Azimuthal anisotropy beneath the deep central Aleutian subduction zone from normal mode coupling

Journal Pre-proof Azimuthal anisotropy beneath the deep central Aleutian subduction zone from normal mode coupling W. Cheng, X.G. Hu, L.T. Liu

PII:

S0264-3707(19)30045-6

DOI:

https://doi.org/10.1016/j.jog.2019.101673

Reference:

GEOD 101673

To appear in:

Journal of Geodynamics

Received Date:

6 March 2019

Revised Date:

18 October 2019

Accepted Date:

30 October 2019

Please cite this article as: Cheng W, Hu XG, Liu LT, Azimuthal anisotropy beneath the deep central Aleutian subduction zone from normal mode coupling, Journal of Geodynamics (2019), doi: https://doi.org/10.1016/j.jog.2019.101673

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Azimuthal anisotropy beneath the deep central Aleutian subduction zone from normal mode coupling

W. Cheng1, 2, X. G. Hu1, and L. T. Liu1 1

State Key Laboratory of Geodesy and Earth's Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuchang, Wuhan, China. 2

University of Chinese Academy of Sciences, Shijingshan, Beijing, China.

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Corresponding author: Xiao Gang Hu ([email protected]) Key Points:

We observe strong coupling of 0 S11  0T12 due to local azimuthal anisotropy in Aleutian subduction zone.

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The fast direction in the depth of 400-700 km is trench parallel measured by normal mode coupling.

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The normal mode coupling can constrain the fast direction for the deep mantle.

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Abstract

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Shear wave splitting measurements show that the anisotropy beneath central Aleutian subduction region is complex. Here we observed strong mixed coupling of 0 S11  0T12 in the vertical component records at ADK station, a station deployed on the central Aleutians subduction zone, after the 2006/04/20 Olyutorsky earthquake, 2013/09/24 Pakistan earthquake and 2017/07/17 Nikol’skoye earthquake. We exclude the factor of Coriolis coupling and verify that the local azimuthal anisotropy is responsible for the strong coupling pair of 0 S11  0T12 . We also estimate the coupling kernels depth and find the peak sensitivity for the coupled 0 S11  0T12 is about 400-700km, which suggest the mantle transition zone(MTZ) beneath the central Aleutians has azimuthal anisotropy. Studies show that the strength of mixed coupling caused by local anisotropy depends on the azimuth of the source-receiver great circle. We compare our result with the fast orientation measured by wave splitting and find the fast direction of trench-parallel appears to be robust for central Aleutian at the 400-700km depth. The local azimuthal anisotropy plays a leading role on the coupled 0 S11  0T12 in the early part records, however, the effects of local anisotropy on long period normal modes coupling are often ignored. Our observations indicate that the local anisotropy can make a noticeable effect on normal modes coupling. Normal modes coupling due to local anisotropy have high depth resolution, which can constrain the depth range of the azimuthal anisotropy mantle. The normal modes coupling signal can be a supplement to shear wave splitting technique in studying the anisotropy of subduction zone.

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Plain Language Summary

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We suggest the trench-parallel anisotropic orientation in the transition zone for central Aleutian arc. However, the relationships between deformation and anisotropy for transition zone remain poorly understand. Multiple observation techniques need to be combined to study the anisotropy at the deep mantle in the future, and the coupling mode can be an important complement.

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Long period normal modes coupling are sensitive to the deep mantle with depth resolution, they have special advantages on probing the anisotropy in the deep mantle compared with shear wave splitting, and the later has poor depth resolution. In this paper the coupling of 0 S11  0T12 present evidence for azimuthal anisotropy at 400-700km beneath central Aleutian arc with higher reliability. The depth of 400-700km is just in the range of transition zone which suggests the MTZ beneath central Aleutian is azimuthal anisotropy. We are confident that our measurements on anisotropy beneath central Aleutian with the coupling of 0 S11  0T12 are robust.

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The anisotropy of MTZ had been confirmed by shear wave splitting measurements [Wookey et al., 2002; Lynner and Long, 2014b], the azimuthal anisotropy of MTZ was also observed by the normal coupling beneath the Tibetan Plateau [Hu et al., 2013]. However, the relationship between anisotropy and MTZ deformation is still difficult to interpret. 1 Introduction

Subduction zones have active geodynamic processes in Earth’s interior, which attract the attention of researchers continually. Seismic anisotropy presents powerful tool in studying the anisotropy caused by mantle deformations in subduction dynamics [Lynner & Long, 2014]. Seismic anisotropy can give us crucial information for the mantle at depths that cannot access for direct observations [Savage, 1999]. Wave splitting measurement [e.g. Silver & Chan,1991; Vinnik et al., 1992] is the most popular tool in probing the anisotropy. Apart from the wave

splitting, the surface wave dispersions and surface polarization are also used to study the anisotropy in the crust and shallow mantle [Park & Yu, 1993; Forsyth, 1975; Montagner & Tanimoto, 1990]. Schulte-Pelkum et. al. [2001] used the P-wave polarization anomaly investigate the anisotropy around the seismic station. Every availability tool has its own practicality. As the shear wave splitting has high reliability and lateral resolution [Silver & Chan, 1991], and it is often used to probe the anisotropy that characterizes subduction systems [Long, 2013]. Especially, the SKS wave splitting techniques have very high lateral resolution in the upper mantle, they can recognize the lateral variations of anisotropy in the 100km range relative to surface wave with several hundred kilometers. But the most notable drawback for shear wave splitting technique is poor depth resolution, and it usually assumes the anisotropy at different depth is same [Rümpker & Ryberg, 2000; Vinnik et al., 2007].

