Along-strike variation in slab geometry at the southern Mariana subduction zone revealed by seismicity through ocean bottom seismic experiments

Abstract

SUMMARYWe have conducted the first passive Ocean Bottom Seismograph (OBS) experiment near the Challenger Deep at the southernmost Mariana subduction zone by deploying and recovering an array of 6 broad-band OBSs during December 2016–June 2017. The obtained passive-source seismic records provide the first-ever near-field seismic observations in the southernmost Mariana subduction zone. We first correct clock errors of the OBS recordings based on both teleseismic waveforms and ambient noise cross-correlation. We then perform matched filter earthquake detection using 53 template events in the catalogue of the US Geological Survey and find >7000 local earthquakes during the 6-month OBS deployment period. Results of the two independent approaches show that the maximum clock drifting was ∼2 s on one instrument (OBS PA01), while the rest of OBS waveforms had negligible time drifting. After timing correction, we locate the detected earthquakes using a newly refined local velocity model that was derived from a companion active source experiment in the same region. In total, 2004 earthquakes are located with relatively high resolution. Furthermore, we calibrate the magnitudes of the detected earthquakes by measuring the relative amplitudes to their nearest relocated templates on all OBSs and acquire a high-resolution local earthquake catalogue. The magnitudes of earthquakes in our new catalogue range from 1.1 to 5.6. The earthquakes span over the Southwest Mariana rift, the megathrust interface, forearc and outer-rise regions. While most earthquakes are shallow, depths of the slab earthquakes increase from ∼100 to ∼240 km from west to east towards Guam. We also delineate the subducting interface from seismicity distribution and find an increasing trend in dip angles from west to east. The observed along-strike variation in slab dip angles and its downdip extents provide new constraints on geodynamic processes of the southernmost Mariana subduction zone.

Along‐Arc Heterogeneity in Local Seismicity across the Lesser Antilles Subduction Zone from a Dense Ocean‐Bottom Seismometer Network

“…Most seismicity beneath the outer forearc is found in the north, where the forearc is less wide, and OBS stations were closer to the trench. We note that more smaller earthquakes may be found using template-matching techniques (e.g., Zhu et al, 2019). Here, we focussed on the larger events with robust arrival time determination, particularly for the generation of a well-constrained 1-D seismic velocity, with less emphasis on the evolution of seismicity in time and space.…”

Diffuse spreading, a newly recognized mode of crustal accretion in the southern Mariana Trough backarc basin

“…2), the Malaguana-Gadao Ridge south of 13°05′N, the diffuse spreading zone, and the Southwest Mariana Rift overlie the Pacific slab at depths of 100 km or less. A local ocean-bottom seismometer study centered over the Southwest Mariana Rift and the forearc found somewhat deeper slab depths in some areas >~80 km from the trench, but also confirmed the presence of the slab beneath the Malaguana-Gadao Ridge and diffuse spreading zone (Zhu et al, 2019). In this setting, abundant slab dewatering is predicted by studies of the metamorphic breakdown of hydrous Research Paper minerals (Peacock, 1990;Schmidt and Poli, 1998), leading to enhanced melt production.…”

“…In this part of the basin, the trench curves to the west and becomes increasingly oriented in the direction of trough opening (Fryer et al, 1998;Kato et al, 2003), creating a strong component of trench-parallel extension within the Mariana platelet forearc (Martinez et al, 2000(Martinez et al, , 2018 that is separate from the trench-normal extension in the backarc basin. The divergent boundary in the backarc curves westward in concert with the trench, and its morphology changes from the magmatically robust seafloor spreading center of the Malaguana-Gadao Ridge (Martinez et al, 2000;Becker et al, 2010) in the northeastern part of our study area to a predominantly tectonic rift at the narrow (~40 km) Southwest Mariana Rift (Becker, 2005;Wan et al, 2019) in the west (Figs. 2 and 3).…”

“…Beneath the central Mariana Trough north of 14°22′N, the slab dips steeply and does not underlie the spreading axis, whereas in the south, the slab dips more gently and underlies the axis and much of the backarc basin (Fig. 2; Hayes et al, 2018;Zhu et al, 2019).…”

Continental Interior and Edge Breakup at Convergent Margins Induced by Subduction Direction Reversal: A Numerical Modeling Study Applied to the South China Sea Margin

“…Statistical analyses of trenches in the western Pacific (Lallemand et al, 2005) and near-field seismic observations (Zhu et al, 2019) show an average slab dip of 28°at shallow depth (Figure 3b), so we initiate the subduction by prescribing a 28°dipping weak fracture zone between the continental and oceanic plates. This weak zone consists of mantle rocks with wet olivine rheology (Chopra & Paterson, 1981;Ranalli, 1995) Figure 2.…”