Questions: GPS Geodesy and Crustal Deformation Monitoring

Question 1

GPS stations near a locked fault show a systematic velocity gradient: stations far from the fault move at the full plate rate (~46 mm/yr), while stations immediately adjacent move much more slowly. What does this gradient reveal about the fault?

• A. The fault is creeping slowly at a rate equal to the velocity deficit observed near the surface
• B. Measurement error accumulates near faults due to ionospheric interference, creating artificial velocity gradients
• C. The locked fault patch is elastically deforming the surrounding crust — surface stations near the fault are dragged along with the locked zone, and the gradient shape constrains the fault's locking depth and extent
• D. The plate motion rate decreases near a fault boundary as tectonic stress is partially released by small earthquakes

Answer: (C)

A locked fault accumulates elastic strain interseismically. The locked patch beneath the surface acts as a physical coupling between two plates; the crust near the locked zone is dragged along with it rather than moving freely. Farther away, the crust moves at the full plate rate unaffected by the lock. The shape of the velocity gradient — specifically how quickly it decays with distance — can be mathematically inverted to estimate the depth of the locked zone and its lateral extent. This interseismic GPS signal is the primary tool for forecasting where strain is accumulating before the next large earthquake.

Question 2

Slow-slip events in the Cascadia subduction zone were first discovered through GPS time-series data, not from seismographs. What property of slow-slip events makes them invisible to seismic networks?

• A. Slow-slip events occur at such great depth that seismic waves are attenuated before reaching surface seismometers
• B. The total energy released is too small — slow-slip events are equivalent to only magnitude 3–4 earthquakes
• C. Slip occurs gradually over days to weeks rather than in seconds, so the slip rate is too slow to generate the high-frequency seismic waves that seismometers detect
• D. Slow-slip is purely horizontal, and most seismometers are designed to detect vertical ground motion

Answer: (C)

Seismic waves are generated by rapid stress changes. An earthquake releases its energy in seconds to minutes, producing ground accelerations that seismometers can detect thousands of kilometers away. A slow-slip event releases the same total energy (equivalent to M6–7) over days to weeks — the stress change per unit time is so slow that no detectable seismic waves are produced. GPS, which measures absolute position changes over time rather than ground acceleration, is uniquely sensitive to this slow, steady surface displacement. This discovery fundamentally changed understanding of how subduction zones work.

Question 3

Postseismic deformation — the continued movement of GPS stations after an earthquake — encodes information about both the mechanical properties of the fault zone and the rheological properties of the lower crust and mantle.

• T. True
• F. False

Answer: True

Postseismic deformation has two primary sources: afterslip (continued slip on the fault itself, concentrated near the rupture zone) and viscoelastic relaxation (the ductile lower crust and mantle flow in response to the coseismic stress change). These two processes produce distinct spatial patterns and decay timescales in the GPS time series, allowing them to be separated mathematically. Afterslip decay times are days to months and reveal fault friction properties; viscoelastic relaxation decays over years to decades and constrains the effective viscosity and thickness of the lithosphere.

Question 4

Seasonal oscillations observed in GPS position time-series represent noise that should be removed before the data can be used to study crustal deformation — they carry no meaningful geophysical information.

• T. True
• F. False

Answer: False

Seasonal signals in GPS time series reflect real physical processes: surface loading and unloading by groundwater, snow and ice, and atmospheric pressure variations deform the crust measurably. These signals ARE geophysically meaningful — they constrain hydrological and ice mass changes, and have been used to study continental water storage variations that complement GRACE satellite gravity data. They are 'noise' only with respect to the tectonic signals being extracted; understood properly, they are independent scientific observations. Separating them from tectonic signals requires modeling the loading process, which turns the 'noise removal' into a scientific analysis of Earth's surface mass redistribution.

Question 5

After a major earthquake, GPS stations continue to deform for months to years. Describe the two main physical processes driving this postseismic motion and explain what each process reveals about Earth structure.

Model answer: The two main processes are (1) afterslip and (2) viscoelastic relaxation. Afterslip is continued fault slip adjacent to the coseismic rupture zone, driven by stress changes imposed by the earthquake; it decays over days to months and reveals the frictional properties and rheology of the fault zone (velocity-strengthening vs. velocity-weakening friction). Viscoelastic relaxation occurs when the ductile lower crust and upper mantle flow in response to the sudden stress change imposed by the earthquake; it decays over years to decades and constrains the effective viscosity and thickness of these layers. GPS time series capture both simultaneously as a spatially and temporally evolving displacement field that can be modeled to separate the two contributions.

Both processes provide information unobtainable from seismology alone. Seismology reveals coseismic slip distribution; GPS time series reveal how the surrounding lithosphere responds over the earthquake cycle, constraining the mechanical properties of Earth at depth. This has direct implications for earthquake hazard: viscoelastic relaxation transfers stress to neighboring fault segments, potentially triggering future earthquakes over timescales of decades.