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The subduction of the Pacific Plate to the North American plate forms Adak Island in the Aleutian arc. The Pacific plate has a speed of 6.5 cm/yr moving beneath to the North American plate, the Aleutian subduction lithosphere age gradually increases from east to west from about 45 Ma to about 80 Ma [Heuret & Lallemad, 2005]. The anisotropy beneath the subduction zone is ubiquitous and the anisotropy beneath central Aleutian subduction has been widely observed and studied by shear wave splitting technique. However, previous studies by observation of S, ScS and SKS splitting show complex anisotropy beneath the central Aleutian arc [Bender et al., 2004; Levin et al., 2006; Lynner & Long, 2014; Nowacki, 2013; Roy et al., 2017]. The fast direction measured by shear wave splitting beneath the central Aleutian arc has some difference by different researchers. On one hand, because splitting in the SKS phase is an accumulation result of anisotropy distributed along the ray path from the core-mantle boundary to the seismic station, it can be thought as a path integrated measurement, and even though multiple phases are selected the depth-resolution of anisotropy is still poor. On the other hand, fast direction variation may indicate that more than one anisotropic system is contributing to the shear splitting signal, the central Aleutian has complex mantle structure.

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Each of Earth’s normal modes has a specific eigenfrequency and only depends on Earth's elastic and density structure. The normal modes can be divided into spheroidal or toroidal according to their particle motion. For a static and spherically symmetric Earth model, the particle motion for toroidal modes are only horizontally polarized, they cannot appear on the vertical component seismic records, but the coupling of fundamental spherical and toroidal (S-T) can cause the toroidal mode signal to appear in vertical recordings. The scattering of energy from toroidal to spheroidal modes will produce an abnormal waveform observed in the vertical component record of a seismometer that is known as a quasi-Love (QL). Studies show that the toroidal modes can couple to neighboring spheroidal modes due to the rotation of the earth, ellipticity, and anisotropy [Dahlen &Tromp, 1998]. When the seismic anisotropy has a horizontal axis of symmetry, we call it azimuthal anisotropy, the observations and theoretical studies both show the S-T coupling is sensitive to the azimuthal anisotropy. Theoretical calculations show that for the frequency lower than 4 mHz the QL wave commonly appears on the vertical seismograms due to spheroidal-toroidal coupling in Earth modal with the azimuthal anisotropy in the mantle [Oda & Onishi, 2001]. QL waveforms can be most efficiently generated when the anisotropy has a horizontal symmetry axis than that with a vertical axes or a lateral variation of isotropic P and S wave speed [Park, 1993]. Strong QL waveform anomalies usually suggest that there is anisotropic structure in the mantle with a horizontal or subhorizontal symmetry axis. The QL waveform will be very weak when the wave

propagation path is parallel or perpendicular to the fast velocity direction, anisotropy with a vertical symmetry axis can hardly generate QL waveform [Yu and Park, 1993, 1994]. Quasitoroidal mode is much more sensitive to the velocity perturbation associated with azimuthal anisotropy than the isotropic perturbation [Oda, 2005]. For frequency higher than 4 mHz, the Quasi-Love waveform anomalies caused by azimuthal anisotropy are often observed [Levin et al., 2007; Kobayashi, 1998]. In practice, Park & Yu, [1994] point out that the Love and Rayleigh conversions below 20 mHz provide strong evidence for the existence of azimuthal anisotropy in the upper mantle, so it is feasible to use low frequency waveform anomalies to study the regional tectonic processes.

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Previous researches on low frequency normal modes coupling always concern on global scale lateral variations in the mantle [Laske & Masters, 1999; Beghein & Trampert, 2003; Deuss et al., 2010], such as the PREM Earth modal uses hundreds of normal modes to constraint the structural heterogeneity [Dziewonski & Anderson, 1981], the effects of local scale anisotropy are less well understood and are often ignored. However, Zürn [2000] argued that the local heterogeneities effectively alter deformation fields, they always cause deviations between observed and synthetic signals. Sharp lateral heterogeneities with a range of hundred kilometers could cause strong normal modes coupling [Neele et al., 1989; Spakman et al., 1988; Snieder et al., 1991]. The S-T coupling due to local azimuthal anisotropy aroused our concern since the work of Hu et al. [2009, 2012]. The strong S-T coupling observed at stations deployed along an equatorial path after the 2005 great Sumatra earthquake suggested the local azimuthal anisotropy in the upper mantle is responsible for the mixed coupling below 4 mHz [Hu et al., 2009]. The observation of normal mode coupling at Taiwan and the Tibet Plateau demonstrated significant S-T coupling can be aroused by local heterogeneity [Hu et al., 2012, 2013]. Low frequency S-T coupling has potential to constrain local anisotropy upper mantle structure.

