Towards seismic cycle modeling of the complex “bookshelf” South Iceland Seismic Zone

The South Iceland Seismic Zone (SISZ) is characterized by a complex "bookshelf" fault system composed of multiple short, parallel, North-South oriented right-lateral and near-vertical strike-slip faults assisting crustal block rotation to accommodate the overall East-West sinistral transform motion. The tectonic strain release across the entire zone is known to occur with 130–140 years intervals, in sequences of moderate earthquakes, up to Mw7. The most recent earthquakes occurred in 2000 and 2008 and reached magnitudes Mw6.3-6.5. Despite its importance for mitigating regional seismic hazards, the seismic cycle in the SISZ remains poorly understood. Thus, we develop a rate-and-state friction-based sequences of earthquakes and aseismic slip (SEAS) model to investigate the long-term seismic behavior of the SISZ.

We utilize the open-source code TANDEM (https://github.com/TEAR-ERC/tandem), a discontinuous Galerkin volumetric solver, and perform 2D simulations on the supercomputer ELJA, operated by the Icelandic High-Performance Computing Centre. Quasi-dynamic simulations with rate-and-state friction are applied to single planar fault models with antiplane shear motion in a homogeneous, isotropic, linear elastic half-space. We model two separate faults within the “bookshelf fault system,” representing the east-western regions of the transform zone. The primary focus of our 2D models is to reproduce the recurrence pattern of the seismic cycle, including hypocentral depth, fault slip, and approximate magnitudes. To configure reliable simulation parameters, we explore diverse models with varying rate-and-state frictional properties, effective normal stresses, and critical slip distances as well as other crucial factors.

Preliminary results indicate recurrence intervals for SISZ earthquakes ranging from 104 to 130 years across the transform zone's western and eastern sections, which agrees well with the observational data. Incorporating varying seismogenic depths in separate models-12 km in the west and 15 km in the east-improves our hypocentral depth predictions. Our study demonstrates the effectiveness of using seismic cycle simulations empowered by high-performance computing with TANDEM, even with a simplified single fault model, to elucidate the seismic processes of the SISZ. The model adequately captures some key characteristics of the seismic cycle of the SISZ, highlighting its potential to inform future seismic hazard assessments in Iceland within the larger scope of the ChEESE-2P project (https://cheese2.eu/).