Releasing step-over · transtension

Right‑Lateral, Right Step‑Over

Two right-lateral fault strands are offset, and the second one picks up where the first ends, stepping to the right. Oblique-normal faults branch from the end of each strand and die out where they intersect the strand on the opposite side. Across the step-over between them, the blocks on either side pull away from each other. That stretching (transtension) drops a diamond-shaped pull-apart basin along normal faults and bends the ground around it down in small steps. The see-through west block shows the faults below, fanning inward and merging at depth into a negative flower structure.

Video coming soon

Legend

Epicenter: the point on the ground directly above the focus

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

Fault surfaces (pink)

Rupture surface: the part of the faults that has slipped

Rupture front, spreading out from the focus

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

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

Small black arrows: how the blocks move along each fault

Black arrows: the blocks pull apart across the step-over

Large blue arrows: shear stress acting on the crust

Bullseye: this block moves toward you

Asterisk: this block moves away from you

Points north; the faults strike north-northwest

Fault strands 1 and 2: the two offset strike-slip faults

Oblique-normal faults branching from each strand end and dying out

Pull-apart basin: the ground drops between the strands, with a sag pond in its low point

Blue marker lines: straight before slip, offset at each fault

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 is a step-over?

A gap where one fault strand ends and a parallel strand nearby picks up the slip. The ground between the two strands has to stretch or squeeze to let the sides keep moving.

Right step

Look along fault strand 1 toward strand 2: the next strand is offset to your right. The answer is the same from either end.

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.

Releasing step-over or bend

The step goes the same way as the slip (a right step on a right-lateral fault), so the sides pull away from each other here. A single fault that curves the same way instead of stepping is a releasing bend, and it does the same thing.

Transtension

Sideways slip plus stretching. The ground between the strands sinks along normal faults, deepest in the middle, often leaving a sag pond.

In cross-section

The normal faults dip inward and merge with the main fault at depth. Geologists call this fan of faults a negative flower structure.

Real examples

Death Valley and the Salton Trough are pull-apart basins between right-lateral faults in California.

Toward or away from you

On the front end of each side block, a bullseye marks the block moving toward you, like the tip of an arrow pointing at you. An asterisk marks the block moving away from you, like the tail of an arrow flying away.

Why it matters in earthquakes

Step-overs wider than a few kilometers often stop a rupture, which limits how big the earthquake gets. Large earthquakes can sometimes jump across.

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