Which of the following statements about megathrust faults is true?

Question

BlackTom AI · Accepted Answer

Let's examine each statement about megathrust faults in turn, weighing what is commonly observed in subduction-zone megathrust events.
Option 1: 'The magnitude of megathrust fault earthquakes typically reach 8.0 Mw.' This is partly misleading. While many megathrust earthquakes exceed 8.0 Mw, the typical maximums often surpass 9.0 Mw, with several iconic events in the 9+ range. Saying they typically reach exactly 8.0 Mw underrepresents the potential size of these earthquakes and can mischaracterize the upper end of the magnitude spectrum.
Option 2: 'When the plates are locked, the coast subsides. When an earthquake occurs, the coasts uplift.' The behavior of the coastline during locking versus rupture is nuanced and location-dependent. Megathrust cycles generally involve elastic strain accumulation during interseismic locking and rupture that can cause vertical displacement; in many regions the overriding plate experiences uplift during rupture, contributing to coastal uplift and occasional tsunami generation, but the wording here asserts subsidence during locking and uplift only during earthquakes, which is an oversimplification and not universally true across subduction zones. This makes the statement unreliable as a general rule.
Option 3: 'Transoceanic tsunamis waves are common following megathrust earthquakes' Indeed, megathrust events often displace large volumes of seawater across the ocean, producing tsunamis that can propagate across oceans. This is a well-documented consequence in many historical megathrust earthquakes, making this statement broadly accurate.
Option 4: 'The depth at which megathrust fault earthquakes occur means that tsunamis rarely form as a result.' Megathrust earthquakes occur at relatively shallow depths along subduction interfaces, which actually facilitates substantial vertical seafloor displacement and commonly leads to tsunamis. The claim that tsunamis rarely form is incorrect.
Option 5: 'Large-scale volcanic eruptions always occur prior to megathrust fault earthquakes.' This is not supported by geologic evidence. Megathrust earthquakes are tectonic in origin and are not preceded by predictable, obligatory large volcanic eruptions. The statement is false as a general rule and mixes two distinct geologic processes.
Option 6: 'Part of the subducting oceanic plate is locked with the continental plate, building up elastic strain before the next earthquake.' This captures a core aspect of megathrust cycles: interseismic locking between the plates stores elastic strain in the overriding plate and the slab, which is released during rupture. This description aligns with the standard model of megathrust behavior and earthquake preparation.

ESS 101 A Examination 2 (Friday December 5th 7:00 - 8:30 PM PST)

Which of the following statements about megathrust faults is true?

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