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Posts tagged with "earthquakes"

China Earthquake

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The most powerful earthquake to hit China in 30 years has killed at least 10,000 people in south-western Sichuan province, with thousands more trapped. The figures are expected to rise dramatically

The M 7.9 strong earthquake was shallow, with a depth of only 10 km, and hit less than 100 km north-east of Chengdu, a city of about 11 million inhabitants. The quake devastated a region of small cities and towns set amid steep hills. The epicentre was relatively far from any plate boundary, but earthquakes in this area are not unknown as can be seen from this USGS map of earthquakes from 1990 to present. Since 1900 the area of this map has known 8 earthquakes larger than M 7.

Kim at All of my Faults are Stress-related has an exellent post on the Tectonics of the May 12 Sichuan earthquake which explains the tectonic situation much better than I would ever be able to, so please go and read it.

The Eastern Sichuan quake ruptured about 275 kilometers of a fault running northeastward between the easternmost mountains of the Tibetan Plateau and the densely populated Sichuan Basin. The violent quake is probably linked to a shift of the Tibetan plateau to the north and east. Earthquakes are frequent and deadly along the fringes of the Tibetan plateau, which was raised when India collided into Eurasia, starting some 50 million years ago.

http://sciencenow.sciencemag.org/cgi/content/full/2008/512/1?rss=1
http://www.abc.net.au/science/articles/2008/05/13/2243183.htm



PS:
A strong aftershock measuring 6.1 on the Richter scale rocked Chengdu itself around 3:10 p.m. Tuesday 13 May 2008. The region has suffered more than 1,950 aftershocks in the past 25 hours, including three over 6 on the Richter scale and 14 between 5 and 6. Heavy rainfall, storm and wrecked roads hamper rescuers' efforts to reach the hardest-hit areas.

PS of 14 may 2008
More about the earthquake including information (and Tectonic Summary) as Reported by USGS at Geology.com




Largest Earthquake Ever Recorded

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The greatest earthquakes occur in subduction zones, where one tectonic plate is sliding beneath another. Virtually all of the big earthquakes, the ones of magnitude eight or nine or above, happen at sea. The largest earthquake ever recorded is no exception to this rule.

The epicentre of the Great Chilean Earthquake of 22 May 1960 was about 160 km off the coast of Chile in the Peru-Chile Trench (39.5° S, 74.5° W) with a focal depth of 33 km. Two days later, on 24 May 1960, Cordón Caulle, a fissure vents system located in the Chilean Lake District, erupted, sending ash and steam as high as 6 km.

At the Peru-Chile trench the Nazca Plate is subducted beneath the South American Plate. In the area hit by the earthquake the dip of the subduction zone is about 30° and the subduction gives rise to an arc of still active volcanoes.

Buildings fell all along the Chilean coast from Conception to the southern end of Isla Chilor. The towns of Valdivia and Puerto Montt were devastated. (The earthquake is also known as the 1960 Valdivia earthquake / Gran terremoto de Valdivia).

The earthquake set off huge landslides and sent rocks and boulders tumbling down the mountain sides. The land around the city of Puerto Montt sank and coastal areas were flooded. Rivers had their courses changed and landslides created new lakes. Many of the landslides occurred in the Chilean Lake District from Lago Villarica to Lago Todos los Santos.

The earthquake set off huge tsunamis which radiated out from the epicentre, travelling at speed of up to 350 km/h, the Chilean coast was devastated by a 25 (or was it 12 ?) m high tsunami which arrived 10 to 15 minutes after the quake. Remains of houses were carried inland as much as 3 km. There was also severe damage in the Philippines, Hawaii and the japan.

Over 2000 people died and 3000 were injured. 2 million people became homeless. There were not extremely large numbers of victims, for such an earthquake, because the population was alerted on that something was going to happen by previous shakes and underground noise.

