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Strong Earthquake - No Damage

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A 6.8-magnitude quake occurred in the Banda Sea, Indonesia, on Friday 28 August at 09:51 local time. It caused no severe damage, and although it was under sea it triggered no tsunami.

If you wonder why? the answer is simple, it was far too deep. And by deep I mean really deep - at a depth of 633.2 km. It is about as deep as earthquakes go. The deepest earthquake ever recorded is 705 km deep, under the Fiji Islands in the Southwest Pacific, on 6 May 2007.

Such deep earthquakes only occur in subduction zones - and only in some old subduction zones.



Looking at a map of historic seismicity from USGS we can see that the quake occurred in a band of former deep earthquakes (marked in red) at a certain distance from the surface frontier of the subduction zone marked as a purple line. In this area it is in fact relatively easy (even though the plate tectonics in the area are rather complex) to follow the inclination of the Benioff zone from shallow earthquakes, 0 – 36 km deep, to the deepest earthquakes 500 – 800 km deep. The earthquake foci normally plot along a dipping plane at an angle of 33 to 60 degrees and such a plane is called a Benioff zone. Earthquakes along Benioff zones define a lithospheric plate that descends into the mantle beneath another, overlying plate. The zone is named after Hugo Benioff, a US seismologist who first described this feature.



In Danish:





Academics

Aleutian Subduction Zone

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First a few words about MARGINS, quoted from their own website:

” What is MARGINS?

Continental margins are the Earth's principal loci for producing hydrocarbon and metal resources, for earthquake, landslide, volcanic and climatic hazards, and for the greatest population density. Despite the societal and economic importance of margins, many of the mechanical, fluid, chemical and biological processes that shape them are poorly understood. Progress is hindered by the sheer scope of the problems and by the space and time scales as well as the complexities of the processes. To overcome these obstacles, the earth science community has identified the outstanding scientific problems in continental margins research and the MARGINS Program is promoting research strategies that redirect traditional approaches to margin studies. In particular, the MARGINS Program will focus on the coordinated, interdisciplinary investigation of four fundamental initiatives; the Seismogenic Zone Experiment, the Subduction Factory, Rupturing Continental Lithosphere, and Sediment Dynamics and Strata Formation (Source to Sink). Each initiative is associated with two focus sites, research locations selected by the community to address the complete range of field, experimental and theoretical studies, over the full range of spatial and temporal scales needed to address fundamental questions associated with each initiative. The MARGINS Focus Sites are shown on this web page. The MARGINS Program is funded by the National Science Foundation (NSF), and is driven by input from, and interaction with the earth science community:“



MARGINS publishes a newsletter bi-annually. Their Newsletter No. 22 (Spring 2009) included a science article on the Aleutian Subduction. The Newsletter can be downloaded from their Newsletters site.

Below an illustration from this newsletter:



and a quote from their poetic introduction to this article:

“Among the most beautifully geometric features of Planet Earth is the string of volcanoes and parallel subduction furrow draped between the facing nations of Russia and the United States. This beauty, carpeted in emerald-green tundra, is both mysterious and dangerous: the Aleutian arc is among the most seismically and volcanically active but poorly understood regions of the two countries. Indeed, the American segment was the site of the world’s largest eruption and second largest earthquake in the 20th century.”



  1. Note 1: The largest volcanic eruption in the world in the 20th century occurred in 1912 at Novarupta (see map above), and I featured it in this post.

  2. Note 2: The great 1964 Alaskan earthquake. The 1964 Alaskan earthquake – the second biggest recorded in history with a magnitude of 9.2 – triggered a series of massive tsunami waves with run up heights of as much as 12.7 metres in the Alaskan Gulf region and 52 metres in the Shoup Bay submarine slide in Valdez Arm. This an other very strong earthquakes are marked by a yellow star on the map above.


I featured tsunami threats like the tsunami triggered by the 1964 Alaskan earthquake in my post on Great Pacific Tsunami Threat.



AcademicsTop Blogs

Great Pacific Tsunami Threat

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Research suggests that future tsunamis could reach a scale far beyond that suffered in the tsunami generated by the great 1964 Alaskan earthquake. The 1964 Alaskan earthquake – the second biggest recorded in history with a magnitude of 9.2 – triggered a series of massive waves with run up heights of as much as 12.7 metres in the Alaskan Gulf region and 52 metres in the Shoup Bay submarine slide in Valdez Arm.