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As Earth’s rotational Coriolis coupling is ubiquitous at frequency below 4 mHz and often cause the couplings of Sm-Tn under the condition of |m-n|=1 according to the rule of angular selection [Masters et al., 1983; Dahlen & Trump, 1998; Zürn et al., 2000; Park, 1986; Hu et al., 2006], so it is difficult to directly observe the low frequency S-T coupling caused by azimuthal anisotropy. But observations by Hu et al., [2009, 2013, 2016] suggest in the early part records the local azimuthal anisotropy is responsible for the coupling of 0 S11  0T12 , 0 S25  0T25 , 0 S 20  0T21 and 0 S17  0T18 . In this paper we observed strong coupling of 0S11-0S12 due to local azimuthal anisotropy at the vertical component in the early seismic records (20 hr) from stations located at Aleutian zone, we use this S-T coupling pair to constrain the anisotropy and estimate trenchparallel orientation beneath the central Aleutian arc at depth range of 400-700 km.

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2 Observations

The Olyutorsky earthquake with magnitude Mw=7.6 occurred in northeastern Russia on April 20 2006. This great earthquake excited Earth's free oscillation recorded by many seismic stations around the world. We observed strong coupling of 0 S11  0T12 in 20hr vertical component records from 4 broadband seismometers located at the Aleutians arc (Fig. 1) after this earthquake. Earth’s rotation, mantle anisotropy and ellipticity all can lead the spherical modes to couple to its nearby toroidal. For normal modes in the frequency band 1.5-3.5 mHz, Coriolis coupling always play an dominant role for the fundamental S-T coupling [Masters et al., 1983].

Aleutian Islands are located in active areas of geodynamics and complex anisotropy in the mantle which has been confirmed by shear wave splitting [Yang et al., 1995; Long and Silver, 2008; Nowacki et al., 2015; Roy et al., 2017]. The coupling of 0 S11  0T12 due to local azimuthal anisotropy has been well observed from seismic stations distributed in the equatorial region [Hu, 2009]. What is the mechanism for the coupling of 0 S11  0T12 observed in Aleutian arc? To determine which effect was responsible for the coupling of 0 S11  0T12 in the 20hr vertical component records, we expand observation. In addition to 4 seismic stations in the Aleutian arc, we investigate the early vertical component records from other 16 seismic stations, these stations are widely distributed in mainland China, the southern United States, and western Russia (Fig. 1).

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In this paper, we rule out the factor of Coriolis force for the coupling of 0 S11  0T12 after Olyutorsky earthquake by extensive observation. We observe the behavior of 0 S11  0T12 both in frequency domain and time domain and quantify coupling effect by measuring the amplitude of beating envelope. Then, we confirm the local anisotropy can explain spherical mode 0 S11 and toroidal mode 0T12 excited by Olyutorsky earthquake. We focus on investigating the early vertical seismic records from ADK station after few great earthquakes with Mw >7.5. ADK station is a permanent station located at the central Aleutian arc with Streckheisen Seismometer (STS). Low frequency seismic records (1Hz sampling) used in this study come from Incorporated Research Institutions for Seismology (IRIS), IRIS data central provides sufficient quality low frequency data with high signal-to-noise ratios from stations deployed around the world.

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To compare observation data with simulation, we synthesize seismograms for each station in the non-rotation, spherically symmetric, radial anisotropy PREM Earth model [Dziewonski & Anderson, 1981] using normal-mode summation with MINEOS software. The Earthquake mechanism and moment in calculating the synthetic normal modes come from Global CMT (https://www.globalc mt.org/CMTsearch.html). We calculate sensitivity kernels for 0 S11 and 0T12 in a PREM earth model via a web version of MINOS (https://igppweb.ucsd.edu/~gabi/r em.dir /surface/minos.H tml) (Gilbert and Masters).

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2.1 Frequency domain observation of coupling 0 S11  0T12

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Each normal mode has specific frequency. When we check the free oscillation signal at the spectrum of seismic records, they will become apparent. We first present observations of amplitude spectra in vertical seismic recordings from ADK, UNV, SPDT, KDAK after 2006 Olyutorsky earthquake, these 4 stations are located in the central and eastern of Aleutians Islands and cover a range of 3,000 km of island arcs (Fig. 1). The lengths of time series are 20hr. Theoretical amplitude spectra for PREM model are also displayed for comparison. Fig. 2 shows the comparison of data and theoretical amplitude spectra from vertical component of 4 stations. Modes appear as distinct peaks in the frequency domain and are good agreement with the synthetic except 0 S11 mode. All records show a clear peak at the frequency of 1.82-1.89 mHz near the 0T12 mode, while the spectrum of 0 S11 mode show anomalous small amplitudes and a little frequency offset from theoretical frequency. We note that the 0 S11 mode is quite small comparing with its neighboring spheroidal modes 0 S10 and 0 S12 ，the two modes are both in good agreement with synthetic amplitude. The vertical components of 4 seismic stations appear

unexpected small but clear peak near mode 0T12 simultaneously, which suggest that this phenomenon are not caused by instruments with uncertain interference. The synthetic seismograms at 4 stations, computed with radial anisotropy PREM Earth model, all show no sign of significant deviate from observed seismogram around 0 S11 mode. Obviously strong coupling of 0 S11  0T12 occurred at all 4 stations.

