Why does the earthquake's little "rattle"
arrive before the big "sway"?
In an earthquake, a small rattle comes first, then, a little later, a big sway hits. This order almost never changes. That's because two kinds of waves set off underground at the same moment, and only one of them arrives first. And this gap of a few seconds is exactly what makes earthquake early warning possible.
When an earthquake hits, it rarely starts with a big jolt. First there's a faint rattling, a clattering tremor — sometimes just enough to make the dishes chatter.
A few seconds after you think "was that an earthquake?", a big sway arrives. That's the main event. It's usually this shaking that knocks things off shelves and makes it hard to stay on your feet.
The farther away the quake, the longer the gap between the two. If you feel a long pause between them, the source is far away.
Why does shaking from a single earthquake arrive in two separate waves?
One is a push-pull wave, the other a side-to-side wave. Both leave the same spot at the same time, but they travel at different speeds.
The push-pull wave arrives first and shakes gently. The side-to-side wave arrives later and shakes hard. That's why the order is always "small, then big."
This time gap isn't random — it comes straight from the physical properties of the ground itself. Let's look at it step by step.
Why is the push-pull wave faster?
A solid like the ground can be deformed in two ways.
- Squeeze or stretch — a deformation that changes volume
- Shear — a deformation that changes shape but not volume
And materials resist being squeezed strongly, but resist being sheared much more weakly. Think of a rubber eraser: hard to crush, easy to twist sideways.
Wave speed increases with stiffness (resistance) and decreases with mass. It's the same reason a tightly-strung wire produces a higher note (a faster wave) than a slack one.
So the push-pull wave, which relies on the strongly-resisted "squeeze," travels fast, while the side-to-side wave, which relies on the weakly-resisted "shear," travels slowly. The order never reverses.
The order they arrive in comes down to how the ground resists being deformed.
Here's where it gets remarkable. The side-to-side wave cannot travel through a liquid. A liquid has no "restoring force" to keep its shape, so when sheared it simply doesn't spring back — no wave forms.
In the early 20th century, researchers collecting seismograph records from stations around the world after major earthquakes noticed something odd. In a certain band on the far side of the Earth, the side-to-side wave never arrived at all.
There's only one explanation: there is a layer of liquid inside the Earth. A structure thousands of kilometres down, which no one has ever seen, was revealed purely from records of ground shaking. It's thanks to this observation that we know the Earth's outer core is liquid.
A gap of a few seconds makes the warning possible
Earthquake early warning systems use exactly this time gap.
A station close to the source picks up the faster push-pull wave first and sends that information out as an electrical signal. Electrical signals travel far faster than ground waves, so the alert can arrive before the big shaking does.
But this method has unavoidable limits.
- If the source is close, there's no time. For a quake directly underneath you, both waves arrive almost together. For a quake right below you, the alert may sound only as the shaking starts, or even after
- The size estimate, made from the first few seconds, can be revised. Some quakes start small and grow; the opening moments alone don't always reveal the final size
- The lead time is, at most, tens of seconds. You need to decide in advance what you'll do in that window
- Protect your head first, and get lowGet under a sturdy table and hold the legs. No table? Move to a spot nothing can fall on, tip onto, or slide into. Stay away from windows, tall furniture, and hanging light fixtures.
- Don't rush outside. Don't run to the stoveFalling debris makes outside dangerous. Turn off the stove only after the shaking stops. Most modern gas appliances already shut off automatically when they detect shaking — approaching one mid-shake is riskier than leaving it.
- Once the shaking stops, check you're safe, then actProtect your feet with slippers or shoes (there may be broken glass), and secure an exit route. Call 119 for anyone injured. In places at risk of tsunami, head for high ground immediately — don't wait for a warning.
The "gap" tells you the distance
Try counting the seconds from when the rattling starts to when the sway arrives. Multiply that number by about 8, and you get roughly the distance to the source, in kilometres.
5 seconds means about 40 km away; 10 seconds means about 80 km. It's a simple rule, but it's a relationship discovered in Japan in the 19th century, and it's still used as a rough guide today.
Conversely, an earthquake with almost no gap at all means the source is very close. A sudden, sharp jolt straight up usually means the quake struck right beneath you or nearby.
Something you can check at home
- Stretch a long spring (a coiled toy spring works) out on the floor and hold one end
- Give the end you're holding a quick push forward and back. A compressed section travels down the spring — that's the push-pull wave
- Give the same spring (or a rope) a single flick left-right. A sideways bulge travels down it — that's the side-to-side wave
- Compare which is faster by tightening or loosening the spring
- Now flick the end of a tray of water left-right. You'll see the sideways disturbance doesn't travel through it at all
The same spring behaves in completely different ways depending on how you flick it. And in water, sideways deformation just doesn't propagate — that's exactly what "the side-to-side wave can't cross a liquid" means, and it's the same clue that revealed the Earth's interior is liquid. Your kitchen tray and the Earth's core follow the same rule.
