Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

Sun et al. 10.1073/pnas.0609143103.

Supporting Figure

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Supporting Figure 5
Supporting Figure 6
Supporting Figure 7
Supporting Figure 8
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Supporting Figure 10

Supporting Figure 5

Fig. 5. Accumulated amplitudes of perturbations of AB-DF differential times as a function of depth from the surface to the core-mantle boundary. The perturbations are calculated by tracing the rays of our data through a tomographic model. (We use Grand's S tomographic model in this calculation.) Plotted are the averages for the ray paths with the total accumulated AB-DF residuals greater than 2 s (solid line), 1 s (dotted line), and 0.5 s (dashed line). The steep increases of the perturbations in the lowermost mantle (mostly the bottom 500 km or so) are the results of (i) greater effect of mantle heterogeneity on the AB travel time than on the DF travel time as they are well separated in the lowermost mantle and the AB travels a longer path, and (ii) increased level of heterogeneity toward the lowermost mantle in tomographic models.

Supporting Figure 6A

Supporting Figure 6B

Fig. 6. Examples of seismograms of PKP(DF) and PKP(AB) waves. (A) The seismograms are from two events in similar locations in South America recorded at China Seismographic Network (CSN): March 15, 2001; lat 32.32°S, long 71.49°W; 37 km; m_b 5.6 (event 1); September 24, 2002; lat 31.52°S, long 69.20°W; 119 km; m_b 6.2 (event 2). The traces are first aligned with the DF arrivals (Left), and then the AB arrivals (Right) are shifted by using the predicted AB-DF times for PREM [Dziewonski AM, Anderson DL (1981) Phys Earth Planet Inter 25:297-356], thus the timing difference of the AB arrivals relative to the zero line represent the AB-DF residuals, i.e., if we assume all of the differential time residuals are attributed to the AB phase, the time shifts of the AB phase represent the AB travel time anomalies relative to PREM. A rapid travel-time change of more than 2 s is clearly visible from azimuth of about -40° to -30°. Some of the AB waveforms (highlighted dark traces) are more complex, compared with the corresponding DF waveforms or the AB waveforms of the same event at other stations. (B) More examples of seismograms of the PKP(DF) and PKP(AB) phases recorded at the CSN from a deep event: April 28, 2003; lat 8.20°S, long 71.59°W; 559 km (event 3). The traces are plotted in the same way as in A. The AB ray paths at azimuths -20° to 20°, which sample the lowermost mantle further to the north than those of events 1 and 2 in A, show even faster arrivals. This is consistent with velocity increase in the lowermost mantle from south to north (CC′ to AA′) under the Central America and the Caribbean. The AB waveforms are much more variable than the corresponding DF waveforms (the more complex AB traces are highlighted).

Supporting Figure 7

Fig. 7. Observed AB-DF residuals (relative to PREM) and predicted perturbations using Grand's S model for the same ray paths as in this study. The linear regression is 0.520 ±0.25 (1 SD) for the slope and 0.47 ± 0.03 for the intercept.

Supporting Figure 8

Fig. 8. Predicted travel-time perturbations for the 3D reference model (Grand's tomographic model scaled by 0.52) for the ray paths from South American earthquakes to the CSN stations. (A) The total AB-DF perturbations through the whole mantle (both the source and station sides) have only a small correlation with cross-correlation coefficient (CC) of -0.25 with the corresponding DF perturbations. (B) The total AB-DF perturbations correlate very well with the corresponding AB perturbations (CC = 0.89), suggesting most of the AB-DF anomalies come from the AB ray paths. (C) The contributions to the total AB-DF perturbations from the source side of the mantle (under the Americas) and the station side of the mantle (under Asia and Western Pacific). Most of the azimuthal variation for azimuths -90° to 0° comes from the American side.

Supporting Figure 9

Fig. 9. Model of P velocity perturbations proposed in this study. It consists of four layers covering the depth range of 2,200 km to the core-mantle boundary. Only the regions inside the dashed lines are constrained by our data; the areas outside are from the reference 3D model.

Supporting Figure 10

Fig. 10. North-south velocity change in the lowermost mantle under the Central America fast anomaly across the profile CC′ to profile AA′ in Fig. 3A. Plotted are observed residuals (filled circles) and predictions (open circles) as a function of latitude at the middle of the AB segment in the D″ region under the Central America. The ray paths are from South America to China at azimuths of -30° to 0°. The longitudes of the AB segments range from -89° to -67°. The observed residuals have been corrected for the Asian side of the mantle using the 3D reference model and the predictions are for the American side of the mantle only using our P model.