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Earth’s rotation, mantle anisotropy and ellipticity can cause the spherical mode to couple its nearby toroidal. The effect for spheroidal-toroidal modes coupling due to ellipticity accounts for less than 10 percent of the toroidal mode signal on the vertical [Zürn, et al., 2000]. Spheroidaltoroidal modes coupling due to ellipticity is very weak and usually can be neglected. We expand observations and investigate records from 16 seismic stations, and these stations are deployed along 4 great circle paths connecting the epicenter of Olyutorsky earthquake and ADK, SPDT, UNV, KDAK. The four great circle paths have an azimuth difference of about ±10° at epicenter (Fig. 1). However, no interference occurred between 0 S11 and 0T12 mode after we carefully check the amplitude spectra of 20hr long vertical records from these 16 stations after 2006 Olyutorsky earthquake. The vertical records with high SNR show no small peak at the frequency of 1.821.89 mHz, which suggest no quasi- 0T12 appeared. Moreover, most observed peaks at 0 S11 mode are well agreed with the theoretical peaks. Close inspection of the spectra from these 16 stations all reveal no strong couplings of 0 S11  0T12 after the Olyutorsky earthquake. Fig. 3 and Fig. 4 show the comparison of observed data and theoretical amplitude spectra from vertical component of these 16 stations.

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We closely inspect the spectra for vertical records from 16 stations located on the four great circle paths (Fig. 1), those four great circles all pass through the epicenter of Olyutorsky earthquake, and they are the direction of the surface wave propagation, no strong coupling of 0 S11  0T12 occurred in the frequency domain. The CRAG, COR, KDAK, MA2, ULN are located at the same great circle, however, the coupling of 0 S11  0T12 was only observed at KDAK deployed on Aleutian arc (Fig. 3, Fig. 4).If the coupling of 0S11-0S12 was caused by Coriolis force, then this phenomenon can be observed in a wide range. The strong couplings are only observed in Aleutian arc suggest the coupling of 0 S11  0T12 cannot be caused by earth rotation. Hence, the most effective mechanism can explain the coupling of 0 S11  0T12 observed in Aleutian arc is anisotropy beneath the stations. Previous studies show that the strong S-T coupling due to anisotropy has the characteristic of small amplitude for spherical mode [Oda, 2005]. The spherical mode has small amplitude near its nodal line and the S-T coupling due to anisotropy can be best observed at the stations near the nodal lines of spherical mode. The obviously nodal nature is a good way to distinguish the azimuthal anisotropy coupling and Coriolis coupling. The 0 S11 modes observed from stations after 2006 Olyutorsky earthquake in Aleutian arc all have small amplitude comparing with its neighboring spheroidal modes 0 S10 and 0 S12 . We also note that 0 S11 has small amplitude at stations COR, MA2, MDJ, MINT, PET, YAK. However, no strong coupling of 0 S11  0T12 was occurred at these stations. The Olyutorsky earthquake had a thrust mechanism with a large s=0 component of the moment tensor, the constant excitation term with source azimuth, the wavelength of 0 S11 is about 3500km. The YAK, MA2, PET, ADK, UNV are located within a radius of 2000km of the epicenter of

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2006 Olyutorsky earthquake ( Fig. 1) and the initial phase of 0 S11 should be consistent at five stations. The behavior for 0 S11 on the vertical component recordings for the five stations makes the anomalous spectral peak at ADK, UNVand SDPT more likely to be caused by a localized structure. Especially, the PET station and ADK station almost locate at a same circle centered on the epicenter of 2006 Olyutorsky earthquake. The distance between two stations is about 1,000 km and the geometric factor for 0S11 mode at two stations are closer, but no coupling of 0 S11  0T12 are identified on the vertical component at PET station. Thus the anomalies strong couplings of 0 S11  0T12 on Aleutian arc are not caused by lateral variation of global mantle structure, but signals of local anisotropy. The strong coupling of 0 S11  0T12 can be clearly observed in amplitude spectral. Our observations above suggest the coupling of 0 S11  0T12 should be caused by a localized structure, however, we conclude that some stations may show weaker coupling of 0 S11  0T12 that cannot be observed in the amplitude spectral. Such as the spectral for ENH station (Fig. 3), we can observe a faint spectrum peak near the 1.85mHz. It is also difficult to quantify coupling effect only by amplitude spectral. Time-domain analyze is requisite for observe the behavior for 0 S11 , as it can exhibit oscillations envelopes "beat" from the imperfect resonance of the quasi-spheroidal and quasi-toroidal modes.

