CHAPTER ONE
INTRODUCTION
1.1 Background Information
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth’s lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling. Earthquake precursors are phenomena which precede earthquakes and enable them to be predicted. A precursor in term of an earthquake is a signal of approaching seismic event.(Pulinets, 2004).
At the Earth’s surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity. . Earthquakes are caused mostly by rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake’s point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocentre (Pulinets and Kirill, 2004).
All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading. These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes. The majority of tectonic earthquakes originate at the Ring of Fire in depths not exceeding tens of kilometres earthquakes occurring at a depth of less than 70 km are classified as ‘shallow-focus’ earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed ‘mid-focus’ or ‘intermediate-depth’ earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, Deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers). These seismically active areas of subduction are known as Wadati–Benioff zones. Deep-focus earthquakes occur at a depth where the subducted lithosphere should no longer be brittle, due to the high temperature and pressure(Oreskes, 2003). A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure.
The Earth crust can be referred to as the rigid external shell of the Earth which consists of the continental and oceanic crust (Pulinets and Kirill, 2004). The crust and the upper layer of the mantle form the lithosphere consisting of semi rigid plates of different sizes. The slow movement of these semi rigid plates of different sizes over the asthenosphere and ocean floor extension is called plate tectonics. The collision of these plates lead to the diving of one plate edge under another (convergent or subduction boundaries).The subduction leads to the formation of different geological structures such as formation of the oceanic trench in the zone of plates contact, formation of new mountains and zones of volcanic activity. Apart from the subduction process there exist other kinds of plate relative motion such as plate separation (divergent boundaries) generating the oceanic crust, and transform boundaries which is as a result of the plates moving relative to one another. All these movements lead to accumulation of damages such as mechanical deformations and crust rupture within the Earth crust, rupture when the deformation exceeds the limit of mechanical strength. The process of rupture is referred to as earthquake. The type and characteristics of earthquake depends on the type of tectonic plate contact. The global distribution of earthquakes is not uniform as they are concentrated at the region close to the tectonic plate’s boundaries (Circum- Pacific Ring of Fire) Pulinets, 1998.
Majority of ionospheric signals resulted from solar or geomagnetic disturbances are correlated spatially over wide region while minority of the signals is thought to originate from the ground(Hayakawa et al., 2010). Generally it is known that the signal from the ground have only local effect therefore, whenever a strong signal originate from the ground, the signal in the region in the ionosphere above it should become de-correlated from the signal in the neighbouring region. This de -correlation is a potential anomaly for earthquake prediction in the ionosphere.
1.2 Statement of Problem
Earthquake is one of the important natural phenomena that have great impact on Earth’s surface. Earthquakes is very hazardous to human life and properties. Due to the complexity of earthquake-preparation processes, identifying true earthquake precursors from anomalous geophysical signals is still a world-class problem. However, the loss of life and properties could be mitigated if there are early warning systems.


1.3 Aim and Objectives of the Study
The aim of this study is to assess for possibleseismo-ionospheric induced variation before the twin earthquake of 25thMarch, 2007 in Vanuatu and West Cost of Honshu. To this, the
objective to determine the ionospheric precursors of the earthquake using Plasma Analyser (IAP) and Langmuir Probe (ISL) sensors on board the Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite.
1.4 Significant of the Study
The significance of this work is to mitigate the loss of lives and properties that occur during seismic event through careful monitoring of the ionosphere.

1.5 Scope of the Study
The study investigates magnitudes 7.3 and 7.1 earthquakes that occurred on 25th March, 2007 using the Plasma Analyser and Langmuir Probe sensors on board the DEMETER satellite.