Geology topics

Anna Ledeczi

Publications and source records attributed to Anna Ledeczi.

3 recordsLinked to original sources

USGS2026

3D broadband ground motion simulations of Cascadia megathrust earthquakes: Full-margin scenarios incorporating variable magnitudes and frontal thrust splay faulting

Great megathrust earthquakes (∼ M w 8–9) that rupture along the Cascadia Subduction Zone pose significant shaking and tsunami hazards to the U.S. Pacific Northwest. Although these large earthquakes are documented in the geologic record, none have been recorded with modern instrumentation, complicating regional seismic hazard assessments. To improve constraints on seismic hazard estimates for future earthquakes, we develop 3D broadband (0–10 Hz) ground‐motion simulations for 36 full‐margin megathrust earthquake scenarios through the Cascadia Coastlines and Peoples Hazards Research Hub. These simulations expand on the M9 Project ( Frankel et al. , 2018 ; Wirth, Frankel, et al. , 2018 ) by incorporating a broader range of rupture parameters—variable magnitudes, splay faulting, and geodetically informed slip distributions. Benchmarking these simulations against the Next Generation Attenuation Subduction (NGA‐Sub) ground‐motion models shows reasonable fits for non‐basin sites, although simulations predict stronger shaking at short (0–2 s) and long (6–10 s) periods. NGA‐Sub basin terms improve fits to synthetic shaking within the Seattle Basin but underpredict amplitudes by a factor of 2 at periods of 2–5 s. We also evaluate how rupture characteristics influence regional inland shaking variability and basin response in the Puget Lowlands. Larger magnitude scenarios generally cause stronger inland shaking due to farther inland extents of slip and larger strong‐motion‐generating fault patches but show little difference in basin amplification within the Puget Lowlands compared to smaller magnitude events. Along‐dip hypocenter location strongly influences overall inland shaking, with offshore (updip) hypocenters generating on average ∼45% higher shaking at long periods (>6 s) compared to onshore (downdip) hypocenters due to rupture directivity effects. Offshore splay faults minimally affect inland shaking but alter offshore deformation important for coupled ground‐shaking and tsunami scenarios. Finally, we show that rupture directivity increases amplification within the Seattle Basin through the generation and interaction of higher order mode surface waves, an effect of 3D basin geometry that cannot be captured using 1D approximations.

British Columbia, California, Oregon, Washington↗

USGS2025

Structural controls on splay fault rupture dynamics during Cascadia megathrust earthquakes

Great subduction earthquakes ( M w ≥ 8.0) can generate devastating tsunamis by rapidly displacing the seafloor and overlying water column. These potentially tsunamigenic seafloor offsets result from coseismic fault slip and deformation beneath or within the accretionary wedge. The mechanics of these shallow rupture phenomena and their dependence on subduction zone properties remain unresolved, partly due to the sparsity of offshore observations of shallow megathrust earthquake deformation. Here, we analyze how offshore structure influences shallow rupture mechanics and slip partitioning using 3D dynamic earthquake simulations of the Cascadia subduction zone (CSZ) megathrust with and without variably dipping seaward- or landward-vergent splay faults in the wedge that sole into the megathrust. Resulting tradeoffs between splay and megathrust slip reveal structural controls on rupture partitioning, with greater splay slip leading to less shallow megathrust slip updip. Gently dipping and seaward-vergent splays host more slip than those with steeper, landward-vergent splays. To isolate the underlying mechanisms, we compare models with Andersonian and plunging principal stresses. Results suggest distinct static and dynamic processes control the dip- and vergence-dependence of splay rupture: static (mis)alignment relative to far-field tectonic loading favors slip on more optimally oriented, shallowly dipping splay faults. In contrast, dynamic stress interactions of an updip-propagating megathrust rupture front with the free surface and potential branch faults favor forward branching onto seaward-vergent splays and inhibit backward branching onto landward-vergent splays. Resulting seafloor displacements suggest splay fault structure may influence coseismic tsunami source processes, highlighting the importance of dynamically viable rupture scenarios in subduction hazard assessments.

Cascadia subduction zone↗

USGS2024

Late-Quaternary surface displacements on accretionary wedge splay faults in the Cascadia Subduction Zone: Implications for megathrust rupture

Because splay faults branch at a steep dip angle from the plate-boundary décollement in an accretionary wedge, their coseismic displacement can potentially result in larger tsunamis with distinct characteristics compared to megathrust-only fault ruptures, posing an enhanced hazard to coastal communities. Elsewhere, there is evidence of coseismic slip on splay faults during many of the largest subduction zone earthquakes, but our understanding of potentially active splay faults and their hazards at the Cascadia subduction zone remains limited. To identify the most recently active splay faults at Cascadia, we conduct stratigraphic and structural interpretations of near-surface deformation in the outer accretionary wedge for the ~400 km along-strike length of the landward vergence zone. We analyze recently acquired high-frequency sparker seismic data and crustal-scale multi-channel seismic data to examine the record of deformation in shallow slope basins and the upper ~1 km of the surrounding accreted sediments and to investigate linkages to deeper décollement structure. We present a new fault map for widest, most completely locked portion of Cascadia from 45 to 48°N latitude, which documents the distribution of faults that show clear evidence of recent late Quaternary activity. We find widespread evidence for active splay faulting up to 30 km landward of the deformation front, in what we define as the active domain, and diminished fault activity landward outside of this zone. The abundance of surface-deforming splay faults in the active outer wedge domain suggests Cascadia megathrust events may commonly host distributed shallow rupture on multiple splay faults located within 30 km of the deformation front.

British Columbia, California, Oregon, Washington↗