CyberShake simulation of strike-slip earthquakes on the Southwest Iceland transform zone

The Statewide Southern California Earthquake Center (SCEC) has designed and implemented CyberShake (CS), a high-performance computing (HPC) workflow to undertake Physics-Based Probabilistic Seismic Hazard Analysis (PB-PSHA) in California (CA). Here, we have ported CS from CA to the South Iceland Seismic Zone (SISZ) and the Reykjanes Peninsula Oblique Rift (RPOR), which experience sinistral transform motion and pose a very high earthquake risk to about 2/3 of the Iceland population. We consider a realization of the 3D SISZ-RPOR fault system, where fault areas are estimated from event magnitude through a scaling law (Mai & Beroza, 2017),  that fits maximum fault extents observed from slip inversions and surface mappings. The magnitude variability across the modeling region (~63.8°- 64.1°N, ~20°-23°W) is Mw 5-7. In this work, we employ CS to model ~2100 kinematic earthquake ruptures and quantify the resulting ground motion (GM) in terms of Pseudo Spectral Acceleration (PSA) intensity measures. An important computational milestone is the software development of an open-source in-house workflow manager at the Marenostrum Supercomputer that replaces the one used in CA by SCEC based on Pegasus and HTCondor. This new workflow manager handles input data (fault-plane geometries, rupture magnitudes, surface stations for GM recording and hazard studies), orchestrates the execution of CS components, and stores results (particle velocity seismograms and hazard curves). Among these components, the Graves-Pitarka (GP) kinematic rupture generator is used to produce finite-fault source descriptions characterized by a few large asperities. The other important component is the open-source fourth-order finite-difference staggered-grid AWP-ODC earthquake simulation code that allows for reciprocity and efficiently simulates rupture and seismic wave propagation in 3D heterogeneous Earth models. CS uses an adjoint computational procedure in which simulations of wave propagation are performed using a polarized delta source to compute the Strain Green Tensors (SGTs) at each fault point. The convolution of SGTs with GP ruptures yields particle-velocity seismograms at each station. SGT time histories are memory demanding, but the adjoint calculations are completely independent and therefore embarrassing parallel, making CS a highly efficient earthquake simulation tool. In this study, SGTs are constructed using a source frequency range of 0-1.0 Hz, generating ground motion synthetics resolved up to 0.5 Hz. CS rotation-invariant PSA values (3 and 5 sec periods) computed from our study show a good agreement with updated Bayesian ground motion prediction equations (Kowsari et al, 2022). This study is a first step towards a PB-PSHA in the SISZ-RPOR region and to routinely apply Cybershake outside of California.

REFERENCES:

Mai, M., & Beroza, G. Source scaling properties from finite-fault-rupture models. Bulletin of the Seismological Society of America, 90(3), 604-615, 2000.

Kowsari, M., Sonnemann, T., Halldorsson, B., Hrafnkelsson, B., Snæbjörnsson, J. &  Jonsson, S. Bayesian inference of empirical ground motion models to pseudo-spectral accelerations of South Iceland Seismic Zone earthquakes based on informative priors. Soil Dynamics and Earthquake Engineering, 132, 106075, 2020.