Elementary Seismology

Physics of the Solid Earth (1)

Dr. William K. Mohanty Associate Professor Department of Geology and Geophysics IIT, Kharagpur Seismology Group IIT Kharagpur

“Seismology 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 them”

Sumatra Earthquake Recorded at IIT, Kharagpur Seismic Observatory

“The joy of being a seismologist comes to you, when you find something new about the earth’s 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 directly”

Keiiti Aki, Presidential address to the Seismological Society of America, 1980

Schematic geometry of seismic experiment

Seismology Group IIT Kharagpur

Introduction to Seismology Basic Concepts: Earthquakes (Passive Source)

Generates Seismic Waves

Propagate away from source and samples the Earth structure Free Surface ground motions caused by these propagating waves Æ recorded at surface detectors (SEISMOMETERS)

Recorded ground motion is SEISMOGRAM

Various Kinds of Seismic Sources SEISMIC SOURCES

Natural Events

Seismology Group IIT Kharagpur

Man-Made Events

Tectonic Earthquakes

Controlled Sources (Explosions, vibrators…)

Volcanic Tremors and Earthquakes

Reservoir Induced Earthquakes

Rock Falls/Collapse of Karst cavities

Mining Induced Rock Bursts/Collapses

Strom Microseisms

Cultural Noise (Industry, Traffic etc.)

Elastic- Rebound Theory “Earthquakes 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 exceeded”

Earthquake Zones Seismology Group IIT Kharagpur

Interior of Earth

Major Tectonic Plates of the Earth

Seismology Group IIT Kharagpur

Frequency of Occurrence of Earthquakes (based on observation since 1900)

Earthquake focus

Seismology (Class 2)

Dr. William K. Mohanty Associate Professor Department of Geology and Geophysics IIT, Kharagpur Seismology Group IIT Kharagpur

Body waves P- Waves S-waves

P-wave velocity (α ) = S-wave velocity (β) =

K+

ρ

μ ρ

Where, K is the bulk modulus or incompressibility, μ the shear modulus or rigidity and ρ the density.

Surface waves Rayleigh Wave Love Wave

4 μ 3

Seismic wave propagation

Long-period vertical component seismogram showing various seismic phases

Ray paths for the seismic phases labeled on the seismogram

Travel-time curves for surface focus

Notation of various phases through Mantle and Core

Earth’s P velocity, S velocity, and density as a function of depth

Earth’s Interior

EARTHQUAKE HAZARDS AND ITS MITIGATION

Dr. William K Mohanty Assistant Professor Department of Geology and Geophysics Indian Institute of Technology, Kharagpur

EARTHQUAKE HAZARDS • • • • • • •

Ground shaking Structural Hazards Liquefaction Landslides Retaining structures failures Lifeline Hazards Tsunami and Seiche Hazards

GROUND SHAKING • •

•

• •

Most important of all seismic hazards When the earthquake occurs, seismic waves radiate away from the source and travel rapidly through the earth’s 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. Size b. Location of earthquake c. 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 source a) The Amplitude b) Frequency content c) Duration of the motion

THE AMPLITUDE

v ( w) = a ( w) / w u ( w) = v( w) / w

where u , v and a are the transformed displacement, velocity and acceleration respectively.

PEAK 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 ACCELERATION

FREQUENCY CONTENT PARAMETERS •

Frequency content describes how the amplitude of a ground motion is distributed among different frequencies

GROUND MOTION SPECTRA α x(t ) = C 0 + ∑ C n sin( wn t + φ n ) n =1

where Cn and Φn are the amplitude and phase angle respectively of the nth harmonic of the Fourier series

RESPONSE 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 spectrum

Vmax/amax • Vmax/amax should be related to the frequency content of the motion • For a simple harmonic motion of period T, Vmax/amax =T/2π • For earthquake motion that include many frequencies, the quantity 2π (Vmax/amax) provides, which periods of the ground motions are most significant

