The University of Malta Seismograph Station at Wied Dalam
The University of Malta has operated various seismographs since the beginning of the 20th century, when seismographic instrumentation was in its early stages. A Milne-Shaw horizontal pendulum seismograph operated from around 1900 to the 1950’s. The seismograms from this instrument are still preserved within the Physics Department, and are indeed being used in a European project EUROSISMOS, which aims to collect, scan and digitize early seismograms belonging to several European institutions.
In 1977, a vertical long-period Sprengnether seismograph, with photographic recording, was installed in an old war shelter in the University grounds. This was replaced some years later by a three-component short period station with analogue paper recording.
The present station is a digital, 3-component, very-broadband (VBB) system. It was installed in June 1995 and consists of an STS-2 triaxial seismometer, and a Quanterra Q680 data acquisition system. The seismograph is housed in a disused tunnel in the limits of Birzebbugia. Its code name is WDD (Wied Dalam). The station forms part of MEDNET – a Mediterranean-wide network of such instruments, managed by the Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome.
The Mediterranean is one of the more seismically active regions in the world and represents a highly complex tectonic environment. MEDNET aims to contribute to the broadband instrumental coverage of the Mediterranean with the aim of improving further our knowledge of the structure and dynamics of this region, and thus work towards the goal of minimization of earthquake losses.
Since 2003 WDD has been upgraded to an online system, that transmits data in real-time to various international data centres. This has been achieved through participation of the Physics Department in an EU-funded project MEREDIAN, coordinated by the ORFEUS Data Centre in de Bilt, the Netherlands (www.orfeus-eu.org). The project came to an end in 2005.
The most important achievement of MEREDIAN has been the setting up of the Virtual European Broadband Seismic Network (VEBSN) which utilizes the SeedLink protocol for real-time transmission of digital seismic data between nodes in this network. The VEBSN incorporates a consortium of the major European seismological research institutions as well as a number of the new EU member states.
The Physics Department is also a member of the Euro-Mediterranean Seismological Centre ( http://www.emsc-csem.org) in France, whose major objective is to establish and operate a system for rapid determination of European and Mediterranean earthquake epicentres and transmitting these results immediately to the appropriate international authorities and to the members in order to meet the needs of protection of society, scientific progress and general information;
The aims of the Malta seismograph station are:
- to continuously monitor and interpret seismic activity in the central Mediterranean, in particular around the Maltese islands, and identify active faults in the sea bed of the Sicily Channel.
- to use this information toward the eventual assessment of the seismic hazard in the Maltese Islands. This assessment will in turn provide needed input to various agencies, such as the Civil Protection unit, insurance agencies, urban planners, etc.
- to enhance the epicentral location capability in the Central Mediterranean
- to contribute to the constant global accumulation of seismic data and to the worldwide coverage of seismic events by as dense as possible a network of digital seismographs. Such coverage continues to yield ever more accurate information about the Earth’s structure
- to make use of the data for further geophysical/geological studies of the Mediterranean basin, in particular the Central Mediterranean.
Since the station’s installation, a significant database has been built up of events that have occurred round the Maltese islands from seismic activity occurring offshore in the Sicily Channel and Malta Escarpment. A study of this seismicity is important because it throws light on the activity and seismogenic potential of offshore fault systems. Many of these are too small to be detected by other European or North African stations, and therefore the presence of a sensitive seismograph on Malta is essential for producing a true picture of the seismicity affecting the islands. This situation also prompted the Seismic Monitoring and Research Unit to develop a single-station location algorithm to get an estimate of the epicentre (latitude and longitude), as well as the exact origin time.
Listed earthquake locations are generated through various steps and several tools are used to help personnel make a final judgement on the authenticity and accuracies of earthquakes.
