A simplified seismicity model of the bookshelf fault system of the Southwest Iceland transform zone

In Iceland, the most seismically active region in Northern Europe, large earthquakes up to ~\({M}_{\text{w}}\)7 repeatedly take place in the two transform zones of the country. Of the two, only the South Iceland Seismic Zone (SISZ) in southwest Iceland is on land and with a large part of the country’s population either collocated or in close proximity to it. Strong earthquake occurrence in the SISZ takes place on a bookshelf fault system, an array of short, vertical, and dextral strike-slip faults oriented perpendicular to the overall transform motion. Importantly, this system has recently been shown to be continuous further towards the west along the entire Reykjanes Peninsula Oblique Rift (RPOR), making the bookshelf fault system approximately twice as long as previously thought. Moreover, a systematic spatial variation of maximum earthquake magnitudes characterizes the SISZ-RPOR system, from ~\({M}_{\text{w}}\)7 down to ~\({M}_{\text{w}}\)5.5 from eastern SISZ to western RPOR, respectively, indicates a subzonation of the seismic region. The above has not been taken into account in past probabilistic seismic hazard assessments (PSHA) and poses a challenge as the historical earthquake catalogue precludes reliable estimates of seismicity parameters for individual subzones of the SISZ-RPOR system. In this study, we address this issue using a recently developed physics-based finite-fault model of the SISZ-RPOR bookshelf fault system, and quantitatively estimate the time-independent magnitude-frequency distributions (MFDs, of the Gutenberg-Richter type) for each subzone. We establish zone-specific distributions representative of long-term fault slip rates and derive the seismicity parameter estimates corresponding to the 2.5, 50, and 97.5 percentiles of fault slip rates along the SISZ-RPOR as predicted by the physics-based model. We present new and quantitative estimates of subzone MFDs and show that the model effectively explains the historical earthquake catalogues. The results of this study not only enable the efficient yet physically realistic and consistent revision of conventional time-independent PSHA for southwest Iceland using e.g., empirical ground motion models, but also a more comprehensive physics-based PSHA from finite-fault rupture modeling and advanced seismic ground motion simulation techniques.