Deep-Focus Earthquakes in Warm Slabs: Seismic Source Parameters in the Peru-Brazil Region

Abstract

Deep-focus earthquakes pose a significant challenge because their occurrence under extreme pressure and temperature conditions should inhibit nucleation through conventional brittle-failure. Transformational faulting is generally accepted as a most plausible mechanism to explain deep-focus seismicity, but it encounters limitations in warm slabs like Nazca because warm temperatures may hinder the preservation of a metastable olivine wedge. Aiming at elucidating the conditions and processes driving deep seismicity in warm slabs, we determined seismic source parameters (stress drop, seismic moment, radiated energy, seismic efficiency) for 13 deep-focus earthquakes (4.8 ≤ Mw ≤ 7.4) in the Peru-Brazil border region. Our results suggest that variations in stress drop can be significant (5–90 MPa) and that scaling between stress drop and seismic moment holds for a wider range of magnitudes (Mw 4.8 to 7.4) than previously reported. Radiated energies are in the 6.8 × 10¹⁰–1.9 × 10¹⁶ J range, with earthquakes in the 6.4–7.4 Mw magnitude range displaying the largest values (4.2 × 10¹⁴–1.9 × 10¹⁶ J). Most importantly, variable radiation efficiencies (0.1–1.4) suggest the coexistence of dissipative and brittle-like ruptures within the slab segment. We propose that these values reflect different degrees of melting involved in the rupture process, possibly controlled by the release of water from hydrous phases in the source region. Moreover, dehydration reactions would be triggered by either the latent heat released from phase transformations or by shear heating, establishing an interplay between thermal runaway enhanced by melting and phase transformations promoted by the release of water.