1. Where a plate descends versus where magma forms

A subduction zone is a convergent boundary where one plate descends beneath another into the mantle. USGS distinguishes earthquakes on the plate interface from those within the descending plate and upper-plate crust. Not every earthquake in a subduction zone is the same kind of interface rupture.

The mantle beneath plates is not an ocean of liquid magma. USGS explains that mantle rock behaves as a solid over short timescales but flows slowly over geological timescales. Long-term flow involved in plate motion is compatible with solid behavior during seismic-wave passage.

2. How locking relates to earthquakes, and water to volcanoes

If friction locks part of the interface while relative plate motion continues, surrounding rock deforms and stress accumulates. Rapid fault slip can release stored energy as seismic waves. Large interfaces can produce large earthquakes, but observing locking does not determine the next earthquake's time and magnitude.

Descending rocks and sediments carry water to depth. Water lowers the temperature at which mantle rock starts melting, contributing to partial melting. Magma produced from that partial melt can rise and build a volcanic arc. Avoid pictures in which the entire mantle melts or the descending plate simply melts directly in every case. No single depth defines magma production in every subduction zone.

Seafloor displacement can generate a tsunami when it moves substantial water. USGS identifies vertical fault-driven seafloor motion and submarine landslides as causes. Earthquake occurrence, seafloor displacement and tsunami generation are not automatically interchangeable events.

3. What deformation observations help infer

USGS's 2021 Cascadia seafloor-geodesy plan explains repeated positioning of seafloor benchmarks to study plate motion and shallow strain accumulation. This source describes a measurement plan and method, not a report that every subsequent result was established.

Benchmark position changes are data; a distribution of locking on a buried fault is an inference using fault geometry and deformation models. Illustratively, even similarly directed movement at several sites would not reveal an exact rupture date from those arrows alone. Check the measurement period, reference frame, model assumptions and uncertainties in a paper or map.

4. Distinguish hazard assessment from predicting a date

USGS's prediction explanation separates specifying time, location and magnitude from estimating occurrence probability for a region and period. Deformation evidence useful for long-term hazard assessment does not verify a large earthquake on a particular date.

Check whether a subduction map shows plate location, measured displacement or inferred locking. For actual earthquake and tsunami response, use local official alerts and evacuation guidance rather than interpreting a research locking map as an immediate warning.

5. Sources

USGS specialist institutional explanations: subduction earthquakes, volcanism and tsunamis; solid mantle behavior; the Cascadia geodesy plan; and prediction versus probability.