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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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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) • Has common recording center • Waves remain coherent as they propagate across the array
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.
Seismic network: • Each station’s clock is independent • Data recording may be at the station or at a common data center • Waves may not be coherent as they propagate across the network
In a network, the location of an earthquake is found by triangulation. Information from all stations is used.
The Earth acts like a filter! High frequency waves attenuate (damp out) rapidly with distance. Low frequency waves diminish in amplitude more slowly with distance traveled.
EQUIVALENT EARTH PEAK ACCELERATION ( 20 LOG M/SEC 2 ) PERIOD (SECONDS) 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.
- USGS Albuquerque Seismological Laboratory - IRIS/IDA Group at IGPP, Scripps Institution of Oceanography University of California, San Diego - University Networks and Affiliates IRIS/USGS GLOBAL NETWORK
GSN Goals: • develop high quality seismic data acquisition equipment • deploy systems globally at 2000 km spacing • make data openly available to all who request a copy
GSN data are used to study the internal seismic structure of the Earth: Courtesy of Adam Dziewonski, Harvard University
Operational Planned Courtesy L. Kong
A design for Sri Lanka: • Use local stations to locate earthquakes in or near Sri Lanka • Use global network data (from Internet) to locate major earthquakes distant from Sri Lanka
A note on noise at seismic stations. The quietest stations in the world are on continents.
An important source of long period noise is changes in air pressure.
If possible, locate seismic stations away from the coast where noise conditions are better (noise is lower).
Another important subject… Remember the DAS?
Other equipment needed: • Clock to time the data • “DAS” to convert electric current to numbers for a computer to process
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.
We “compress” 20 numbers into 3 (a, b, c): 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.
