Phantom earthquakes and triggered aseismic creep: Vertical partitioning of strain during earthquake sequences in Iran

We present evidence for significant aseismic fault slip at shallow depth above a pair of mainshock-aftershock sequences in the Zagros Mountains of Iran. The two Mw 5.9 earthquakes are each spanned by high-quality geodetic imagery and have well-recorded sequences of aftershocks that occurred beneath a salt decollement. Earlier studies of the geodetic data inferred that the mainshocks were located above the decollement, requiring a similar to 10 km spatial separation between aftershock cluster and earthquake centroid. We find that the geodetic data simultaneously allow two slip sources of similar magnitude: one within the basement, collocated with aftershocks, and one shallow source (also equivalent to Mw 6) responsible for the primary signal apparent in the geodetic imagery. Should this phenomenon be widespread in the Zagros, it would partially explain a previously noted discrepancy between observed seismic moment release in the Zagros and current convergence rates between the Arabian and Eurasian plates.