Hardware and Software of the Hong Kong Short-period Seismograph Network

The equipment and software of the network can broadly be divided into three parts: the seismograph stations, the central data reception and processing system, and the central data analysis system.

Each seismograph station is equipped with a robust seismometer, a digitizer and an interface cabinet.  The seismometers used in all eight stations are piezoelectric sensors which provide velocity and acceleration outputs. They have a dynamic range of about 100 dB, frequency response of 1 to 30 Hz, and are capable of detecting ground motions as small as 10^-9 metres. The seismometers can be placed at any angle and are suitable for installation in boreholes.  Signals recorded by the seismometers are converted to digital signals by the digitizer at a rate of 100 samples per second and time-marked by a Global Positioning System (GPS). The signals are then transmitted through a dedicated telephone line at 9600 baud to the central station at the Hong Kong Observatory for data storage and analysis. The transmission of data in digital form lowers the chance of signal degradation.

The central data reception and processing system receives and stores the incoming data automatically. Sample data from seismological stations are ingested into the processing system for storage as continuous files of 15 minutes each.  The system automatically calculates the short term average (STA) and the long term average (LTA) of seismic signals of each station.  If the ratio STA to LTA exceeds a pre-set value, the station is said to be triggered.  If three stations are triggered simultaneously, the data is automatically stored as an event file and transferred to the computer workstation of the central data analysis system for further analysis.

The central data analysis system consists of a computer workstation connected to the data processing system through local area network.  Event files are transferred to the data analysis system for analysis using the SEISAN software package.  SEISAN consists of a set of computer programs and a database.  It can analyze both analogue and digital data.  It possesses automatic phase picking and magnitude calculation functions and can readily determine the location and depth of epicentre for plotting on a map.