A seismic refraction survey indicates a moderately large offset in bedrock overlain by uniform velocity soils. Which step would help evaluate recency of fault movement?

• A. Drilling deep exploratory wells
• B. Measuring magnetic anomalies
• C. Review aerial photographs and excavate a trench
• D. Installing a GPS network

Answer: (C)

Determining whether a fault moved recently requires direct evidence of the most recent surface rupture. Reviewing aerial photographs helps reveal fresh fault-related expressions on the landscape, such as new scarps, offset features, or misaligned drainage patterns, which can indicate recent movement. Excavating a trench across the fault then exposes the fault plane and the stratigraphy on both sides, allowing you to see precisely where and when layers were displaced and to identify the last rupture by dating the last undisturbed layer above the fault versus the displaced units below. This paleoseismic approach provides a time estimate for the most recent faulting event, which is exactly what you’re after.

Deep exploratory wells sample subsurface conditions but don’t directly establish when the fault last moved. Magnetic anomaly measurements reflect lithology and magnetization contrasts rather than the timing of faulting. Installing a GPS network can track current deformation rates but doesn’t date the most recent rupture, so it’s less effective for assessing recency.