Summary
The rattle arrives first because the ground resists being squeezed strongly, but resists being sheared only weakly. The wave that relies on the stronger resistance travels faster and arrives first. This gap of a few seconds is what makes earthquake early warning possible — and the very same property once revealed the structure deep inside the Earth.
The first rattle is a warning.
Protect your head before you start counting.
Even after the big shaking stops, the ground itself can weaken in reclaimed land through a process called "liquefaction." We explain how that works in our article on liquefaction. And the reason small tremors can keep following a big quake for days is covered in "Why do aftershocks keep coming for days after a big earthquake?"
Want to go deeper? — Terms, formulas, and how this connects to the textbooksWe've labelled each part by level, from middle-school science to open research questions
- MSCovered in middle-school science
- HSCovered in high-school "Basic Physics" / "Basic Earth Science"
- HS+Covered in high-school "Physics" / "Earth Science," or treated as advanced/sidebar content in textbooks
- Univ.Not taught in high school — university-level specialist material (seismology, elasticity theory)
- ResearchNot yet settled even at university level — an open question researchers are actively investigating
MSTerms: earthquake vocabulary
- P wave: the "push-pull wave" from the main text. P stands for Primary. It's a longitudinal wave that vibrates along its direction of travel, and it causes the initial faint tremor.
- S wave: the "side-to-side wave" from the main text. S stands for Secondary. It's a transverse wave that vibrates at right angles to its direction of travel, and it causes the main shaking.
- Initial micro-tremor duration: the time between the P wave's arrival and the S wave's arrival. Also called the P-S time.
- Seismic intensity vs. magnitude: intensity is how strongly a given location shakes, and varies from place to place. Magnitude is the size of the earthquake itself — one earthquake has one magnitude value.
- Hypocentre and epicentre: the hypocentre is the underground point where the rock first ruptures; the epicentre is the point on the surface directly above it.
HSWorking it out: counting the small shaking gives you the distance
When an earthquake hits, there's first a small rattling tremor, followed by a bigger shaking. Just by counting the seconds between them, you can work out the distance to the source.
distance = speed × time
| Fast wave (P wave) | about 7 km/s |
| Slow wave (S wave) | about 4 km/s |
| Gap in seconds | duration of the small shaking [seconds] |
At the exact same moment, from the exact same spot, two waves of different speeds set off. The farther they travel, the wider the arrival gap grows — just like a fast runner and a slow runner pulling apart over a longer race.
In other words, the "gap in seconds" is itself a ruler for distance.
Let the distance to the source be D, and the duration of the small shaking be t. The two arrival times are D ÷ 7 and D ÷ 4. Their difference is t. Rearranging gives:
D = t × (7 × 4) ÷ (7 − 4)
| Numerator | 7 × 4 = 28 |
| Denominator | 7 − 4 = 3 |
| Divide | 28 ÷ 3 ≒ 9.3 |
| Summary | D ≒ t × 9.3 (km) |
The well-known "seconds × 8" rule is this 9.3, adjusted to match real underground structure (wave speed changes with depth). "Roughly seconds × 8–9 km" is close enough to remember.
| Small shaking lasted 3 seconds | 3 × 8 = 24 km |
| Small shaking lasted 10 seconds | 10 × 8 = 80 km |
| Small shaking lasted 30 seconds | 30 × 8 = 240 km |
Here's the important part to read correctly. A long gap doesn't mean "far away, so safe." If you can feel shaking from that far away, it must be a large earthquake.
Conversely, if the big shaking hits before you even have time to count, the source is close. Either way, only start counting after you've protected yourself.
Early warning works by detecting the P wave first and issuing an alert before the S wave arrives. The time it buys comes from the same calculation as in ②.
| Distance to source | assume 100 km |
| Time for P wave to arrive | 100 ÷ 7 ≒ 14 sec |
| Time for S wave to arrive | 100 ÷ 4 = 25 sec |
| Difference | 25 − 14 = 11 sec |
11 seconds. And since it takes a few more seconds to process the observation and issue the alert, the time you actually have to act is even shorter.
And the closer the source, the smaller this gap becomes. At 20 km: 20 ÷ 7 ≒ 3 sec versus 20 ÷ 4 = 5 sec, a gap of just 5 − 3 = 2 seconds. For a quake directly below you, the warning simply cannot beat the shaking. That's not a technology problem — it's a limit set by the physics of wave speed.
So waiting to think until after the alert arrives is too late. Securing furniture in advance, keeping nothing that could fall near where you sleep — measures you take beforehand are the ones that reliably work.
Magnitude is a scale where each +1 means about 32 times more energy. It grows by multiplication, not addition.
| Difference between M7 and M8 | 32× |
| Difference between M7 and M9 | 32 × 32 = 1024× |
"M7 and M9 differ by just 2" is misleading — it's about 1000 times more. Because the numbers look small, it's easy to underestimate the gap. Knowing what the scale actually means changes how you read the same announcement.
HSWhere does "seconds × 8" come from?
