Earthquakes + What Causes a Tsunami?
most famous faults -- the San Andreas. However, few Californians have experienced a major San Andreas earthquake. In Northern California, years ago in 1906. Over 3,000 people were killed and 225,000 people were left homeless. In Southern California, the last major earthquake on the San Andreas fault was more than 150 years ago (1857), rupturing the fault from Central California to San Bernardino. Few people lived in the area, so there was very little damage.
Further south along the San Andreas fault, from San Bernardino through the Coachella Valley to the Salton Sea, more than 320 years have passed since the last major earthquake (around 1680). Another major earthquake is likely to happen on this section of the fault within our lifetime, and will shake all of Southern California. A study led by the U.S. Geological Survey describes in great detail the extensive damage and casualties that result from such an earthquake, and recommends many ways that we can keep this natural disaster from becoming a catastrophe.
While the San Andreas is most likely to be the source of our largest earthquakes, there are hundreds of other faults throughout Southern California that could also cause damaging earthquakes. Some may happen before the next San Andreas earthquake and could be even more destructive if they occur directly beneath densely populated areas.
When it comes to disaster, there are simple things you can do to make yourself safer. The information on this page is designed as a step-by-step guide to give you details on what to do before, during, and after an earthquake. Start with the simple tips within each step so that you can build on your accomplishments.
An example of this in Step 1 is moving heavy, unsecured objects from top shelves onto lower ones. This will only take minutes to complete and you are safer from that hazard!
The information in the steps linked below will help you learn how to better prepare to survive and recover, wherever you live, work, or travel.
|Step 1: Secure your space by identifying hazards and securing moveable items.|
|Step 2: Plan to be safe by creating a disaster plan and deciding how you will communicate in an emergency.|
|Step 3: Organize disaster supplies in convenient locations.|
|Step 4: Minimize financial hardship by organizing important documents, strengthening your property, and considering insurance.|
SURVIVE AND RECOVER
|Step 5: Drop, Cover, and Hold On when the earth shakes.|
|Step 6: Improve safety after earthquakes by evacuating if necessary, helping the injured, and preventing further injuries or damage.|
|Step 7: Reconnect and Restore Restore daily life by reconnecting with others, repairing damage, and rebuilding community.|
Subduction Zones are Potential Tsunami LocationsMost tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck".
Accumulated Seismic EnergyAs the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plage. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries.
Earthquake Causes TsunamiEnergy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. At the same time, inland areas of the overriding plate are suddenly lowered.
Tsunami Races Away From the EpicenterThe moving wave begins travelling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline.
Tsunamis Travel Rapidly Across Ocean BasisTsunamis travel swiftly across the open ocean. The map below shows how a tsunami produced by an earthquake along the coast of Chile in 1960 traveled across the Pacific Ocean, reaching Hawaii in about 15 hours and Japan in less than 24 hours.
Tsunami "Wave Train"Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated large waves.