Aftershocks rattle Napa Valley: What causes them?

The 6.0-magnitude earthquake that rocked the Napa Valley in California over the weekend, causing dozens of injuries and extensive damage in the city of Napa, delivered several encore performances Tuesday in the form of aftershocks.

Four minor temblors were reported in Napa County Tuesday morning ranging from magnitude 2.7 to 3.9, according to the US Geological Survey (USGS). As many as 65 aftershocks occurred in the first 24 hours following Sunday’s quake. Officials have warned Napa residents to be alert in the coming days for additional aftershocks that could dislodge debris on structures damaged by the main quake.

Aftershocks are a frequent occurrence following a major earthquake. They tend to be smaller than the main quake and can persist over a period of weeks, months, or even years.

The ground shaking experienced during an earthquake is the result of a sudden slip along a fault. The quake occurs when strain along the fault is large enough to overcome friction between the two sides of the fault. When the fault ruptures, the energy that had been stored by the buildup of strain is suddenly released, setting up ground motions that radiate away from the fault and produce shaking.

The ensuing temblor not only rattles buildings, roads, and nerves, but can also shift strain to nearby faults. Once the mainshock has subsided, additional, smaller-magnitude quakes occur as the displaced earth reaches new resting states. These are called aftershocks.

Aftershocks are by definition less powerful than mainshocks. If something initially identified as an aftershock occurs that is of greater magnitude than the preceding quake, the USGS recategorizes the first quake as a foreshock and the greater temblor as the main quake.

To be classified as an aftershock, the quake must occur within a specified radius of the main quake, known as the “aftershock zone.” The zone is most commonly measured as one fault-rupture length of the mainshock.

Aftershocks can continue for centuries, some scientists believe. There is a debate among seismologists over whether recent tremors that have occurred in a region south of New Madrid, Mo., known as the New Madrid Seismic Zone, could actually be aftershocks of a devastating sequence of quakes that rocked the central United States in 1811 and 1812. However, new research from the USGS published in January posits that the region should be considered an active earthquake zone.

One-third of US states face a “relatively high likelihood of experiencing damaging ground shaking,” according to the USGS’s earthquake hazard map that was updated in July.

Sunday’s quake occurred near the West Napa fault, which runs parallel to the San Andreas fault. No deaths have been reported, but more than 200 people were injured and property damage could exceed $1 billion.