Magnitude is not one measurement
Moment magnitude replaced Richter for anything of consequence, but a global magnitude 2.5 and above feed is dominated by the older local scales, and that tension is the substance of this layer.
Saturation is why Richter was retired for large events. The local magnitude scale, short-period body wave magnitude and surface wave magnitude are all amplitude measurements at a particular period. Once the rupture duration and dimension exceed what that period can sample, the number simply stops increasing with the actual size of the earthquake. Body wave magnitude pegs around 6.5 and 20-second surface wave magnitude around 8.3. Moment magnitude is derived from seismic moment and does not saturate, which is why it is the authoritative value for significant events.
A global M2.5+ feed is a mixture of magnitude types. Below about magnitude 4 the authoritative value is usually a local magnitude, a regional moment magnitude or a duration magnitude, not a true moment magnitude. Displaying "M3.2" without saying which scale it came from quietly conflates a Richter-lineage local magnitude with a regional moment inversion. They are not the same measurement, and the feed carries the type, so this layer shows it.
What the 2.5 threshold really means
This is the most important honesty item on the page, and the publishing agency states it directly.
The catalog publishing criteria are magnitude 2.5 and above or felt in the US outside California, 3.0 and above in California, and 4.0 and above or felt everywhere else in the world. The stated goal of those criteria is about magnitude 4.5 global completeness and about magnitude 3.0 US completeness.
So a worldwide "M2.5 and above" feed is not a complete picture of global M2.5 seismicity. It is complete to roughly 4.5 globally, and the smaller events it does contain come overwhelmingly from densely instrumented regions. Expect this map to be dominated by California, Nevada, Alaska, Oklahoma, Puerto Rico and Hawaii, while mid-ocean ridges and sparsely instrumented continental interiors show only larger events.
There are quality gates on top of the magnitude threshold. A published event needs at least five phase timing observations, a travel-time residual below 2.0, and reasonable station coverage. Events failing those are not published even when above threshold, with the explicit consequence that small earthquakes in sparsely instrumented regions may never be published or even discovered.
The correct reading, then: a cluster of small events indicates a well-instrumented region at least as much as an active one, and an empty area is not evidence of quiet ground. For scale, roughly 30,000 earthquakes are located and published in a year, out of an estimated several million that occur.
Why the numbers change after you first see them
Magnitudes, locations, depths and even the existence of events change after first publication. This is not error, it is the catalog working.
- The authoritative magnitude type changes as better data arrive. An initial local or body wave magnitude is superseded by a regional or W-phase moment magnitude, and the printed number moves with it.
- Events below the rapid response thresholds enter catalog processing, which can take up to ten weeks, though most finalize within six.
- Events are withdrawn when they turn out to be false triggers, quarry or mining blasts, or duplicates. A marker disappearing is a normal outcome.
Response targets are regional: 20 minutes for magnitude 3.0 and above in the eastern US, 4.0 and above in the western US, Hawaii, Puerto Rico and populated Alaska, and 5.0 and above elsewhere in the world. That target can slip on late-arriving data, and for slow, low-frequency earthquakes it can be days or weeks before an event is discovered at all.
Several quality fields travel with each event and are worth reading before believing a dot: the travel-time residual, the largest azimuthal gap between stations (a large gap means a poorly constrained epicenter), the distance to the nearest station (which strongly controls depth resolution), and the station count. Depth is often more consequential than magnitude for surface effects, and it is also the least well-constrained parameter in a sparse network.
Why it matters to aviation
Airport infrastructure. Strong shaking triggers runway, taxiway, apron and terminal inspections, and can damage navaids, approach lighting and air traffic facilities as well as cutting power and water. The January 2024 Noto Peninsula earthquake cracked Noto Airport's runway, stranded around 500 people when access roads were blocked, and cancelled flights into late January.
Tsunami. Coastal airports sit at low elevation. Sendai Airport was damaged by the 2011 Tohoku earthquake and then inundated, with water across the apron, taxiways and runway and reaching the second level of the terminal. Warnings also trigger precautionary closures far from the epicenter, so an earthquake can affect airports thousands of kilometers away.
Volcanic association, stated carefully. Shallow volcano-tectonic earthquake swarms are a recognized precursor of magma movement, so seismicity near a volcanic edifice is worth watching. But the aviation hazard is ash, which abrades surfaces, can melt and accrete in the hot section leading to flameout, and can block pitot-static systems. The authoritative channel for that is the network of Volcanic Ash Advisory Centers together with SIGMETs, which is the aviation hazards layer. An earthquake feed is not a volcanic ash product, and there is no reliable general relationship by which a distant large earthquake predicts an eruption.
Disaster response. Major earthquakes generate relief flights and humanitarian air operations, and in US airspace temporary flight restrictions under the same disaster-relief provision that covers wildfires.
What this layer does not tell you
Every data source has an edge. These are the ones that matter for reading this layer correctly.
- Despite the 2.5 threshold, the catalog is complete only to about magnitude 4.5 globally and about 3.0 in the US. Small events appear overwhelmingly where instrumentation is dense.
- An empty region is not evidence of quiet ground, and a cluster indicates good station coverage as much as high activity.
- Magnitudes are a mixture of scales. Below about magnitude 4 the value is usually a local, regional moment or duration magnitude, not a true moment magnitude.
- Magnitudes, depths and locations are revised after publication, sometimes for weeks, and events are occasionally withdrawn entirely as blasts or false triggers.
- Depth is the least well-constrained parameter in a sparse network, and it often matters more than magnitude for surface effects.
- A marker carries magnitude, location, depth and time. It says nothing about whether any airport is open, any runway is serviceable or any airspace is restricted. NOTAMs are the authority for that.
- Tsunami warnings come from the Tsunami Warning Centers, not from a flag on an earthquake record.
AeroScope is an situational awareness and research tool. Nothing here is approved for air traffic control, navigation, flight planning or collision avoidance. For operational decisions, use the authoritative source named above.