Map of some of the places mentioned and the most important volcanoes in the district. In 2005 we made our way from Puerto Montt to San Carlos de Barriloche in Argentina through the Chilean Lake District (bus, boat, bus, boat, bus) - as many tourists do. Under way I photographed the following volcanoes: Osorno (famous for its Fujiyama look), Puntiagudo ("Volcán Puntiagudo" (Spanish for "Sharp-pointed Volcano") is a stratovolcano with a prominent 2,493 m high sharp-pointed summit that results from glacial dissection and gets its name from this feature), and Tronador. See the 3 photos below.

http://earthquake.usgs.gov/regional/world/events/1960_05_22.php
http://en.wikipedia.org/wiki/Great_Chilean_Earthquake
http://www.gochile.cl/html/ChileValdivia/Chile-Valdivia-Terremoto.asp
http://www.geophys.washington.edu/tsunami/general/historic/chilean60.html
http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=154





Notes:
The volcano Puyehue is often cited as the volcano that erupted on 24 May, but actually it was the nearby fissure volcano Cordón Caulle. Although Cordón Caulle is sometimes listed as part of Puyehue volcano, it is tectonically and magmatically distinct from Puyehue. No historical eruptions are known from Puyehue, and eruptions in 1921-22 and 1960 listed in some sources actually occurred at Cordón Caulle volcano located to the Northwest.

As far as I know the epicentre of the main quake was at 39.5° S, 74.5° W - some maps however show it inland (including the USGS map). Well of course there were more than one shock, but even then?

Can earthquakes trigger volcanic eruptions? The volcanic eruption 2 days after the 1960 Chilean earthquake has been taken as evidence, but that could still be a coincidence, and the question is still debated. That volcanoes, on the other hand, can cause earthquakes, is well known.

What I wanted to stress here is the role of subduction zones for important natural hazards like earthquakes, volcanoes, tsunamis and landslides.



Archaeoseismology

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New scientific disciplines are popping up all the time - like helioseismology (the study of the propagation of pressure waves in the Sun) and astroseismology (the study of the propagation of pressure waves in stars). At some time I posted about geomythology and geoarchaeology.

A relatively new interdisciplinary discipline is archaeoseismology. I am all in for interdisciplinarity, and when I was busy with my post on the Great Lisbon Earthquake in 1755
I realised that there might certainly be work to be done within this discipline.

During the 2008 Annual Meeting of the Seismological Society of America (SSA) these days (16 - 18 April 2008) Santa Fe, New Mexico, a special session is dedicated to Archaeoseismological Methodologies: Principles and Practices. Archaeoseismology is a young scientific discipline that studies past earthquakes in the archaeological record. It has the potential to bridge the gap between instrumental and historical seismology, on the one hand, and palaeoseismology and earthquake geology, on the other hand. There is still much to be known about ancient earthquakes. The instrumental record for seismology is short, going back 100 years. The historical seismology record is much longer, including written documentation such as news accounts and diaries, which vary widely by culture and region. The archaeoseismic record serves as the bridge between historical accounts and the palaeoseismic record of Earth’s history.

Seismology (from the Greek seismos = earthquake and logos = word) is the scientific study of earthquakes and the propagation of elastic waves through the Earth. Hopefully we can learn from history, the seismological history.

http://www.physorg.com/news127583258.html
http://www.freerepublic.com/focus/f-news/2002718/posts
http://www.eurekalert.org/pub_releases/2008-04/ssoa-uco040708.php





Great Lisbon Earthquake 1755

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I am back home from a week in Lisbon, where I was several times reminded of the Great Lisbon Earthquake.

On 1 November 1755 Lisbon was shaken by a violent earthquake. It occurred at 9:40 in the morning during High Mass - a mass to celebrate All Saints' Day, also known as All Hallows or Hallowmas, an important religious holiday in this strong Roman catholic country.

Geologists today estimate the Lisbon earthquake approached magnitude 9 on the Richter scale, with an epicentre in the Atlantic Ocean about 200 km west-south-west of Cape St. Vincent (See map). Estimates place the death toll between 60,000 to 100,000 people, making it one of the most destructive earthquakes in history.

More than 20 churches collapsed, and due to the candles lit for the celebration fire quickly broke out, and flames raged for five days. Gigantic fissures up to five metres wide appeared in the city centre. Survivors rushed to the open space of the docks for safety and watched as the water receded revealing the sea floor. Approximately forty minutes after the earthquake, an enormous tsunami engulfed the harbour and downtown, rushing up the Tagus river. It was followed by two more waves.

