Along Strike Variation of Seismic Moment Release and Coupling Along Oceanic Transform Faults

University Of South Florida, Tampa FL

Investigators

Abstract

Earth's tectonic plates that are created at mid ocean ridge spreading centers are dissected by a global network of oceanic transform faults that offset the mid-ocean ridge segments. The motion along these oceanic transform faults is lateral and they are analogs to continental transform faults. Continental transform faults, such as the San Andreas Fault in California, pose a serious risk to population centers and a better understanding of their seismic behavior would considerably aid hazard estimation in high-risk population centers near these faults. While oceanic transforms are remote making it difficult to study their seismological behavior, the material properties of oceanic crust and mantle are much more homogenous than continental lithosphere. Furthermore, large earthquakes that periodically rupture the same fault patches have recently been identified on some oceanic transform faults. These repeating fault patches could be considered a laboratory for studying earthquake nucleation and rupture. In this project, we will study a global set of oceanic transforms to find out how common repeating rupture patches are. The project will characterize their size and distribution, and identify parameters such as plate rate, temperature or geometrical fault zone complexity that might affect fault behavior. On broader impacts, it is anticipated that results from oceanic transforms will lead to improved seismic hazard estimates of continental transforms. Results from the project will also guide selection of which oceanic transform faults to study in the future including the placement of ocean bottom seismometers. The project will support two early career researchers and a female graduate student. The remoteness of oceanic transform faults has hindered a systematic, detailed study of their along strike seismic behavior. Prior studies estimated the average behavior over entire oceanic transforms and found that a considerable amount of the plate motions is accommodated by aseismic creep. However, recent studies reveal spatially highly variable seismic moment release and identify fault patches where large earthquakes occur with quasi-periodic recurrence times. Large earthquakes account for the full plate motion and fault patches on which they occur are termed fully coupled. Fully coupled patches seem to be surrounded by regions of low coupling, where fault motion is accommodated mainly by aseismic creep. This project will study a global set of 30 oceanic transform faults spanning a wide range of plate motion rates, fault length and morphological complexity to determine the ubiquity of repeating rupture patches and along-strike variations of seismic coupling. Data for the project will be drawn from readily available global earthquake catalogs, seismic waveforms and high-resolution bathymetry to relate the spatial seismic moment release pattern to seismicity and fault zone geometry. The project will provide a first order characterization of parameters that affect size and likelihood of occurrence of fully coupled patches such as plate rate, thermal structure and fault zone geometrical and material complexity. The existence of strong fault patches with repeating earthquakes would indicate that strong and weak parts of a fault zone remain in place over many seismic cycles. The boundaries between strong and weak patches may control rupture nucleation and arrest and knowing their location would allow targeted studies to search for potential changes in seismicity and fault zone properties preceding large ruptures. Overall, results from this project will lead to a better understanding of different modes of seismic energy release and earthquake ruptures and are potentially transferrable to the structurally more complex continental transform faults.