More big earthquakes, or a different ruler?

“There are more earthquakes than there used to be” gets said after every bad one. The standard answer — from the US Geological Survey, from IRIS, from the British Geological Survey — is no: large earthquakes have not become more common, and what has grown is the number of seismometers. None of those pages says how big a change the record could have detected. I wanted that number, so I wrote a test, and wrote the rule down before fetching anything.

The rule, before the data

The version worth testing is not the one people point at. The USGS catalog obviously holds more earthquakes each year: at magnitude 4.5 and up, the last ten years average 2.2 times as many a year as 1973–82. By every account that is mostly the network, not the planet, and nobody disputes it. The claim is about the planet, and that can only be asked at a size the network could always see. So I sealed it at magnitude 7 — about fourteen a year worldwide — over 1973–2025, the span of the USGS’s continuous global catalog:

• survived if the 95 % interval on the change per decade sits above zero;
• killed if it rules out a rise of 10 % a decade (+66 % over the record);
• inconclusive otherwise.

Before fetching a single count I ran the rule on two thousand made-up catalogs for each assumed truth, to see how likely each of those three outcomes was. If the true rate is flat, it kills the claim about nine times in ten and wrongly confirms it about three times in a hundred. If there is a real rise of 10 % a decade, it confirms nine times in ten. I committed that table, the rule and the fetcher, and pushed them before the fetch ran.

It survived

729 earthquakes of magnitude 7 or more, 1973–2025. The rate rose 6.6 % a decade, with a 95 % interval of +1.6 % to +11.8 %. The rule says survived. 1973–82 averaged 10.2 a year; 2016–25 averaged 13.4.

That is an outcome I had given three chances in a hundred if the authorities were right. So either I was looking at the three, or something in the instrument had moved. I had named the likeliest suspect in the rule’s own notes, as the reason for starting in 1973 and leaving magnitude 6 out: magnitude is not one scale.

The ruler changed under the rule

Every event in the catalog records which kind of magnitude its size came from. Of the 78 magnitude-7 events the USGS lists for the 1970s, 72 are sized as Ms — surface-wave magnitude, the older standard. The share falls to 37 % in 1980–84 and 2 % in 1985–89; the last magnitude-7 event sized that way is from 1987. Everything after is moment magnitude, Mw, which is computed from the physical size of the rupture.

The two scales do not fail in the same places. Surface-wave magnitude is read from waves that run along the surface, and an earthquake 300 km down makes few of them. So count the deep ones. The catalog lists no earthquake of magnitude 7 or more deeper than 300 km anywhere in the world from 1973 to 1979 — then 7 in the 1980s, and 16, 15 and 17 in the three decades after. Deep earthquakes did not begin in 1980. The ruler learned to see them.

Measured the other way

So I ran a second check, after the data, and it is labelled that way wherever it appears. The Global CMT project has computed moment magnitude for every large earthquake since 1976 with one method throughout. Same rule, unchanged, on the years both catalogs cover:

The largest gap is the first bar: for 1976–80 the USGS catalog holds 51 magnitude-7 events and Global CMT holds 68. Through the late eighties and early nineties the gap runs the other way (61 against 52, 75 against 61), and from 2001 the two agree within three events a period. That later agreement is not independent evidence: for many large events from the 1980s on, the USGS took its moment magnitude from Global CMT. The years where the two catalogs are genuinely separate witnesses are the ones where they disagree most.

So the ruler change is real, it pulls the old end of the USGS line down, and it does not explain the rise away. Over the same fifty years, measured one way throughout, the rate still rose — 5.7 % a decade instead of 6.2 %, with a lower bound three tenths of a percent above zero.

The obvious third witness, the ISC-GEM catalog, which re-measured exactly the 1970s and early 1980s, is handed out through a form with a CAPTCHA, so I did not use it.

Where the rise lives

The busiest ten years in both catalogs are 2007–2016: 173 magnitude-7 events in the USGS catalog, 17.3 a year against a 1973–2025 mean of 13.75. Picking the busiest window after looking is a trap, so the comparison keeps the picking: in a hundred thousand simulated records with a constant rate, the busiest decade anywhere in the record reaches 173 about 4 % of the time (3 % for Global CMT’s 170). Unusual, not impossible, and a look I took after the data.

And it is fragile. Start the USGS count in 1983, once the old scale has mostly gone, and the same rule says killed: +1.9 % a decade, −4.3 % to +8.5 %. Start Global CMT in 1983 and it says survived: +7.3 %, +0.5 % to +14.6 %. The two catalogs disagree most in the 1980s and early 1990s, which are exactly the years a later start leans on. Both start years were chosen after seeing the data, which is why neither can be the verdict — and why the verdict should carry no more weight than its lower bound, which is between 0.3 and 1.6 percent a decade depending on the ruler and the first year.

What that leaves

Two sentences get said about big earthquakes, and neither comes through intact.

“There are more and more” imagines a surge. No version of this test puts the top of the interval above 15 % a decade; the rise that is there is single digits, concentrated in 2007–2016, and on the USGS catalog it is gone if the clock starts ten years later.

“Large earthquakes have not become more common” is closer, and it is also more than this record shows: over fifty years, measured two ways, a rule written before the data says the rate went up, modestly. The sentence the numbers allow is duller than either — roughly constant, with one unusually busy decade that nobody can yet call a trend.

The one thing to keep

When a count rises, ask whether the measurement changed — and then actually measure it the other way, because the answer can be yes and still not be the explanation. Here the first decade was sized on a scale that nearly missed a whole class of earthquake, and a catalog built one way throughout took about half a point a decade off the slope and left the rest.

Corrected the same afternoon. The first version of this entry, public for under an hour, stopped at “on a catalog measured one way throughout, the same rule cannot tell”. That was true only for 1976–2020, the span of Global CMT’s main file. With its monthly files for 2021–2025 added — a busy five years, 75 magnitude-7 events in both catalogs — it says survived too. The check that agreed with the story I was telling was the one I should have extended before publishing.

The register files the study as survived, because that is what I committed to, with both catalogs beside it and the three branch probabilities printed next to the rule. Counts, code, the rule as committed and the second check are in the pack. Counts come from the USGS ComCat count service; moment magnitudes from the Global CMT catalog (Dziewonski et al. 1981; Ekström et al. 2012).