Ways to measure deformation

relative plate velocities based on seafloor spreading rates and directions plus directions from earthquake slip vectors

Ways to measure deformation

Triangulation network in Mexico

Laser-based total station surveyingSatellite Laser Ranging

Very Long Baseline Interferometry

VLBI

Scale 10 mm/yr

Very Long Baseline Interferometry (VLBI)

determinations of plate velocities

HARTRAO

SLR velocities compared to NUVEL velocities from seafloor spreading and earthquake data

More ways to measure deformation

•Types of GPS: •Hand-held: accuracy ~10-30 m instantaneous•Campaign mode: accuracy ~1-5 cm/yr•Continuous mode: accuracy ~1-3 mm/yr

More GPS

•Plate motions measured by GPS•Problem, only possible to measure deformation at specific points)

No matter how measured, deformation measurements can be used to create a velocity field. Now what?

Regional velocity field

SCEC Velocity model 3.0

Make profiles to study fault slip rates

• How do we know where to make profiles?

• Largest recent earthquakes on previously unrecognized or under-appreciated faults!

•

Regional velocity field

Bourne et al., 1998

Microplates in western U.S.

Seth Stein’s webpage

Stable Sierra Nevada block

Colorado Plateau

McCaffrey et al., 2003

Measured displacements

Remove subduction zone inter-seismic signal

Deformation in eastern Mediterranean

Anatolia rotates as a rigid microplate, about pole near Sinai

Aegean interpreted as diffuse extension, shown by steadily increasing rates NUBIA SINAI

ARABIA

EURASIA

Seth Stein’s website

Deformation in eastern Mediterranean

But deformation can also be described by several microplates

Nyst & Thatcher, 2004

Seafloor Geodesy

Attach stations to seafloor

Seafloor stations communicate with float with communicates with GPS satellites

From: Gagnon et al., 2005

Example of subduction deformation on seafloor off Peru

Synthetic Aperture Radar

InterferomteryInSAR

Both from: JPL

From: H. Zebker

Satellites: Repeat passFly over once, repeat days-years later* Measures deformation and topography

Space shuttle:Shown here: Shuttle Radar Topography Mission (SRTM)

Measures topography, deformation with other missions

Aircraft: Shown here: AIRSAR

Measures topography, ocean currents

Age, Ma

moho

Seafloor spreading is a tape recorder of the geomagnetic field!

crust

upper mantle

The recording head of the tape recorder

The tape drive

The recorded reversal chronology

Ocean Ridge system

Eas

t Pac

ific

Ris

e

Mid-A

tlantic Ridge

Modern view of ocean bathymetry derived from satellite altimetry. see EXPLORING THE OCEAN BASINS WITH SATELLITE ALTIMETER DATA Global Bathymetric Prediction for Ocean Modelling and Marine Geophysics

Global bathymetry

Map shown in next slide

Ship tracks across the East Pacific Rise which obtained the magnetic anomalies shown in the next slide. The measurements were made in the 1960’s by the Columbia University research vessel Eltanin.

21

20

19

The Eltanin 19 profile is among the most influential geophysical profiles ever published. It provided the “smoking gun” evidence for seafloor spreading, evidence that turned a majority of skeptics into a majority of believers. The profile was published together with three others in 1966 by Pitman and Heirtzler in “Magnetic Anomalies over the Pacific-Antarctic Ridge” (Science, 154, 1164-1171). The figures above comes from that paper. The track lines (ELT 19-21) of the research vessel Eltanin are shown together with the correlated magnetic anomalies (numbered dashed lines) and the 2000 fathom bathymetric contour. The crosses are earthquake epicenters. The inferred active spreading center would be between anomalies 1 and 1’. The voyage occurred in 1965.

The famous Eltanin 19 profile

The four profiles show total intensity anomalies and bathymetry (ocean depth in km) along the four tracks shown on the previous map. Note that track 20 crosses the ridge system twice.

The vertical scale for total intensity anomaly, F, is shown in “gammas”. This is the same as nanoTeslas or nT. The horizontal lines are at zero anomaly; the scale is thus minus 500 to plus 500 nT.

Eltanin profiles of magnetic anomalies

The incredible symmetry of the Eltanin 19 profile

ESEWNW

total intensity anomaly calculated from model

WNWESE

measured profile of total intensity anomalies

mirror image of measured profile to show symmetry

+500

-500

+500

-500

0

0

Map of magnetic anomaly numbers

Deep Sea Drilling sites

magnetic anomaly number

Age (Ma) from geomagnetic reversal chronology extrapolated in South Atlantic assuming constant rate of spreading

pale

onto

logi

cal a

ge,

Ma

Seafloor ages from deep sea drilling versus geomagnetic reversal chronology

Deep sea drilling in the South Atlantic Ocean

Chronology of geomagnetic field reversals

magnetic anomaly “number”

Ocean floor age, millions of years (Ma), determined largely from deep sea drilling

Geologic time scale

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

4600 My 600 My

Age range of modern ocean floor

Transform faults