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2.2 Time-domain observation of 0 S11  0T12

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Fourier transform is a basic tool to analyze frequency characteristics for time series, and it is easy to identify normal modes in the frequency domain as modes appear as distinct peaks. However, only spectral amplitude analysis may be insufficient for normal mode analysis, splitting and coupling of some normal modes may be indistinguishable but will be clear when isolating normal modes to time domain with narrow band filter. Such as the splitting of 0T3 and 0T4 excited by 1960 Chilean earthquake are not obvious in frequency domain and are confirmed in time domain [Stein and Geller, 1978]. It is difficult to quantify the coupling effect according to spectral amplitude. If two waves with close frequencies oscillating in the same direction, then a beat envelope will be created with a frequency equal to the difference between the two original frequencies. For the Earth’s free oscillation, when mixed mode coupling is isolated in frequency, the beating envelope may be taken as evidence of modal coupling [Park, 1990].

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In this paper, we isolate the coupling pair 0 S11  0T12 from vertical records and observe the behavior of the coupling pair in time domain. We isolate envelope of coupled 0 S11  0T12 with narrow band filter and then remove the decay to observe the beating caused by the mixed coupling. We quantify coupling effect by measuring the amplitude of beating envelope. We observe the behavior of 0 S11  0T12 for vertical records from YSS, YAK, TIXI, PET, ADK, ADPT, UNV, WMQ, WUS and ENH. The ten stations are deployed around the epicenter of the Olyutorsky earthquake. Fig. 5 shows the result for the time domain signal in frequency range 1.82 mHz -1.9 mHz with a band-pass filter and observe the behavior of 0 S11  0T12 . Then we calculate the envelope for the time signal with Hilbert Transforms [Kak, 1970]. We normalize the amplitude for the time signal and quantify coupling effect by measuring the amplitude of beating envelope. The theory beat frequency is the absolute value of the frequency difference of 0 S11 and 0T12 . As the frequency difference for 0 S11 and 0T12 is about 0.004 mHz, We obtain the beating envelope by remove decay

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from envelope with a band pass filter for carrier frequency 0.004 mHz. The ADK, UNV and SDPT stations present strong coupling of 0 S11  0T12 in frequency domain, beating envelope for those stations are also strong with the beating envelope amplitudes 0.283 0.241 0.1554 respectively. The YAK, ENH and YSS stations show no clear coupling of 0 S11  0T12 in frequency domain, however, small beating envelope can be observed after remove decay, their beating envelope amplitudes are 0.05 0.113 0.02 respectively. The PET and TIXI stations are located close to AKD, and their envelopes correspond to simple decaying sinusoids with beating envelope amplitudes 0.007 0.018 respectively. Our observations of the coupling pair of 0 S11  0T12 in time domain demonstrate the coupling is a local phenomenon. Two stations deployed at close range can vary greatly for 0 S11 and 0T12 such as PET ADK. Time domain analysis for coupling pair represents a good way to estimate the effect of coupling according to its beating envelope. 2.3 Observation at ADK station after other events

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Our expand observations show the Coriolis force is not responsible for the couplings of 0 S11  0T12 after the 2006 Olyutorsky earthquake. The local anisotropy should be the dominant mechanism for the strong coupling of 0 S11  0T12 observed in Aleutian arc, however both the azimuthal and radially anisotropy structure can cause S-T coupling. Theoretical studies show that the S-T coupling can be most efficiently generated by the anisotropy with a horizontal axis of symmetry rather than that possessing vertical axes of symmetry [Park & Yu, 1992; Park, 1993]. As a onedimensional model with vertical-axis anisotropy will allow the toroidal and spheroidal equations to separate, meaning that there will be no coupling in their synthetics [Park & Yu, 1992], our synthetic seismograms for the four stations in Aleutian also show no sign of coupling of 0 S11  0T12 . The intensity of S-T coupling due to azimuthal anisotropy coupling is strong related to the angle between the symmetry axis of anisotropy and the propagation path [Yu &Park, 1993, 1994]. We will further confirm the azimuthal anisotropy plays a leading role on the coupling of 0 S11  0T12 observed at ADK station according to the azimuthal characteristic of the coupling of 0 S11  0T12 .

