Oblique strain in the lithospheric mantle and lower crust commonly partitions into nearly pure tangential and normal strain components in the upper seismogenic crust. This phenomenon, termed "strain partitioning," produces subparallel strike-slip faults and dip-slip faults with associated fold belts. The occurrence and scale of partitioning is important for correctly identifying and characterizing potential seismic sources, and thus is significant for assessing seismic hazard. The scale of strain partitioning is a continuum from regional to local. Regional strain partitioning originates beneath or within the lower part of the seismogenic crust where large-magnitude (ms 5.5+) earthquakes typically nucleate. The regionally partitioned structures may be expressed by subparallel faults and folds from 5 km to hundreds of kilometres apart and should be treated as independent seismic sources in terms of characterizing earthquake magnitude, recurrence, slip rate, and sense of slip. Local strain partitioning occurs primarily within the upper part of the seismogenic zone above the region where large-magnitude earth-quakes nucleate. Although these structures may be expressed by subparallel faults and folds up to 6 km apart, they should be characterized collectively in order to estimate seismic source parameters of the underlying seismogenic source. Lack of recognition of strain partitioning can, and probably will, lead to an underassessment of slip rate and earthquake recurrence and an inaccurate assessment of sense of slip and fault geometry at seismogenic depth.

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