SLOW MOTION IN A STATION RECORD — READING WORKSHEET
All values below are INVENTED. They illustrate a surface station record, not fault slip.
No earthquake forecast or current hazard assessment can be made from this exercise.

Day:                      0     5     10     15     20     25     30
East position (mm):       0.0  -0.5   -1.0    1.5    4.0    3.5    3.0
Assumed background drift:-0.1 mm/day

1. Subtract the predicted background position (−0.1 × day) from each observation.
   Residual position (mm): ______________________________________________
2. In which interval does the residual rise? How much does it rise in total?
3. Why is surface-station displacement not numerically identical to slip on a buried fault?
4. Would one station be enough to rule out a local monument problem? Explain.
5. If tremor is absent for part of the interval, does that prove no slow slip occurred?

ANSWER KEY
1. Residuals: 0,0,0,3,6,6,6 mm.
2. Days 10–20; 6 mm total residual displacement.
3. The surface deformation depends on fault geometry, position, elastic response and
   slip distribution. An inverse model connects station motion to buried slip.
4. No. Neighboring stations and instrument/site checks help separate coherent
   regional deformation from local motion or a recording problem.
5. No. Tremor and geodetically inferred slow slip are related observations, but not
   interchangeable. A fault segment can slip with or without detectable tremor.

Primary background:
https://www.usgs.gov/publications/slip-rate-and-tremor-genesis-cascadia
https://www.washington.edu/news/2011/05/22/scientists-find-odd-twist-in-slow-earthquakes-tremor-running-backwards/
https://pubs.usgs.gov/publication/fs20253050/full

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