(See also my post on Largest Earthquake Ever Recorded - with a magnitude of 9.5)

The study published in the journal Quaternary Science Reviews shows that the potential impact in terms of tsunami generation, could be significantly greater if both the 800-km-long 1964 segment and the 250-km-long adjacent Yakataga segment to the east were to rupture simultaneously. The data indicate that two major earthquakes have struck Alaska in the last 1,500 years, and according to the findings a bigger earthquake and a more destructive tsunami than the 1964 event are possible in the future.

The two mentioned earthquakes ca. 900 and ca. 1500 years ago simultaneously ruptured adjacent segments of the Aleutian megathrust and the Yakutat microplate, with a combined area ca. 15% greater than 1964, giving an earthquake of greater magnitude and increased tsunamitriggering potential.

Reference:
Shennan et al.
Multi-segment earthquakes and tsunami potential of the Aleutian megathrust
Quaternary Science Reviews: Volume 28, Issues 1-2, January 2009, Pages 7-13
doi:10.1016/j.quascirev.2008.09.016

http://www.dur.ac.uk/news/newsitem/?itemno=8306
http://www.unews.utah.edu/p/?r=071709-1
http://www.redorbit.com/news/science/1723347/pacific_tsunami_threat_greater_than_expected/index.html?source=r_science
http://www.sciencedaily.com/releases/2009/07/090720083421.htm
http://www.eurekalert.org/pub_releases/2009-07/du-ptt072009.php
http://www.wired.com/wiredscience/2009/07/tsunami/

In Danish:
http://politiken.dk/videnskab/article755056.ece
http://ing.dk/artikel/100164-forskere-har-undervurderet-tsunami-risiko-for-usa?utm_medium=rss&utm_campaign=nyheder



AcademicsTop Blogs

Earthquake in New Zealand

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Propably the largest quake of 2009 to date happened today off the southern tip of New Zealand. The first report that I received from USGS said: Magnitude: 7.8 Mw and Depth: 35 km, but it was probably weaker.

So far only little damage has been reported. The affected region is unpopulated (0 people/km²) and has high resilience for natural disasters. A tsunami warning was in effect for about 2 1/2 hours after the quake, but then dropped. Sea-level readings indicated a tsunami had been generated by the temblor, though there was no report of any substantial impact. At Jackson Bay, the sea had risen 0.17 m. The coastal area near the epicenter is sparsely populated and includes Fiordland National Park.

Tectonic Summary from USGS:

“The North and South Islands of New Zealand straddle the boundary between the Australian and Pacific plates. Along this boundary through southern New Zealand, the Australian Plate moves to the northeast at a rate of 35-45 mm/yr relative to the Pacific plate. In Southwestern South Island, this motion is accommodated by oblique convergence at the Puysegur Trench, where the Australian Plate subducts beneath the Pacific Plate. Further north along the South Islands’ west coast, relative motion is accommodated via oblique strike-slip movement along the Alpine Fault, driving the uplift of the Southern Alps.

The recent (09/07/15) thrust earthquake occurred near the southern tip of South Island in a region known as Fiordland, in a complex area of transition in plate boundary structure from Puysegur subduction to Alpine fault strike-slip motion. The preliminary location, depth, and estimate of fault orientation of this event are consistent with the earthquake having resulted from slip on the subduction thrust interface between the Pacific and Australian plates. The deformed Australian plate beneath Fiordland is highly active both along its’ interface with the Pacific plate and internal to the subducted Australian plate. Over the past two decades, several large earthquakes have occurred in Fiordland, predominantly in a cluster to the northeast of today’s earthquake. Though the faulting mechanisms of those events are similar to today’s earthquake, their slip vectors are rotated clockwise with respect to plate motions, and thus may not represent exactly the same style of faulting.

The most recent of these previous large events occurred in August of 2003 when a magnitude 7.2 earthquake approximately 100 km to the northwest caused minor damage in Otago and Southland, and numerous landslides across the Fiordland region. A magnitude 7.0 event on August 10, 1993 caused power outages in the Te Anau area and was felt throughout South Island and as far away as Sydney, Australia. A magnitude 6.4 earthquake struck on May 31, 1989 and was felt strongly in the southwestern part of South Island and a magnitude 6.7 quake struck on June 3, 1988.”



