The radiation patterns of P-waves are used to construct a graphical representation of earthquake faulting geometry.
The symbols are called “ Focal Mechanisms ” or “ Beach Balls ”, and they contain information on the fault orientation and the direction of slip.
Introduction to Seismology-KFUPM
Review: Earthquake Fault Mechanism The type of faulting may be identified (reverse, normal, strike-slip) but the particular fault plane cannot be determined. So they both are shown as the possible solutions. The three stresses: two horizontal plus the vertical. Why is the significance to understand the style of faulting? Because, they are also telling us about the stresses acting within Earth. Introduction to Seismology-KFUPM
Let’s come back to discuss earthquake fault mechanism b d Introduction to Seismology-KFUPM
Cross section of reverse fault, earthquake focus and quadrants of compression and dilatation. First motions observed at the surface reveal patterns of compression and dilatation. Introduction to Seismology-KFUPM
Focal mechanism solution for a fault is commonly a lower focal sphere projection . For a dip-slip fault, this projection is equivalent to the compression/dilatation pattern viewed by a bird flying over the earthquake focus. Black = Compression White = Dilatation Introduction to Seismology-KFUPM
Cross sections of Dip-Slip Faults C= Compression D= Dilatation The focal mechanism solution for a reverse fault (a) has a compression (black) in the inside portion of the circle, surrounded by regions of dilatation (white) The opposite pattern is observed for a normal fault (b) Introduction to Seismology-KFUPM
Introduction to Seismology-KFUPM A=? B1=? B2=? C=?