|Observatories and Research Facilities for EUropean Seismology|
|Volume 4, no 2||September 2002||Orfeus Newsletter|
Coordinated seismic experiment in the AzoresGraça Silveira1, Suzan van der Lee2, Eleonore Stutzmann3, Luis Matias1, David James4, Peter Burkett4, Miguel Miranda1, Luis Mendes Victor1, João Luis Gaspar5, Luisa Senos6, Sean Solomon4, Jean-Paul Montagner3 and Domenico Giardini2
In collaboration with other running projects, this experiment has three goals: (1) study of the Azores plume: its structure, its origin at depth and its interaction with the Mid-Atlantic Ridge; (2) investigation of the structure and deep interactions of the plate boundaries between Eurasia, Africa and North America; (3) at a local scale, monitoring of seismic and volcanic activity for a better determination of hazard by combining broadband data with existing volcano and earthquake surveillance networks.
Several Broadband stations have been installed in the Atlantic area (e.g. BORG in Iceland, TBT in Canaries, ASCN in Ascens\343o) during the last few years, but the source-receiver geometry distribution is still insufficient to get a good azimuthal coverage of the wave paths beneath the Azores. One permanent VBB 3-component station has been installed in the Azores archipelago in 1997, the IRIS-GSN CMLA station of S\343o Miguel island which is co-operated by IM. The data will be available through internet at IRIS DMC. The COSEA experiment has been started for the purpose of better understanding the Azores area at different scales.
Figure 1. Geographical distribution of the COSEA VBB seismic stations.
COSEA has coordinated three individual experiments:
Figure 2. Deployment of the FLO (Flores) VBB seismic station; left - shelter used for the TITAN 3NT data aquisition system; right - preparing the STS 2 thermal insulation.
Figure 3. Deployment of the CDRO (Faial) VBB seismic station; left - digging the sensor vault; middle - installing the STS 2 in the concrete pier; right - preparing the TITAN-3NT acquisition parameters.
Figure 4. Left and middle - deployment of the PSCM (Terceira) VBB seismic station; right - PSJO (Pico) VBB seismic station after deplyment.
A first phase of data collection will run for a minimum one year, with the possibility of renewal for a second year, corresponding to the deployment period of the DTM/ETHZ experiment; a second phase of scientific investigation will last for a minimum of two years beyond the end of the data collection period. All stations are equipped with broadband digital equipment, with 24-bit ADC electronics and broadband (STS-2) sensors.Temporary Networks following their format specifications. The IRIS DMC will provide restricted access to the complete data set to all partners and to collaborating European data centers (but also with restricted access to the COSEA partners). Two years after the end of the field experiment, the data will be made available to the worldwide scientific community. The specific data to be jointly assembled (channels, events, continuous wave forms) have been defined by the scientific teams depending upon the scientific goals to be reached. Figure 5 shows an example of the an earthquake near Coast of Peru, recorded at CDRO seismic station.
Figure 5. Earthquake of 7th July 2001 near coast of Peru earthquake recorded at CDRO (Faial). Both channels MH (1.25 sps) and BH (20 sps) are presented.
Regarding the determination of the deep origin and structure of the Azores plume and its interaction with the Mid Atlantic Ridge, data collected from the permanent VBB stations and from the temporary VBB station will be processed using surface and body techniques. Fundamental and higher-mode surface wave tomography will also be used with earthquakes from the Mid-Atlantic ridge and from along the Eurasia-Africa plate boundary in order to image the uppermost-mantle structure in the region of the latter plate boundary.
This study will be complemented with other running projets at the CGUL. Using data from both VBB temporary network and from the local short period SIVISA network, we wish to study the seismicity pattern of the Azores islands in terms of: 1) crustal seismic anisotropy; 2) source mechanisms and 3) seismic strain rate.