Published on March 12, 2008
ESYS 150LECTURE 7BASIC PRINCIPLES OF EARTHQUAKE GEOLOGY: ESYS 150 LECTURE 7 BASIC PRINCIPLES OF EARTHQUAKE GEOLOGY BASIC GEOLOGICAL CONCEPTS ELEMENTARY SEISMOLOGY GROUND MOTION TSUNAMI BASIC GEOLOGICAL CONCEPTSEARTHQUAKECaused by a fault: BASIC GEOLOGICAL CONCEPTS EARTHQUAKE Caused by a fault Fault is a fracture along which the earth moves. Offsets of tilled farmland by 1979 movement of the Imperial fault. View is to the east. West side of the fault has moved north. BASIC GEOLOGICAL CONCEPTSFAULTS AND GEOLOGICAL MAPPINGThree Basic Principles (Steno 1669): BASIC GEOLOGICAL CONCEPTS FAULTS AND GEOLOGICAL MAPPING Three Basic Principles (Steno 1669) Law of Original Horizontality - sediments are deposited or settle out of water in horizontal layers. Law of Superposition - in an un-deformed sequence of sedimentary rocks each successive layer is deposited on top of a previously formed, and hence older, layer. Law of Original Continuity - sediment layers are continuous ending only by butting up against a topographic high, pinching out or changing from one sediment type to another. BASIC GEOLOGICAL PRINCIPLESFAULTS AND GEOLOGICAL MAPPINGThe Grand Canyon: BASIC GEOLOGICAL PRINCIPLES FAULTS AND GEOLOGICAL MAPPING The Grand Canyon Lowermost rocks are the oldest. Buried at great depths and altered by metamorphism, then exposed at surface by erosion. Younger marine sediments were deposited as layers, tilted and then eroded. Then the seas returned and horizontal layers were deposited. Finally whole region was uplifted and the Colorado river cut through to form gorge GEOLGICAL CONCEPTSTYPES OF FAULTDescription of fault surface: GEOLGICAL CONCEPTS TYPES OF FAULT Description of fault surface A 75 My old exposed sandstone layer at La Jolla Bay exposed. The sea surface forms a horizontal plane against the inclined bed. The strike of the bed is the angle the horizontal makes with north. The dip is the angle between the perpendicular to the strike and the bed GEOLOGICAL CONCEPTSTYPES OF FAULTFootwall/Hanging wall from Mining geology: GEOLOGICAL CONCEPTS TYPES OF FAULT Footwall/Hanging wall from Mining geology Diagram showing miners excavating ore that precipitated in an old fault zone. Rock layers are not continuous across the fault. Miners put their lamps on the hanging wall and walked on the foot wall GEOLOGICAL CONCEPTSTYPES OF FAULTNormal and Reverse faults: GEOLOGICAL CONCEPTS TYPES OF FAULT Normal and Reverse faults Cross section of a normal fault. Hanging wall moved downwards. Zone of omission indicates extensional forces active Cross section of a reverse fault. Hanging wall has moved upwards. Zone of repetition indicates compressional forces active. GEOLOGICAL CONCEPTSTYPES OF FAULTStrike-slip fault: GEOLOGICAL CONCEPTS TYPES OF FAULT Strike-slip fault The right hand side of the fault moves towards the man when he straddles the fault. When he turns around the right hand side will still have moved closer to him. Called a right lateral fault GEOLOGICAL CONCEPTSTYPES OF FAULTLeft stepping right lateral fault: GEOLOGICAL CONCEPTS TYPES OF FAULT Left stepping right lateral fault A left step in a right lateral fault. The land is pushed together when the fault moves. Eventually could grow into a mountain. (b) Example shows land offset along the Superstition Hills right lateral fault during the 16 November 1987 earthquake. GEOLOLOGICAL CONCEPTSTYPES OF FAULTRight stepping right lateral fault: GEOLOLOGICAL CONCEPTS TYPES OF FAULT Right stepping right lateral fault The land is pulled apart at the fault bend whenever the fault moves. It creates a hole the could become a basin. Land offset along Superstition Hills right lateral fault during the rupture. Note the right step and the pull apart at the bend. GEOLOGICAL CONCEPTSTYPES OF FAULTFault surface: GEOLOGICAL CONCEPTS TYPES OF FAULT Fault surface Diagram of the fault surface. Hypocenter where rupture began: Epicenter point on Earth’s surface above the hypocenter. Due to dip of the fault, the epicenter does not plot on the trace of the fault at the surface. SEISMOLOGYBASICSSeismometer: SEISMOLOGY BASICS Seismometer The basic principles. The earth moves moving the framework of the seismometer. The suspended heavy weight and then pen beneath remain relatively stationary. As the concrete base moves the stationary pen draws an ink line on paper on the rotating drum. SEISMOLOGYBASICSSeismogram: SEISMOLOGY BASICS Seismogram Seismogram recorded on Russia of a 1909 earthquake in Asia Minor. Three types of wave are observed P (Primary), S (Secondary) and L (surface or Love) waves. Note the difference in time between the P and S waves. SEISMOLOGYBASICSBody waves.: SEISMOLOGY BASICS Body waves. Two types of waves move through the earth (Body waves) P waves. Travel the fastest. Like the push-pull motion of a slinky toy. (Granite 4.8 km/sec) S waves. Move in an up and down motion perpendicular to the direction of advance. Like the waves in a shaken rope. (Granite 3 km/sec) SEISMOLOGYSEISMIC WAVESSurface waves: SEISMOLOGY SEISMIC WAVES Surface waves Surface waves created when body waves hit the surface of the earth. They are similar to the waves created by throwing a rock into a lake. Consist of Love waves which are horizontally polarized and Raleigh waves which cause both vertical and horizontal movement. They