VẬT lý địa CHẤN class3 earthquakes

This map was made with a combination of satellite predicted bathymetry and topography from various sources It gives us a chance to make some sweeping tectonic statements Can you identify many of the features? What type of regional geophysical data might allow us to make better educated guesses at what some of these features might be? Seismology – What is a Wave Seismology is primarily concerned with determining the structure of the earth – on all scales To accomplish this it uses the ability of seismic waves to propagate through the earth What is a wave? Wavelength: The length between two crests or troughs Amplitude: The maximum height relative to the zero position Frequency: The number of “wavelenghts” that pass a point in one second From: Mussett and Khan, Looking into the earth It is important to note that although the waves travels through the earth, the material itself does not – in the same way as water Frequency v = f *λ Velocity = frequency x wavelength Frequencies in seismology range from less than 10 to perhaps 100s of Hz (cycles per second) From Tarbuck and Lutgens, 2005 Ancient Seismographs The Chinese made the first seismographs over 2000 years ago This seismograph will tell the observer the direction of the first motion of the ground during an earthquake From Tarbuck and Lutgens, 2005 Earthquake Nowadays, if an earthquake occurs, how we detect it, and how the waves travel – in the next few slides I will attempt to answer these questions Detecting Seismic Waves It is easy to visualize the motion of an ocean wave, and how we might measure its wavelength, amplitude, and frequency, but how we this with the earth? On the left are two basic seismometers When the ground moves the pendulums moves either horizontally (top) or vertically (bottom) The movement is damped so that subsequent motion of the earth is not obscured by the pendulum oscillating On the right is a more conventional seismometer Motion of the magnet through the coil generates a current in the coil which is amplified and recorded Three seismometers oriented vertically, N-S and E-W are required to measure the full ground motion Figures from: Mussett and Khan, Looking into the earth Building A Seismometer A basic seismometer is actually very simple Shown here is a simple seismometer that with the addition of some electronics (amplifier etc) will happily record earthquakes Refer to http://www.iris.edu/edu/AS 1.htm for more details Snell’s Law The earth is not a uniform sphere Broadly speaking, it is made up of layers When wave fronts cross from one rock type into another with a higher velocity they turn From Kearey et al., 2002 From Mussestt and Khan, 2000 Wavefront The time between successive wavefronts remains unchanged, so the wavelength must increase in the second rock in proportion to the increase in velocity v1 λ1 BB' = = v2 λ2 AA' BB' sin i = Trigonometry AB' tells us that: AA' sin i2 = AB' Rearranging gives: AB' = BB' AA' = sin i1 sin i2 Snell’s Law As BB’ and AA’ are in proportion to the velocities v1 and v2, the equation can be rearranged to sin i1 sin i2 = v1 v2 Snell’s Law From Mussestt and Khan, 2000 sin 37 o sin i2 = 5 sin i2 = sin 37 o So i2 = 48.8o Answer the following question: A ray traveling in a rock with a seismic velocity of km/s encounters an interface with a rock of km/s at an angle of 45o At what angle from the normal does it leave the interface? Snell’s Law – Multiple Horizons From Mussestt and Khan, 2000 sin i1' sin i2' = v1 v2 sin i2' sin i3 = v2 v3 As i’1 = i1, I’2 = i2, and so on sin i1 sin i2 sin i3 = = = constant v1 v2 v3 The ratio (sin i/v) thus remains unchanged How Deep was that Earthquake? The depth of the hypocenter below the epicenter can be found by measuring the difference in arrival of the direct P-wave and the wave that reflects from the surface, pP As the depth increases the pP-P difference increases From Mussestt and Khan, 2000 Fault Plane Solutions Simply put, a fault plane solution tells us the orientation and nature of the fault that caused an earthquake From Mussestt and Khan, 2000 In the simple case above, imagine a peg in the ground that is struck by a hammer from the north Immediately following the impact the ground directly to the north of the peg experiences compression, and that to the south experiences dilation The magnitude of the compression and dilation decreases off axis (b) S-waves are also generated in the east and west directions Fault Plane Solutions From Mussestt and Khan, 2000 In the above example there is a N-S trending right-lateral (dextral) strike slip fault surrounded by a circle of seismometers When this fault moves, the seismometers will record a “first motion” In the top left quadrant this is +ve, or up, in the bottom left quadrant this is –ve, or down By mapping out the first motions on these seismometers, we can derive what sort of earthquake it is However, there is ambiguity, as a left lateral strike slip fault trending E-W would also fit these first motions This is where we might also consider the local geology – are there any dominant trends? Is there a cluster of aftershocks that illuminates a particular plane? Beachballs From Mussestt and Khan, 2000 Traditionally an earthquake fault plane solution is displayed as a beach ball (b) Here we are looking down onto the lower hemisphere of a sphere (a) To create this beachball, earthquakes are plotted an an equal area Lambert projection net using the azimuth, take-off angle, and sense of the earthquake The above beachball defines a fault plane with a roughly NW-SE strike and a dip of either 25o ~NE or 65o ~SW Making A Beachball From Mussestt and Khan, 2000 First plot azimuth, take-off angle and sense of earthquake Find a plane that splits two areas of compressional and dilational earthquakes Plot the pole (P) to that plane, and then find another plane that also splits two area of compressional and dilational earthquakes, but also passes through the pole to the previous plane (ensuring that both planes are perpendicular) Beachballs for Various Faults From Mussestt and Khan, 2000 Earthquakes in PNG From Tarbuck and Lutgens, 2005 Earthquake Intensity The Mercalli Intensity Scale is a measure of what people reported This is useful for areas where there were few seismometers, or in study of ancient earthquakes Earthquake Magnitude In 1935 Richter