Elsevier

Earth and Planetary Science Letters

Volume 402, 15 September 2014, Pages 305-312
Earth and Planetary Science Letters

Distinct compositional thin layers at mid-mantle depths beneath northeast China revealed by the USArray

https://doi.org/10.1016/j.epsl.2013.02.015Get rights and content

Abstract

We observe a clear seismic arrival at ∼35–45 s after the direct P wave in USArray recordings of two deep earthquakes occurring beneath northeast China. Velocity-spectrum and beam-forming analyses reveal that this arrival has a lower slowness value than the direct P wave and a back azimuth slightly different from the great-circle direction. The measured slowness and arrival time indicate that it is a transmitted S to P conversion from structures below the sources. We employ the common-conversion-point (CCP) stacking and diffraction migration methods to determine the location and geometric features of the seismic structures. The CCP stacking image indicates that the structure is a localized discontinuity at ∼1000 km with a dimension at ∼200 km by ∼50 km along the E–W and N–S directions, respectively. It is located at ∼150 km northeast to the two events. The 2D migrated images, on the other hand, indicate that the sources structure are reflectors dipping northeastwards by ∼17° at a slightly shallower depths. The reflectors have a length scale of ∼100 km ant their centers are ∼50 km away from the epicenters of the two earthquakes. Forward waveform modeling suggests that the dipping reflectors may be thin layers with a thickness of few kilometers. The layers have a lower shear velocity and a higher density than that of the surrounding mantle, which matches well with those predicted for mid-ocean-ridge basalt (MORB) at mid-mantle depths, according to a recent ab initio study. Combined with the results from previous studies, our observations here suggest that the former oceanic crust may be ubiquitously present in the lower mantle beneath subduction zones.

Highlights

► USArray recorded anomalous seismic arrivals from two deep quakes in NE China. ► The source structures are seismically distinct thin layers in the lower mantle. ► The thin layers could be subducted oceanic crust. ► Oceanic crust may be ubiquitously present in the lower mantle of subduction zones.

Introduction

Many seismic studies have revealed that the Earth's lower mantle is featured by two large lower velocity structures beneath Africa and central Pacific, known as the Africa and Pacific Large-Low-Shear-Velocity Provinces (LLSVPs), surrounded by seismically fast anomalies that are interpreted as the subducted slabs in later Mesozoic and Cenozoic (Dziewonski, 1984, Su et al., 1994, Grand et al., 1997, van der Hilst et al., 1997, Ritsema et al., 1999). The origin and evolution of the two LLSVPs, however, have remained poorly understood (e.g., Ohta et al., 2008, Schuberth et al., 2009, Zhang et al., 2010, Dziewonski et al., 2010, Nakagawa and Tackley, 2010). There are speculations that the LLSVPs are piles of dense mid-ocean-ridge basalt (MORB) accumulated above the core–mantle boundary (CMB) (Ohta et al., 2008, Nakagawa and Tackley, 2010). Mapping subducted oceanic crust in the lower mantle thus is crucial to understanding whether oceanic crust can sink to the CMB to form the two large piles.

Kaneshima and Helffrich, 1998, Kaneshima and Helffrich, 1999 observed strong coda waves from teleseismic recordings in western US from deep earthquakes occurring in Mariana and interpreted them as the S to P converted waves from scatterers in the mid-lower mantle depths. The scatterers appeared to be thin lower velocity layers with high dipping angles, which led them to speculate that the scatterers may be former oceanic crust subducted into the lower mantle. In this paper, we define a volumetric anomaly with an arbitrary shape as a scatterer, and a thin layer with distinct velocity/density as a reflector. As such a reflector is a special type of the broadly defined scatterers. Niu et al. (2003) found a strong seismic reflector at ∼1100 km deep beneath the Mariana subduction zone. The reflector is a ∼12 km thick layer with an S-wave velocity of 2–6% lower and a density 2–9% higher than those of the surrounding mantle. The difference in P-wave velocity is rather trivial (<1%). Due to the lack of data on the elasticity of oceanic crust at lower mantle condition at the time, it was not conclusive whether the observed reflector is a piece of subducted oceanic crust or not. In addition to these studies, several recent studies also suggested these reflectors exist over a wide range of depth (∼800–1850 km) beneath various subduction zones surrounding the Pacific (e.g., Kaneshima and Helffrich, 2003, Kaneshima and Helffrich, 2009, Kaneshima and Helffrich, 2010).

Besides the lower mantle reflectors, there were also reports of the existence of sharp discontinuities at depths from 900 to 1100 km in the middle mantle (Kawakatsu and Niu, 1994, Niu and Kawakatsu, 1997, Vinnik et al., 1998, Vanacore et al., 2006). Possible causes of these mid-mantle discontinuities, however, have remained poorly understood. It is unclear whether these discontinuities are related to the seismic reflectors as mentioned above. In this study, we analyze an unusual later arrival recorded by the USArray from two deep earthquakes occurring in NE China. We employ seismic imaging techniques to locate the source structures and utilize forward waveform modeling to estimate the seismic properties of these structures. With the newly available mineral physics data, our goal is to better constrain the nature of the lower mantle reflectors, as well as to understand whether the two types of seismic structures being reported previously are caused by resolution ambiguity in the short-period data.

Section snippets

Data and analysis

The seismic data used in this study are recorded by the transcontinental USArray (Fig. 1), which consists of 400+ transportable broadband seismic stations. Although the primary goal of the USArray deployment is to obtain high-resolution seismic images of the North American lithosphere, with an aperture of more than 2000 by 1000 km, the USArray is also ideally suited for detecting weak signals associated with small-scale 3D structures within Earth's deep interior around the globe. We use

Results and discussion

The vespagram of the 05/19/2008 event is shown in Fig. 3a. The ∼40 s later arrival exhibits a negative relative slowness of 0.21 s/deg. with respect to the P wave, suggesting that it is indeed an S to P converted phase from an anomalous structure below the source. The vespagram also shows another S to P conversion phase at 12.5 s with a slightly negative slowness, −0.03 s/deg. (Fig. 3a). We use iasp91 velocity model and the relocated focal depth of 520 km, which was determined by Wang and Niu (2010)

Conclusion

We observe an unexpected later arrival at the USArray records of two deep earthquakes beneath NE China. Using array processing techniques, we find that the arrival is best explained by an S to P conversion from a dipping structure at about ∼1000 km. While synthetic tests suggest either a discontinuity or a thin low velocity layer can explain the observed short-period data, we argue that the latter is the more physically likely source for the S to P conversion, as the required seismic properties

Acknowledgments

We thank IRIS and EarthScope for providing the data used in this study, Steve Grand, George Helffrich for helpful discussion, and two anonymous reviewers for their constructive comments and suggestions. This research is supported by the NSF grant EAR-0748455.

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