Seismic velocity structure at Deep Sea Drilling Project site 504B, Panama Basin: Evidence for thin oceanic crust

Abstract
We present an analysis of wide‐angle reflection/refraction data collected in the immediate vicinity of Deep Sea Drilling Project hole 504B in the Panama Basin, currently the deepest drill hole (1.288 km) into oceanic crust. The data were acquired with a 1785 inch3 air gun array and fixed‐gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths. Near‐normal‐incidence, multichannel seismic (MCS) reflection data were acquired simultaneously. Observed P and S wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness. Comparison of the travel times and amplitudes of the P and S wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method. Forward modeling shows that in contrast to standard oceanic velocity models, a velocity‐depth profile that better explains the observed data is characterized by high‐velocity gradients (up to 0.6 km/s/km) in the middle crust, a 1.8‐km‐thick low‐velocity zone (Vp = 7.1–6.7 km/s) immediately above Moho, and a total crustal thickness of only 5 km. Interpretation of the high‐velocity gradients in the middle crust is constrained by the observation of P wave amplitude focusing at ranges of 16–19 km. Although not as well developed in comparison to the P wave arrivals, S wave arrivals show similar focusing. Total crustal thickness is constrained by the combined interpretation of a P wave, wide‐angle reflection event observed at a range of 16–28 km and an MCS reflection event with a crustal travel time of 1.4–1.5 s. Although these events cannot be directly correlated, their travel times are consistent with the assumption that both have a common origin. Amplitude modeling of the wide‐angle event demonstrates that these events are generated at the Moho.