Hidden thrust · folds the surface

Blind Thrust Fault

A thrust fault that stops below the ground. The slip dies out at the buried fault tip, so instead of breaking, the rock above bends into a fold that grows a little with each earthquake. The front face shows the fault in cross-section.

Video coming soon

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

Large blue arrows: compressional stress acting on the crust

Thrust fault: the gently dipping fault seen on the front face

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)

Fault tip (buried): where the thrust stops below the ground

Road

Aqueduct (concrete channel)

Railroad

Power line

Electrical arcing where the power line snaps

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

Leeper's Awesome 2016 Tacoma

Passenger train: a diesel engine pulling three passenger cars

What is it

A thrust fault whose upper tip is buried. There is no fault line at the surface to map.

Folding above the tip

Rock ahead of the tip bends upward into a fold. Over many earthquakes the fold builds low hills, which are often the only clue at the surface.

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.

Why it matters

Blind thrusts are found by drilling, seismic surveys, or after an earthquake. Several lie under the Los Angeles basin, including the Puente Hills thrust beneath downtown.

Examples

1987 Whittier Narrows, M5.9 · 1994 Northridge, M6.7 · 1983 Coalinga, M6.4

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