Seeing what an intensity value means

Strong ground motion is a record, not a single value

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Why a simulation, not just a prediction equation

A ground motion prediction equation (GMPE) answers a narrow question: given a magnitude, a distance and a site condition, what is the median peak acceleration — or spectral acceleration at one period — and how much scatter is there around it? The answer is a handful of numbers at a point. That is enough to shade a map, and it is what most rapid shaking products report.

A ground motion simulation answers a larger one. It returns a full three-component time series — the actual motion of the ground, second by second, in all three directions. Everything a scalar leaves out is in there:

• Duration. Two sites can share a peak acceleration and shake for ten seconds or for sixty. Damage does not care only about the largest cycle.
• Frequency content. A structure responds at its own period. A time series says what energy arrived near that period; a single peak value cannot.
• Arrival and sequence. P-wave, S-wave, surface waves, and the gap between them — which is what makes a simulated record look and behave like a recording rather than a bar chart.
• Near-field and directivity effects. Rupture propagating toward a site produces large velocity pulses that a distance-based regression averages away.
• Basin response. Sedimentary basins trap and amplify long-period energy in a way that depends on the wave path, not just on the site's V_S30.

That output is what an engineer needs for a response-history analysis, and it is what drives the shaking animations published here — neither is possible from a peak value alone. A physics engine has to be told where the ground is at every frame; there is nothing to hand it in a single number.

There is a second difference, about where each method's confidence comes from. A GMPE is a regression fitted to recorded earthquakes, so it is best constrained where recordings are plentiful — moderate magnitudes at moderate distances — and thinnest exactly where the stakes are highest: large ruptures, close in. A Broadband Platform seismogram is instead computed, propagating a kinematic rupture model through a regional crustal velocity model, so it is not limited to the magnitude-distance range that happens to be well sampled.

The simulated seismograms behind these animations are produced on the SCEC Broadband Platform through the BBP API, and the verification and validation evidence for that service is published separately.

One standard environment

Every animation uses the same scene, so the only thing that differs between two videos is the ground motion driving them. A wooden kitchen table carrying familiar objects of known size and weight: put two animations side by side and any difference you see is the earthquake, not the set dressing.

The objects are simulated as rigid bodies with their real masses, friction, and centers of mass. Nothing is animated by hand: the floor and table are driven by the ground motion time series, and everything on the table responds to that. Full technical description →

Three types of animation

Same scene, three different questions about the same earthquake.