Restraining step-over · transpression

Right‑Lateral, Left Step‑Over

Two right-lateral fault strands are offset, and the second one picks up where the first ends, stepping to the left. Oblique-reverse 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 converge. That squeezing (transpression) pushes up a diamond-shaped pop-up along reverse faults and bulges the ground around it up in small steps. The see-through west block shows the faults below, fanning inward and merging at depth into a positive 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)

Triangles: reverse or thrust fault, on the hanging wall (indicating the uplifted block)

Small black arrows: how the blocks move along each fault

Black arrows: the blocks converge 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-reverse faults branching from each strand end and dying out

Pop-up: the ground rises between the strands

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.

Left step

Look along fault strand 1 toward strand 2: the next strand is offset to your left. 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.

Restraining step-over or bend

The step goes the opposite way from the slip (a left step on a right-lateral fault), so the sides push into each other here. A single fault that curves this way instead of stepping is a restraining bend, and it does the same thing.

Transpression

Sideways slip plus squeezing. The ground between the strands is pushed up along reverse faults, highest in the middle, building a pop-up.

In cross-section

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

Real examples

The Santa Cruz Mountains, where the 1989 Loma Prieta M6.9 earthquake struck, are rising at a restraining bend in the right-lateral San Andreas Fault.

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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