Northwestern University researchers have answered the question why earthquakes happen in clusters.

The clusters are explained as localized seismic events like quakes happening in quick sequences of short time spans such as days, weeks, or months.

In a computer model, the researchers noted that earthquake faults have a role on it as they keep a better memory of the latest event paving way for a recurrence.

"If it's been a long time since a large earthquake, then, even after another quake happens, the fault's 'memory' sometimes isn't wiped out, so there's still a good chance of having another," said Seth Stein, the senior author of the study and Professor of Geological Sciences at the Weinberg College of Arts and Sciences.

The research paper S44B­08: "Are Earthquake Clusters/Supercycles Real or Random?" will be presented on Thursday, Dec. 15, at the American Geophysical Union (AGU) meeting in San Francisco by Leah Salditch, the lead author.

Secret Behind Earthquake Recurrence

The model shows swarms or clusters happen on faults with long-term memory. This means even after a bigger earthquake, chances of another one happening are quite high. That is because an earthquake may not release all the pressure at the fault in one event and the left over strain can trigger another one.

The 1906 earthquake in San Francisco was a trend setter in this regard. Seismologists started predicting possible quakes on the basis of a fault's memory of the previous earthquake irrespective of the past quakes there.

The premise of fault's memory in forecasting future earthquakes and hazard mapping goes a long way in planning for buildings that can resist earthquakes with appropriate capacities.

Salditch, a graduate student at Stein's research, threw more light into the phenomenon and said there had been earthquakes having short time spans separating them punctuated with longer times of lull.

San Andrea's clusters had a 50 years gap in big earthquakes while clusters were separated by hundreds of years.

Clusters on the fault at Cascadia near Oregon, British Columbia, Washington and the Dead Sea fault are also possible examples.

One of the authors of the study, Edward M. Brooks, said the study is valuable in forecasting future earthquakes by tracking the last incident.

Earthquake App Helpful

Meanwhile, the University of California, Berkeley's worldwide network of smartphone earthquake detectors recorded nearly 400 earthquakes since the MyShake app was released in February. Among the spots, the most active seismic ones were Oklahoma fracking fields.

The ground motion is measured by the Android app using a smart phone's motion sensors and the data is sent to the Berkeley Seismological Laboratory for analysis.

The alerts give users the warning time to save or switch off damageable equipment. Already, 200,000 people have downloaded the app whose new version was out on Dec. 14 with options for notifications of quakes nearby.

Richard Allen, UC Berkeley professor of earth sciences and the project leader said the technology would make alerts faster to improve real-time detection with MyShake.

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