GEOL 170 Professor Charles M. Rubin. Lecture 01. Syllabus & organization of course. Reading for this week: Ch 1 & 2 List of terms.

Transcription

1 Professor Charles M. Rubin Lecture 01 Syllabus & organization of course Reading for this week: Ch 1 & 2 List of terms. What is an earthquake? Types of seismic waves seismic waves are acoustic waves body wave, primary or P wave shear wave, secondary or S wave surface wave, including Love wave, Rayleigh wave and 1906 San Francisco earthquake 1964 Good Friday Alaska earthquake 1994 Northridge earthquake 1999 Izmit earthquake, Turkey

2 LECTURE 2. Where earthquakes occur Earthquakes in the past: historical and pre-historical earthquakes earthquake catalog seismograph; seismometer focus, hypocenter epicenter aftershock foreshock b values (G-R relations) moonquakes The global mosaic of earthquakes Seismicity occurs in belts Belts are characterized by depth and concentration and magnitude belts separate topographic or tectonic provinces mid-ocean ridge-transform systems oceanic trench and volcanic arc systems some diffuse or concentrated zones of seismicity within continent plate boundaries or within plate deformation association with volcanoes (Hawaii) magma migration, edifice collapse association with engineering projects. Ring of Fire Depth of seismicity more difficult to determine Shallow (<70 km) Intermediate km deep > 300 km

3 LECTURE 3: Measuring earthquakes How we observe and record earthquake waves 132 A.D. Chang Heng s seismoscope. Balls were held in dragons mouths by lever devices connected to an internal pendulum. Mass on a freely movable support detects both vertical and horizontal shaking of the ground. Spring for vertical pendulum for horizontal must be dampened modern instruments - electronic Sensitive vs. strong motion seismographs Accelerometer Three waves recorded Primary wave Secondary wave surface waves records period, frequency and timing How to locate an earthquake Relies on the difference in travel time between the P and S waves triangulation differences in velocity structure depth issues Seismograph arrays central base station that provides timing variations in seismic shaking over short distance Kingdome SHIPS allows imaging of shallow earth Differences in velocity structure of the earth

4 LECTURE 4: Using earthquakes to explore inside the earth British geologist R.D. Oldham pointed out that we could discover the interior of the Earth through earthquakes. E. von Rebeur Paschwitz noted waves registered by delicate horizontal pendulum in Potsdam and Wilhelmshaven, Germany about half an hour after a great earthquake in Tokyo Eby 1900 seismographs had detected both P and S waves. sources (eq s and seismometers were needed to record global seismicity Seismic waves travel along ray paths in the Earth By 1906, Oldham recognized that the Earth must have a central core Shadow of S waves at 110 arc. In 1914, Beno Gutenberg (Gottingen, Germany) fixed the depth of this boundary at 2900 kilometers. This has held up over the years. P waves became weak at 105 arc. Experienced 5 minutes delay Reflected waves were recognized. Each time a P or S wave encoutners the Earth s surface or an boundary between two rock types in the interior, it is both reflected and refacted. Seismic tomography Like a CAT scan or ultrasound Interior Earth Structure Four main shells Crust 25 to 60 km thick under continents, 4-6 under oceans varied complex internal structure Mantle from base of crust to 2885 km dense silicate rocks solid and strong on short time scales convecting on long time scales. sub-shells and anomalous regions 100 km lithosphere (includes crust) 400 km asthenosphere NOT well mixed Outer core Depths: to 5100 Mainly iron, oxygen, and silicon Strongly reflective boundary But no S-wave propagation Inner core Depths: 5100 to 6316 km Danish seismologist, Inge Lehmann

5 Observed waves from a New Zealand earthquake not explained by 3 shell structure Explained by an inner core with a higher velocity Fine Mapping of deep Earth structure Locations of epicenters define active faults global scale regional scale Deep earthquakes on active faults? changes in state? (requires implosion or explosion