Many people at the time saw the disaster a God’s punishment - because the town was too rich, because of the inquisition (the Spanish Inquisition was established in 1478), because of idolatry or other sins? Apart from theological, philosophical (including work by Voltaire), and literary discussions, however, scientists got involved. It was the first earthquake studied scientifically for its effects over a large area, and it led to the birth of modern seismology.

I stayed at a hotel a few hundred metres from the Santa Justa Lift (also known as the Carmo Lift). The Santa Justa Lift was designed by an apprentice of Gustave Eiffel (the one with the Eiffel Tower). The iron lift is 45 metres tall and it brought me from the downtown streets to the uphill Carmo Square. From the roof of the lift construction (with a bar, where I had an espresso) there is a nice view over downtown Lisbon and the Carmo Convent. This mediaeval convent was ruined in the Earthquake, and the ruins of its Gothic church are the main trace of the great earthquake still visible in the city. The ruins were preserved to remind Lisboners of the destruction.

I went down to the docks and took the train to see one of the most impressive monuments in Lisbon, the Jeronimos Monastery. The vaulting in the church withstood the earthquake of 1755, which probably says something about the architecture. In the same district - called Belém, which is in fact Portuguese for Bethlehem, and pronounced more or less as “blem” - there is an old tower, the Torre de Belém. The tower was built in the same style as the Monastery between 1515 and 1519 in the middle of the river Tagus to defend Lisbon and the monastery. Today, however it stands on the water’s edge practically moored to the north bank, the river having altered course during (and after) the earthquake and tsunami of 1755.

From there the train moves on to the romantic fishing port - and holiday resort with yacht harbour - of Cascaias, about half an hour’s ride from Lisbon (30 kilometres west of Lisbon). A large portion of the village was destroyed during the earthquake in 1755. But today it is bustling, and I had a nice evening meal at the beach with cockles and local wine.



PS of 17 April 2008
The position of the epicentre is disputed (See comments). The source mechanism seems to require generation at a subduction zone, but where would that be?

Owen Fracture Zone - Recent Earthquake

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On Monday, 3 March 2008, a Magnitude 5.4 earthquake struck the Owen Fracture Zone Region (according to USGS). This reminded me of a paper in the first issue of Nature Geoscience.

In the Arabian Sea the 1,100 km long Owen fracture zone marks the boundary between the Indian and Arabian tectonic plates (see map). Both plates are colliding with the southern edge of Eurasia but the Arabian plate is generally considered to be moving north-eastward slightly faster than the Indian plate, and it is this difference in motion that is accommodated by the Owen fracture zone. This motion seems to have started between 3 and 8 million years ago.

The 3 March earthquake is marked with an orange coloured star on the USGS map of historic earthquakes shown below.

On this map the Sheba Ridge and the Owen transform fault down to the Carlsberg ridge are clearly delineated by historic earthquakes (1990 - present). Lesser earthquakes are seen along the Owen fracture zone (green line) - this is after all one of the slowest plate boundaries on Earth with a moving rate estimated as only about 2 mm/year. There is a conspicuous seismic gap in the southern end of “the green line” (marked with a question point on my map above) with some diffuse earthquakes West of the line.

Fournier et al. treated this area in a paper titled “In situ evidence for dextral active motion at the Arabia–India plate boundary” published in Nature Geoscience 1, 54 - 58 (2008) - Published online: 2 December 2007 | doi:10.1038/ngeo.2007.24 .

Their study suggest that a wedge of the Arabian plate, approximately corresponding to my orange coloured triangle, has been transferred to the Indian plate at some time in the last 10 million years. This is now a 50 km wide pull-apart basin, where an ultraslow divergent boundary (“spreading ridge”) has been developing. As the diffuse earthquakes seem to show the deformation is not yet clearly localised, but correspond to a transient state preceding the birth of a new plate boundary, and a new triple junction approximately where the earthquake on Monday occurred (orange star). A more stable ridge-ridge-ridge triple junction than the recent situation.

http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.24.html
http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.56.html



Svalbard Earthquake

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The Norwegian Arctic archipelago of Svalbard was hit by a major earthquake in the early hours of Thursday morning 21 February 2008. The quake measured 6.2 on the Richter scale. This is the strongest quake in Norwegian history. The focus of the earthquake was localised on the sea floor, about 10 km down into the earth's crust. 15 aftershocks was registered shortly after.

