EART 118 Seismotectonics MWF D250 9:30-10:40 am; Th D250 2:00-4:00 pm Prof.: Thorne Lay, C382 EMS, Office Hours 11:00-12:00 MWF TA: Lingling Ye, Office.

EART 118 SeismotectonicsMWF 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 MWFTA: Lingling Ye, Office Hours 11:00-12:00 MF

SEISMOTECTONICS: Study of the relationshipbetween earthquakes, active tectonics, faultsand deformation in a region. Earthquakecharacteristics (location, size/energy release,faulting mechanism) are obtained by analysisof seismic waves recorded by ground motionsensing instruments called seismometers.

Plate boundaries are a planet-wide network of faultswhere 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

SCSCA 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

ML local magnitudemb body wave magnitudeMs 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 & TECTONICSLocations 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 componentseismogram 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 --> 10000 mm / 20 mm/yr = 500 yrLonger if some slip is aseismic

Faults aren’t exactly periodic, likely because chaotic nature of rupture controls when large earthquakes occur

Stein & Wysession, 2003 4.5-14

INDIA BURMA

Tsunami generated

SUMATRA TRENCH

Focal mechanisms indicate where stick-slip fault sliding occurs. InSubduction 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 ChileMw 8.8

Filling the 1835seismic gap?But it went wellbeyond that…

Updated From: Lay et al., GRL, 2010

c

Variable frictional properties seem ubiquitous