Environmental Agency of the Republic of Slovenia, Seismology and Geology Office , Dunajska 47/VII, 1000 Lubljana, Slovenia
SNRS (Seismic Network of the Republic of Slovenia)
After the M=5.8 earthquake on 12 April 1998 in the Krn mountain region the Government of the Republic of Slovenia secured the funds and appointed the Seismology and Geology Office (USG) of the Environmental Agency of the Republic of Slovenia (EARS), at that time organized as Geophysical Survey of Slovenia, to build a new national seismic network. After comprehensive site selection studies and an international bid for the instrumentation the network has gradually been built with the first new stations put into permanent operation at the beginning of 2002. At present (December 2006) 23 new permanent seismological stations are operating (Fig. 1) while three more are already built but not yet connected. All are broadband stations sending data in real time to the SNRS (Seismic Network of the Republic of Slovenia) data centre at the USG in Ljubljana. The standard equipment is Quanterra Q730 datalogger and Guralp CMG-40T seismometer. To secure on-scale seismograms in case of a strong earthquake five stations have also Episensor accelerometer (Tab.1). Due to thick weathered surface layer four stations use shallow borehole seismometres. Sampling rates are 200sps, 20sps and 1sps. At each site Flash memory in Q730 accommodates approx. 90 min data as data buffer in case of downlink. At the end of the project of the modernization of the SNRS, the network will consist of 26 broadband stations (Fig.1). For the financial reasons, lower quality and narrower band Guralp CMG-40T seismometers were originally purchased. Afterwards, three Streckeisen STS-2 and one Guralp CMG-3ESP seismometers were bought and installed.
The USG still operates old six station Nanometrics digital network, a few portable stations and a small network of 8 digital strong motion instruments.
(Near) real-time data transfer
All stations of the SNRS are protected by the firewall of the Governmental INTRANET. Real time data acquisition is done using the Antelope software package. The data from most stations is transferred by use of the leased phone lines and from seven locations by use of the HSCSD mobile phone connection. In the Data Centre in Ljubljana the data are integrated with the data of the old Nanometrics digital six station network (Fig. 2).
After the colaboration agreement was signed we are also receiving the data in real time from the selected stations from Central Institute for Meteorology and Geodynamics (ZAMG), Austria, the Department of Earth Sciences of the Trieste University (DST) and from the National Institute of Oceanography and Experimental Geophysics (OGS) from Trieste (Fig.2). Using Antelope orb2orb connections (orb = object ring buffer) we receive the data from the DST broadband network, as well as from OGS broadband and short period networks. We also installed guralp2orb process that collects data from several stations in Croatia. This data exchange is the result of the activities within the EC MEREDIAN project and the waveforms are regularly forwarded to the ORFEUS data centre. Some of the SNRS stations are part of the European VEBSN.
Table 1. Seismic stations and instrumentation of the SNRS.
Figure 1. Seismic Network of the Republic of Slovenia (SNRS).
The two basic conditions for fluent data processing and archiving are good data flow and flawless operation of all the computers involved. The data flow is influenced by network traffic and by occasional remote hardware malfunction. When this is the case, a swift reaction is important since it can significantly reduce data loss.
Data flow is being monitored by periodic five minute reading of the latency of the data coming to the main ORB. The latencies are then checked against the conditions with increasing severity and a SMS message is sent to several recipients when some or all data latencies exceed a certain amount of time (typically some minutes). To ensure SMS delivery we use two service providers and two GSM modems attached to two workstations. In case of failure non-delivered SMS messages are thus automatically forwarded to another provider.
The network software runs on three Sun workstations. That enables us some redundancy in most critical functions as well as simple replacement in case of major failure in the acquisition server. One of the workstations is configured in-between the firewalls and is dedicated to data exchange tasks. The status of the workstations is being periodically monitored with several tests. Main methods are:
- each machine checking that all the others are on and that acquisition and alerting software is running,
- moving failed SMS to the send queue of the other of the two computers equipped with wireless modems and notifying if the number of failed SMSs exceeds a specified threshold.
Besides real-time problems detection, regular check of log and message files is essential.
Figure 2. Schematic representation of data acquisition and exchange at the SNRS data centre.
Data loss of the SNRS network is shown in figure 3. In average, there is 6.6% of unretrieved data for HH channels with 200 Hz sample rate, mostly as occasional dataloss of a single station for a period of several days. The difference of data loss between HH channels and BH channels with 20 Hz sample rate is related primarly to communication problems. Typically, the Quanterra ORB can hold approximately one hour of HH channels data whereas BH channels data are retrievable also after more than 10 hours of communication breakdown. The larger data loss in the period between June and August 2005 for example is due to three stations (out of 21) not operating most of the time.
Figure 3. Percentage of data loss of Seismic Network of the Republic of Slovenia between January 2004 and December 2006. HH channels represent 200 Hz sample rate data and BH channels represent 20 Hz sample rate data.
Data analysis, storage, archiving and availability
The main task of the SNRS is fast alerting automatic procedure. Automatic detection and location algorithms are part of the Antelope Real Time System (ARTS) package. Antelope algorithm for automatic location of earthquakes uses pre-computed travel times for a given velocity model and a number of grids. When an event occurs, the node of the grid which is in accordance with the largest number of detections and gives the lowest RMS is reported as a location of the event. In comparing automatic and manual locations one has to bear in mind that manual locations done with HYPOCENTER cover the space uniformly. With the grid of 2 by 3 km for automatic location of local events even in an ideal case, where automatic epicentre is always selected at the node which is closest to the manual epicentre, the average distance between these two epicentres would be about 1 km.
Although the configuration and the detection and location capabilities of the network are continuously changing by adding new stations, some preliminary results of the location accuracy can be evaluated. For the period between 1 January 2004 and 30 June 2005 we made analysis for the restricted area lying inside the polygon formed by the stations that were operating during the whole interval (CEY, OBKA, PERS, GROS, GOLS, GCIS and CRES).
We compared 351 events with both automatic and manual epicenter calculated. Median distance between manual and automatic location was 5.15 km while 56 events had a distance greater than 25 km (Fig 4).
Figure 4. Relative mislocations of the automatic location algorithm – automatic locations are plotted relative to the manual location which is considered as the co-ordinates origin (0,0).
Collected data are daily manually analysed with the Antelope software package. For detailed analysis of teleseismic events we use Seismic Handler. The waveform data is stored in the Antelope database and available through autodrm service (firstname.lastname@example.org). The continuous data are daily automatically archived on DLT tapes and selected event segments are periodically archived on CDs. The parametric data are stored locally in the SEISAN database (from the year 1996). Our final monthly bulletins are also available in the ISC database.
Seismology and Geology Office regularly contributes parameter and waveform data to the international seismological data centres. We send automatic detections to EMSC, our weekly bulletins to neighboring countries, EMSC and NEIC, and final monthly bulletins to ISC.