Social Sciences and History / Unit 10 SSH 0923 worked example, 60 to 90 seconds
Faults, Focus, Epicenter: Naming the Fault From the Motion
The idea
The exam describes rock motion and asks the fault type, or describes an earthquake and asks where it began, and Dastrup's terms answer both. Stress is force per area; strain is the response: elastic, reversible; ductile, permanent bending; brittle, breaking. Faults are where brittle deformation lets two blocks move. Normal and reverse faults show vertical, dip-slip motion between a hanging wall above the fault plane and a footwall below, the miner's lantern and the miner's feet: in a normal fault the hanging wall drops; in a reverse fault it rises. A strike-slip fault moves horizontally. The focus, or hypocenter, is the point of first rupture, always at depth; the epicenter is the surface point directly above it, the location news reports give. Seismic waves spread from the focus as body and surface waves.
In practice
Given "the block above the fault slid down", normal. Given "the block above was pushed up over the block below", reverse. Given "the fence line is offset sideways across the fault", strike-slip. Given "the rupture began eight kilometers down", that depth is the focus, and the map point above it the epicenter.
Where people slip
The trap is the epicenter as the origin; the origin is the focus, below. The second trap is hanging wall: it is the block above the plane whatever the motion.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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