Primary Wave and Secondary Wave,

These propagate as waves of earthquake shake. For a while after an earthquake occurrence, small shakes continue and then a major shake can occur.

Preliminary Tremor…The first small tremors

　　　　Convey the initial tremor…Primary Wave

Principal motion…Shaking followed by a large

　　　　Tell the principal motion…Secondary Wave

Aftershocks

After an earthquake occurs, smaller earthquakes than the original earthquake occur nearby. The first major earthquake is called the “principal earthquake” and the following earthquakes are called “aftershocks”.

---

Aftershock Area

The area where aftershocks occur.

---

Nature of Aftershocks

1. Aftershocks frequently occur right after the principal earthquake and gradually reduce with time.
2. Large intensity aftershocks do not frequently occur, but small intensity aftershocks are common. When the magnitude of the earthquake increases by 1, the number of aftershocks occurrence is about one tenth.
3. The average magnitude of the largest aftershock is smaller by 1 than the magnitude of the principal earthquake.