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Moderate mag. 4.0 earthquake - Aegean Sea, 15 km northeast of Thera, Nomos Kykladon, South Aegean, Greece, on 20 Apr 11:21 am (GMT +3) -

Moderate mag. 4.0 earthquake - Aegean Sea, 15 km northeast of Thera, Nomos Kykladon, South Aegean, Greece, on 20 Apr 11:21 am (GMT +3) Moderate mag. 4.0 earthquake - Aegean Sea, 15 km northeast of Thera, Nomos Kykladon, South Aegean, Greece, on 20 Apr 11:21 am (GMT +3)

Moderate magnitude 4.0 earthquake at 10 km depth

20 Apr 08:27 UTC: First to report: GFZ after 5 minutes.
20 Apr 08:27: Now using data updates from NOA
... [show all] ...
20 Apr 09:17: Hypocenter depth recalculated from 10.5 to 10.4 km (from 6.5 to 6.5 mi).
Update Tue, 20 Apr 2021, 18:55
Quake hits near active submarine volcano off Santorini Island: should we worry?
Seismic station on Thirasia recording the earthquake (big blue signal) and many smaller and larger fore-and aftershocks (image: National Observatory of Athens)
Seismic station on Thirasia recording the earthquake (big blue signal) and many smaller and larger fore-and aftershocks (image: National Observatory of Athens)
This morning, a magnitude 4.0 earthquake occurred 10 km northeast of Santorini Island. The quake hit at 11.21 am local time. It was felt by many people on Santorini, especially in the northern part of the island in the town of Oia, mostly as weak shaking that lasted a few seconds.
The quake was preceded and followed by a series of smaller fore- and aftershocks, with magnitudes of up to 3.4. During the past months, seismic activity in this area had been somewhat higher, but the most interesting aspect is likely that the quakes occurred very close at or directly under an active submarine volcano located 8 km northeast of Santorini island.
Called Kolumbo, it is considered the potentially most dangerous active volcano in Greece. This is first because of its underwater setting, it is capable of triggering tsunamis, and second, because it has the potential to produce large explosive eruptions. It did so during its only known historic eruption in 1650, which devastated Santorini and killed many people and thousands of livestock by toxic gasses. A large tsunami invaded the island's flat coasts, which back then were uninhabited, but now quite crowded with hotels and towns enjoying the location at Santorini's famous black-sand beaches.
So, the question asked is whether the current quakes might be a sign of the volcano becoming more active?
Since it is hidden under the water and only has had one eruption in historic times, Kolumbo is certainly the least known of the active volcanoes in the country, which makes interpreting the potential significance of such events more difficult.
According to a post by Gerasimos Papadopoulos, Research Director of Seismology at National Observatory of Athens, there is currently no sign that the volcano might be becoming restless, and remains in its normal dormant state.
While earthquakes like today's, directly under the volcano, can be caused by volcanic processes that eventually might precede a new eruption, they by no means are necessarily so and most of time are purely tectonic in nature, not affecting the state of the volcano.
In any case, its status merits close monitoring: a volcano getting restless close to popular Santorini island, one of Greece's main tourist attractions, would certainly not be good news, especially now that the country is preparing to open its tourist season in May.

