Elementary Seismology

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Physics of the Solid Earth (1)Physics of the Solid Earth (1)Dr. William K. MohantyAssociate ProfessorDepartment of Geology and GeophysicsIIT, KharagpurSeismology Group IIT KharagpurSeismology is the study of the generation, propagation, and recording of elastic waves in the Earth (and other celestial bodies) and of the sources that produce themSumatra Earthquake Recorded at IIT, Kharagpur Seismic ObservatoryThe joy of being a seismologist comes to you, when you find something new about the earths interior from the observation of seismic waves obtained on the surface, and realize that you did it without penetrating the earth or touching or examining it directlyKeiitiKeiitiAki, Presidential address to the Aki, Presidential address to the Seismological Society of America, 1980Seismological Society of America, 1980Schematic geometry of seismic experimentSeismology Group IIT KharagpurIntroduction to SeismologyBasic Concepts:Generates Seismic WavesPropagate away from source and samples the Earth structureRecorded ground motion is SEISMOGRAMEarthquakes (Passive Source)Free Surface ground motions caused by these propagating waves recorded at surface detectors(SEISMOMETERS)SEISMIC SOURCESNatural Events Man-Made EventsTectonic Earthquakes Controlled Sources (Explosions, vibrators)Volcanic Tremors and EarthquakesReservoir Induced EarthquakesRock Falls/Collapse ofKarst cavitiesMining InducedRock Bursts/CollapsesStrom MicroseismsCultural Noise(Industry, Traffic etc.)Various Kinds of Seismic SourcesSeismology Group IIT KharagpurEarthquakes to the progressive accumulation of strain energy in the rock mass surrounding a pre-existing fault and the sudden release of this energy by faulting when the fracture strength is exceededElasticElastic-- Rebound TheoryRebound TheoryEarthquake ZonesSeismology Group IIT KharagpurInterior of EarthMajor Tectonic Plates of the EarthSeismology Group IIT KharagpurFrequency of Occurrence of Earthquakes(based on observation since 1900)Earthquake focusSeismology (Class 2)Seismology (Class 2)Dr. William K. MohantyAssociate ProfessorDepartment of Geology and GeophysicsIIT, KharagpurSeismology Group IIT KharagpurBody wavesP- WavesS-wavesRayleigh WaveLove Wave Surface wavesP-wave velocity ( ) = 34+ KS-wave velocity () = Where, K is the bulk modulus or incompressibility, the shear modulus or rigidity and the density.Seismic wave propagation Long-period vertical component seismogram showing various seismic phasesRay paths for the seismic phases labeled on the seismogramTravel-time curves for surface focus Notation of various phases through Mantle and Core Earths P velocity, S velocity, and density as a function of depthEarths InteriorEARTHQUAKE HAZARDS AND ITS MITIGATIONDr. William K MohantyAssistant ProfessorDepartment of Geology and GeophysicsIndian Institute of Technology, KharagpurEARTHQUAKE HAZARDS Ground shaking Structural Hazards Liquefaction Landslides Retaining structures failures Lifeline Hazards Tsunami and Seiche HazardsGROUND SHAKING Most important of all seismic hazards When the earthquake occurs, seismic waves radiate away from the source and travel rapidly through the earths crust. Produce shaking at the ground surface, which may last from few seconds to minutes Strength and duration of shaking at a particular site depends on a. Sizeb. Location of earthquakec. Characteristics of the site Final portion of the trip of seismic waves form source to the ground surface often through soil Soil deposits act as filters.GROUND MOTION PARAMETERS Strong ground motion data are essential to understand the high-frequency nature of crustal seismogenic failure processes, the nature of seismic radiation from the source, and the nature of crustal wave-propagation phenomena near the sourcea) The Amplitudeb) Frequency contentc) Duration of the motionTHE AMPLITUDEw w a w v / ) ( ) ( =w w v w u / ) ( ) ( =where , and are the transformed displacement, velocity and acceleration respectively.u v aPEAK HORIZONTAL ACCELERATION (PHA)PEAK HORIZONTAL VELOCITY (PHV) PHV characterize ground motion amplitude accurately at intermediate frequencies. Structures or facilities (tall or flexible buildings, bridges etc.), PHV provide accurate indication of the potential damage.PEAK DISPLACEMENT Associate with low frequency. Difficult to determine accurately. Less commonly used as a measure of ground motion.EFFECTIVE ACCELERATIONFREQUENCY CONTENT PARAMETERS Frequency content describes how the amplitude of a ground motion is distributed among different frequenciesGROUND MOTION SPECTRA= + + = 10) sin( ) (n n n n t w C C t xwhere Cnand nare the amplitude and phase angle respectively of the nthharmonic of the Fourier seriesRESPONSE SPECTRA The response spectra