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It is difficult to directly observe the mixed coupling due to azimuthal anisotropy; three preconditions must be satisfied simultaneously for a station: local azimuthal anisotropy, appropriate azimuthal arrival and small vertical amplitude for spherical mode. The spectral peaks do not correspond to simple decaying sinusoids, but rather to oscillations whose envelopes beat from the imperfect resonance of the quasi-spheroidal and quasi-toroidal modes, when mixed mode coupling can be clearly observed in frequency, then the beating envelope may be taken as evidence of modal coupling [Park, 1990]. Spectral analysis and time-domain analysis are all need to evaluate effect of mixed coupling pair. To analyze the effect of local azimuthal anisotropy on the couplings of 0 S11  0T12 , we search for more events and focus on the ADK station, a permanent station that has been working continuously for many years and can provide high SNR seismic data. We analyze the vertical records at ADK station after 11 great earthquakes (Mw> 7.5) with different azimuth to ADK, and eventually the couplings of 0 S11  0T12 are again observed after 2013/09/24 Mw=7.7 Pakistan earthquake and 2017/07/17 Mw=7.8 Nikol’skoye earthquake (Fig. 6). Pakistan earthquake occurred in South Asia is thrust mechanism and the Nikol’skoye earthquake occurred on Bering Island is strike slip mechanism.

Fig. 7 shows the amplitude spectra for the vertical component records from ADK station after 2013 Pakistan earthquake and 2017 Nikol’skoye earthquake, with a feature of small amplitude for 0 S11 mode. We note that the azimuth of two earthquakes are 310° and 290° relative to ADK station respectively, which are close to the azimuth of Olyutorsky earthquake and the later have a azimuth of 322°. We select plenty of events to investigate the coupling of 0 S11  0T12 at ADK station, but only three earthquakes excite strong coupling of 0 S11  0T12 , they both have a similar azimuth relative to the ADK stations. Our observations demonstrate the existence of local azimuthal anisotropy beneath the ADK station.

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We also investigate the vertical records from 6 stations that around ADK station after Nikol’skoye earthquake (Fig. 6). Fig. 8 shows the amplitude spectra of vertical records after Nikol’skoye earthquake from stations YSS, PET, MA2, UNV, SDK and SPIA. The six stations are distributed around ADK station. The amplitude spectra of vertical records from those six stations all show clear spectral peaks at 0 S11 mode. No interference occurred between 0 S11 and 0T12 mode in the amplitude spectra. PET, UNV, SDK and SPIA are very close to ADK, however, after carefully check the amplitude spectra of 20hr long vertical records from these four stations we don’t identity the coupling of 0 S11  0T12 . Similar to the 2006 Olyutorsky earthquake, The distance between ADK and PET is about 1,000 km and the geometric factor for 0 S11 mode at two stations are closer, but no coupling of 0 S11  0T12 are identified on the vertical component at PET station. 3 Discussion

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Previous studies show that for the frequency band 1.5-3.5 mHz Coriolis coupling always plays an dominant role for the fundamental S-T coupling [Masters et al., 1983]. However, we rule out the effect of Earth’s rotation on the strong coupling of 0 S11  0T12 in vertical seismic recordings at ADK station by extending observations to nearby stations. The strong coupling of 0 S11  0T12 only occurred on the local area that around ADK station suggests that the local azimuthal anisotropy beneath ADK station is the major cause for generating toroidal mode signal on the vertical records. Cross couplings due to local azimuthal anisotropy are not observed for the first time. Strong couplings of 0 S25  0T25 , 0 S 20  0T21 and 0 S17  0T18 associated with local anisotropy are widely observed at 18hr vertical spectra from stations located at Taiwan and Tibetan plateau [Hu 2013， Hu et al., 2009]. Coupling of 0 S11  0T12 is observed at near-equatorial stations after 2004 Sumatra earthquake, which suggest the local azimuthal anisotropy in the mantle is the most effective mechanism for the anonymous coupling [Hu et al., 2008]. Significant coupling of 0 S 15  0T16 and 0 S 20  0T21 were observed at ASCN station, which suggest the asthenosphere beneath the stations may contain azimuthal anisotropy, and the azimuth anisotropic structure may be formed by the related plate movement and mantle density change [Behn et al., 2004]. Every normal mode has its corresponding energy distribution in Earth’s interior. Using MINOS software we calculate the sensitivity kennels for 0 S11 and 0T12 with PREM earth model. For the mode 0 S11 , the peak of compressional energy density locates at depth range about 350-700 km, the 0T12 mode has a great sensitivity to the region of upper mantle. The coupling of 0 S11  0T12 is sensitive to six elastic parameters describing azimuthal anisotropy in the upper mantle, and the azimuthal anisotropy lies deeper than 400km depth [Beghein et al., 2008]. So we infer that it is the local anisotropy

that causes strong coupling of 0 S11  0T12 and the maximum likelihood depth range of azimuthal anisotropy beneath the ADK station is 400-700km. The mixed couplings of free oscillation due to azimuthal anisotropy are sensitive to the symmetry axis of azimuthal anisotropy (orientation of the fast velocity axis). Assuming the orientation of a horizontal symmetry axis is w0, and when the w0 is at 45° strike to the great circle the strength of S-T coupling will be maximal, no strong coupling occurs if the fast direction is parallel or perpendicular to the great circle path connecting epicenter and station [Park, 1997].