6 LECTURE 5: faults in the earth Recognized by discontinuities in depositional layers or juxtaposition of rocks along a fractured boundary Active vs. Inactive faults Interpreting evidence for recent motions sag ponds lines of springs fault scarps Timing of displacement soils trenches to expose faults (paleoseismology) GPS geodesy Three types of faults Strike and dip of a plane fault types: Dip slip Normal fault thrust fault strike slip right lateral left lateral Three types of faults and earthquakes Kobe earthquake, Northridge earthquake, Borah Peak earthquake fault types: Dip slip Normal fault thrust fault strike slip right lateral left lateral

7 LECTURE 6: Causes of Earthquakes Earthquakes occur in large scale patterns associated with global sized features such as mountains, rift valleys, mid-oceanic ridges, and ocean trenchers Types of Earthquakes tectonics earthquakes volcanic earthquakes collapse earthquakes caverns mines landslides explosion earthquakes detonation of chemicals or nuclear devices Geodesy (surveying) and the slow build-up of energy optical methods GPS geodesy Predictive tool Elastic rebound strike slip fault scenario (Reid and the 1906 San Francisco earthquake) thrust fault Nisqually earthquake slide Changes in the rocks wet rocks may actually increase volume under pressure dilatency Effects of water on triggering earthquakes Denver What producessseismic waves rupture initiation asperities irregularities in ruptures rupture termination Difference between propagation and slip direction (p 125) predicting the sign of the seismic waves

8 Focal mechanisms quadrants of push and pull beach balls Develop examples for each kind of fault, Relate to T axis

9 LECTURE 7: Plate tectonics Earthquakes occur in large scale patterns associated with global sized features such as mountains, rift valleys, mid-oceanic ridges, and ocean trenchers Driving forces and recognition of the global patterns grounds for a unifying theory Plate tectonic theory Spreading zones, mid-ocean ridges or divergent boundaries transform faults and transform boundaries subduction zones, Benioff-Wadati zone, convergent boundary Benioff-Wadati zone Interplate earthquakes Intraplate earthquakes within plate margins away from plate margins Predictions Spreading zones, mid-ocean ridges or divergent boundaries transform faults and transform boundaries subduction zones, convergent boundary deep earthquakes Benioff-Wadati zone

10 LECTURE 8: Size of an earthquake Earthquakes size is related to the area that broke and the amount of dislocation on that plane. Scaling relationships Felt size also depends on depth and the substrate you are standing on. Intensity of shaking equal intensity = isoseismal lines to first order, shakes more near earthquake exceptions: Loma Prieta also varies with depth Mercalli intensity scale definition by experience/damage useful for pre=instrumental events initial estimates Earthquake magnitude: need scale independent of social factors, depth traditional Richter magnitude depends on wave amplitude superceded by moment magnitude Richter magnitude the logarithm to base 10 of the maximum seismic wave amplitude in thousandths of a mm recorded on a standard seismograph at a distance of100 km from the earthquake epicenter Thus, for each unit of Magnitude, wave amplitude is 10 times bigger This is now called M L or local magnitude also requires standard seismograph calibrated to a certain mass provided a purely instrumental scale for California earthquakes relatively modest at outset practical limits are strength of rock and detectability of wave misleading when broadly applied depth and occurrence of surface waves are other variables. (largest of three waves is used and thus amplification of surface waves can affect call 1960 Chile Alaska 8.6

11 How to modify the Richter scale? typical record shows P wave, S wave and a train of Rayleigh waves - so why not measure all three? P-wave magnitude is called m b For shallow earthquakes, surface wave train yeilds Ms These two can be quite discrepant. (Ms 8.6, mb 6.5 for Alaska eq) Energy Thus, Energy is measure of choice equal and opposite forces are a force couple size of the couple is its moment: the numerical value is the product of the value of one of the two forces and the distance between them good measure of earthquake source Unfortunately, the units are unfamiliar and thus not widely adopted. Seismic energy release absorbed by fracture and friction irregularities in ruptures rupture termination Seismic moment So, the moment is related to a measure that is more similar to the magnitude for use in the public arena. This system is gaining popularity over the Ms and Ml systems Mw is based on a measure of the whole dimension of the slipped fault one unit of Mw equals 30 times the energy release Acceleration of ground shaking ground acceleration is another measure of size some argument about the utility of acceleration vs. peak velocity bracketed duration (how long it shook above a certain acceleration)

