relative plate velocities based on seafloor spreading rates and directions plus directions from earthquake slip vectors
Jan 11, 2016
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