Seafloor sediment thickness beneath the VoiLA broad-band ocean-bottom seismometer deployment in the Lesser Antilles from P-to-S delay times

Abstract

Summary Broadband ocean-bottom seismometer (OBS) deployments present an opportunity to investigate the seafloor sediment thickness, which is important for constraining sediment deposition, and is also useful for subsequent seismological analyses. The Volatile Recycling in the Lesser Antilles (VoiLA) project deployed 34 OBSs over the island arc, fore- and back-arc of the Lesser Antilles subduction zone for 15 months from 2016-2017. Using the amplitudes and delay times of P-to-S (Ps) scattered waves from the conversion of teleseismic earthquake P-waves at the crust-sediment boundary and pre-existing relationships developed for Cascadia, we estimate sediment thickness beneath each OBS. The delay times of the Ps phases vary from 0.20 ± 0.06 s to 3.55 ± 0.70 s, generally increasing from north to south. Using a single-sediment and single-crystalline crust Earth model in each case, we satisfactorily model the observations of 8 OBSs. At these stations we find sediment thicknesses range from 0.43 ± 0.45 km to 5.49 ± 3.23 km. To match the observations of 9 other OBSs, layered sediment and variable thickness crust is required in the Earth model to account for wave interference effects on the observed arrivals. We perform an inversion with a two-layer sediment and a single-layer crystalline crust in these locations finding overall sediment thicknesses of 1.75 km (confidence region: 1.45-2.02 km) to 7.93 km (confidence region: 6.32-11.05 km), generally thinner than the initial estimates based on the pre-existing relationships. We find agreement between our modelled velocity structure and the velocity structure determined from the VoiLA active-source seismic refraction experiment at the three common locations. Using the Ps values and estimates from the VoiLA refraction experiment, we provide an adjusted relationship between delay time and sediment equations for the Lesser Antilles. Our new relationship is ${\boldsymbol{H\ }} = {\boldsymbol{\ }}1.42{\boldsymbol{d}}{{\boldsymbol{t}}^{1.44}}$, where H is sediment thickness in kilometres and dt is mean observed Ps delay time in seconds, which may be of use in other subduction zone settings with thick seafloor sediments.

Variation in Upper Plate Crustal and Lithospheric Mantle Structure in the Greater and Lesser Antilles From Ambient Noise Tomography

“…11a). The thickness is in general agreement with point measurements derived from scattered phases and refraction work in the region (Allen et al, 2019;Chichester et al, 2020;Padron et al, 2020), showing a correlation coefficient of r = 0.80, which gives confidence in the choice of this velocity as our estimated proxy (Fig. 11b).…”

“…11c). We acknowledge that slightly different choices, such as gradients or other nominal velocity values are possible, but our choices compare well with estimates from previous studies of sediment thickness (Chichester et al 2020;circles in Figs. 11a,b) and the Moho depth (Schlaphorst et al, 2018;Melekhova et al, 2019;symbols in Fig.…”

“…10a) the depth-velocity profile resembles that in the northern km/s surface as an estimate of sediment limit. Circles represent depth estimates from previous study (Chichester et al, 2020), the size represent quality of the results. b) Pointwise comparison between previous and current studies, also showing the correlation coefficient (r2.0).…”

“…In our study, data are recorded by a dense network of permanent land stations and additionally an array of oceanic bottom seismometers deployed during the VoiLA experiment (VoiLA; Goes et al., 2019), which increases resolution in the fore‐ and back‐arc. Previous studies have investigated the crustal structure in this area via active source imaging and receiver functions (e.g., Allen et al., 2019; Arnaíz‐Rodroguez et al., 2016; Boynton et al., 1979; Chichester et al., 2020; Christeson et al., 2008; Dorel et al., 1974; Kopp et al., 2011; Melekhova et al., 2019; Padron et al., 2020; Schlaphorst et al., 2018; Sevilla et al., 2010) but the inter‐island subsurface is poorly studied so far. Our aim is to image the shallow mantle and crust, investigating variations in sediment accumulation, crustal thickness and the velocity structure in the uppermost mantle across the arcs.…”

Shedding New Light on an Enigmatic End Member of Back‐Arc Spreading: The Structure of the Grenada Basin in the Lesser Antilles

“…Extreme thicknesses of sediment in the south of the basin, which in places may exceed 12 km as a result of the large sedimentary outflow from the adjacent South American continent (Aitken et al., 2011; Chichester et al., 2020), not only limit the usefulness of potential‐field data (particularly gravity surveys), but also make it impossible to directly sample the basement lithology in‐situ. The only hard‐rock evidence for back‐arc spreading comes from mid‐Eocene‐age pillow basalts sampled on the islands of Mayreau (Mayreau basalt) and Carriacou (Cherry Hill basalt).…”