Space‐Time Stress Variations on the Palu‐Koro Fault Impacting the 2018 Mw 7.5 Palu Earthquake and Its Seismic Hazards

Abstract In this study, we calculated the Coulomb stress change before and after the 2018 Palu earthquake (Mw 7.5) induced by historical large earthquakes on and around the Palu‐Koro fault (PKF) within Sulawesi Island, Indonesia. We found that the 1996 earthquake (Mw 7.9) on the Minahassa thrust likely promoted the 2018 Palu earthquake by stress loading on its hypocenter. Stress shadows in the PKF impacted the 2018 earthquake rupture’s southward unilateral propagation and termination. Stress increases in the two seismic gaps to the north and south ends of the PKF have resulted in increased seismic hazards, calling close attention to hazard prevention in central Sulawesi. This indicates that fault‐interaction‐induced stress variations on the PKF significantly controlled the 2018 Palu earthquake and its seismic hazards. Our study is important for understanding stress triggering between the subduction earthquakes and intraplate earthquakes in thrust and strike‐slip faults systems globally

The stress shadow problem in physics-based aftershock forecasting: Does incorporation of secondary stress changes help?

International audienceMain shocks are calculated to cast stress shadows across broad areas where aftershocks occur. Thus, a key problem with stress‐based operational forecasts is that they can badly underestimate aftershock occurrence in the shadows. We examine the performance of two physics‐based earthquake forecast models (Coulomb rate/state (CRS)) based on Coulomb stress changes and a rate‐and‐state friction law for their predictive power on the 1989 Mw = 6.9 Loma Prieta aftershock sequence. The CRS‐1 model considers the stress perturbations associated with the main shock rupture only, whereas CRS‐2 uses an updated stress field with stresses imparted by M ≥ 3.5 aftershocks. Including secondary triggering effects slightly improves predictability, but physics‐based models still underestimate aftershock rates in locations of initial negative stress changes. Furthermore, CRS‐2 does not explain aftershock occurrence where secondary stress changes enhance the initial stress shadow. Predicting earthquake occurrence in calculated stress shadow zones remains a challenge for stress‐based forecasts, and additional triggering mechanisms must be invoked

Triggered earthquakes suppressed by an evolving stress shadow from a propagating dyke

Large earthquakes can generate small changes in static stress: increases that trigger aftershock swarms, or reductions that create a region of reduced seismicity—a stress shadow1, 2. However, seismic waves from large earthquakes also cause transient dynamic stresses that may trigger seismicity3, 4. This makes it difficult to separate the relative influence of static and dynamic stress changes on aftershocks. Dyke intrusions do not generate dynamic stresses, so provide an unambiguous test of the stress shadow hypothesis. Here we use GPS and seismic data to reconstruct the intrusion of an igneous dyke that is 46 km long and 5 m wide beneath Bárðarbunga Volcano, central Iceland, in August 2014. We find that during dyke emplacement, bursts of seismicity at a distance of 5 to 15 km were first triggered and then abruptly switched off as the dyke tip propagated away from the volcano. We calculate the evolving static stress changes during dyke propagation and show that the stressing rate controls both the triggering and then suppression of earthquake rates in three separate areas adjacent to the dyke. Our results imply that static stress changes help control earthquake clustering. Similar small static stress changes may be important for triggering seismicity near geothermal areas, regions being hydrofractured and deflating oil and gas fields