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Seismology- is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake environmental effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes (such as explosions).
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ELEMENTS OF SEISMOLOGY: EARTHQUAKES FAULTS, THEORY OF PLATE TECTONICS
Seismology- is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake environmental effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes (such as explosions). A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of earth motion as a function of time is called a seismogram. A seismologist is a scientist who does research in seismology.
TYPES OF SEISMIC WAVE
Seismogram records showing the three components of ground motion. The red line marks the first arrival of P-waves; the green line, the later arrival of S-waves.Seismic waves are elastic waves that propagate in solid or fluid materials. They can be divided into body waves that travel through the interior of the materials; surface waves that travel along surfaces or interfaces between materials; and normal modes, a form of standing wave.
Body waves
Types of body waves:
1.) P-waves: Pressure waves or Primary waves (P-waves), are longitudinal waves that involve compression and rarefaction (expansion) in the direction that the wave is traveling. P-waves are the fastest waves in solids and are therefore the first waves to appear on a seismogram.
2.) S-waves: S-waves, also called shear or secondary waves, are transverse waves that involve perpendicular motion to the direction of propagation. S-waves are slower than P-waves. Therefore, they appear later than P-waves on a seismogram. Fluids cannot support this
perpendicular motion, or shear, so S-waves only travel in solids. P-waves travel in both solids and fluids.
Surface wavesPrimary types of surface waves:1.) The Rayleigh waves- which has some compressional motion, Rayleigh waves can be explained
theoretically in terms of interacting P- and S-waves of vertical polarization that are required to satisfy the boundary conditions on the free surface.
2.) The Love wave- which does not have compressional motion. Love waves can exist in the presence of a subsurface layer, and they are formed by S-waves of horizontal polarization only. Surface waves travel more slowly than P-waves and S-waves; however, because they are guided by the surface of the Earth (and their energy is thus trapped near the Earth's surface) they can be much larger in amplitude than body waves, and can be the largest signals seen in earthquake seismograms. They are particularly strongly excited when their source is close to the surface of the Earth, as in a shallow earthquake or explosion.
Normal modesBoth body and surface waves are traveling waves; however, large earthquakes can also make the Earth "ring" like a bell. This ringing is a mixture of normal modes with discrete frequencies and periods of an hour or shorter. Motion caused by a large earthquake can be observed for up to a month after the event.
ENGINEERING SEISMOLOGY
Engineering seismology is the study and application of seismology for engineering purposes. It generally applied to the branch of seismology that deals with the assessment of the seismic hazard of a site or region for the purposes of earthquake engineering. It is, therefore, a link between earth science and civil engineering.
Two principal components of engineering seismology:
1.) Firstly, studying earthquake history (e.g. historical and instrumental catalogs of seismicity) and tectonics to assess the earthquakes that could occur in a region and their characteristics and frequency of occurrence.
2.) Secondly, studying strong ground motions generated by earthquakes to assess the expected shaking from future earthquakes with similar characteristics. These strong ground motions could either be observations from accelerometers or seismometers or those simulated by computers using various techniques.
Elements of seismology: Elastic waves- a type of mechanical wave that propagates in elastic or viscoelastic materials Longitudinal wave- also known as "l-waves", are waves in which the displacement of the
medium is in the same direction as, or the opposite direction to, the direction of travel of the wave.
Transversal waves seismogram- also known as transverse waves, because particle motions are transverse waves, because particle motions are transverse to the direction of movement of the wave front, or perpendicular to the ray.
Seismographs Engineering seismology
o Estimation of seismic hazard and risko A seismic building
Observational seismologyo Macro seismology (observing earthquakes effects)o Micro seismology (recording earthquakes)
EARTHQUAKES AND FAULTS
What is a fault?Earthquakes occur on faults. A fault is a thin zone of crushed rock separating blocks of the earth's crust. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. Faults can be centimeters to thousands of kilometers long. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. Faults can extend deep into the earth and may or may not extend up to the earth's surface.How do we know a fault exists?
Past fault movement has brought together rocks that used to be farther apart; Earthquakes on the fault have left surface evidence, such as surface ruptures or fault scarps
(cliffs made by earthquakes); Earthquakes recorded by seismographic networks are mapped and indicate the location of a
fault.Some faults have not shown these signs and we will not know they are there until they produce a large earthquake. Several damaging earthquakes in California have occurred on faults that were previously unknown.
Types of FaultsFaults Are Classified According to the Kind of Motion That Occurs on Them
• Joints - No Movement • Strike-Slip - Horizontal Motion
- Similar in movement, a strike slip fault occurs through shearing when two blocks move in horizontal but opposite direction of each other. Depending on the direction of offset, it can be a right lateral offset or a left lateral offset.
• Dip-Slip - Vertical Motion - Normal faults, cause by blocks of crust pulling apart under the forces of tension.- Reverse faults, faults caused by blocks of crust colliding under the forces of compression.
Strike-Slip Fault – Left Lateral
Strike-Slip Fault – Right Lateral
Tectonic plates
Tectonic plates are large parts of litosphere ‘floating’ on the astenosphereConvective currents move them around with velocities of several cm/year.The plates interact with one another in three basic ways:
1. They collide2. They move away from each other3. They slide one past another
• Earthquakes occur at FAULTS.• Fault is a weak zone separating two geological blocks.• Tectonic forces cause the blocks to move relative one to another.