Seismic Waves.pdf

EARTHQUAKE GEOLOGY: SEISMIC WAVES Okan Tüysüz 2006

A disturbance that carries energy

energy is transferred by molecules hitting one another like dominoes

There are 3 types of waves: „ Transverse „ Compessional „ Surface

Wave

TRANSVERSE WAVES „

Definition: The wave energy causes matter in the medium to move at right angles to the direction the wave travels.

TRANSVERSE WAVES „ „ „

Only travels through solid and gas (not liquid) Can travel through space Examples: light „ radio „ Earthquake “S” waves „

COMPRESSIONAL WAVES „

Definition: Matter in the medium move forward and backward in the same direction that the wave travels

COMPRESSIONAL WAVES „ „

Travels through solid, liquid and gas Examples: Sound „ Sonar „ Earthquake “P” waves „

SURFACE WAVES „

Definition: When a transverse wave and a compressional wave combine at or near the boundary between two media „ the particles move in a circle „

SURFACE WAVES „ „

Found only at or near the earth’s surface Examples: Oceans (boundary of water and air) „ Earthquake Surface Wave (boundary of land and air) „

What are Seismic Waves ??? „

„

Seismic waves are the vibrations from earthquakes that travel through the Earth They are the waves of energy suddenly created by the breaking up of rock within the earth or an explosion .They are the energy that travels through the earth and is recorded on seismographs

Elastic Rebound Theory 1. stress on rock masses

2. rock will bend elastically

3. when stress overcomes rock strength, fault will slip 4. stored energy is released

Seismic Waves

History „

„

„

„

Around 132 AD, Chinese scientist Chang Heng invented the first seismoscope, an instrument that could register the occurrence of an earthquake. They are recorded on instruments called seismographs. Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, location and magnitude of an earthquake can be determined from the data recorded by seismograph stations.

Seismometers and Seismographs „ „ „ „ „

Seismometers are instruments for detecting ground motions Seismographs are instruments for recording seismic waves from earthquakes. Seismometers are based on the principal of an “inertial mass” Seismographs amplify, record, and display the seismic waves Recordings are called seismograms

Types of Seismic Waves „ Body

waves

Travel through the earth's interior „ Surface Waves

Travel along the earth's surface - similar to ocean waves

P-Wave (Body Wave) Primary or compressional (P) waves a) The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave. b) The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. c) It pushes and pulls the rock it moves through just like sound waves push and pull the air. d) Highest velocity (6 km/sec in the crust)

P-Wave

Secondary Wave (S Wave) „

„

„

Secondary or shear (S) waves a)The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. b) An S wave is slower than a P wave and can only move through solid rock. (3.6 km/sec in the crust) c) This wave moves rock up and down, or side-to-side.

S-Wave

P-waves

S-waves

Surface waves „ „

propagate along a surface cause the most structural damage

L-Wave „ „

„

Love Waves The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side.

L-Wave

Rayleigh Waves „

„

„

The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

Rayleigh Waves

Earthquake waves are not measured everywhere on the surface of the Earth AREA OF NO WAVE DETECTION

EPICENTER

S and P WAVES DETECTED P WAVES ONLY DETECTED

P WAVES

NO WAVES DUE TO REFRACTION

S WAVES

NO WAVES DUE TO PRESENCE OF LIQUID LAYER

Velocity of Waves Speed of sound: 1. Air = 331 m/sec at 0 degrees C. 2. Water = 1,498 m/sec 3. Iron = 5,200 m/sec

(P wave 4,000-8,000 m/sec)

Increases why?

Properties of seismic waves 1. Velocity depends on density and elasticity; faster in dense, rigid material; slower in less dense, plastic material. 2. Velocity increases with depth, because rocks are denser at depth 3. P-waves travel through solids and liquids 4. S-waves cannot travel through liquids 5. P-waves travel faster than S-waves. 6. When seismic waves pass from one material to another, the path of the wave is refracted (bent).

Into the Earth „

„

The Earth is made up of three distinct layers The three main layers are the crust, the mantle and the core.

Crust

„

„

„ „

„

The crust is the thin, solid, outermost layer of the Earth. The crust is thinnest beneath the oceans, averaging only 5 kms thick. It is thickest beneath large mountain ranges. Continental crust varies in thickness, but averages about 30-35 km. Beneath large mountain ranges, (Himalayas or the Sierra Nevada), the crust reaches a thickness of up to 100 km.

Upper Mantle

„

„

„

„

„

The layer below the crust is the mantle The mantle has more iron and magnesium than the crust, making it more dense The uppermost part of the mantle is solid and, along with the crust, forms the lithosphere The rocky lithosphere is brittle and can fracture. This is the zone where earthquakes occur. It’s the lithosphere that breaks into the thick, moving slabs of rock, tectonic plates.

