D" Discontinuity Structure Beneath the North Atlantic Based on Observations from the Deep 2010 Spanish Earthquake

Abstract:

The D" discontinuity is typically observed as an increase in P- and S-wave velocity of 1-3% roughly 150-300 km above the CMB. The discontinuity shows strong laterally variability in spatial location and height above the CMB. Previous studies have revealed strong evidence for the existence of the discontinuity under North Central Asia, Alaska, the Arctic, Australasia, and Central America, but only a handful of observations have been made beneath the North Atlantic due to the limited numbers of deep earthquakes in Europe. We collected transverse component recordings from all available broadband stations in the USArray to examine the D" discontinuity structure under the North Atlantic using array processing techniques. We searched for earthquakes in the European region between Jan. 2005 and Jun. 2014 with moment magnitudes between 5.5 and 7.5, event depths greater than 75 km, and epicentral distances from 55° to 90°. A total of five events were found matching these criteria. We collected a total of 2077 transverse component seismograms. We inspected each trace manually and removed traces without clear S and ScS arrivals. The remaining traces were aligned and normalized to unity on the S-wave arrival and collected into 3° geographic bins. We calculated velocity seismograms (vespagrams) for each geographic bin and screened vespagrams based on signal-to-noise ratio and slowness resolution of S and ScS. Only the 616 km deep M6.3 southern Spain event of April 11^th, 2010 demonstrated high enough data quality. A total of 372 transverse traces from this event were collected into 39 3° geographic bins. Clear Scd arrivals indicative of the D" discontinuity were identified on 20 out of 39 vespagrams. We calculated the height of the D" discontinuity above the CMB for each Scd observation based on the travel time difference between S and Scd. The results indicate a D" discontinuity with an average thickness of 261 km above the CMB beneath the North Atlantic between 45°-60° N and 45°-55° W. We present a new map with discontinuity location and the S-wave velocity structure for this region. We model waveforms for this event using the axi-symmetric finite-difference method SHaxi. The discovery of D" discontinuity beneath North Atlantic is important to advance our understanding of the potential subduction history of the Farallon plate in the Mesozoic.