6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND...

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

Transcript of 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND...

Page 1: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Earthquake source mechanics

Lecture 6Seismic moment

Page 2: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Earthquake magnitudeRichter magnitude scale

M = log A(∆) - log A0(∆)where A is max trace amplitude at distance ∆and A0 is at 100 km

Surface wave magnitude MSMS = log A + α log ∆ + βwhere A is max amp of 20s period surface waves

Magnitude and energylog Es = 11.8 + 1.5 Ms (ergs)

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Seismic moment

Seismic intensity measures relative strength of shaking locallyInstrumental earthquake magnitude provides measure of size on basis of wave motionPeak values used in magnitude determination do not reveal overall power of the sourceSeismic Moment: measure of quake rupture size related to leverage of forces (couples) across area of fault slip

F

F

L

Moment = FL

- two equal & opposite forces= force couple

- size of couple = moment- numerical value = product ofvalue of one force times distance between

Applying couple to fault

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Seismic Moment II

Can be applied to seismogenic faultsElastic rebound along a rupturing fault can be considered in terms of resulting from force couples along and across itSeismic moment can be determined from

fault slip dimensions measured in field or from aftershock distributionsanalysis of seismic wave properties (frequency spectrum analysis)

F

F

Stress &strainaccumulation

Fault ruptureand rebound

Applying couple to fault

Page 5: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Seismic moment

Empirical relation between moment and magnitude islog10 M0 = c M + d

or Mw = 2/3 log10 M0 – 6(Moment-magnitude scale (Kanamori)

Seismic moment = Area of fault plane x stress drop of earthquake x coseismic slip

[NB: Area x stress = forceforce x distance = moment]]

provides estimate of overall size of the seismic sourceUnits: units of moment = newton-metres = Nm =

= joule = J = unit of energySo seismic moment is also a measure of the energy of the earthquake

Page 6: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

A few great fracturing events totally dominate the earthquake seismic moment released (moments more realistic than comparing magnitudes)The moment release, even at the major plate boundaries is distributed very unevenlyAbout ¼ of the seismic moment released between 1904-86 was by the great Chilean earthquake of 1960 that ruptured 100km of the subduction zone interface at the Peru-Chile trenchSan Francisco 1906 doesn’t even get a mention!

Seismic moment

Seismic moment does not saturate

e.g. Alaskan earthquake: Ms=8.4; Mw=9.2

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Seismic moment tensor

⎟⎟⎟

⎜⎜⎜

⎛=

333231

232221

131211

MMMMMMMMM

M jk

The nine different force couples that make up the components of the moment tensor:

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Seismic moment tensor

⎟⎟⎟

⎜⎜⎜

⎛=

0000000

0

0

MM

M

where Mo is defined as the scalar seismic moment:

M0 = µ A s

where µ is the rigidity modulus, A is the area of the fault and s is the slip or displacement of the fault

Example of right lateral movement on a strike-slip fault in the x1direction corresponds to:

Note Mij = Mji

Page 9: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Scalar seismic moment

Define the scalar seismic moment (Aki)M0 = µ A sµ - rigidityA - fault areas - slip (vector)

Units: force x length (Nm or J)

l

wµs

Aspect ratio:

usually l/w ∼ 2,

except for long strike-slip faults

Page 10: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Far-field radiation pattern for double couple source

The orientation of the small arrows shows the direction of first motion; their length is proportional to the wave amplitude. The P-wave first motions are outward in the compressional quadrant and inward in the dilatational quadrant, with nodal lines in between. S-wave first motions are generally away from the pressure axis and toward the tension axis; there are 6 nodal points and no nodal lines in S

P-waves

S-waves

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Example of focal spheres

Example of focal spheres and their corresponding fault geometries. The lower half of the focal sphere is plotted to the left, with compressional quadrants shaded. The block diagrams show th two fault geometries (the primary and auxillary fault planes) that could have produced the observed radiation pattern.

Page 12: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Far-field pulse shapesThe near-field displacements caused by an earthquake will be permanent. M(t) would look like this

In the far field there is a displacement pulse. dM/dt is proportional to the far-field dynamic response, such as observed with P-wave arrivals.

Note most seismometers measure velocity or acceleration rather than displacement.

Page 13: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Seismic moment from seismograms

Define seismic momentM0 ∝ area under pulseRupture length ∝ duration of pulse : l ∝ τ

Time domain - instrument corrected pulse

Pulse duration

0

30

Time

Am

plitu

dedM

/dt

τ

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Seismic moment from seismograms

Define seismic momentM0 ∝ A0

Rupture length ∝ 1/ frequency: l ∝ 1 / f0

Frequency domain

Frequency

Am

plitu

de

f0

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Earthquake characteristics

M0 = µ A s ∼ µ 2 l sby dimensional analysis slip is:

s = 2 M0 / µ l2

stress drop with earthquake:∆σ = 2 M0 / 2π w2 l

Note can get ∆σ from seismogram as rupture length,l ∝ τ , pulse duration and M0 is proportional to

amplitudeStress drops in range 1-30 MPa (Kanamori)

Page 16: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Stress drop in earthquakes

Stress drop is typically a small fraction of total stresses in double couple earthquakes

Varies according to crustal properties, fault maturity

Single-force earthquakes (landslides) have much larger stress drops

Fault length versus moment for large earthquakes (Scholz et al. 1986): larger stress drop produces more seismic moment for a given rupture area

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Tectonics

Infer seismic and tectonic slip rateslip rate = (sum of M0) / (µ l.w T)

M0 - summing and earthquake cataloguel.w - overall tectonic settingT - duration of earthquake catalogueFind active slip rate (SAF) is 3 cm/yrAseismic (creep) rate is 3 cm/yrCompare with satellite data, GPS

Page 18: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Seismic moment - from fault area

Scalar seismic moment (Aki)M0 = µ A s

l

wµs

Get area of fault plane from aftershocks

Measure slip in the field

Page 19: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Regional tectonics from seismotectonics

Fault plane solutionsType of faultingSlip directionStress field orientation

Quantitative seismotectonicsSeismic momentSlipStress dropSeismic and tectonic slip rate

Page 20: 6 Seismic moment - UCL Seismic moment.pdf · GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake source mechanics Lecture 6 Seismic moment

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Moment tensor inversions

1. NEIC fast moment tensors - from teleseismic P waveforms

http://gldss7.cr.usgs.gov/neis/FM/fast_moment.html

2. Harvard CMT solutions - Centroid momen-tensor (from long perdiod body waves)

http://www.seismology.harvard.edu/projects/CMT/

3. EMSC rapid source parameter determination - European-Mediterranean Seismological Centre - uses P & S waves – results in 24 hours

http://www.emsc-csem.org/

4. NEIC broadband depths and fault-plane solutionshttp://neic.usgs.gov/neis/nrg/bb_processing.html