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Mode of slip and crust–mantle interaction at oceanic transform faults


Oceanic transform faults, connecting offset mid-ocean spreading centres, rupture quasi-periodically in earthquakes up to about magnitude M 7.0 that are often preceded by foreshocks. In addition to seismic slip, a large portion of slip takes place as aseismic creep, which likely influences initiation of large earthquakes. Although oceanic transform faults are one of the major types of plate boundaries, the exact mode of slip and interaction between the seismic and aseismic motion remains unclear. Here we present a detailed model of the mode of slip at oceanic transform faults based on data acquired from a recent temporary deployment of ocean-bottom seismometers at the Blanco Transform Fault and existing regional and teleseismic observations. In the model, the crustal part of the fault is brittle and fully seismically coupled, while the fault in the mantle, shallower than the depth of the 600 °C isotherm, creeps partially and episodically. The creep activates small asperities in the mantle that produce seismic swarms. Both mantle and the crustal zones release most of the plate-motion strain during large-magnitude earthquakes. Large earthquakes appear to be preceded by a brief episode of shallow mantle creep, accompanied by seismic swarms, which explains the observation of foreshocks and shows that mantle creep likely influences initiation of large seismic events.

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Data availability

The OBS dataset is archived at the IRIS Data Management System ( X9 is the network code for the Plate Boundary Evolution and Physics at an Oceanic Transform Fault System project32; 7D is the network code for the Cascadia Initiative Community Experiment – OBS component data33. Raw bathymetry data, used in Fig. 1 and Supplementary Fig. 1, are available from the authors.

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The seismic stations for the project were provided by the Ocean Bottom Seismograph Instrument Pool (, funded by the National Science Foundation (NSF). This research was supported by NSF grants OCE-1031858, OCE-1131767 and OCE-1737073. We thank the crews of RV Melville and RV Oceanus, OBSIP technicians and volunteers who contributed to data collection.

Author information

J.L.N. and J.B. designed the study and collected the dataset. V.M.K. and J.L.N. created the seismicity catalogue. V.M.K. conducted the data analysis. All authors discussed results and contributed to the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Václav M. Kuna or John L. Nábělek.

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Further reading

Fig. 1: Bathymetric map of the BR with locations of earthquake epicentres and OBS stations.
Fig. 2: Seismicity of the BR.
Fig. 3: Mantle swarms versus crustal aftershock sequences.
Fig. 4: Mode of slip of the BR transform fault.