A single earthquake can be described with several numbers: magnitude, depth, intensity, review status and sometimes an impact color. Those fields do not measure the same thing. Magnitude estimates the size of the source. Depth places the start of rupture below the surface. Intensity describes shaking at a particular location. Volcano scales and alert colors answer different questions again.
This guide explains how the main scales work, why modern seismology uses moment magnitude for large earthquakes and how to read a reported event without turning one field into a prediction or safety guarantee.
Quick answer
- Magnitude is one estimate of an earthquake’s source size.
- Moment magnitude, Mw, derives from seismic moment and is generally the most reliable size estimate for large earthquakes.
- Richter magnitude usually means local magnitude, ML, developed for nearby earthquakes in southern California.
- Reported depth is the distance from the surface to the hypocenter, where rupture starts.
- Intensity describes shaking and effects at a location; it can differ from one place to another.
- VEI compares the explosive size of volcanic eruptions; it is not a warning level.
- PAGER, GDACS colors, Volcano Alert Levels and Aviation Color Codes are impact or status systems, not substitute magnitude scales.
See all earthquakes above M 5.0 reported in the past 24 hours on the map →
Magnitude and intensity are not the same
Magnitude is a source estimate. A catalog may revise it when more stations, waveforms or source models become available, but the earthquake does not receive a different magnitude in every city.
Intensity is local. It describes shaking or observed effects at a particular place. One earthquake can produce higher intensity near part of the rupture, lower intensity farther away and locally amplified motion where geology or basin structure changes the waves. Construction and vulnerability influence observed effects too.
The phrase “How strong was it?” can therefore ask three different questions:
- How large was the earthquake source?
- How strong was shaking at a specified location?
- What human or economic effects may be plausible?
Magnitude, intensity and rapid impact products answer those questions differently.
What the Richter scale actually measures
Charles Richter developed a logarithmic local-magnitude scale in the 1930s for earthquakes in southern California. It is written ML, where “L” means local. The original method compared measured wave amplitude after correcting for distance, using a specified instrument and calibration.
That method made nearby earthquakes comparable with one number, but it was designed for particular instruments, frequencies and distance ranges. Global networks later developed magnitude types for other waves and distances. “Richter scale” remains common in everyday language, yet a modern catalog may report ML, mb, Ms, Mw or another calibrated magnitude. Calling every value “Richter” can be technically wrong.
ML is not obsolete. It is still useful for many small and regional earthquakes when the network and calibration fit the event. Earthquake24 preserves the source’s magnitude type rather than relabeling it.
Why moment magnitude Mw is preferred for large earthquakes
Some amplitude-based scales saturate: beyond part of their useful range, the measured signal does not grow in a way that reliably distinguishes the largest ruptures. Moment magnitude, Mw, is based on seismic moment, a physical description of the source. It represents very large ruptures more consistently and is therefore preferred for comparing large earthquakes globally.
Mw was scaled to be broadly comparable with older magnitude scales where their useful ranges overlap. An early catalog value can still change from mb or ML to Mw as broadband waveform or source information becomes available. Mw is not necessarily the first value published, and many smaller regional events appropriately remain ML or another type.
How seismic moment and Mw are calculated
Seismic moment, M0, combines three physical properties:
M0 = rigidity × fault area × average slip
- Rigidity describes the rock’s resistance to deformation.
- Fault area is the portion of the fault that slipped.
- Average slip is the mean movement across that area.
When M0 is expressed in newton-metres, the USGS relation is:
Mw = 2/3 × (log10(M0) − 9.1)
Scientists estimate seismic moment from seismic waveforms and, when available, geodetic observations of ground deformation. It remains an estimate because rupture is complex, Earth models are simplified and solutions can improve as data arrive. Earthquake24 does not calculate Mw from a marker or simplified map geometry; it displays the source’s value and type.
