So much for a honeymoon period; no sooner do I arrive at Scienceblogs than commentators start making demands. Fortunately, the story I’ve been ordered to write about was already in my ‘of interest’ pile, so in this instance it’s not too much trouble to oblige. I just hope I’m not setting a dangerous precedent.
The recent reports on the research of Jesse Lawrence and Michael Wysession highlight a novel application of seismic tomography, which uses the seismic energy generated by earthquakes to peer at variations in the Earth’s internal structure. Tomography requires a set of earthquakes where the time of rupture is precisely known, and a network of seismograph stations. Usually, researchers then look at the differences in the arrival times of seismic waves from a particular earthquake at neighbouring stations to identify places where the waves are traveling unusually fast or slowly: for example, if a stations starts picking up the waves much earlier than its neighbour, then the earthquakes are traveling through the rocks beneath that station unusually fast, and if they arrive later, they are traveling unusually slowly. Such variations are generally thought to correspond to temperature differences – the hotter the rocks are, the slower the waves will travel through them, and vice versa.
Lawrence and Wysession decided to look at something else as well; the differing degree of attenuation, or energy loss, of the seismic waves as they passed through the earth. This gives you information not about changes in temperature, but changes in strength; because they deform more easily, weaker rocks will tend to absorb more seismic energy as earthquake waves pass through them, increasing the observed attenuation.
Here’s what they found: a large zone of abnormally high seismic attenuation in the uppermost lower mantle (700-1400 km) beneath East Asia. The first images has removed the top 1000km of their 3D model to show the lateral distribution of this ‘Beijing anomaly’; the second shows a cross-section through it.
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Nice plan for content warnings on Mastodon and the Fediverse. Now you need a Mastodon/Fediverse button on this blog.