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

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.

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…

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!

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?

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

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?

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?

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?