Soru

### Deep-Focus Earthquakes

Deep-focus earthquakes occur at depths between $300\text{ km}$ and $700\text{ km}$ below Earth's surface. At these depths, high pressures and temperatures should theoretically cause mantle rocks to deform ductilely (flowing like a highly viscous liquid) rather than fracturing brittlely. Three geophysicists propose different mechanisms to explain how deep-focus earthquakes can occur.

Geophysicist 1
Deep-focus earthquakes are triggered by *dehydration embrittlement*. Subducting tectonic slabs carry hydrous minerals, such as serpentine, deep into the mantle. Past a depth of $300\text{ km}$, the increasing temperature and pressure cause these minerals to destabilize and dehydrate, releasing high-pressure water. This fluid pressure offsets the confining pressure of the mantle, reducing the friction along faults and allowing sudden brittle failure. For this process to occur, hydrous minerals must survive to these depths, which requires the slab core to remain relatively cool, below $600^\circ\text{C}$. If the slab core temperature exceeds $600^\circ\text{C}$, dehydration occurs prematurely at shallower depths, and deep-focus earthquakes will not occur.

Geophysicist 2
Deep-focus earthquakes are caused by *transformational faulting* (anticrack shear) of metastable olivine. As a cold slab subducts rapidly, the olivine in its core remains metastable past its normal stability depth. At depths of $350\text{ km}$ to $700\text{ km}$, this metastable olivine undergoes a rapid phase transition to the high-pressure polymorphs wadsleyite or ringwoodite. Under shear stress, this transition initiates along thin, localized bands. The volume decrease associated with the phase change causes structural collapse and localized shear heating, leading to a runaway slip event. This mechanism requires the presence of metastable olivine, which can only survive in subducting slab cores that remain below $700^\circ\text{C}$. At temperatures of $700^\circ\text{C}$ or higher, olivine transforms to its high-pressure phases under equilibrium conditions without generating sudden shear instability.

Geophysicist 3
Deep-focus earthquakes are caused by a *thermal runaway* instability. When subducting slabs deform under high shear stress, the mechanical energy is converted into heat. Because silicate rocks are poor conductors of heat, this thermal energy cannot dissipate quickly, raising the local temperature of the shear zone. Since rock viscosity decreases exponentially with temperature, this heating softens the rock, localizing the deformation further and generating even more heat. This positive feedback loop leads to thermal runaway and sudden shear slip. This mechanism can only initiate if the slab's core is cold enough (below $800^\circ\text{C}$) to support the high initial shear stresses necessary to trigger thermal runaway. In slabs with core temperatures at or above $800^\circ\text{C}$, the rock is too ductile to support high shear stresses, preventing the initiation of thermal runaway.

Based on the descriptions provided, all three geophysicists would agree that deep-focus earthquakes are unlikely to occur within a subducting slab if the slab's core temperature is: