238-6 When Earth Writes Its Own Story: Unraveling the Hidden Complexity of Coastal Earthquake Records Through Monte Carlo Stratigraphy

Session: Recent Advances and New Voices in Marine and Coastal Geoscience

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Abstract:

We developed a quantitative framework to model coastal stratigraphic formation under earthquake-induced uncertainty using Monte Carlo simulations. Our approach centers on accommodation space theory, where normalized accommodation space A* = (z_s - z_sf)/z_s controls sediment deposition patterns. The model incorporates co-seismic subsidence followed by post-seismic recovery governed by logistic differential equations, simulating crustal movements over 2,000-year timescales.

We implemented four uncertainty scenarios combining constrained/unconstrained earthquake slip magnitudes and timing: (A) unconstrained slip and timing, (B) constrained slip with unconstrained timing, (C) unconstrained slip with constrained timing, and (D) both constrained. Each scenario utilized a slip-budget framework distributing total subsidence across multiple seismic events. Sediment deposition rates for four stratigraphic units (mud, marsh, upper marsh, soil) were randomized within literature-derived ranges (0.0004-0.15 m/yr for mud, 0.00068-0.0203 m/yr for marsh, 0.0011-0.006 m/yr for upper marsh, 0.0005-0.009 m/yr for coastal soils). Tsunami events deposited 0.2 m sand layers. Each scenario was simulated 1,000 times to generate robust statistical ensembles.

The Monte Carlo simulations revealed significant variability in stratigraphic outcomes even under identical earthquake sequences. Hill numbers analysis (N₀ richness, N₁ Shannon entropy, N₂ Simpson index) quantified stratigraphic complexity, showing that N₁ exhibits higher sensitivity to rare deposits like tsunami sands compared to N₂. Complexity patterns varied systematically with elevation, reflecting the interplay between earthquake timing, magnitude, and environmental factors.

Our age-matching methodology successfully constrained event timing by querying simulation ensembles for specific stratigraphic signatures. For example, tsunami deposits at 0.9-1.5 m and 4.0-4.5 m depths yielded robust age probability distributions. Results demonstrate that multiple distinct earthquake sequences can produce remarkably similar stratigraphic columns, while identical earthquake sequences generate diverse stratigraphic expressions depending on environmental variability.

The framework addresses fundamental questions in coastal stratigraphy: quantifying how many different stratigraphic columns represent the same earthquake sequence, and conversely, how many different earthquake sequences produce similar geological records. This work provides the first systematic application of Hill numbers to sedimentary diversity analysis and establishes a robust framework for interpreting coastal paleoseismic records under uncertainty.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8421

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