devised the Richter magnitude scheme for describing the size of earthqauke •Measured amplitude in microns of the largest oscillation of a particular type of seismometer 100 km from the source •The amplitudes have a very large range, so he took the logarithm (to base 10) to make the numbers more manageable An increase of in magnitude means the amplitude is 10 times greater (energy release is 30 times greater) •Magnitude = log10(max amplitude of oscillation, in units of 10-6m) •-ve values are possible (oscillations < one millionth of a meter) Many –ve magnitude earthquakes have been recorded at the HUGO seismic station half way between Hawai’i and the mainland •The scale was originally designed for shallow earthquakes near the receiver and a particular type of seismometer It has been modified to deal with this •It underestimates the biggest earthquakes – many seismometers are not as sensitive to the lowest frequencies From Tarbuck and Lutgens, 2005 Earthquake Magnitude Seismic Moment Though the Richter magnitude scale is the most commonly quoted, a later and better measure is the seismic moment, Mo Just before a fault ruptures, the shear forces on either side of the fault exert a couple, whose size, or moment, equals the product of the shear forces and the perpendicular distance between them The force is dependant on the strain, the area of rupture, A, and the rigidity modulus, μ The strain depends on the fault offset and the width of the strained volume From Mussestt and Khan, 2000 moment of couple = F * 2b d As F = µA * strain, and strain = , 2b moment of couple = µAd = M o How we determine rupture area? •Aftershocks What is the maximum seismic moment of an earthquake limited by? From Tarbuck and Lutgens, 2005 Seismic Moment Here we can see both the along strike extent of the subducting plate (shown by the earthquake distribution), and the down-dip distribution The area of the plate rupturing in a given earthquake is a limiting factor in the moment magnitude of the earthquake From Tarbuck and Lutgens, 2005 Seismic Moment Fortunately, only distinct parts of the subduction zone slip at one time, limiting the size of the earthquake Risks and Mitigation From Tarbuck and Lutgens, 2005 What are some of the earthquake risks? What are some of the ways that we can minimize damage? Tsunamis and tsunami warning systems Using automated seismic triggers to slow trains, etc (Bullet Train in Japan) Build Sensibly References Used Basic seismic theory: • Kearey, P., M Brooks, and I Hill, An Introduction to Geophysical Exploration, 3rd edition., pages 21-30, 2002 Basic theory, seismology, and earthquakes: • Mussett, A.E and M.A Khan, Looking into the earth: An introduction to geological geophysics, pages 24-64, 2000 Really basic theory: • Tarbuck, E.J and F.K Lutgens, Earth: An introduction to physical geology, chapter 11, 2005 Plus additional html references as listed in this presentation [...]... splits two areas of compressional and dilational earthquakes Plot the pole (P) to that plane, and then find another plane that also splits two area of compressional and dilational earthquakes, but also passes through the pole to the previous plane (ensuring that both planes are perpendicular) Beachballs for Various Faults From Mussestt and Khan, 2000 Earthquakes in PNG From Tarbuck and Lutgens, 2005... (oscillations < one millionth of a meter) Many –ve magnitude earthquakes have been recorded at the HUGO seismic station half way between Hawai’i and the mainland •The scale was originally designed for shallow earthquakes near the receiver and a particular type of seismometer It has been modified to deal with this •It underestimates the biggest earthquakes – many seismometers are not as sensitive to the... an earthquake Earthquakes that arrive at a distance of greater than 18o are termed teleseismic These are important as they not only sample deep parts of the earth, but they come back to the surface at a steep angle, spending as little time as possible in the highly variable crust 2002 Denali Earthquake From: http://www.citiesoflight.net/AlaskaQuake.html Note: Animation only visible in Earthquakes From... From Mussestt and Khan, 2000 Traditionally an earthquake fault plane solution is displayed as a beach ball (b) Here we are looking down onto the lower hemisphere of a sphere (a) To create this beachball, earthquakes are plotted an an equal area Lambert projection net using the azimuth, take-off angle, and sense of the earthquake The above beachball defines a fault plane with a roughly NW-SE strike and... calculated for the distance to actual seismic receivers and compared with observed times The difference between the two is then minimized by adjusting the velocity-depth curve This is repeated for millions of earthquakes and hundreds of seismometers all over the world From this, a velocity depth model can be derived From Mussestt and Khan, 2000 Body Waves There are two types of body wave (waves which travel... From Tarbuck and Lutgens, 2005 Earthquake Intensity The Mercalli Intensity Scale is a measure of what people reported This is useful for areas where there were few seismometers, or in study of ancient earthquakes Earthquake Magnitude In 1935 Richter devised the Richter magnitude scheme for describing the size of earthqauke •Measured amplitude in microns of the largest oscillation of a particular type... arrival time difference of 6.5 minutes equates to a epicentral angle of 46o We also know that the earthquake occurred about 8 minutes before the first P-arrival If this procedure is repeated for multiple earthquakes we can triangulate the location From Mussestt and Khan, 2000 How Deep was that Earthquake? The depth of the hypocenter below the epicenter can be found by measuring the difference in arrival ... the P- and S-wave (the thunder and the lightning)? As S-waves travel more slowly than P-waves, the more distant the earthquake from the receiver, the greater the lag of the S- after the P-arrival... 2000 Body Waves S-Waves cannot travel through water The passage of an S-wave depends on the medium restoring its shape after initially being sheared Water does not this The S-wave velocity is... P-wave velocity (vs = 0.55 vp) From Mussestt and Khan, 2000 reflection : sin i1P sin i1S = v1P v1S refraction : sin i1P sin r2 S = v1P v2 S As the velocity of the S-wave is different to the P-wave,