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 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. Strength envelope gives strength vs depth Shows effects of material, pore pressure, geotherm, strain rate Brace & Kohlstedt, 1980 BRITTLE DUCTILE

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

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. Waveforms from the Global Seismographic Network (GSN) of the Sumatra Earthquake

EARTHQUAKE MAGNITUDE Earliest measure of earthquake size Dimensionless number measured various ways, including M L local magnitude m b body wave magnitude M s surface wave magnitude M w moment magnitude Easy to measure Empirical - except for M w, 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 thermo- mechanical structure of lithosphere PACIFIC NORTH AMERICA San Andreas Fault, Carrizo Plain 36 mm/yr

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. P WAVE FIRST MOTIONS

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: India subducts beneath Burma at about 20 mm/yr Fault interface is locked EARTHQUAKE (COSEISMIC): Fault interface slips, overriding plate rebounds, releasing accumulated motion and generating tsunami HOW OFTEN: Fault slipped ~ 10 m --> 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 Stein & Wysession, INDIA BURMA Tsunami generated SUMATRA TRENCH

Focal mechanisms indicate where stick-slip fault sliding occurs. In Subduction Zones, this is mainly thrust faulting on the plate boundary.

Lay et al., EPS, 2011 Aseismic model with near-trench slip can fit GPS statics well. Quasi-seismogeodesy.

Feb. 27, 2010 Chile M w 8.8 Filling the 1835 seismic gap? But it went well beyond that… Updated From: Lay et al., GRL, 2010 c

Variable frictional properties seem ubiquitous