Metodi Potenziali
2015-2016
Intro – Relevance of determining
Earth Physical Parameters
Prof. Carla Braitenberg
Trieste University, DGM
Home page: http://www2.units.it/~braitenberg/
e-mail: berg@units.it
01.10.2015
2hs
Physics of the solid Earth
• Object of Physics of the solid Earth: analyze
solid part of the Earth regarding physical
properties and physical processes.
• Determine structure of the Earth in terms of
physical properties: rheologic parameters
(Young modulus, shear modulus, viscosity,
density, magnetic susceptibility, electrical
resistivity, thermal conductivity).
Variations of physical properties
• First order variation: homogeneously layered
structure: Nucleus, mantle, Crust. Lithosphere:
crust and upper mantle.
• Refinement 1: increase number of layers in
each unit.
• Refinement 2: determine lateral variations,
laterally variable thickness of layers.
Model of the P and S
wave veocities and
density in the Earth.
(Anderson, 1989)
Physical properties detected by
physical fields
• The physical properties can be sensed by physical
fields.
• Seismic wave propagation: seismic velocity, depends
on elastic parameters and density
• Gravity field: density
• Magnetic field: magnetization, electrical currents
induce magnetic field. Susceptibility and electrical
resistivity.
• Electric field: electrical currents, resistivity, time
variable magnetic field.
• Magnetotellurics: observation of time variable
magnetic field and earth electrical currents. Electrical
resistivity
Physical processes
• Analyze physical processes that determine Earth
evolution.
• Earthquakes evidence dynamic state of outer
layer. Breaking of crust along faults. GPS shows
continuous movement of lithosphere. Relative
movement causes deformation and stresses.
• Relation between strain and stress: elastic
constants (Young and shear modulus).
Understanding requires calculation of forces
acting from below (lithosphere drag) and
between moving lithospheric blocks.
Velocity of plate movements for Asia.
(Wang et al., Science, 2001)
The long term deformations
• Today’s movement observed by GPS is a snapshot of the
movement of lithospheric blocks that has continued since
the formation of the crust.
• Reconstruction of previous assemblements of lithospheric
plates through geological and biological mapping:
analogies between rock formations and biological
evolution allow to recover previous assemblements.
• For ocean floor: observation of magnetic field is crucial in
reconstructing the plate movement through the stripes of
thermo-residual magnetization of oceanic crust.
• For continents: measurement of orientation of
magnetization of volcanic rocks allows to determine
position of earth magnetic dipole at age of formation.
Paleomagnetism contributes to reconstruct plate
movements.
Reconstructions of Euler poles and magnetic
anomalies for North Atlantic
a) Detail of the rift (MOR) and drawing
of one magnetic isochron. Background
is vertical gravity gradient to enhance
fracture zones. EP: Euler Pole
Torsvik et al., 2008, Reviews of Geophysics.
Ocean Magnetic anomaly map. COB:
continent-Ocean Boundary. Vectors:
relative movement at different times.
White lines: position of Eurasia at 55 and
57 Ma.
Reconstruct plate positions from calculated paleopositions of magnetic poles.
Paleomagnetic South
Poles from a selection of
observations from
different continents.
South Africa
coordinates.
Torsvik et al., 2009
Relevance of Solid Earth Sciences to
Humanity
• Industrialization and Urbanization increases
link of human well-being to Earth processes.
• Vulnerability: increased vulnerability to
seismic and volcanic events. Densely
populated areas are in enormous danger. Fast
growing countries put priority on fast
construction and development but not on
safety issues.
• Example: earthquake Haiti.
Earthquake of Haiti, M=7.0 Jan12,
2010
Port Au Prince and surroundings were destroyed:
• Casualties: 316 000
• 97 294: number of destroyed buildings
• 188 383: number of damaged buildings
• Source: http://earthquake.usgs.gov/earthquakes/
Haiti 2010 earthquake record in Trieste Carst
with horizontal ultra broad band pendulums
(Trieste, Grotta Gigante)
See documentation on NEIC, USGS
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The location and focal mechanism of the earthquake are consistent with the event having
occurred as a combination of reverse and left-lateral strike slip faulting on the EnriquilloPlantain Garden fault system. The overall Enriquillo-Plantain Garden fault system
accommodates about 7 mm/y of motion, nearly half the total oblique convergence between
the Caribbean and North America plates. The January 12 main shock did not produce
observable surface displacement on the geomorphologically well-expressed main-strand of
the Enriquillo-Plantain Garden fault system, but appears instead to have primarily involved
rupture of a fault or faults distinct from the previously mapped principal strand, causing
significant uplift of the Léogâne delta.
The Enriquillo-Plantain Garden fault system has not produced a major earthquake in recent
decades. The EPGFZ is the likely source of historical large earthquakes in 1860, 1770, and
1751, though none of these has been confirmed in the field as associated with this fault.
Sequence of events possibly associated with the Enriquillo fault in 1751-1860 are as follows.
October 18, 1751: a major earthquake caused heavy destruction in the gulf of Azua (the
eastern end of the Enriquillo Fault) which also generated a tsunami. It is unclear if the
rupture occurred on the Muertos thrust belt or on the eastern end of Enriquillo Fault.
Nov. 21, 1751: a major earthquake destroyed Port-au-Prince but was centered to the east of
the city on the Plaine du Cul-de-Sac.
June 3, 1770: a major earthquake destroyed Port-au-Prince again and appeared to be
centered west of the city. As a result of the 1751 and 1770 earthquakes and minor ones in
between, the authorities required building with wood and forbade building with masonry.
April 8, 1860: there was a major earthquake farther west accompanied by a tsunami.
Caraibic plate
Tarr et al., 2007, USGS
Legend previous figure
• Seismicity for time
interval 1900-2007
Relative movement of plates
Tarr et al., 2007, USGS
Historic seismicity Caraibic plate
Tarr et al., 2007, USGS
Parallel faults to the plate boundary
Prentice et al., JGR,2003.
Distribution of aftershocks
NEIC, USGS
Extension of the fault
NEIC, USGS
Model movement on the fault: requires knowledge of
rheological parameters, stresses, dislocation on fault. Relevant:
has elastic energy been released, or will fault move on part that
was not affected by main shock.
NEIC, USGS
Causes that led to these extremely
high numbers of destruction
• Rapid development of city
• Missing engineering-technical criteria for earthquake
risk estimation.
• Missing knowledge of present tectonic movements.
• Missing knowledge of structure of the crust. Local
terrain can amplify seismic response (sediments,
contact between sediments and basement)
• Missing knowledge in early warning, monitoring of
seismicity.
• Missing constructin codes in earthquake prone area.
• Underestimation of seismic risk due to relatively lower
seismic activity in the past decades in the western part
of island.
What actions should be taken, using
Haiti as example to other areas
• Increase terrestrial data on crustal structure and
seismic monitoring.
• Utilize satellite observations. Deformation, rock types
can be monitored.
• Integration of satellite data with targeted terrestrial
measuring campaigns
• Teaching of investigation techniques. Monitoring of
earth processes must be done by local personnell.
• Increase transfer of knowledge to save lives by better
understanding of earth processes and earth structure.