Earthquake Geoscience: Seismology Lecture Notes PDF

Geos 218: Geological Disasters & Society Unit 5a: Earthquakes Seismology P-waves, S-waves, Surface waves Locating EQs Magnitude vs Intensity 1 http://www.iris.edu/hq/inclass/anima Types of seismic waves tion/1component_seismogram_buildi...

Geos 218: Geological Disasters & Society Unit 5a: Earthquakes Seismology P-waves, S-waves, Surface waves Locating EQs Magnitude vs Intensity 1 http://www.iris.edu/hq/inclass/anima Types of seismic waves tion/1component_seismogram_buildi ng_responds_to_p_s_surface_waves P wave Fast S wave Intermediate Surface Waves Slow 2 1  Seismic Waves n Waves from large earthquakes can pass through the entire Earth. Waves do not follow straight paths through the Earth but change velocity and direction as they encounter different layers of different density Fig. 3.19 3 Travel Time Graph P waves are P wave faster than S wave S waves P waves arrive before S waves Difference S-P interval in the arrival depends on times of P distance to & S waves with earthquake distance 4 2  Locating an EQ Must determine distance of EQ from at least three seismic stations. Intersection of the circles gives the location. Fig. 3.23 5 On-Line Lecture n Part 1: Earthquake basics n Part 2: Faults (3 kinds) n Part 3: Seismic Waves n Part 4: Locating Earthquakes n Part 5: Magnitude vs Intensity 6 3  Compare Number of EQs with Magnitude ~1 every 3 years ~1 each year ~1 every 4 months ~1 each month ~1 each week ~every other day ~1 every 12 hrs ~1 every 90 min ~1 every 10 min ~1 every 90 sec ~1 every 10 sec 7 Compare Energy of EQs with Magnitude MOST of the energy is released in ~20 magnitude 7 and larger EQs each year 8 4  Northridge – Mag 6.7 Energy released for various Loma Prieta – Mag 6.9 earthquakes ~ 5.4 Northridges Great SF - Mag 7.8 = ~9.25 Loma Prietas = ~50 Northridges Sumatra – Mag 9.3 = ~200 San Frans = ~1850 Loma Prietas = ~10,000 Northridges 9 Foreshocks - Mainshock - Aftershocks Large earthquakes are not just single events but part of series of earthquakes over months or years –Largest event in series is mainshock –Smaller events preceding mainshock are foreshocks –Smaller events following mainshock are aftershocks Large event may be considered mainshock, then followed by even larger earthquake, so then re- classified as foreshock 10 5  Mainshock Aftershocks Mainshocks are followed by many smaller events that define the rupture area 1994 Northridge M=6.7 500 aftershocks in first 24 hours 3000 aftershocks with M > 1.5 in first 3 weeks 11 Sa n Fe rn ge an i d hr do rt o N 12 6  Earthquake Magnitude n Richter Magnitude à a logarithmic scale, so a magnitude 7 EQ has 10 times larger ground motion than magnitude 6 EQ, and magnitude 7 EQ has 30 times more energy than magnitude 6 EQ. n Largest recorded EQ - 1960 Chilean EQ - offset up to 20 meters - major tsunami - 9.8 magnitude (on Richter, “only” 9.5 on modern scale!) - Fault length 1000 km 13 The Richter Scale What is the Richter } { magnitude of this EQ? Defined magnitude as ‘logarithm of maximum seismic wave amplitude recorded on standard seismogram at 100 km from earthquake’, with corrections made for distance S-P = 26 sec { Amplitude=23mm } Magnitude = 5 14 7  The Richter Scale But what if it took longer or shorter to get there but had the same amplitude? 