Question

### Deep-Focus Earthquakes

Most earthquakes occur at shallow depths (less than $70\text{ km}$) where rocks are cold and brittle. However, deep-focus earthquakes occur at depths of $300\text{ to }700\text{ km}$, where high temperatures and pressures should cause rocks to deform plastically rather than fracture. Two scientists propose different mechanisms for how these deep-focus earthquakes occur.

*Scientist 1*
Deep-focus earthquakes are caused by dehydration embrittlement. As a subducting oceanic slab sinks, hydrous minerals (such as serpentine) within the slab break down under increasing temperature and pressure, releasing liquid water. This high-pressure fluid fills pre-existing micro-fractures in the surrounding rock, reducing the effective normal stress and allowing frictional sliding along fault planes that would otherwise be locked by extreme pressure.

*Scientist 2*
Deep-focus earthquakes are caused by transformational faulting associated with a metastable phase change. At depths of $300\text{ to }700\text{ km}$, the mineral olivine in the subducting slab is thermodynamically unstable and should transform into the denser polymorph ringwoodite. However, the cold interior of the slab keeps olivine in a metastable state. As stress increases, micro-shear zones form where olivine rapidly transforms to ringwoodite. This sudden volume change creates localized shear instability, triggering the earthquake.

According to Scientist 2, deep-focus earthquakes are initiated primarily by which of the following processes?