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EART 118 Seismotectonics MWF D250 9:30-10:40 am; Th D250 2:00-4:00 pm Prof.: Thorne Lay, C382 E&MS, Office Hours 11:00-12:00 MWF TA: Lingling Ye, Office Hours 11:00-12:00 MF SEISMOTECTONICS: Study of the relationship between earthquakes, active tectonics, faults and deformation in a region. Earthquake characteristics (location, size/energy release, faulting mechanism) are obtained by analysis of seismic waves recorded by ground motion sensing instruments called seismometers. Plate boundaries are a planet-wide network of faults where most earthquakes and volcanoes are located. Material is perfectly elastic until it undergoes brittle fracture when applied stress reaches f Material undergoes plastic deformation when stress exceeds yield stress 0 Permanent strain results from plastic deformation when stress is raised to 0 ‘and released Strength envelope gives strength vs depth Shows effects of material, pore pressure, geotherm, strain rate BRITTLE DUCTILE Brace & Kohlstedt, 1980 Strength increases with depth in the brittle region due to the increasing normal stress, and then decreases with depth in the ductile region due to increasing temperature. Hence strength is highest at the brittle-ductile transition. Strength decreases rapidly below this transition, so the lithosphere should have little strength at depths > ~25 km in the continents and 50 km in the oceans. Earthquake Explanation • An earthquake is the process of sudden, shearing displacement on a fault (a surface of contact between two rock masses) combined with resultant vibrations (seismic waves) • Earthquakes ‘catch up’ with prior large-scale crustal motions: strain and stress in rock change (reduce) • Earthquakes are frictional sliding instabilities. Repeated stick-slip behavior is observed. Friction depends on pressure, temperature, fluids, slip velocity, fault history, and material properties in the fault zone. Reid, 1910 From Keller & Pinter Waveforms from the Global Seismographic Network (GSN) of the Sumatra Earthquake SC A record section plot of vertical displacements of the Earth's surface recorded by seismometers around the world. Time is on the horizontal axis, and vertical displacements of the Earth on the vertical axis. EARTHQUAKE MAGNITUDE Earliest measure of earthquake size Dimensionless number measured various ways, including ML local magnitude mb body wave magnitude Ms surface wave magnitude Mw moment magnitude Easy to measure Empirical - except for Mw, no direct tie to physics of faulting Note; not dimensionally correct EARTHQUAKE FREQUENCY MAGNITUDE LOG-LINEAR Gutenberg-Richter RELATION MOST OF THE LARGEST EARTHQUAKES ARE AT SUBDUCTION ZONES AND RESULT FROM THRUST FAULTING AT THE PLATE INTERFACE Kanamori, 1978 Much of what is known about the geometry and mechanics of the interaction between plates at subduction zones comes from the distribution and focal mechanisms of shallow earthquakes at the interface between the plates EARTHQUAKES & TECTONICS Locations map plate boundary zones & regions of intraplate deformation even in underwater or remote areas Focal mechanisms show strain field Slip & seismic history show deformation rate Depths constrain thermomechanical structure of lithosphere 36 mm/yr NORTH AMERICA PACIFIC San Andreas Fault, Carrizo Plain P WAVE FIRST MOTIONS Polarity of first P-wave arrival varies between seismic stations in different directions. First motion is compression for stations located such that material near the fault moves ``toward'' the station, or dilatation, where motion is ``away from'' the station. When a P wave arrives at a seismometer from below, a vertical component seismogram records up or down first motion, corresponding to either compression or dilatation. Seismograms recorded at various distances and azimuths used to study geometry of faulting during an earthquake, known as the focal mechanism. Use fact that the pattern of radiated seismic waves depends on fault geometry. Simplest method relies on the first motion, or polarity, of body waves. More sophisticated techniques use waveforms of body and surface waves. EARTHQUAKE FOCAL MECHANISM STUDY Focal mechanisms reveal tectonic faulting orientations: EARTHQUAKE CYCLE INTERSEISMIC: SUMATRA TRENCH BURMA INDIA India subducts beneath Burma at about 20 mm/yr Fault interface is locked Tsunami generated EARTHQUAKE (COSEISMIC): Fault interface slips, overriding plate rebounds, releasing accumulated motion and generating tsunami Stein & Wysession, 2003 4.5-14 HOW OFTEN: Fault slipped ~ 10 m --> 10000 mm / 20 mm/yr = 500 yr Longer if some slip is aseismic Faults aren’t exactly periodic, likely because chaotic nature of rupture controls when large earthquakes occur Focal mechanisms indicate where stick-slip fault sliding occurs. In Subduction Zones, this is mainly thrust faulting on the plate boundary. Aseismic model with near-trench slip can fit GPS statics well. Quasi-seismogeodesy. Lay et al., EPS, 2011 Feb. 27, 2010 Chile Mw 8.8 Filling the 1835 seismic gap? But it went well beyond that… c Updated From: Lay et al., GRL, 2010 Tremor (and slow slip) vs LFE vs earthquake 6 Gomberg and Peng, 2010 Variable frictional properties seem ubiquitous