1. Geodesy & Crustal Deformation
Geology 6690/7690
Geodesy & Crustal Deformation
11 Sep 2017
Geodesy is the measurement and representation of the
shape of the Earth and its gravity field
Among the most important (& growing!) applications of
geodesy monitoring/studies of climate (sea/lake/river level,
water & ice mass/movement), and water resource monitoring
Terrestrial Positioning
• Geodetic measurements of position rely on measuring
angles &/or measuring distances, and space-based
methods also require measurement of time
• Earliest measured angles to stars/sun (astrolabe) plus time
• Triangulation measures angles line-of-sight; requires a
distance measurement on (at least) one ray of network;
can achieve ~1 cm accuracy
Read for Fri 22 Sep: Luttrell et al (GRL 2013)
© A.R. Lowry 2017

2. Geodesy & Crustal Deformation
Geology 6690/7690
Geodesy & Crustal Deformation
Last time cont’d: Terrestrial Positioning
• Leveling measures differences in height from an instrument
to a level rod or staff, relative to the geoid; can be accurate
to < 1 mm…
• Electronic Distance Measurement accurate to ~ 1 cm
(mm’s for e.g. two-color laser which reduces refraction error).
• Space-based positioning (GPS, InSAR) is cheaper and as or
more accurate, so dominates modern tectonic geodesy.

3. Few (if any) of these terrestrial measurement techniques
are still used for academic research because GNSS
measurements are more accurate (with the possible
exception of leveling) and much cheaper (leveling is very
expensive and GNSS is approaching that height accuracy).
Other important terrestrial geodetic measurements that
still provide (research-grade) information we can’t
duplicate with GNSS include
• long-baseline tiltmeters,
• borehole strainmeters
• borehole tiltmeters
All of these are much more sensitive to deformation than is
GNSS positioning, which also makes them somewhat more
finicky/problematic to include in deformation modeling
studies…

4. Quick note on the difference between GPS and GNSS:
GPS (Global Positioning System) is the original US-based,
DoD-funded satellite navigation and positioning network;
GNSS (Global Navigation Satellite System) is a catch-all
term for all of the various navigation satellites including
GLONASS (Russia; completed), BDS (BeiDou System;
China; projected completion in 2020) and Gallileo (EU;
projected operational in 2020) networks.
The design of these systems are similar although with
differences in some particulars. This overview emphasizes
GPS because it is fully operational (and generally less
klugy than GLONASS), and I am most familiar with it… But
there is great excitement around the improvements
expected from having additional satellites!

5. Read for Fri (22 Sep)
Luttrell, K., Mencin, D., Francis, O., & Hurwitz, S. (2013).
Constraints on the upper crustal magma reservoir beneath
Yellowstone Caldera inferred from lake‐seiche induced
strain observations. Geophysical Research Letters 40(3)
501–506.
Discussion lead should be prepared with
• Slides with each of the important figures from the paper
• Summary slides of the observations, methodology,
& results
• Supporting slides from other sources that help to illustrate
important or unfamiliar tools, concepts, ideas
• Critical thinking skills switched to “ON”
Who wants it?

6. Critical Thinking Skills (I):
When reading ANY paper, it’s important to make certain
you understand the terminology being used, e.g.:
• Seiche
• Borehole strainmeter
• Load
• Spectrogram

7. Critical Thinking Skills (II):
Always Look Carefully at the Data Analysis:
• Is there a better way to quantify the data?
• Is there a potential for multiple signal sources?
• Is there an approach to analysis that might remove
signals that aren’t relevant for processes you wish
to understand?
• Are there quantitative tools that might have been
used to extract more information from the data?

8. Critical Thinking Skills (III):
When reading a paper that involves modeling,
it’s always helpful to think about:
• What are the assumptions of boundary conditions
(and do they matter?)
• What are the assumptions of initial conditions
• What physical processes are being modeled
(and are there neglected physical processes that
could conceivably be important?)
• What observations are being modeled
(and how are they related? Qualitative or quantitative?
Comparison or inversion? What is the criterion for a
“Good Match” to the observations? Are there other
observations that might be relevant?)
• Does the model make testable predictions? How might
you test it?

9. Critical Thinking Skills (IV):
Assumptions matter!
• What was assumed about the system being studied in
order to simplify or approximate the dynamics? Are
these assumptions reasonable?
Numbers matter!
• What are the assumptions of rock material properties?
Do they gibe with laboratory measurements?
Do they gibe with geophysical measurements of
in-situ properties?
• What are the numerical values of other physical
properties? Are they reasonable? Is there
observational support?