Earthquakes magnitude, frequency and scalingx

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Transcript Earthquakes magnitude, frequency and scalingx

Pelatihan :
Techniques in Active Tectonic Study
Juni 20-Juli 2, 2013
Instruktur: Prof. J Ramon Arrowsmith (JRA)
Dari Arizona State University (ASU) - US
Tempat Pelaksanaan:
Ruang Pangea, Laboratorium Gempabumi (LabEarth) –
Puslit Geoteknologi LIPI dan Kuliah lapangan akan dilakukan
disekitar Sesar Lembang, Jawa Barat.
* Lebih jelas baca TOR/KAK dan daftar acara
Earthquake Magnitude
Outline of this lecture
• Crustal faults (and another example of rupture
pulse model)
• Seismic moment and moment magnitude
• Examples: Haiti/Chile and Japan (along with
seismic waves)
• Empirically defined moments
• Threshold for surface rupture
Pollard and Fletcher, 2005
Three views of a crustal-scale strike-slip fault. Map view illustrates the fault as a zone of
deformation. Cross section A-A' in the fault plane includes a contour map of the slip (u) which goes
to zero at the fault tipline and is greatest near the hypocenter (star). Cross section B-B'
perpendicular to the fault plane suggests that slip mechanisms are frictional resistance (FR) in the
upper part of the crust and localized quasi-plastic flow (QP) in the lower part. The graph at the right
indicates a linearly increasing resistance to shearing with depth to the brittle-ductile transition, and
then a non-linear decreasing resistance to shearing with depth. Reprinted from (Sibson, 1989)
Pollard and Fletcher, 2005
Map and cross sections of the Imperial Fault and the Brawley Fault for the October 15,
1979 earthquake in southern California (Archuleta, 1984). a) Map of the rupture trace. b)
– e) vertical cross sections parallel to the fault trace with contours of the model rupture
time, slip duration, strike-slip offset, and dip-slip offset.
Magnitude of
Earthquakes
Moment Magnitude Scale:
• Seismic moment (Mo)
– Measures amount of strain energy
released by movement along whole
rupture surface; more accurate for
big earthquakes
– Calculated using rocks’ shear
strength times rupture area of fault
times displacement (slip) on the
fault
• Moment magnitude scale uses
seismic moment:
– Mw = 2/3 log10 (Mo) – 6
Pollard and Fletcher, 2005
Schematic diagram of four different methods for estimating the slip on a fault (Thatcher
and Bonilla, 1989). The actual slip is contoured on the fault surface in a). Illustrations b)
– d) show how geologists, geodesists, and seismologists gather data (left column), and
graphical representations of these data are shown to the right. e) Interferometric
synthetic aperture radar (InSAR) data provide the field of displacement at the surface
near a fault which can be inverted to estimate the slip distribution.
How do we
determine that the
recent Chile
earthquake was “500
times larger than”
the recent Haiti
earthquake?
http://abcnews.go.com/GMA/Chile_Earthquake/chileearthquake-chilean-earthquake-strongerhaitis/story?id=9963716
Tohoku, Japan Earthquake: Aftershock (and Foreshock) Sequence,
03/08/11 - 03/16/11
Tohoku, Japan Earthquake: Aftershock (and Foreshock)
Sequence, M:Time History
Simple view of the seismic wave propagation
Movie 3
http://www.iris.edu/hq/retm/#1328;
Seismic wave propagation
across US Movie 4
http://www.iris.edu/hq/retm/#1328;
Evolving displacement field
Movie 5
http://gps.alaska.edu/ronni/sendai2011.html
Tohoku, Japan Earthquake: Finite Fault Model USGS V1 - 7 hrs after OT
•Compact rupture, mostly bilateral about
epicenter, peak slip up dip of hypocenter.
•Rupture was likely restricted to the
shallow trench, and GPS vectors suggest
slip did not reach the plate boundary
beneath the coastline.
•Peak slips closer to 40+ m, inferred from
updated modeling.
•Box is 580 km long and 190 km wide
•Zone of main slip is 300 km long and
~150 wide
•M0= mu*A*u_bar
-mu = 30 GPa (3x1010 N/m2)
-A is surface area that slipped (m2)
[width x length]
-u_bar is the average slip in meters
japan = mu.*500.*km.*200.*km.*11 =
3.3000e+022 Nm
japan_mw=(2./3).*log10(japan)-6 =
9.0123
Empirically determined moments
Rupture map
Slip distribution
Empirically determined moments
Pollard and Fletcher, 2005
Three views of a crustal-scale strike-slip fault. Map view illustrates the fault as a zone of
deformation. Cross section A-A' in the fault plane includes a contour map of the slip (u) which goes
to zero at the fault tipline and is greatest near the hypocenter (star). Cross section B-B'
perpendicular to the fault plane suggests that slip mechanisms are frictional resistance (FR) in the
upper part of the crust and localized quasi-plastic flow (QP) in the lower part. The graph at the right
indicates a linearly increasing resistance to shearing with depth to the brittle-ductile transition, and
then a non-linear decreasing resistance to shearing with depth. Reprinted from (Sibson, 1989)
Circular (‘penny-shaped) crack
m slip or opening
1 Mpa stress drop
Earthquake Magnitude
Outline of this lecture
• Crustal faults (and another example of rupture
pulse model)
• Seismic moment and moment magnitude
• Examples: Haiti/Chile and Japan (along with
seismic waves)
• Empirically defined moments
• Threshold for surface rupture