The following map from USGS gives a lot of information about the earthquake:


Another map from the USGS Earthquake Seismicity webpage shows the New Zealand transform faults (Alpine fault) in a greater tectonic perspective. Earthquakes occur regularly in New Zealand . About 14,000 earthquakes, most of them minor, are recorded each year. About 200 of these are strong enough to be felt. Fewer than 10 annually cause damage. Most earthquakes in New Zealand occur along the main ranges running from Fiordland in the southwest to East Cape in the northeast. This axis follows the boundary between the Indo-Australian and Pacific plates. Large earthquakes are less common along the central Alpine Fault, where the plates are not subducting and the forces are accommodated in different ways.

Some of the links below are rather short-lived
http://news.xinhuanet.com/english/2009-07/15/content_11713337.htm
http://www.bloomberg.com/apps/news?pid=20601087&sid=aB8ek3ijRBiM
http://www.gdacs.org/reports.asp?eventType=EQ&ID=61348&system=asgard&location=NZL&alertlevel=Green&glide_no=&TsID=1479
http://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/us2009jcap.php
http://shakingearth.blogspot.com/2009/07/largest-quake-of-year-in-new-zealand.html
http://news.bbc.co.uk/2/hi/asia-pacific/8151530.stm
http://tvnz.co.nz/national-news/factbox-new-zealand-s-biggest-earthquakes-2848956
http://www.huffingtonpost.com/2009/07/15/new-zealand-earthquake-tr_n_232996.html



AcademicsTop Blogs

Solar Eclipses and Earthquakes?

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As I have been directly asked, I would like to point out that in my view there is no scientific evidence to support the rumour that an earthquake-tsunami in South East Asia or Japan linked to the solar eclipse on 22 July 2009 will occur.

“That a Tokai Earthquake could occur at any moment” as I wrote in a post here already a couple of years ago is another matter, but putting a specific date to it for whatever reason is going too far. So far scientists have not found (or proved) any statistically significant correlation between solar eclipses and earthquakes.

http://www.earthobservatory.sg/news/2009/20090415-July22-Tsunami-Hoax/index.php



AcademicsTop Blogs

Vrancea Earthquake - Continued

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The Romanian Cutremur.net has a nice figure showing the detached slab beneath Vrancea at
http://www.cutremur.net/modelul-cel-mai-larg-acceptat-pentru-zona-vrancea/
(Cutremur is Romanian for earthquake).
The slab is shown in light blue (marked as oceanic lithosphere).

This is a continuation of my post on the Vrancea earthquake Saturday 25 April 2009.



Romanian Earthquakes

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On Saturday, 25 April 2009 at 17:18 an earthquake of magnitude 5.3 occurred in Romania. In itself nothing very spectacular, but looking at the historical seismicity in the earthquake area is interesting.

The area around this new quake is obviously a very earthquake-prone area. The seismicity beneath Vrancea is dominated by intermediate depth earthquakes in a well-defined volume. The epicentral area is confined to about 40 km x 80 km. Most earthquakes occur between 70 and 180 km depth (green and blue dots on the USGS map) within an almost vertical column. Deeper and shallower events have also been recorded but only with small magnitudes (Mw < 5.5). Strong earthquakes in the Vrancea area have caused a high toll of casualties and extensive damage over the last several centuries. With a moment magnitude of 7.4, an earthquake on Friday, 4 March 1977 killed about 1,570 people, the majority of them in Bucharest, and wounded more than 11,000. About 35,000 buildings were damaged. The depth was 94 kilometres. The Bucharest is among the most vulnerable European capitals to earthquake, due to the seismic activity in the Vrancea region.

Four major earthquake events occurred during the last century
(Oncescu and Bonjer, 1997):
Date............Depth (km)..........Size (Mw)
Nov. 10, 1940.........150...................7.7
March 4, 1977.........95....................7.4
Aug. 30, 1986.........130...................7.1
May 30, 1990..........90....................6.9


The earthquakes are located in a confined, isolated focal volume, beneath the Eastern Carpathians Arc bend.

The focal mechanism for the largest Vrancea shocks are typically of reverse faulting. The earthquakes are probably caused by an old sinking slab. The tectonic evolution of the Carpathians started with the subduction of oceanic lithosphere 22 to 10 Million years ago with the direction of subduction first towards the southwest and later towards the west and northwest. The main driving mechanism during that time was the gravitational sinking of the subducted slab which had a higher density than the surrounding mantle material. After consumption of the oceanic lithosphere, continental collision started, resulting in overthrusting and uplift. Around 10 million years ago the sinking slab broke off and sank further downwards more or less vertically, due to gravitation. The situation today is sketched in the image below taken from a brochure from the Karlsruhe University that can be downloaded here: www-sfb461.ipf.uni-karlsruhe.de/brochures/brochure2002.pdf (the above overview map of the Carpathians is also from this brochure.




