Fault interactions and Coulomb stress-triggering in complex fault networks

Earthquakes on normal faults cause negative Coulomb stress transfer (CST) onto receiver faults located across-strike, which, in theory, delays or prevents rupture. Nevertheless, earthquakes on such across-strike faults are frequently observed. This study explores why, and how, large earthquakes can be triggered on faults repeatedly receiving negative coseismic CST. Two triggering mechanisms are hypothesised: (1) Positively stressed patches or segments of faults, resulting from heterogeneous stress transfer, act as triggers to rupture earthquakes on faults that are on average negatively stressed. (2) Negative coseismic CST is compensated by interseismic loading and other processes building up positive stress on the source faults. To test both hypotheses, a 400-year earthquake sequence is modelled, located in the fault network of the Western Anatolian Extensional Province (SW Türkiye). The fault network features multiple active faults located along-strike and across-strike of another, as well as faults in a variety of orientations, suitable to explore stress-triggering mechanisms in a structurally complex setting. Detailed information on fault location, geometry, and mechanism is compiled from field investigations, literature review, and geodetic data. Based on instrumental and historical earthquake catalogues, and the suitability of the source fault network, a sequence of earthquakes is determined to investigate the two hypotheses by comparing the effects of coseismic CST and interseismic loading.

Results show that, out of 28 modelled large (MW ≥6) earthquakes, 6 were triggered on faults receiving significant negative coseismic CST. For five of these, negative coseismic CST is compensated by processes increasing CST. Only for one studied example, highly stressed positive fault segments on an otherwise negatively stressed fault could have been the driving mechanism leading to rupture of a large earthquake. Given all model uncertainties, stress-heterogeneities cannot be validated as a probable triggering mechanism for faults in the stress shadow of neighbouring faults. In contrast, earthquakes on normal faults located across-strike of another can only delay, but in most cases not prevent failure, as interseismic loading usually exceeds negative coseismic CST.

To reinforce these results, the impact of the modelled fault geometry and slip rates, used to calculate interseismic loading, is evaluated. Models of strike- and dip-variable faults, following the actual surface fault traces, are compared with simplified, planar fault models. Simplified models feature exaggerated areas of positive and negative CST on source faults prior to earthquakes, essentially distorting the stress field and causing stress-heterogeneities that are less pronounced in more realistic models. This observation highlights the necessity of modelling fault geometry as realistic as possible, especially when models are used in fault-based SHA. The impact of slip rates on model results is less drastic, so long as slip rates are used that are determined on similar time scales as the model duration. For the studied fault network short-term (geodetic) and long-term (‘geologic’) slip rates vary from the ‘Holocene’ slip rates by an order of magnitude. If used for modelling interseismic loading, the stress state and recurrence intervals of faults would be drastically under- or overestimated.