Metodi Potenziali2015-2016

Intro – Relevance of determining Earth Physical Parameters

Prof. Carla BraitenbergTrieste University, DGM

Home page: http://www2.units.it/~braitenberg/e-mail: berg@units.it

01.10.20152hs

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

Torsvik et al., 2008, Reviews of Geophysics.

a) Detail of the rift (MOR) and drawing of one magnetic isochron. Background is vertical gravity gradient to enhance fracture zones. EP: Euler Pole

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 paleo-positions of magnetic poles.

Torsvik et al., 2009

Paleomagnetic South Poles from a selection of observations from different continents.South Africa coordinates.

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• 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 Enriquillo-Plantain 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.