Massimo Cocco

Aftershocks driven by a high-pressure CO2 source at depth

In northern Italy in 1997, two earthquakes of magnitudes 5.7 and 6 (separated by nine hours) marked the beginning of a sequence that lasted more than 30 days, with thousands of aftershocks including four additional events with magnitudes between 5 and 6. This normal-faulting sequence is not well explained with models of elastic stress transfer, particularly the persistence of hanging-wall seismicity that included two events with magnitudes greater than 5. Here we show that this sequence may have been driven by a fluid pressure pulse generated from the coseismic release of a known deep source of trapped high-pressure carbon dioxide (CO2). We find a strong correlation between the high-pressure front and the aftershock hypocentres over a two-week period, using precise hypocentre locations and a simple model of nonlinear diffusion. The triggering amplitude (10–20u2009MPa) of the pressure pulse overwhelms the typical (0.1–0.2u2009MPa) range from stress changes in the usual stress triggering models. We propose that aftershocks of large earthquakes in such geologic environments may be driven by the coseismic release of trapped, high-pressure fluids propagating through damaged zones created by the mainshock. This may provide a link between earthquakes, aftershocks, crust/mantle degassing and earthquake-triggered large-scale fluid flow.

Fluid flow and seismicity pattern: Evidence from the 1997 Umbria‐Marche (central Italy) seismic sequence

[1]xa0We model the spatial and temporal evolution of seismicity during the 1997 Umbria-Marche seismic sequence in terms of subsequent failures promoted by fluid flow. The diffusion process of pore-pressure relaxation is represented as a pressure perturbation generated by coseismic stress changes and propagating through a fluid saturated medium. The values of isotropic diffusivity range between 22 and 90 m2/s. The calculated value of anisotropic diffusivity (Daniso = 250 m2/s) is largest along the average strike (N140°) direction of activated faults. Our results suggest that the observed spatio-temporal migration of seismicity is consistent with fluid flow.

Modeling seismicity rate changes during the 1997 Umbria‐Marche sequence (central Italy) through a rate‐ and state‐dependent model

[1]xa0We model the spatial and temporal pattern of seismicity during a sequence of moderate-magnitude normal faulting earthquakes, which struck in 1997 the Umbria-Marche sector of Northern Apennines (Italy), by applying the Dieterich (1994) rate- and state-dependent constitutive approach. The goal is to investigate the rate of earthquake production caused by repeated coseismic stress changes computed through a 3-D elastic dislocation model in a homogeneous half-space. The reference seismicity rate is assumed time independent, and it is estimated by smoothing the seismicity that occurred in the previous decade without declustering. We propose an analytical relation for deriving the stressing rate directly from the reference seismicity rate. This allows us to perform a tuning of the constitutive parameter Aσ (where A accounts for the direct effect of friction in the rate- and state-dependent model and σ is the effective normal stress) into the Dieterich model through a maximum likelihood method, which yields for this seismic sequence a best fitting value equal to 0.04 MPa. Our computations show that, although seven out of eight main shocks are located in areas of increased rate of earthquake production, numerous aftershocks are located in seismicity shadows. Our simulations point out that the adopted value of Aσ strongly affects the pattern of both seismicity shadow and areas of enhanced rate of earthquake production. We conclude that solely accounting for static stress changes caused by the main shocks of this seismic sequence is not sufficient to forecast the complex spatial and temporal evolution of seismicity.

A global search inversion for earthquake kinematic rupture history: Application to the 2000 western Tottori, Japan earthquake

[1]xa0We present a two-stage nonlinear technique to invert strong motions records and geodetic data to retrieve the rupture history of an earthquake on a finite fault. To account for the actual rupture complexity, the fault parameters are spatially variable peak slip velocity, slip direction, rupture time and risetime. The unknown parameters are given at the nodes of the subfaults, whereas the parameters within a subfault are allowed to vary through a bilinear interpolation of the nodal values. The forward modeling is performed with a discrete wave number technique, whose Greens functions include the complete response of the vertically varying Earth structure. During the first stage, an algorithm based on the heat-bath simulated annealing generates an ensemble of models that efficiently sample the good data-fitting regions of parameter space. In the second stage (appraisal), the algorithm performs a statistical analysis of the model ensemble and computes a weighted mean model and its standard deviation. This technique, rather than simply looking at the best model, extracts the most stable features of the earthquake rupture that are consistent with the data and gives an estimate of the variability of each model parameter. We present some synthetic tests to show the effectiveness of the method and its robustness to uncertainty of the adopted crustal model. Finally, we apply this inverse technique to the well recorded 2000 western Tottori, Japan, earthquake (Mw 6.6); we confirm that the rupture process is characterized by large slip (3-4 m) at very shallow depths but, differently from previous studies, we imaged a new slip patch (2-2.5 m) located deeper, between 14 and 18 km depth.

