Interpreting a Dipping Earthquake Zone and Arc Volcanism
A summer research intern is given a spreadsheet from a remote continental margin. It contains 47 earthquake hypocenters: a shallow cluster at 5-15 km depth near the coast, a dipping band of events at 40-80 km, and isolated events at 120-190 km farther inland. Volcanic rocks sampled along the arc are mostly andesite and dacite, with minor rhyolite; basalt does not dominate the arc front. Two GPS stations show horizontal velocities of 31 mm/yr and 28 mm/yr toward N35E relative to the interior, with the coastal station moving faster. The intern has no seismic-reflection profiles and no drilled samples from the downgoing plate. Her supervisor asks her to classify the plate boundary and to state what the data do and do not justify about earthquake and eruption hazards. A news reporter has already described the deep earthquakes as a sign that 'the volcano will erupt next week.'
1. Classify the plate boundary using the earthquake-depth pattern, rock compositions, and GPS vectors. Explain why a transform boundary is a poor fit.
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The data fit a convergent, subduction-zone boundary. The earthquake hypocenters define a dipping zone: shallow events near the coast, intermediate events at 40-80 km, and deep events at 120-190 km farther inland. That geometry is consistent with a downgoing slab, not a single vertical or horizontal fault. Intermediate and deep earthquakes are especially diagnostic because transform faults are generally confined to the shallow brittle crust.
The andesite-dacite suite with minor rhyolite is typical of arc magmatism produced when water released from a subducting slab lowers the mantle melting temperature. A transform boundary would not produce a linear volcanic arc of that composition. The GPS vectors show convergence toward N35E, and the coastal station moving faster than the interior is consistent with strain accumulation near the plate interface.
A transform boundary is a poor fit because it would predict dominantly shallow, strike-slip earthquakes, no dipping deep seismic zone, and no arc volcanism. The intern should still note that focal mechanisms are absent, so she has not directly demonstrated thrust faulting; the case is strong but not complete.
2. Evaluate the reporter's statement that the deep earthquakes predict an eruption next week. What can and cannot be inferred from the data?
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The statement is not justified. The deep earthquakes are located within the subducting slab, tens of kilometers below the volcanic arc. They indicate ongoing subduction and brittle failure in the slab, not movement of magma beneath a specific volcano. A cluster of deep earthquakes does not establish a short-term eruption timeline.
What can be inferred is a long-term hazard: the region is seismically and volcanically active because subduction is ongoing. The data support preparing for earthquakes and eruptions in general. They do not support a 'next week' forecast.
A short-term eruption forecast would require different evidence, such as shallow volcanic seismicity, ground deformation, changes in gas emissions, thermal anomalies, or seismic tremor. The reporter has confused a persistent tectonic process with a precursor. The intern should also avoid the opposite error of saying the volcanoes are harmless; the correct conclusion is that the data establish long-term potential, not imminent activity.
3. Name two additional data sets the intern should request and explain how each would test her interpretation.
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Two useful additions would be earthquake focal mechanisms and seismic tomography or receiver-function imaging.
Focal mechanisms would show whether shallow earthquakes are thrust events consistent with convergence, strike-slip events consistent with a transform boundary, or normal events consistent with extension. A mix of thrust mechanisms along a dipping plane would strengthen the subduction interpretation. If the shallow events were predominantly strike-slip, the intern would need to reconsider the boundary type or the role of sliver rotation.
Tomography or receiver functions would image the subducting slab's geometry and depth extent. A high-velocity dipping slab extending from the trench to the deep earthquake zone would directly test the inference drawn from hypocenters alone. It could also reveal a slab gap, a flat-slab segment, or a seamount, any of which would change the hazard interpretation. Either data set would move the conclusion from a strong inference based on indirect evidence toward a more direct test.