The structural engineer must account for potential seismic forces. Because California is a particularly active earthquake zone, the engineers of SWSE always attend to this risk. Follow the links below to learn about historical seismic events, up-to-date scientific perspectives and preparation for future seismic activity.
Earthquake Facts and History
Various historical and scientific data:
1. The earliest reported earthquake in the current geographical region of California occurred in 1769 and was reported by Gaspar de Portola while on an expedition southeast of Los Angeles. The first reported earthquake in the current geographical region of America occurred in 1663.
2. The San Andreas fault system experiences a relative motion of approximately 56 mm per year, at which rate Los Angeles and San Francisco would meet in some fifteen million years. It is not one fault, but a system of faults over eight hundred miles in length occurring between the Pacific Plate and the North American Plate. The San Andreas is a right-lateral strike-slip fault, meaning that the plates move horizontally to one another, and the Pacific Plates moves north as compared to the North American Plate.
3. Southern California has a reputation as an area of notable seismic activity. Each year, approximately ten thousand earthquakes are recorded in the region each year, of which fifteen to twenty are of magnitude 4.0 or greater. These earthquakes constitute two percent of the world's annual quakes.
4. Seismic activity is recorded according to magnitude, a measurement of the size of earthquake, and intensity, which is dependant upon the location of the observer.
5. Human recordings of earthquakes date back to the second millennium BC, but it was not until 1760 AD that their cause was correctly identified, by British engineer JohnMitchell.
6. The largest recorded earthquake in the world was a magnitude 9.5 Mw earthquake in Chile on May 22, 1960. The largest in US history since 1900 was a 9.2 Mw quake in Alaska on March 28, 1964.
Earthquakes and Engineering
Measuring the Earthquake
In accordance with the Richter scale, the magnitude of a seismic event is calculated using the logarithm of wave amplitude as recorded by seismographs, taking into account the distance from the location of the measurement to the location of the earthquake. As a result of the logarithmic scale, a whole number increase in Richter magnitude corresponds to a tenfold increase in amplitude.
Alternatively, earthquake magnitude can be reported according to the Modified Mercalli Scale (see below), which is based upon observed effect.
Knowing the Risks, Designing the Structure
During the design process, the engineers of SWSE take into account such factors as: the proximity of the structure to the local fault zones; the type of soil on which the structure will rest; and the intended use of the building, with such critical structures as hospitals designed with the greatest resilience to seismic forces. Data such as the maximum historical ground acceleration at the building site can be obtained using seismic maps from previous earthquakes.
Modified Mercalli Scale
1. Not felt except by a very few under especially favorable conditions.
2. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
3. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
4. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
5. Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
6. Felt by all, many frightened some heavy furniture moved; a few instances of fallen plaster. Damage slight.
7. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
8. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
9. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
10. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rail bent.
11. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
12. Damage total. Lines of sight and level are distorted. Objects thrown into the air.