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By
Nicholas Opoku
Seismologist, Ghana Geological Survey
CTBTO SCIENCE AND TECHNOLOGY 2013 CONFERENCE
VIENNA
GHANA DIGITAL SEISMIC NETWORK – THE
WAYFORWARD AND THE CHALLENGES
The closest plates boundary to Ghana are:
The spreading at the mid Atlantic ridge which is separating
the South American continent from the African continent.
And also the East African rift system where the Nubian
(West Africa) and the Somalian (East Africa) plates are
slowly drifting away from each other.
Most earthquakes all over the world are known to occur
along plate boundaries
INTRODUCTION
Location of Ghana relative to major plate boundaries within and around Africa.
Ghana
East African
Rift Mid Atlantic
Ridge
Arabian Plate
Nubian Plate
Somalian Plate
Even though Ghana is far from these major plate boundaries,
significant earthquakes have struck and continue to strike Ghana
Seismic research in Ghana dates back to the colonial days. The first
documented earthquake occurred in the year 18th Dec. 1636.
Other significant events have occurred in the years 1862, 1858, 1863,
1906, 1907, 1911, 1918, 1919, 1923,1930, 1933, 1935, 1964, 1969, and
more recently 1997, 2003 and 2006, with magnitudes ranging between
4 to 6.5.
It is the southern part of Ghana, especially the capital city Accra
which has been subjected to these significant number of earthquakes.
Prominent amongst them is the 22nd June, 1939 earthquake of
magnitude 6.5 which shook the entire country and caused major
destruction and loss of lives.
Intra-Plate Seismic Activities In Ghana
This earthquake occurred at approximately 7:20pm (GMT). It was
felt for about 20-30seconds at Accra.
The intensity of the shock was greatest in the capital city Accra (IX
on the modified mercalli scale).
The greatest damage to life and property occurred in Accra, where 16
people were killed and 133 injured, and hundreds of thousands of
pounds damage was done to buildings.
The shock occurred at a time when everyone was awake and many
were out of doors, hence, the low recorded deaths and injuries caused
by the severity of the shock wthin a population of about 77, 000 people
in Accra. Also there were no outbreak of fire as usually accompany
severe earthquakes.
SOME OF THE EFFECTS OF THE 22ND JUNE
1939 EARTHQUAKE IN GHANA
Low level reservoir showing thrust
of concrete slabs at Weija in Accra Cracks in C. I. D. Building in
Accra
Landslide caused by the earthquake
in Accraian shales east of Bishops
church in Accra
Nyanyanu village in Accra
competely destroyed
SOME OF THE EFFECTS OF THE 22ND JUNE
1939 EARTHQUAKE IN GHANA
Ghana lies on the south eastern margin of the West African Craton
(stable old crust) which is in contact with the Pan African mobile belt
Intra-Plate Seismic Activities In Ghana
The Akuapim Mountain Range (fault) which lies along this
contact zone is responsible for most earthquakes in Ghana.
Location of Ghana
with respect to the
west Africa Craton.
Geological Setting / Tectonics in Ghana
The Akuapim fault in Ghana run
through Accra, the Akwapim
Mountains, Ho and to Togo and
meets the coastal boundary fault
which lies along the coast near
Accra
It forms part of the mobile belt /
mountain range that marks the
south-eastern margin of the west
African Craton.
The metamorphosed rocks
found at this fault zone shows
intense breciation and folding as
evidence of faulting
Fault zone defined by abrupt changes
in dip at Larteh on the Akwapim Mt. Range
Tight synclinal and open anticlinal fold along
Mamfe-Larteh road on the Akwapim Mt. Range
Some of the Geological Evidences of Faulting
along the Akuapim Fault Zone (Mountain Range)
Instrumental Recording of Earthquakes in Ghana
A Milne’s single-boom seismograph was the first Seismic
Instrument installed in the year 1914 by the colonial Government
to record earthquakes. It Operated until 1937.
Geotech Long Period Z component Seismograph was later
installed in March, 1973 at Kukurantumi.
An Analog seismic network which were all S-13 short period
seismometers were procured from Teledyne Geotech (U.S.A)
and were installed in June 1987.
System Components of the Real Time Analog
Seismic Network
S-13 Short Period Seismometer
in a Vault at Shai Hills . Booster Station at Amanokrom
Central Recording Staion at GGS
HeadQuarters, Accra Analog seismic Network
The Government of Ghana in 2010 gave budgetary support to GGS to
fund the purchase and installation of six remote stations digital
broadband seismic network to replace the defunct analogue seismic
network.
The package also included ten stand alone strong motion
accelerometers and the building of a new Seismological Observatory.
The Digital Seismic Network equipment were procured from
Nanometrics Inc., Canada.
