Present-day kinematics at the Azores-Gibraltar plate boundary from GPS observations R.M.S. Fernandes (PhD student)
[email protected]
DEOS, Faculty of Aerospace Engineering, Delft University of Technology The general objective of this Ph.D. research project is to model the present-day tectonics of the AzoresGibraltar region (i.e., the western segment of the Eurasia-Africa plate boundary) using Global Positioning System (GPS) data. It is investigated with particular detail the driving tectonic forces that are presently acting in the Azores Archipelago region. This area is known as the Azores Triple Junction (ATJ) since it is the confluence of three major tectonic plates: Eurasia, Africa and North America. This study expects to contribute for the understanding of the geophysical phenomena influencing this region. The acquired information may possible help to improve our knowledge about the natural hazards in the Azores-Gibraltar, which affect permanently the safety of the people living there. Introduction The improvement of modern spatial geodetic techniques in the recent years, particulary GPS, has provided a very important contribution for a better understanding of contemporary motion of rigid tectonic plates and deformation areas. In the last years, several plate kinematic models were computed based on several spacegeodetic data sets. A continuous improvement in the quality of these models is observed due to the existence of more data, with larger time-spans and a better coverage of the major tectonic plates. However, although these models can describe the motion of the stable plates reasonably well, they only provide boundary conditions on the current deformations that occur in the plate boundaries. For these regions, specific models has to be produced. One of these regions is the so-called Azores-Gibraltar. Since 1988, several geodetic campaigns (GPS and gravimetric) have been carried out in this region in the frame of TANGO project. Furthermore, the network of continuous operating GPS stations are growing permanently. The major goal of this research is to derive and interpret an extensive velocity field for the Azores-Gibraltar region by processing the complete available data set of GPS observations (see Figure 1-left for the location of the Azores sites). Consequently, the investigation of the best approach to estimate the site motions from the GPS signals in a consistent reference frame is a fundamental key in this research project. Results in 2001/2002 During this period, it has been continued the research on applying and refining the most recent methodologies to process and analyze continuous and sporadic GPS data. Concerning the continuous data, it has been implemented the necessary methodologies to setup and automatically process two GPS permanent networks in the Iberian region. One of them was created after the invitation, made to our group by the Central Bureau of the European Reference Network (EUREF), to become an official Local Analysis Center. In the Azores Archipelago, one permanent station has been installed in the Terceira Island in October 2001 and sporadic data acquired during two campaigns that were carried out in April and October 2001. These campaigns were processed and their results included in the analysis of the velocity field for Azores (see Figure 1-right). Furthermore, it has been developed a new plate tectonic model, based in the latest ITRF realization: the ITRF2000 solution. This model (DEOS2k) is being used in the analysis of the discrepancies between the Azores-Gibraltar sites and the stable Eurasian, Africa and North American plate velocities, because it has been proven to be more precise than other models for the most of the geodetic sites.
32˚W 40˚N
30˚W CORV FLOR
28˚W
26˚W
24˚W
Mid Atlantic Ridge GRAC JORG FAIM
40˚N
TERC
CORV
EURA
FLOR GRAC
PICO 38˚N
JORG
NOAM SMIB
Graciosa (3)
FAIM
TERC PICO
38˚N
39˚N
SMAR
Terceira (22) Faial (20)
Permanent since Oct 2000
29˚W
SMIB
Horizontal Velocities w.r.t. Africa
S. Jorge (6)
36˚N
Gloria Fault
Permanent since Oct 2001
Pico (13) 28˚W
SMAR
DEOS2k
Installed in 1988 Installed in 1999 Installed in 2001
NUVEL
AFRC
5mm/yr (95% confid. ellipse)
27˚W
32˚W
30˚W
28˚W
26˚W
36˚N 24˚W
Figure 1. On the left: GPS sites with available data in the Azores Archipelago (re-)observed in the frame of this Ph.D. project in 2001. On the right: horizontal relative motions for the significant stations (with more than 2 years of time-span) with respect to stable Africa (as predicted by DEOS2k and NUVEL-1A) for the initial TANGO sites in the Central and Oriental groups and predicted motions for points located in stable Eurasia. In the Azores Archipelago, it is possible already to conclude that the relative displacement between Eurasia and Africa along the Azores plate boundary is mostly accommodated within a relatively short lithospheric band that follows the islands emplacement and the neighboring topographic ridges. End members of this system appear to be the S. Maria and Graciosa islands, which display ”African” and ”Eurasian” behavior, respectively. All other sites are located along the presently active interplate deformation zone. Concerning the Gibraltar region, some trends can already be extracted from the analysis of the permanent network. The Central and Eastern parts of Iberian Peninsula appear to be part of stable Eurasia. The DEOS solution indicates a significant westward motion of the western stations with respect to the rest of Iberia, suggesting that this region is in the deformation zone between the African and Eurasian plates. Contributed papers in 2001/2002 Publications Fernandes R.M.S., J.M. Miranda, J. Catal˜ao, J.F. Luis, L. Bastos and B.A.C. Ambrosius (2002), Coseismic Displacements of the MW =6.1, July 9, 1998, Faial Earthquake (Azores, North Atlantic), Geophys. Res. Lett., 29, doi:10.1029/2001GL014415. Fernandes R.M.S. and B.A.C. Ambrosius (2001), Analysis by DEOS of the SW Part of the EUREF Network, Reports on Geodesy No. 3, Institute of Geodesy and Geodetic Astronomy, Warsaw, Poland, 58, 35-42. Presentations Fernandes, R.M.S., L. Bastos, B.A.C. Ambrosius, R. Noomen, W. Simons, J.M. Miranda and W. Spakman, Defining the plate boundaries in the Azores region, 11th General Assembly of the Wegener Project, Athens, Greece, 16 June, 2002. Fernandes, R.M.S., L. Bastos, B.A.C. Ambrosius and R. Noomen, The permanent GPS network in the Iberian Peninsula, XXV EGS General Assembly, Nice, France, 25 April, 2002. Fernandes, R.M.S., B. Ambrosius, L. Bastos, R. Noomen, W. Spakman and S. Matheussen, Kinematics of the Azores Triple Junction in the frame of Global Plate Tectonics, IAG 2001 Scientific Assembly, Budapest, 6 September, 2001.
