The term earthquake describes both the sudden slip on a fault and the radiated seismic energy and ground shaking caused by the slip. It also covers ground shaking caused by volcanic or magmatic activity and other sudden movement due to stress changes in the earth.
Actually, many earthquakes happen in the Bay Area happen every day. Most of them are too small to feel. On average, there is an earthquake that a few people will feel every 2-3 weeks and one that will be felt by many people every year. Visit this website to see a map of the earthquakes that have occured in the Bay Area in the past 7 days. We cannot predict exactly when the next big earthquake will happen in the Bay Area, or anywhere else; we can only give a probability. Within the next 30 years, the probability is 62% that we will experience an EQ with magnitude 6.7 or greater on one of our faults. Southern California is in a similar boat: Within the next 30 years, there is a 60% chance that people there will have an EQ with magnitude 6.7 or greater.
Maybe. Or maybe not. A lot of other things besides earthquakes can cause local shaking - trucks driving by, thunder, etc. Check out this map. It displays recent seismic activity in California and is updated every time we detect and locate an earthquake. Also, this page shows a recent list of earthquakes in Northern California. If you felt an earthquake, help us out and report it in the ``Did You Feel It'' section.
http://www.earthquake.usgs.gov/regional/nca/ucerf/
If you look at earthquake statistics in most regions of the world, including California, you will find that for every magnitude 5 earthquake, there are about 10 that have a magnitude of 4, and for each magnitude 4, there are 10 with magnitude 3. Unfortunately, this means there are not enough small earthquakes to relieve enough stress to prevent the large events. In fact, it would take 32 magnitude 5's, 1000 magnitude 4's, or 32,000 magnitude 3's to equal the energy produced in one magnitude 6 event.
Only if you live in an old, unreinforced adobe house. In modern homes, doorways are not stronger than any other parts of the house, and the doors in them may swing and injure you. You are safer practicing ``DROP, COVER, AND HOLD ON'' under a sturdy piece of furniture. Every year, institutions and agencies that deal with earthquake hazard encourage everyone to participate in a preparedness drill. Find out more and sign up at http://www.shakeout.org. Another tidbit of information: if the shaking stops before you have had a chance to decide what to do, then you didn't need to do anything. If you need to ``drop, cover, and hold on,'' you'll have plenty of time to decide.
The Golden Gate Bridge is currently undergoing a sequence of retrofits, some of which have already been completed. The process has reached the point that the Golden Gate Bridge no longer faces the potential for collapse. Until all the retrofit steps have been completed, the Main Suspension Bridge may be damaged significantly in a major earthquake. Check for updates at the website of the Golden Gate Bridge. The new eastern section of the Bay Bridge, which is expected to open in 2013, is being built to withstand a major earthquake. The retrofit of the western segment has been completed. The Bay Bridge is considered an emergency ``lifeline'' route to be used in disaster response activities. Thus, when completed, it will be able to reopen quickly following an earthquake. For more information, visit http://www.baybridgeinfo.org/faqs.
No. The San Andreas Fault System, which crosses California from the Salton Sea in the southeast to Cape Mendocino in the north, is the boundary between the Pacific Plate and North American Plate. The Pacific Plate is moving northwest with respect to the North American Plate at approximately 46 millimeters, or about 2 inches per year (the rate your fingernails grow). The strike-slip earthquakes on the San Andreas Fault are a result of this plate motion. The plates are moving horizontally past one another, so California is not going to fall into the ocean. However, in the very distant future, Los Angeles and San Francisco may one day be adjacent to one another!
There are two reasons for this. One is the time between quakes on these two faults. The 1868 earthquake on the Hayward Fault was considered the ``Great San Francisco earthquake'' until the San Andreas Fault ruptured in 1906. Before then, we did not know about the hazard of either fault. Our newest scientific studies show that earthquakes on the San Andreas Fault happen about every 200 years, while they happen on average every 140 years on the Hayward Fault. Do the math: 1868 + 140 = 2008, while 1906 + 200 = 2106. The San Andreas Fault appears not to be ``due'' as soon as the Hayward. The second reason is vulnerability: The San Andreas Fault runs along the hills of the Peninsula and is offshore from Pacifica to Stinson Beach. The Hayward Fault passes along the foot of the East Bay Hills, through Fremont, Hayward, Oakland, etc. Many more people live directly on the Hayward Fault than on the San Andreas Fault. In addition, much of the infrastructure that supports the people in the Bay Area crosses the Hayward Fault: water for San Francisco and EBMUD, power lines, freeways, BART ...
Intensity describes how much the ground shakes during an earthquake. For a given earthquake, intensity is different from one place to another depending on the shaking, but the quake has only one magnitude. Factors that determine intensity at a specific spot include the rock and soil under it and how far it is from the epicenter of a quake. The intensity is usually given on the Modified Mercalli Intensity (MMI) Scale, a qualitative measure based on groups of standard observations, such as "Difficult to stand" or "Shutters, pictures move". To distinguish it from magnitude, intensity is assigned Roman numerals I-XII, with XII being the highest. If you recently experienced an earthquake and want to contribute to an intensity study, fill out a felt report.
Credits
Berkeley Seismological Lab
215 McCone Hall, UC Berkeley
Berkeley CA 94720-4760
outreach@seismo.berkeley.edu
