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Three earthquake sources matter: the offshore Cascadia megathrust, deep events inside the Juan de Fuca Plate, and shallow upper-plate faults, including the Seattle Fault. Shaking can damage buildings, bridges, ports and utility networks, interrupting transport, electricity and water supply. Soft basin sediments can amplify or prolong shaking; saturated loose fill can undergo soil liquefaction and lateral spreading. Steep slopes can fail as landslides. The same regional event can therefore have very different consequences on bedrock and on artificial fill.^[ref]

Large Cascadia tsunamis chiefly threaten the outer Pacific coast. Seattle's inland waterways also have local tsunami hazards from crustal faults and underwater landslides; one should not equate a Seattle waterfront site with an open-ocean coastal site. Cascade eruptions can affect the region through volcanic ash and transport disruption. Lahars are strongly channelled by river valleys, so their exposure is location dependent rather than uniform across Seattle. These geological differences influence infrastructure design, land-use decisions and emergency planning.^[ref][ref]

A surface-reaching normal fault earthquake commonly produces a down-facing step towards the hanging wall, accompanied by extension across the break. A fresh free face can expose bedrock, polish or striations, roots or offset soil. The actual rupture is usually irregular: fault slip changes along strike, separate strands overlap or step, and tension fissures, small grabens and secondary cracks distribute part of the extension.

Exposed competent rock or very thin cover: a relatively narrow, sharp break can closely follow the principal fault plane. A steep free face can be recognizable immediately, then weather and shed debris. The face need not be vertical or maintain the same height along strike.

Loose granular or poorly consolidated cover: the main shear can splay upwards through alluvium or regolith. Open fissures, step-like scarps, rotated blocks, synthetic and antithetic normal faults, and a small intervening graben can form across a wider zone. Collapse and raveling quickly build colluvium at the toe. A single rupture event can therefore create several parallel breaks; counting them does not count earthquakes.

Thick weak or cohesive cover: deeper fault slip can be transferred into distributed shear strain, tilted beds, broad warping or monoclines, with tension cracks above a propagating tip. A discrete break may be attenuated or absent if the fault remains blind. Cover thickness alone is not enough to predict the response: cohesion, grain size, layering, saturation, rupture displacement and fault depth also matter. Saturated loose sediment can additionally develop soil liquefaction, sand blows or lateral spreading; these are secondary shaking effects and do not independently locate the tectonic rupture.

The observed vertical step, crack aperture and total width of disturbed ground should be measured separately. Reconstruct the total displacement across all strands and account for the original ground slope. Landslides or gravitational spreading can imitate normal-fault scarps, so map connections to the main fault and displaced markers rather than assuming every open crack is primary tectonic slip.

Excavate across the mapped fault scarp, approximately perpendicular to its strike, at a site where young sediments record and preserve deformation. On cleaned trench walls log bedding, soil horizons, shear zones, fissure fills, buried surfaces and colluvial wedges. Correlate the same marker beds across the fault before reconstructing individual displacements.

Look for event horizons, not merely a stack of sediments:

In the drawing, A was faulted in event 1; W1 accumulated; B buried and stabilized that surface. Event 2 displaced A, W1 and B, after which W2 accumulated and C sealed the fault tip. Thus A records two events, B one and C none. This combination of offset and sealing relationships is stronger evidence than identifying two wedge-shaped units alone.

Bracket event ages with multiple samples just below and above the event horizons. Suitable short-lived organic material can be dated by radiocarbon dating; optically stimulated luminescence dating can constrain deposition of appropriate mineral grains. Reworked old charcoal, younger roots, incomplete bleaching, erosional gaps and time elapsed between rupture and wedge deposition must enter the age interpretation. For calendar dates, $t_{\mathrm{youngest}\mathrm{cut}}<t_{\mathrm{event}}<t_{\mathrm{oldest}\mathrm{seal}}$ only when those deposits genuinely bracket the rupture; a radiocarbon age in years before present reverses the numerical ordering.^[ref]

One earthquake can generate several debris pulses and wedges; repeated earthquakes may reuse one strand without leaving separable deposits. Erosion, bioturbation, nondeposition, lateral facies changes and upward-dying splays can hide events or mimic terminations. Confirm relationships on both trench walls and, when possible, in adjacent exposures. The result is a minimum supported event sequence with age intervals, not an assumption of perfectly periodic recurrence or one earthquake per wedge.

For a surface-breaking reverse earthquake, expect a fault scarp or disrupted belt with the hanging wall displaced upward and commonly over the footwall. Small thrust faults, splays and backthrusts may repeat or override shallow deposits. Shortened soil, roads or other markers may buckle, while soft sediment can form pressure ridges and compression folds. Near the scarp, gravitational collapse and local bending can also produce tensile fissures and small normal faults; a reverse event need not create compression everywhere at the surface.

For a blind reverse rupture, expect coherent uplift and warping, newly tilted surfaces, or a fold scarp without a throughgoing primary fault offset. Small flexural-slip surfaces along bedding, secondary thrust faults or backthrusts, and opening cracks or small normal faults over the bent outer arc of the anticline can accompany the folding. Deformation may be distributed or localized at fold hinges. Thus a blind main fault does not imply an entirely unbroken ground surface, and secondary cracking must be distinguished from the principal rupture by its geometry and displacement pattern. Shaking-triggered landslides or soil liquefaction can accompany either case and should not automatically be mapped as primary tectonic rupture.^[ref]

A surface deposit of sand and fluid expelled during soil liquefaction. It can indicate past strong shaking but does not by itself identify rupture on the local fault.