olelog

A strong earthquake with a preliminary magnitude of 7.4 has struck in the Pacific Ocean off southern Japan, at the Izu-Bonin Trench where the Pacific plate is subducting beneath the Philippine Sea Plate.





Japan's Meteorological Agency issued a tsunami alert of up to 2 m for nearby islands and warnings of milder tsunami for the southern coasts on the main Japanese island. A minor swelling of waves of about 30 cm was observed on the island's shorelines about 40 minutes after the quake, the agency said. Japan later downgraded the alert to a warning of small waves.

• http://www.kansascity.com/2010/12/21/2534854/strong-earthquake-hits-off-southern.html
• http://news.yahoo.com/s/afp/20101221/wl_asia_afp/quakejapan

More than a year ago some mysterious earthquakes in Saudi Arabia were discussed in the Geoblogosphere, a.o. by Ralph Harrington and Erik Klemetti. In the meantime they have been the topic of a paper published in the journal Nature Geoscience.

Both geobloggers have of course duly commented the new findings here and here, which makes my job easy - just read their blogs, and you are up to date, that is what I highly recommend.

Nevertheless just briefly this:
In 2009 (between April and June), more than 30,000 earthquakes struck an ancient lava field in Saudi Arabia - the extensive Harrat Lunayyir lava province formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was so far regarded as seismically quiet.

The Red Sea rifting pushed the African and Arabian tectonic plates apart, and the rifting is still going on, so that within many million years the two continents will have a large oceans between them.

Here is a Google situation map:


Location of Harrat Lunayyir: Latitude: 25.17°N * Longitude: 37.75°E (if the map shouldn’t turn up!)

The authors used geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. They documented a surface fault rupture that is 8 km long with 91 cm of offset. This deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10 km long. Sensors show that magma has risen to roughly 2 km below the surface of the Earth, and eruptions remain possible. Rather than extension being accommodated entirely by the central Red Sea rift axis, the authors suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Their analyses allowed them to forecast the likelihood of a future eruption or large earthquake in the region.

The lava field of Harrat Lunayyir is part of a "lava province" roughly 180,000 square kilometres in size. Harrat Lunayyir is a basaltic volcanic field east of the Red Sea port of Umm Lajj. It contains about 50 volcanic cones. The hazard from these volcanoes is low, given the remoteness of the site and the type of eruption expected. Volcanic eruptions in Saudi Arabia are rare and only occur every few hundred years. According to contemporary accounts, the best known volcanic event in the region occurred in 1256, which sent flows of lava "like a red-blue boiling river" for 52 days into the holy city of Medina.

• http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo966.html
• http://www.csmonitor.com/Science/2010/0926/Ancient-volcanic-field-reawakens-in-Saudi-Arabia
• http://www.nature.com/news/2010/100926/full/news.2010.494.html
• http://www.redorbit.com/news/science/1922646/saudi_arabia_earthquakes_caused_by_volcano/
• http://www.msnbc.msn.com/id/39380701/ns/technology_and_science-science/blogblogs

Much has been written and said about the Zagros Fold Belt in Iran, not least because the Zagros fold and thrust belt region is known as a major petroleum source. It is however an area with a complex geology and geologic history with many secrets still uncovered. I would exaggerate, if I said that a recent study published in the October 2010 issue of the journal Basin Research has added much to our knowledge about the area, but that is the way science usually works, adding bits and pieces to get a better overall picture. The paper won’t reach the front page of any newspaper, but that doesn’t mean that it is not important, and the thermocronological approach is interesting. The study (“Insights in the exhumation history of the NW Zagros from bedrock and detrital apatite fission-track analysis: evidence for a long-lived orogeny”) has mainly evidenced that the Zagros orogeny was long-lived and multiepisodic, implying that the timing of accretion of the different tectonic domains that form the Zagros Mountains requires cautious interpretation.



The Zagros Fold Belt extends for approximately 1,500 km in length and borders Iran to the southwest along the eastern edge of the Persian Gulf.



The mountains (or Fold-Thrust Belt) were formed by collision of the Eurasian and Arabian tectonic plates. Recent GPS measurements in Iran have shown that this collision is still active. A relatively dense GPS network which covered the Zagros in the Iranian part also proves a high rate of deformation within the Zagros. The GPS results show that the current rate of shortening in SE Zagros is ~10 mm/yr and ~5mm/yr in the NW Zagros.



The activity manifests itself in plenty of earthquakes. A historical seismicity map from USGS shows that these are generally relatively shallow.



Earthquakes are spread over the whole with of 200 km of the Zagros Fold Belt. They mostly have estimated depths that range from 8 to 13 km, and surprisingly enough do not show any real tendency to deepen towards the Main Zagros Thrust (often depicted as an active subduction zone).- actually most of the earthquakes may have occurred in the overriding sediments and not in the basement.



Brief description of possible tectonic evolution:

Prior to the collision between the Arabian and Eurasian continents the Neo-Nethyan oceanic basin rifted apart an Iranian microcontinent from the rest of the Southwest Iran. The separation of Arabia from Africa and its subsequent collision with Eurasia was the last of a series of separation/collision events, all of which combined create the extensive Alpine–Himalayan orogenic system. This continental breakup began in Northwestern Iran during the Permian to Triassic time period, but began later in the Sanadaj-Sirjan Zone during the Late Triassic. (The Sanadaj–Sirjan zone, on the North East side of the thrust zone), is a region of polyphase deformation, the latest reflecting the collision of Arabia and Eurasia and the subsequent southward propagation of the fold–thrust belt). The Arabian plate then began to converge closer to the Eurasian continent, possibly due to the rifting apart of the Arabian continent from Africa at the Red Sea and the Gulf of Aden. The Arabian continent began to subduct below the Eurasian continent beginning in the Late Jurassic. Following the time of subduction (80-95 million years ago), ophiolites were possibly generated from island arc collisions with the Arabian passive margin and placed on the edge of the Arabian continent at the end of the Cretaceous. During the continental collision, these ophiolites formed along the suture. Another deformation event took place at the Eocene as subsequent ocean crust was placed on the Arabian margin and then folded. Gabbroic intrusions were then placed approximately 40-38 million years ago and intruded the Sanadaj-Sirjan Zone and folded Paleocene-Eocene volcanics. The complete late Cretaceous and Tertiary history of the collision is however still debated, because multiple collisions of small continental blocks and island arcs against the North East Arabian margin are recorded. The new study helps constrain the different cooling phases and denudation episodes.