Daily automatic analysis is at 4am local time while manual verification of earthquakes is conducted within 24 hours. Best efforts are made to locate earthquakes recorded on WDD giving a high priority to earthquakes occurring in the Sicily Channel. Stations CEL and IDI have been included in the system to help locate better regional earthquakes. Small local earthquakes recorded on CEL and IDI are not considered of important relevance for the SMRU operations and are most of the time left unverified.
Each station is processed independently using LESSLA producing a list of events for a particular day. Events are added to a central database which can then be viewed from the ‘For Seismologist Only’ section. Please note that this automatic analysis may give false or incorrect earthquake detections and locations.
Single-Station Earthquake Location
Events listed in the database are categorised as either an Earthquake or a Blast. P and S picks are manually checked and adjusted to calculate the distance from a WDD calibrated S-P travel-time graph. The station-to-epicentre direction (back-azimuth) is selected from the most stable azimuth of the three different streams: LH (1 sample per second), BH (20 samples per second) and HH (80 or 100 samples per second). Back-Azimuth estimations are produced by LESSLA and compared to SeisGram2k. The correctness of the back-azimuth depends highly on the P pick accuracy and on the signal-to-noise ratio of the seismogram.
The SMRU have set three confidence grades for the manual location of earthquakes. Quality grade (A) indicates a very confident location with an agreement between BH and HH azimuth approximations or has a good LH solution. Quality (A) earthquakes will also have certain P and S picks. A quality grade (B) means that there is little or no confidence in the epicentre location usually resulting from different BH and HH azimuth approximations. A quality grade (C) is set to unreliable earthquake locations with poor or no P pick.
Multi-Station Earthquake Location
Listed events from different stations are at times referring to the same earthquake. These epicentres are viewed on a map simultaneously helping personnel select a more precise epicentre. This epicentre is most of the time the selected location for verified earthquakes.
Verified and Published Earthquakes
When the information available is reliable an earthquake is verified and marked in red in the ‘For Seismologist Only’ section. Verified earthquakes are published on the public pages, Welcome Page and Recent Earthquake List, depending on public relevance.
LESSLA stands for Local Earthquake Single Station Location Analyser. It is a system developed originally for an M.Sc. dissertation. It consisted of the design and implementation of an automated system for detecting, identifying and locating local and regional earthquakes, using 3-component single-station polarization analysis.
The system utilises the three components (north, east and vertical) from each of the three sampling streams available from the station data acquisition system, to identify, and pick, major arrivals such as P and S. Local and regional earthquake distances are calculated from measured S-P times, while event azimuth is estimated using wave polarization from three components. The system analysis one whole day of data and issues a detailed daily bulletin, containing: pick times, event distance, azimuth and location, magnitude, epicentre map, PDF files of event seismograms, links to relevant international bulletins and other analysis information. The report may also be sent by email, allowing the user to visually analyse the event seismograms and make judgement on the location reliability.
LESSLA has enhanced considerably the daily routine analysis of seismic data for WDD. It has also helped identify earthquakes which are not listed in international online bulletins or inaccurately located because they are poorly recorded on any station other than WDD, on Malta.
The project runs on Linux and communicates with several other programmes such as Seismic Analysis Code (SAC) and the Standard for the Exchange of Earthquake Data (SEED).
In March 2015 the Seismic Monitoring and Research Unit purchased state-of-the art equipment
These instruments are used to establish the Malta Seismic Network, and also complement the increasing number of sophisticated geophysical equipment now used by the SMRU to conduct its research.
The network will consists of three broadband seismographs located in Malta and Gozo, together with other temporary installations. The new seismic stations, which continuously transmit recorded signals to the University data centre, help locate regional earthquakes more accurately, study local seismic site effects, and also enhances real-time earthquake monitoring for the central Mediterranean.
Funding for the equipment came from the Integrated Civil Protection System for the Italo-Maltese Cross-Border Area (SIMIT) project and European Research Development Funds (ERDF 310 – Expanding the Physics and Applied Interdisciplinary Research Capabilities at the Faculty of Science).