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Earthquake data

Earthquake details

Date & timeApr 20, 2021 08:21:55 UTC -
Local time at epicenter20 Apr 11:21 am (GMT +3)
StatusConfirmed
Magnitude4
Depth10.4 km
Epicenter latitude / longitude36.5273°N / 25.5272°E (Aegean Sea, Greece)
Antipode36.527°S / 154.473°W
Shaking intensityLight shaking
Felt12 reports
Primary data sourceNOA (National Observatory of Athens)
Nearest volcanoKolumbo (1 km / 1 mi)
Nearby towns and cities13 km (8 mi) NNE of Monolithos (pop: 181) | Show on map | Quakes nearby
15 km (9 mi) ENE of Oia (pop: 3,380) | Show on map | Quakes nearby
32 km (20 mi) SE of Ios (pop: 2,020) | Show on map | Quakes nearby
58 km (36 mi) S of Filoti (pop: 1,480) | Show on map | Quakes nearby
66 km (41 mi) SSE of Naxos (pop: 7,070) | Show on map | Quakes nearby
70 km (44 mi) SSE of Paros (pop: 5,290) | Show on map | Quakes nearby
138 km (86 mi) NNE of Heraklion (pop: 137,200) | Show on map | Quakes nearby
160 km (99 mi) NE of Rethymno (pop: 32,500) | Show on map | Quakes nearby
176 km (109 mi) NE of Chania (pop: 53,900) | Show on map | Quakes nearby
227 km (141 mi) SE of Athens (pop: 664,000) | Show on map | Quakes nearby
29 km (18 mi) NW of Anaphi Island (pop: 271) | Show on map | Quakes nearby
136 km (85 mi) NNE of Kreta Island (pop: 623,100) | Show on map | Quakes nearby
Weather at epicenter at time of quakeFew Clouds 16.1°C (61 F), humidity: 59%, wind: 9 m/s (18 kts) from W
Estimated seismic energy released6.3 x 1010 joules (17.5 megawatt hours, equivalent to 15.1 tons of TNT) | about seismic energy
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Data for the same earthquake reported by different agencies

Info: The more agencies report about the same quake and post similar data, the more confidence you can have in the data. It takes normally up to a few hours until earthquake parameters are calculated with near-optimum precision.
Mag.DepthLocationSource
4.010.4 km18 Km NNE From Santorini, GreeceNOA
4.310 kmDodecanese Islands, GreeceGFZ
4.02 kmDODECANESE ISLANDS, GREECEEMSC
3.95 kmMEDITERRANEAN SEA, GreeceKOERI-RETMC
3.910 kmMediterranean Sea, 595 Km N of Marsa Matruh, GreeceENSN

User reports for this quake (13)

User reports estimate the perceived ground shaking intensity according to the MMI (Modified Mercalli Intensity) scale.
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Εξο Γιαλός / Light shaking (MMI IV) / 2-5 s : Was laying on the bed and all the sudden felt a strong jolt and heard creaking but it only lasted a few seconds | 15 users found this interesting.  
Kaisariani, Nomarchía Athínas, Attica (223 km NW of epicenter) [Map] / not felt (reported through our app)
Εξο Γιαλός / Light shaking (MMI IV) / 2-5 s : Was laying on the bed and all the sudden felt a strong jolt and heard creaking but it only lasted a few seconds  
Εξο Γιαλός / Light shaking (MMI IV) / 2-5 s : Was laying on the bed and all the sudden felt a strong jolt and heard creaking but it only lasted a few seconds  
Εξο Γιαλός / Light shaking (MMI IV) / 2-5 s : Was laying on the bed and all the sudden felt a strong jolt and heard creaking but it only lasted a few seconds  
Santorini (19.7 km SSW of epicenter) [Map] / Weak shaking (MMI III) / single vertical bump / 1-2 s
Εξο Γιαλός / Light shaking (MMI IV) / 2-5 s : Was laying on the bed and all the sudden felt a strong jolt and heard creaking but it only lasted a few seconds  
Mesaria / Weak shaking (MMI III) / single lateral shake
Didim (180.1 km ENE of epicenter) [Map] / Light shaking (MMI IV) / vibration and rolling / 15-20 s (reported through our app)
Oia Santorini / Moderate shaking (MMI V) / 2-5 s
Finikia, Santorini / Light shaking (MMI IV) / rattling, vibrating / very short
ΙΟΣ (ΝΗΣΟΣ) / Weak shaking (MMI III) / rattling, vibrating / 1-2 s
Kamari / Light shaking (MMI IV) / both vertical and horizontal swinging / 2-5 s
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