References:
Homke et al.
Insights in the exhumation history of the NW Zagros from bedrock and detrital apatite fission-track analysis: evidence for a long-lived orogeny
BasinResearch (2010) 22, 659–680
doi: 10.1111/j.1365-2117.2009.00431.x

Nadine McQuarrie
Crustal scale geometry of the Zagros fold–thrust belt, Iran
Journal of Structural Geology 26 (2004) 519–535

Mike Molnar
Tertiary Development of the Zagros Mountains
Geol 418 – Earth History

Useful links:

• http://knol.google.com/k/geology-of-iran#
• http://pubs.usgs.gov/of/1997/ofr-97-470/OF97-470G/iranGmap.html
• http://earthsciences.dal.ca/people/Visiting_Scientist/zarasvandi/zarasvandi.html
• http://sepmstrata.org/nassir-thesis-site/chapt04.html

Continued in next post: http://my.opera.com/nielsol/blog/2010/09/27/zagros-mountains-continued-from-yesterday-with-new-earthquake-of-today

To-days story probably started (or at least brought forward to a greater public) by an article in Los Angeles Times of 18 July 2010. They wrote a.o.

“The Mogi doughnut hypothesis, developed by a Japanese seismologist, holds that earthquakes occur in a circular pattern over decades, building up to one very large temblor in the doughnut hole.”

In Terra Daily a day later they came up with this comment:

“A new earthquake theory suggests doughnut-shaped patterns of temblors build up over decades to a final large earthquake in the doughnut "hole," scientists say.”

The circular pattern theory, called a Mogi doughnut after the Japanese seismologist who proposed it, may lead to improved earthquake forecasts, the Los Angeles Times reported Sunday.”

To call it a “new theory” may not be quite correct as the Japanese seismologist K. Mogi suggested the idea back in 1969, which isn’t exactly last week.

The concept holds that earthquakes sometimes occur in a circular pattern over decades -- building to one very large quake in the doughnut hole. The idea behind the doughnut is relatively straightforward: Earthquakes in California are basically caused by tectonic movements in which the Pacific plate slides northwest relative to the North American plate. As the plates move, stress builds up along both sides of cracks in the Earth's crust, as if a giant sheet of peanut brittle were being shoved in two directions. Tectonic stress will first cause ruptures on the smaller faults, because they need less pressure before they break and thus produce small earthquakes. When they do rupture, the tectonic pressure gets transferred somewhere else, moving along like a crack in a windshield. Ultimately, the stress moves closer to bigger faults that need more pressure to erupt, thus creating larger and larger earthquakes until the "Big One" happens.

Take it for what it is: Just another unproven and not universally accepted hypothesis (and not even new). Why shouldn’t random earthquakes sometimes randomly create a certain recognisable pattern (circle, square, triangle …).

• http://articles.latimes.com/2010/jul/18/local/la-me-earthquake-forecast-20100718
• http://www.latimes.com/news/custom/scimedemail/la-me-earthquake-forecast.eps-20100718,0,1446387.graphic
• http://www.terradaily.com/reports/Doughnut_hole_quake_theory_studied_999.html
• http://www.gi.alaska.edu/ScienceForum/ASF3/315.html
• http://enr.construction.com/yb/enr/article.aspx?story_id=147546660

An earthquake of magnitude 7.2 has struck offshore near the Indonesian island of Sumatra, near Aceh province. The quake struck 214 km south of Aceh's capital of Banda Aceh. A local tsunami alert was issued and later lifted by the Pacific Tsunami Warning Center. The site is very near that of 2004's 9.2 magnitude earthquake.

In a later post today (I have to lave my computer now for a few hours) I intend to comment more on the tectonic setting of this earthquake. Just for now a tectonic summary from USGS:

“The northern Sumatra earthquake of May 9, 2010 occurred as a result of thrust faulting on or near the subduction interface plate boundary between the Australia-India and Sunda plates. At the location of this earthquake, the Australia and India Plates move north-northeast with respect to the Sunda plate at a velocity of approximately 60-65 mm/yr. On the basis of the currently available fault mechanism information and earthquake depth, it is likely that this earthquake occurred along the plate interface.
The subduction zone surrounding the immediate region of this event slipped during the devastating Mw 9.1 earthquake of December 2004, and today's event appears to have occurred within the rupture zone of that earthquake. Today's earthquake is the latest in a sequence of large ruptures along the Sunda megathrust, including a M 7.8 in April of this year, approximately 200 km to the south of today's event; two M 7.4 earthquakes beneath Simeulue approximately 100 km to the south in 2002 and 2008; a M 8.6 210 km to the south in 2005; a M 7.5 650 km to the south near Padang in 2009; and two events of M8.5 and M7.9 approximately 1000 km to the south in 2007.”

• http://news.bbc.co.uk/2/hi/asia-pacific/8670593.stm
• http://www.gdacs.org/reports.asp?eventType=EQ&ID=86296&system=asgard&location=IDN&alertlevel=Red&glide_no=&TsID=1991