EGU2020-5809
https://doi.org/10.5194/egusphere-egu2020-5809
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Earthquake rupture properties and tsunamigenesis in the shallowest megathrust

Valenti Sallares1, Cesar R. Ranero1,2, Manel Prada1, and Alcinoe Calahorrano B.1
Valenti Sallares et al.
  • 1Institute of Marine Sciences, ICM-CSIC, Barcelona, Spain (vsallares@icm.csic.es)
  • 2ICREA, Barcelona, Spain

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

How to cite: Sallares, V., Ranero, C. R., Prada, M., and Calahorrano B., A.: Earthquake rupture properties and tsunamigenesis in the shallowest megathrust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5809, https://doi.org/10.5194/egusphere-egu2020-5809, 2020

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Presentation version 2 – uploaded on 02 May 2020
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  • CC1: Comment on EGU2020-5809, Dietrich Lange, 07 May 2020

    Dear Valenti,

    congratulations on this nice work on velocity models and depth variablilty of rigidity.

    I would like to refer to a paper which focus on the same topic, but more from seismological observations (Sen et al. (2015), BSSA (doi: 10.1785/0120140123)). There, we did source time durations for smaller events for aftershocks of the Maule 2010 rupture (similar to Bilek and Lay (1999, Nature)). Although we were looking at regional and smaller events, the result was that source time for smaller events increases as well with decreasing depth. Furthermore, we found a  similarity of the depth dependence of normal and thrust events and between smaller and larger magnitude earthquakes we suggested that the depth-dependentvariation of rigidity, rather than frictional conditional stability at the plate interface, is primarily responsible for the observed pattern.

    Kind regards

    Dietrich

    • AC1: Reply to CC1, Valenti Sallares, 07 May 2020

      Dear Dietrich,

      many thanks for your comment and for the reference that is most welcome. 

      I think that depth-dependent rigidity and, more in general, the -heterogeneous- distribution of elastic rock properties should be considered to interpret many aspects of seismic rupture -including duration as you say-.  Elastic properties can be extracted -or derived- from seismic tomography models. Combining geophysical models and seismological observations to estimate source properties (e.g. slip distribution or vertical displacements) for individual events appears to me as a way to go.

       

Presentation version 1 – uploaded on 02 May 2020 , no comments