50 sec means its further away which means it must be LARGER! (6.0) 6 sec means its closer which means it must be SMALLER! (4.0) S-P = 26 sec Amplitude=23mm Magnitude = 5 15 Earthquake Magnitude Richter scale is useful for determining magnitude of shallow, small-to-moderate, nearby earthquakes Does not work well for distant or large earthquakes –Short-period waves do not increase amplitude for bigger earthquakes –Richter scale: 1906 San Francisco earthquake was magnitude 8.3 1964 Alaska earthquake was magnitude 8.3 –Other magnitude scale: 1906 San Francisco earthquake was magnitude 7.8 1964 Alaska earthquake was magnitude 9.2 (100 times more energy) 16 8  Seismic moment (Mo) – Measures amount of strain energy released by movement along whole rupture surface; Mo= µ*A*D more accurate for big earthquakes – Calculated using rocks’shear µ= rigidity of the rock strength times rupture area of A= fault area fault times displacement (slip) on D= displacement the fault – 1 magnitude per quake Moment Magnitude Mw = 2/3*log10 Mo – 10.7 (constants picked so that the new scale agrees w/ Richter for smaller EQs) 17 http://www.iris.edu/hq/inclass/animation/magnitudes_moment_magnitude_explained 18 9  Earthquake Rupture Length Fault-rupture length (area in this case, not slip) greatly influences EQ magnitude 0.1 km (100m) long fault rupture à magnitude 4 EQ 1 km long fault rupture à magnitude 5 EQ 10 km long fault rupture à magnitude 6 EQ 100 km long fault rupture à magnitude 7 EQ 19 Magnitude vs. Intensity n The Magnitude of an EQ is a measure of how big it is – how much energy is released. n (1 magnitude value per earthquake) n The Intensity (Modified Mercalli) of an EQ is a measure of how much shaking occurs at any place and how much damage it does. n (Intensity varies a lot across the “felt” area”) 20 10  Ground Motion During Earthquakes Buildings are designed to handle vertical forces (weight of building and contents) so that vertical shaking in earthquakes is usually safe Horizontal shaking during earthquakes can do massive damage to buildings Acceleration (like the feeling of stepping on your gas pedal!) – Measure in terms of acceleration due to gravity (g) – (g) is 9.8 m/sec2 (32 ft/sec2) free falling body due to gravity – Weak buildings suffer damage from horizontal accelerations of more than 0.1 g (= 0.98 m/s2) – At isolated locations, horizontal acceleration can be as much as 1.8 g (Tarzana Hills in 1994 Northridge, California earthquake) (FYI: 1g accelerates you to 22 mph after 1 sec) 21 1971 San Fernando EQ M=6.5 EQ Intensity is what you feel. What controls the intensity? n Magnitude n Distance n Geology n Building style n Duration of shaking 22 11  http://www.iris.edu/hq/inclass/animation/earthquake_intensity 23 Modified Mercalli Intensity I. Not felt except by a very few under especially favorable conditions. II. Felt only by a few persons at rest, especially on upper floors of buildings. III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Vibrations similar to the passing of a truck. IV. Felt indoors by many, outdoors by few. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. VII. Damage negligible in buildings of good design and construction; considerable damage in poorly built or badly designed structures. VIII. Damage slight in specially designed structures; considerable damage in well- built buildings with partial collapse. Damage great in poorly built structures. IX. Damage considerable in specially designed structures; Damage great in well-built buildings, with partial collapse. Buildings shifted off foundations. X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. XII. Damage total. Objects thrown into the air (acceleration > g!) 24 12  Modified Mercalli Intensity Mercalli intensity depends on: Earthquake magnitude –Bigger earthquake, more likely to cause death and damage Distance from hypocenter –Usually (but not always), closer earthquake à more damage Type of rock or sediment making up ground surface –Hard rock foundations vibrate from nearby earthquake body waves –Soft sediments amplified by distant earthquake surface waves –Steep slopes can generate landslides when shaken 25 Intensity Intensity Map for the New Madrid Earthquakes in 1811-12 Why are there earthquakes here in Missouri? More in Unit 6 26 13  Quick Review Which is correct? A. Magnitude is a measure of how hard the shaking feels. B. Intensity is a measure of how hard the shaking feels. C. Both are the same 27 14