GPS at IPOC

The observation of the present-day deformation of the Earth's surface with high spatial and temporal resolution makes up a major part of IPOC. For this purpose, continuously recording GPS stations were installed in Northern Chile. While seismological data shed light on just short time scales and geological records capture only very long time scales, precise geodetic measurements are capable of acquiring a detailed picture of the entire earthquake cycle. This includes not merely the interseismic compression but also a possible "slow" earthquake that may occur in different depth along the seimogenic zone. Moreover, the redistribution of strain connected with the spatio-temporal clustering of earthquakes can also be observed.

Aim of this component is to provide a continuous stream of GPS time series for joint interpretation with conventional seismic data. Traditional seismic data obtain the surface displacement by a double integration of acceleration measurements (strong motion instruments) or by deconvolution of the instrument response and integration of the velocity measurements (broadband instruments). The accuracy of absolute displacements derived with seismic instruments is poor due to the bandwidth and the dynamic range limits of seismometers. In contrast geodetic methods (GPS instruments) do not have this problem: here the ground displacement is measured directly. Unlike the seismometer, the GPS receiver never saturates, however, seismic measurement provides a powerful constraint on the much noisier GPS measurements.

The combination of high-rate GPS displacements and traditional seismic data will extend the frequency range of surface displacement data that classically produced daily coordinate time series. Contrasting to seismic data, geodetic data requires extensive processing before geophysical interpretations can be made. This includes the production of time series, velocities and strain maps.

In addition to the continuous GPS installations that are integral part of IPOC, various GPS surveys have been and are being carried out for more than a decade. Information on these are given in the Associated Projects section.

A total of 13 IPOC stations (PB01 – PB09, PB11, HMBCX, PSGCX and MNMCX) are equipped with permanent recording GPS receivers (TOPCON GB-1000). They continuously measure at sampling rates of both, 1 second and 30 second. Measurements at 30 second are stored externally - the memory last for at least 16 month. Measurements at 1 second are stored internally on a 'first in first out' basis and are transmitted jointly with seismic data - if data communication is available. 1-second-data are overwritten after 16 days in the absence of a communication link.

The GPS Metadata and Data Portal holds metadata and data for all cGPS installations in South America. This includes GPS time series from IPOC Partners and Associated Partners.

The metadata includes the full description of the sites, ownership, instruments and setup, including the detailed history of changes in the installations.The data may be visualised in the map view as velocity vectors of one or several stations that must be selected beforehand. In addition horizontal and vertical components of the GPS time series may be visualised in the "Plots" section of the portal (for one site at a time, please first select the station). Here, it is further possible to additionally visualise earthquakes together with the GPS time series.

GPS sites may be selected via the map or the list in the upper left. There are various filter options for selecting sites, data types and availability, magnitude and depth of earthquakes, GPS velocities, etc.

The Continuous Mode GPS data of the IPOC multiparameter stations PB01-PB09 and PB11 (managed by GFZ since 2002) have been published with a DOI by GFZ Data Services (http://doi.org/10.5880/GFZ.1.1.2017.001). Detailed information about data availability, metadata and site descriptions of the cGPS network at IPOC can be found here (subset of the GPS Metadata and Data Portal)

People involved: Zhigou Deng, Marcos Moreno, Anne Socquet, Chistophe Vigny, Frédérick Boudin, Markus Ramatschi, Jürgen Klotz, Günter Asch, Mitja Bartsch

Partner institutions: École Normale Supérieure, Université Montpellier 2, ISTerre, GFZ German Research Centre for Geosciences

Bedford, J., M. Moreno, B. Schurr, M. Bartsch, and O. Oncken (2015), Investigating the final seismic swarm before the Iquique-Pisagua 2014 Mw 8.1 by comparison of continuous GPS and seismic foreshock data, Geophysical Research Letters, 42(10), 3820-3828, doi.org/10.1002/2015gl063953

Béjar-Pizarro, M., A. Socquet, R. Armijo, D. Carrizo, J. Genrich, and M. Simons (2013), Andean structural control on interseismic coupling in the North Chile subduction zone, Nature Geoscience, 6(6), 462-467, doi.org/10.1038/ngeo1802

Li, S., M. Moreno, J. Bedford, M. Rosenau, and O. Oncken (2015), Revisiting viscoelastic effects on interseismic deformation and locking degree: A case study of the Peru-North Chile subduction zone, Journal of Geophysical Research: Solid Earth, 120(6), 4522-4538, doi.org/10.1002/2015jb011903

Metois, M., A. Socquet, C. Vigny, D. Carrizo, S. Peyrat, A. Delorme, E. Maureira, M. C. Valderas-Bermejo, and I. Ortega (2013), Revisiting the North Chile seismic gap segmentation using GPS-derived interseismic coupling, Geophysical Journal International, 194(3), 1283-1294, doi.org/10.1093/gji/ggt183

Métois, M., C. Vigny, and A. Socquet (2016), Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38°–18°S), Pure and Applied Geophysics, 173(5), 1431-1449, doi.org/10.1007/s00024-016-1280-5

Moreno, M., et al. (2012), Toward understanding tectonic control on the Mw 8.8 2010 Maule Chile earthquake, Earth and Planetary Science Letters, 321-322, 152-165, doi.org/10.1016/j.epsl.2012.01.006

Moreno, M., et al. (2012), Toward understanding tectonic control on the Mw 8.8 2010 Maule Chile earthquake, Earth and Planetary Science Letters, 321-322, 152-165, doi.org/10.1016/j.epsl.2012.01.006

Ruiz, S., M. Metois, A. Fuenzalida, J. Ruiz, F. Leyton, R. Grandin, C. Vigny, R. Madariaga, and J. Campos (2014), Intense foreshocks and a slow slip event preceded the 2014 Iquique Mw 8.1 earthquake, Science, 345(6201), 1165-1169, doi.org/10.1126/science.1256074

Schurr, B., et al. (2014), Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake, Nature, 512(7514), 299-302, doi.org/10.1038/nature13681

Socquet, A., J. P. Valdes, J. Jara, F. Cotton, A. Walpersdorf, N. Cotte, S. Specht, F. Ortega-Culaciati, D. Carrizo, and E. Norabuena (2017), An 8 month slow slip event triggers progressive nucleation of the 2014 Chile megathrust, Geophysical Research Letters, 44(9), 4046-4053, doi.org/10.1002/2017gl073023