Q: How do you know whether an earthquake is an aftershock or not?
Large earthquakes often trigger smaller earthquakes as aftershocks. The aftershocks occur because the earth has to readjust to the new stress condition produced by the fact that the large earthquake happened. Aftershocks exhibit a particular pattern with time. If a small earthquake fits this pattern that is part of this increased earthquake activity following a large event, we call it an aftershock. If it does not fit the pattern, then we say it is just another small earthquake that is part of the natural background rate of earthquake activity. Calling a small earthquake an aftershock simply draws attention that it is related in space or time to the large mainshock.
Q: Up to what wave intensity is the rictor scale used? And what scale is used for larger earthquakes?
The magnitude scale for measuring the size of an earthquake is based on the amplitude (not intensity) of a seismic wave measured at a certain wave period (in seconds) or frequency (in hertz). The original Richter magnitude (designated the Local Magnitude scale) was designed to work only in California. It is based on measuring the amplitude of the seismic waves near 1 Hz or 1 sec period. Because larger earthquakes release more of their energy at longer periods (lower frequencies) than smaller earthquakes, measuring the amplitude near 1-sec period waves is not always a valid measure of the true "size" of the earthquake. Because of this, the "Richter magnitude" is thus said to saturate at about magnitude 7.0 to 7.5. This means that even though the earthquake is larger, the amplitude near 1-sec period does not significantly change. For larger earthquake (> M7.5) we use other magnitude scales which measure either the amplitude of seismic waves at longer periods (anywhere from 20 seconds to 100 seconds), or we use a completely different estimate for the size that takes into account the surface area of rupture along the fault and the amount of slip (displacment) on the fault. The latter is called the seismic moment.
Q: How many earthquakes happen per year throughout the world?
There are approximately 6,000 earthquakes per year greater than magnitude 4.0, and hundreds of thousands of smaller earthquakes that mostly go undetected. In southern California, where there are hundreds of sensitive monitoring stations, this network records between 2,000 and 10,000 earthquakes per year with magnitudes greater than about M 1.0.
Q: When do most earthquakes occur?
Earthquakes happen all the time. The earthquake itself is thought to occur when the state of stress exceeds the strength of the fault (or rock), the fault slips suddenly, and the 'earthquake' or ground shaking begins.
Q: How does the depth of the earthquake effect the amount of damage?
The depth of the earthquake has a very strong effect on the amount of damage. The 1994 M6.7 Northridge earthquake started at a depth of about 18 km (12 miles) and ruptured up to a depth of 5 km (3 miles). It caused damaged estimated to be between $20 billion and $40 billion and killed about 60 people. In contrast, the 2001 M6.8 Nisqually earthquake near Seattle occurred at a depth of 51 km (33 miles). No one was killed and damage was on the order of a few $million. A large part of this difference was the greater depth of the 2001 earthquake
Q: What do people do to try to minimize the damage caused by earthquakes?
There are many things people do to minimize the damage caused by earthquakes. The first is to recognize where large earthquakes occur and to restrict building in these areas, and require new buildings in earthquake prone areas to meet higher standards to resist damage and collapse from earthquake ground shaking. Older buildings are required to be reinforced with added bracing, typically steel reinforcement. There are also a lot of little things (like strapping down your hot-water heater, putting latches on kitchen cabinent doors, etc.) that you can do to your home to minimize damage.
Q: Are animals able to detect earthquakes before they happen?
This is a frequent question because of the many accounts of animals acting unusually or being very agitated prior to earthquakes. In nearly all well documented cases, the answer is no.
Earthquakes, like lightening that produces both light and sound (thunder), produces two kinds of waves, P-waves that travel with the speed of sound in rock and S-waves that travel at about 1/2 this speed. P-waves are also often much smaller that the slower S-waves. What is actually happening is that the animals, which are more sensitive to small ground motions, are feeling either small foreshocks, or the first-arriving P-waves that are too small for humans to feel. The larger main shock or the later-arriving S-waves then occur, and the humans notice that the animals were agitated before they were able to feel the earthquake themselves. In either case, the animals are still experiencing earthquakes, not predicting them. The animals are just experiencing the earthquakes sooner, as they are more sensitive to the first-arriving seismic waves the earthquake (or earthquake sequence) can produce.
Q: Q: an earthquake, how deep does the crevice go when the earth opens
A popular literary device is a fault that opens during an earthquake to swallow up an inconvenient character. But unfortunately for principled writers, gaping faults exist only in novels. The ground moves across a fault during an earthquake, not away from it. If the fault could open, there would be no friction. Without friction, there would be no earthquake. This information was obtained from: http://www.scec.org/education/public/allmyths.html