Why a one-unit magnitude increase is a large change
Magnitude is logarithmic. A one-unit increase corresponds to about 10 times the measured wave amplitude in the traditional relation and approximately 32 times the released energy. A magnitude 7 event therefore represents roughly 100 times the amplitude and about 1,000 times the energy of a magnitude 5 event under the standard approximation.
| Difference | Approximate amplitude ratio | Approximate energy ratio |
|---|---|---|
| M4 to M5 | 10× | 32× |
| M5 to M6 | 10× | 32× |
| M6 to M7 | 10× | 32× |
| M5 to M7 | 100× | about 1,000× |
Energy is not the same as damage. Depth, distance to the rupture, duration, frequency, local ground, structures, exposure and secondary processes all affect outcomes. Very small events can have magnitude zero or a negative value because a logarithmic signal can fall below its reference level; “negative magnitude” does not mean negative energy.
Why several magnitude types still exist
| Symbol | Name | Common measurement | Typical role |
|---|---|---|---|
| ML | Local or Richter-type magnitude | Nearby amplitude with distance correction | Small and regional earthquakes |
| mb | Body-wave magnitude | Short-period P waves | Rapid teleseismic estimates for moderate events |
| Ms | Surface-wave magnitude | Long-period surface waves | Secondary estimate for large shallow events |
| Mw | Moment magnitude | Seismic moment from waveform/source analysis | Large earthquakes and source comparison |
| Md | Duration magnitude | Duration of the decaying signal | Some local networks |
The useful range and preferred type depend on the network. Values for the same event can differ because they use different wave periods, station coverage, calibration and source assumptions. Earthquake24 displays M 6.1 when no clear type is supplied and Mw 6.1 only when the source reports a moment-magnitude type.
How earthquake depth is measured
The point inside Earth where rupture begins is the hypocenter, or focus. The epicenter is the point on the surface above it. Catalog depth is hypocentral depth, not the deepest or shallowest part of the complete fault rupture.
Seismic stations record different wave arrivals. P waves generally arrive before S waves. Automatic systems and analysts compare observed arrival times at several stations with predicted travel times through a seismic-velocity model, adjusting latitude, longitude, depth and origin time to reduce the mismatch.
Depth is often less constrained than horizontal position. Nearby stations, good geometry around the source, clear phase picks and identifiable depth phases can improve it. A catalog can publish depth uncertainty, and a reviewed value can differ from the first automatic result.
Shallow, intermediate and deep earthquakes
USGS uses three broad scientific depth groups:
| Earthquake class | Earthquake24 display boundary | Meaning |
|---|---|---|
| Shallow | 0 to <70 km | Near the surface or upper lithosphere |
| Intermediate | 70 to <300 km | Commonly within descending lithosphere |
| Deep | 300 to about 700 km | Deep within a subducting slab |
The boundaries do not overlap: 70 km begins the intermediate class and 300 km begins the deep class. Earthquakes deeper than 70 km occur within slabs of lithosphere descending into the mantle at subduction zones.
These categories are not a danger scale. A shallow source can produce concentrated nearby shaking; a large intermediate or deep source may be felt across a broad region. Neither statement alone determines consequences.
View recent earthquakes with reported depth →
Why depth changes the shaking people experience
Seismic waves lose strength as they travel. If everything else were equal, a deeper source would place more distance between rupture and the surface and would often reduce the strongest motion near the epicenter. In real earthquakes, everything else is not equal.
Shaking also depends on magnitude, rupture dimensions and direction, fault mechanism, wave frequencies, duration, crustal structure, sediments, basin geometry and topography. Observed effects additionally depend on structures and exposure. This is why Earthquake24 does not draw a generic impact or “safe” circle from magnitude and depth.
Intensity scales: MMI, JMA and EMS-98
Modified Mercalli Intensity (MMI) describes effects at a place with Roman numerals. Lower degrees focus on whether motion is felt and objects move; higher degrees include damage and structural effects. One earthquake can have many MMI values across its affected area.
The Japan Meteorological Agency seismic intensity scale has ten steps: 0, 1, 2, 3, 4, 5 Lower, 5 Upper, 6 Lower, 6 Upper, 7. It is instrumentally measured at locations and is not calculated by simply converting magnitude.