Lower Mantle

„

„

„

As we descend into the Earth temperature rises and we reach part of the mantle that is partially molten, the asthenosphere. As rock heats up, it becomes pliable or ‘plastic’. Rock here is hot enough to fold, stretch, compress, and flow very slowly without fracturing. The plates, made up of the relatively light, rigid rock of the lithosphere actually ‘float’ on the more dense, flowing asthenosphere

Core

„

„ „

„

At the center of the Earth lies the super-dense core. It has a diameter of 3486 kms. The core is larger than the planet Mars The core of the Earth is made up of two distinct layers: „

„

A liquid outer layer and a solid inner core. Unlike the Earth’s outer layers with rocky compositions, the core is made up of metallic iron-nickel alloy. It’s hard to imagine, but the core is about 5 times as dense as the rock we walk on at the surface!

Trends in the Earths Interior „

„

As you progress further and further into the Earth the more dense the material becomes. This is do in part to: „

„

the weight of material overhead. Getting closer to the Earths Core

Trends in the Earths Interior „

„

As you progress further and further into the Earth the hotter the material becomes. This is do impart to: „

„

„

Boyles Law: Compression causes heating. Heat lost by the Earth’s core Radio-active decay giving off heat.

Density of the Earth -Mantle Density

Top = 3.5 g/cm3 Bottom = 5.5 g/cm3

-Whole Earth Density = 5.5 g/cm3 -Core Density must be ~ 9 -12 g/cm3

On Web

Refraction of Earthquake Waves

Incoming Ray

Low Speed Material High Speed Material

Low to High Away from Normal Normal

Refraction of Earthquake Waves Incoming Ray

High Speed Material Low Speed Material

High to Low Toward Normal

Normal

Refraction of Waves

On Web

Low density = low speed High density = high speed

Low to high - away low high

High to low - toward

Low Velocity Zone 0

P-Wave Velocity crust

100

Low Velocity Zone 300

500 upper mantle km

equations for velocities

Vp =

Vs =

k + 4/3µ ρ

µ ρ

1/2

ρ density µ

shear modulus (rigidity)

k

bulk modulus (rigidity)

1/2

because shear modulus (rigidity) for fluid is zero, S waves cannot propagate through a fluid consequence of equations is that P waves are 1.7x faster than S can infer physical properties from P and S waves

Locating and Measuring Earthquakes „

„

Determining the Location of an Earthquake Measuring the Size of an Earthquake

Measuring Earthquakes „

Seismograph = instrument used to record the movement of the Earth

Vertical Component Seismometer

Horizontal Component Seismomter

Distance - Time Relations

First Arrivals - Seismographic Records

Locating the epicenter 1. 2.

3.

P-waves travel faster than S-waves Difference in arrival time increases with distance from epicenter Epicenter location can be determined with seismograph array

P vs. S Wave Travel Time Curves

Earthquake Location by Range

Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter. „ Step 1: „ Determine the difference in arrival time for your Pwave, and your S-wave.

Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter. Step 2: „

„

Open to the Travel Time Graph.

Step 3: „

Use the vertical scale (time) to mark off the difference in arrival time on a scrap sheet of paper.

Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter. „ Step 4: „

Make sure to keep your scrap paper vertical! Slide it along the curves until you find the place where your marks each line up on one of the curves.

Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter. „ Step 5: „

Read off the distance from the horizontal axis that corresponds to this spot. This is the distance between the epicenter and your seismograph location.

Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter. „

Step 6: Repeat steps 1-5 for at least 2 other locations. „ It is necessary to have at least 3 stations, if you do not you can not be sure of the exact location of the epicenter. „

Locating the Epicenter of an Earthquake Part II Locating the Epicenter „ Step 1: „ Find the location of your first seismographic station on the map. „ Step 2: „ Use a compass or string to create a circle with its center at your seismograph location, and a radius equal to the distance you found.

Locating the Epicenter of an Earthquake Part II: Locating the Epicenter „ Repeat this procedure around two other seismographic stations. „ Where the three circles all intersect (cross) is where your epicenter is located. „ If the circles do not all intersect, but form a small triangle, the epicenter is the center of the triangle.

Locating the Epicenter of an Earthquake Part III: Origin time of the Earthquake „ From the distances determined in Part I, determine how long it would take a P wave to travel that distance.

Origin Time of Earthquake „

Step 1: „ „

„

Find the distance on the horizontal axis. Go up to the point where you hit the P-wave travel line. Go over to the vertical axis and read off the time.

