Deep-Focus Earthquakes and Subduction Zones: The Link Between Deep Quakes and Ocean Trenches

Deep-focus earthquakes (300–700 km depth) don't happen randomly — they're the **seismic signature of subducting slabs** plunging into the mantle. The prominent ocean floor feature they're associated with: **oceanic trenches**.

The Subduction Zone Factory

1. Oceanic plate (cold, dense) converges with another plate
2. Plate bends at **trench** — deepest ocean points (Mariana: 11,034 m)
3. Slab descends into mantle at 2–10 cm/yr
4. Metastable minerals (olivine → spinel → perovskite) transform under pressure
5. Transformational faulting + **dehydration embrittlement** → earthquakes at depth

Wadati-Benioff Zones

Deep quakes define **dipping seismic planes** tracing the slab:

• Shallow (0–70 km): Thrust faulting at plate interface
• Intermediate (70–300 km): Dehydration of serpentine, lawsonite
• Deep (300–700 km): Olivine → spinel transition (phase change faulting)

Key Examples

Why Not Deeper?

Below ~700 km, slab warms to mantle temperature → ductile deformation replaces brittle failure. Also, slab may flatten/stagnate at 660-km discontinuity.

Tsunami Connection

Deep quakes **rarely cause tsunamis** — too deep to displace seafloor significantly. Shallow megathrust quakes (0–50 km) at trench are the tsunami generators.

Deep-focus earthquakes = **subduction zone's deep heartbeat**. The trench is the surface expression; the quakes map the descent.