How long to expect aftershocks

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. Credit: USGS. The new results will help investigators in both understanding earthquakes in continents and trying to assess earthquake hazards there. Instead of just focusing on where small earthquakes happen, we need to use methods like GPS satellites and computer modeling to look for places where the earth is storing up energy for a large future earthquake.

We don't see that in the Midwest today, but we want to keep looking. Helens Books Commemorate Mount St. Load comments. Get smart. Sign up for our email newsletter. Sign Up. Read More Previous. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Decrease your risk of damage and injury from an earthquake by identifying possible home hazards before the next damaging earthquake.

CEA offers earthquake home insurance premium discounts for houses and mobilehomes that have been properly retrofitted. Be prepared with emergency plans and supplies.

Sign up for the MyShake app today. Remember to Drop, Cover, and Hold On. Find an affordable earthquake insurance policy to protect your property when the big one hits. CEA earthquake insurance can help you recover. Loss of Use coverage helps with the costs of temporary shelter and additional living expenses so that families can get back on their feet quicker. Without earthquake insurance, you place yourself at risk of losing everything or sustaining damages to your personal property that you cannot afford to repair.

CEA has made earthquake insurance easy to buy. Be prepared for what happens after an earthquake. CEA works with 25 residential insurance companies that serve the majority of California homeowners. Phone: Translate Share. Contact Us. Our forecast is based on a statistical model of the behavior of past aftershock sequences in similar tectonic settings. The rate of aftershocks usually follows a few general rules:.

The initial forecast after an earthquake occurs is calculated using parameters that worked for previous earthquakes in that region or similar regions around the world. As time goes by and we observe how many aftershocks are happening we use parameters that are a combination of the initial parameters and parameters determined from the current sequence of earthquakes.

The initial forecast uses only the mainshock magnitude, and therefore can be released soon after the mainshock, and before many aftershocks have occurred. Because the initial forecast depends a lot on the mainshock magnitude, we wait at least 30 minutes after the event occurs before issuing a forecast, to allow the preferred mainshock magnitude to stabilize. We also update the forecast if the mainshock magnitude significantly changes after the initial forecast.

Screenshot of the Overview webpage for an earthquake in Alaska. Commentary Tab: A Starting Point for Basic Information 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. Forecast Tab: Tables Showing the Detailed Forecast The Forecast tab presents the forecast as tables, covering a range of aftershock magnitudes and time frames.

Model Tab: Scientific Details This tab shows what model was used to compute the forecast, as well as the model parameter values. How the Aftershock Forecasts Work Our forecast is based on a statistical model of the behavior of past aftershock sequences in similar tectonic settings.

The rate of aftershocks usually follows a few general rules: Larger mainshocks trigger more aftershocks than smaller mainshocks, and the aftershock productivity for a given mainshock magnitude also varies between tectonic regions.

For instance, mainshocks along the San Andreas fault in California generally produce fewer aftershocks than mainshocks of the same magnitude where the Pacific plate collides with and subducts under Alaska. The rate of aftershocks decreases with time, such that the earthquake rate is roughly inversely proportional to the time since the mainshock. The magnitudes of the aftershocks do not get smaller with time, only their rate changes. Two example aftershock sequences, from southern California, that illustrate the three general properties of aftershocks.