Origin Time of Earthquake „

Step 2: Taking the time found in step 1, subtract this from the arrival of the P wave and that is the original time of the earthquake. „ (3:21pm)-(7 min 40 seconds) „

How do we measure earthquake size?

Earthquake Measurement Three types of scales are commonly used to measure the intensity of earthquakes: • the Modified Mercalli scale • the Richter Magnitude scale • the Moment Magnitude scale

Modified Mercalli Index „ „

„ „

Measures damage to human (man made) structures. Intensity depends on: reporting accuracy, population, development, building codes, and enforcement. Intensity Scale is I - XII. Useful for all pre-instrumental events. The few seismographs operating in the early part of the last century were isolated and uncalibrated. Calibration with explosions occurred from the 1930’s to present.

A map of Modified Mercalli intensity for the 1994 Northridge, California, earthquake.

Modified Mercalli Scale

EARTHQUAKE SIZE „ „

„

„

Richter or Magnitude Scale Measures actual energy release - developed in 1930's Scale -2 to 12. -CALIBRATION: if A is 1 micron at instrument 100 km from event, then the Magnitude = -2 - there is an increase of 32x as much energy from one integer to the next. (Therefore, a magnitude 4.1 is about 3x the size of a 4.0)

Richter Magnitude

„

Richter Magnitude scale

„

- based on maximum size of wave measured on a seismograph, corrected for distance from the seismograph to the epicenter - ranges from 1.0 (smallest) to 9.0 (largest) logarithmic scale: each increase of 1.0 corresponds to 10 times more ground shaking Magnitude – amount of energy released by earthquakes

„ „ „

Richter Magnitude

Moment Magnitude „

Moment Magnitude scale

„

- most accurate and widely used scale today - based on the total amount of energy released during quake, which is related to three factors:

„

1) the average amount of movement on the fault 2) the total area of fault surface that moved 3) the rigidity of the rock

SEISMIC MOMENT Seismic Moment, M0 = k*Dav*A K = rigidity modulus Dav = average displacement (slip) of one side of fault relative to the other A = area of fault surface that ruptured Rock Basalt Granite Sandstone Lithosphere Water

k 2.38*1010 Pa 1.38 1.81 3.3 0

Earthquake Magnitude „ „

M0 is not directly observable Use amplitudes of certain types of waves to estimate magnitude E.g., Ms = log(A/T) + 1.66 log(D) + 3.3 „ Max. surface wave amp: A „ Period of wave: T „ Distance: D „

SEISMIC MOMENT, ENERGY, AND MAGNITUDE

„

Energy release can be estimated from the seismic moment: E in dyne-cm = M0/20,000

„

Mw = 2/3(log10M0 in dyne-cm) - 16

„

Earthquake Magnitude „ „

Increase by 1 in magnitude implies a factor of 10 increase in amplitude Energy scales with squared amplitude „

Increase of 1 in magnitude implies a factor of ~30 increase in energy

DIFFERENT MAGNITUDE TYPES: „ „ „ „

log10 A of P wave = Mb scale -good for 0 - 7 log10 A of S wave = Ms scale -good for 3 - 7 log10 A of L wave = ML scale -surface waves 3 - 7 For all very large earthquakes occurring since 1973, seismologists currently integrate the area under curve of all waves on the seismogram. This results in the Mw scale, which is more accurate for very large (7.5 and up) events

„

Great Chilean Earthquake in May of 1960 is the largest earthquake ever recorded. Its Mw = 9.5 Rupture length = 1000 km. Here the Nazca Plate under the ocean collides with the South American Plate at 9.7 cm/yr. Currently the world’s fastest rate.

There are no 10.5s! Magnitude and earthquake energy scale are related: M = 2/3 log(M0) - 10.7 M is magnitude, M0 is seismic moment (energy in ergs)

There are no 10.5s! Seismic moment is given by: M0 = μDS Where μ measures shear strength, D is average fault slip and S is fault area.

There are no 10.5s! What slip is required to make a 10.5? 10.5 = 2/3 log(M0) - 10.7 -> M0 = 6.3 • 1031 μ ~ 2 • 1011 (dyne/cm2) S = 1000 km * 15 km = 1.5 • 1014 cm2 D = 2.1 • 106 cm (21 km!)

Magnitude-Frequency „ „

Small earthquakes are more numerous than large Globally (and locally) earthquakes closely obey: „ „ „ „

„

Log N = a - b M N = number of eqs with magnitude ≥ M a = “productivity” b = “b” (alas)

b is usually close to 1, implying a factor of 10 increase in eq frequency with decrease of 1 in magnitude

Magnitude-Frequency

Earthquake Depth