Lecture8 - Geologic deformation - EARTHQUAKES pattern...

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Unformatted text preview: Geologic deformation - EARTHQUAKES pattern recognition Geologic structures: dynamically-produced patterns or arrangements of rock or sediment that result from tectonic forces within the Earth • produced as rocks change shape, location & orientation in response to applied stress • structural geology is the study of the shapes, arrangement, and interrelationships of bedrock units and the forces that cause them • "forces"? - that which change motion or orientation of a body could be bedrock/tectonic plate 1 Stress is a force per unit area • three basic types of stress (in geology) 1) compressional – shortens 2) tensional – elongates 3) shear – parallel slippage • rocks subjected to stresses greater than their own strength begin to deform • continuum of folding / flowing or fracturing / rupture compression depend on rates - slow rate - flow - fast - breakage or rupture tension shear slippage along discrete planes on a rock body 2 Rocks can behave 3 ways: 1) if deformed materials return to original shape after stress removal, they are behaving elastically ..rubber band • however, once the stress exceeds the elastic limit of a rock, it deforms permanently - malleable 2) ductile deformation involves bending plastically - essentially where 3) brittle deformation involves fracturing/rupture we get earthquakes 1) - after earthquake - small earthquakes - relaxation of the rest of the rock body - trying to return to original shape • if stress results in irreversible changes in shape or size of rock… • strain is a change in size or shape in response to stress stress is the applied force • strain is the deformation brittle ductile change of shape rock has shattered and broken 3 Rocks behave as elastic, ductile or brittle depending on: rapid strain - things fracture and break • amount & rate stress Low strain rate promotes ductile deformation ..glass window • type of rock Mechanically weak rocks tend to deformation through ductile processes ..shale • temperature & (confining) pressure High T promote ductile deformation High confining P promotes ductile deformation - bottom of the rock cycle, high confining pressure and temp ..migmatite Ductile deformation (flow) & folding Folds • wave-like undulations in rock layers • mm-km scale features • result from horizontal or lateral compressional forces • described by the orientation of axial plane axis of the fold COMPRESSIONAL FORCES! 4 Ductile deformation (flow) & folding km-scale folding shape of A also defined by axial plane - axial plane - where most curvature occurs 5 6 axial plane has a bit of a plunge to it, like an arrow pointing into the ground A series of anticlines & synclines 7 circular or slightly elongated structures • dome - interference of • upwarped displacement of rocks folds • basin • downwarped displacement of rocks interference of synclines basin 8 Brittle deformation (rupture) & faulting • loss of cohesion of a body under the influence of deforming stress • usually occurs along sub-planar surfaces that separate zones of coherent material fault zone - leads to EARTHQUAKES! lithified Brittle deformation (rupture) & faulting Joints are fractures or cracks in bedrock along which essentially no movement has occurred • multiple parallel joints are called joint sets due to compression uplifting of the rock 9 Brittle deformation (rupture) & faulting Faults are fractures in bedrock along which movement has occurred • considered "active" if movement has occurred within the last 15,000 years (last ice age) displacement fault terminology: hanging wall & footwall most ores occur along the fault zone - if you were a miner, would hang your light on the hanging wall, and put your feet on the footwall block 10 Fault classification displacement or slip of fault 1) Dip-slip faults have movement parallel to the dip (angle from horizontal) of the fault plane angle of the fault plane from the horizontal a) Reverse faults • HW moves up with respect to the FW • Older stratigraphic units are placed on top of young • Results in duplication of stratigraphic units and thickening of crust (compressional forces) - dip-slip - reverse faults are the same as thrust faults - low angle, usually thrust fault - high angle - reverse fault Rocky Mountains - result of compression - thickening 11 Rocky Mountains - older material on top of younger b) Normal faults • HW moves down with respect to the FW • Younger stratigraphic units are placed on top of old • Results in loss of units from stratigraphic section & thinning of the crust (extension) 12 normal fault Basin & Range basin and range are the result of extension - lower than rest of land 13 Brittle deformation (rupture) & faulting 2) Strike-slip/transform fault • dominant displacement is horizontal; HW & FW slide past each other in horizontal plane • produced in shear setting • stratigraphic order is uninterrupted • types of strike-slip faults – right-lateral – left-lateral - sliding on a vertical axis A block diagram showing the features along a strike-slip fault - when canoeing upstream, would have to look to the right relative to each block fault plane 14 The San Andreas fault system is a major transform fault -Pacific plate is moving to the northwest relative to the rest of California - a number of faults, not just one - dark yellow line is the San Andreas fault, but the others are similar faults - San Gabriel Mountains result of transforming effects 15 - fault material tends to be very brittle - tends to have lot of clay particles --> sag ponds - right lateral fault traces 16 Brittle deformation (rupture) & faulting ..back to EQ.. lecture 3 - extensional fault - where the earthquake actually occurs is the Focus - epicenter is the point above on the map What is an earthquake • produced by the movement of rock bodies past other .. the stress has to exceed the strength of the rocks in brittle manner (cohesion is lost) • the loci of the earthquake movement are faults, i.e., brittle deformation zones • faults come at all scales, mm to separation of lithospheric plates (e.g., San Andreas)… earthquakes can come at all scales as well 17 What is an earthquake San Andreas fault undergoing this Faults & elastic rebound • fault creep (slow migration of crust along fault plane; weak vibrations) - must have enough time/space • rocks bend & store elastic energy (build-up of strain) cohesion • frictional resistance holding the rocks together is overcome = earthquake – release of strain (slippage) earthquake – elastic rebound of rock (as rock tries to return to original shape) What is an earthquake Vibration of Earth produced by the rapid release of energy stored in rock subjected to stress • energy released radiates in all directions from its source, the focus or hypocenter • location on the surface directly above the focus = epicenter • energy is in the form of waves measured as magnitude; felt/observed as intensity 18 - yellow circles -- energy radiating from the focus Seismology seismology - study of earthquake waves • instrument that record seismic waves called seismographs/seismometer >1 type of seismograph is needed to record need at least three both vertical & horizontal ground motion • records of seismic waves from seismographs = seismograms rely on inertia of suspended weight to record motion 19 old school seismograph measure up and down as well as left and right - energy produced from earthquakes - seismic waves - P wave- primary waves - S wave - secondary - Surface waves 20 Seismology Types of seismic waves … REVIEW! Body waves 1) P waves (compressional) 6–8 km/s. Parallel to direction of movement (slinky); also called primary waves. Similar to sound waves. 