Aftershocks

            The aftershocks that follow a major earthquake are always frightening to those who experienced the major quake, and can also be destructive, depending on the aftershock magnitude.  Where do aftershocks come from and why are some stronger than others?

            Aftershocks occur in the same rupture zone area and at the same depth as the original quake.  The rupture zone area includes the original fault line and associated faults that intersect or run parallel to the main fault within a few kilometers of the epicenter.  The main shock seems to destabilize the fault structures in the immediate and nearby areas, resulting in small fault line slippages of as little as a few centimeters on both the main fault and associated minor faults.

            A study conducted by a Cal Tech research team of the 1992 Landers earthquake in California indicates that minor afterslip occurred as far away as 20 kilometers (12 miles) from the main shock epicenter.  Only 8% of the aftershocks occurred within 0.5 kilometers of the main shock epicenter, and 23% within 1.5 km, which means that 69% of the aftershock sources were outside the 1.5 km radius.  However, the closer the aftershock location is to that of the main shock, the stronger it is.  In the Landers study, 60% of the aftershock strength, or energy release, took place within 1.5 km of the main shock location.

            How much plate movement takes place in the main shock and in the aftershocks?  Aftershock fault slippage is usually only a small fraction of that of the main shock slippage.  In the Landers study, it ranged from 1% to 1/10%.  Using that scale, if the main shock fault slippage is 5 meters, for example, the aftershock slippage may be as little as few centimeters.  But plate slippage of even a few centimeters may produce a sudden energy release that can cause heavy localized shaking, especially when the epicenter is near the surface.

            The January 12, 2010 Haiti earthquake had a main shock magnitude of 7.0.  The strongest aftershock registered 6.1.  The energy release of the main quake was 10 times stronger than the 6.1 aftershock, but because the aftershock occurred at a shallow depth and probably close to the original epicenter, the ground shaking was still quite violent.

            Haiti’s January 12 earthquake is turning out to be one of modern history’s most costly natural disasters in terms of lives lost and property destroyed.  As of this writing, the death toll estimate is 150,000, but it is believed that many more than that may still be discovered.  Aid from the United States and many other countries around the world has been flowing in and helping to provide food, water, medical care, and temporary shelter to millions of Haiti’s traumatized people.