Employment of a Permanent Monitoring GPS Network at the Seismic Area of Volvi, Greece

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Employment of a Permanent Monitoring GPS Network
at the Seismic Area of Volvi, Greece

• P. D. Savvaidis, I. M. Ifadis
• Aristotle University of Thessaloniki,
• Department of Civil Engineering,
• Laboratory of Geodesy, GR-54006 Thessaloniki

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• the western termination of the North Anatolian
  fault, that accommodates lateral motion of
  Anatolia to the west
• an N-S extension that could result from the
  spreading of the Aegean region over the subducted
  oceanic Mediterranean lithosphere

The present-day active geodynamics of the
Northern Aegean is governed by
GREECE - ????S
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• The Mygdonian graben in the northern part of
  the Aegean region is one of the active structures
  that accommodate the N-S extension.
• The formation of the graben is recent and
  active, as demonstrated by the important
  historical seismicity on this structure.

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• In 1978, severe earthquakes damaged an extensive
  area of Northern Greece including the city of
  Thessaloniki.
• 45 people killed
• 220 injured
• 9,480 buildings collapsed
• 91,000 buildings with moderate to minor damages

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• The main shocks came from the seismic zone of
  Volvi, NE
• of the city of Thessaloniki.

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• The main shock
• (Ms 6.5) occurred on June 20, 1978
• It was preceded by several foreshocks the largest
  of which occurred on May 23, 1978 (Ms 5.8)
• and June 19, 1978 (Ms 5.2)

The epicentres were located in the region between
the Langada and Volvi lakes
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Three particular fault zones were observed
after the earthquakes

• The first family of cracks followed the southern
  boundary of the graben, striking roughly N80 from
  Loutra Volvi in the east to Stivos, Nikomedino
  and Yerakarou in the west.
• The other two fault zones, striking N130 and
  N110 respectively, cut the Mygdonian valley from
  Stivos in the SSE towards the Langada lake in the
  WNW near Skolari and as far as Analipsis

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• A 16 station geodetic control network was
  established in 1979 using classical geodetic
  techniques
• The measurements were repeated in 1980, 1981,
  1982, 1983, 1989, and 1990

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• In 1994, a new survey of the Volvi network was
  carried out using GPS technology
• European Project EURO-SEISTEST Volvi
  Thessaloniki, a European Test Site for
  Engineering Seismology, Earthquake Engineering
  and Seismology.

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• In 1997 the GPS network was further extended with
  the addition of six more stations and
  re-measured, now covering the area from the
  outskirts of the city of Thessaloniki to the
  east, near to the boundary of the Mygdonian
  graben.

EURO-SEISTEST
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• Comparison of epochs 1979 and 1994
• 8 cm of N S extension corresponding to a
  horizontal deformation rate of more than
  5 mm/year
• Comparison of epochs 1994 and 1997 Indication
  of a N - S motion of the order of 1.5 cm

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• The results of the terrestrial and GPS campaigns
  suggest that a motion has been continuous with
  time since 1978
• This observed displacement field could be
  explained by different approaches (e.g. long term
  postseismic relaxation on the 1978 rupture plane,
  or continuous aseismic motion on the fault etc.),
  but in all cases more measurement data are needed

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• In 1999, the decision was taken for the
• establishment of a permanent monitoring GPS
  network in the area under investigation. This
  network, the
• VOLVI Array
• would consist of a limited number of permanent
  GPS receivers continuously monitoring data in
  locations, which have shown evidence of motion

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• STEP 1
• Establishment and initialisation of a Continuous
  Reference GPS Station in the area of Thessaloniki
  
• STEP 2
• Establishment of two permanent GPS stations at
  the north and south of the area between the lakes
  
• STEP 3
• Addition of two more permanent GPS stations in
  the area

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STEP 1

• A Continuous Reference GPS Station started to
  operate at the facilities of the Laboratory of
  Geodesy in autumn 1999
• Raw data are collected at hourly intervals and
  stored for future computations
• RTCM corrections are being transmitted through a
  UHF radio network for Real Time Kinematic GPS
  applications covering the area of greater
  Thessaloniki

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• A preliminary estimation of the current ITRF
  co-ordinates of station "THES" was done using
  data from selected IGS sites in the surrounding
  region

The ITRF coordinates of THES (upd. 3/2000) X
4463666.6966 0.008 m Y 1890982.2682
0.006 m Z 4131186.0218 0.007 m
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STEP 2
?
Profitis
Wide Area Computer Network
?
ISDN
?
Station (THES)
Stivos
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STEP 3
Evangelismos
Profitis
Stivos
Yerakarou
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• SUMMARY
• The Volvi GPS array will consist of five
  permanent monitoring GPS receivers, one of them
  being the Continuous Reference GPS Station THES
  in operation at the Laboratory of Geodesy
• At present, another GPS receiver is in
  experimental operation at Stivos and the wide
  area computer network is being tested for optimum
  performance
• It is expected that in a few months the array
  will be operational and the measurements will be
  used for the computation of ground deform-ation
  in the area of Volvi