An Yin pioneered an interdisciplinary approach to tectonics that combined acute field observations with geochemical analyses and continuum mechanics to create incisive tectonic interpretations that often transcended conventional wisdom. His field area ranged from the North American Cordillera to the Indoasian collision to southern Africa, and, remotely, to the far reaches of the solar system.

An began his career applying elasticity theory to the formation of low-angle faults before migrating to Indoasian tectonics where he developed the tectonic reconstruction for Asia that has been the starting point of geological research there for 30 years. He took advantage of the opening of Tibet to undertake a vast range of studies that revealed a multitude of unexpected geologic and tectonic phenomena – including an extruded blueschist terrane in central Tibet and a 2000-km-long thrust system at collision front – that established him as the great authority on this key region. His Himalayan synthesis provided the first internally consistent explanation for its 3D geological development and the astonishing proposition that the world’s most spectacular normal fault is in reality a backthrust. While the principal scientific focus on the Indo-Asian collision has been on the dramatic topography of Tibet and the Himalaya, its distal effects have resulted in far more earthquake-related deaths. Returning to his hometown of Tianjin, An documented a 160-km-long seismic gap that has not ruptured for over 8,000 years and is capable of generating a similar magnitude quake to that in nearby Tangshan that killed >250,000 people. Upon the recognition of slow earthquakes, he used diffusion-induced pressure-wave theory to relate for the first time slow earthquakes to tectonic tremor propagation. Over the past decade, he investigated a range of extra-terrestrial tectonic processes leading to the provocative proposal that Mars experienced localized plate tectonics and an ingenious kinematic model for the formation of tiger-stripe fractures on the saturnian moon Enceladus.

An Yin’s sudden passing on July 12, 2023 while instructing undergraduate field camp in eastern California was felt acutely across the geologic world, mitigated only by the model he left us of how the combination of intellectual rigor, originality, and passion can lead to stunning new insights into how our planet works.