Pulling apart · tension

Normal Fault

When the brittle crust is extended and thinned due to tensional stress, it breaks along a normal fault. The block above the fault (the hanging wall) drops down relative to the block below (the footwall).

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Legend

HW: hanging wall, the block above the fault

FW: footwall, the block below the fault

Epicenter: the point on the ground directly above the focus

Focus (hypocenter): location on the fault where the rupture initiates

Small black arrows: how the blocks move relative to each other

Hatch marks: normal fault, on the hanging wall (indicating the block that drops)

Large blue arrows: tensional stress acting on the crust

Fault plane (pink)

Rupture surface: the part of the fault that slipped

Rupture front, spreading out from the focus

Fault slip magnitude arrows: blue (most slip) to yellow (least)

Road

Aqueduct (concrete channel)

Railroad

Power line

Electrical arcing where the power line snaps

Flooding from the broken aqueduct

House: a wood-frame home, flexible enough to sway and ride out the shaking

Brick building: three stories of unreinforced brick and masonry, brittle, so it collapses into rubble during strong shaking

Industrial complex: built right across the fault, so the rupture tears it in half and the bays beside the break collapse

Leeper's Awesome 2016 Tacoma

Passenger train: a diesel engine pulling three passenger cars

What moves

The hanging wall drops down along the dip of the fault.

What causes it

Tensional stress. The crust is being pulled apart and gets longer and thins.

Seismic waves

As the rupture spreads, every part of the fault that slips generates seismic energy that radiates out from the fault surface as seismic waves.

What it builds

Valleys and tilted mountain blocks, like the Basin and Range of Nevada and Utah.

Examples

1959 Hebgen Lake, Montana, M7.3 · 1983 Borah Peak, Idaho, M6.9

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