No damage or injuries were reported but people panicked and left their homes. The earthquake was also felt in northern Bulgaria and in neighbouring Moldavia.

http://www-sfb461.physik.uni-karlsruhe.de/pub/web/sfb-www/main/general/vrancea.html
http://social.moldova.org/news/april-25-earthquake-in-romania-felt-in-moldova-and-bulgaria-198951-eng.html

PS: Discussion continues here


Solomon Quake 2007

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As plate tectonics are the cause of most earthquakes, it is no wonder that those earthquakes tell us a lot about plate tectonics. Sometimes, however, it takes a long time to read the message.

Two years ago I wrote a minor post about Solomon earthquakes that occurred in April 2007. In the meantime these earthquakes have been thoroughly studied, and a paper on some of the findings has just been published in the journal Science. The earthquake is sort of special, a.o. because it occurred at a three-plate boundary, or triple junction. Below is a simple situation map.


The New Georgia Island Group of the Solomon Islands is one of four places where an active or recently active spreading ridge has subducted beneath an island arc. The spreading ridge pushing the small Solomon Sea Plate and the Australian Plate apart is being subducted beneath the Pacific Plate. The situation is rather complicated because the two plates descend beneath the overriding plate at different rates and directions.

According to the study the event began in the Australian Plate and moved across into the Solomon Sea Plate and had two centers of energy separated by lower energy areas, which is noticeable as we normally think earthquakes should stop at the plate boundaries. When the earthquake moved from one plate to the other, it quickly changed direction, mimicking the different plate motion directions of the plates involved. The authors are confident that the fault slip in the two main locations are different by 30 to 40 degrees. That behaviour during an earthquake has probably never been observed before, but it most certainly has happened here before, according to the authors.

Before about half a million years ago, the easternmost segment of the Woodlark Basin spreading ridge was subducting beneath the westernmargin of the Solomon Islands, and the ridgetrench triple junction migrated northwesterly at around 110 to 120 mm/year. The differences in plate subduction rates and directions produced a slab window, which today lies beneath the southern New Georgia Islands. (When a mid-ocean spreading ridge subducts, it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within an inter-slab gap called a slab window).

A cartoon in the paper shows how the Solomon Sea Plate and the Australian Plate subduct as two different layers (slabs) beneath the overriding Pacific plate, with a slab window at depth (shown as increasing empty space between the two coloured slabs). The surface subduction boundary is marked by a red line.




http://www.sciencemag.org/cgi/content/short/324/5924/226
http://news.yahoo.com/s/ap/20090409/ap_on_sc/sci_solomon_tsunami_1
http://www.eurekalert.org/pub_releases/2009-04/ps-sie040609.php
http://www.geologytimes.com/research/Solomon_Islands_earthquake_sheds_light_on_enhanced_tsunami_risk.asp
http://earthquake.usgs.gov/eqcenter/eqinthenews/2007/us2007aqbk/finite_fault.php
http://earthquake.usgs.gov/eqcenter/eqinthenews/2007/us2007aqbk/
http://walrus.wr.usgs.gov/tsunami/solomon07/index.html



L'Aquila Earthquake - Followup

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At a press conference in L'Aquila today (7 April 2009) the Italian Premier Silvio Berlusconi said 207 people were confirmed dead, while 15 people were still missing, after the earthquake that hit the city of L'Aquila. Berlusconi said slightly more than 1,000 people were wounded following the quake and that 150 had been found alive. Other media now speak about 1,500 injured and some 17,000 homeless after Monday's quake.

There has been a lot of fuss about a so-called prediction of the quake. I would like to remind you what earthquake prediction is about. An earthquake prediction is a prediction that an earthquake in a specific magnitude range will occur at a specific place at a specific time. As Amphibol (in German) rightly reminds us, an earthquake was predicted to occur at Sulmora about 30 km from L’Aquila on Sunday 29 March - but it didn’t! A week later, however, on Monday 6 April an earthquake did occur, but at L’Aquila. What would have happened if the inhabitants of Sulmora had been evacuated or had fled to L’Aquila for shelter. Just think about it - and let’s face it, so far measurements of radon emissions are more or less worthless as earthquake prediction tool for all the other reasons we have heard the last couple of days.

In my post yesterday I mentioned that the quake occurred at a normal fault. When continents (like Africa and Europe) collide you would normally expect compression of the crust, but normal faulting is an expression of extension. I didn’t go into this apparent paradox, and I no longer need to as Kim Hannula at All of My Faults Are Stress Related did that extremely well in her post here.

