We estimated the QScS values to be 324 ± 34 and 218 ± 20 at PATS and CTAO, respectively, for the 1996 event, which is the event used by Gomer and Okal (2003). These values are close to the value of 366 (error bar from 253 to 634) at PATS and 177–200 at CTAO, as obtained by Gomer and Okal (2003). The relatively small difference between the two studies is due mainly to difference in of the methodologies of the two studies, such as a time-windowing method.
Since there was no previous QScS study beneath the OJP in the past, except for Gomer and Okal (2003), we compared QScS values obtained in previous studies with those of the present study in broader regions, including the OJP. Sipkin and Jordan (1980) and Chan and Der (1988) estimated the QScS values from Fiji–Tonga events to central Japan as 173 ± 37 and 214 ± 42, respectively, which are higher than the average QScS value of 156 in the western Pacific (Sipkin and Jordan 1980). The QScS values obtained by the present study (309 ± 55) are even higher than those obtained by previous studies, probably because multiple ScS waves analyzed by the two previous studies sampled a broader region from the Fiji–Tonga region to Japan, including the OJP, than the present study. The QScS beneath the OJP is close to those beneath the stable continents of 280–333 (Chan and Der 1988; Revenaugh and Jordan 1991; Sipkin and Revenaugh 1994). The QScS values beneath the Solomon subduction zone and the Coral Sea are 100–200, which is significantly lower than those of the OJP. Very low QScS (109) are observed at the COEN station in the present study, probably because the multiple ScS phases travel long distances in the tectonically active region of the Papua New Guinea.
While it is difficult to estimate Qs of the upper and lower mantle separately from multiple ScS waves of nearly vertical paths, the spectral ratio and travel time difference of sScS and ScS waves bear information on the Qs and S velocity anomaly in the upper mantle above the 2016 event (Additional file 1: Table S1, Figure S1). The average Qs in the upper 500 km above the source is estimated to be 98 ± 15, which is close to the Qs of 104 for the PREM. The travel time residual of sScS–ScS is 5.5 ± 0.6 s at stations located toward the OJP from the source, which corresponds to S velocity lower by 2.4% than that of the IASP91 model in the upper 500 km of the 2016 event. However, these values may not represent the upper mantle beneath the OJP, because the Qs and travel time residual from the sScS and ScS pairs are affected by the Papua New Guinea and Solomon Islands subduction zones with presumably strong lateral heterogeneities. Next, we referred to the existing Qs model of the lower mantle to estimate the Qs of the OJP upper mantle from the QScS value of 309 using a ray theory. Using the PREM Qs value of 312 for the lower mantle, the Qs of the upper mantle is estimated to be 303. Since the LLSVP is seated in the OJP lower mantle, the effect of the LLSVP on attenuation should be taken into consideration. Konishi et al. (2017) determined one-dimensional Vs and Qs models at depths greater than 2000 km using a waveform inversion technique beneath the western Pacific region, including the OJP. The Qs values beneath the OJP are approximately 260 at depths from 2000 km to 2850 km and 216 at depths from 2850 to the core–mantle boundary. Konishi et al. (2017) attributed the low Qs values to thermo-chemical anomalies of the Pacific LLSVP. Using the Qs of 260 at depths from 2000 km to the core–mantle boundary and the Qs of PREM in the rest of the lower mantle, we obtained a Qs value of 367 for the OJP upper mantle. The Qs in the OJP upper mantle is higher than those computed from one-dimensional models, such as the PREM (134) and the laterally averaged SEMUCB-WM1 model (130, Karaoğlu and Romanowicz 2018). Considering the similarity of the QScS values beneath the OJP by the present study and those of stable continents reported in previous studies, as mentioned above, we estimated the upper mantle Qs values by a ray theory beneath the continents from the QScS values obtained in the previous studies (Chan and Der 1988; Revenaugh and Jordan 1991; Sipkin and Revenaugh 1994) by assuming a lower mantle Qs to be that of PREM and compared these values with the Qs estimated for the OJP upper mantle. The QScS of 280–333 for the stable continents resulted in an upper mantle Qs of 244–395, which is close to the Qs of the OJP upper mantle estimated above (303 and 367). The upper mantle Qs of the OJP is in the estimated range of those for stable continents.
The travel time residuals of multiple ScS waves are 6.0 ± 0.8 s, which indicates that the average velocity of the entire mantle beneath the OJP is low. The LLSVP is located in the lower mantle at depths from 1000 km to the core–mantle boundary, and the observed positive residuals should be attributable, at least partially, to the low velocities of the LLSVP. We examined how large residuals can be explained by the LLSVP by calculating the theoretical residuals with the three-dimensional S velocity model S40RTS (Ritsema et al. 2010) using a ray theory. Although a geographical pattern of the observed residuals is reproduced in the theoretical residuals (largely positive residuals beneath the OJP and less positive residuals outside the OJP), the theoretical residuals are 3.3 ± 1.1 s, which is approximately half of the observed residuals. While the upper mantle of the S40RTS model has strong high-velocity (1.5–3% in the shallowest 100 km) and low-velocity anomalies (− 1 to − 3% at depths from 200 km to 300 km), the net effects on multiple ScS waves are negligible, because the effects are canceled for nearly vertical paths of the multiple ScS waves. The theoretical residuals are therefore mainly due to the broadly low-velocity anomalies of the LLSVP. Assuming that the remaining 2.7 s occurs in the entire upper mantle and the top 300 km depths, the velocity anomalies are − 0.9 ± 0.4% and − 1.9 ± 0.8%, respectively. There are two ways to interpret the remaining positive residuals. One is the positive residuals caused by the low-velocity zone in the upper mantle obtained by Richardson and Okal (2000), and the other is the positive residuals due to underestimated correction of the LLSVP. Amplitudes of velocity anomalies estimated by seismic tomography are well known to depend on the parameterization and regularization used in tomography. Some models have velocities as low as approximately − 2.5 to − 3% in the LLSVP (e.g., Lu and Grand 2016), whereas the S40RTS has velocities as low as approximately − 1.5 to − 2%. Determining the velocity structure in the upper mantle using multiple ScS studies is difficult. Seismic tomography using data from the OJP array is expected to provide a tight constraint on the velocity structure beneath the OJP.
In summary, the QScS in the mantle beneath the northern OJP is estimated to be 309 ± 55, which is consistent with the result of Gomer and Okal (2003). This is higher than the average QScS in the western Pacific and that calculated from global one-dimensional Qs models and is close to the QScS beneath stable continents. Assuming the lower mantle Qs based on existing Qs models with LLSVP effects accounted for, the seismic attenuation in the upper mantle is probably weak beneath the OJP, as is that of stable continents. The travel times of multiple ScS waves are as large as 6 s. While the positive residuals are at least partially explained by the effect of the Pacific LLSVP in the lower mantle, it is difficult to conclude whether the low-velocity zone in the upper mantle is required to explain the positive residuals, which remains to be concluded by seismic tomography using data from in situ stations such as the OJP array.