Focal Mechanisms Explained: What are those “beach balls”? (Educational)

Summary

This video script explains the mechanisms behind seismic waves and the patterns of P-wave arrivals caused by different types of earthquakes. It covers strike-slip, normal, and thrust faults, with detailed examples such as the San Andreas Fault and the Kane Fracture Zone. The script illustrates how seismic waves radiate in different directions, showing compressional and dilatational movements on focal spheres. Using diagrams and real-world examples, it explores how first-arriving P waves can help determine fault types, aiding in the interpretation of earthquake data and improving our understanding of seismic events.

Takeaways

• 😀 Seismologists use focal mechanisms, known as 'beach balls,' to visualize the faulting motions that cause earthquakes.
• 😀 The direction of the first-arriving P-wave helps determine the faulting mechanism for an earthquake.
• 😀 Strike-slip earthquakes involve seismic waves radiating outward, compressing in some quadrants and stretching in others.
• 😀 The quadrants of compressional arrivals are shaded, while dilatational quadrants are left unshaded in the focal mechanism pattern.
• 😀 Right-lateral and left-lateral strike-slip faulting can produce similar P-wave arrival patterns, requiring geological knowledge to identify the correct fault plane.
• 😀 For example, a focal mechanism from the San Andreas Fault would likely show a north-south fault plane.
• 😀 A right-lateral strike-slip earthquake on the Kane Fracture Zone creates compression in the Northeast and Southwest quadrants and dilatation in the Northwest and Southeast.
• 😀 The P-wave first arrival pattern is observed in distant stations using the concept of a focal sphere, which represents seismic ray paths in three dimensions.
• 😀 Cross-sections of seismic waves help identify compressions and dilatations at different locations, such as Lima, Madrid, Seattle, and Cape Town.
• 😀 Normal fault earthquakes, like those in the Basin and Range Province, show compressions and dilatations in a pattern similar to an 'orange-slice' due to extensional forces.
• 😀 Thrust fault earthquakes, like those beneath the Sierra Pie de Palo in Argentina, have a similar focal mechanism to normal faults, but with compressions in the center and dilatations on the outside due to compressional forces.

Q & A

What is the purpose of a focal mechanism in seismology?

A focal mechanism is used by seismologists to illustrate the faulting motions that cause an earthquake. It is often represented informally as a 'beach ball' graphic that shows the direction of compressional and dilatational movements from the earthquake's epicenter.

What are P-waves, and why are they important for determining earthquake faulting mechanisms?

P-waves (Primary waves) are the fastest seismic waves and are the first to arrive at seismograph stations. They provide critical information about the direction of faulting because their first arrivals indicate whether the rock has been compressed or stretched, which helps in identifying the type of faulting mechanism.

How does a strike-slip fault affect P-wave arrivals?

In a strike-slip fault, P-waves cause compression in certain quadrants (e.g., northwest and southeast) and dilatation in others (e.g., northeast and southwest). The compression causes the first arriving P-waves to push away from the epicenter, while the dilatation causes them to pull toward the epicenter.

How can a seismologist distinguish between different types of strike-slip faults?

By examining the focal mechanism and the direction of P-wave arrivals, seismologists can distinguish between right-lateral and left-lateral strike-slip faults. The pattern of compression and dilatation, combined with geological knowledge of the region, helps determine whether the fault is oriented in a north-south or east-west direction.

What is a focal sphere and why is it important in earthquake analysis?

A focal sphere is an imaginary sphere surrounding the earthquake hypocenter. It helps visualize the three-dimensional geometry of seismic wave propagation. P-waves radiate from the hypocenter through the lower hemisphere of this sphere, and by examining their travel to distant stations, seismologists can determine the pattern of compressions and dilatations.

What are the differences in P-wave behavior when studying a right-lateral strike-slip earthquake on the Kane Fracture Zone?

In a right-lateral strike-slip earthquake, compressional P-waves radiate to the northeast and southwest, while dilatational P-waves radiate to the northwest and southeast. These patterns can be observed at distant stations like Lima and Madrid, helping to characterize the fault's behavior and orientation.

What is the pattern of P-wave arrivals for a normal fault, such as the one at Steen's Mountain?

In a normal fault, compressional P-waves radiate at shallow to intermediate downward angles toward the east and west, while dilatational P-waves radiate at steeper downward angles. The resulting focal mechanism shows an 'orange-slice' pattern, with compressions on the outside and dilatations in the center.

How do thrust faults affect the P-wave arrival patterns?

In a thrust fault, compressional P-waves radiate at steep downward angles, while dilatational P-waves radiate at shallow to intermediate downward angles. The focal mechanism appears similar to a normal fault but with the roles of compression and dilatation reversed: compressions are in the center, and dilatations are on the outside, forming a 'cat's eye' pattern.

What is the significance of the 'orange-slice' pattern in focal mechanisms?

The 'orange-slice' pattern is characteristic of both normal and thrust faults. It represents the division of the focal sphere into quadrants of compressions and dilatations. For normal faults, compressions are on the outer quadrants, while for thrust faults, compressions are in the center.

How can a seismologist determine which nodal plane represents the fault plane in an earthquake?

By analyzing the focal mechanism and using geological knowledge of the region, a seismologist can determine which nodal plane is the fault plane. The orientation of the fault is inferred based on patterns of P-wave arrivals and the known geological features at the earthquake’s location.