Where Does Earthquake Data Come From? AFAD, Kandilli, USGS

By Nesim Razon, independent Android developer · 2026-11-10

Open two earthquake lists right after a quake and you may see two different magnitudes, two slightly different times or places. Nothing is broken. Recent earthquake lists come from seismological networks, and each agency runs its own stations, its own software and its own review process.

Seismic networks: the source of every list

An earthquake list is not made by an app. It starts at a seismological network: a set of seismometers spread across a region, sending their recordings to a processing center. Software and analysts there find the time, the location, the depth and the magnitude of each event, and the agency publishes the result.

Turkey has several such sources. Two public ones are AFAD's Earthquake Department, part of the national disaster management authority, and the Kandilli Observatory and Earthquake Research Institute at Boğaziçi University. Outside Turkey, global agencies such as the US Geological Survey (USGS) and the European-Mediterranean Seismological Centre (EMSC) publish their own solutions, especially for larger events.

What Son Depremler actually uses

Son Depremler reads data directly from two public services: AFAD's event service and the Kandilli Observatory's last-earthquakes page. You can switch between the two sources with one tap and compare them. The app is not affiliated with either institution, and it does not use USGS or EMSC data.

From AFAD the app requests the last 30 days of events and applies the time and magnitude filters on the phone. Kandilli publishes its list of the most recent 500 earthquakes, which covers roughly a week, so a longer period is only available from AFAD in the app.

The first result and later revisions

The first solution after an earthquake is usually computed quickly, often automatically, from the stations that have already received the waves. Analysts then review the recordings, add more stations, pick the wave arrivals more carefully and recompute. Location, depth and magnitude can all change a little in that process.

You can even see this in Kandilli's list: each line has a solution quality column, which shows a preliminary solution (marked as such) or a revised one with a revision number and date. The practical lesson is that a number you see a few minutes after an earthquake can still be updated, and a list you open a day later can differ from the one you saw first.

Why magnitudes differ between agencies

• Different magnitude types: one agency may report a local magnitude (ML), another a moment magnitude (Mw) or a duration magnitude (MD). They are designed to be close but are not identical.
• Different stations: each network uses its own stations and their distance to the earthquake, which affects the estimate.
• Different methods and software: calculation details and corrections vary.
• Different timing: one list may show an automatic result and the other a reviewed one.
• Rounding and display: values are reported to one decimal and may be rounded in different ways.

Kandilli's list contains several magnitude columns. The app shows Kandilli's ML when it exists, and otherwise Mw and then MD. AFAD's records also carry a magnitude type, so the number you see is not always the same kind of magnitude on both sources.

How to read the differences

A difference of a few tenths between two agencies is common and does not mean either one is wrong. If one list says 4.6 and the other says 4.4, both are describing a moderate earthquake. What matters more is the order of magnitude, the depth and the location, which usually agree closely.

Times need attention as well. AFAD's service provides times in UTC, while Kandilli lists local Turkish time, and the app converts them for display. If you compare with another source, check which time zone it uses.

Following one earthquake step by step

Imagine a moderate earthquake in the middle of the night. Within a short time, the seismic networks detect it and produce a first automatic estimate. If you open a list at that moment you may see a first value, or the quake may not appear yet because the service has not published it. Over the following minutes and hours the entry may be refreshed with a reviewed location or magnitude, and smaller aftershocks begin to appear as separate lines.

If a list seems behind, the reason is often just timing: the agency has not published yet, or the app is showing its last saved copy. Refreshing in the app asks the source again.

Tips for comparing sources

• Look at the time first: if two entries are within a few seconds and a few kilometers of each other, they are almost certainly the same earthquake.
• Check which magnitude type each source reports before concluding that they disagree.
• For small earthquakes, expect larger relative differences, because few stations may have recorded them.
• For a strong earthquake, wait for the reviewed values before quoting a number to others.
• Use the official statements of the agency, rather than screenshots, when you need an exact figure.

What a list can and cannot do

A recent earthquake list is a record of what has already happened. The data reaches the agencies after the earthquake, usually within minutes, and can be revised afterwards. It cannot predict earthquakes and it is not an early warning system.

Use lists to learn what happened, compare sources and see the pattern of small earthquakes in your region. For official statements and safety information after a strong event, follow AFAD.