Meteorological Cooperative › Warning systems › Seismicity of the Republic

Seismicity of the Republic

The peninsula sits on the Fluid plate, a small block wedged between the larger masses to the north and south and counted with the Eurasian group. Almost all of its earthquakes originate on the southern margin, where the Fluidian and Aporean microplates meet beneath the Belvacius hills. The network locates several thousand events a year. Fewer than fifty of them are felt by anybody, and the last event above magnitude 6 occurred in 1934.

Plate settingThe Fluid plate, called the Fluidian microplate in seismological usage, within the Eurasian group

Principal sourceFluidian and Aporean microplate boundary, beneath the Belvacius hills in the south-east

Northern marginPassive; the drowned coastline of Merrisoma and Lindoma

Southern marginCompressional; source of the western uplands and the south-eastern hills

Located eventsAbout 2,600 a year at magnitude 0.8 and above

Events felt anywhereAbout 45 a year, most of them at REP 2

Largest instrumental eventMagnitude 6.4, Belvacius, 11 October 1934

Building standardFixed centrally under Article 178 (1)(b) and identical throughout the territory

Tectonic setting

The Eurasian group includes a number of small blocks along its southern edge, and the Fluid plate is one of them. Seismological writing calls it the Fluidian microplate, and the register calls it the Fluid plate; both name the same block. It moves north-north-west at about 4 mm a year with respect to the Eurasian interior. That movement is slow by the standards of an active boundary, and it accounts for both the mildness of the seismicity and its persistence.

The northern boundary of the microplate is passive. Nothing there converges, the coast is a drowned landscape of estuaries, spits and shallow bays, and seismic release along it is negligible. The harbour at Lindoma and the tidal bay at Merrisoma are both drowned valleys of that margin.

The southern boundary is compressional. The Aporean microplate, which underlies Reformed Luces Aporia and the sea between, converges against the Fluidian block along a line running east-north-east beneath the Belvacius hills. Convergence there built the south-eastern hills, contributed to the western uplands, and produced the mineral deposits on which the economy of the Republic has rested. Roughly 88 per cent of the seismic energy released inside the territory since 1901 originated within 40 km of that line.

Rate of occurrence

Seismicity in the Republic is common when measured and rare when experienced. Both statements describe the same record. The network locates an event roughly every three hours, and almost every one of those events is too small for any person to notice. The rate is comparable to that of other slow-converging margins of the group, and the distribution of magnitudes follows the usual power law with a b-value near 1.05.

Located events by magnitude, annual mean over the instrumental record since 1962
Magnitude band	Events a year	Mean interval	Ordinarily perceptible
0.8 to 1.9	2,290	4 hours	No
2.0 to 2.9	276	32 hours	Near the epicentre only, at REP 2
3.0 to 3.9	31	12 days	Yes, locally, ordinarily REP 2 or REP 3
4.0 to 4.9	2.8	4 months	Yes, over a sub-region
5.0 to 5.9	0.28	3.6 years	Yes, over a region
6.0 and above	0.012	85 years	Yes, nationally

Highest intensity reached anywhere in the territory, annual frequency
REP	Step	Occasions a year	Mean interval	Warning issued
2	Slight	45	8 days	No
3	Light	9	6 weeks	Yes
4	Moderate	2.4	5 months	Yes
5	Rather strong	0.6	20 months	Yes
6	Strong	0.12	8 years	Yes
7	Very strong	0.033	30 years	Yes
8	Severe	0.011	90 years	Yes
9	Violent	No instance since 1901	-	Yes
10	Extreme	No instance in the record	-	Yes

A warning is issued wherever the estimated intensity exceeds REP 2. The REP intensity scale gives the effect at each step.

The events of record

Events of record since 1901
Date	Epicentre	Magnitude	Depth	Highest REP	Effect recorded
11 October 1934	Belvacius, 14 km south of Corcina	6.4	18 km	8	Masonry damage at Corcina and Cortius; nine deaths; the reference event for the building standard
3 March 1958	Belvacius margin, offshore	5.8	27 km	6	Chimneys down at Corcina; no sea disturbance observed
22 July 1971	Ravelluri, inland of the lagoon	5.1	11 km	6	Cracked plaster over four sub-regions; no structural damage
9 December 1989	Belvacius, beneath the hills	5.6	22 km	7	Damage to older stone building at Corcina; the last event above REP 6
16 April 2004	Quintorum, southern edge	4.9	9 km	5	Widely felt across the eastern region; no damage
28 February 2019	Belvacius, beneath the hills	5.2	19 km	4	REP 4 at Corcina, felt in fourteen sub-regions, no damage; the event that established the case for a warning system
3 August 2026	Southern margin, beneath the Belvacius hills	5.4	24 km	5	REP 5 in Belvacius, REP 3 as far north as Leriduson, REP 1 at Lindoma; no damage; the first event for which REP-KJS reached the whole territory

Depth and mechanism

Focal depths on the Belvacius margin lie mostly between 10 and 35 km, with a mean near 21 km. Depth of that order attenuates the shaking before it reaches the surface, and it is one reason the intensities observed in the Republic stay below what the magnitudes alone would suggest. Focal mechanisms are predominantly reverse, consistent with convergence across the boundary, with a strike-slip component on the eastern segment beneath the sea.

Two minor sources exist away from the margin. Shallow events below magnitude 3.5 occur along the inland fault system beneath Ravelluri and Quintorum, and their small depth makes them locally perceptible at magnitudes that would pass unnoticed on the margin. Induced events are recorded around the workings at Ferrista and Corcina and around the northern reservoirs above Ilvara, none above magnitude 2.9, and each is identified as induced in the catalogue.

The sea

Coastal disturbance

A destructive sea wave requires a large, shallow, vertically displacing event beneath water. The Belvacius margin lies mostly inland, its offshore segment is deep and predominantly strike-slip, and no sea disturbance from a local earthquake appears anywhere in the record of the Republic.

REP-KJS nonetheless computes a coastal assessment for every event and states the result in every broadcast, including the ordinary result that no coastal warning is in force. The assessment is retained because its cost is negligible and because the settlements of the eastern shore stand at low elevation.

Distant sea waves generated outside the territory would reach the eastern shore through the corridor toward Reformed Luces Aporia. Notice of such an event arrives through exchange with the regional centres and travels on the same channel.

Events reaching beyond the frontiers

The Aporean boundary lies close to the sea corridor toward Sluland, and an event on its eastern segment shakes ground in Reformed Luces Aporia as well as in the Republic. REP-KJS estimates intensity across the whole affected area, the frontier being no part of the calculation, and broadcasts the estimate for Aporean territory alongside the estimate for the Republic.

The arrangement is reciprocal and rests on an exchange of station data agreed in 2023. Aporean stations feed the detection network of the Republic and the stations of the Republic feed theirs, so an event on either side is located from both sets of records. The estimate for foreign ground is stated as such in the broadcast and follows the same scales.

Building and the standard

Construction standards are fixed centrally under Article 178 (1)(b), one of the few matters the Constitution requires to apply identically everywhere in the territory. The standard is set against the 1934 event and against a return period of 475 years, and it applies at Lindoma exactly as it applies at Corcina, though the expected ground motion at Lindoma is a fraction of that on the margin.

The cooperative supplies the hazard maps on which the standard rests and takes no part in setting or enforcing it. The panel building stock of 1958 to 1989 was designed to earlier figures, and the assessment of that stock against the current hazard maps is published with the maps.