Lec.24 - 24. Faults(I) I Main Topics A Why...

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Unformatted text preview: 11/15/11 24. Faults (I) I Main Topics A Why are faults important? B Fault geometry C Fault kinemaEcs D Fault classificaEon E IntroducEon to fault mechanics 11/15/11 GG303 1 24. Faults (I) Collapsed Cypress Structure, Oakland, California, aVer Loma Prieta Earthquake, 1989 hMp://upload.wikimedia.org/wikipedia/commons/1/18/Cypress_structure.jpeg 11/15/11 GG303 2 1 11/15/11 24. Faults (I) II Why are faults important? A Faults generate earthquakes hMp://en.wikipedia.org/wiki/File:Sanandreas.jpg 11/15/11 GG303 3 24. Faults (I) II Why are faults important? B Faults bound the tectonic plates of the Earth hMp://geology.com/plate ­tectonics.jpg 11/15/11 GG303 4 2 11/15/11 24. Faults (I) II Why are faults important? C Faults create mountains hMp://marlimillerphoto.com/SrA ­17.html 11/15/11 GG303 5 24. Faults (I) StraEgraphy at PalleM Creek, California II Why are faults important? D Faults deform the Earth’s surface and affect sedimentaEon hMp://3dparks.wr.usgs.gov/pp1515/chapter4/fig4 ­19.jpg 11/15/11 GG303 6 3 11/15/11 24. Faults (I) II Why are faults important? (cont.) E Fluid transport in the Earth's crust 1 Water 2  Magma 3  Hydrocarbons 4  Hydrothermal fluids [ore minerals]) 11/15/11 Fish Springs Fault, California hMp://geology.csupomona.edu/docs/redcindersm.jpg GG303 7 24. Faults (I) II Why are faults important? F Faults are zones of weakness to account for in engineering projects Fault, Homestake Mine, South Dakota 11/15/11 GG303 8 4 11/15/11 24. Faults (I) III Fault geometry 1 Thin relaEve to their in ­plane dimensions 11/15/11 GG303 9 24. Faults (I) III Fault geometry 2 Bounded in extent 11/15/11 GG303 10 5 11/15/11 24. Faults (I) III Fault geometry 3 Commonly grossly planar (at least locally) 11/15/11 GG303 11 24. Faults (I) IV Fault KinemaEcs A RelaEve (not absolute) displacement (slip) of originally neighboring points (or “piercing points) is essenEally parallel to the fault B Piercing points mark intersecEon of a line with a fault C The slip vector connects offset piercing points D Slip is not the same as "movement" or "displacement" 11/15/11 GG303 12 6 11/15/11 24. Faults (I) IV Fault KinemaEcs A RelaEve (not absolute) displacement (slip) of originally neighboring points (or “piercing points) is essenEally parallel to the fault B Piercing points mark intersecEon of a line with a fault C The slip vector connects offset piercing points D Slip is not the same as "movement" or "displacement" 11/15/11 thulescienEfic.com GG303 13 24. Faults (I) V ClassificaEon of faults •  Strike ­slip –  Right ­lateral A Geologic classificaEon –  LeV ­lateral 1 Based on orientaEon of slip •  Dip ­slip –  Normal vector relaEve to –  Reverse (thrust) the strike and dip of a fault •  Oblique (combinaEon of strike ­slip and dip ­slip) 11/15/11 GG303 14 7 11/15/11 24. Faults (I) 2 Strike ­slip a Slip vector is predominantly horizontal (i.e., parallel or anE ­parallel to the line of strike) b Sense of slip i Right lateral: in map view across a fault, a marker is offset to the right ii LeV lateral: in map view across a fault, a marker is offset to the leV 11/15/11 GG303 15 24. Faults (I) 3 Dip ­slip fault a slip vector is parallel (or anE ­parallel) to dip b Sense of slip i Normal: hanging wall moves down ­dip relaEve to footwall ii Thrust fault: hanging wall moves up ­dip relaEve to footwall *Deeper (older) rocks thrust over shallower (younger) rocks* 11/15/11 GG303 16 8 11/15/11 24. Faults (I) B Slip vs. SeparaEon 1 Slip: True relaEve displacement of originally neighboring points 2 SeparaEon: Apparent relaEve displacement of an offset feature as seen in a map or a cross ­secEon 11/15/11 GG303 17 24. Faults (I) C The amount and direcEon of slip can change with Eme and/ or posiEon along a fault 11/15/11 GG303 18 9 11/15/11 24. Faults (I) VI IntroducEon to fault mechanics (2D) A Total stress field = ambient stress field + stress perturbaEon due to fault slip σijTotal = σij0 + Δσij B Fault slip requires faults strength (shear tracEon fault withstands) to decrease 11/15/11 GG303 19 24. Faults (I) VI IntroducEon to fault mechanics (2D) C General relaEons 1  Slip = Δu 2  At peak Δu a ∂u/∂x = 0 b exx = 0 3  Δu0 at fault ends, but ∂u/∂x, exx, and σxx probably high 11/15/11 GG303 20 10 11/15/11 24. Faults (I) Displacement DisconEnuity VI IntroducEon to fault mechanics (cont.) D Long strike ­slip fault 1  Model is (c) 2  w = b(θA ­θB)/2π 3 σ xz = rA −Gb y −Gb y − 2π rA2 2π rB2 rB 4 At x = 0 a rA = rB b σxz = 0 5 Can use displacement disconEnuity in an elasEc model of surface ­breaking strike ­slip faults 11/15/11 σ xz = GG303 −Gb y −Gb y − 2π rA2 2π rB2 21 11 ...
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