1. The seismic sequence of the 2016 Mw 7.8 Pedernales, Ecuador earthquake
Sergio Leon-Rios1*, Ana Luiza Martins1, Amaya Fuenzalida Velasco1, Lidong Bie1, Tom Garth1, Pablo Gonzalez1, James Holt1, Andreas Rietbrock1, Benjamin Edwards1, Marc Regnier2, Diego Mercerat3, Michel Pernoud3, Matthieu Perrault4, Javier Santo4, Alexandra Alvarado4, Susan Beck5
1. Liverpool Earth Observatory, School of Environmental Sciences, University of Liverpool, UK. * 2. Géoazur, Université Nice, IRD, CNRS, OCA, Nice, France. 3. CEREMA, CNRS, Nice, France 4. Instituto Geofísico de la Escuela Politécnica Nacional, Quito, Ecuador 5. Department of Geosciences, University of Arizona, Tucson, Arizona, USA
3. Methodology and dataset
Using data recorded both on the permanent and the recently deployed network we manually picked, located and calculated the moment tensor solutions for the foreshock event, and the large aftershocks (M > 5).
For picking and location we used Seismic Data eXplorer (SDX) software [1]. In order to compare our results with other solutions we also computed the location using NonLinLoc software [2].
To obtain the focal mechanism of the events we utilized ISOLA software [3] which performs a least-squares minimisation of synthetic and observed waveforms along with a depth grid-search to find the best centroid solution.
We tested different velocity models in order to constrain our solutions.
5. Aftershock sequence
First three months of aftershocks show a distribution that is coherent with the north and south boundaries of the coseismic slip model from Nocquet et al (In press) [7]. However there is not a clear pattern with the updip and downdip limits of the rupture.
Inset figure shows the distribution in depth for the aftershocks where is possible to observe the slab interface. An accurate 1D velocity model (in preparation) will help us to gain better resolution after relocating these events.
1. Introduction
On the 16th April 2016, a Mw 7.8 mega-thrust earthquake occurred in Northern Ecuador, close to the city of Pedernales.
The event ruptured an area of 120 x 60 km and was preceded by a Mw 4.8 foreshock, located only ~15 km west of the epicentre, and registered 10 minutes before the main event.
Fig 1 Earthquake report generated by the Instituto Geofisico de la Escuela Politecnica Nacional (IGEPN). (Left) Map view of the epicentre. (Right) Focal mechanism solution using SWIFT software.
Aftershocks 15May – 09Aug 2016
No resolution close to the trench
4. Foreshock event
Figure 3 (top) presents different solutions for location and RMT for the foreshock using the velocity models indicated in the legend. (bottom left) Cross section in the foreshock area (AA’) showing how the depth location changes according to the velocity model. Slab1.0 [4] is plotted as a reference. (bottom right) Fitting of the waveforms for the solution using SDX location and Font (2013) [5] velocity model. A A’
2. Emergency deployment
A few weeks after the main event a large array of instruments was deployed by a collaborative project between the Geophysical Institute of Ecuador (IGEPN), IRIS (USA), Geoazur (France) and the University of Liverpool (UK).
This dense seismic network, with more than 70 stations, includes broadband, short period and strong motion instruments and is currently recording the aftershock activity of the earthquake.
Aftershocks 15May – 09Aug 2016
Coulomb stress change was calculated using the parameters of the main shock as an input for the receiver fault at a depth of 25 km.
Stress distribution after the earthquake, at 25 km depth, increases to the north and south of the rupture where aftershocks are concentrated.
Fig 2 (Right) Current seismic network after the emergency deployment. Different colours indicate the institutions collaborating on this project. (Left) Collage of pictures taken during the deployment and maintenance fieldwork showing the instruments, deployment and the importance to share science with the affected people.
6. Discussion and conclusions
This study highlights the importance of collaboration for quick deployment of large network arrays in mega-thrust earthquake projects.
Differences in location and RMT solution for one event show how sensitive can be the proper use of a good velocity model. IASP91 [6] works well for a generic solution, Ecuador1D is based on IASP91 so similar solution is expected. Font (2013) [5] performed a good job and is the most reliable solution at the moment. About foreshock’s in depth location it is possible to classified this event as interplate, however further analysis is needed in order to constrain in a better way the slab in the area of study. In order to do that, next step is to get a 1D velocity model using P and S arrival times.
Aftershock distribution shows a pattern that surround the coseismic slip and are coherent with the Coulomb stress change at 25 km depth. More depths need to be computed in order to check its relation with the post seismic process.
References:
Acknowledgments:
[1] Heath A E (2009). SDX (Seismic Data Explorer) (Version 1) [Internet (free access)]. Retrieved from
[2] Lomax A, Virieux J, Volant P, Berge-Thierry C (2000). Probabilistic earthquake location in 3D and layered models. Advances in seismic event location, 101–134. DOI: / _5
[3] Sokos E N, Zahradnik J (2008). ISOLA a Fortran code and a Matlab GUI to perform multiple-point source inversion of seismic data. Computers & Geosciences, 34(8): DOI: /j.cageo
[4] Hayes G P, Wald D J, Johnson R L (2012). Slab1.0: a three‐dimensional model of global subduction zone geometries. J. Geophys. Res., 117:B DOI: /2011JB008524
[5] Font Y, Segovia M, Vaca S, Theunissen T (2013). Seismicity pattern along the Ecuadorian subduction zone: New constrains from earthquake location in a 3D a priori velocity model. Geoph. J. Int., 193(1): DOI: /gji/ggs083
[6] Kennett B L N and Engdahl E R (1991). Travel times for global earthquake location and phase association. Geoph. J. Int., 105: DOI: /DP/
[7] Nocquet et al. (In press). Supercycle at the Ecuadorian subduction zone revealed after 2016 Pedernales earthquake. Nature geoscience.
Becas de Doctorado en el extranjero, Becas Chile, convocatoria 2015 de la Comisión Nacional de Investigación, Ciencia y Tecnología (CONICYT - Chile).
NERC grant under the contract NE/P008828/1 for the project “Rapid deployment of a seismic array in Ecuador following the April 16th 2016 M7.8 Pedernales earthquake”.
Instituto Geofisico de la Escuela Politecnica Nacional, Ecuador.
To all the people in Ecuador who allowed us to deploy our instruments in their houses.