Broadband Station Network ROA/UCM/GFZ in South Spain and Northern Africa E. Buforn, A. Udias Departamento de Geofisica y Meteorologia, Universidad Complutense, Madrid J. Martin D vila Real Observatoriode la Armada, San Fernando, Spain W. Hanka GeoforschungZentrum,Potsdam, Germany A. Pazos Real Observatoriode la Armada, San Fernando, Spain The seismotectonic importance of the Eurasian-African plate boundary in the western part of the Mediterranean region near the Iberian peninsula has motivated the installation of a new network of broadband and seismographic stations in this area. At present, the network consists of four stations located in San Fernando (C~idiz), SFUC; Cartagena, CART; Mah6n (Menorca), MAHO; and Melilla (North Africa), MELI (Figure 1). The first station, SFUC, was installed in 1996 and the rest from 1997 to 1999. There are plans to install a fifth station in Ceuta (North Africa). The stations are run by a joint operation of Real Observatorio de la Armada, San Fernando, C~idiz (ROA), Universidad Complutense, Madrid (UCM); and GeoforschungZentrum, Potsdam, Germany (GFZ). The stations are part of the GEOFON Permanent Network of broadband seismographic stations, and their data can be obtained through the GEOFON Data Center (Hanka and Kind, 1994). The Institute of Geophysics of the ETH in Zurich participated in the operation of the MELI station until December 2001. At every station there are also permanent GPS stations, which have been operating since 1995 in SFUC, 1997 in CART, 1998 in MELI, and 1998 in MAHO (Martfn-Davila et al., 2000). The stations are instrumented with three-component Streickheisen seismometers, STS-2's, and Quanterra Data Acquisition System Q380 in SFUC and MELI and 0._4124 in CART and MAHO. The systems have four channels of digital data (VH, LH, and BH) with sampling rates of: 0.1, 1, and 20 Hz at all stations. Additionally, SFUC and MELI have a channel (HH) of 80 Hz, and CART and MAHO of 100 Hz. Channels VH, LH, and BH record continuously on tape, while channel H H is triggered when signal amplitude exceeds a predetermined threshold value. Stations can be called from Madrid, San Fernando, or Potsdam via telephone modem. At present the system is being changed to establish a direct connection through the Internet. The response curve of the system for the four stations is flat to ground velocities between 0.001 Hz and 100 Hz. Stations SFUC, CART, and MAHO are installed inside tunnels at Spanish Army and Navy facilities, while MELI is at an unused mine gallery. The GPS stations consist of "Trimble 4000 ssi" receivers and Choke-ring type antennas. Stations are linked to ROA via telephone modem. The GPS station at San Fernando (SFER) has been an International GPS Service (IGS) station since 1996. Apart from the microseismic peaks, which are higher than average due to the site locations close to the sea, the stations, with the exception of MELI, show rather low noise levels. The location of MELI may be changed in the future. As examples of the quality of the stations, Figures 2 and 3 show two records of earthquakes at teleseismic distance at MELI and a local event at CART. An analysis of the seismic noise has been made for SFUC for the complete year of 1997 (Cesca et al., 2001) using the IRIS method for broadband stations based on the power spectrum energy (IRIS, 1993). The robust estimation of the noise level, as shown in figure 4, for periods between 0.2 and 1,000 seconds has the same characteristics and falls between the maximum and minimum acceptable levels according to the generally adopted standards of Peterson's curves (Peterson, 1993). Typical single- and double-frequency peaks (fundamental and first harmonic of oceanic noise) are found for approximately 7 and 14 seconds Seismological ResearchLetters March/April2002 Volume73, Number2 173 _10 ~ _5 ~ 0o 5~ 45 ~ 45 ~ SPAIN 40 ~ 40 ~ MAHO 0 c T 35 ~ 35 ~ MOROCCO _10 o _5 ~ 0o 5~ ,A Figure 1. Location of the ROA/UCM/GFZ broadband stations. Circles represent installed stations and the triangle the planned station to be installed at Ceuta. EW, NS and Z components for MELI station 23//06/01; 20:44:44 ' 1 - • I I • _ ~ , , , i I , , , , ' I ' ' ' ' I ' ' ' i ~ ~ ~ ~ i , ~ ~ ~ i , , ~1~ 10 15 I I I I I I I - i , ~ , ~ i , ,- 2-- 0 O -2 X 0 5 20 25 30 X 10+2 A Figure 2. Seismograms from the MELI station of the three components (EW, NS, Z) of the LH channel for the earthquake in Peru, 23 June 2001, M = 7.9, epicentral distance 9,343 km. The horizontal scale is given in seconds and the vertical in counts (one count equals approximately a nanometer/second). 174 Seismological Research Letters Volume73, Number2 March/April 2002 EW, NS and Z components for CART station 22//04/01 . 17:29:48 ~ o "r,-r, I "W"~IllP ",:::t- .