The effects of deep anomalies on upper mantle structures resolved by ACH-based body wave teleseismic tomography method

Teleseismic body wave tomography beneath a profile of portable seismic stations using the ACH method (named after authors Aki, Christoffersson, and Husebye) is generally based on relative residual data from teleseismic earthquakes. The relative residuals are inverted to retrieve the two dimensional structure of the velocity perturbation relative to a spherical reference Earth model (for example, IASP91) in the structure of interest beneath the profile. This method tries to minimize the influence of extraneous factors, such as errors in earthquake location or origin time and ray paths from the source to the base of the target volume, by subtracting the mean of the arrival-time residuals for each event, since only the velocity deviations in the target model are investigated. The data are then corrected for crustal travel-time variations a perior inversion. Because travel time perturbation reflects the velocity perturbation integrated along the ray path, some perturbations in the target model may be caused by deeper structures in the upper mantle. This paper intends to study whether the de-meaning process used in the ACH method can remove the effects of deeper mantle anomalies (especially those located directly underneath the target region) or deeper heterogeneities that may leak into the velocity structure of the region of interest. Therefore, considering some different hypothetical velocity structures, including positive and negative anomalies (with relatively high and low velocities, respectively), this study targets an area approximately 1% immediately underneath the base of the model at a depth of 460 to 660 km in an attempt to determine how the velocity structure of the upper mantle beneath a profile would be affected by the presence of possible anomalies in greater depths. Hypothetical tests were applied using teleseismic data recorded in a profile across the Zagros collision zone. The Zagros seismic experiment comprised 66 short-period, medium- and broad-band stations deployed along a NE-SW transect from Bushehr to Posht-e-badam in the southwestern part of Central Iran between November 2000 to April 2001. The profile is believed to be almost perpendicular to the main tectonic units of the Zagros collision zone. For the target model, a simplified P-wave structure to a depth of 460 km based on the tectonic observations and previous tomographic results consisting of two relatively high and low velocity anomalies of approximately 3% at depths of 120 to 300 km, respectively, beneath the Zagros zone and Central Iran were embedded within the reference Earth model. These two anomalies were separated by a sharp sub-vertical transition. Using the target model structure and postulating different anomalies underneath the base of the model, the relative residuals were inverted. The results indicate that these hypothetical heterogeneities in the mantle below the base of the model leads to some effects in the velocity structure of depths lower than 300 km, which have lower resolution. These effects could be attributable to insufficient resolution of the target model at these depths due to a low number of criss-crossing rays. Moreover, there are some deviations in depths of 120 km up to Moho. However, all models retrieve the major features (including transitions and major blocks) available in the hypothetical model, albeit with underestimated amplitude due to the regularization parameters and model parameterization.