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Magnitude 6.7 quake strikes south of Crete

Posted by feww on July 1, 2009

Strong 6.7 Mw quake strikes south of Crete, Greece

Magnitude 6.7 mainshock followed by at least one aftershock measuring 4.9 Mw struck south of the island of Crete, Greece.

10-degree Map Centered at 35°N,25°E

us2009ina8
Location Map. Note location of Santorini. Original Map: USGS

greece seis haz map
Major Tectonic Boundaries: Subduction Zones -purple (Source: USGS)

historic seismicity crete area

7_legend seismicity
Major Tectonic Boundaries: Subduction Zones -purple (Source: USGS)

Details of the Mainshock:

  • Magnitude: 6.7
  • Date-Time:
    • Wednesday, July 01, 2009 at 09:30:12 UTC
    • Wednesday, July 01, 2009 at 12:30:12 PM at epicenter
  • Location:  34.188°N, 25.426°E
  • Depth:  38 km (23.6 miles)
  • Region CRETE, GREECE
  • Distances
    • 130 km (80 miles) S of Iraklion, Crete, Greece
    • 195 km (120 miles) SE of Chania, Crete, Greece
    • 270 km (165 miles) NNE of Tubruq, Libya
    • 450 km (280 miles) SSE of ATHENS, Greece
  • Location Uncertainty:  horizontal +/- 7.1 km (4.4 miles); depth +/- 12.8 km (8.0 miles)
  • Parameters:  NST= 51, Nph= 51, Dmin=131.6 km, Rmss=1.19 sec, Gp= 72°, M-type=teleseismic moment magnitude (Mw), Version=7
  • Source: USGS NEIC (WDCS-D)
  • Event ID:  us2009ina1

What’s happening at Santorini?

For Background Information See: Volcanoes, Santorini Eruption and Crops Failure in China

Santorini

Country: Greece
Subregion Name: Greece
Volcano Type: Shield volcanoes
Volcano Status: Historical
Last Known Eruption: 1950
Summit Elevation: 367 m 1,204 feet
Latitude: 36.404°N 36°24’13″N
Longitude: 25.396°E 25°23’47″E
Source: GVP

Renowned Santorini (Thera), with its steep-walled caldera rim draped by whitewashed villages overlooking an active volcanic island in the center of a caldera bay, is one of the scenic highlights of the Aegean. The circular island group is composed of overlapping shield volcanoes cut by at least four partially overlapping calderas. The oldest southern caldera was formed about 180,000 years before present (BP), followed by the Skaros caldera about 70,000 years BP, and then the Cape Riva caldera about 21,000 years BP. The youngest caldera formed about 3600 years BP during the Late-Bronze-Age Minoan eruption that forced abandonment of the thriving Aegean Sea island. Post-Minoan eruptions beginning in 197 BC constructed a series of lava domes and flows that form two islands near the center of the caldera. A submarine eruption took place in 1650 AD outside the caldera NE of Thera. The latest eruption at Santorini produced a small lava dome and flow in 1950, accompanied by explosive activity. Photo by Lee Siebert, 1994 (Smithsonian Institution). Caption: Global Volcanism Program.

Geographic setting

Santorini is an active volcano in the South Aegean Sea about 120 km north of
Crete, its location being 36.4oN, 25.4oE, and belongs to the Cycladic islands.
Santorini is a group of 5 islands:
1. The main island Thira (75,8 km², ca. 7000 inhabitants)
2. Thirasia (9,3 km², ca. 250 inhabitants)
3. Aspronisi (0,1 km², uninhabited)
4. Palea Kameni (0,5 km², 1 inhabitant)
5. Nea Kameni (3,4 km², uninhabited  (http://www.geophysicsonline.gr)

On June 13, 2009 GR Reporter (www.GRREPORTER.INFO) wrote:

columbusComputer generated image of Columbus. Source: GR Reporter

The submarine volcano, Columbus, located about 6.5 km south-east of Santorini in the Aegean Sea, is now the focus of “great interest” for Greek and German geologists.
“They have registered constant earthquakes of 4 Richter, hot air eruptions and continuous changes in the sea floor around the crater. The volcano is 470 meters high and reaches down to 17 meters beneath the sea floor. Its crater’s width is out of proportion- 1.5 kilometers. Complex submarine equipment has shown that Columbus’s volcanic activity never stops. It is the reason for the frequent earthquakes and constant changes in the surface around the crater.”

“The distortion of the sea floor is minor but it can be seen on the walls of the crater and in the 10-15-kilometers perimeter around it,” says Martin Heds from the Geology and Seismology institute to the Hamburg University.

According to Heds, “this does not indicate an eruption in near future.” The last known eruption of Columbus occurred in 1650.

“‘Reservoirs’” filled with hot water, reaching 200 degrees centigrade, resembling under-water fire places and releasing different kinds of gases- mostly carbon dioxide and hydrogen sulphide,” were discovered in the vicinity of Columbus.

A 5-km wide magma chamber is believed to be situated under the volcano. Lava is constantly spewed out of the chamber, GR Reporter suggested.

Tectonic Setting

The volcanic complex of Santorini is the most active part of the South Aegean Volcanic Arc. This volcanic arc is about 500 km long and 20 to 40 km wide and extends from the mainland of Greece through the islands of Aegina, Methana, Poros, Milos, Santorini, Kos, Yali, Nisyros and the Bodrum peninsula in Turkey. It is characterized by earthquakes at depths of 150-170 km that mark the subduction of the African underneath the Eurasian plate, more precisely the Aegean subplate, at a rate of up to 5 cm per year in a northeasterly direction.  (http://www.geophysicsonline.gr)

Hellenic Arc
The South Aegean Volcanic Arc and the tectonic setting of Santorini. (http://www.geophysicsonline.gr) Click Image to Enlarge.

Geology of Santorini

Non-volcanic rocks are exposed on Santorini at the Profitis Ilias Mountain, Mesa Vouno, the Gavrillos ridge, Pirgos, Monolithos and the inner side of the caldera wall between Cape Plaka and Athinios.

They represent a former non-volcanic island of about 9×6 km extension similar to neighboring Cycladic islands like Anaphe, Ios or Amorgos. The rocks consist of metamorphosed limestone and schist from Triassic to Tertiary time folded during the Alpine folding. The observed metamorphose grade is a blue-schist faciesresulting from tectonic deformation by the plate collision in the Oligocene to Miocene. At Athinios a 9.5 million year old Miocene granite intrusion has been found; it is part of the Cycladic Granitic Province and is the source of ore minerals including talc, chalcopyrite, chrysocolla, magnetite and others.(http://www.geophysicsonline.gr).

Santorini Geological setting
Simplified geologic map of Santorini. (http://www.geophysicsonline.gr  Sourced  From  http://www.geo.aau.dk/palstrat/tom/santorini_homepage/santorini_geology.htm).
Click Image to Enlarge.

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2 Responses to “Magnitude 6.7 quake strikes south of Crete”

  1. cr said

    [please see blog policy concerning commercial ads. Moderator]

  2. a friend in crete said

    [Thank you for kind words. Moderator]

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