|Observatories and Research Facilities for EUropean Seismology|
|Volume 4, no 1||May 2002||Orfeus Newsletter|
The DESERT passive seismic experiment 2000/2001 in the Middle-EastM. Weber1, K. Abu-Ayyash2, A. Abueladas2, H. Al-Amoush1, A. Agnon3, Y. Bartov4, M. Baumann5, Z. Ben-Avraham6, G. Bock1, J. Bribach1, M. Daoud2, R. El-Kelani7, A. Förster1, H.J. Förster5, U. Frieslander8, Z. Garfunkel3, S. Grunewald1, H.J. Götze9, V. Haak1, Ch. Haberland1, A. Hofstetter8, M. Hassouneh2, A. Hördt10, K.-H. Jäckel1, D. Kesten1, R. Kind1, N. Maercklin1, J. Mechie1, A. Mohsen1, M. Neubauer10, R. Oberhänsli5, I. Qabbani2, O. Ritter1, G. Rümpker1, M. Rybakov8, T. Ryberg1, F. Scherbaum5, A. Schulze1, S. Sobolev1, M. Stiller1, J. Stoll11, H. Thoss1, U. Weckmann1, K. Wylegalla1.
The aim of the interdisciplinary and multi-scale Dead Sea Rift Transect (DESERT) project (DESERT Group, 2000) is to shed light on the question of how large shear zones work. DESERT consists of several geophysical sub-projects that are carried out by partners in Germany, Israel, Jordan and Palestine. Principal investigators are Michael Weber in Germany, Zvi Ben-Avraham in Israel, Khalil Abu-Ayyash in Jordan, and Radwan El-Kelani in the Palestine Territories. One of the sub-projects was a large-scale passive seismic experiment which was conducted in Israel, Jordan, and the territory of the Palestinian Authority. Aims of the project are (a) study of crust and mantle structure with the receiver function (RF) method, (b) travel-time tomography, (c) to investigate azimuthal anisotropy in crust and upper mantle from shear wave splitting, and (d) the study of local seismicity. In this note, we give a brief overview on the field experiment and the data archiving procedure.
Figure 1. Station distribution of the DESERT passive seismic array.
The passive seismic experiment was organized by GeoForschungsZentrum Potsdam (GFZ), Germany. The data loggers and seismometers were provided by the GFZ Geophysical Instrument Pool. The following persons from GFZ participated in the actual fieldwork: Günter Bock, Rainer Kind, Ayman Mohsen, Georg Rümpker, Kurt Wylegalla. Other participants came from the Geophysical Institute of Israel, Lod, Israel (Rami Hofstetter); Natural Resources Authority Amman, Jordan (Abdel-Qader F. Amrat, Walid Abdel-Hafiz, Muhamed Hijazi, Bassam Al-Bis, and Khamis Rizik); An-Najah University, Nablus, Palestinian Authority (Radwan El-Kelani, Ayman Mohsen).
All seismometers were three-component. Mark L4-3D short-period sensors were used. Broad-band seismometers used in the experiment were 12 Guralp 40-T, 8 Guralp CMG-3T, and 9 Streckeisen STS-2. All stations were equipped with Reftek data loggers, and recording was continuous in compressed mode at 50 Hz sample frequency. Depending on the noise conditions, about 20-30 Mbyte of data were accumulated per station per day. The data were stored on disks whose capacity varied between 2 and 4 Gbyte. Service visits to the stations were carried out once every 3-4 months.
Seismic stations were powered by one or two 12V batteries of 60 Ah capacity each that were recharged by solar panels of 50-60 Watt capacity, or by an electrical charging unit in cases where 220V mains power was available at the site. A regulator was used to switch off the data logger if the voltage fell below 11.8V. This prevents drainage of batteries in cases where recharging of the batteries failed. The data logger switches on again automatically as soon as the battery voltage reaches 12.6V. For safety reasons stations were set up mainly at police stations, existing sites for the national seismograph networks, schools, government offices, and water reservoirs. The amount of vandalism and losses by theft after more than 1 year of field operation were consequently relatively minor. Unfortunately, as a result of the deteriorating political situation, the stations in Gaza and Hebron could not be maintained after August 2000.
Back in the labarotory at GFZ Potsdam, the data were read from the tapes, quality checked and converted to 24-hour day files in Miniseed format using the extr_file routine written by W. Hanka. The 24-hour files are stored as zipped tar files in the GEOFON data archive. Full Seed volumes can be extracted via breqfast requests from the archive.
At this time, the data are for the exclusive usage of the DESERT group. It is anticipated that access to the data will be open three years after the end of the field experiment, i.e. June 2004. Data requests can then be submitted via the GEOFON web page at GFZ Potsdam.
Figure 2. Record example of a local earthquake (ML = 2.2) on May 5, 2000, 21:41:36.1 UTC, in the Arava valley. Focal depth was 18 ± 2 km, i.e. at the base of the upper crust. Interpretation of P- and S-arrival in a Wadati diagram revealed a ratio of 1.74 for the VP/VS ratio.
Figure 3. Example of receiver function processing for the Sulawesi MW = 7.4 earthquake of May 4, 2000, 04:21 UTC origin time and recorded at a temporary broadband station of the DESERT passive seismological experiment located in Jordan at 89° epicentral distance and back azimuth 92°. The components are indicated on the left-hand side of the panel. Amplitudes of N, E, Q and T are enlarged threefold relative to Z or L. L, Q, T is a ray-based coordinate system with the P-wave mainly on the L-component, and SV and SH on the Q- and T-component, respectively. Traces a-c are the raw data; d-f bandpass-filtered in the frequency band 0.02-0.2 Hz, this step reduces the high-frequency parts of the signal; g-i traces after rotation of d-f to the LQT coordinate system; j-l LQT traces after deconvolution with the P wave of the L-component seismogram (trace g). The source-equalized receiver function is given by the Q-component seismogram trace k. Some phases in trace k have been marked: 1 = Ps phase from the Moho beneath the station; 2 = Ppps multiple between Moho and surface; 3 = Ppss multiple between Moho and surface. The strong negative phase directly arriving after 1 is probably a P-S conversion from a discontinuity in the mantle where velocity decreases with increasing depth.