Subduction Zone Evidence and Tsunami Hazard
Along a 900-km east-west transect in the western Pacific, a research vessel records the following. The trench is at 0 km. Earthquake foci lie at 20 km depth beneath 50 km east of the trench, 90 km beneath 180 km, 180 km beneath 300 km, and 240 km beneath 420 km. No earthquakes deeper than 250 km are detected east of 450 km. On the incoming oceanic plate west of the trench, seafloor age increases from 5 million years at 900 km west of the trench to 40 million years at the trench, and sediment thickness increases from 0.2 km to 1.5 km over that distance. The trench floor itself has only a thin, disrupted sediment cover. Two GPS stations record horizontal motion: the station on the incoming plate moves 60 mm/yr east, and the station on the overriding plate moves 20 mm/yr east. Coastal communities report repeated large tsunamis.
1. Identify the plate boundary type and the subducting plate. Use the earthquake-depth pattern and the GPS vectors, not just the trench, to justify your answer.
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The boundary is convergent, and the incoming oceanic plate is subducting. The earthquake foci deepen eastward from 20 km to 240 km, defining a Wadati-Benioff zone that dips east beneath the overriding plate. That depth progression requires a descending slab; a transform boundary would produce shallow earthquakes at similar depths, not a systematic increase with distance from the trench. The GPS vectors show both plates moving east, but the incoming plate moves at 60 mm/yr while the overriding plate moves at 20 mm/yr. The difference is 40 mm/yr of convergence. The seaward plate is the one entering the trench, and its earthquakes trace the descending slab. The trench alone could be ambiguous, but the earthquake depths and relative motion together confirm subduction.
2. Explain why the incoming plate is older and has thicker sediment near the trench than 900 km west of it, and why the trench floor itself has only a thin sediment cover.
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The incoming plate formed at a spreading ridge west of the transect and has moved east since then. Seafloor near the trench is therefore about 40 million years old, while seafloor 900 km west is only about 5 million years old. Older seafloor has had more time to accumulate sediment, so the incoming plate is thickest near the trench. The trench floor is different because sediment is removed or deformed there. Some sediment is scraped off and added to the overriding plate, and some is carried down the subduction zone. The high convergence rate and the disrupted cover are consistent with this removal. Thus the thick sediment on the incoming plate does not simply pile up in the trench; it is partly accreted or subducted.
3. A planner says that because deep earthquakes stop at about 250 km, the subduction zone cannot produce a great earthquake or tsunami. Evaluate this claim.
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The claim is not supported. The 250-km cutoff for earthquakes does not show that the shallow part of the subduction zone is inactive. The cutoff may reflect the thermal structure of the slab, such as a young or warm slab in which brittle failure stops at a shallower depth. Great megathrust earthquakes nucleate mainly in the shallow locked zone, where elastic strain accumulates. Tsunamis are generated by sudden seafloor displacement during such shallow rupture, not by the deepest earthquakes. The 40 mm/yr convergence, the trench, and the reported tsunamis all indicate significant hazard. A proper assessment would also need geodetic data on locking, trench morphology, and paleotsunami records. Deep-earthquake depth alone is insufficient to rule out great earthquakes.