describes the maximum response of a single-degree-of-freedom (SDOF)PREDOMINANT PERIOD The predominant period is defined as the period of vibration corresponding to the maximum value of the Fourier amplitude spectrumVmax/amax Vmax/amax should be related to the frequency content of the motion For a simple harmonic motion of period T, Vmax/amax =T/2 For earthquake motion that include many frequencies, the quantity 2 (Vmax/amax) provides, which periods of the ground motions are most significantSite Condition Vmax/amaxRock 5.5 cm/sec/g = 0.056 secStiff soils (7.4LOCATION OF CHAMOLI EARTHQUAKESTATIONS WHICH RECORDED THE MAINSHOCK AND THE AFTERSHOCKS OF CHAMOLI EARTHQUAKEOBSERVED PEAK GROUND MOTION (TRIANGLES) VS. HYPOCENTRAL DISTANCE R DURING THE CHAMOLI EARTHQUAKEEAST-WEST COMPONENT OF ACCELERATION AND VELOCITY TRACES AT SITES IN DELHI DURING THE CHAMOLI EARTHQUAKESPECTRAL RATIOS OF SOFT SITES TO RIDGE OBSERVATORYOBSERVED AND PREDICTED HORIZONTAL AmaxAND VmaxAS FUNCTION OF MwAT DELHI SITESPredicted Peak Ground Motion at Sites in DelhiSimulated Horizontal Ground Motion at CPCB and ROHypocentral location in 3-Dimension Distance = A = ts-tp Azimuth = Where Three Component Single Station o u + = Z|.|

\|= NSEWAA1tan u ( )090 s uDirection of first motion Angle o Vertical E-W N-S up W S 00 down E N 00 up W N 900 down E S 900 up E N 1800 down W S 1800 up E S 2700 down W N 2700 ocan be determined from this table Locating an epicenter LOCATING EARTHQUAKES Forward-Modeling Ti predicted = f(xi,v)=tiobserved F(M) = d Inverse Modeling d= Gm Magnitude Magnitude is a measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. It is a function of amount of energy released at focus and is independent of the place of observation. General form of all magnitude scales M = log (A/T)max + f (A, h) + Cs + Cr Where A = max. amplitude in thousandths of mm, T = period of the seismic wave in seconds, f = correction factor for epicentral distance (A) and focal depth (h), Cs = correction factor for the seismological station, and Cr = regional correction factor. Magnitude Scales Magnitude scales are based on a few simple assumptions - For a given source-receiver geometry larger events will produce wave arrivals of larger amplitudes at the seismic station The decay of ground displacement amplitudes with epicentral distance and their dependence on source depth h, i.e. the effects of geometric spreading and attenuation of the considered seismic waves, is known at least empirically in a statistical sense. It can be compensated by a so-called calibration function (, h). The latter is the log of the inverse of the reference amplitude Ao(, h) of an event of zero magnitude, i.e. (V,h)= log Ao(, h). The logarithm is used because of the enormous variability of earthquake displacement amplitudes Magnitudes should be a measure of seismic energy released and thus be proportional to the velocity of ground motion, i.e. to A/T with T as the period of the considered wave The the maximum value (A/T)max in a wave group for which (, h) is known should provide the best and most stable estimate of the event magnitude The effects of prevailing azimuth dependent source directivity can be corrected by a regional source correction term Cr and the influence of local site effects or amplitudes depending on local crustal structure, near-surface rock type, soft soil cover and/or topography may be accounted for by a station correction Cs Local Magnitude, ML ML = log Amax - log A o ML = log A - 2.48 + 2.76 log A MD = a o + a 1 log D+ a 2 A Duration Magnitude, MD Body Wave Magnitudes (MB) Mb = log (A/T) + Q (h, A ) Where A = actual ground motion amplitude in micrometer, and T = corresponding period in second. Surface-Wave Magnitude (Ms ) Ms = Log (A/T) + 1.66 log A + 2.0 Where A = Spectral amplitude, the horizontal component of the Rayleigh wave, with a period of 20 s, measured on the ground surface in micron, T = Period of seismic wave in second, and A = Epicentral distance in degree. Moment Magnitude, Mw Where M0 is in Nm. 0 . 6 log320 10 = M MWA = fault area (length x depth) m2 d = longitudinal displacement of the fault , m and u = modulus of rigidity (app. 3 x 1010 Nm-2 for the crust and 7 x 1010 Nm-2 for the mantle Relationship between different magnitude scale (Gutenberg and Richter, 1956) MB = 0.63MS + 2.5 MS = 1.27 (ML 1) 0.016M2L Log Mo = 1.5 MS + 16.1 Modified Mercalli Intensity (MMI) Scale Intensity is a measure of the effect that an earthquake produces at a given location. Intensity Isoseismal map for the Arkansas earthquake of December 16,1811 Isoseismal map of Kutch (Bhuj) earthquake of 26 January 2001. log amax = Io/3-1/2 M = 1+ 2/3 Io Energy-Magnitude Relations Log Es=2.4m-1.2 (Es in joule) Log Es = 1.5Ms+4.8 Log Es=1.96Ml+2.05 Magnitude Vs Ground motion and