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4 settembre 11.00-12.00 - Carlo Doglioni & Massimo Cocco: “The 2016-2017 seismic sequence of Amatrice and Norcia”

 

5 settembre 14.30-15.30 -Martin Engi: "Petrochronology – deciphering the temporal archive in rocks"

 


The 2016-2017 seismic sequence of Amatrice and Norcia

Cocco M.*1 & Doglioni C.*2-1

1 Istituto Nazionale di Geofisica e Vulcanologia, Roma
2 Dipartimento di Scienze della Terra, Sapienza Università di Roma

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1) Source complexity inferred from rupture models and seismicity evolution (Massimo Cocco)

The seismic sequence that struck the sector of the Central and Northern Apennines (Italy) comprised between the Amatrice, Accumoli, Norcia and Visso towns consists of a series of moderate-to-large magnitude earthquakes (5.0 < MW < 6.6) occurred within a few months and activating a nearly 70 km long normal fault system oriented in the Apennines direction. The seismicity is relatively shallow (depths < 10 km) and the largest shocks nucleated at a depth of nearly 8 km. The main shocks and most of the aftershocks show NNW–SSE striking focal mechanisms in agreement with the current NE-SW extensional tectonic setting of Central and Northern Apennines.
The sequence began on August 24th with a MW 6.0 earthquake, which struck the region between Amatrice, Accumoli and Norcia and caused 299 fatalities and extensive damages in the urban and rural surrounding areas. On October 26th 2016, another MW 5.9 main shock occurred near Visso and Ussita at the northern edge of the aftershock zone that followed the August 24th event extending the activated seismogenic area toward the NW. Four days after the second main shock and more than two months since the beginning of the sequence, on October 30th 2016, a third larger earthquake (MW 6.5) occurred near Norcia, roughly midway between Accumoli and Visso, severely damaging the already afflicted towns and villages in this sector of the Apennines. On January 2017 four moderate-magnitude (5.0 < ML < 5.5) earthquakes occurred in the southern part of the activated seismogenic volume near Montereale and Campotosto. All the main shocks nucleated at the base of a SW dipping normal fault system, segmented by the presence of crosscutting compressional structures.
Field observations, GNSS and InSAR data and seismic waveforms reveal the heterogeneity of the rupture process during individual earthquakes and the complexity of the activated fault system. The August 24th MW 6.0 earthquake ruptured a nearly 20 km long normal fault with a quite heterogeneous slip distribution characterized by two shallow slip patches located up-dip and NW from the hypocenter. For this earthquake fault dimensions and peak slip values are larger than expected for such a moderate-magnitude event. The October 26th MW 5.9 main shock consists of a double event rupturing contiguous patches on the fault segment (SW dipping) of the normal fault system. The rupture history during the largest main shock of the sequence (MW 6.5), occurred on October 30th 2016, reveals an extraordinary complexity: the coseismic rupture propagated on a normal fault and on a blind fault inherited from compressional tectonics.
Geodetic and seismological observations corroborate the interpretation of a seismic sequence characterized by complex multi-fault coseismic ruptures and heterogeneous distribution of slip on individual segments. These earthquakes raise serious concerns on our understanding of fault segmentation and emphasize the importance of strain localization in the shallow crust and dynamic control on the activation of fault segments during sequences of normal faulting earthquakes.

2) Origin of the seismicity in the Apennines (Carlo Doglioni)

