1. Sri Lanka Earthquake & Tsunami Warning Training Program Session III.3 Global and Local Arrays April 5, 2006 CETRAC, Sri Lanka

2. Sri Lanka Earthquake & Tsunami Warning Training Program Topics covered:  Types of seismic arrays  How arrays are used  Example global networks

3. Sri Lanka Earthquake & Tsunami Warning Training Program Seismic array:  Has common time base (one clock)  Has common recording center  Waves remain coherent as they propagate across the array

4. Sri Lanka Earthquake & Tsunami Warning Training Program 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.

5. Sri Lanka Earthquake & Tsunami Warning Training Program 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

6. Sri Lanka Earthquake & Tsunami Warning Training Program In a network, the location of an earthquake is found by triangulation. Information from all stations is used.

7. Sri Lanka Earthquake & Tsunami Warning Training Program High frequency waves attenuate (damp out) rapidly with distance. Low frequency waves diminish in amplitude more slowly with distance traveled. The Earth acts like a filter!

8. Sri Lanka Earthquake & Tsunami Warning Training Program IRIS GSN SYSTEM EQUIVALENT EARTH PEAK ACCELERATION ( 20 LOG M/SEC 2 ) PERIOD (SECONDS) Global networks choose instruments capable of recording long period waves. Local arrays may use short period sensors to record local earthquakes.

9. Sri Lanka Earthquake & Tsunami Warning Training Program The 2004 Sumatra- Andaman Isl quake radiated very long period seismic waves that were recorded by global networks.

10. Sri Lanka Earthquake & Tsunami Warning Training Program IRIS/USGS GLOBAL NETWORK - USGS Albuquerque Seismological Laboratory - IRIS/IDA Group at IGPP, Scripps Institution of Oceanography University of California, San Diego - University Networks and Affiliates

11. Sri Lanka Earthquake & Tsunami Warning Training Program 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

12. Sri Lanka Earthquake & Tsunami Warning Training Program GSN data are used to study the internal seismic structure of the Earth: Courtesy of Adam Dziewonski, Harvard University

13. Sri Lanka Earthquake & Tsunami Warning Training Program

15. German GEOFON Network

16. Sri Lanka Earthquake & Tsunami Warning Training Program French GEOSCOPE Network

17. Sri Lanka Earthquake & Tsunami Warning Training Program Japan FNET

18. Sri Lanka Earthquake & Tsunami Warning Training Program Chinese Digital Seismic Stations

19. Sri Lanka Earthquake & Tsunami Warning Training Program

20. Operational Planned Courtesy L. Kong

21. Sri Lanka Earthquake & Tsunami Warning Training Program 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

22. Sri Lanka Earthquake & Tsunami Warning Training Program A note on noise at seismic stations. The quietest stations in the world are on continents.

23. Sri Lanka Earthquake & Tsunami Warning Training Program Stations near the ocean are noisier.

24. Sri Lanka Earthquake & Tsunami Warning Training Program This peak is caused by ocean waves breaking on coastlines.

25. Sri Lanka Earthquake & Tsunami Warning Training Program This peak is caused by ocean waves also.

26. Sri Lanka Earthquake & Tsunami Warning Training Program An important source of long period noise is changes in air pressure.

27. Sri Lanka Earthquake & Tsunami Warning Training Program If possible, locate seismic stations away from the coast where noise conditions are better (noise is lower).

28. Sri Lanka Earthquake & Tsunami Warning Training Program Another important subject… Remember the DAS?

29. Sri Lanka Earthquake & Tsunami Warning Training Program Other equipment needed:  Clock to time the data  “DAS” to convert electric current to numbers for a computer to process

30. Sri Lanka Earthquake & Tsunami Warning Training Program 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.

31. Sri Lanka Earthquake & Tsunami Warning Training Program The DAS converts electrical current into numbers.

32. Sri Lanka Earthquake & Tsunami Warning Training Program How the DAS bundles the data depends upon the computer programs running inside the DAS. Some DASs produce a record containing data for a fixed period of time. Other DASs produce a record of a fixed size. In this case, the period of time contained varies from record to record because the data are compressed.

33. Sri Lanka Earthquake & Tsunami Warning Training Program This “seismogram” is very simple:

34. Sri Lanka Earthquake & Tsunami Warning Training Program We “compress” 20 numbers into 3 (a, b, c): X = a T 2 + bT + c

35. Sri Lanka Earthquake & Tsunami Warning Training Program 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.

36. Sri Lanka Earthquake & Tsunami Warning Training Program Data center software is quite complex. Several packages exist, including this commercial one.

37. Sri Lanka Earthquake & Tsunami Warning Training Program NRTS_utilities: a simple IDA display program

38. Sri Lanka Earthquake & Tsunami Warning Training Program You can display data from PALK and other stations

39. Sri Lanka Earthquake & Tsunami Warning Training Program The buttons at the top control the display