Rupture Process of the 2014 Iquique Chile Earthquake in Relation With the Foreshock Activity

Wednesday, 17 December 2014
Yuji Yagi, Ryo Okuwaki, Bogdan Enescu and Shiro Hirano, University of Tsukuba, Tsukuba, Japan
On April 1, 2014 a great thrust earthquake (Mw 8.1) occurred off Iquique, northern Chile. A remarkable foreshock sequence was observed about 2 weeks before the mainshock. Seismic source models are essentially important to reveal the relationship between the mainshock and foreshock activity. However, seismic source models for the Iquique earthquake are quite different from one another. To grasp features of this earthquake, we compared its teleseismic P-waves and found a clear directivity effect, showing that the main rupture started about 50 km southward from the mainshock epicenter. On the other hand, there is no clear directivity effect of the main phase, indicating that it is generated by a circular crack-like rupture. Our observations also suggest that the Iquique earthquake has a back propagating rupture. Therefore, it is difficult to obtain a stable rupture process using conventional inversion schemes. In this study, we estimate the rupture process of the Iquique earthquake from teleseismic P-wave data applying a novel formulation that takes into account the uncertainty of Green’s function. To image the complex rupture features, including the back propagating rupture, we applied the multi-time window inversion scheme with a broad modeling time window for each space knot. The estimated source process is characterized by unilateral rupture propagation. During the first 20 s the dynamic rupture front propagated from the hypocenter to the large asperity located about 50 km southward, crossing a remarkably active foreshock area at high velocity, but small and irregular seismic moment release rate. The main rupture started 20 s after the initial break and the back propagation was observed in the remarkably active foreshock area. Our result implies that the 20 s long initial phase was likely influenced by the stress drop due to the foreshock activity near the mainshock hypocenter. Moreover, the 2-weeks long swarm-like foreshock activity migrating roughly at 5 km/day towards the mainshock hypocenter, and possibly associated slow-slip, contributed to the stress accumulation prior to the Mw 8.1 megaquake. The mainshock initial rupture phase might have triggered the rupture of the large asperity, which had large fracture energy.