What does the beach ball diagram of an earthquake describe?

A focal mechanism, or “beachball”, is a graphic symbol that indicates the type of slip that occurs during an earthquake: strike-slip, normal, thrust (reverse), or some combination. It also shows the orientation of the fault that slipped. Let’s look at the simplest example, a strike-slip fault.

How do you find the focal mechanism?

Focal mechanisms are derived from a solution of the moment tensor for the earthquake, which itself is estimated by an analysis of observed seismic waveforms. The focal mechanism can be derived from observing the pattern of “first motions”, that is, whether the first arriving P waves break up or down.

What is the beach ball theory?

“Beach ball” is an informal term. In technical contexts, call them focal mechanisms. The waves from an earthquake radiate outwards, with a pattern of 4 quadrants. In two quadrants the first motion will be compressional (a push), and in the other two extensional (a pull).

How does a beach ball work?

The ball is locked in position by the air flow surrounding it; the fast moving air is at a lower pressure than the still air; should the ball move from the stream, the higher pressure of the still air forces it back in.

What are the three types of fault lines?

There are three main types of fault which can cause earthquakes: normal, reverse (thrust) and strike-slip. Figure 1 shows the types of faults that can cause earthquakes. Figures 2 and 3 show the location of large earthquakes over the past few decades.

What is dip-slip?

Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse. A thrust fault is a reverse fault with a dip of 45 degrees or less.

Why do some geoscientists refer to focal mechanisms as beach balls?

When an earthquake occurs, seismologists create graphics of focal mechanisms, informally referred to as beach balls,to show the faulting motions that produce the earthquake. Simply put, the focal mechanisms are based on the direction of the first arriving P wave.

What do the beachballs mean in an earthquake?

A focal mechanism, or “beachball”, is a graphic symbol that indicates the type of slip that occurs during an earthquake: strike-slip, normal, thrust (reverse), or some combination. It also shows the orientation of the fault that slipped.

What kind of diagram is a beach ball?

These diagrams are called focal mechanism diagrams, or sometimes “beach-ball” diagrams. These circular diagrams shown in the following figure are able to illustrate the strike of a plane with respect to North and its dip, where the straighter the line the more vertical the dip.

How is the focal mechanism of an earthquake obtained?

Details. Earthquake focal mechanisms plots display the geometry of an earthquake fault, the direction of slip, and the deformation within the source region in a single diagram. This diagram, also called a beach ball plot by seismologists, is obtained by projecting the intersection of the fault plane and a perpendicular auxiliary plane…

What does the beach ball tell you about a strike slip fault?

For a strike slip fault, the beach ball will indicate whether the fault is right or left lateral. The motion will be toward the black back (compressional) quadrant. This will be a left lateral fault (stand at the fault plane facing the other side, and you would have to go to the left to find the offset features).