Excavating the Past
A paleoseismic trench is an archaeological dig for earthquakes. By cutting a 3–5 m deep excavation perpendicular to an active fault, geologists expose a vertical cross-section through thousands of years of earthquake history. Each rupture event displaces sediment layers, creates a surface scarp that erodes into a distinctive wedge-shaped deposit, and is eventually buried by ongoing sedimentation. Reading this stratigraphic record requires the skills of both a structural geologist and a sedimentologist.
Event Identification
Recognizing individual earthquakes in a trench wall is detective work. Key indicators include fault strands that terminate upward at a specific horizon (the event horizon), colluvial wedges of scarp-derived sediment burying the pre-event surface, fissure fills where coseismic cracks opened and filled with surface material, and abrupt changes in layer thickness across the fault. Each indicator alone is ambiguous; together they build a convincing case for a specific event.
Dating the Record
Radiocarbon dating of detrital charcoal, seeds, or organic-rich sediment above and below event horizons brackets earthquake ages. With careful stratigraphic ordering and Bayesian statistical models (like OxCal), paleoseismologists can constrain event dates to within decades, even for earthquakes thousands of years old. The sedimentation rate determines how much material accumulates between events and thus how resolvable the record is.
From Trench to Hazard
Trench data feed directly into seismic hazard analysis. The number of events, their ages, and displacement per event constrain recurrence intervals, slip rates, and characteristic earthquake magnitudes for a fault. This simulation lets you build a virtual trench — adjusting displacement, sedimentation, and erosion to understand how the geological record captures (or fails to capture) the earthquake history.