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).