Progressive weakening of a fault by seismic waves revealed by dynamically triggered slow slip events on the North Anatolian Fault

Seismic waves from distant earthquakes are known to trigger earthquakes and slow slip events on active faults. However, the underlying physics of such interaction is poorly understood. Dynamic rearrangement of grains in a granular medium, pore pressure changes within that same gouge or the response of a frictional interface have been proposed to explain such distant triggering. The main issue is the lack of observations at various scales of the same triggered slip event in nature, from seconds to years and from the local meter-scale to a full-scale image of induced slip on the fault.

We use data from a dense seismogeodetic network, InSAR imagery and four creepmeters located at Ismetpasa along the North Anatolian Fault to quantify the temporal and spatial evolution of a sequence of transient shallow slow slip events triggered by the passage of the waves from the 2023 Kahramanmaras doublet. Cumulative slip amounts to 1 cm over a few months and accumulates in the form of multiple mm-amplitude slip events lasting from minutes to weeks. Slip nucleated initially during the passage of surface waves from the M7.8 Kahramanmaras and M7.5 Elbistan earthquakes. High-rate (1s) GNSS time series were used to derive complete time series of dynamic aerial strain and stress tensors to compute the dynamic change in Coulomb stress change during the passage of surface waves. Although the timing of dynamic Coulomb stress change is consistent with the initiation of the first slip events, it is difficult to physically quantitatively relate the amplitude of the shaking with the amplitude of the slip events. Three of the creepmeters subsequently recorded multiple creep events with logarithmically decaying fault slip (Tau> 1.7 hours) following an initial offset of a few tens of microns.  We note that the amplitude of the second slip event, triggered by the M7.5 Elbistan earthquake,  is larger than that of the first one, triggered by the M7.8 Kahramanmaras earthquake. Subsequent events in the following weeks initiate when local atmospheric pressure drops severely, a feature that was not observed prior to the sequence. Finally, we observe that, for all these slow slip events, slip rate is slowed down by the local increase in atmospheric pressure with a logarithmic relationship between slip rate and pressure.

We interpret these slow slip events as the signature of the progressive weakening of the fault zone, weakening first initiated dynamically by the passage of the surface waves from distant earthquakes and progressively continued with the cumulation of slip along the fault. The first incoming waves from the Kahramanmaras earthquake dynamically re-arrange material in the fault gouge, initiating a slip instability at depth, potentially further facilitated by elevated pore pressure, then progresses to the surface and expands along the fault. Each wave train further weakens the fault plane allowing for more slip, which itself further weakens the fault. Once no strain is available, the fault recovers and regains its strength over time. Our study provides a first view of the dynamics of triggered events by combining seismological, geodetic and atmospheric observations.