Earthquake aftershocks how long

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.

However, unlike your fingers, the whole fault plane does not slip at once. The rupture begins at a point on the fault plane called the hypocenter, a point usually deep down on the fault. The epicenter is the point on the surface directly above the hypocenter.

The rupture keeps spreading until something stops it exactly how this happens is a hot research topic in seismology. Part of living with earthquakes is living with aftershocks. Earthquakes come in clusters. In any earthquake cluster, the largest one is called the mainshock; anything before it is a foreshock, and anything after it is an aftershock. Aftershocks are earthquakes that usually occur near the mainshock. The stress on the mainshock's fault changes during the mainshock and most of the aftershocks occur on the same fault.

Sometimes the change in stress is great enough to trigger aftershocks on nearby faults as well. An earthquake large enough to cause damage will probably produce several felt aftershocks within the first hour.

The rate of aftershocks dies off quickly. The day after the mainshock has about half the aftershocks of the first day. Ten days after the mainshock there are only a tenth the number of aftershocks. An earthquake will be called an aftershock as long as the rate of earthquakes is higher than it was before the mainshock. Following a significant earthquake, this aftershock forecast can provide situational awareness of the expected number of aftershocks, as well as the probability of subsequent larger earthquakes.

Specifically, we forecast:. We forecast aftershock activity over future time intervals of a day, a week, a month, and a year. We use the behavior of past aftershock sequences to forecast the likelihood of future aftershocks.

As an aftershock sequence progresses, our forecast also incorporates information about the behavior of that specific sequence. There are higher thresholds of M6 or 6.

We also compute forecasts for some smaller earthquakes that are of particular public interest, for example earthquakes in densely-populated areas. We will not usually compute aftershock forecasts for earthquakes that are themselves aftershocks of a prior larger earthquake, or for earthquakes that occur as part of volcanic activity.

Forecasts are updated regularly. The rate of aftershocks changes with time, generally decreasing, although sometimes temporarily increasing after a significant aftershock. Therefore, the forecasts are updated to keep current with the changing aftershock rate. We also update the forecasts over time to incorporate more information about the specific behavior of the aftershock sequence.

We update at least once within the first day, again within the first week, and again within the first month. The time that the current forecast was released, and the planned time of the next forecast update, are included in each forecast.

Clicking on the card will take the user to the Aftershock Forecast. The Commentary tab describes the aftershock forecast in simple language, starting with the concept that larger earthquakes could follow and that aftershocks will be continuing for some time; and some safety information is included.

The subsequent information is a simple summary of the forecast, followed by what has already happened, and ending with a more quantitative version of the forecast.

Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. 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. They soon "reload" the fault, dampen the aftershocks, and return the status quo within 10 years. In the middle of continents, faults move at less than a millimetre every year. In this slow lane, things can take a century or more to return to normal after a big quake, and aftershocks stick around for that duration.

Again, New Madrid proves the principle - a cluster of large earthquakes hit the area in the past thousand years, but the crust shows no sign of recent deformation according to two decades of GPS measurements. It seems that recent activity really is the legacy of centuries-old quakes, a threat that has since shut down. They happen on the faults we think caused the big earthquakes in and , and they've been getting smaller with time.

To test this idea, Stein and Liu used results from lab experiments on how faults in rocks work to predict that aftershocks would extend much longer on slower moving faults. They then looked at data from faults around the world and found the expected pattern. For example, aftershocks continue today from the magnitude 7.

This might be of some comfort to residents near the epicenter of the Hebgen Lake Quake. Then again, it might not. It's rather hard to feel comforted by the fact that the fault moves slower than the San Andreas, and therefore shall have aftershocks longer, when the last big quake took down a mountainside, ripped open roads, created a new lake, and left fault scarps all over the danged place, right?

The Hebgen Lake earthquake tore Highway to shreds.