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As the strength of coupling depends on the azimuth of the source-receiver great circle, we can roughly estimate the fast direction beneath the central Aleutian according to coupling of 0 S11  0T12 excited by numerous direction earthquakes distributed around ADK station. Couplings of 0 S11  0T12 at ADK station are noticeable after 2006 Olyutorsky earthquake, 2013 Pakistan earthquake and 2017 Nikol’skoye earthquake. The azimuths of the three great circle paths connecting epicenters of those earthquakes at ADK station are 322°, 310° and 290° respectively. The three great circle intersect about -53° azimuth at ADK station (Fig.5). Even though we investigate lots of earthquakes, we observed strong coupling of 0S11-0S12 at ADK station only after that three events. According to the azimuthal dependence that angle at about 45° the fundamental S-T is strongest, we thus infer the azimuthal anisotropy in the depth 400-700km have a symmetry axis nearly parallel to the plate motion direction or parallel to the trench. The fast axis are roughly constrained to East-West line or North-South line beneath the central Aleutian subduction, The black dashed lines in Fig.6 are the possible fast orientations constrained, which orientation is the most likely fast axis direction?

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Observation of shear splitting suggested the seismic anisotropy is ubiquitous beneath the central Aleutian subduction zone. Using data from a long-running seismic observatory on the Adak Island, Bender et al., [2004] carried out a study of birefringence in 41 core-refracted shear waves (SKS, SKKS, PKS) observed between 1994 and 2003 and inferred the fast directions in the 120°150° SE range, the fast shear wave speed direction of ~130° SE appears to be very robust. The birefringence in core-refracted teleseismic shear waves (SKS) in the central Aleutians using data from ADK suggests that the fast polarization of shear waves in the central Aleutians Islands is closely aligned with oblique convergence between Pacific and North American plates [Levin et al.,2006]. Nowacki[2013] estimated the fast direction with SKS splitting measurements at ATKA station that located in the central Aleutian, the result show the fast direction is dominantly 17° North-East beneath central Aleutian arc, however, the source side shear wave splitting results for deep earthquake beneath central Aleutian show the fast orientation at deeper than 200km is closely to trench parallel [Nowacki et al., 2015]. Observation of wedge anisotropy with sourceside splitting measurements in Aleutian subduction zone indicates trench parallel or oblique fast direction at depth of about 100km, the sub-slab mantle beneath Aleutian exhibits trench parallel fast direction at depth of about 300km[Long and Silver 2008]. Lynner and Long[2014] analyzed the anisotropy beneath the center Aleutian subduction zone with source-side shear wave splitting technique and found that the fast polarization azimuths(FPA) are scattered between 175° E to 160° E, but are roughly parallel to the motion of the Pacific plate. Wave splitting measurements by Roy show an abrupt change in FPA (fast polarization azimuths), two anisotropy orientations are obtained for the central Aleutian subduction: one along the plate motion direction (NW-SE) suggesting 2-D entrained flow and the other orthogonal (NE-SW) to it. That implies that flow in the sublab mantle beneath Adak Island is complex [Roy et al.,2017]. There are some deviations for fast orientation measured by shear wave splitting conducted by different researchers beneath

central Aleutian arc. The most popular phase used in the shear wave splitting technique is SKS. SKS measurements have poor depth resolution and reflect anisotropy anywhere along the raypath from CMB to station. The source-side shear wave splitting technique uses teleseismic S phase come from earthquakes that occurred in the subducting slab and measured at distant stations; it has good depth resolution but has high dependence on the depth of event. The potential contamination of the source-side signal from receiver-side anisotropy must be given special concern [Lynner & Long, 2014a].

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According to the simplest LPO model, the fast splitting orientation is roughly parallel to the motion of the downgoing plate (Trench-normal). However, observations from many subduction zones deviate from the simple prediction; most subduction systems exhibit a complicated mix of fast splitting directions [Long, 2013]. The coupling of 0 S11  0T12 is sensitive to the depth range 400-700km, which is the depth of transition zone. Roy et al. [2017] observed trench-parallel anisotropy orientation in the transition zone for the Izu-Bonin subduction zones. Nowacki et al. (2015) rule out the LPO of transition zone minerals as the likely case of anisotropy. They suggest a highly anisotropic phase of dense hydrous magnesium silicates or a hydrated layer is the likely cause of anisotropy in the transition zone. The trench-parallel fast azimuth beneath the central Aleutian arc may be associated with trench-parallel maximum elongation directions due to deflection around the sides of the deep slab. We are inclining to think that the strong couplings of 0 S11  0T12 may be caused by the trench-parallel anisotropy for the MTZ of central Aleutian.

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Tomographic images of shear-wave show that between 164E and 173E beneath the Aleutian arc there is a slow seismic velocity area relative to surrounding mantle, this speed anomaly area forms a “slab portal” and may facilitate the production of adakites [Levin et al., 2005]. Adak Island is the home of unusual adakite volcanism that has been associated with the melting of the subducting oceanic crust at the lateral edge of the Pacific slab [Yogodzinski, 2001]. ADK station lies in an area that has unusual plate subduction and the “slab portal” lies further west of ADK, the coupling of 0 S11  0T12 observed at ADK may also affected by the “slab portal”. The edge of the Pacific slab might lead to unusual strain in the mantle subduction zone beneath ADK. Although this correlation without a clear explanation from geodynamic models. More observations are needed in the future to explore the correlation between this phenomenon and slab edge.

