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 !