However, when the largest aftershock is bigger than Båth’s formula, or accompanies several larger-magnitude earthquakes, it moves out of pure aftershock territory into what would be considered “triggered mainshocks.” In 1931, for example, 10 days after an Mw7.8 earthquake in Hawkes Bay, New Zealand, there was a second major earthquake only 0.6 magnitude units smaller. Therefore, the largest aftershock typically has about one-fortieth the energy release of the mainshock.īåth’s Law is a rule of thumb and not a prescription. The Richter magnitude scale is logarithmic, and one magnitude unit step reflects a thirtyfold increase in energy release. In the 1960s, Swedish seismologist Markus Båth (pronounced “Bort”) declared what is now known as “ Båth’s Law”: The largest aftershock is typically 1.1 to 1.2 magnitude units smaller than the mainshock, irrespective of the mainshock size. If the activity passes into an area already primed for fault rupture, earthquakes may be more energetic than typical aftershock sequences. As the exponential decay proceeds, an increasing proportion of the seismicity would have happened anyway.Īlso, perhaps one should consider typical and atypical aftershock behavior. Preexisting background seismicity has been supplemented by the triggered seismicity. However, there’s no way to actually point to a specific earthquake and have total confidence that it is a true aftershock – that is, an event triggered by the mainshock that would not otherwise have occurred at the time. Aftershocks are said to have dissipated once activity has returned to background levels. The population of aftershocks reflects a frequency distribution that is richer in smaller shocks, with a lower magnitude-frequency gradient (popularly known as the “Gutenberg-Richter b-value”) than the total population of seismicity in that region. The largest aftershock can occur months after the mainshock, as seen with the Christchurch event. While there is exponential decay in numbers of events, there is no exponential decay in magnitude. This delayed response is attributed to slow “stress corrosion” associated with the mechanical or chemical weakening of rock barriers, which then allow a fault to slip, generating an aftershock.Īftershocks occur in a volume of radius one to two rupture lengths from the original mainshock, and they may be aligned and concentrated around one or both ends of the original fault rupture. Omori identified how, following an earthquake, there is a crowd of smaller shocks in the vicinity of the original fault rupture that manifests exponential decay of activity through time, in what is now known as “Omori’s Law.” For a major earthquake such as Darfield, this activity can continue for months.Īftershock physics explains that the mainshock fault rupture perturbs the stress field over the surrounding volume of rock, within which many smaller fault adjustments then take place. ![]() Japanese seismologist Fusakichi Omori studied aftershocks at the end of the nineteenth century. Now, 10 years on from Christchurch, what lessons are to be learned from the 2010–11 experience – especially with regard to the impact of aftershock events, with potential application to other cities around the globe? What did RMS® learn from Christchurch, and how has this been applied to our modeling? The Christchurch event followed the Darfield quake some five-and-a-half months later but was an unusually strong aftershock Christchurch was just 0.9 moment magnitude units less than Darfield. This event was part of the 2010–11 Canterbury Earthquake Sequence, which started with the Mw7.1 Darfield Earthquake on September 4, 2010. This was the Mw6.2 event that occurred on February 22, 2011, in Christchurch, the capital of South Island, New Zealand, with an epicenter at Port Hills on the southern edge of the city. Since the year 2000, only one city located in an advanced market economy has been severely impacted by an earthquake.
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