Posts

SCIENCE: Illuminating seafloor faults and ocean dynamics with dark fiber distributed acoustic sensing

/0 Comments/in /by

Nathaniel J. Lindsey, T. Craig Dowe, and Jonathan B. Ajo-Franklin

Abstract

Distributed fiber-optic sensing technology coupled to existing subsea cables (dark fiber) allows observation of ocean and solid earth phenomena. We used an optical fiber from the cable supporting the Monterey Accelerated Research System during a 4-day maintenance period with a distributed acoustic sensing (DAS) instrument operating onshore, creating a ~10,000-component, 20-kilometer-long seismic array. Recordings of a minor earthquake wavefield identified multiple submarine fault zones. Ambient noise was dominated by shoaling ocean surface waves but also contained observations of in situ secondary microseism generation, post–low-tide bores, storm-induced sediment transport, infragravity waves, and breaking internal waves. DAS amplitudes in the microseism band tracked sea-state dynamics during a storm cycle in the northern Pacific. These observations highlight this method’s potential for marine geophysics.

doi: 10.1126/science.aay5881

Nature Geoscience: Seismic velocity reduction and accelerated recovery due to earthquakes on the Longmenshan fault

/0 Comments/in /by

Shunping Pei, Fenglin Niu, Yehuda Ben-Zion, Quan Sun, Yanbin Liu, Xiaotian Xue, Jinrong Su, and Zhigang Shao

Nature Geosci. 12 (2019) 387-392.

DOI: 10.1038/s41561-019-0347-1

Abstract

Various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults. Although there are growing numbers of observations on coseismic velocity reductions, generally there is a lack of detailed observations of the healing phases. Here we report on a pronounced coseismic reduction of velocities around two locked sections (asperities) of the Longmenshan fault with a large slip during the 2008 Mw 7.9 Wenchuan earthquake and subsequent healing of the velocities. The healing phase accelerated significantly at the southern asperity right after the nearby 2013 Mw 6.6 Lushan earthquake. The results were obtained by joint inversions of travel time data at four different periods across the Wenchuan and Lushan earthquakes. The rapid acceleration of healing in response to the Lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes. We suggest that stress redistribution plays an important role in rebuilding fault strength.