The common concept assumes that earthquakes in Svalbard are due to its location in an active seismic zone along the Mid-Atlantic Ridge. The epicentre in Storfjorden lies close to a relatively large fault running in north-south direction . Through Svalbard's geological history this fault has repeatedly been the focus of seismic activity.

The reason was, however, according to Odleiv Olesen from NGU (Norwegian Geological Survey) the large displacement of sediments that took place in the last 1.1 million years near Svalbard. During the glaciations large masses of sediments were eroded from the bottom of Storfjorden on the East coast of Svalbard, and deposited elsewhere. This process changed the tension in the Earth crust and resulted in movements of the crust and earthquakes. This process is also known to cause minor earthquakes along the coast of mainland Norway.


There is an ongoing discussion about the role of change of gravity forces during the moon eclipse in the earth crust displacements. The total moon eclipse took place on the same night as the last large earthquake in Svalbard (21 February 2008).


See also my post on Svalbard Terranes.




http://www.ssf.npolar.no/pages/news153.htm
http://www.norwaypost.no/cgi-bin/norwaypost/imaker?id=131274
http://npweb.npolar.no/english/articles/1203601816.69



Bucaramanga Earthquake

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Maps can be treacherous, and interpretations should be handled with care.

The first map shows that the epicentre of the earthquake near Bucaramanga (Colombia) on Sunday 17 February 2008 occurred very close to a transform fault. The depth was however an estimated 150 km. At transform faults, that is faults where two continents slide past each other, like at the San Andreas Fault, you can only get shallow earthquakes. The earth is too hot below the asthenosphere for there to be earthquakes. Temperature increases as you go deeper into the earth. Most earthquakes stop happening at depths of about 20 km because the rock is too hot. The asthenosphere begins at about 100 km into the earth.

The map also shows a subduction zone - far to the west in the Pacific. In a subduction zone, a cold piece of crust is being pulled underneath other crust. This piece of crust is relatively cold, so that earthquakes can occur much deeper.

The second map shows historic seismicity in the area. I would like to point to three distinct features:
  1. A lot of shallow earthquakes show up along the transform fault.
  2. From west to east other earthquakes occurred deeper and deeper (following the Benioff zone - the seismic zone on top of the subducting slab).
  3. A small, but very active zone of intermediate-deep seismicity (blue dots) catch the eye near Bucaramanga. That is the so-called Bucaramanga earthquake nest.


The Bucaramanga earthquake nest is situated at a depth of about 160 km beneath Colombia at 6.8° N, 73° W, and it produces about eight earthquakes of M 4.7 or more each year from a source region having dimensions of about 10 km or less. The nesting of earthquakes beneath the Bucaramanga area may be due to a collision between two subducting slabs, a northern slab with a dip angle of about 25° and a southern slab with a 50° dip angle, while the dip in the Bucaramanga nest is about 29° (according to this abstract). The Bucaramanga earthquake nest is of course also discussed in other paper as well.

The earthquake earthquake near Bucaramanga (Colombia) on Sunday 17 February 2008 was NOT a transform fault earthquake, BUT a subduction earthquake.
All maps should be treated with care - two maps are often better than one.

A final note is that the shallow earthquakes are the dangerous ones. Deep earthquakes don’t give rise to much damage.

http://news.trendaz.com/index.shtml?show=news&newsid=1137573&lang=EN



PS of 19 Feb. 2008: I ought to have made a "depth" diagram illustrating the principle. Well, here it comes. (not to scale and not in accordance with actual relief)







Kivu Graben Earthquakes

Two strong earthquakes in the Kivu region on Sunday 3 February 2008 killed at least 40 people in Rwanda and D.R. Congo.

Kivu is a rift lake. The Kivu Graben is part of the great western East African Rift System. The graben, approximately 90 km wide and 200 km long, trends NNE-SSW and straddles both Rwanda and the Democratic Republic of Congo. Structurally, Kivu Graben is the southern extension of the Albertine Graben in Uganda. The western border fault (with tilted fault-block mountains rising on the west side) is also marked by destructive active volcanoes like Nyamulagira and Nyiragongo (approximate location shown as red dots on map).

