A wiggle nobody could explain for 15 years
The 2011 Tōhoku-Oki earthquake is the most heavily instrumented quake in history. Hundreds of papers, thousands of monitoring stations. And buried in all that data sat one small signal that wouldn't go away: a sudden eastward step, recorded by GPS stations from Hokkaido to Kyushu at the same moment, about 13 minutes after the mainshock — with no aftershock to explain it.
The first answer (GFAS)
The obvious explanation was a data-processing artifact — a glitch. The team corrected for every glitch they could model. The shift stayed put. It was real, and it was permanent. So they kept going.
Next tab: build the wave that did it, and watch its 13-minute round trip to the core.
Fire the quake. Follow the echo.
Your QuakeSimulator shows shaking spreading across the map. This shows the part the map can't: the wave going down. An ScS wave is a shear wave (S) that travels to the core-mantle boundary, reflects (because shear waves can't pass through liquid metal), and returns as another shear wave (S). Hit play.
Read the dial
Drop the magnitude or steepen the fault and watch the verdict flip. Most giant quakes never do this — their core-reflected wave comes home too weak to matter. Tōhoku was the rare case where size and geometry both lined up.
One source is a guess. Three is engineering.
This is why the Tōhoku echo is a perfect Three Gauge Test case. No single instrument could have closed it. The eastward step only became a discovery when three independent reads agreed and the easy answers got ruled out.
| Gauge 1 — what the GPS says | A clean, simultaneous 6 mm eastward step across all of Japan, ~13 min after the shock. |
| Gauge 2 — what the second read says | The step lands exactly when the ScS waveform returns from the core-mantle boundary. |
| Gauge 3 — believe the patient | Landslide? Too localized. Aftershock? None logged. Glitch? It survived every correction. The data didn't blink. |
GFAS, defused
Good First Answer Syndrome isn't beaten by being smarter on the first guess. It's beaten by refusing to let the first plausible answer end the search. The glitch hypothesis was reasonable. It was also wrong. Two more gauges is what told them which.
Run the simulator at Mw 9.0 / shallow dip until you trigger the slip. Now find the edge: drop the magnitude one notch at a time until the verdict fails. Write the smallest quake that still rings the core hard enough to move a country — and explain which gauge goes dark first, and why.
The danger window doesn't close when the shaking stops
Here's the part that turns a cool fact into a hazard. The returning wave acted like a gentle shove on faults that were already loaded to the edge by the mainshock. It triggered slip across the plate boundaries — quietly this time. Next time it might not be quiet.
| Round trip to the core | ~5,800 km, about 13–15 minutes |
| Permanent shift | 5–6 mm eastward, nationwide |
| Energy of the triggered slip | ~Mw 7.5 equivalent — spread so wide it barely shook |
| Rupture footprint | ~3,000 km — claimed as the broadest slip event documented |
| The first of its kind | First core-reflected wave ever tied to triggering fault slip |
How it ties to the QuakeSimulator
The QuakeSimulator answers “if it shook here, who feels it.” This lab adds a second clock: after the shaking stops, a core echo can re-load the same faults about 13 minutes later. A future build of the sim could drop a pin at the epicenter that, minutes after the main event, pulses a second, fainter ring — the echo coming home.
The honest asterisk: this is one team’s best-fit model of a subtle signal, peer-reviewed in Science but not yet confirmed in a second quake. Replication is the missing gauge — so the next move is to hunt the same fingerprint in the great-quake archives: 2004 Sumatra, 1964 Alaska, 1960 Chile.
So what is the fingerprint? It comes in two halves. The cause — did the rupture’s geometry funnel enough shear energy straight down to launch a strong core-reflected ScS in the first place? That’s a screen you can run on any quake from its fault angle and mechanism (Tōhoku’s shallow megathrust was geometry-perfect). The effect — did something slip again 13–15 minutes later, right as that wave came home? In 2011, Japan’s “effect” was a clean GPS step, because it sits under the densest GPS network on Earth. But 1960 and 1964 happened before GPS — there is no shift to look for. So you go to the old long-period seismograms, strainmeters, and tide-gauge records and hunt for a second, fainter burst of slip in the recordings from roughly 10–20 minutes after the main shock. The GPS step was the luxury that let us catch it the first time; the seismic echo landing in that 13–15‑minute window is the fingerprint that travels to every quake in the archive.