Fault zone properties affecting the rupture evolution of the 2009 (Mw 6.1) L'Aquila earthquake (central Italy): Insights from seismic tomography

[1]xa0We have inverted P- and S-wave travel times from seismograms recorded by a dense local network to infer the velocity structure in the crustal volume where the April 6th 2009 main shock nucleated. The goal is to image local variations of P-wave velocity and Poisson ratio along the main shock fault zone for interpreting the complexity of the rupture history. The initial stages of the mainshock rupture are characterized by an emergent phase (EP) followed by an impulsive phase (IP) 0.87 s later. The EP phase is located in a very high VP and relatively low Poisson ratio (ν) region. The IP phase marks the beginning of the large moment release and is located outside the low ν volume. The comparison between the spatial variations of VP and Poisson ratio within the main shock nucleation volume inferred in this study with the rupture history imaged by inverting geophysical data allows us to interpret the delayed along-strike propagation in terms of heterogeneity of lithology and material properties.

Coulomb stress changes caused by repeated normal faulting earthquakes during the 1997 Umbria-Marche (central Italy) seismic sequence

[1]xa0We investigate fault interaction through elastic stress transfer among a sequence of moderate-magnitude main shocks (5 < Mw < 6) which ruptured distinct normal fault segments during a seismic sequence in the Umbria-Marche region (central Apennines). We also model the spatial pattern of aftershocks and their faulting mechanisms through Coulomb stress changes. We compute stress perturbations caused by earthquake dislocations in a homogeneous half-space. Our modeling results show that seven out of eight main shocks of the sequence occur in areas of enhanced Coulomb stress, implying that elastic stress transfer may have promoted the occurrence of these moderate-magnitude events. Our modeling results show that stress changes caused by normal faulting events reactivated and inverted the slip of a secondary N-S trending strike-slip fault inherited from compressional tectonics in its shallowest part (1–3 km). Of the 1517 available aftershocks, 82% are located in areas of positive stress changes for optimally oriented planes (OOPs) for Coulomb failure. However, only 45% of the 322 available fault plane solutions computed from polarity data is consistent with corresponding focal mechanisms associated with the OOPs. The comparison does not improve if we compute the optimally oriented planes for Coulomb failure by fixing the strike orientation of OOPs using information derived from structural geology. Our interpretation of these modeling results is that elastic stress transfer alone cannot jointly explain the aftershock spatial distribution and their focal mechanisms.

A frictional population model of seismicity rate change

[1]xa0We study models of seismicity rate changes caused by the application of a static stress perturbation to a population of faults and discuss our results with respect to the model proposed by Dieterich (1994). These models assume a distribution of nucleation sites (e.g., faults) obeying rate-state frictional relations that fail at constant rate under tectonic loading alone, and predicts a positive static stress step at time t0 will cause an immediate increased seismicity rate that decays according to Omoris law. We show one way in which the Dieterich model may be constructed from simple general ideas, illustrated using numerically computed synthetic seismicity and mathematical formulation. We show that seismicity rate changes predicted by these models (1) depend on the particular relationship between the clock-advanced failure and fault maturity, (2) are largest for the faults closest to failure at t0, (3) depend strongly on which state evolution law faults obey, and (4) are insensitive to some types of population heterogeneity. We also find that if individual faults fail repeatedly and populations are finite, at timescales much longer than typical aftershock durations, quiescence follows a seismicity rate increase regardless of the specific frictional relations. For the examined models the quiescence duration is comparable to the ratio of stress change to stressing rate Δτ/, which occurs after a time comparable to the average recurrence interval of the individual faults in the population and repeats in the absence of any new load perturbations; this simple model may partly explain observations of repeated clustering of earthquakes.

