Luca Dal Zilio
Earthquakes at mountain belts––one of the most harmful natural disasters––are transient phenomena that vary in space and time. Although these variations are commonly attributed to factors such as tectonic structures and rock heterogeneities, what controls the frequency-size distribution and the maximum magnitude of earthquakes is still debated. Here, seismic data and numerical models suggest that plate convergence rate is the first-order controlling parameter of the frequency-size distribution of earthquakes, irrespective of inherited tectonic structures. Plate convergence rate controls the brittle strength envelope through a rheological feedback with temperature and strain rate. Faster convergence leads to cooler temperatures and wider seismogenic domains, thereby increasing both maximum earthquake magnitude and the relative amount of large earthquakes.
◼︎ Subduction dynamics
◼︎ Lower mantle subduction
◼︎ Supercontinent breakup
◼︎ Solid-solid phase transitions
◼︎ Collisional orogeny and mountain building
Seismic cycle modeling:
◼︎ Himalayan seismotectonics
◼︎ Seismic behaviour of mountain belts
◼︎ Megathrust seismicity and segmentation
◼︎ Interseismic coupling
Long-term (a) and short-term (b-d) characteristics of reference models for a plate convergence rate of 50 mm/yr.
The 2015 magnitude 7.8 Gorkha earthquake is the latest large earthquake in the Himalaya. There are indications that potentially much larger earthquakes have occurred in that area and could possibly occur anywhere in the Himalaya. Yet, the timing, magnitude, and spatial extent of such potential future earthquakes, and their relation to more moderate earthquakes—like the Gorkha earthquake—remain enigmatic.
Using a newly developed 2D numerical model, here we examine the factors that regulate the bimodal seismicity: large blind earthquakes (M7+) tend to cluster in the downdip part of the seismogenic zone, whereas infrequent great earthquakes (M8+) propagate up to the Himalayan frontal thrust. Our simulations successfully match the interseismic strain and produce a realistic earthquake cycle. Most importantly, we find that the bimodal behaviour emerges as a result of relatively higher friction and a non-planar geometry of the Main Himalayan Thrust (MHT) fault. These models provide new ways to evaluate seismic rupture patterns that the MHT fault can produce based on its friction and geometry, and suggest that a major earthquake after partial ruptures is unavoidable. Thus, our simulations strongly support the emerging view that the next large earthquake in Nepal may rupture an area significantly greater than the section from the 2015 Gorkha earthquake.
Plate convergence rate controls seismic behaviour of mountain belts
Dal Zilio, L., van Dinther, Y., Gerya, T. V., & Pranger, C. C.
Earth and Planetary Science Letters, 482, 81-92.
Bimodal seismicity in the Himalaya controlled by fault friction and geometry
Dal Zilio, L., van Dinther, Y., Gerya, T., & Avouac, J. P.
Nature communications, 10(1), 48.
Interseismic behaviour computed in the 2D model. a, Observed (error bars) vs. synthetic (solid lines) present-day velocity fields. b, Elastic strain regime across the Himalaya inferred over an interseismic period of 350 years and orientation of principal compressional axes (blue bars).
Megathrust behaviour computed in the 2D model over 10,000 years. a, Spatiotemporal evolution of slip on the MHT for the reference model. Red lines show slip during the simulated earthquakes. b, Time evolution of downdip rupture width.
Continents are subjected to continuous events of aggregation and dispersal through geological time. However, rifting and break-up of supercontinents into smaller continental plates at different times, and their subsequent drift are not fully explained at present. Here we carried out a 2D numerical study suggesting that subduction of old and dense oceanic lithosphere and its coupled mantle flow, provide the key force to trigger rifting and break-up of continents.
The timescale and location of the subduction-induced continental rifting depend on the style of subduction. When subduction remains confined in the upper mantle typical marginal seas are formed, with back-arc basin extension, instead extension localizes at 2800-3500 km in the continental plate interiors, when slabs penetrate deeper in the lower mantle. This model reconciles Pangaea dismembering with the images of slab subducted in the Earth’s deep mantle.
World topographic map and schematic cross sections of subduction zones extrapolated from P-wave tomographic models.
The role of deep subduction in supercontinent breakup
Dal Zilio, L., Faccenda, M., & Capitanio, F. (2018).
Tectonophysics, 746, 312-324.
Subduction-related opening of marginal and distal basins. Panels (a) and (b) show the composition field of upper mantle confined and lower mantle subduction, respectively.
© Luca Dal Zilio. All rights reserved.
Slab Rollback Orogeny model for the evolution of the Central Alps:
Dal Zilio, L., Kissling, E., Gerya, T., van Dinther, Y.
Nowadays, the scientific dispute as to whether vertical or horizontal forces are the primary drivers of mountain building seems settled in favor of horizontal forces. For the central European Alps, however, this concept fails to explain first-order observations of a mountain belt. Recent stratigraphic, palaeo-altimetry and lithosphere structural evidence suggest that a rollback orogeny model is capable of explaining the construction of thick nappe successions and the large-scale evolution of the Swiss Alps.
Here we investigate this hypothesis using a high-resolution, rheologically consistent, two-dimensional visco-elasto-plastic thermo-mechanical numerical model. We conduct a set of numerical experiments in which we systematically vary several major parameters responsible for the degree of rheological coupling between plates during collision. The driving forces of orogeny are solely provided by the structure within the model, i.e., by the oceanic slab and the buoyancy of the crust.
Our model reproduces the self-consistent stages of oceanic subduction, continent-continent collision, spontaneous slab breakoff and subsequent evolution of the orogen. This leads to the coeval stacking of the crustal root in the lower plate and widening of the orogen. In particular, we discuss how the current crustal seismicity pattern implies the occurrence of extensional forces at work beneath the Molasse Basin and within the Alps. Our results thus support the emerging hypothesis that the remaining slab exerts a first-order control on the motions and deformations of the orogen.
A: Inset map illustrating the Alps. B: Teleseismic tomography cross-section illustrating European slab geometry beneath the central Alps. C: Topographic signal and vertical cross-section approximately perpendicular to the strike of the Central Alps. Transparent gray circles represent hypocenters. Dashed black lines indicate the Moho.
Post-collisional evolution with superimposed outlines of the slab geometries 1 and 10 My after slab breakoff. Compositional map shows the final structure 20 My after slab breakoff. Evolution of the orogen is driven by slow slab rollback and delamination of crustal material.
A: Composition and spatial distribution of seismicity and the inferred focal mechanisms display a broad pattern of different style of faulting, which are consistent with the local tectonic regime. B: Histograms of all events and the corresponding magnitude.