Deep-focus earthquakes occur at depths of to , where high temperatures and pressures are expected to cause mantle rocks to deform plastically rather than fracture. Two scientists propose different hypotheses to explain the initiation of these deep earthquakes.
Scientist 1
Deep-focus earthquakes are caused by *dehydration embrittlement*. Subducting tectonic plates carry hydrous minerals (such as serpentine) deep into the mantle. As temperature and pressure increase with depth, these minerals undergo dehydration reactions, releasing liquid water. This pressurized fluid fills microscopic pore spaces, reducing the effective normal stress on pre-existing faults. The reduction in friction allows the rock to suddenly slip, generating an earthquake.
Scientist 2
Deep-focus earthquakes are caused by *transformational plasticity*. The mantle mineral olivine () undergoes a phase transition to a denser polymorph, wadsleyite, at a depth of approximately . Within cold subducting slabs, this transition is delayed, creating a metastable region of olivine. Under high shear stress, small zones of metastable olivine undergo a rapid, runaway transformation into wadsleyite. This phase change weakens the shear strength of the rock along narrow shear zones, causing localized instability and sudden displacement.
Based on Scientist 1's hypothesis, which of the following conditions must be met for a deep-focus earthquake to occur at a depth of ?
- AThe surrounding temperature and pressure must decrease as depth increases from to .
- Hydrous minerals must be carried by the subducting tectonic plate to that depth.Cevap
- CMetastable olivine must undergo a rapid phase transition to wadsleyite at that depth.
- DThe released liquid water must increase the friction and effective normal stress along the fault.