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Seismology: Fundementals 2


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  • 1. Introduction to Seismology Introduction to Seismology-KFUPM Strong Motion Seismology Ali Oncel Department of Earth Sciences KFUPM Introduction to Seismology-KFUPM Previous Lecture Richter's Local Magnitude Earthquake Magnitudes Magnitude Saturation What causes saturation? Are Mb and Ms still useful? What is the better estimate of M? Moment magnitude Strength of Earthquake Magnitude-Energy Correlation Introduction to Seismology-KFUPM How to compute the energy released by an earthquake? E = Mo/(2 x 104) log E = 1.5 Mw + 11.8 Where Es is seismic energy This figure was produced in cooperation with the US Geological Survey, and the University of Memphis private in ergs. Then, foundations log Mo - log(2 x 104) = 1.5 Mw + 11.8 Mw = (log Mo - 16.1) / 1.5 1 erg= 10-7 Joule From: Kanamori, H., 1977, The Energy Release in Great Earthquakes, Journal of Geophysical Research, v82, p. 2981- 2987. 1
  • 2. Introduction to Seismology-KFUPM Example: Energy for Mw=4.0? The total seismic energy radiated from the source, E(4), would be: E(4) = 10**(1.5*4 + 11.8) = 10**17.8 ergs = 6.3 x 1011 Joules 1 erg= 10-7 Joule The moment, Mo(4), would be: Mo(4) = E x (2 x 104) = 1.26 x 1015 Joules It has been found that a 1 kton explosion will generate seismic waves approximately equivalent to a magnitude 4 earthquake. The energy released by TNT (trinitrotoluene) and the TNT equivalent of the Hiroshima nuclear bomb (McGraw-Hill Encyclopedia of Science and Technology, 1992): Energy per ton of TNT = 4.18 x 109 Joules Earthquake Damage A rescue worker stands by a crack in Parks Collapse of Fourth Avenue near C Street, Anchorage, due to Highway caused by a 7.9 earthquake that rocked earthquake caused landslide. Before the earthquake, the a sparsely populated area of Alaska. By Jimmy sidewalk at left, which is in the graben, was at street level on Tohill, AP the right. The graben subsides 11 feet in response to 14 feet o horizontal movement. Anchorage district, Cook Inlet region, Alaska. 1964 A crack system destroys driveway adjacent to summit road 0.8 km (1/2 mi) southeast of Highway 17. [J.K. Nakata, U.S. Geological Survey] Aerial view of collapsed sections of the Cypress viaduct of Interstate Highway 880. [H.G. Wilshire, U.S. Close-up view of tsunami damage along the waterfront at Geological Survey] Kodiak. Introduction to Seismology-KFUPM A Given Earthquake has: Only one magnitude. A certain amount of energy was released. Parks and Plates Different intensity. Depends on your location. ©2005 Robert J. Lillie Intensity is a qualitative (non-numerical) rating of the ground shaking at a given place. It is inferred from the effects of the earthquake on the land and any buildings in the area. First-hand accounts from people are also used. Some scales of intensity are the Modified Mercalli, JMA, and MSK Intensity Scales. Quantitative (numerical) measures of ground motion from accelerograms are descriptions of shaking “severity” and should not use the word “intensity”. 2
  • 3. Introduction to Seismology-KFUPM HARRY O. WOOD and FRANK NEUMANN, Modified Mercalli intensity scale of 1931, pp.277-283, BSSA Introduction to Seismology-KFUPM Felt Report Form F e for a New Event l t R e p DID YOU o r FEEL IT? t F REPORT IT o r HERE! m f o r a N e w E v e From: n Introduction to Seismology-KFUPM M=5.6, July 8, 1986, Palm Spring Earthquake From: 3
  • 4. Introduction to Seismology-KFUPM Earthquake Fa Intensity Generally ultl Increases with ine Distance from Epicenter Parks and Plates ©2005 Robert J. Lillie Earthquake Intensity Generally Increases with Distance from Epicenter Parks and Plates ©2005 Robert J. Lillie Introduction to Seismology-KFUPM INTENSITY Factors that determine how intense the shaking was at a given location: New Madrid Earthquakes (1811-12) Magnitude of the earthquake. Distance from the earthquake focus. Local soil conditions. Earthquake Intensity Database Search 1638-1985 U.S. Geological Survey map From: 4
  • 5. Introduction to Seismology-KFUPM Animated Example from Hayward Fault This sequence of intensity maps shows how the intensity grows as a fault rupture increases in length. This sequence, shown for the northern Hayward fault, starts from a hypothetical magnitude 5 and grows to a magnitude 7.1 earthquake. The dynamic rupture process in this hypothetical earthquake would occur in about 8 seconds. Introduction to Seismology-KFUPM Damage Scenario A damage scenario is a representation of the possible damage caused by an earthquake to the built environment in an area. It is in terms of parameters useful for economical and engineering assessment or postearthquake emergency management. A probabilistic earthquake scenario is a representation of earthquake effects. The earthquake has a specified probability of exceedance during a prescribed period in an area. Adapted from the International Handbook of Earthquake and Engineering Seismology, Aki and Lee[1] From the International Handbook of Earthquake and Engineering Seismology, Aki and Lee[1] Used at Los Angles Co. Emergency Operations Center, By LA County Office of Emergency Management for Training Scenario, November 9, 2000. LA County Emergency Operations Center 5
