Focal mechanism

An earthquake shakes the ground.

It moves the rocks in different ways. Scientists look at how the ground moves. They use tools to see the waves.

These waves can push up or pull down. Scientists draw a special map to show this. The map looks like a colorful beach ball. This helps them see the fault. A fault is where the rocks slip. The map shows how the earth moved. It is a way to solve a puzzle.

When an earthquake happens, the ground shakes. Scientists want to know how the rocks moved. They study a focal mechanism. This is a way to describe how the earth broke.

Scientists look at seismic waves. These are waves of energy from the shake. They watch the first motion of these waves. This means they see if the wave pushes up or pulls down.

They use this data to make a map. This map is called a beachball diagram. It looks like a colorful beach ball. The colored parts show where the ground pushed together. The white parts show where it pulled apart.

This map helps find the fault plane. A fault plane is the surface where the rocks slipped. The map also shows the slip vector. This is the direction the rocks moved. This work helps us understand how the earth works deep down. It even helps us tell the difference between earthquakes and explosions.

An earthquake is a sudden movement of rocks deep underground. Scientists want to know exactly how those rocks moved. They use something called a focal mechanism to describe this. This tool shows the shape of the deformation in the source region. It tells us about the orientation of the fault plane that slipped. It also shows the slip vector, which is the direction of the movement.

To find this, scientists study seismic waves. These waves travel through the Earth after a shake. One way is to look at the first motions of P waves. They check if the first wave breaks up or down. This means they see if the ground pushed or pulled. They can also use a moment tensor solution. This is a math way to look at the waveforms.

Scientists use a special graph to show their results. It is called a beachball diagram. This diagram is a lower-hemisphere stereographic projection. They plot data from stations using angles. The take-off angle is the angle from the vertical. It shows how the seismic ray emerges from the earthquake focus. Filled symbols show where the motion was up. Hollow symbols show where the motion was down.

These diagrams help us learn about our planet. For example, they helped prove the idea of seafloor spreading. Scientists looked at oceanic transform faults to see this. They found the motion was opposite to what older ideas suggested. They also used them to study deep earthquake zones. They found some zones are under compression. Other zones are under tension.

This work is very important for safety and science. It helps people tell the difference between earthquakes and explosions. This is used to monitor the Comprehensive Test Ban Treaty. An underground nuclear explosion has a different pattern. It is called an isotropic pattern. This makes it easy to distinguish from a natural earthquake. Scientists can even use software like BBC to make these diagrams.

A focal mechanism describes the specific deformation that occurs in the source region of an earthquake. This mechanism is essential because it explains how the Earth's crust moved to generate seismic waves. In cases involving a fault, the focal mechanism defines the orientation of the slipped fault plane. It also identifies the slip vector, which is the specific direction of the movement. Scientists often refer to this process as a fault-plane solution.

To understand how these mechanisms are created, we must look at how seismic data is processed. One traditional method involves observing the pattern of "first motions" in P waves. Scientists check whether the first arriving waves show an upward or downward motion. This method was used before digital recording became common. It is still useful today for very small earthquakes. Modern researchers primarily use semi-automatic analysis of recorded waveforms to find a moment tensor solution. The moment tensor is a mathematical description of the earthquake's energy radiation.

Results are displayed using a graphical tool called a beachball diagram. This diagram is a lower-hemisphere stereographic projection of the data. Scientists plot individual seismic records using two specific angles: the azimuth and the take-off angle. The take-off angle is the angle from the vertical as a seismic ray emerges from the focus. To build the diagram, researchers use standard tables to calculate these angles based on distance. Filled symbols represent stations where the P wave first motion was compressive, or upward. Hollow symbols represent tensional, or downward, motion.

If there are enough observations, scientists can draw two orthogonal great circles. These circles are known as nodal planes. They divide the diagram into compressive and tensional quadrants. By convention, the compressive quadrants are color-filled, while the tensional quadrants remain white. The two nodal planes intersect at a point called the N-axis, or neutral axis. Scientists also plot the P and T axes. The P-axis is placed in the center of the white segment. The T-axis is placed in the center of the color-filled segment.

Determining which nodal plane is the actual fault plane can be difficult. One plane is the fault plane, while the other is the auxiliary plane. A focal mechanism alone cannot tell them apart. Scientists must use other geological or geophysical evidence to resolve this ambiguity. For example, in the 2004 Indian Ocean earthquake, the solution showed two nodal planes. One plane dipped northeast at 6 degrees, and the other dipped southwest at 84 degrees. Because of plate tectonic models, scientists knew the shallow northeast plane was the correct fault.

Focal mechanisms are vital for studying areas where we cannot see the surface. They are useful for defining faulting in deep volumes or under the ocean. These solutions helped prove the hypothesis of seafloor spreading. By studying oceanic transform faults, scientists found the motion was opposite to classical interpretations. The beachball plots showed strike-slip motion that matched seafloor spreading models. Additionally, these tools helped discover that deep earthquake zones in subducting slabs experience different forces. Some zones are under compression, while others are under tension.

Beyond geology, focal mechanisms serve a critical role in global security. They allow scientists to distinguish between natural earthquakes and underground nuclear explosions. An explosion has an isotropic moment tensor, which creates a different energy pattern. This ability to discriminate between events is essential for monitoring the Comprehensive Test Ban Treaty. To assist in this work, software programs like BBC are available. This MATLAB-based toolbox helps researchers prepare professional beachball diagrams automatically.