California's three major fault lines have been uncharacteristically quiet over the past century, leading experts to fear that a massive earthquake might be long overdue.

Scientists from the United States Geological Survey examined various seismic activities involving the Hayward, San Jacinto, and San Andreas fault lines over the past millennium.

They found that no major earthquakes have occurred between these three rifts in the last 100 years, something that has never happened before in California's history.

This relatively brief break from earthquakes might mean that the Golden State is in for some serious ground-shaking in the next few decades.

California's Earthquake Hiatus

California experiences an average of 10,000 earthquakes every year. However, many of these tremblors are spread throughout the state's 500 active fault lines and are considered only minor seismic activities.

Scientists use the Richter Magnitude Scale to measure the amount of seismic energy the Earth releases during quakes. Tremors with magnitudes of 2.5 to 5.5 on the scale can be felt by people aboveground, though they are not strong enough to produce any major damages.

Meanwhile, those with 5.5 and 6.5 magnitudes can cause visible damaging to roads and buildings.

Earthquakes that register magnitudes of 6.5 and higher are the ones that cause major damages to properties. Severe ground-shaking on this scale almost always prove fatal, especially if they occur in highly populated areas.

The USGS researchers found that the Hayward, San Jacinto, and San Andreas fault lines usually experience earthquakes with 6.5 or higher magnitudes between three to four times every 100 years. However, this trend suddenly stopped over the past century.

The last major tremor involving one of these fault lines was the 1918 San Jacinto earthquake that devastated the San Jacinto area in California.

In an interview with LiveScience, USGS geophysicist and study co-author Glenn Biasi discussed the recent inactivity of the three major fault lines.

"We're unusually quiet," Biasi said. "The biggest faults and the faults carrying most of the slip have not ponied up."

Worn-Out Fault Lines

What could have caused the earthquake hiatus? Biasi and his colleague, paleoseismologist Katherine Scharer, believe the major fault lines might be worn out from all the seismic activities they had throughout the 19th century.

From 1800 to 1918, there have been eight major tremors that occurred along the Hayward, San Jacinto, and San Andreas rifts. These included the 1857 Fort Tejon and the 1906 San Francisco quakes, both of which registered magnitudes of about 7.8.

However, the three main fault lines might have already accumulated enough seismic energy possibly for another big quake during the 100-year interlude. These rifts are the ones carrying most of the tectonic activity in California, and they are bound to slip sooner or later, according to Biasi.

If their findings prove to be correct, the researchers said the Earth can expect more massive quakes over the next 100 years. Californians can expect the succeeding century to be more like the period that ended in 1918. As many as six big quakes could occur in the coming decades.

The findings of the USGS study are featured in the journal Seismological Research Letters.

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