Characteristics of seismicity associated with the 2014/04/01 Northern Chile (Pisagua) earthquake (Mw8.2)

Nelson Pulido, National Research Institute for Earth Science and Disaster Prevention (April 18/2014)

A megathrust earthquake occurred in Northern Chile in 2014/04/01 (Mw8.2), in a region that had not experienced a major earthquake since the great 1877 (~M8.6) event (Figure 1). This area had been already identified as a mature seismic gap due to fast subduction of the Nazca plate beneath the South American plate (6-7 cm/year), as well as to a strong interseismic coupling inferred from geodetic measurements (Chlieh et al. 2011 and Metois et al 2013). This region has also experienced two M7.7 earthquakes in 2005 (at a depth of ~100km) and 2007, east and south of the Pisagua mainshock respectively (Figure 1).

In Figure 1 we show the characteristics of seismicity from 3 months before to 11 days after the occurrence of the mainshock (2014/04/01, 23:46:45 UTC). We use events obtained from the earthquake catalogue of the Seismological Network of Chile University, which contains 1566 events in the Pisagua mainshock source region (2014/01/01 to 2014/04/11), with magnitudes starting at 1.8(Ml) and for depths shallower than 70 km. In figure 1A we can observe a cluster of foreshocks of the M8.2 event which started with a M6.7 event (2014/03/16, 21:16:29) 50 km to the south, and then migrated towards the mainshock. In figure 1B we can see the aftershocks of the Pisagua earthquake which spanned a region of ~200km south of the mainshock epicenter. In figure 1C we plot the whole seismicity from 2014/01/01 to 2014/04/11. Events with magnitudes larger than 6.0 are displayed as stars. Circles sizes are proportional to magnitude.

In order to see the temporal and spatial variation of seismicity associated with the Pisagua earthquake, in Figure 2 we plotted chronologically the events from 2014/01/01 to 2014/04/11 in terms of their distance along trench (North-South) relative to the mainshock epicenter. In Figure 2A we can clearly observe an earthquake migration toward the mainshock, starting with the M6.7 event. This sequence started 16 days before the mainshock and propagated at a speed of nearly 4 km/day (Figure 2A). The foreshocks sequence was followed by 6 days of reduced seismicity just before the occurrence of the mainshock (Figures 2A, 2B). This type of earthquake migration before a large magnitude earthquake was also observed before the 2011 Tohoku earthquake (M9.0) (Kato et al. 2012). Kato et al. (2012) found that the Tohoku foreshock sequence migrated towards the mainshock with velocities between 2-5km/day, starting 24 days before the mainshock. The observation of a very similar seismicity pattern preceding the 2014 Pisagua earthquake suggests that this type of slip behavior might be a universal feature before the occurrence of megathrust earthquakes.

Acknowledgments

The seismicity data used in this work was downloaded from the online catalogue of the Chile Seismological Network managed by Chile University.

References

Chlieh, M., et al. (2011). Interseismic coupling and seismic potential along the Central Andes subduction zone, J. Geophys. Res., 116, B12405, doi:10.1029/2010JB008166.

Metois, M. et al. (2013). Revisiting the North Chile seismic gap segmentation using GPS-derived interseismic coupling, Geophys. J. Int., 194, 1283–1294 doi: 10.1093/gji/ggt183

Kato, A., et al. (2012). Propagation of Slow Slip Leading Up to the 2011 Mw 9.0 Tohoku-Oki, Science 335, 705, doi: 10.1126/science.1215141

Figure 1. A) Foreshocks and B) Aftershocks ofthe 2014/04/01 Pisagua earthquake. C) Seismicity between 2014/01/01 to 2014/04/11 in the source region of the Pisagua earthquake. Stars correspond to earthquakes with magnitude larger than 6.

Figure 2. A) Temporal characteristics of seismicity 1 month before and 11 days after the 2014/04/01 Pisagua earthquake. B) Same for a period 3 months before to 11 days after the mainshock. Stars correspond to earthquakes with magnitude larger than 6.