Learning Objectives

By the end of this lecture:

• [LO-1.1] State a precise definition of solid Earth geophysics and explain why indirect observation is the defining epistemological constraint
• [LO-1.2] Classify any geophysical study into geodynamics, hazards, or resource management — with process and observable named
• [LO-4.1] Identify the physics domain governing a given observable and the Earth property it senses
• [LO-6.1] Identify four stakeholder communities and their distinct requirements from geophysical knowledge

The Defining Constraint

January 26, 1700 — the entire Cascadia fault ruptured

• 1,300 km rupture — no instrumental seismic record existed
• Date reconstructed from:
  • Japanese tsunami records
  • Drowned coastal forests (tree rings stop 1699)
  • Coastal sand sheets from tsunami inundation
  • Indigenous oral traditions of Cascadia peoples

Everything known about this event is geophysical inference from indirect evidence.

Key point: The inability to sample directly is not a technology failure — it is a permanent physical constraint.

Definition

Physical fields measured at or above the surface:

Every observable is indirect. The Earth property is always inferred through a physical model.

Three Motivating Contexts

The same methods frequently serve multiple contexts simultaneously.

Key point: The motivation determines the required precision and what counts as a satisfactory answer.

Geodynamics: How the Planet Works

Figure 1.1 — Three motivating contexts. Python-generated · assets/scripts/fig_three_motivations.py

• Juan de Fuca Plate subducts at ~3 cm/yr — measured by GPS, not drilling
• Slab geometry at depth — imaged by seismic tomography, not sampling

Natural Hazards: Physics for Public Safety

• ShakeAlert (Alaska → California): real-time P-wave detection → shaking estimate → warning before S-wave arrives
• Lead time: seconds to tens of seconds
  • Slow high-speed trains · open fire station doors · trigger industrial shutoffs

Uncertainty matters directly:

Underestimate shaking → inadequate building design → casualties
Overestimate shaking → wasted resources, public distrust

Key point: The wave propagation physics in Weeks 2–5 is the direct scientific foundation for this operational system.

Resource Management: Knowing What Is Underground

• Before drilling: seismic reflection surveys image stratigraphy kilometers below the seafloor
  • Marine airguns → elastic pulses → reflections off rock interfaces → subsurface image
• Same methods now serve the clean-energy transition:
  • Geothermal resource assessment
  • Critical minerals for batteries
  • CO₂ storage monitoring

Global exploration geophysics: tens of billions of dollars annually

Key point: The motivation changes (petroleum → climate); the physics does not.

Five Physics Domains

These equations will be derived in detail during each module.

Key point: Every observable connects to an Earth property through a physical operator. This is the forward problem.

Passive vs. Active Surveys

CASIE21 (2021): Combined active airguns + passive ocean-bottom seismometers on Cascadia margin — a paradigmatic integrated survey.

Key point: Active surveys image structure; passive networks monitor behavior. Rigorous campaigns integrate both.

Who Uses Geophysics?

• Research universities — physical models, open datasets, trained scientists
• Government agencies — USGS (hazard maps), NOAA (tsunami warning), CTBTO (nuclear monitoring)
• Engineering firms — site characterization, Vs30, dam safety, tunnel routing
• Energy / mining — petroleum, geothermal, critical minerals, CO₂ storage
• Environmental agencies — groundwater, contaminant mapping, GPR
• Defense — nuclear test monitoring, underground facility characterization

Key point: Geophysics graduates work across all these sectors. Physical reasoning, signal processing, and Python are directly transferable.

Worked Example: Precision Matters

Magnetic lineation survey, Juan de Fuca Ridge:

• Reversal boundary at 21 km from ridge axis
• Brunhes–Matuyama reversal age: 0.78 Ma

km has 2 significant figures →

The trailing digits encode no physical information. Overconfident precision propagates into all downstream analysis.

Concept Check

Discuss in pairs — 3 minutes

1. The magnetic lineation method exploits a naturally occurring signal. What category of survey is it, and what is the natural source it exploits?

2. If greater precision on the spreading rate were needed, which input — distance or age — would be more productive to refine, and on what physical or practical grounds?

3. The calculation assumes the reversal boundary is uniformly 21 km from the ridge axis. What geological processes could violate this assumption?

Summary: Lecture 1

• Solid Earth geophysics infers interior structure from surface observations — indirect measurement is unavoidable
• Three motivating contexts: geodynamics · natural hazards · resource management
• Five physics domains connect observables to Earth properties: mechanics · waves · gravity · EM · heat
• Passive methods: continuous, global, source-limited resolution
• Active methods: high-resolution snapshots, controlled source, high cost
• Significant figures = minimum expression of measurement uncertainty

Further Reading & Lab 1

Reading:

• Lowrie & Fichtner (2020) §1.1–1.3 — free via UW Libraries
• MIT OCW 12.201: ocw.mit.edu/courses/12-201
• Mousavi & Beroza (2022) Deep-learning seismology, Science: doi.org/10.1126/science.abm4470
• PNSN real-time seismicity: pnsn.org

Lab 1 (Friday):

1. Install ObsPy
2. Fetch a PNSN seismogram via IRIS FDSN client
3. Identify P and S arrivals by eye
4. Compute source distance from