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Sri Lanka Earthquake & Tsunami Warning Training Program. Session III.3 Global and Local Arrays April 5, 2006 CETRAC, Sri Lanka. Topics covered:. Types of seismic arrays How arrays are used Example global networks. Seismic array :. Has common time base (one clock)
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Global and Local Arrays
April 5, 2006
CETRAC, Sri Lanka
An array can be used as an antenna to determine the direction from which the seismic waves arrive. This process, called beamforming, tells where the earthquake is located.
In a network, the location of an earthquake is found by triangulation. Information from all stations is used.
High frequency waves attenuate (damp out) rapidly with distance. Low frequency waves diminish in amplitude more slowly with distance traveled.
IRIS GSN SYSTEM
Global networks choose instruments capable of recording long period waves. Local arrays may use short period sensors to record local earthquakes.
The 2004 Sumatra-Andaman Isl quake radiated very long period seismic waves that were recorded by global networks.
- IRIS/IDA Group at IGPP, Scripps Institution of Oceanography
University of California, San Diego
- University Networks and Affiliates
IRIS/USGS GLOBAL NETWORK
Courtesy of Adam Dziewonski, Harvard University
Courtesy L. Kong
The quietest stations in the world are on continents.
Remember the DAS?
The DAS converts electrical current from the seismometer into numbers to be processed by the conputer.The electrical current from the seismometer is continuous, but the DAS produces numbers only at set intervals of time, the digitization rate. The DAS bundles these numbers into a packet called a data record.
How the DAS bundles the data depends upon the computer programs running inside the DAS.Some DASs produce a record containing data for a fixedperiod of time. Other DASs produce a record of a fixedsize. In this case, the period of time contained varies from record to record because the data are compressed.
X = a T2 + bT + c
Data records are passed from computer to computer using data telemetry methods. Data centers may pass data to computers from other organizations. By sharing data, both organizations obtain more information.
Data center software is quite complex. Several packages exist, including this commercial one.