Modelling earthquake rates and associated uncertainties in the Marmara Region, Turkey

Thomas Chartier,Hélène Lyon-Caen,James H Dieterich,Oona Scotti,Keith Richard-Dinger,Bruce E Shaw

doi:10.5194/nhess-21-2733-2021

Abstract

Abstract. Modelling the seismic potential of active faults and the associated epistemic uncertainty is a fundamental step of probabilistic seismic hazard assessment (PSHA). We use SHERIFS (Seismic Hazard and Earthquake Rate In Fault Systems), an open-source code allowing us to build hazard models including earthquake ruptures involving several faults, to model the seismicity rates on the North Anatolian Fault (NAF) system in the Marmara Region. Through an iterative approach, SHERIFS converts the slip rate on the faults into earthquake rates that follow a magnitude frequency distribution (MFD) defined at the fault system level, allowing us to model complex multi-fault ruptures and off-fault seismicity while exploring the underlying epistemic uncertainties. In a logic tree, we explore uncertainties concerning the locking state of the NAF in the Sea of Marmara, the maximum possible rupture in the system, the shape of the MFD and the ratio of off-fault seismicity. The branches of the logic tree are weighted according to the match between the modelled earthquake rate and the earthquake rates calculated from the local data, earthquake catalogue and palaeoseismicity. In addition, we use the result of the physics-based earthquake simulator RSQSim to inform the logic tree and increase the weight on the hypotheses that are compatible with the result of the simulator. Using both the local data and the simulator to weight the logic tree branches, we are able to reduce the uncertainties affecting the earthquake rates in the Marmara Region. The weighted logic tree of models built in this study will be used in a following article to calculate the probability of collapse of a building in Istanbul.

Highlights

The North Anatolian Fault system (NAFS) runs through the north of Turkey for a distance of more than 1500 km (Fig. 1), accommodating the westward extrusion of the Anatolian plate with a right lateral motion of around 25 mm/yr (e.g. Reilinger et al, 2006)
We calculated earthquake rates in the Marmara Region relying on the fault slip rate and geometry as primary information
We combined two innovative approaches: the SHERIFS approach that relies on statistical rules and the RSQSim approach that relies on physical rules

Summary

Introduction

The North Anatolian Fault system (NAFS) runs through the north of Turkey for a distance of more than 1500 km (Fig. 1), accommodating the westward extrusion of the Anatolian plate with a right lateral motion of around 25 mm/yr (e.g. Reilinger et al, 2006). Along the western portion of the NAFS, this right lateral motion is partitioned between two fault branches, with the northern branch accommodating most of the motion (Reilinger et al, 2006) This northern branch of the NAFS crosses the Sea of Marmara 20 km south of the city of Istanbul (Fig. 2). Six earthquakes larger than magnitude 7.0 are in the catalogue, and it can be argued that the observed MFD is a result of the incomplete sample of the phenomenon and that the long-term MFD should follow a GR distribution. This debate on the catalogue MFD concerns the Californian earthquake catalogue as well (Page and Felzer, 2015; Parsons et al, 2018)

Objectives

Findings

Discussion

Conclusion