Open a Japanese earthquake alert and you will see a number that is not magnitude: 震度 (shindo) — seismic intensity on the Japan Meteorological Agency scale. Open a USGS ShakeMap and you will see Roman numerals: Modified Mercalli Intensity (MMI). Both answer “how hard did it shake here?” Neither is magnitude. They are cousins with different dialects.
If you follow Tremr’s live monitor for global magnitude and then read Japanese TV or JMA maps, the mismatch can feel like two different earthquakes. It is usually one event, two intensity languages. This article is the decoder — a companion to Magnitude vs Intensity and How to Read a ShakeMap.
Intensity Is Local; Magnitude Is One Number
Magnitude estimates the size of the earthquake at the source. Intensity estimates shaking at a place. A single Mw 7.0 can produce shindo 1 in a distant prefecture and shindo 6+ near the rupture. That is why Japanese bulletins lead with both: magnitude for the event, shindo for cities.
MMI does the same job for much of the Americas and for USGS products. The philosophy matches. The steps, labels, and measurement culture do not.
What Shindo Actually Is
Japan’s scale runs from 0 to 7, with fine steps at the top: 5-lower / 5-upper (5弱 / 5強) and 6-lower / 6-upper (6弱 / 6強). It is not a felt-report-only scale. JMA intensity is computed from strong-motion seismometers using a measured index that maps to those steps — then checked against human and building effects in the published descriptions.
Rough lived experience (simplified):
- 0–2: mostly unnoticed or slight indoor motion
- 3–4: felt by many; hanging objects swing; sleepers may wake
- 5-lower / 5-upper: walking is hard; unsecured furniture moves; light damage possible
- 6-lower / 6-upper: standing is difficult; heavy furniture topples; serious structural damage becomes likely
- 7: thrown by shaking; most buildings suffer severe damage or collapse depending on construction
When Japanese headlines say “最大震度6強,” they are not saying magnitude 6. They are saying the worst observed (or estimated) intensity step was 6-upper.
What MMI Is
Modified Mercalli Intensity uses Roman numerals I–X+ (sometimes quoted to XII in older tables). It began as a macroseismic scale — observers describing effects on people, objects, and buildings — and still lives that way in Did You Feel It? reports. ShakeMaps paint MMI from instruments plus models plus those reports.
Familiar bands:
- I–III: not felt to weak
- IV–V: light to moderate; felt indoors; objects rattle
- VI–VII: strong to very strong; damage to ordinary buildings begins
- VIII–IX: severe to violent; considerable damage
- X+: extreme
There is no perfect one-to-one converter between shindo and MMI. Rough bridges exist in research tables, but a shindo 5-upper is not “exactly MMI VII” in every building and soil. Treat conversions as approximate orientation, not legal equivalence.
Why Japan Built a Different Ladder
Japan’s dense population, wood-and-steel building stock, and national early-warning culture demanded a scale that stations could compute in seconds and that TV could color on a prefecture map. Shindo is wired into EEW messages, train stop logic, and public muscle memory. Ask a Tokyo resident what “震度5弱” feels like and you will get a clearer answer than asking what “MMI VI” feels like.
MMI’s strength is international familiarity and its tie to damage descriptions used in historical earthquake studies. USGS ShakeMaps speak MMI so a global audience shares one color language. Neither scale is “more scientific.” They optimize for different networks and publics. See also The Earthquake Directly Below Tokyo for why local intensity literacy matters in Japan’s capital.
Reading Both on the Same Event
Take a large Japanese inland quake. JMA will publish maximum shindo quickly, then a colored intensity map. USGS may publish a ShakeMap in MMI for the same rupture. The hot spots should rhyme — soft basins and near-fault zones light up on both — but the numbers will not match digit-for-digit.
Practical reading order:
- Magnitude + depth on Tremr or USGS for the source size
- Shindo map if you care about Japanese cities and alerts
- MMI ShakeMap if you are comparing globally or reading USGS products
- Soil and building context before deciding what “5-upper” or “VII” means for a specific address
Common Mix-Ups
“Shindo 7 means magnitude 7.” No. Magnitude can be 6.something with shindo 7 nearby, or 7+ with lower shindo far away.
“MMI X is ten times shindo 5.” Scales are not linear twins. Do not multiply across systems.
“The first shindo number is final.” Early maxima can be revised as more stations report — same patience you bring to first-hour ShakeMaps.
“If USGS shows MMI V, Japan must be wrong about 6-lower.” Different algorithms, station sets, and site effects. Compare maps, not a single converted digit.
How This Shows Up on Tremr
Tremr’s live feed is built for fast global awareness: magnitude, depth, place, map. Intensity products — shindo maps in Japan, MMI ShakeMaps at USGS — are the next click when you need “how bad where.” Learn both dialects and you stop arguing with the headline. Magnitude tells you the earthquake had size. Shindo and MMI tell you where that size became shaking people could feel and buildings could fail.
The Bottom Line
Shindo and Mercalli are two intensity languages for the same idea: shaking at a place, not size at the source. Japan’s 0–7 ladder is instrumental, fast, and culturally fluent. MMI’s Roman numerals are the global ShakeMap tongue. Read the local scale for local decisions — and never confuse either one with magnitude.