Common belief holds that once a big earthquake strikes, the major damage is done. Aftershocks are believed to diminish in intensity. A normal aftershock occurs on a connected fault line, transferring stress to interconnected fault lines.
However, a new study reveals that this isn’t the case. Scientists are worried about the transfer stress from one fault to another hundreds of miles away. These faults can be geologically disconnected. Actually, a seismic wave can almost instantly transfer stress between faults around the world. Aftershocks could also be triggered months after the main earthquake.
Scientists from the Scripps Institution of Oceanography at the University of California, San Diego (UCSD) studied 48 previously unexamined aftershocks over an eleven-year span (2004-2015) from earthquakes registering 7 to 8.0 Magnitude. These aftershocks occurred seconds after the main event and took place miles away from the “mainshock rupture.”
Co-author of the recent study, Wenyuan Fan, a graduate student at UCSD says,
These aftershocks miles away reveal that stress can be transferred almost simultaneously by the passing seismic waves from one fault to another within the earthquake fault system.
One example is along the Sundra arc subduction zone, the region that generated the devastating Indonesian tsunami in 2004. In addition to claiming 230,000 lives, this 7.0 M earthquake triggered aftershocks over 200 kilometers (124 miles) away.
Another example is the major earthquake that took place in April of 2005 in Nepal. After the quake, a large section of the fault did not move or rupture, remaining aseismic. Every time a major earth movement occurred, it remained stuck. The fear is that the built up pressure would one day rupture, causing another catastrophic earthquake.
The infamous San Andreas Fault in California has similarly been stuck. One 360-kilometer (225-mile) long section hasn’t budged sine 1857. One fear is if an 8.0 M quake occurs elsewhere in the country, its seismic waves me reach the fault, causing a major earthquake. In turn, the seismic waves from this quake may trigger additional seismic waves across the country like a domino effect.
Scientists agree that “The Big One,” a major earthquake along the San Andreas Fault is long overdue. The U.S. Geological Survey estimates that a 7.8 M earthquake could cause 1,800 deaths, 50,000 injuries, 300,000 damaged buildings, 1,600 fires and cause more than $200 billion in damage. The seismic wave aftershocks could increase these numbers substantially.
The study, written by Scripps geophysicist Peter Shearer and graduate student Fan states,
These findings have important implications for earthquake hazard-prone regions like California, where ruptures on complex fault systems may cascade and lead to mega-earthquakes.