Featured Research

from universities, journals, and other organizations

Tracking Down Strange Seismic Waves

Date:
December 11, 2008
Source:
ETH Zurich
Summary:
Seismic waves generated by earthquakes pass through the earth. Changes in their direction or velocity indicate variations in the materials through which they pass. Geophysicists have now been able to show in a model exactly what happens at zones where crustal plates subduct below one another.

The schematic diagram shows the subducting oceanic plate with the pathway of the seismic waves in red. The red double arrow corresponds to the propagation direction of the fast seismic wave parallel to the fault.
Credit: Image courtesy of ETH Zurich

Seismic waves generated by earthquakes pass through the earth. Changes in their direction or velocity indicate variations in the materials through which they pass. ETH Zurich geophysicists have now been able to show in a model exactly what happens at zones where crustal plates subduct below one another.

As a rule, oceanic crust is subducted under the adjoining continent or under other oceanic crust at the major fault zones bordering and traversing the Pacific. Seismic waves generated by earthquakes and passing through the earth’s interior provide information about the underground structure of such regions, and with few exceptions they run according to specific patterns. Accordingly, the waves pass through an anisotropic region by splitting up and propagating at right angles to one another in two different directions and at two different velocities. In subduction zones, the fast component of shear waves alignes normally parallel to the trench. This is usually interpreted as due to flow of the mantle parallel to the trench in the region below the slab. However, numerical models show that the flow is perpendicular to the slab.

Cause identified

In his doctoral dissertation, Manuele Faccenda from the Institute for Geophysics at ETH Zurich has now studied the reason why the faster of the two seismic waves propagates in the known directions. Together with Luigi Burlini and Taras Gerya, Senior Lecturers at ETH Zurich, and David Mainprice of the University of Montpellier, he has now published the results of the study today in the scientific journal “Nature”.

Until now it was assumed that the cause of the splitting of the waves – the anisotropy – is situated in the earth’s mantle, but satisfactory replication of this in models was unsuccessful. Therefore the scientists have now included new aspects in their model. The study was based on seismic wave data already recorded. To this were added various parameters representing the conditions in the fault zone, e.g. the subduction angle and the age of the plates, their thickness and the orientation of the minerals to be expected in this crustal region. The expected water content of the minerals was also taken into account.

Hydrated plates as the decisive factor

In their model the researchers were now able to show that the source of the anisotropy lies in the subducting plate and not in the mantle. The computer simulation they developed reveals exactly what happens when an oceanic crustal plate subducts below a neighbouring plate. It is apparent that cracks form in it, depending on its age, depth, subduction angle and pressure. The minerals of the oceanic plate can be hydrated along these cracks by water penetrating down to a depth of 40 kilometres. This forms highly anisotropic platelet-shaped minerals aligned parallel to the cracks.

The seismic waves attain their highest velocity when they penetrate through these platelets longitudinally. In this case the fast seismic waves propagate parallel to the fault. The overall results of the study show that the orientation patterns of the seismic waves relative to the faults match the patterns observed around the Pacific well. The only case in which the researchers were unable to simulate the known pathway of the seismic waves by their model was for a quite young subduction zone in which the plate is still very flexible and subducts at only a gentle slope, and in which consequently scarcely any fractures and hydrated minerals occur. Faccenda explains that in this case probably the cause of the anisotropy really is to be found in the mantle.


Story Source:

The above story is based on materials provided by ETH Zurich. The original article was written by Simone Ulmer. Note: Materials may be edited for content and length.


Journal Reference:

  1. Faccenda M, Burlini L, Gerya & Mainprice D. Fault-induced seismic anisotropy by hydration in subducting oceanic plates. Nature, 2008; 455 (7216): 1097 DOI: 10.1038/nature07376

Cite This Page:

ETH Zurich. "Tracking Down Strange Seismic Waves." ScienceDaily. ScienceDaily, 11 December 2008. <www.sciencedaily.com/releases/2008/11/081129173952.htm>.
ETH Zurich. (2008, December 11). Tracking Down Strange Seismic Waves. ScienceDaily. Retrieved September 20, 2014 from www.sciencedaily.com/releases/2008/11/081129173952.htm
ETH Zurich. "Tracking Down Strange Seismic Waves." ScienceDaily. www.sciencedaily.com/releases/2008/11/081129173952.htm (accessed September 20, 2014).

Share This



More Earth & Climate News

Saturday, September 20, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Raw: Wildfires in CA Burn Forest Asunder

Raw: Wildfires in CA Burn Forest Asunder

AP (Sep. 18, 2014) An out-of-control Northern California wildfire has nearly 2,800 people from their homes as it continues to grow, authorities said Thursday. Authorities said a man has been arrested on suspicion of arson for starting the fire on Saturday. (Sept. 18) Video provided by AP
Powered by NewsLook.com
Raw: Elephant Undergoes Surgery in Tbilisi Zoo

Raw: Elephant Undergoes Surgery in Tbilisi Zoo

AP (Sep. 18, 2014) Grand the elephant has successfully undergone surgery to remove a portion of infected tusk at Tbilisi Zoo in Georgia. British veterinary surgeons used an electric drill to extract the infected piece. (Sept. 18) Video provided by AP
Powered by NewsLook.com
Raw: Scientists Examine Colossal Squid

Raw: Scientists Examine Colossal Squid

AP (Sep. 16, 2014) Squid experts in New Zealand thawed and examined an unusual catch on Tuesday: a colossal squid. It was captured in Antarctica's remote Ross Sea in December last year and has been frozen for eight months. (Sept. 16) Video provided by AP
Powered by NewsLook.com
Man Floats for 31 Hours in Gulf Waters

Man Floats for 31 Hours in Gulf Waters

AP (Sep. 16, 2014) A Texas man is lucky to be alive after he and three others floated for more than a day in the Gulf of Mexico when their boat sank during a fishing trip. (Sept. 16) Video provided by AP
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins