Anything we can do to increase the chances of successful earthquake prediction could help save a lot of lives. And it allows us to rest easier when we find out that those little temblors are just past earthquakes saying "So long, and thanks for all the fish. The views expressed are those of the author s and are not necessarily those of Scientific American. A confirmed adorer of the good science of rock-breaking, Dana Hunter explores geology with an emphasis on volcanic processes, geology news, and the intersection of science and society.
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Discover World-Changing Science. Stein and Liu analyzed earthquake data gathered worldwide. For major quakes that occurred where the sides of a fault moved past each other at average rates of more than 10 millimeters per year — as the two sides of many tectonic boundaries do — aftershocks died off after a decade or so. But for faults where the sides scraped past each other at just a few millimeters per year, aftershocks lasted about years, the researchers reported.
The longest series of aftershocks, some which have lasted several centuries, were triggered by quakes that occurred in continental interiors along slow-moving faults. Large earthquakes are often followed by aftershocks, the result of changes in the surrounding crust brought about by the initial shock. Aftershocks are most common immediately after the main quake. As time passes and the fault recovers, they become increasingly rare. This pattern of decay in seismic activity is described by Omori's Law but Stein and Liu found that the pace of the decay is a matter of location.
At the boundaries between tectonic plates, any changes wreaked by a big quake are completely overwhelmed by the movements of the plates themselves. At around a centimetre per year, they are regular geological Ferraris. The national meteorological agency warned that aftershocks could strike for up to a week following the main event. Repairs continued for years after the deadly Christchurch earthquake. The magnitude 6. The rarity of big quakes, however, makes it difficult to document and statistically model how large earthquakes interact with each other in space and time.
Aftershocks could offer a workaround. As a result, interactions between the largest earthquake in a sequence, known as a mainshock, and its aftershocks may hold clues to earthquake interactions more broadly, helping to explain how changes on a fault induced by one earthquake may affect the potential site of another.
The stress is what drives earthquakes. Scientists have noted a tendency for aftershocks to occur where two types of stress act on a fault change. The first type is called is normal stress, which is how strongly two sides of a fault are pushing together or pulling apart. The second type is called shear stress, or how strongly the two sides are being pushed past one another, parallel to the fault, by remote forces. Decreases in the normal stress and increases in the shear stress are expected to encourage subsequent earthquakes.
Measures of these changes in the volume of rock around a fault are combined into a single metric called the Coulomb failure stress change. There are components of stress that are different from shear stress and normal stress. After big earthquakes, we say them. But what do these terms mean? What do they mean for what we felt and what we will feel the next time?
Do we really understand what seismologists are saying? This section describes how earthquakes happen and how they are measured. It also explains why the same earthquake can shake one area differently than another area. It finishes with information we expect to learn after future earthquakes.
An earthquake is caused by a sudden slip on a fault, much like what happens when you snap your fingers. Before the snap, you push your fingers together and sideways.
Because you are pushing them together, friction keeps them from moving to the side. When you push sideways hard enough to overcome this friction, your fingers move suddenly, releasing energy in the form of sound waves that set the air vibrating and travel from your hand to your ear, where you hear the snap. The same process goes on in an earthquake.
Stresses in the earth's outer layer push the sides of the fault together. The friction across the surface of the fault holds the rocks together so they do not slip immediately when pushed sideways. Eventually enough stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.
Just as you snap your fingers with the whole area of your fingertip and thumb, earthquakes happen over an area of the fault, called the rupture surface. For example, the number of aftershock will decrease to one-tenth in the first 10 days, whereas it will only decrease to one-half in the next 10 days.
This is the reason why we feel like the aftershock lasts for a long time. In addition, the larger the magnitude of the main-shock is, the longer it takes for the aftershock to settle. Frequency of aftershocks differs according to their magnitudes.
For instance, the number of aftershocks with magnitude of 5 is about 10 times larger than that of aftershocks with magnitude of 6.
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