Technical Information

Geographic Coverage

Supported region: latitude 28–50°N, longitude 125–113°W (Western US and northern Mexico), covering California, Nevada, Oregon, and the Mexican states of Baja California and Sonora. Custom location queries outside this region will be rejected.

Data Source: USGS ShakeMap

All automated event reports are derived from USGS ShakeMap data. ShakeMap produces a spatial grid of estimated instrumental intensity (PGA, PGV, MMI) by interpolating from seismometer recordings and applying regional ground-motion models.

Intensity Lab downloads the ShakeMap grid for each event and uses scipy.interpolate.RegularGridInterpolator to estimate MMI at each gazetteer site. ShakeMap data improve over time as more seismic station records and Did You Feel It? reports are incorporated.

In addition to interpolated city estimates, Intensity Lab also ingests the ShakeMap station list (stationlist.json) to obtain observed MMI values directly at seismic stations. Unlike gazetteer city values, station MMI is not interpolated — it is calculated directly from the ground motion recorded at each instrument using the Worden et al. (2012) GMICE (PGA or PGV to MMI conversion), or from the intensity value reported by ShakeMap if available. Station observations are identified on maps with a diamond marker (♦) to distinguish them from the circle markers used for gazetteer cities.

Station data availability: Seismic station observations are only shown when USGS ShakeMap includes a stationlist.json for the event. This file is typically present for M3.5+ events in California and other well-instrumented networks. For smaller events, events in sparsely instrumented regions, or events where ShakeMap was generated without a station list, the Stations view will indicate that no observed data are available. In those cases, the Cities view (ShakeMap interpolated) remains the best available estimate of shaking at named locations. Intensity Lab does not synthesize or model station MMI — if no observed recordings are available, none are shown.

GMICE: PGA to MMI Conversion

When converting GMPE output (PGA) to MMI, Intensity Lab uses the Worden et al. (2012) Ground-Motion–to–Intensity Conversion Equation (GMICE). PGA is expressed in %g (percent of gravitational acceleration).

The conversion uses a two-branch piecewise linear formula in log space:

Low branch (log₁₀(PGA_%g) < 0.584): MMI = 2.27 + 3.93 × log₁₀(PGA_%g)

High branch (log₁₀(PGA_%g) ≥ 0.584): MMI = 3.66 + 1.55 × log₁₀(PGA_%g)

MMI values are clamped to the range [1, 10]. This conversion is consistent with the approach used by USGS ShakeMap for instrumental intensity.

ShakeMap Interpolation Method

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

Download the ShakeMap grid file (XML format) for the event.
Extract the MMI field and its latitude/longitude axes.
Construct a RegularGridInterpolator over the grid.
Query the interpolator at each gazetteer 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.

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.

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 gazetteer sites by epicentral distance until at least 3 sites 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.

Seismic station display: The same MMI ≥ 4.0 and minimum 3-site rules apply to seismic stations when the Stations view is selected. If fewer than 3 stations exceed the threshold, the nearest recorded stations are shown as padding, regardless of their MMI level.

When the Stations view shows “No station intensity data available yet,” it means no observed seismic recordings were available in the USGS ShakeMap for this event — not that seismic stations did not record it. The station list is populated only from data explicitly provided in ShakeMap’s stationlist.json. Intensity Lab does not estimate or infer station values.

Vs30: Site-Specific Soil Conditions

V_s30 is the time-averaged shear-wave velocity in the upper 30 meters of the Earth's surface, measured in m/s. It is the primary proxy for local site amplification in GMPE calculations: softer soils (low V_s30) amplify shaking; hard rock (high V_s30) attenuates it.

Data source priority for each site:

CGS Geology — California Geological Survey geology-based map via SCEC UCVM (California sites)
Wald Topography (UCVM) — Topographic slope proxy via SCEC UCVM (Nevada, Oregon, northern Mexico)
Wald Topography (USGS) — USGS global V_s30 raster (Wald & Allen 2007) fallback
Default (760 m/s) — Rock-site reference when no data is available

The API exposes site-specific V_s30 for any location in the supported region: GET /api/v1/vs30?lat={lat}&lon={lon}

Ground Motion Prediction Equations (GMPEs)

GMPEs are empirical equations fit to large databases of recorded ground motions. They predict PGA (and other parameters) as a function of earthquake magnitude, source-to-site distance, and site conditions. Intensity Lab uses four NGA-West2 models:

Key	Authors	OpenQuake Class
ask14	Abrahamson, Silva & Kamai (2014)	AbrahamsonEtAl2014
bssa14	Boore, Stewart, Seyhan & Atkinson (2014)	BooreEtAl2014
cb14	Campbell & Bozorgnia (2014)	CampbellBozorgnia2014
cy14	Chiou & Youngs (2014)	ChiouYoungs2014

GMPE calculations are implemented via GEM OpenQuake (openquake.hazardlib). PGA output is converted to MMI using the Worden et al. (2012) GMICE.

Required Input Parameters

Each GMPE requires earthquake source parameters, distance metrics, and site characterization. Many of these are not directly reported by USGS for smaller events; the table below documents every parameter, the value used, and the source or justification.

