Enhanced shallow slip and tsunamigenesis of subduction earthquakes

24 Gener 2020
Sala d'Actes
Impartida per: 
Dr. Valentí Sallarès
Departament de Geociències Marines, Institut de Ciències del Mar


Seismological data provide compelling evidence of a depth-dependent rupture behavior of megathrust earthquakes. Relative to deeper events of similar magnitude, shallow earthquake ruptures have larger slip and longer duration, radiate energy that is depleted in high frequencies and have a larger discrepancy between their surface wave and moment magnitudes (MS and MW, respectively). These source properties make them prone to generating devastating tsunamis without clear warning signs. The origin of the observed differences has been a long-lasting matter of debate. Here we first show that the overall depth trends of all these observations can be explained by worldwide average variations of the elastic properties of the rock body overriding the megathrust fault, which deforms by dynamic stress transfer during co-seismic slip, and we discuss some general implications for tsunami hazard assessment. Second, we test this conceptual model for the particular case of the 1992 Nicaragua tsunami earthquake (MS7.2 and MW7.8). This event nucleated at ~20 km-deep but it appears to have released most of its seismic moment near the trench. This earthquake caused mild shaking and little damage, so that tsunami hazard based on human perception was underestimated and the destructive tsunami hit the coast unexpectedly. We use a set of 2D seismic data to map the P-wave seismic velocity above the inter-plate boundary, and we combine it with previously estimated moment release distribution to calculate slip and stress drop distributions and moment-rate spectra that are compatible with both the seismological and the geophysical data. The models confirm that slip concentrated in the shallow megathrust, with two patches of maximum slip exceeding 10-12 m in the near-trench zone that can explain the observed tsunami run-up, while the average stress drop is ~3 MPa. The low rigidity of the upper plate in the zone of maximum slip explains the high frequency depletion and the resulting MW-MS discrepancy without need to consider anomalous rupture properties or fault mechanics.


Brief biography

I graduated in Physics at the U. Barcelona in 1994, and I had my in PhD in Physics at the same university in 1999. I was awarded the García-Siñeriz Foundation Award for the best DEA work in Applied Geophysics in 1997 and to the best PhD in Geophysics in 2000. Between 2000 and 2004 I worked at the IRD-Géosciences Azur, first as Marie Curie postdoc, and then as Chargé de Recherche. In 2004 I got a Científico titular position at the CSIC, first in the UTM and since 2012 in the ICM. My research interests focus on marine geophysics, including the methodological development of seismic tomography and its applications to the study of the structure and processes that govern the evolution of subduction zones, with especial emphasis to earthquake rupture and seismogenesis, continental margins and back-arc basins. Another line of research is seismic oceanography, an interdisciplinary approach that combines geophysical and hydrographic methods to investigate the structure and dynamics of the oceans. I am the author of 87 articles in magazines and books (65 SCI, 44 Q1). I have participated in 33 scientific projects funded by public organizations and private entities. I have been chief scientist of 6 oceanographic surveys in Spanish oceanographic vessels, and I have participated in 5 other marine and 5 land ones. I am vice-director of the ICM-CSIC (2018-), where I coordinate the Scientific Strategy Commission. I have been coordinator of the Earth Sciences Area of the Access Committee to the Spanish Supercomputing Network (2013-2019).