The geodynamics of the Apennines is controlled by the "easterly" retreat of the Adriatic-Ionian subduction zone. This mechanism provides contractional tectonics in the frontal thin-skinned accretionary prism and contemporaneous thick-skinned backarc extension along the Apennines and Tyrrhenian Sea. Local transfer zones of differential slab retreat, salients and recesses in the accretionary prism, transfer zones within the dilatational backarc basin are rather characterized by strike-slip tectonics. This scenario is shaped by different geotherms that generate variable depth of the brittle-ductile transition (BDT), hence controlling the volumes that can be activated during the seismic cycles. The largest extensional earthquakes occur where the BDT is deeper along the Apennines belt. This happens where the topography is higher and the lithostatic load (sigma 1) is therefore greater, increasing the differential stress. Vice-versa, the most energetic contractional earthquakes generate where the topography is low, since the lower the lithostatic load (sigma 3), the larger the differential stress. All earthquakes are associated to the propagation of elastic waves. However, they are fueled by different types of energy. In contractional and strike-slip settings, the earthquakes dissipate elastic energy accumulated within a volume above the creeping layer of the crust. In extensional settings, earthquakes are rather the result of gravitational collapse of the brittle upper crustal prisms. Since the evolution and energy accumulation of earthquake preparation and nucleation between normal fault and thrust-related earthquakes are different, we need to distinguish the different processes, i.e., graviquakes and elastoquakes. Graviquakes have the maximum depth of the seismogenic zone about one third with respect to the length of the volume affected by the collapse. The dimension of the volume dictates the length of the fault system that allows the crustal volume to fall and deform into a sag basin. InSAR data of the 2016 Amatrice-Norcia sequence show that the subsided area during the coseismic stage is about 10 times larger than the uplifted volume (180-230 Mm3 vs. 15-20 Mm3). This supports the notion that extensional earthquakes are due to the closure at depth of dilated crustal volumes throughout microfractures in the brittle upper crust during the interseismic stage. Gravitational energy is hundreds of times larger with respect to the earthquake energy, confirming that it is far than enough to generate the earthquake, plus folding, fracturing and shearing rocks. This may also explain why aftershocks last longer along normal faults, since the crust will continue to move in favour of gravity until the equilibrium will be reached, whereas along thrusts, the aftershocks are inhibited because the volume has to move against gravity. Seismic precursors, if any, may then have different sign and this can be one of the reasons why they have not yet been recognized.

 


 

Petrochronology – deciphering the temporal archive in rocks

Engi M.*1

1 Institut für Geologie, Universität Bern, Switzerland
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Not long ago, rocks had an age – even an “absolute” age – as detemined by geochronological methods based on one or more radioactive decay clocks (e.g., U-Pb, Th-Pb, Nd-Sm). Recent advances have lead to a paradigm shift: As ever smaller sample volumes have become datable with increasing precision, results often showed not one age but a range of dates. Interpreting such results demands an integrative approach that combines petrogenetic analysis with local chronometry.
Petrochronology is this ambition of deciphering and dating detailed stages in a rock’s evolution (Engi et al., 2017). In essence, this involves four steps: (1) investigation of petrographic textures and mineral assemblages with the aim of establishing a relative chronology; (2) microchemical analysis of coexisting minerals by EPMA (electron microprobe) and LA-ICP-MS (for trace elements); (3) petrological quantification of P-T conditions by thermodynamic (or kinetic) modeling; (4) micro-dating of individual growth zones in one or more datable minerals from the assemblages analyzed (in 1-3).
Sounds like a lot of work – what’s the benefit? Whether we aim to understand the formation of magmas – their chemical and physical evolution with time – or to trace metamorphism in a subduction factory, the rock archive preserves evidence of how the tectonic engine has changed thermal and baric conditions. Insight into the duration and rates of geological processes demands a detailed temporal sequence of well delimited events. While numerical models help us sharpen the questions we address when studying select samples, in turn the relevance of such models can and must be tested by comparison to the rock record. It is particularly critical to compare the rates – of heating and cooling, (de)compression and strain – determine in sample-scale studies with those used in numerical models or obtained from these. It’s a two-way test: Petrochronological data can provide critical tests to our understanding of Earth dynamics; conversely, the significance of individual age data needs testing in the context of a tectonic model.
This lecture highlights the state of the art and outlines current limits, both technical and conceptual. The key in petrochronology is establishing context: We must aim to link age data obtained by LA-ICP-MS or ion probe (SIMS, SHRIMP) reliably to the minerals or assemblages used to quantify physical growth conditions. Petrology now uses sophisticated tools, e.g. inclusion barometry, thermometry in chemical domains or using trace elements in accessory minerals. Diffusion has long been seen as essentially a limitation or obstacle to age dating, while its utility is only starting to be realized, notably for constraining the duration of processes (Kohn & Penniston-Dorland, 2017).
Select studies of applied petrochronology will be presented, covering a wide range of geological contexts.

Engi, M., Lanari, P., Kohn, M.J. (2017); Significant ages - An introduction to petrochronology: Rev. Mineral. Geochem.83, 1-12.
Kohn, M.J. & Penniston-Dorland, S.C. (2017): Diffusion: obstacles and opportunities in petrochronology: Rev. Mineral. Geochem.83, 103-152.