SCGSN Nov96

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

• Types of seismic arrays
• How arrays are used
• Example global networks

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Seismic array

• Has common time base (one clock)
• Has common recording center
• Waves remain coherent as they propagate across
  the array

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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.
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Seismic network

• Each stations 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

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In a network, the location of an earthquake is
found by triangulation. Information from all
stations is used.
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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.
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IRIS GSN SYSTEM
Global networks choose instruments capable of
recording long period waves. Local arrays may
use short period sensors to record local
earthquakes.
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The 2004 Sumatra-Andaman Isl quake radiated very
long period seismic waves that were recorded by
global networks.
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IRIS/USGS GLOBAL NETWORK
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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

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GSN data are used to study the internal seismic
structure of the Earth
Courtesy of Adam Dziewonski, Harvard University
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German GEOFON Network
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French GEOSCOPE Network
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Japan FNET
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Chinese Digital Seismic Stations
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Courtesy L. Kong
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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

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A note on noise at seismic stations.
The quietest stations in the world are on
continents.
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Stations near the ocean are noisier.
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This peak is caused by ocean waves breaking on
coastlines.
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This peak is caused by ocean waves also.
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An important source of long period noise is
changes in air pressure.
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If possible, locate seismic stations away from
the coast where noise conditions are better
(noise is lower).
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Another important subject
Remember the DAS?
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Other equipment needed

• Clock to time the data
• DAS to convert electric current to numbers for
  a computer to process

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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.
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The DAS converts electrical current into numbers.
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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.
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This seismogram is very simple
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We compress 20 numbers into 3 (a, b, c)
X a T2 bT c
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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.
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Data center software is quite complex. Several
packages exist, including this commercial one.
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NRTS_utilities a simple IDA display program
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You can display data from PALK and other stations
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The buttons at the top control the display