A First Motion Study looks at whether the very first movement of a P-wave on a seismogram is an “up” (compression) or “down” (dilatation). By mapping this across many stations, scientists can determine if the fault was a normal, reverse or strike-slip fault without ever seeing the fault itself. ANSWER: (B) Analyzing the direction of the initial P-wave to determine the fault type.
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This is also known as a “Focal Mechanism” solution. When a fault breaks, it pushes the Earth in some directions and pulls it in others. Stations “pushed” by the fault show an initial upward pulse, while those “pulled” show a downward pulse. By plotting these on a “beachball diagram,” seismologists can reconstruct the geometry of the earthquake. This is how we know the orientation of faults deep underground or in the middle of the ocean where they cannot be visually inspected.