How many seconds each Romanian town gets before the strong wave of a Vrancea earthquake

Why Vrancea

Nearly all of Romania's large earthquakes start in the same small place#

Beneath the bend of the Carpathians, a body of cold, dense lithosphere hangs almost vertically in the mantle and breaks at depths where few places on Earth have earthquakes; where it came from is still debated. The dots are the sources of the 206 deep Vrancea earthquakes in the ISC Bulletin, 1961–2024, larger for stronger quakes: all of them fit in an area about 69 km from west to east and 57 km from north to south.

Because the source is always in roughly the same place, and the rupture starts deep, the stations above it feel an earthquake before the rest of the country does. That is what makes a warning possible.

Two waves

The first wave is fast and weak, the second slow and strong#

The P wave pushes and pulls the rock along its path and crosses the crust at about 6–8 km a second. The S wave shakes it sideways at a little over half that speed and carries most of the motion that brings buildings down.

On the map, the thin ring is the P wave at the surface, the thick ring the S wave. For an earthquake 130 km deep, the P wave reaches the ground above the source after 18 seconds and the S wave after 31.

The message

Three stations in Vrancea raise the alarm#

INFP's warning system relies first on the stations of Vrâncioaia (VRI), Ploștina (PLOR) and Muntele Roșu (MLR). In a 2011 paper, its authors write that the system can "send earthquake warning to Bucharest in real time, in about 5 s after detection in the epicenter".

The message travels through the network almost instantly. The map assumes the message leaves 5 seconds after detection; how long it takes to reach a person is not measured here. From the moment it is sent, the time left before the S wave differs from place to place: a few seconds near Vrancea, more than a minute in the west. The dots that light up later have more time.

The whole country

Every town's seconds, for an earthquake 130 km deep#

The colour shows how many seconds pass between the warning message and the arrival of the strong wave, for an earthquake at the usual depth of the Vrancea nest. Nine in ten people in Muntenia, the south (Bucharest included), and two in three in Moldavia, the east, get 10 to 30 seconds. Suceava and Botoșani in the north-east get about 35–45 seconds, the west more than a minute, but the wave arrives there much weaker.

At this depth, the least time goes to Gura Teghii (Buzău county): 8.1 seconds after the message.

Depth

Near Vrancea, depth changes the time left a lot; in Bucharest, very little#

Above a deeper earthquake the gap between the P and the S wave is longer: both waves have farther to travel to the surface, and the faster P wave pulls further ahead. At VRI and Ploștina, the stations above the sources, 9.5 seconds passed between the two waves for quakes shallower than 100 km (12 quakes) and 16 seconds for those deeper than 140 km (59 quakes).

For quakes 90–150 km deep, 13 to 17 seconds pass in Focșani between detection of the first wave in Vrancea and the arrival of the S wave, and 27 to 29 seconds in Bucharest, because the way to the capital is long at any depth. The depth selector on the map also shows scenarios for shallower and deeper quakes, backed by fewer recordings.

Bucharest

Bucharest: 22 to 32 seconds from the first wave in Vrancea to the S wave, over 123 earthquakes#

For every earthquake with readings at stations in the city, the time between the first P arrival at any of VRI, PLOR and MLR and the S arrival in Bucharest was computed. It was usually 27 seconds; in one quake in ten, under 24.

The page of INFP's warning system gives Bucharest approximately 25 to 28 seconds between detection in the epicentral area and the arrival of the S wave: the records bear that out for a typical quake. Take away the 5 seconds that, according to INFP's 2011 paper, pass before the message is sent, and 17–27 seconds remain; INFP writes 20–27. In 26 of the 123 quakes, the time left would have been under 20.

The 1999 study that proposed the system wrote that the gap between the two waves is "always greater than 25 seconds". At the city stations it was under 25 in 26 of the 123 quakes; at Măgurele, on the southern edge, where the S wave arrives 1.8 seconds later, in 8 of 108. The difference may come from the stations and the earthquakes used in 1999; only the authors can say.

The test

The map was checked on stations left out one at a time#

At 150–220 km from the epicentre, Bucharest among them, ak135, the model of the Earth's interior used worldwide, makes the time between detection of the P wave and arrival of the S wave 1.4 seconds longer than was recorded. The map corrects the times with the differences between calculation and record at nearby stations, on other earthquakes.

For every reading checked, the correction was computed without that station's data and without that earthquake. Within 300 km of the epicentre the map's typical error was 1.0 seconds (quakes of 2005–2024); in one reading in ten it was more than 2.9 seconds. Farther away the typical error grows: 2.3 seconds at 300–450 km, 3.9 seconds beyond 450 km.

On the map, the squares are stations with at least five readings: white where the typical error was under a second, orange under two seconds, red from two seconds up. A model of the Earth's interior built for Vrancea (Koulakov and colleagues, 2010) had an error of 1.1 seconds without the correction and 1.0 with it, close to ak135: the correction from the stations matters more than the choice of model.

4 March 1977

In 1977, Bucharest had a higher intensity than Focșani#

The dots are observations of the 1977 earthquake's effects, gathered afterwards and expressed as intensity on the MSK scale. Intensity was VIII in Bucharest, about 167 km from the epicentre in the ISC Bulletin, and VII in Focșani, about 46 km away (other catalogues place the epicentre nearer Focșani: the page on the depth of Vrancea quakes gives 158 and 34 km); IX was reported in the epicentral area. Although farther from the epicentre, Bucharest had stronger reported effects than Focșani. In the quakes of 1986 and 1990 it was the other way round: VIII in Focșani, VII in Bucharest.

The time left before the S wave and how hard the ground moves are two different things. The towns in the west have plenty of time and weak waves; Bucharest has little time and strong ones.

What the page does not say

What these seconds do not measure#

The times on the map run from the sending of the message to the arrival of the S wave. On the records the seismological agencies read to compute magnitudes (213 readings in the bulletin), the largest swing usually came 2.7 seconds after the start of the S wave, and between 1.0 and 5.3 seconds in half of them.

The earthquakes in the test have magnitudes from 2.4 to 7.2, nearly all below 6. For the large ones of 1977, 1986 and 1990 the stations recorded the first wave, but the bulletin holds almost no S-wave readings in Romania. A magnitude 7 rupture lasts tens of seconds. How long a message takes to reach a phone was not measured here.

This page is not an alert service. INFP sends early seismic notifications through the Telegram app (infp.ro); what to do in an earthquake is explained by Romania's Department for Emergency Situations at fiipregatit.ro (in Romanian).