Evaluation Of Probable Effect Of Earthquake Within An Investigated Site Located At Northern Part Of Otuoke Using Seismic Refraction Tomography

CHAPTER ONE

1.0                                                        INTRODUCTION

1.1                                           BACKGROUND OF THE STUDY

The uncontrolled growth of the global population led to an increase in annual earthquake-related losses from US$ 14 billion in 1985 to more than US$ 140 billion in 2014. Similarly, the average affected population rose from 60 million to over 179 million within the same period. Earthquakes constitute approximately one fifth of the annual losses due to natural disasters, with an average death toll of over 25,000 people per year according to University Corporation for Atmospheric Research (2010).

Earthquakes may cause liquefaction, landslides, fire, and tsunami which would lead to far higher level of damage and losses.

As a result of the plate tectonics revolution in the 1960’s and the ensuing decades of intensive research in theearth sciences, the long term earthquake potential of most parts of the world, especially near plate boundaries, isfairly well understood.   As a result, earth scientists have a partial understanding of the long-term seismicbehavior of some of the more active faults (Michetti et al., 2017).However, with few exceptions, large earthquakes do not appear tooccuratuniformtimeintervals,ortoruptureexactlythesamesegmentofafaultfromoneearthquaketothenext.Consequently, even for sites near the best understood faults, earth scientists are not able to predict whereandwhen thenext large earthquake is going tooccur, andsoa probabilistic approach toseismichazardevaluationismostcompatiblewithourstateofknowledgeofseismicpotential.Evengreateruncertaintyexistsin most other regions of the earth, where the locations of potentially active faults are poorly known or unknown,and theirseismicpotentialispoorlyunderstood (University Corporation for Atmospheric Research, 2010).

There is also a large degree of uncertainty in the level of the ground motion that a specified earthquake willgenerate at a particular site, and this uncertainty usually dominates the uncertainty in the seismic hazard estimateat the site.Developments in theoretical and computational seismology during the past two decades haveprovided significant insights into the causes of this variability in ground motions, and are now being used toreduceit(Michetti et al., 2017).

Conventional ground motion models predict ground motion parameters using a simplified model in which theeffectsoftheearthquakesourcearerepresentedbyearthquakemagnitude;theeffectsofwavepropagationfromtheearthquake source to the site region are specified by a distance; and the effects of the site are specified by a sitecategory.Thesegroundmotionmodelshavealargedegreeofuncertaintybecauseotherconditionsthat are known to have an important influence on strong ground motions, such as near-fault rupture directivity effectsandtheresponseofsedimentarybasins,arenottreatedasparametersofthesesimplemodels.Inordertoreducetheuncertainty in ground motion prediction at a given site, the parameterization of ground motion models is beingaugmentedtoincludemorerealisticrepresentationsofsource,pathandsiteeffects(Michetti et al., 2017).

For some purposes, the strong ground motions expected at a site are represented by those resulting from a singleearthquake.For example, ground motion maps of future scenario earthquakes have played an important role inurban planning and mitigation activities.Also, seismic hazards for the design of critical facilities are sometimescharacterizedusing a deterministic approach,whichusually consistsofaworstcasescenarioearthquake.Deterministicestimatesofgroundmotionsaretypicallymadeforasinglecontrollingearthquakewhosemagnitude(andpossiblyothersourceparameters)andclosestdistancearespecified.Sincethegroundmotionsfromonlyoneearthquake are considered, the uncertainty in the estimated ground motions depends on detailed aspects of theearthquakesource,thewavepropagationpathbetweenthesourceandthesite,andthesiteresponse(Michetti et al., 2017).

Giventheuncertaintyinthetiming,locationandmagnitudeoffutureearthquakes,formostengineeringpurposesitis more meaningful to use a probabilistic approach to characterizing the ground motion that a given site willexperience in the future than to use a scenario earthquake.A probabilistic seismic hazard analysis takes intoaccount the ground motions from the full range of earthquake magnitudes that can occur on each fault or sourcezone that can affect the site.This information is numerically integrated using probability theory to produce theannualfrequencyofexceedanceofeachdifferentgroundmotionlevelforeachgroundmotionparameterofinterest(Tuttle et al., 2019).

Theprobabilisticapproachtoseismichazardcharacterizationisverycompatiblewithcurrenttrendsinearthquakeengineering and the development of building codes, which have embraced the concept of performance baseddesign. Incontrasttothetraditionalbuildingcodeapproach,performance-based design requires an explicit prediction of the structure’s performance at each of several groundmotion levels corresponding to a set of performance objectives.The performance objectives may range fromcontinuedfunctionofthebuildingduringrelativelysmall,frequentgroundmotions;tolimitingdamagebelowthelife safety threshold in severe, less frequent ground motions; to prevention of collapse for very severe, infrequentground motions.Each performance objective is associated with an annual probability of occurrence, withincreasingly undesirable performance characteristics caused by increasing levels of strong ground motion havingdecreasingannualprobabilityofoccurrence(Tuttle et al., 2019).

Performancebaseddesignrequiresamorecomprehensiverepresentationofgroundmotionsthandomostcurrentdesign procedures. It requires the specification of the ground motions at multiple annual probability levels. Also,thegroundmotionsmayneedtobespecifiednotonlybyresponsespectrabutalsobysuitesofstrongmotiontimehistoriesforinputintotime-domainnon-linearanalysesofstructures(Tuttle et al., 2019).Thisisbecauseresponsespectrumanalysis,uponwhichnearlyallcurrentstructuraldesignisbased,usesalinearelasticmodelandthereforedoesnotaddressthenon-linearresponsethatistheessenceofbuildingdamageandfailure.

STATEMENT OF THE PROBLEM

The concepts and trends in seismic hazard characterization that have emerged in the past decade, and identifies trends and concepts that are anticipated during the coming decade. New methods have been developed for characterizing potential earthquake sources that use geological and geodetic data in conjunction with historical seismicity data. Scaling relationships among earthquake source parameters have been developed to provide a more detailed representation of the earthquake source for ground motion prediction.

AIM AND OBJECTIVES OF THE STUDY

The main aim of this work is to to Evaluate the probable effect of earthquake within an investigated site located at northern part of otuoke using seismic refraction tomography. The objectives of the study are:

1. To assess the probable effect of earthquakeat northern part of otuoke using seismic refraction tomography.
2. To study th characteristics of earthquake.

• To study the causes of earthquake.

SCOPE OF THE STUDY

Earthquakes are the result of sudden movement along faults within the Earth. The movement releases stored-up ‘elastic strain’ energy in the form of seismic waves, which propagate through the Earth and cause the ground surface to shake. The scope of this work covers evaluating the  effect of earthquake within an investigated site located at northern part of otuoke using seismic refraction tomography.

RESEARCH QUESTIONS

At the end of this work answers to the following questions shall be provided:

1. What are some effects of earthquakes on the natural and built environments?
2. What building characteristics are significant to seismic design?

• What are the consequences of earthquakes?

SIGNIFICANCE OF TH STUDY

This study is important to both the student involves and scientists. For the student, this study will help him to have deep knowledge about earthquake and its effects. To scientists,with this study scientistsare not able to predict whereandwhen thenext large earthquake is going tooccur, andsoa probabilistic approach to seismichazardevaluation.