Site Condition

Vmax/amax

Rock

5.5 cm/sec/g = 0.056 sec

Stiff soils (7.4

where R is the closest distance to seismic rupture in km. The source term F, takes on value of 0 for strike-slip and normal faulting and 1 for reverse, reverse-oblique and thrust fault SSR = 1 for soft-rock site (sedimentary deposit of Tertiary age and crystalline rock) SHR = 1 for hard-rock (older sedimentary, metamorphic and crystalline rock) SSR = SHR = 0 for alluvium site

LOCATION OF CHAMOLI EARTHQUAKE

STATIONS WHICH RECORDED THE MAINSHOCK AND THE AFTERSHOCKS OF CHAMOLI EARTHQUAKE

OBSERVED PEAK GROUND MOTION (TRIANGLES) VS. HYPOCENTRAL DISTANCE ‘R’ DURING THE CHAMOLI EARTHQUAKE

EAST-WEST COMPONENT OF ACCELERATION AND VELOCITY TRACES AT SITES IN DELHI DURING THE CHAMOLI EARTHQUAKE

SPECTRAL RATIOS OF SOFT SITES TO RIDGE OBSERVATORY

OBSERVED AND PREDICTED HORIZONTAL Amax AND Vmax AS FUNCTION OF Mw AT DELHI SITES

Predicted Peak Ground Motion at Sites in Delhi

Simulated Horizontal Ground Motion at CPCB and RO

Hypocentral location in 3-Dimension

Three Component Single Station Distance =  = ts-tp Azimuth = Z     Where   tan 1  AEW A  NS  



  90  0

can be determined from this table Direction of first motion

Angle 

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

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 (, h) + Cs + Cr Where A T f Cs Cr

= max. amplitude in thousandths of mm, = period of the seismic wave in seconds, = correction factor for epicentral distance () and focal depth (h), = correction factor for the seismological station, and = 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 σ (∆, h). The latter is the log of the inverse of the reference amplitude Ao(∆, h) of an event of zero magnitude, i.e. σ (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 σ (∆, 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 = log A - 2.48 + 2.76 log 

ML

Duration Magnitude, MD

MD = a o + a 1 log D+ a 2 

Body Wave Magnitudes (MB) Mb

= log (A/T) + Q (h,  )

Where A = actual ground motion amplitude in micrometer, and T = corresponding period in second.

Surface-Wave Magnitude (Ms ) Where

Ms = Log (A/T) + 1.66 log  + 2.0

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  = Epicentral distance in degree.

Moment Magnitude, Mw

2 M W  log 10 M 0  6.0 3 Where M0 is in Nm.

A = fault area (length x depth) m2 d = longitudinal displacement of the fault , m and  = 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

Intensity Intensity is a measure of the effect that an earthquake produces at a given location.

Modified Mercalli Intensity (MMI) Scale

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 Energy

Magnitude of Earthquake and their effects

Aftershocks and Fault Area Omori’s Law:

n

C

K  t 

p

Where , n = frequency of aftershocks at time t after the main shock.K, C, and P are constants that depend on the size of the earthquake, and P value is usually close to 1.0 – 1.4

log A 1.02 M S  6.0 Where , A is measured in sq-cm

Attenuation Relation • LnY =c1+c2M-c3lnR-c4R+c5F+c6S+   r  c7 exp (c8 M ) R 2 2   r  c7  exp c8 M  Log a = -1.02 + 0.249M – log R – 0.00255 R R2 =D2 + H2 ; H=7.3 km, D=closest distance to surface projection of the source in km M=Moment magnitude (Joyner and Boore, 1981)

Earthquake Prediction  Long Term – Years in advance  Intermediate- Weeks in advance  Short Term- Hours or days in advance Fault Characteristics Long Term

Intermediate and Short Term

Recurrence Interval (Seismic Gap) Time of last earthquake

Precursory phenomena

Precursory phenomena • • • • • • • • •

Change in / Increase in seismic activity Emission of the radioactive gas Ground water fluctuation Changes in taste and temperature in wells and springs Teleseismic P-wave travel time delays Variation of geomagnetic, geoelectric fields, resistivity Geodetic, leveling measurement Animal behavior