When a relatively large earthquake occurs, a series of smaller earthquakes will follow continuously around the epicenter of the first earthquake. We call this first large earthquake “mainshock” and the subsequent “aftershocks”. This occurrence pattern of seismic activity is called as “mainshock-aftershock type”. A large earthquake whose epicenter is shallow accompanies aftershocks in most cases.

Besides mainshock-aftershock type stated above, there is another seismic activity called “swarm type”. Unlike mainshock-aftershock type, it does not have particular large earthquakes; however its seismic activity lasts for a long period of time, varying its level of the activities.

It is the area where aftershocks occur. Till about the first 24 hours after the occurrence of a main-shock, its aftershock area almost accords with the hypocentral area, an area destroyed by the main-shock. However, the aftershock area gradually extends to a wider area thereafter.

When the main-shock occurs, the rock around the epicenter becomes dynamically unstable. It is considered that aftershocks occur in order to adjust for this dynamical instability.

An aftershock has the following characteristics.

[1] The number of aftershocks will be most immediately after the occurrence of main-shock but will gradually decrease over time.

The number of aftershocks will decrease to approximately the one-tenth in 10 days whereas one-hundredth in 100 days. The decreasing rate of aftershocks is high immediately after the main-shock but will become low over time. For example, the number of aftershock will decrease to one-tenth in the first 10 days, whereas it will only decrease to one-half in the next 10 days. This is the reason why we feel like the aftershock lasts for a long time. In addition, the larger the magnitude of the main-shock is, the longer it takes for the aftershock to settle.

[2] A larger (smaller) aftershock occurs less (more) frequently.

Frequency of aftershocks differs according to their magnitudes. The number of aftershocks with magnitude of M is about 10 times larger than that of aftershocks with magnitude of M+1. (For instance, the number of aftershocks with magnitude of 5 is about 10 times larger than that of aftershocks with magnitude of 6).

[3] Generally, a magnitude of the largest aftershock is smaller by 1 than that of the main-shock.

[4] The largest aftershock, in many cases, occurs within 3 days after the occurrence of the mainshock in the case of inland earthquake. As for those occurred in sea area, the largest aftershock generally occur within about 10 days.

For instance, as the largest aftershock of the Great-Hanshin Earthquake in 1995 occurred two hours after the main-shock, while that of the Mid Niigata Prefecture Earthquake in 2004 occurred in 38 minutes (both of them are inland earthquake). Meanwhile, the largest aftershock of Sanriku-Haruka-Oki Earthquake in 1994, which is known as an undersea earthquake, took place 9.5 days after the mainshock.

[5] A large aftershock tends to take place in the edge and around the “aftershock area”.

[6] Quake of large aftershock may become comparable to that of main-shock depending on its location.

For instance, the earthquake which took place in Satsuma-Region in Kagoshima- Prefecture on March 26 1997 (M6.6) triggered the largest aftershock on April 3 (M5.7). In Sendai-City, Kagoshima prefecture, seismic intensities of both earthquakes were 5-upper on the JMA scale. As for the 2003 Tokachi-Oki Earthquake (M8.0), the largest aftershock (M7.1) occurred an hour after the main-shock; however in Urakawa-Town, Hokkaido, seismic intensities of both earthquakes were 6-lower on the JMA scale.