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

]Acknowledgments We would like to thank IRIS for the seismic data used in this paper. We use the GMT software to prepare the figures (Wessel and Smith, 1991), we also thank the National National Sciences Foundation of China (grants Nos 41174022,41321063, 41021003, 41674100 and 41374029).

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Fig. 1. Distribution of seismic stations and event used in this paper. The black dashed lines indicate the great circles connecting the epicenter of 2006 Olyutorsky earthquake and stations of ADK, SPDT, UNV, and KDAK.

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Fig. 2. Comparison of amplitude spectra of 20hr vertical records from four stations after 2006 Olyutorsky earthquake. The couplings of 0 S11  0T12 are clearly identified at all 4 stations. The synthetic spectrum (dashed blue line) are computed by the normal modes summation. The observed data are Hanning windowed before spectral analysis and the starting time of records is 0 hr after Olyutorsky earthquake. The vertical dashed lines are the theoretical frequency for modes calculated by PREM Earth modal [Dziewonski and Anderson, 1981].

Fig. 3. Amplitude spectrum of 20hr long vertical component from seismic stations located at the four great circle paths after 2006 Olyutorsky earthquake. No strong couplings of 0 S11  0T12 are identified. The synthetic spectrum (dashed blue line) is computed by the normal modes summation. The observed data are Hanning windowed before spectral analysis and the starting time of records is 0 hr after Olyutorsky earthquake. The

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vertical dashed lines are the theoretical frequency for modes calculated by PREM modal [Dziewonski and Anderson, 1981].

Fig. 4. Amplitude spectrum of 20hr long vertical component from seismic stations located at the four great circle paths after 2006 Olyutorsky earthquake. No strong couplings of 0 S11  0T12 are identified. The synthetic spectrum (dashed blue line) is computed by the normal modes summation. The observed data are Hanning windowed before spectral analysis and the starting time of records is 0 hr after Olyutorsky earthquake. The vertical dashed lines are the theoretical frequency for modes calculated by PREM modal [Dziewonski and Anderson, 1981].

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Fig. 5 Data in time domain filtered from vertical records from YSS, YAK, TIXI, PET, ADK, ADPT, UNV, WMQ, WUS and ENH using narrow band filter with band-pass frequency range of 1.82-1.9 mHz. The red line is envelope of the time series. The yellow line is beating envelope after remove decay with a band pass filter with frequency 0.0035-0.01 mHz. The start time for the data is 5 hours after earthquake; we cut 5 hours long records to avoid the interferon by edge effect caused by filer.

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Fig. 6 Distribution of seismic stations and events used in this paper. The numbers 1, 2, 3 represent the earthquake epicenters of Olyutorsky, Nikol’skoye and Pakistan respectively. The red dashed lines are great circles paths connecting the epicenter and ADK station. The white arrows indicate the motion of the subduction. The black dashed lines are the possible fast orientations constrained by the coupling of 0 S11  0T12 .

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Fig. 7 Amplitude spectrums of 20 hr long from vertical component of ADK station after Nikol’skoye and Pakistan earthquake. The left Fig. is amplitude spectrum of vertical record after Nikol’skoye earthquake and the right Fig. is amplitude spectrum of vertical record after Pakistan earthquake. Obviously the couplings of 0 S11  0T12 are clearly identified after both events. The synthetic spectrum (dashed blue line) is computed by the normal modes summation. The observed data are Hanning windowed before spectral analysis and the starting time of records is 0 hr after Nikol’skoye earthquake. The vertical dashed lines are the theoretical frequency for modes calculated by PREM modal [Dziewonski and Anderson, 1981].

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Fig. 8 Amplitude spectrum of 20 hr long vertical component from seismic stations YSS PET MA2 ADK UNV SDK SPIA after Nikol’skoye earthquake. The six stations located around the ADK station, but no strong couplings of 0 S11  0T12 are identified. The observed data are Hanning windowed before spectral analysis and the starting time of records is 0 hr after Nikol’skoye earthquake. The vertical dashed lines are the theoretical frequency for modes calculated by PREM modal [Dziewonski and Anderson, 1981].

Table 1. The fast orientation measured by different researcher for central Aleutian Primary sources

Type of data

Bender et al.,2004

SKS, SKKS, PKS

fast axis

depth

~130° SE

NON

Levin et al.,2006

SKS

oblique

NON

Nowacki,2013

SKS

~17° N-E

NON

Nowacki et al.,2014

S

trench parallel

Long and Silver,2008

S

Long and Silver,2008

S S

Roy et al.,2017

S

trench parallel

about 100km about 300km

trench perpendicular

NON

complex

coupling of 0 S11  0T12

NON

trench parallel

400-700km

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This paper

trench parallel or oblique

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Lynner and Long,2014

more than 200km