-.to - 5 _ I I I 4 2 0 2 "~-4 (2) "- 6 2 -''''1'''''''''''''''''''''t 0 ~-2 o X 10 20 30 40 50 60 ~i, Figure 3. Seismograms from the CART station of the three components (EW, NS, Z) of the HH channel for the local earthquake in Alicante, Spain, 22 February 2000, M = 3.3, epicentral distance 51 km. --80 I - a _ l o o I I .... I I I II 1 ...................... , . . . . I . . I . I 11 II I . I I I t 111 I I I I I I II I I I I Jllll ,. .............................................................................................................................................. -r,r" "'-, ~ 1. +......", ..................................................... i.............................................. -/- .............i............................... -,-....................... ~.................... i ~ ,'" "i', -120 ............................................................ '.............. .,,'" ', - 1 4 0 ................................................................... ..'..................................... .-, ,.~[ ....J..................................... ~........~............................................................... i ............../ - E ................. /,,/,/ ; - - 1 6 0 ...................................................................................................................... O ." LU or) _ 180 13_ m Station S F U C -- .................................................... 0-24h i i , .. ,, !,- .................................................... - ....................... -'.~...................................... ,............................................................ .j..f- ................................................................. ; ................... L-_ 111/97-31112/97 Robust Spectra C o m p o n e n t noise 200 I 1 0-1 I I I I I I i I 10~ I I I I 1 I I I I I I I I 1 01 I I I I ! 102 Period I I I I It i I I 103 I I i I I I 104 (s) ,& Figure 4. Power spectrum energy curves for the seismic noise at BB Station SFUC (Spain) during 1997. Continuous lines correspond to the three components (NS, EW, and Z)and dashed lines to Peterson's curves (Cesca, 2001). Seismological Research Letters March/April 2002 Volume73, Number2 175 periods. The vertical component shows higher noise levels than the horizontal for periods less than 2 seconds and lower for larger periods. Higher estimations of noise levels for periods larger than 40 seconds are due to the lower amount of data of the VH channel used in the analysis with respect to BH and LH channels. A spike present at 0.4 seconds is due to a local site effect. For periods above 0.6 seconds higher noise levels are observed in fall and winter, and the lowest levels are observed in summer. Daily analysis reveals maximum noise levels during daytime hours from 6:00 AM to 6:00 I'M, and minimum from midnight to 6:00 AM. The influence of oceanic storms and atmospheric pressure changes on daily and seasonal variation of noise levels has been observed (Cesca et al., 2001). Continuous data for the channels VH, LH, and BH are stored in the G E O F O N Data Center and may be obtained in SEED format through its Web site (http:ffwww.gfz-potsdam.de/geofon/www_req/) or by "breq_fast" e-mail requests (breq_fast@gfz-potsdam.de). Event data for earthquakes with magnitudes larger than 5 are available with minimum delay from SPYDER databases at G E O F O N and ORFEUS (http:/ /offeus.knmi.n[/) Data Centers in SAC format. For earthquakes of magnitudes less than 5, data may be obtained from UCM or ROA in SAC format. A catalogue of wave forms for local and regional earthquakes with magnitudes lower than 5 in the period 1997-2000 has been collected. The highest number of local events is recorded at CART and the lowest at MAHO. For more information you may contact E. Buforn (figeoO7@sis.ucm.es) or J. Martfn D~ivila (mdavil.a@roa.es). REFERENCES Cesca, S., E. Buforn, and J. Martfn Dfivila (2001). Analysis of seismic noise at SFC BB station (Spain), European Geophysical Society, XXVI General Assembly, Nice, March 2001. Hanka, W. and R. Kind (1994). The GEOFON Program, Annali di Geofisica37, 1,060-1,065. IRIS (1993). Federation of Digital SeismographNetworks Station Book, IRIS, 203 pp. Mart[n-D~ivila, J., J. Garate, A. Pazos, M. Catalan, A. Udfas, M. Berrocoso, W. Hanka, E. Buforn, P. Pdrez, J. Fermindez, J. Prian, J. Quijano, J. A. Pefia, J. Gallego, and G. M. Delgado (2000). Geophysical/geodetical activities of the Royal Naval Observatory in San Fernando (ROA) to study the Eurasia-Africa plate boundary zone in the Ibero-Maghrebian region, The Tenth General Assembly of the WEGENER Project (WEGENER 2000) San Fernando (Boletin ROA 3/2000), 72-75. Peterson, J. (1993). Observation and Modeling of Seismic Background Noise, USGS Open File report 93-322, 95 pp. ACKNOWLEDGMENTS This work is partially supported by Ministerio de Ciencia y Tecnologfa, Project REN 2000-0777-C02. 176 SeismologicalResearchLetters Volume73,Number2 March/April2002 Departamento de Geoflsicay Meteorologla Universidad Complutense Madrid Spain figeo12@sis.ucm.es (E.B., A. U.) Real Observatorio de la Armada San Fernando, Cddiz Spain (J.M.D.) GeoforschungZentrum Potsdam Germany ~.H.) Real Observatorio de la Armada San Fernando, CAdiz Spain (A.P.)