Building for Earthquakes • Chances are 2 out of 3 that you’ll be home when the next earthquake strikes, and 1out of 3 that you’ll be in bed. So your home’s ability to withstand an earthquake affects not only your pocketbook but also your life and the lives of those who live with you. Collapse of brick structure during 1983 Coalinga Earthquake
Imagine for a moment that your house is anchored to a flatcar on a moving train. Suddenly the train collides with another train and the flatcar stops abruptly. What happens to your house? If it’s a wood-framed house, as most houses in California are, it probably would not collapse, although your brick chimney might topple over. Damage to chimneys resulting from magnitude 7.1, 1992 Petrolia Earthquake
This analogy introduces an important concept. The jolt to your house during the train wreck is analogous to the shocks the house would receive during a large earthquake. The response of the house and its contents (including you) to these jolts follows the principle of inertia. The principle of inertia says that a stationary object will remain stationary. A home not bolted to its’ foundation will slip of the foundation because of its’ inertia. This house along Jefferson Street in the Marina District shifted more than 10 cm on its foundation due to 1989 Loma Prieta Earthquake Wooden structure located on Jefferson Street in Watsonville has shifted on its foundation due to 1989 Loma Prieta Earthquake
Damage to unreinforced masonry structures in Los Gatos (above) and San Jose (below) resulting from the 1989 Loma Prieta Earthquake • Ductile buildings such as wood and steel-frame structures tend to bend and sway during an earthquake. In contrast, brittle structures made of brick or concrete block joined together with mortar, or adobe buildings from California’s pioneer days are unable to deform during an earthquake with out collapsing.
The Armenian SSR Earthquake • On December 7, 1988, at 11:41 a.m. local time a magnitude 6.9 earthquake shook northwestern Armenia and was followed four minutes later by a magnitude 5.8 aftershock. Swarms of aftershocks, some as large as magnitude 5.0, continued for months in the area around Spitak. The vast majority of the damage occurred in unreinforced masonry structures.
In this earthquake both design deficiencies and flawed construction practices were blamed for the large number of building collapses and resulting deaths. Many of the modern multi-storied buildings did not survive. Twenty-five thousand were killed and 15,000 were injured by the earthquake. In addition 517,000 people were made homeless.
During the 1994 Northridge Earthquake, similar damage patterns to those in Armenia were observed throughout the San Fernando Valley. Partial collapse of Bullocks Department Store Collapsed roof near Northridge Mall
A common failure in California’s recent earthquakes was the two- or three-garage with living space overhead. Many condominiums have most of the first floor devoted to parking, with apartment space in the upper floors. The large amount of empty space at the garage door means less bracing against earthquake forces than in standard walls, so these open areas are the first to fail in an earthquake. Detail of shoring to garage area in building on Beach Street in the Marina District. The practice of using the first floor for garages left the building with inadequate lateral bracing on the ground level.
Carport beneath the collapsed apartment building in Northridge. Collapse of buildings into carports was a common cause of apartment damage in the epicentral area. The carports, because they were open on one side, did not have the resistance to shaking. This apartment building in Reseda collapsed over the garage due to 1994 Northridge Earthquake
Similar problems arise, although on a smaller scale, with large picture windows, sliding-glass patio doors, double doors or patio covers. Porch Damage, Wood Frame House, Santa Cruz Mountains, Loma Prieta Earthquake, 1989 Total Collapse of Front of Residence, Coalinga Earthquake of 1983
Soil Types and Shaking Amplification • Because of certain conditions, seismic waves may cause certain areas to shake up to 10x harder during an earthquake, this is called site amplification. The chief contributor to the site amplification is the velocity at which the rock or soil transmits shear waves (S-waves). Shaking is stronger where the shear wave velocity is lower.
The National Earthquake Hazards Reduction Program (NEHRP) has defined 5 soil types based on their shear-wave velocity (Vs). We have modified these definitions slightly, based on studies of earthquake damage in the Bay Area. The modified definitions are as follows: A) Vs > 1500 m/sec (This soil type occurs infrequently in the bay area. We consider it with type B. They are both represented by the color blue on the map). Soil type A includes unweathered intrusive igneous rock. Soil types A and B do not contribute greatly to shaking amplification. B) 1500 m/sec > Vs > 750 m/sec. Soil type B includes volcanics, most Mesozoic bedrock, and some Franciscan bedrock. (Mesozoic rocks are between 245 and 64 million years old. The Franciscan Complex is a Mesozoic unit that is common in the Bay Area.) C) 750 m/sec > Vs > 350 m/sec. Soil type C includes some Quaternary (less than 1.8 million years old) sands, sandstones and mudstones, some Upper Tertiary (1.8 to 24 million years old) sandstones, mudstones and limestone, some Lower Tertiary (24 to 64 million years old) mudstones and sandstones, and Franciscan melange and serpentinite. D) 350 m/sec > Vs > 200 m/sec. Soil type D includes some Quaternary muds, sands, gravels, silts and mud. Significant amplification of shaking by these soils is generally expected. E) 200 m/sec > Vs. Soil type E includes water-saturated mud and artificial fill. The strongest amplification of shaking due is expected for this soil type.
Fire! • There is also a formidable threat of fire, such as resulted from the 1906 San Francisco, the 1923 Tokyo, and the 1995 Kobe earthquakes. Citizens of San Francisco watch fires burn out of control following the 1906 earthquake Fires burn in the city of Kobe following the 1995 quake there
This panoramic view shows San Francisco in flames, five hours after the earthquake. The photograph was taken from Mason Street at 10:00 A.M., April 18, 1906. There is little evidence of earthquake damage. Most of the city's downtown buildings appear to be intact, yet these were later partially or wholly destroyed by flames. The fire continued unchecked for three days. This view of the San Francisco ruins shows many square blocks completely leveled. The photograph was taken on May 1, 1906, almost a month after the disaster. Much of the debris had already been hauled away leaving only empty ash-blackened blocks. Rebuilding of small buildings had begun.
Great Kanto quake (Tokyo-Yokohama) 1923 • killed at least 140,000 • tens of thousands burnt to death • Great Hanshin quake (Kobe) 1995 • fires started in old, cramped parts of city • many wooden buildings • 146 fires started • 23,000 homes destroyed • Tokyo today • ~ 1 million wooden homes Kobe Tokyo