Measurements of the derivative of the p wave travel time curve by means of an array network

Abstract

Data from the four UKAEA seismic arrays have been combined to measure the slope of the P-wave travel time curve (dT/d Δ , or slowness) of events occurring at distances Δ = 30 to 104. Slowness is the quantity that enters into any calculation of the compressional velocities to give the main source of direct detailed information regarding the mantle. Multiple regression analysis was used and corrections, which are azimuthally dependent, estimated to correct for the near surface geology under the arrays. By using all available events and removing the bias introduced by the array geology, the slowness-distance curve should represent the best average for the world. Anomalous features in the slowness curve occur at distances of around 35-36, 48-49 , 60 , 68-70 and 84-85 . These correspond to high velocity gradients within the lower mantle near the depths of 900, 1200, 1550, 1900 and 2500 kms., and support the hypothesis that the mantle is inhomogeneous at depth. A comparison is made between these features and regions considered to be inhomogeneous found at similar distances by other studies. The site corrections obtained for each seismometer are attributed to inhomogeneities in the sub-array geology. The correct ions derived for the arrays situated in Canada (YKA) and India (GBA) are small in magnitude and show the crustal layering to be essentially horizontal. The corrections at the array in Scotland (EKA) are shown to be related to the relative altitudes of the instruments. A velocity of 2.94 km/sec. was derived for the velocity in the top 170 m of crust. The corrections for the Australian array (WRA) show that to a first approximation, the layering in the crust is dipping at 3.6 in the direction N195 E. The corrections have a large azimuthal component and show the presence of an anomaly near the cross-over point of the array.