EarthArXiv

Various earthquake source models predict that aseismic slip modulates the seismic rupture process. However, observations of aseismic slip associated with earthquakes are scarce, which has left the earthquake source model controversial. Here, we characterise seismic and aseismic processes for 3 days during the 2014 Iquique earthquake sequence in northern Chile by analysing seismicity and crustal deformation time series measured by high-rate Global Positioning System (GPS). We demonstrate that the early afterslip started immediately after the M 8.1 mainshock and led to the largest M 7.6 aftershock 27 hours later, located 120 km to the south. At the mainshock latitude, the interevent early afterslip is located downdip of the mainshock rupture, and is associated with aftershocks. These afterslip and aftershocks exhibit a rapid temporal decay. In contrast, south of the mainshock slip patch, a peak of afterslip separates the mainshock rupture from the largest aftershock, suggesting that this area acted as a barrier to the southward propagation of the mainshock rupture. Seismicity count and moment accelerate in this southern area during the interevent stage. We conclude that the largest aftershock nucleation was driven by the interevent afterslip. The mechanical connection between sequential great earthquakes can therefore be mediated by aseismic slip.

Figure caption: (a) Cleaned GPS time series (coloured dots) with a trajectory model fit (black curve). Red and purple vertical lines indicate timing of the 2014 Iquique mainshock and the largest aftershock. The site location is shown as a red solid square in (b). (b) Intervevent GPS displacements between the two earthquakes (black vectors) and model displacements (blue vectors) from afterslip model shown in (c). Red, purple and orange stars indicate the epicentres of the mainshock, the largest aftershock and an M 6.1 aftershock, respectively (Soto+2019JGR). Black dots indicate moderate aftershocks (McBrearty+2019). (c) Slip distribution of the mainshock (red), interevent afterslip (blue), the largest aftershock (purple) and 2-day afterslip following the largest aftershock (green). Black dots indicate GPS sites used for the slip inversion. (d-e) afterslip distribution for the first and second halves of the interevent period (blue contours) with outlines of the mainshock and the largest aftershock slip area (red and purple curves). Blue, green and black dots indicate moderate aftershocks during the corresponding windows (see (f) for meaning of colours). (f) Latitude-time evolution of interevent moderate aftershocks. (g) Temporal evolution of seismicity count (top) and geodetic and seismicity moments (bottom) in the two regions north and south of 20.2°S as labelled. Each curve is normalised with respect to their cumulative value. This figure is modified from Itoh+2023EarthArXiv.

Updated on 3 April 2023