travel slower than body waves but with less attenuation because of long wavelength and low frequency. SEISMOLGYEARTHQUAKE LOCATIONTravel time versus distance plots: SEISMOLGY EARTHQUAKE LOCATION Travel time versus distance plots Can construct distance versus travel time for earthquakes knowing properties of earth’s interior Note that with increasing distance the arrival times for the various waves separates. The 3 min 45 sec difference between the arrival time of the P and S waves corresponds to a distance of 2000 km. SEISMOLOGYLOCATING EARTHQUAKESTriangulation: SEISMOLOGY LOCATING EARTHQUAKES Triangulation Need three different seismic stations to locate an earthquake. Measure the P-S arrival times and convert these to distance. Triangulate using radius given by P-S travel times. Triangulation for position of an earthquake near New Madrid using stations in Columbus, St Louis and Memphis SEISMOLOGYMAGNITUDEProperties of waves: SEISMOLOGY MAGNITUDE Properties of waves Important terms are Amplitude - height of wave above the starting point Wavelength - the distance between successive waves Period - the time between successive waves = 1/ Frequency Body waves - 0.5 to 20 Hz Surface waves - 0.005 to 0.1 Hz. SEISMOLOGYMAGNITUDEScale: SEISMOLOGY MAGNITUDE Scale Richer scale Based on the log to the base ten of the amplitudes of a standard seismometer 100 km from the earthquake center. For every ten fold increase in amplitude the Richter magnitude increases by one. However the energy release goes up by a much large factor Quick and easy to use SEISMOLOGYMAGNITUDENomograph of the Richter scale: SEISMOLOGY MAGNITUDE Nomograph of the Richter scale Read the difference in travel time between P and S waves and plot. Read the maximum amplitude of the S wave and plot. Draw a line between the two points and gives the magnitude. SEISMOLOGYMAGNITUDEEarthquakes in the world each year.: SEISMOLOGY MAGNITUDE Earthquakes in the world each year. Earth is shaken by millions of quakes each year. Most are too small to be felt. To be damaging most have to be above 6.0 Above 7 they can cause major damage. Note the decrease in frequency as the magnitude increases SEISMOLOGYMAGNITUDEComparison of Earthquakes: SEISMOLOGY MAGNITUDE Comparison of Earthquakes Comparison of Northridge, Loma Prieta, San Francisco and Sumatra 2004 earthquakes. Compute seismic moment and then convert to Moment Magnitude Moment is more reliable measure for large earthquakes as it measures directly the amount of strain energy released by movement along rupture surface. GROUND MOTIONIMPORTANT CONCEPTSAcceleration and Resonance: GROUND MOTION IMPORTANT CONCEPTS Acceleration and Resonance Buildings built to handle vertical forces such as weight but not horizontal forces. Acceleration compared to gravity 9.8 m/sec2 Acceleration - .1g weak buildings break up 1g nothing stands up Resonance - ground has natural frequency buildings have natural frequency 1 story .1s/ 30 story 3 secs / .1sec per story GROUND MOTIONIMPORTANT CONCEPTSEarthquake Intensity: GROUND MOTION IMPORTANT CONCEPTS Earthquake Intensity Want quantitative method of measuring effect of earthquakes. Use descriptive Mercalli scale Major variables - Magnitude - Distance from hypocenter - Foundation materials/slopes - Building style - Duration of shaking GROUND MOTIONEARTHQUAKE INTENSITYMercalli Scale: GROUND MOTION EARTHQUAKE INTENSITY Mercalli Scale Earthquake magnitude asses energy released by earthquakes. Earthquake intensity measures the effect on people and buildings Used to asses historical records GROUND MOTIONEARTHQUAKE INTENSITYBuilding collapses: GROUND MOTION EARTHQUAKE INTENSITY Building collapses Especially dangerous are buildings with “soft” first stories. (a) No bracing on the first floor and no shear walls. Stresses are concentrated at the joints between the between the first and second floors. They give way causing the floor to flatten. (b) Building on Washington Street in San Diego GROUND MOTIONEARTHQUAKE INTENSITYBridges collapse: GROUND MOTION EARTHQUAKE INTENSITY Bridges collapse Poorly designed buildings collapsed due to columns moving out from underneath the top heavy road bed. Led to major retrofitting of all bridges in California. Strengthening columns and improving connections to the road way. TSUNAMIHOW CREATED: TSUNAMI HOW CREATED Biggest and most feared wave, harbor wave - Japanese Due to sub sea movements that disturb water mass, most often earthquakes. Analogous to throwing a stone into a pool, ripples - tsunami waves Period 10 - 60 min, Wavelength 100 - 800 km, amplitude 1 -2 m, velocity 230 m/sec (500 mph) At coast waves slow down and pile up and come on shore like a very rapidly rising tide. TSUNAMINOTALBLE ONES BEFORE SUMATRAN TSUNAMI: TSUNAMI NOTALBLE ONES BEFORE SUMATRAN TSUNAMI Many tsunami’s in the Pacific. Mainly from sea bottom motion during earthquake. Velocity = √(g x D) where D is the depth of the ocean in meters. = 230 m/sec Moves quite slowly relative to P and S waves TSUNAMIEARLY WARNING SYSTEM: TSUNAMI EARLY WARNING SYSTEM P, S and surface waves move at between 10 - 5.5 km/sec. Much faster than tsunami velocity of 230 m/sec. Body waves arrive much sooner than the tsunami waves. Have enough time to determine size and position of earthquake before the tsunami arrives.This is the basis of the early warning system.