Intensity Information & Examples

Two Kinds of Observational Data

Intensity on this site comes from two distinct kinds of underlying observation, and it matters which one you're looking at:

• Interpolated ShakeMap grid values — a spatial estimate of shaking at a named city or gazetteer site, produced by USGS ShakeMap from a network-wide interpolation of nearby recordings. This is what populates the Cities view on an event page.
• Direct station observations — an intensity value computed straight from the waveform recorded at one specific instrument, with no interpolation. Two kinds of instrument feed this: broadband seismometers (velocity sensors, typically the HH*/BH* channels) and strong-motion accelerometers (HN*/EN* channels), which do not clip at high shaking levels the way broadband sensors can. This is what populates the Stations view, marked with a diamond (♦) rather than a circle.

Intensity Lab does not synthesize or model station values — if USGS ShakeMap did not publish a station list for an event, no station intensities are shown, and the interpolated Cities view remains the best available estimate.

GMICE: PGA/PGV to MMI Conversion

Whenever a peak ground motion value (PGA or PGV) needs to become an MMI number — converting GMPE output, or deriving intensity directly from a station's recorded acceleration — Intensity Lab uses the Worden et al. (2012) Ground-Motion–to–Intensity Conversion Equation (GMICE). This is the same conversion USGS ShakeMap itself uses for instrumental intensity, so values here are directly comparable to a ShakeMap's own instrumental intensity layer.

For PGA (expressed in %g, percent of gravitational acceleration), the conversion is a two-branch piecewise linear formula in log space:

MMI values are clamped to the range [1, 10]. Station recordings that report PGV instead of PGA use the equivalent Worden et al. (2012) PGV branch; when ShakeMap itself already publishes an intensity value for a station, that reported value is used directly instead of re-deriving it.

ShakeMap Interpolation Method

USGS ShakeMap provides MMI values on a regular geographic grid. To estimate MMI at a specific named location (a gazetteer site or a registered user site), Intensity Lab uses bilinear interpolation:

1. Download the ShakeMap grid file (XML format) for the event.
2. Extract the MMI field and its latitude/longitude axes.
3. Construct a scipy.interpolate.RegularGridInterpolator over the grid.
4. Query the interpolator at each site coordinate.

Sites outside the ShakeMap grid boundary receive a default MMI of 1.0 (not felt). Grid resolution varies by network and event — typically 0.01° for California events. ShakeMap data improve over time as more seismic station records and Did You Feel It? reports are incorporated, so re-checking an event a day later can show a refined estimate.

Site Selection for Intensity Reports

After ShakeMap interpolation, Intensity Lab applies display rules to determine which gazetteer sites appear in event reports and on the homepage featured event card. The same rules apply to seismic stations when the Stations view is selected.

Primary rule: All sites with interpolated MMI ≥ 4.0 (Light shaking or stronger) are included, ranked by MMI descending. This threshold is why the displayed cities are not simply the nearest ones to the epicenter — a distant city on soft soil may appear while a closer city on hard rock does not.

Minimum sites rule: If fewer than 3 sites exceed the MMI 4.0 threshold (common for small or deep events), the list is padded with the nearest sites by epicentral distance until at least 3 are shown. These padding sites may have MMI below 4.0.

Coverage boundary: Only sites within the ShakeMap grid extent are evaluated. Sites outside the grid boundary receive a default MMI of 1.0 and are excluded from the ranked list.