Let the distance to the source be d, and the P- and S-wave speeds be Vp and Vs. The arrival-time difference is:
t = d/Vs − d/Vp
Solving for d gives d = t × Vp·Vs/(Vp − Vs). Plugging in numbers:
| P-wave speed | about 8 km/s |
| S-wave speed | about 4 km/s |
| Coefficient Vp·Vs/(Vp−Vs) | 8 × 4 ÷ (8 − 4) = 8 |
| If the P-S time is 10 seconds | d ≒ 8 × 10 = 80 km |
※ Actual speeds vary with underground structure, so the coefficient is generally taken to fall between 6 and 8. Treat it as a rough guide.
This relationship is known as the Omori formula, named after Fusakichi Omori and colleagues, who worked it out in Japan during the Meiji era. It's still taught as a basic tool in earth science today, since it lets you find the distance to a quake's source with nothing but a stopwatch.
HS+What determines wave speed
The speed of a wave travelling through an elastic body roughly follows the form v = √(elastic modulus / density). Stiffer means faster; heavier means slower.
The two wave types draw on different kinds of "stiffness."
- P wave: Vp = √((K + 4μ/3) / ρ) — both resistance to volume change K and resistance to shape change μ come into play
- S wave: Vs = √(μ / ρ) — only resistance to shape change μ matters
The term inside the P-wave formula is always larger, so Vp > Vs is guaranteed mathematically. There's no way for the order to reverse. In typical rock, the ratio is roughly 1.7.
And in a liquid, μ = 0, so Vs = 0. The S wave cannot travel. Everything said in the main text traces back to this one line.
Univ.Seeing inside the Earth through its shaking
Seismic waves refract and reflect at boundaries where the underground material changes — the same way light bends passing through glass. Using this property, researchers can work backward from arrival times recorded at stations worldwide to reconstruct the velocity structure inside the Earth.
One of the earliest major discoveries was the S-wave shadow zone. In the 1910s, researchers found a band roughly 103 to 143 degrees from the epicentre where S waves never arrived at all, confirming the existence of a liquid outer core. This was followed by the discovery of the (solid) inner core in 1936, by Inge Lehmann.
Today, seismic tomography combines data from many earthquakes and stations to reconstruct three-dimensional velocity structure. It's used to trace the path of subducting plates and estimate mantle flow — the same principle as a medical CT scan.
ResearchWhat we still don't know
- Earthquake prediction still isn't possible. No scientifically established method exists for pinpointing in advance when, where, and how large a quake will be. This has been studied for decades in Japan and elsewhere, but the current consensus is that practical short-term prediction hasn't been achieved. Working from the premise that prediction isn't possible, effort instead goes into advance preparation and immediate post-event information such as early warning.
- We don't know whether a quake's opening moments determine its final size. Large and small earthquakes often start with very similar-looking waveforms. One view holds that "whether a small start grows into something big is essentially random"; another holds that "the final size is already set at the very start." The question remains unresolved. This bears directly on how accurate early warning can ever be — if the second view is correct, it should be possible to estimate final size more precisely from the opening signal.
- The meaning of slow-slip events is also still being worked out. In the 2000s, researchers discovered plate-boundary slip that unfolds slowly over days to months, along with faint accompanying tremor. Its relationship to major earthquakes draws a lot of attention, but whether it acts as a precursor remains unsettled.
- Predicting long-period shaking is also difficult. High-rise buildings can sway strongly even far from the source, and this depends heavily on the structure of underground sediment layers. Our knowledge of underground structure is limited, so real uncertainty remains in these predictions.
How this maps onto the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science — how earthquake shaking travels / initial micro-tremor duration | P waves and S waves, intensity vs. magnitude, seconds × 8 |
| HS | Basic Physics — waves (longitudinal and transverse) / Basic Earth Science — Earth's interior | Differences between the two wave types, deriving the Omori formula |
| HS+ | Physics — elasticity and wave speed (often treated as advanced) | v = √(elastic modulus/density), why S waves can't cross a liquid |
| Univ. | Seismology, elasticity theory, geophysics | The S-wave shadow zone, discovery of the outer and inner core, tomography |
| Research | Seismology (unresolved) | Limits of prediction, how rupture grows, slow slip |
| — | Disaster-preparedness education | Protect your head, don't go outside, act after the shaking stops |
- Japan Meteorological Agency (気象庁) explanatory materials on earthquakes and the mechanism and limits of earthquake early warning.
- Shearer, P. M., Introduction to Seismology (a standard seismology textbook).
- Lehmann, I., P', Publications du Bureau Central Séismologique International, 1936 (discovery of the inner core).
- Obara, K., Nonvolcanic deep tremor associated with subduction in southwest Japan, Science 296, 1679–1681, 2002 (discovery of deep low-frequency tremor).
- Materials from Japan's Cabinet Office (内閣府) and local governments on how to act and prepare for earthquakes.
※ Seismic wave speeds and coefficients vary with underground structure. This article presents commonly used rough guides.
※ This article is a general-audience science explainer. Follow the instructions of the Japan Meteorological Agency, fire services, local authorities, and your regional disaster plan for what to do during an earthquake. In areas at risk of tsunami, head for high ground immediately without waiting for a warning. The figures given here are rough guides meant to aid understanding of the underlying mechanism.