Microseismicity Indicates Atypical Small‐Scale Plate Rotation at the Quebrada Transform Fault System, East Pacific Rise.

Closely spaced, multi‐strand ridge transform faults (RTFs) accommodate relative motions along fast spreading mid‐ocean ridges. However, the relations between RTFs and plate spreading dynamics are poorly understood. The Quebrada system is one of the most unique RTF systems at the East Pacific Rise, consisting of four transform faults connected by three short intra‐transform spreading centers (ITSCs). We use seven‐months of ocean bottom seismograph data to study the Quebrada system, and find abundant earthquakes unevenly distributed among three active faults. We identify two deep, diffuse seismicity clouds at the inside corners of the ITSC‐transform fault intersections, and one seismically active fracture zone. The observations suggest a complex regional plate‐motion pattern, including possible heterogeneous rotations within the Quebrada system. Evolution of multi‐strand RTFs may have resulted from a strong three‐dimensional local thermal and fluid effects, while the RTFs may have also regulated regional tectonics, forming an intricate feedback system. Plain Language Summary: Mid‐ocean ridge transform faults (RTFs) are plate boundaries that offset adjacent mid‐ocean ridges. At fast spreading mid‐ocean ridges, such as the East Pacific Rise (EPR), closely spaced, multi‐strand RTFs are often connected by two or more short intra‐transform spreading centers (ITSCs). However, physical processes accommodating plate spreading along such multi‐strand RTF systems and the inter‐relations between the fault system and the tectonic dynamics are not well understood. Quebrada is one of such multi‐strand RTFs at the EPR. We utilize seven‐month seismic data from ocean bottom seismographs of a 2008 experiment to investigate the seismotectonics of the region. We find intriguing, abundant seismicity on one of the fracture zones, contradicting the traditional view that fracture zones are seismically quiescent. Further, we identify two diffuse seismicity clouds penetrating the uppermost mantle at the inside corners of the ITSC‐transform fault intersections, implying complex interactions among ITSCs, transform faults, and their surrounding structure. From these observations, we infer that there are rotational motions within the Quebrada fault system, which have caused slip along the fracture zone and facilitated fluid circulations to produce deep, diffuse seismicity. We speculate that there is a complex feedback system between the multi‐strand RTFs and local three‐dimensional tectonic processes. Key Points: Four segments of Quebrada transform system show significantly different seismic intensitiesOne fracture zone segment appears to be seismogenic indicating local plate rotationDeep clouds of seismicity extend into the uppermost mantle at spreading center‐ transform fault intersections [ABSTRACT FROM AUTHOR]