Classifying a Plate Boundary from Earthquake Depth Data
A class examines a U.S. Geological Survey earthquake catalog for a 500-km-long region. The extract lists 18 earthquakes within 50 km of the coast at depths of 5–25 km, 11 earthquakes 150–250 km inland at depths of 80–180 km, and 3 earthquakes 300–350 km inland at depths of 400–620 km. Magnitudes range from 4.2 to 6.8. Focal mechanisms show reverse faulting near the coast and normal faulting in the deepest zone. The map in the catalog shows no active volcanoes in the region, and the nearest plate boundary is offshore. Students are asked to classify the boundary as transform, continental rift, or subduction zone and to justify the choice with at least two independent lines of evidence. One group argues that the deep earthquakes rule out a transform boundary but says the reverse faults could also occur in a rift. Another group points to the lack of volcanoes and claims subduction is unlikely. The teacher has 20 minutes for a whole-class consensus and wants students to distinguish evidence that is diagnostic from evidence that is merely consistent.
1. Which boundary type is best supported by the earthquake data? Justify the choice with at least two independent lines of evidence and address the lack of volcanoes.
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The best classification is a subduction zone. The strongest diagnostic evidence is the systematic increase in earthquake depth inland: 5–25 km near the coast, 80–180 km at 150–250 km inland, and 400–620 km at 300–350 km inland. That pattern defines a Wadati–Benioff zone, which forms where an oceanic plate descends beneath another plate. Deep earthquakes rule out a transform boundary, which is typically confined to the shallow crust, and a continental rift, which does not produce earthquakes at 600 km depth. Reverse faulting near the coast is consistent with compression at a convergent margin. The lack of volcanoes is not decisive: the extract is limited, and volcanic arcs can be absent or not shown. Students should distinguish the depth pattern, which is diagnostic, from the fault type and volcano absence, which are consistent but not sufficient alone.
2. Evaluate the two groups' arguments. Which evidence is diagnostic, which is merely consistent, and what additional data would strengthen the interpretation?
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The rift argument is weak because continental rifts produce shallow normal-fault earthquakes, not a deep planar zone. The reverse faults near the coast could occur in a rift, but the depth progression cannot. The no-volcano claim is also weak because absence in a limited catalog is not evidence of absence in the region, and subduction zones can have gaps in volcanism. The teacher should ask students to rank evidence by diagnostic power: earthquake depth pattern > focal mechanisms > volcano presence. A stronger test would be to add seismic tomography, GPS strain, or a wider earthquake catalog. The class consensus should state the boundary type with confidence and identify what additional data would change the interpretation, modeling how geologists use multiple lines of evidence rather than a single feature.
3. Design a short classroom task that helps students separate diagnostic evidence from consistent evidence in this scenario.
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Use a two-column chart: 'diagnostic' and 'consistent but not decisive.' Students place the depth progression in the first column and the fault types, volcano absence, and coastal location in the second. Then ask each group to write a falsification statement: 'This would not be a subduction zone if...' Acceptable answers include a lack of deep earthquakes along the entire boundary or a depth pattern that does not deepen inland. This assesses whether students can separate correlation from causation and whether they understand that a plate-boundary classification is an inference from multiple data sets. The teacher can close with a quick vote and a one-sentence justification, then compare the class's reasoning with the actual catalog interpretation.