Four Blues and the Red Line

Structural lore — the One Piece world’s geographic skeleton, established on-page in Ch. 51 page 4.

What we know (as of Ch. 51)

Ch. 101 page 7 — Nami’s three-band Calm-Belt diagram establishes the world’s full sea-region count as at least six: East Blue + West Blue + North Blue + South Blue + Grand Line + two Calm Belts (north and south of the Grand Line). The Grand Line sandwiched between two other seas cosmology is on-page canon. Cross-link Calm Belt First Named On-Page.
Ch. 101 page 12 — Currents from all four seas converge at Reverse Mountain because of the Red Line’s barrier-effect. The Red Line is here operationalized as a planetary-scale hydrographic barrier, not just a continental strip. Pre-loads Reverse Mountain Current System.
Ch. 101 page 14 — The Red Line is visually established as vertically dramatic: a colossal jagged mountain-range with summits lost in the storm-cloud canopy. The Red-Line-as-mountain-range register pre-loads Mariejois / Sky-Island altitudes.
Ch. 104: West Blue is named on-page as Laboon’s home origin — first application of West Blue as a populated character-origin (the Ch. 51 reference was geographical-only). Implies trans-Red-Line traversal exists — Laboon and his crew traversed from West Blue to the Grand Line via what is later revealed as Reverse Mountain’s four-rivers-up system.
Ch. 104: Calm Belt named on-page for the first time (Nami, page 6) — described only as the obstacle to fleeing the Grand Line. Mechanics (no-wind dead-zone + Sea Kings) not yet on-page.

The world is a globe divided by two great geographic features: the Red Line (a continental land-belt running N-S) and the Grand Line (a great-circle oceanic route running E-W).
The two features intersect, quartering the world into four oceanic regions named for cardinal directions: North Blue, East Blue, South Blue, West Blue.
East Blue is the tamest of the four (per Mihawk’s narration). Implication: the other three are not as tame, by some metric.
The Grand Line (the not-tame sea) is the world’s recognized threshold-ocean. Aspirants leave their home-Blue to enter the Grand Line.

What metric makes East Blue the tamest? — bounty-totals? weather? predator-density? government-control? all of the above? (Forward-payoff: every Grand-Line-island weather-shock pays this off; the Calm Belt reveal further refines.)
What is on the Red Line? — pending Reverie / Mariejois / Fishman Island.
What is below the Grand Line? — pending (Calm Belts, Fishman Island, etc.).
Why are the four oceans named Blue? — pending (paratext / SBS).

Grand Line — the not-tame sea
East Blue — the tamest sea (the saga-opener home-region)
What is the One Piece — the destination-thread the geography services
Frog In Puddle Doctrine — the saga’s structural-thesis built on this geography
Grand Line Map — the in-universe MacGuffin-map that charts (part of) this geography