12 LECTURE 9: Volcanoes, Tsunamis, and Earthquakes Relationship between seismicity and volcanism Plate scale volcano scale Some volcanoes an absence of precursory seismicity Mt Pelee erupted without warning in 1902 no felt shaking pyroclastic flow on Martinique glowing avalanche of hot gases, steam and rock debris followed by many explosive episodes Since this time, seismic networks that monitor volcanic islands have provided important mitigation. La Sourfriere eruption in 1979 Montserrat July successfully evacuated Hawaii Seismicity also associated with mid-plate volcanism 11/29/75 5 km deep, 45 m south of Hilo, SE coast Two compaion events 14 seconds of shaking 5-6 meter tsunami The same day, Kilauea erupted. within an hour 18 hours later, eruption as over $4M damage from earthquake Use this to explore the relationship between earthquakes, tusnamis and eruptions Volcanic earthquakes fault rupture may precede the motion of magma and eruption of lava disrupt magma chamber soda pop, superheated steam gas release disturbs the unstable equilibrium of the magma many opportunities for seismic energy release P- and S-wave events or P-wave only (cuz of fluid zones encoutered) Volcanic hazards lava flow calaclysmic erputions fertile soils Sometimes seismicity is a valuable precursor to volcanic eruption, can chart depth of magma

13 Mt Pinatubo 1991 Philippines previously erupted at 500, 3000, 5500 years ago Seismic swarms felt in March --> seismic and geodetic monitoring by May and June, gas emissions climactic eruption of ash cloud 400 km wide x 34 km high, only 600 killed 1980 Mt Saint Helens May 18, 1980 Composite volcano summit caldera cater produced by post-eruption collapse Plinian eruptions such as Vesuvius in 79 A.D. in which the Roman scholar Pliny the elder died March 1980 earthquake swarm hundreds of earthquakes each day mid-april the bluge on the north summit flank magma migration? forest and resort on north flank were evacuated small eq (5.5) w km below the mtn ground shaking triggered a massive landslide, depressured the magma chamber Plinian cloud was 20 km high, 30 across killed 65 people globally, barographs recorded the fluctuations in the air pressure. Mammoth Lakes Earthquake Swarm 1980 seismic episode Long Valley caldera Composite volcano summit caldera crater produced by post-eruption collapse Bishop tuff at 760,000 y b.p.; 600 km3 Tsunamis Series of catastrophic waves that inundate the coast directivity height open water height vs. run-up (maximum of meters) on a low-lying coast, this might cover a huge amount of area (Florida..) before it looses energy or encounters by topography Shape of sea floor at coast

14 Caused by displacements of rock on the sea floor (usually vertical), triggering ocean wave Earthquakes especially subduction zone develop elastic rebound theory for thrust fault Slow earthquake Submarine Landslide Series of catastrophic waves that inundate the coast directivity height Crescent City, CA 1964 great Alaska earthquake of 1964 (Good Friday) $10 M in damage in California 75% in CC First, two smaller waves destroyed 30 square blocks; the third washed 500 m inland; and fourth Implications for the Pacific Northwest Subduction zone event distance from coast to the fore-bulge Newport: about 45 minutes Cape Blanco: 18 minutes

15 LECTURE 10: Events that precede an earthquake Efforts at earthquake forecasting Clues for recognizing impending earthquakes earthquakes and prediction in china fossil earthquake Cascadia Parkfield: failed but postponed Calculating the odds Plate scale volcano scale Some volcanoes an absence of precursory seismicity Mt Pelee erupted without warning in 1902 no felt shaking pyroclastic flow on Martinique glowing avalanche of hot gases, steam and rock debris followed by many explosive episodes