Meteorological Cooperative › Warning systems › REP-KJS › REP intensity scale

The REP intensity scale

Intensity describes what an earthquake does at a place. The Republic uses a ten-step scale derived from the Japanese shindo scale, which is itself descended from Mercalli. The two shindo steps that admit a minus and a plus, 5 and 6, are expanded into four whole steps here, and shindo 7 is divided into two. Magnitude, by contrast with intensity, describes the event itself and takes one value everywhere.

RangeREP 1 to REP 10, whole numbers only

Derived fromThe Japanese shindo scale, itself derived from Mercalli

ExpansionShindo 5-, 5+, 6- and 6+ become REP 5, 6, 7 and 8; shindo 7 becomes REP 9 and REP 10

Computed fromInstrumental intensity, filtered three-component acceleration, rounded down to the step

Warning thresholdA warning is issued wherever the estimated step exceeds REP 2

Also statedShindo, MSK-64, EMS-98 and Modified Mercalli, in every broadcast

Why the scale was expanded

The shindo scale distinguishes 5-minus from 5-plus and 6-minus from 6-plus, and the distinction matters, since the damage at 6-plus differs from the damage at 6-minus by more than the damage at 4 differs from that at 5. Suffixed steps travel poorly. They defeat numeric comparison in firmware, they require a convention for the suffix in every text encoding, and they oblige every device manufacturer to handle a value that sorts as a string.

The Republic therefore promoted each suffixed step to a whole number. The correspondence is exact and reversible, so a REP value converts to shindo and back without loss. Shindo 7 is open-ended, and the cooperative divided it at 6.0 m/s² into REP 9 and REP 10, so that the top of the scale distinguishes severe destruction from total destruction. No event in the record of the Republic has reached either.

The scale

REP intensity, correspondence and observed effect
REP	Name	Shindo	MSK-64	EMS-98	Mercalli	Peak acceleration	Effect at the place
1	Imperceptible	0 to 1	I to II	I to II	I to II	below 0.025 m/s²	Recorded by instruments. Felt by nobody, or by a few persons at rest on an upper floor.
2	Slight	2	III	III	III	0.025 to 0.08	Felt indoors by some persons at rest. Hanging objects swing a little. Sleepers are rarely woken. No warning is issued at this step.
3	Light	3	IV	IV	IV	0.08 to 0.25	Felt indoors by most persons and outdoors by a few. Hanging objects swing, glass and crockery move, sleepers wake. No damage to structures.
4	Moderate	4	V	V	V	0.25 to 0.80	Felt by everybody indoors and by many outdoors. Unstable objects fall, doors swing, hanging lamps swing strongly. Slight cracking of old plaster.
5	Rather strong	5-	VI	VI	VI	0.80 to 1.4	Alarm is general and many leave buildings. Furniture shifts, unsecured shelving empties, tiles fall. Cracks appear in weak masonry.
6	Strong	5+	VI to VII	VI to VII	VI to VII	1.4 to 2.5	Standing becomes difficult. Heavy furniture moves, chimneys and parapets fall, unreinforced masonry cracks widely. Reinforced structures are undamaged.
7	Very strong	6-	VII to VIII	VII to VIII	VII to VIII	2.5 to 3.15	Standing without support is impossible. Weak masonry buildings sustain serious damage and some partially collapse. Cracks in reinforced concrete walls. Ground fissures on soft soil.
8	Severe	6+	VIII to IX	VIII to IX	VIII to IX	3.15 to 4.0	Many weak masonry buildings collapse. Reinforced buildings sustain structural damage. Underground pipes break. Landslide on steep ground is common.
9	Violent	7	IX to X	IX to X	IX to X	4.0 to 6.0	Most masonry buildings collapse. Reinforced buildings lean or fail. Rails bend, roads rupture, extensive ground deformation. No instance since 1901.
10	Extreme	7	X to XII	X to XII	X to XII	above 6.0	Destruction is general and includes structures built to the standard. Ground surface visibly deformed. No instance in the record of the Republic.

Acceleration bands are indicative. The step is computed from instrumental intensity and not from peak acceleration alone, so a value near a boundary may fall on either side according to the frequency content and duration of the motion.

How the value is computed

The station computes an instrumental intensity from the three components of ground acceleration. The three components are filtered to the frequency band that shakes buildings and combined into a single motion. The size that motion reaches, for a total of 0.3 s during the event, is taken as the amplitude. That amplitude enters a logarithmic expression producing a continuous number, which is rounded down to the whole step.

I_R = 2 * log10(a) + 0.94         a in m/s^2, vector amplitude
REP = floor(I_R)                  clipped to the range 1 to 10

The continuous value is retained everywhere it is used. A broadcast states the whole step, and the archive, the shake maps and the conformance vectors hold the continuous number to two decimal places.

Intensity and magnitude

Two different quantities

Magnitude describes the event and takes a single value. Intensity describes a place and takes a different value at every place. The event of 3 August 2026 was magnitude 5.4 everywhere; its intensity was REP 5 in the Belvacius sub-region, REP 4 at Cortius, REP 3 at Leriduson and REP 1 at Lindoma.

The value stated first in a broadcast is the intensity estimated for the receiving device, since that is the number governing what a person should do. Magnitude follows, together with the highest intensity estimated anywhere in the territory.

Correspondence with the other scales

MSK-64, EMS-98 and Modified Mercalli are macroseismic scales assessed from observed effect, and shindo and REP are computed from instrument records. The correspondence between the two families is approximate at every step and is stated as a range wherever a REP step spans two macroseismic degrees. It rests on the pairs observed in the Republic since 1962, supplemented by the published Japanese correspondence for shindo.