The first quake which measured 6.1 on the Richter scale occurred 20 km N of Bukavu, D.R. Congo - marked with orange-coloured square on map. The second quake with a magnitude of 5 struck 40 km W of Butare, Rwanda, and 55 km E of Bukavu (See map). The quakes wsere followed by several aftershocks.

Hundreds of people were wounded. Many homes, as well as schools and churches, were damaged.

http://www.terradaily.com/reports/Africa_quakes_kill_at_least_40_officials_hospitals_999.html
http://www.foxnews.com/story/0,2933,327930,00.html
http://news.bbc.co.uk/2/hi/africa/7225896.stm

Ole

Earthquakes and Lichenometry

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Geology is a multidisciplinary science. It doesn’t even keep within its own boundaries, but interacts with other natural sciences - like biology for instance.

To get a better understanding of how earthquakes work, it is necessary to study past earthquakes. Here it is important to estimate when the earthquake in question actually took place, which may be difficult for prehistorical events. Several dating methods are available, like tree-ring dating, varve chronology and others. But they are not all even accurate or precise.

Lichenometry is accurate and has tightly clustered precision. It can be used on rockfalls caused by earthquakes. Lichenometry is the study of dating a surface using lichens as age markers: lichens increase in size radially as they grow. Measuring the diameter of the largest lichen on a rock surface can thus be used to determine the time the rock has been exposed. Lichen can be preserved on old rock faces for up to 10,000 years, providing the maximum age limit of the technique. The use of lichenometry is of increased value for dating deposited surfaces over the past 500 years as radiocarbon dating techniques are less efficient over this period. Different lichens have different growth rates, and the local climate also influences the growth rate. Slow growing lichens date older events than fast growing lichens. Obviously it is necessary to determine the lichen species and local growth rate, before measuring the diameter makes any sense.

Lichens are plant-like colonies of fungi and algae that grow together (in symbiosis) on the exposed surface of rocks. Lichens are often the first to settle in places lacking soil.

Lichenometry can, of course, also be used to date rock art or stonewalls, if you should be interested - see A Study of Lichens and Lichenometry. Lichens are, by the way, very sensitive to pollution from the atmosphere, which causes them not to grow in highly polluted areas. “Luckily” we have lichens on beautiful monumental stones in our garden, but my wife doesn’t seem to be so happy about that.




White Lava

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Why should an earthquake swarm in Tanzania remind me of a place in the Rhine valley in Germany?



The highest magnitude of the earthquakes was 6.0 on the Richter scale registered on Tuesday 17 July 2007, which was felt throughout northern Tanzania and parts of neighbouring Kenya. The swarm is situated close to Mt. Oldonyo Lengai, a volcano in northern Tanzania, which erupted on Friday 20 July 2007, spewing small amounts of smoke and lava.

Although volcanic eruptions are often preceded and accompanied by earthquake swarms, most earthquake swarms are not associated with volcanic eruptions.



Oldonyo Lengai (or Ol Doinyo Lengai) is unique among active volcanoes in that it produces natrocarbonatite lava, a unique occurrence of volcanic carbonatite. Due to this unusual composition, the lava is erupted at relatively low temperatures (approximately 500-600°C). This temperature is so low that the molten lava appears black in sunlight, rather than having the red glow common to most lavas. It is also much more fluid than normal (silicate) lavas. The carbonate minerals of the lavas formed by Oldonyo Lengai are unstable at the Earth's surface and susceptible to rapid weathering, quickly turning from black to grey in color. The resulting volcanic landscape icolourferent from any other in the world.

Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time.

Ol Doinyo Lengai is located in the eastern branch of the African Rift Valley and this brings me to another rift, however failed, the Rhine graben. here are the only carbonatites that I have personally ever seen - at the Kaiserstuhl. In the southern part of the Upper Rhine Graben the most prominent example of rift related magmatic activity is represented by the partly eroded Miocene Kaiserstuhl volcanic complex. Subvolcanic intrusions of carbonatites are now exposed in the center of the Kaiserstuhl. The activity took place somewhere between 16 and 14 million years ago.





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