The Place in the Crust where the Movement of Earthquakes Starts is called the Focus

The focus of an earthquake, also known as the hypocenter, is the point within the Earth's crust where the release of energy occurs, initiating the seismic activity. It is the actual location where the rocks break and slip, resulting in seismic waves that propagate through the Earth.

Understanding Earthquakes
Earthquakes are caused by the sudden release of energy in the Earth's crust. This release of energy occurs due to the movement of tectonic plates, which are large sections of the Earth's crust that float on the semi-fluid mantle beneath. When stress builds up along the boundaries of these plates, it eventually exceeds the strength of the rocks, causing them to break and slip. This slipping of rocks generates seismic waves that radiate outwards from the focus.

The Focus
The focus is the exact point within the Earth's crust where the initial rupture occurs, leading to the generation of seismic waves. It is typically located underground, at varying depths depending on the specific tectonic activity and the type of fault. The depth of the focus can range from a few kilometers to several hundred kilometers below the Earth's surface.

Types of Foci
1. Shallow Focus: Shallow focus earthquakes occur at depths of less than 70 kilometers below the surface. These earthquakes are often associated with divergent plate boundaries, where two tectonic plates are moving away from each other, or transform plate boundaries, where two plates are sliding past each other horizontally.

2. Intermediate Focus: Intermediate focus earthquakes occur at depths ranging from 70 to 300 kilometers below the surface. These earthquakes typically occur in subduction zones, where one tectonic plate is being forced beneath another plate.

3. Deep Focus: Deep focus earthquakes occur at depths greater than 300 kilometers below the surface. These earthquakes are commonly associated with subduction zones, where one tectonic plate is subducting beneath another at a steep angle.

Relationship with the Epicenter
While the focus is the actual point of energy release, the epicenter is the point on the Earth's surface directly above the focus. When an earthquake occurs, seismic waves radiate outwards from the focus and reach the surface, causing the ground to shake. The epicenter is determined by triangulating the arrival times of these seismic waves at different seismograph stations.

In conclusion, the focus of an earthquake is the specific point within the Earth's crust where the initial rupture and energy release occur. It is underground and can vary in depth depending on the tectonic activity. The epicenter, on the other hand, is the point on the Earth's surface directly above the focus.