Finally a satellite image of central Italy that shows the rugged topography in the vicinity of L’Aquila.


Lower elevations are shown in green, while higher elevations are light brown and off-white. The steepness of the mountain slopes is indicated by shading: darker shading means steeper slopes. L’Aquila is nestled in the central Apennine Mountains, which run the length of Italy like a spine. The mountains are crisscrossed by dozens of faults. Faults are not always visible at the surface, but in this part of the Apennines, many of them are revealed by steeply sloped fault scarps. A major fault system is revealed by a scarp running north-northwest from the Focino Plain nearly to L’Aquila, passing along the eastern foothills of Mt. Velino and Mt. Ocre. L’Aquila is wedged between a pair of parallel faults running toward the northwest and a long, broken fault extending toward the east.



Italian earthquake

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An earthquake hit L'Aquila at 3.32 am local time today, 6 April 2009, killing at least 150 people and leaving up to 50,000 homeless. Preliminary estimates suggest between 10,000 and 15,000 buildings have been destroyed or damaged beyond repair in the region. It is however much to early for a final balance. The village of Onna was competely razed to the ground. It was was the country's deadliest since the Irpinia quake in the south in November 1980, which killed more than 2,500 people.

According to USGS the epicentre was at 42.423°N, 13.395°E. The depth was only about 10 km. Shallow earthquakes are usually the most damaging.

Carved up by two major fault lines, Italy has gained a reputation as one of the most earthquake prone countries in Europe. The Eurasian and African plates meet along a line which runs through North Africa and crosses the Mediterranean near southern Italy and Greece. Africa is moving northward at about 2 cm a year. As a result two main cracks - or fault lines - cut across the Italian peninsula, one running north-south along the spine of the Apennine mountains (where Aquila is situated) and another crossing east-west south of Rome and north of Naples. The city of L’Aquila (founded in 1245) is thus located in a tectonic basin bounded to the north by a northwest–southeast-trending active normal fault. (The earthquake was in fact a shallow normal-faulting event). The city sits in a valley in the central Apennines north of Rome and is built on a basin of sediments which has attracted geological interest in the past. It was hit by earthquakes repeatedly in its history including one in 1703 which flattened the centre.

Here are a few links to the first media reports from the disaster.

http://news.bbc.co.uk/2/hi/europe/7985958.stm
http://news.bbc.co.uk/2/hi/europe/7984867.stm
http://www.gdacs.org/reports.asp?eventType=EQ&ID=54774&system=asgard&location=ITA&alertlevel=Green&glide_no=
http://www.terradaily.com/reports/Strong_6.7_magnitude_quake_strikes_Italy_Rome_fire_service_999.html
http://www.latimes.com/news/nationworld/world/la-fg-italy-quake7-2009apr07,0,1185599.story
http://www.telegraph.co.uk/news/worldnews/europe/italy/5114636/LAquila-earthquake-damaged-ancient-baths-in-Rome.html
http://www.telegraph.co.uk/news/worldnews/europe/italy/5114139/Italian-earthquake-experts-warnings-were-dismissed-as-scaremongering.html

In Danish:
http://www.dr.dk/Nyheder/Udland/2009/04/06/110312.htm?nyhedsbrev
http://ing.dk/artikel/97716-usaedvanlig-forklaring-bag-jordskaelv-mikroplader-blev-revet-fra-hinanden

In Dutch
http://www.knack.be/nieuws/europa/aardbeving-in-italie--zeker-40-doden/site72-section25-article31778.html

In German:
http://www.zeit.de/online/2009/15/bg-erdbeben

And a few links to fellow bloggers’ posts so far about the event:

* http://amphibol.blogspot.com/2009/04/erdbeben-in-italien.html
* http://ontario-geofish.blogspot.com/2009/04/large-earthquake-hits-italy.html
* http://daveslandslideblog.blogspot.com/2009/04/first-take-on-italy-earthquake.html
* http://geology.about.com/b/2009/04/06/italy-struck-by-m-63-quake.htm
* http://www.goodschist.com/2009/04/06/manitude-63-earthquake-in-laquila-italy/
* http://seismo.berkeley.edu/blogs/seismoblog.php/2009/04/06/title-2
* http://aldopiombino.blogspot.com/2009/04/il-terremoto-dellaquila-tra-annunci.html

more will no doubt follow.



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