Transcript EART 118 Seismotectonics

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