Questions: Subduction Zone Seismic Architecture and Slab Imaging

Question 1

Seismic tomography images of subducting slabs show them as high-velocity anomalies in the mantle. What physical property primarily causes this elevated seismic velocity?

• A. The slab is composed of denser minerals than the surrounding mantle, and denser minerals always transmit seismic waves faster
• B. The slab contains abundant water bound in hydrous minerals, which dramatically stiffens the rock and increases wave speed
• C. The slab is significantly colder than the surrounding mantle, and seismic velocity increases as temperature decreases
• D. The slab has been metamorphosed to eclogite, which has an intrinsically higher elastic modulus than peridotite

Answer: (C)

Temperature is the dominant control on seismic velocity in the mantle. Cold rock is stiffer (higher elastic moduli), so seismic waves travel faster through it. The subducting slab is old, cold oceanic lithosphere that descends faster than it can equilibrate thermally with the surrounding hotter mantle, so it remains a cold, high-velocity body for millions of years. While mineralogy (e.g., eclogite transformation) also affects velocity, the primary signature in tomographic images is thermal — the slab appears fast because it is cold, not because of a specific mineral assemblage.

Question 2

The double seismic zone (DSZ) in subducting slabs consists of two planes of earthquakes separated by 20–40 km. What causes the UPPER plane of seismicity?

• A. Unbending stresses as the slab straightens from its curved geometry at the trench
• B. Dehydration of hydrous minerals releasing water that locally weakens rock and triggers brittle failure in the former oceanic crust and uppermost mantle
• C. Frictional slip along the interface between the subducting and overriding plates
• D. Thermal contraction of the cold slab as it heats up in the surrounding mantle

Answer: (B)

The upper DSZ plane is attributed to dehydration reactions. As the slab descends, increasing pressure and temperature destabilize hydrous minerals (serpentine, chlorite, amphibole) that formed during seafloor hydrothermal alteration. When these minerals break down, they release water — and locally elevated pore fluid pressure reduces effective normal stress on pre-existing fractures, triggering brittle failure even at depths where ambient conditions would normally prevent it. This is distinct from the lower DSZ plane, which is attributed to unbending stresses or dehydration of serpentinized lithospheric mantle deeper in the slab.

Question 3

Slab stagnation at the 660-km discontinuity occurs because an endothermic phase transition at that depth provides resistance to the slab's negative buoyancy, sometimes causing slabs to spread laterally rather than sinking directly into the lower mantle.

• T. True
• F. False

Answer: True

The 660-km discontinuity corresponds to a phase transition from ringwoodite (spinel structure) to bridgmanite plus ferropericlase. This transition is endothermic — it absorbs heat — which means that in the cold slab, the transition is delayed to greater depth. The boundary in the cold slab is depressed, creating a buoyancy effect that resists slab penetration. In some subduction zones (notably Izu-Bonin and parts of the western Pacific), the slab flattens and spreads laterally along the 660-km boundary rather than penetrating directly into the lower mantle, creating the stagnated slab geometry imaged by tomography.

Question 4

The two planes of seismicity in the double seismic zone of subducting slabs are caused by the same mechanism — both result from dehydration of hydrous minerals releasing fluids that weaken rock.

• T. True
• F. False

Answer: False

The two planes have different causes, which is part of what makes the DSZ scientifically valuable. The upper plane is primarily attributed to dehydration of hydrous minerals in the former oceanic crust and uppermost slab mantle. The lower plane is thought to arise from unbending stresses — as the slab transitions from the curved geometry at the trench to a straighter descent, bending stresses imposed on the lower half of the slab drive the lower seismicity. Alternatively, serpentinized mantle beneath the oceanic Moho may dehydrate at greater depth, contributing to the lower plane. The between-plane gap is relatively aseismic, reflecting the neutral stress zone of the bending model.

Question 5

What does the presence of a mantle wedge with low seismic velocity and high seismic attenuation above a subducting slab tell us about the physical processes occurring there?

Model answer: Low seismic velocity and high attenuation in the mantle wedge indicate elevated temperatures, partial melting, and the presence of fluids. The subducting slab continuously releases water from dehydrating hydrous minerals; this water migrates upward into the overlying mantle wedge peridotite, lowering its melting point (flux melting). The combination of hot mantle wedge temperatures and fluid influx produces partial melts, which reduce seismic velocity (melt is less rigid than solid rock) and increase attenuation (seismic energy is absorbed by the melt and fluid-filled pores). These partial melts rise through the wedge and erupt at the surface as the volcanic arc that characteristically sits above subduction zones.

The mantle wedge is the engine of arc volcanism: the seismic signature — low velocity, high attenuation — is a direct proxy for the conditions (partial melt + fluids) that generate the magmas erupting at arc volcanoes. This connects the deep subduction zone structure to the surface expression of subduction, illustrating how seismology reveals not just rock velocities but also the fluid and melt distribution that drives geological processes.