water pressure in the soil to increase pushing apart soil particle to the point they can t hold weight The Marina district of the San Francisco suffered extensive liquefaction in the 1989 Loma Preita Earthquake The port of Kobe Japan suffered major damage due to liquefaction during the 1995 Earthquake Improperly engineered landfill is often significantly more damaging to structures than even some of the poorest alluvial soils Landfills are also frequently full of organic matter which decays and creates voids and weak spots prone to settlement The flat alluvial lands along the shores of SF bay Santa Ana and the Sacramento River deltas are composed largely of thick deposits of a soft silty clay that is highly compressible and unstable and has a high water content all poor characteristics in high intensity earthquake regions Hillside buildings are normally graded and filled If these fills are poorly compacted these sites can be very risky If landslide risk is absent or can be eliminated then hillside properly represents one of the best possible investments in earthquake country Buildings Two semi attached or adjacent buildings with no or little gap between their walls can seriously damage each other during an earthquake As the two buildings are structurally different they vibrate differently at different frequencies and can hence pound against each other The points of connectivity within the structure need to be specifically designed to be able to withstand these forces and deformation demands Lateral bracing reduces the displacements that the building will experience Steel framed bracing is common in large buildings Steel framed buildings are extremely flexible which can be a disadvantage when the building is swaying Sheer wall bracing is a solid continuous wall of plywood or reinforced concrete wall attached to the framing of the buildings This wall is very still and unbending thereby reducing the lateral deformations Diagonal or X bracing stiffens the framing of the buildings against deformations The shear wall bracing of large buildings is made of steel reinforced concrete Steel diagonal bracing is used in steel framed buildings and to strengthen older frame buildings Wood and steel are preferred in buildings because they are light hence lessening the inertial force and are extremely flexible and can deflect and flex without breaking If wood sills are not bolted down to the concrete foundation crawl space walls or cripple walls are not braced with plywood or the walls are made of glass and no wood then wood buildings can suffer serious damage Practically all buildings that are built on unstable or soft ground have a heavy roof such as clay tile having a soft ground story will suffer damage during a moderate high earthquake Any bracing will only be effective if its nailing is adequate Wood diagonal bracing is not as strong as shear wall bracing When the hollows of concrete blocks are properly reinforced with vertical and horizontal steel rods and then carefully grouted with poured concrete buildings walls made of such blocks form solid and continuous shear wall units Reinforced concrete buildings can be severely damaged if the concrete does not fill all the cavities if steel reinforcing is inadequate and if their connections within the building s diaphragms are weak or insufficient Reinforced masonry buildings are made of two separate layers of brick laid with connecting steel ties embedded in the mortar and horizontal and vertical steel reinforcements are then inserted in the space between the two layers These buildings resist at least moderate earthquake forces Steel buildings tend to be relatively earthquake resistant due to their light frame and ability to absorb a great amount of energy and deformation Failure in connection details and welding procedures contribute to damage in steel framed buildings The structure might also pose a disadvantage if it is too flexible Concrete shear wall bracing reduced flexibility In non ductile concrete buildings there can be many design flaws Among them are insufficient ties holding the vertical reinforcing steel together and confining the concrete in columns and poor reinforcing of beam to column joints These buildings can also suffer damage if the beam to column connections are not exactly 90o URM buildings are brittle and cannot deform without being damaged by the lateral thrusts of an earthquake Their