  • 6. Introduction to Seismology-KFUPM Earthquake Damage Scenario Shake Map: Example for Verdugo Fault Introduction to Seismology-KFUPM EXPLANATION of the online "Intensity Maps" activity Source : Assignment: Measuring Earthquakes Due to April 15, Group Based Work is suggested. Exercises I. Finding Isoseisms II. Locating an Epicenter by Creating an Intensity Map 6
  • 7. Introduction to Seismology-KFUPM Previous Lecture Magnitude How to compute the energy released due to an earthquake Earthquake Damage Magnitude-Intensity Mercalli Scale of arthquake Intensity Data Acquisition for Earthquake Example: Palm Spring Earthquake, M5.6, July 8, 1986 Relationship between Intensity and Distance Animated Example of Intensity: Hayward Fault Damage Scenario Earthquake Planning Scenario: Example for Verdugo Fault Introduction to Seismology-KFUPM Project and Presentation Introduction to Seismology-KFUPM Faults A fault is a fracture across which there has been sliding parallel to the Parks and Plates ©2005 Robert J. Lillie fracture surface. When people stand across the fault, their heads are next to the hanging wall, their feet next to the foot wall. a) Normal fault: A normal fault is a dip-slip fault where the rock above the fault moves down compared to the rock below the fault. b) Reverse fault: A reverse fault is a type of dip-slip fault. The hanging-wall fault block of a reverse fault moves upward relative to the footwall fault block. (A thrust fault is a low-angle reverse fault.) c) Strike-slip fault: A strike-slip fault is a fault where two plates slide horizontally past each other. It is called a strike-slip fault because the “slip” of the fault is parallel to the “strike” of the fault. 7
  • 8. Introduction to Seismology-KFUPM Faults Strike-slip faulting Strike- animation Introduction to Seismology-KFUPM Strike-Slip Fault Source: Photo taken from Martin Miller's website, University of Oregon. 8
  • 9. Introduction to Seismology-KFUPM Normal Faulting animation Introduction to Seismology-KFUPM Thrust Faulting animation Source: University of Victoria. 9
  • 10. Introduction to Seismology-KFUPM Focal Mechanisms There are special graphics, called focal mechanisms or “beach balls” that we use as shorthand to describe the style of faulting. Introduction to Seismology-KFUPM Seismographs A seismograph is an instrument which writes a permanent continuous record of earth motion.[1] Measuring the three orthogonal components of ground motion at a seismic station. Introduction to Seismology-KFUPM Simplified motions of seismic waves Seismic waves are waves of energy that travel through Earth and along its surface. The waves are produced by earthquakes, explosions, or some other disturbance. Seismic waves are studied to locate and understand earthquakes and to determine the structure of Earth's interior. 10
  • 11. Introduction to Seismology-KFUPM a)Body and surface wave paths from an earthquake located SSE of a station. b) Seismograms from each of the three seismometers, responding to arrivals of the body (P= compressional, S= shear) and surface (L=Love, R= Rayleigh) waves Introduction to Seismology-KFUPM Initial Ground Motions and Z-component seismograms for P-waves Pushes the ground up Initial arrival as a compression pushes the ground up; Z-component shows an upward first motion. Introduction to Seismology-KFUPM Initial Ground Motions and Z-component seismograms for P-waves Pulls the ground down Initial arrival as a dilatation pulls the ground down; Z-component shows an downward first motion 11
  • 12. Compression Dilatation Introduction to Seismology-KFUPM Initial P-wave Radiation Pattern: from an Earthquake occurring on a reverse fault a)Waves radiate outward in quadrants of compression and dilatation b)The Z-component seismograms for the three stations highlighted in (a). Introduction to Seismology-KFUPM a) Map view of radiation pattern for right-lateral, strike- slip fault occurring along the San Andreas transform plate boundary b) First-motion information for arrival at stations indicated in (a), plotted as a focal mechanism solution. 12
  • 13. Introduction to Seismology-KFUPM c) Auxiliary fault interpretation of the first-motion in (a), showing that the same radiation pattern could have resulted from a left-lateral fault. d) Focal mechanism solution for (c) is exactly the same as that resulting from right-lateral fault in (a). Introduction to Seismology-KFUPM Case Work: Fault interpretation May 1, 2003 May 1, 2003- Bingol Earthquake (Mw=6.4) Mw=6.4 What is the strike of fault? Introduction to Seismology-KFUPM Case Work: Fault interpretation TUBITAK Earthquake Stations 13
  • 14. Introduction to Seismology-KFUPM Case Work: Fault interpretation Earthquake Aftershocks Introduction to Seismology-KFUPM Case Work: Fault interpretation Earthquake Aftershocks Depth Distribution Introduction to Seismology-KFUPM Observed surfaced deformation due to Bingol Earthquake 14
  • 15. Introduction to Seismology-KFUPM Homework: VIRTUAL SEISMOLOGY You have learned how to pinpoint the location of an earthquake by measuring the speed of seismic waves radiating away from the focus of the earthquake. Now, we can determine an earthquake's magnitude by measuring the strength of ground shaking as you did for global earthquakes. Learn how to do both these things by visiting the virtual earthquake web page. e/VQuakeExecute.html and completing the exercise. It should take you about 30 minutes. Turn in your certificate of completion at the beginning of class on Monday, 26 March. 15