Parameter	Symbol	Value Used	Source / Rationale
Moment magnitude	M	Event magnitude	USGS ComCat
Fault rake	λ	0° (strike-slip)	Default for uncharacterized Western US faults
Fault dip	δ	90° (vertical)	Conservative for uncharacterized faults
Hypocentral depth	h	Event depth, or 10 km	USGS ComCat; 10 km default if unknown
Rupture width	W	10^{(0.32M − 1.01)}	Wells & Coppersmith (1994) all-mechanism scaling
Top of rupture depth	Z_tor	max(0, h − W/2)	Estimated from depth and rupture half-width
Joyner-Boore distance	R_jb	Epicentral distance	Point-source approximation (haversine)
Rupture distance	R_rup	√(R_jb² + h²)	Hypocentral distance (3-D Pythagorean)
Horizontal distances	R_x, R_y0	0.0	Not applicable; no rupture-plane geometry available
Shear-wave velocity	V_s30	Site-specific	UCVM (CGS / Wald), USGS raster, or 760 m/s default
V_s30 measured flag	—	False	All values are model estimates, not field measurements
Depth to V_s = 1000 m/s	Z_1.0	ASK14 regression from V_s30	Abrahamson et al. (2014), Eq. 31
Depth to V_s = 2500 m/s	Z_2.5	CB14 regression from V_s30	Campbell & Bozorgnia (2014), Eq. 33

Fault Geometry: Rake and Dip

The NGA-West2 models require fault rake (slip direction) and dip (fault plane angle). For most Western US events, focal mechanism data is not available in real time. Intensity Lab assumes rake = 0° (pure strike-slip) and dip = 90° (vertical fault plane). Strike-slip faulting is the dominant mechanism in California's transform plate boundary setting. For thrust or normal events this will introduce some error; the GMPEs are relatively insensitive to rake for sites at distances greater than ~20 km from the rupture.

Distance Metrics: R_jb and R_rup

NGA-West2 models use two distance measures: the Joyner-Boore distance (R_jb), the closest horizontal distance to the surface projection of the rupture, and R_rup, the closest 3-D distance to the rupture plane. Because rupture geometry is not available for most M3–5 events, Intensity Lab approximates R_jb as the epicentral distance and R_rup as the hypocentral distance (√(R_epi² + depth²)). For sites more than ~20 km from the epicenter — the majority of all 2,002 gazetteer sites for any given event — this point-source approximation introduces less than 5% error in predicted PGA.

Rupture Width and Z_tor

Rupture width W is estimated from earthquake magnitude using the Wells & Coppersmith (1994) all-mechanism regression: W = 10^{(0.32M − 1.01)}. This yields W ≈ 3 km for M 4.0 and W ≈ 8 km for M 6.0. The top of rupture depth is then Z_tor = max(0, h − W/2), where h is the catalog hypocenter depth.

Basin Depth Parameters: Z_1.0 and Z_2.5

Several NGA-West2 models require the depth to the V_s = 1000 m/s horizon (Z_1.0) and depth to V_s = 2500 m/s (Z_2.5), which describe sedimentary basin depth. These are estimated from site V_s30 using published regression equations: Z_1.0 from Abrahamson et al. (2014) Equation 31 and Z_2.5 from Campbell & Bozorgnia (2014) Equation 33. These regressions represent average basin depth for a given V_s30 in California; sites in deep basins (e.g., Los Angeles Basin) may have basin depths significantly larger than these estimates.

Regional caution: The NGA-West2 models were developed primarily from California strong-motion recordings. Estimates for regions with different crustal properties — such as Oregon, Nevada, or central Mexico — should be interpreted with caution and treated as approximate.

Email Notifications

Registered users can opt into two types of automated email notifications. All intensity estimates are computed and stored for every qualifying event regardless of notification delivery — data is always viewable on the website after logging in.

Site-Specific MMI Report

Sent to users who have registered a custom location when that site falls within the ShakeMap coverage area. Includes the ShakeMap-interpolated MMI, PGA, and PGV at the registered location, plus a comparison across four NGA-West2 GMPE models and nearby community shaking estimates.

Regional Event Summary

Sent to users subscribed to all regional events when an M3.5+ earthquake produces moderate or stronger shaking somewhere in the region (MMI ≥ V). Includes a map, nearby community MMI table, and a distance-intensity comparison plot. Both notification types are evaluated independently — a user may receive both a site-specific report and a regional summary for the same event.

Notification Thresholds

Each notification type has an independent MMI threshold:

Site-specific reports: sent when the ShakeMap-interpolated MMI at your registered location is ≥ IV (Light — generally felt indoors).
Regional summaries: sent when the highest estimated MMI across all monitored communities is ≥ V (Moderate — felt by everyone, minor damage possible).

Events below these thresholds are still fully processed and all intensity data remains accessible on the website and via the API. No email is generated for events that do not reach the threshold.

Manage your notification preferences →

API Access

The Intensity Lab API is available at /api/v1/. Public endpoints (earthquake list, event detail, site intensities) require no authentication.

The POST /api/v1/ground-motion/ endpoint for on-demand queries at custom coordinates requires a JWT bearer token. Create a free account to get started.

View full API documentation →

About Intensity Lab

Questions or feedback? Submit feedback. For more about the project, see the About page.