Sensitivity tests. (a) Results of linear sensitivity analysis. Site numbers are indicated above black triangles. (b) Variation in RMS misfits by further sensitivity analyses for each case. (c) Sensitivities of the TM phase at sites 001 and 111 by further sensitivity analysis for the cases where blocks X1 and Y are replaced. (d) Sensitivities of the TM phase at site 102 by further sensitivity analysis for the case where block X2 is replaced.