Earth, Molnar 2015 4x

Mid-ocean ridge orientation

Mostly trend north–south globally; continuous volcanic systems where new oceanic crust is formed

Subduction zone morphology

Arcuate island arcs + deep ocean trenches + outer topographic rise + inclined earthquake zone (Wadati–Benioff zone)

Earthquake distribution at subduction zones

Shallow quakes at trench, intermediate + deep quakes (to ~700 km) in dipping Benioff zone

Trench position

Trench always lies seaward because continental crust is buoyant and does not subduct

Lithosphere recycling balance

Lithosphere created at ridges = lithosphere destroyed at subduction zones (mass balance)

Thermal control on subduction asymmetry

Hot ridge lithosphere is weak; cold oceanic lithosphere is strong and resists bending

Trench formation mechanism

Downward bending of cold dense slab due to gravity forms deep-sea trench before subduction

Outer topographic rise

Lithosphere seaward of trench flexes upward due to bending of rigid plate (elastic flexure)

Age effect on flexure

Older lithosphere = thicker, produces wider but lower outer bulge due to rigidity differences

Earthquake generation at trenches

Bending causes extensional normal faulting seaward + compressional deeper quakes landward

Underthrusting evidence

Shallow focus focal mechanisms show slip on gently dipping plane = oceanic plate subducting beneath continent

Underthrusting process

Oceanic plate slips beneath overriding plate → deformation causes uplift/sinking of surface regions

Seismology use in structure analysis

Seismic wave speed + attenuation used to infer internal layering and detect subduction geometry

Q factor definition

Measure of seismic attenuation: high Q = low energy loss; low Q = high energy loss

Lithosphere vs asthenosphere waves

Lithosphere transmits high-Q waves efficiently; asthenosphere attenuates waves due to partial melt/weakness