Our Earthquake Zones Now Follow the Shaking, Not a Formula

When an earthquake appears on the Calchis map, it gets four zones: Catastrophic, Severe, Moderate and Affected, each tied to a level of shaking on the Modified Mercalli scale. Where the U.S. Geological Survey has published a ShakeMap for the event, the zones are ShakeMap's own contours. This post is about what happened before ShakeMap arrived, and why we changed it.

What was wrong

Every earthquake had a Catastrophic zone. Before ShakeMap, the zones came from a scaling formula that drew all four for any earthquake from magnitude 2.5 up. A magnitude 4.0 got a one-kilometer “Catastrophic Impact Zone”; a 5.5, one of 5.6 kilometers. Of the 37 recent magnitude-4-and-up earthquakes for which ShakeMap had been published, ShakeMap showed the shaking that zone stands for, intensity VIII, for none. One, a magnitude 5.5 in Alaska on October 6, peaked at intensity 3.5: weak shaking, felt by some, damaging to nothing.

ShakeMap rarely replaced it. Zones were drawn once, when the earthquake first appeared, minutes after it happened. ShakeMap usually comes later, so most earthquakes kept the formula's zones: 295 of the 329 magnitude-5-and-up events of the previous 60 days.

The formula was not what its label said. It called itself a USGS ShakeMap scaling formula, and our methodology page credited a published attenuation study. We could not trace the formula to either.

What replaced it

Where ShakeMap has no seismic stations, it estimates shaking from a ground-motion model and converts that to intensity. Calchis now does the same before ShakeMap is published. Each zone's edge is where the median shaking predicted by a published ground-motion model (Boore, Stewart, Seyhan and Atkinson, 2014) falls to the zone's intensity, converted by the relation ShakeMap uses (Worden and others, 2012). Distance is measured from the rupture rather than the epicenter, using a published rupture length for the magnitude (Wells and Coppersmith, 1994), and a deep earthquake's depth counts. A zone whose intensity the shaking never reaches is not drawn. A magnitude 5 at 10 kilometers now has no Catastrophic zone at all.

And when ShakeMap is published within three days of an earthquake, its zones are rebuilt from ShakeMap and the homes counted inside them are recounted.

How we checked it

Check	Result
37 recent earthquakes with a ShakeMap	Catastrophic zone drawn for none, as ShakeMap; Moderate zone a median 0.91× ShakeMap's radius, Affected 0.74×
Northridge, 1994, M6.7	Catastrophic zone 18 km (ShakeMap 26); Severe 39 km (65)
South Napa, 2014, M6.0	No Catastrophic zone (none); Severe 23 km (27)
Ridgecrest, 2019, M7.1	Catastrophic zone 31 km (31); Severe 56 km (58)

What it still is not

Circles, not shapes. A real rupture runs along a fault, and minutes after an earthquake its direction is not known. ShakeMap's contours replace the circles when they arrive.

Rock, not soil. The estimate assumes firm ground everywhere. Soft soil shakes harder, so the estimate can read low on it.

One choice was fitted. How much a deep earthquake's depth counts was chosen from three options against the same 37 earthquakes, and we say so in the documentation.

An estimate, until ShakeMap. It is the median: half of real earthquakes shake harder than this at a given distance, and half less. Every zone records which method drew it.

Sources: USGS ShakeMap contours for the events named, via the USGS ComCat event pages; Boore, Stewart, Seyhan & Atkinson (2014), Earthquake Spectra 30(3); Worden, Gerstenberger, Rhoades & Wald (2012), Bulletin of the Seismological Society of America 102(1); Wells & Coppersmith (1994), Bulletin of the Seismological Society of America 84(4). The full method and checks are in our methodology.