2) S waves (shear) 4–5 km/s. Perpendicular to direction of movement (rope); also called secondary waves. Result from the shear strength of materials. Do not pass through liquids. Seismology Types of seismic waves 3) surface waves (Two types) results in horizontal & vertical-orbital motions with long period & great amplitude; travel along outer part of Earth a) love (L) waves – side-to-side 21 Seismology Types of seismic waves 3) surface waves results in horizontal & vertical-orbital motions with long period & great amplitude; travel along outer part of Earth b) rayleigh (R) waves (RA- lee) – like an ocean wave – most destructive - rolling roller coaster - difficult for us to build buildings, bridges, etc. to withstand 22 Locating the source of earthquakes epicenter is located using the difference in velocities of P & S waves locating the epicenter of an earthquake • a circle with a radius equal to the distance to the epicenter is drawn around each station • the point where all three circles intersect is the earthquake epicenter P always first (slower) arrival of energy to seismograph (slower still) - larger lag time between S and P - will be able to tell how far from epicenter the city is based on lag tim between S and P waves 23 - going to fit where the first P arrives and where the first S arrives - preexisting, predetermined graph determines distances from epicenter 24 radius is how big the distance is from epicenter - this is why you need at least three seismometers to measure and locate the epicenter 25 Locating the source of earthquakes Earthquake belts • 95% of the energy released by earthquakes Ring of Fire originates in a few relatively narrow zones • major earthquake zones include the CircumPacific belt, Alpine - Himalayan chain, and the oceanic ridge system …plate boundaries Distribution of >M5 earthquakes, 1980 - 1990 Ring of Fire 26 - most of the active geology occurs at plate boundaries 90% of quakes <100 km depths - deeper you go, the more likely the rock will flow rather than break subduction zone seismicity in E On & W Qc …not plate boundary? - kind of line up on NW/SE plane - not on a plate boundary --- BUT isostatic rebound - we were under 2 km of ice - land trying to rebound - reactivation of preexisting structures - 1 Ga, on the scale of the Himalayas - reactivating ancient fault structures 27 Measuring the size of earthquakes Two measurements that describe the size of an earthquake are 1) intensity – measure of the degree of earthquake shaking at a given locale based on the amount of damage 2) magnitude – estimates the amount of energy released at the source of the earthquake 28 Modified Mercalli Intensity Scale was developed using California buildings as its standard - still used in some places, especially building code... etc. 29 1811 Earthquake, New Madrid, Missouri Modified Mercalli Intensity Scale - stops here - barely anyone in the Western region - couldn't record Measuring the size of earthquakes Magnitude scales 1) Richter scale - more quantitative • based on the amplitude of the largest seismic wave recorded 100 km from epicenter • accounts for the decrease in wave amplitude with increased distance • magnitudes < 2.0 are not 'felt' • 1 unit of Richter magnitude increase = 10x increase in wave amplitude (shaking) & 33x energy increase 30 - interval between P and S and Amplitude plotted and then where they cross on Richter magnitude scale is the measure Measuring the size of earthquakes Magnitude scales 2) moment-magnitude scale – gauges quakes total energy total length of fault rupture x depth of fault rupture x total amount of slip along rupture x strength of rock (stress:strain) moment - take a lot more complex calculations - but more precise 31 Earthquake destruction amount of damage attributable to earthquakes • intensity & duration of the vibrations • nature of the material upon which the structure rests (sitting on bedrock, or clay, soil, hill) • design of the structure can move and shift destruction from earthquakes: • ground movement & fire (building collapse) • floods (dam break & courses of rivers change) 1906 San Francisco 32 1964 Anchorage Earthquake destruction Destruction from earthquakes causes density liquefaction • inversion – unconsolidated materials saturated with water converts into a mobile fluid; no water added clays and soils • seiches …say: saysh – the rhythmic sloshing of water in lakes, reservoirs, & enclosed basins (10 m hi) – waves can weaken reservoir walls and cause destruction 33 Earthquake destruction Destruction from earthquakes: • tsunamis, or seismic sea waves • generate speeds up to 500–800 km/hr in open ocean • in the open ocean height is usually less than 1 meter - can barely see it coming • in shallower coastal waters the water piles up to heights that occasionally exceed 30 meters • can be very destructive when at beach, the water will start to disappear Formation of a tsunami offset along fault due to earthquake - water volume shifts 34 Earthquake predictions rocks under stress begin to dilate (expand in volume) = produces physical changes in crust short term predictions (unreliable) • swarms of micro-foreshocks • tilt/bulge in crust • changes in wave velocity • changes in gas & GW level & chemistry • strain monitors watches strain accumulating Earthquake predictions long term predictions (more reliable?) • seismic gaps locked segments of a fault which have been ’quiet’, use of time series analysis • strain monitors & ground deformation - San Andreas - building up strain - creeping a bit - eventually will need to release all that strain 35 Potential Earthquake Hazard, Worldwide - topographically unstable? what type of material buildings are made on - seismically active? 36 ...
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