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Basalt-eclogite transition in the subducting oceanic
crust detected by high frequency seismic waves
Wenbo Wu and Jessica C. E. Irving
October 6, 2016
The occurrence of intermediate-focus and deep earthquakes is still mysterious, although some physical models, such as dehydration and thermal runaway, have been proposed. Three intermediate-focus (140-160km) earthquake nests have been found beneath
Japan and attributed to the stress changes associated with the basalt-eclogite transition
in subducted oceanic crust (Nakajima et al., 2013). However, there has not yet been
conclusive evidence to support the basalt-eclogite transition happening at this depth for
this particular region. We investigated seismic waves at the Hi-net seismic array stations
from more than 450 earthquakes within and close to the nests. A special kind of P-wave,
following the first arrival P-wave, is observed in records of more than 100 earthquakes
in the subducted oceanic crust. These special, or delayed, P-waves have greater high
frequency components (>20Hz) than the first P-waves. Considering the tectonic context
of subduction, we explain the delayed P-wave as trapped seismic waves in the oceanic
crust. They are sensitive to the seismic structure of the oceanic crust and therefore can
be used to detect the seismic velocity change due to the basalt-eclogite transition.
More quantitative analysis relies on numerical modeling. Among the numerical simulation tools of seismic wave propagation, the Spectral Element Method (SEM) has the
advantages of geometrical flexibility, high accuracy and scalability, thus fits our problem
well. However, solving our problem in 3D domain is expensive, in terms of computational time. In order to mathematically represent wavefield well, the mesh (composed
1
of 5 grid points for one dimension) size in SEM needs to be comparable to or even
smaller than the shortest wavelength. The highest frequency in our data is higher than
20Hz and the corresponding shortest wavelength is ∼170m. If the 3D model dimension is 280km×280km×280km, the total elements would be more than 4×109 . Thus,
solving our problem in 3D domain would be far beyond the capability of computation
resource available for us. Instead of 3D simulations, we simplify the problem to 2D modeling, which becomes computationally tractable. Combining the geological context and
geophysical studies of subduction beneath Japan, we build a 2D subduction model and
adopt SPECFEM2D to calculate synthetics. The total number of elements in 2D model
is reduced to about 5×104 and one simulation takes about only 26 hours using 320 cores
of the Princeton’s Tiger cluster.
Finally, the simulation results support a basalt-eclogite transition at a depth around
140-155km. Specifically, the more high frequency components in the delayed P-wave
signals could be explained with low-attenuation oceanic crust. Relative to the onsets
of the first P-waves, the delay times of the second P-wave signals first increase and then
decrease with the depths of the earthquakes. A 5-9% P-wave velocity change can replicate
the pattern of changing delay times. This positive velocity change is consistent with the
basalt-eclogite transition. Thus, the intermediate-focus earthquake nests beneath Japan
might be associated with the gradually developing basalt-eclogite transition.
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