Chapter 7 Scaling of Slip Weakening Distance with Final Slip during Dynamic Earthquake Rupture

We discuss physical models for the characteristic slip weakening distance Dc of earthquake rupture with particular focus on scaling relations between Dc and other earthquake source parameters. We use inversions of seismic data to investigate the breakdown process, dynamic weakening, and measurement of Dc. We discuss limitations of such measurements. For studies of breakdown processes and slip weakening it is important to analyze time intervals shorter than the slip duration and those for which slip velocity is well resolved. We analyze the relationship between Dc and the parameters and , which are defined as the slip at the peak slip velocity and the peak traction, respectively. We discuss approximations and limitations associated with inferring the critical slip weakening distance from c D′ a D c D′ . Current methods and available seismic data introduce potential biases in estimates of Dc and its scaling with seismic slip due to the limited frequency bandwidth considered during typical kinematic inversions. Many published studies infer erroneous scaling between Dc and final slip due to inherent limitations, implicit assumptions, and poor resolution of the seismic inversions. We suggest that physical interpretations of Dc based on its measurement for dynamic earthquake rupture should be done with caution and the aid of accurate numerical simulations. Seismic data alone cannot, in general, be used to infer physical processes associated with Dc although the estimation of breakdown work is reliable. We emphasize that the parameters Tacc and peak slip velocity contain the same dynamic information as Dc and breakdown stress drop. This further demonstrates that inadequate resolution and limited frequency bandwidth impede to constrain dynamic rupture parameters. Cocco et al. Scaling of dynamic slip weakening distance p. 1

On the scale dependence of earthquake stress drop

We discuss the debated issue of scale dependence in earthquake source mechanics with the goal of providing supporting evidence to foster the adoption of a coherent interpretative framework. We examine the heterogeneous distribution of source and constitutive parameters during individual ruptures and their scaling with earthquake size. We discuss evidence that slip,xa0slip-weakening distance and breakdown work scale with seismic moment and are interpreted as scale dependent parameters. We integrate our estimates of earthquake stress drop, computed through a pseudo-dynamic approach, with many others available in the literature for both point sources and finite fault models. We obtain a picture of the earthquake stress drop scaling with seismic moment over an exceptional broad range of earthquake sizes (−8xa0<xa0MWxa0<xa09). Our results confirm that stress drop values are scattered over three order of magnitude and emphasize the lack of corroborating evidence that stress drop scales with seismic moment. We discuss these results in terms of scale invariance of stress drop with source dimension to analyse the interpretation of this outcomexa0in terms of self-similarity. Geophysicists are presently unable to provide physical explanations of dynamic self-similarity relying on deterministic descriptions of micro-scale processes. We conclude that the interpretation of the self-similar behaviour of stress drop scaling is strongly model dependent. We emphasize that it relies on a geometric description of source heterogeneity through the statistical properties of initial stress or fault-surface topography, in which only the latter is constrained by observations.

Complex Fault Geometry and Rupture Dynamics of the MW 6.5, 30 October 2016, Central Italy Earthquake

We study the October 30th 2016 Norcia earthquake (MW 6.5) to retrieve the rupture history by jointly inverting seismograms and coseismic GPS displacements obtained by dense local networks. The adopted fault geometry consists of a main normal fault striking N155°and dipping 47° belonging to the Mt. Vettore-Mt. Bove fault system (VBFS) and a secondary fault plane striking N210° and dipping 36° to the NW. The coseismic rupture initiated on the VBFS and propagated with similar rupture velocity on both fault planes. Up-dip from the nucleation point, two main slip patches have been imaged on these fault segments, both characterized by similar peak-slip values (~3 m) and rupture times (~3 s). After the breakage of the two main slip patches, coseismic rupture further propagated southeastward along the VBFS, rupturing again the same fault portion that slipped during the August 24th earthquake. The retrieved coseismic slip distribution is consistent with the observed surface breakages and the deformation pattern inferred from InSAR measurements. n nOur results show that three different fault systems were activated during the October 30th earthquake. The composite rupture model inferred in this study provides evidences that also a deep portion of the NNE-trending section of the Olevano-Antrodoco-Sibillini (OAS) thrust broke co-seismically, implying the kinematic inversion of a thrust ramp. The obtained rupture history indicates that, in this sector of the Apennines, compressional structures inherited from past tectonics can alternatively segment boundaries of NW-trending active normal faults or break co-seismically during moderate-to-large magnitude earthquakes.