Modified Mercalli Intensity Scale — Accumath Toolchest

Dataset: accumath-mercalli-scale.csv — 12 rows, intensities I–XII.

Source

USGS (U.S. Geological Survey — US government, public domain):
Scale basis and history: "The Modified Mercalli Intensity Scale," usgs.gov (Earthquake Hazards).
Level descriptions: the abridged description of the 12 levels printed in the USGS publication "The Severity of an Earthquake," USGS General Interest Publication 1989-288-913 (pubs.usgs.gov) — the same abridgment the USGS scale page cites.
Magnitude comparison: USGS "Magnitude / Intensity Comparison" table ("intensities typically observed at locations near the epicenter").
The scale was developed in 1931 by Harry Wood and Frank Neumann from Mercalli's earlier scale.

How the columns were built

description — a compact paraphrase, in our own wording, of the USGS-published effects for that level. USGS's paragraphs are not reproduced verbatim.
name — USGS's perceived-shaking descriptors, as printed on USGS ShakeMap / intensity-scale graphics: I Not felt; II–III Weak; IV Light; V Moderate; VI Strong; VII Very strong; VIII Severe; IX Violent; X–XII Extreme. The abridged scale text itself assigns no names; the names are recorded from USGS's own graphics, as stated here so nobody mistakes them for part of the 1931 text.
typical_magnitude_note — from the USGS comparison table, which is published by magnitude band, not per intensity:

Magnitude	Typical maximum MMI
1.0–3.0	I
3.0–3.9	II–III
4.0–4.9	IV–V
5.0–5.9	VI–VII
6.0–6.9	VII–IX
7.0 and higher	VIII or higher

The CSV column records the lowest band in which that intensity appears as a typical maximum (so VII reads 5.0–5.9 even though it also appears in the 6.0–6.9 band, and X–XII read "7.0 and higher" because the top band is open-ended). It is a rough guide to typical maximum intensity near the epicenter — not a conversion.

Caveats

Intensity is observed effects, not an instrument measurement. The scale has no mathematical basis; it is a ranking based on what was felt and what was damaged at a particular place. One earthquake has one magnitude but many intensities.
It depends on the site. Intensity at a location varies with distance from the epicenter, focal depth, local soil/rock conditions (soft ground amplifies shaking), and the quality of construction — which is why the descriptions for VII and up are written in terms of building classes.
Lower levels (I–VI) are defined mainly by how people feel the shaking; higher levels (VII–XII) mainly by structural damage. USGS notes that structural engineers usually contribute information for assigning intensities of VIII or above.
Maximum observed intensity generally occurs near the epicenter and decreases outward, but ground conditions can produce local exceptions.

Legal basis

USGS is a US federal agency; its published material is public domain under 17 U.S.C. § 105. Descriptions here are paraphrased facts with the source named on every row.