EMS-98, the European Macroseismic Scale, has twelve degrees for assigning macroseismic intensity. Its treatment of observed effects explicitly considers building vulnerability and damage grades.
| Scale | Describes | Key distinction |
|---|---|---|
| MMI | Shaking and observed effects at a place | Roman-numeral intensity, not magnitude |
| JMA intensity | Instrumental motion at a Japanese observation site | Ten steps; no direct magnitude conversion |
| EMS-98 | Macroseismic effects in an area | Twelve degrees with vulnerability classes |
The systems describe local shaking or effects but are not interchangeable labels.
ShakeMap, Did You Feel It? and PAGER
These products answer distinct questions:
| Product | Main question |
|---|---|
| Magnitude | How large was the earthquake source? |
| ShakeMap or intensity | How was shaking distributed across the region? |
| Did You Feel It? | What shaking did participants report? |
| PAGER | What fatality and economic-loss ranges may be plausible? |
PAGER combines estimated shaking, population exposure and country-specific loss models. Its green, yellow, orange or red level reflects the higher of modeled fatality and economic-loss alert levels. It is rapid and uncertain: not a magnitude, not a casualty count and not a local order.
Volcano scales: VEI, alert levels and aviation color codes
Earthquake magnitude and volcanic eruption size are not measured by the same system.
The Volcanic Explosivity Index (VEI) is a semi-quantitative comparison of explosive eruptions. Evidence includes erupted tephra volume, column height, duration and qualitative description. It is usually shown from 0 to 8, although the scale is open-ended; much of it increases by a factor of ten in ejecta volume between levels, with exceptions among the smallest classes.
VEI does not measure every volcanic process. It does not directly state lava-flow reach, gas concentration, lahar extent or current status. Lava volume is not its primary criterion.
USGS uses separate ground and aviation systems:
| System | Terms | Meaning |
|---|---|---|
| Volcano Alert Level | NORMAL, ADVISORY, WATCH, WARNING | Ground-focused status terminology |
| Aviation Color Code | GREEN, YELLOW, ORANGE, RED | Aviation and ash-threat status |
| VEI | Usually 0–8; open-ended | Explosive eruption size, generally assigned from evidence |
Identical colors across systems do not make their meanings identical.
Explore reported volcanic activity →
Why GDACS colors mean something different
GDACS uses green, orange and red to indicate potential humanitarian impact and possible relevance to international coordination. Its models can combine hazard severity, exposed population, vulnerability and coping capacity.
A GDACS red event is therefore not “VEI red”, “magnitude red” or a local instruction. GDACS describes its products as potentially automatic or semi-automatic, uncertain and subject to error. Earthquake24 explains GDACS only as an external system here; GDACS is not an active Earthquake24 production source as of the review date.
How to read an Earthquake24 event
The fields are easiest to interpret in this order:
- Source and source time: who published the record, and when the event began.
- Review status: automatic or reviewed source metadata.
- Magnitude and type: M, Mw, ML, mb or another supplied type.
- Reported depth: the hypocentral estimate and its depth class.
- Location uncertainty: where the source exposes it.
- Intensity, ShakeMap or PAGER: source products distinct from raw magnitude.
- Tsunami field: source metadata; not itself a warning.
- Processed time: when Earthquake24 validated and published that source version.
Marker size represents magnitude only. Marker color distinguishes the site’s selected display rule; it does not show the area affected. Automatic does not mean false, reviewed does not mean final, and an absent marker does not prove an event did not occur.
Open the latest reported events map →
Common questions
Is the Richter scale wrong?
No. ML was a major scientific advance and remains useful within its calibrated range. The error is calling every modern magnitude “Richter” without checking the source type.
Is Mw the same as energy?
No. Mw derives from seismic moment. Radiated seismic energy is related but physically distinct; the familiar 32-times figure is an approximation per magnitude unit.
Can a magnitude be negative?
Yes. A logarithmic signal below the scale’s reference level can receive zero or a negative value.
Does a deeper earthquake always have smaller effects?
No. Depth matters, but magnitude, distance, rupture, geology, structures and exposure also affect observed outcomes.
Does magnitude determine a tsunami?
No. The USGS tsunami field is source metadata, not a warning, and magnitude alone cannot determine tsunami generation.
Is VEI a current volcano warning?
No. VEI compares explosive eruption size. Status systems and observatory reports have different meanings.