Sri Lanka Earthquake & Tsunami Warning Training Program

Sri Lanka Earthquake & Tsunami Warning Training ProgramSri Lanka Earthquake & Tsunami Warning Training ProgramSri Lanka Earthquake & Tsunami Warning Training Program

Session III.3

Global and Local Arrays

April 5, 2006

CETRAC, Sri Lanka

Sri Lanka Earthquake & Tsunami Warning Training Program

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.


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!


IRIS GSN SYSTEME

QU

IVA

LEN

T E

AR

TH P

EA

K A

CC

ELE

RA

TIO

N (

20 L

OG

M/S

EC

2 )

PERIOD (SECONDS)

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/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


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




German GEOFON Network


French GEOSCOPE Network


Japan FNET


Chinese Digital Seismic Stations



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.


Stations near the ocean are noisier.


This peak is caused by ocean waves breaking on coastlines.


This peak is caused by ocean waves also.


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.


The DAS converts electrical current into numbers.


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.


This “seismogram” is very simple:


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.


NRTS_utilities: a simple IDA display program


You can display data from PALK and other stations


The buttons at the top control the display

Documents

Sri Lanka Earthquake & Tsunami Warning Training Program