Background Information on Aftershocks
Like most earthquakes, the recent earthquake in Turkey is expected to be followed by numerous aftershocks. Aftershocks are additional earthquakes that occur after the mainshock and in the same geographic area. Usually, aftershocks are smaller than the mainshock, but occasionally an aftershock may be strong enough to be felt widely throughout the area and may cause additional damage, particularly to structures already weakened in the mainshock. As a rule of thumb, aftershocks of magnitude 5 and larger are considered potentially damaging. In Turkey, where the infrastructure damage is so vast, magnitude 5 and larger aftershocks should definitely be considered potentially dangerous. However, the damage caused by aftershocks is usually limited to a relatively small region localized about the aftershock epicenter.
Aftershocks are most common immediately after the mainshock; their average number per day decreases rapidly as time passes. Aftershocks are most likely to be felt in the first few days after the mainshock, but may be felt weeks, months, or even years afterwards. In general, the larger the mainshock, the longer its aftershocks will be felt. Aftershocks tend to occur near the mainshock, but the exact geographic pattern of the aftershocks varies from earthquake to earthquake and is not predictable. The larger the mainshock, the larger the area of aftershocks. While there is no "hard" cutoff distance beyond which an earthquake is totally incapable of triggering an aftershock, the vast majority of aftershocks are located close to the mainshock.
As time goes on, the aftershocks will subside. As they do, we will revise the table below to reflect the diminishing chances of large aftershocks.
|Time interval||Probability of M>6 Aftershock||Probability of M>7 Aftershock|
|September 16, 1999 through September 23, 1999||less than 10%||less than 10%|
|September 16, 1999 through October 16, 1999||about 20%||less than 10%|
|September 16, 1999 through March 16, 2000||about 40%||less than 10%|
The California Generic Aftershock Model uses standard statistical models of earthquake occurrence fit to observations of aftershock sequences in California. The temporal decay of the aftershock rate is described by Omori's Law, a power law, while the distribution of aftershock sizes is based on the Gutenberg-Richter relation, an exponential distribution. Because the North Anatolian fault in Turkey is similar to the San Andreas fault in California, we believe it is appropriate to use the California model to describe the Izmit, Turkey, aftershock sequence. The model is described in:
Reasenberg, P.A. and L.M. Jones, "Earthquake Hazard After a Mainshock in California", Science, Volume 243, pp. 1173-1176, 1989
Reasenberg, P.A. and L.M. Jones, "Earthquake Aftershocks: Update", Science, Volume 265, pp. 1251-1252, 1994.