The network consist of six remote sites digital broad band stations
transmitting real time seismic data to the central Observatory Accra.
The New Digital Seismic Network
Trillium 120 PA Trillium Compact
Trident305 Digitizer Trident305 Digitizer
Cygnus205 Transceiver Cygnus205 Transceiver
Carina105 Transceiver
Ethernet
GGS LAN
Data Acquisition Servers
GGS Workstations
Event Detection & Processing, Network & Data
Management, Data & SOH Monitoring, etc.
GGS Acquisition Hub
3 x Trillium Compact Stations
(Transmitted over V-Sat Telemetry
3 x Trillium 120PA Stations
(Transmitted over V-Sat Telemetry)
System Components
and Setup
Remote And Central Recording Stations Facilities
Remote Station Map of stations lcation
Seismological Observatory Satellite Receiver (bigger) at Seismological Observatory
CRF for Data Acqusition, Event Processing,
Publication, Notification & Storage
UPS
All data from the Remote Sites are Received by the 3.8meter Satellite
antenna on to the Carina
The Carina streams the data onto the Apollo Server.
The Apollo server temporary stores and displays the waveform.
The Hydra Server detects and processed any events in real time
automatically. It then post the event and processing history to
Athena Web page for display, permanent storage and e-mail notification.
Athena Server
Carina
Apollo Server Hydra Blue Server
Hydra Red Server
In the event of a moderate earthquake the depth of faulting
and the faulting type can be constrained through moment
tensor inversion.
Software Package
Locate all earthquake hypocenters in Ghana and beyond
start time (origin time),
magnitudes determination and
frequency of occurrence
Apollo Waveform Software
The Apollo Waveform software displays seismic traces from the local
area network stations and that of the GSN stations.
Apollo Poject software retrives and
permanently stores waveform data
in seed format.
Apollo Project and Atlas Softwares
Atlas software is
used to view
waveforms and to
post process local
events
Atlas Event Bulleting
Event Post Processing by Hydra
It is possible to review picks to fine tune automatic processed events by
Hydra software.
Athena Web Page Displays Processed Events
by Hydra and Atlas
LOCATION OF GSN STATIONS TO IMPROVE THE
AUTOMATIC LOCATION PROGRAMME
But due to very poor seismic network coverage in the West Africa (W/A)
Sub-Region, most events outside our network area may have large
estimation error within W/A Region, especially micro-seismic events.
10 x Strong Motion Stations
(Data recorded to local CF
media)
Taurus Digitizer
Titan Accelerometer
Buffered operations
mode assures
minimal
power consumption.
Requires scheduled
station visits to swap
media before
reaching
full recording
capacity
Ten standalone strong motion accelerometers installed on
critical or lifeline structures such as electrical power facilities
(Akosombo and Akuse dams) and water supply and sewage
treatment facilities (Weija dam).
The Purpose of this strong motion accelerometers among
others is to determine:
The nature of earthquake ground motion and its impact on
structures
The duration of the shaken
The frequencies of the motion
The Strong Motion Equipments
Expected Aims and Objectives to be Achieved
from the Entire Seismic Network
The seismic (earthquake monitoring) data which is being generated will
be utilised to obtain Ground Motion Estimates to generate a new
National Seismic Hazard map to serves as basis for the sustainable socio-
economic mitigation of geo-hazards through:
proper and effective Land Use Planning,
for a revised Building codes formulation.
for Design Standards for Critical or lifeline structures such as Bridges,
all types of Dams, Overhead Transportation Systems, Nuclear Power
Plants.
based on the technical consideration and our economic circumstances
for Disaster Mitigation Strategies to reduce the effect of future
earthquakes.
for Post earthquake reconstruction purposes in order to avoid a
repetition of the failed reconstruction programme.
for well defined insurance Policies for protection against
earthquakes
And for research into the internal composition of the earth and
exploration purposes and to contribute towards Global Seismological
research.
Extend the seismic network to cover other places
to have a good coverage area in Ghana.
Co-operation, sharing of data and extension of
the network within the entire West African Sub-
region.
Efforts are underway to integrate our system into
the Global Seismic Network for the sharing of
information.
Sustenance of capacity building of Seismologists,
Earthquake Engineers, Technicians and other
supporting staff to keep them abreast with time.
Way Forward
Challenges Funding for the mentainance of the Seismic Network?
Funding of activities to be carried out in the West African
Sub-Region to better study events within the Sub-Region?
Rapid access to spare parts for replacement when there is a
break down (especialy batteries and solar panels) to avoid data
lost over a long period of time?
Funding for Training of young scientists (Seismologists and
other Technical Staff)?
The above outline are the key challenges that we have to
overcome.
THANK YOU
FOR
YOUR ATTENTION !
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