Petrophysical, geochemical, and hydrological evidence for extensive fracture-mediated fluid and heat transport in the Alpine Fault's hanging-wall damage zone

© 2017. American Geophysical Union. All Rights Reserved. Fault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging-wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP-2). We present observational evidence for extensive fracturing and high hanging-wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP-2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging-wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off-fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation.

Petrophysical, geochemical, and hydrological evidence for extensive fracture-mediated fluid and heat transport in the Alpine Fault's hanging-wall damage zone

© 2017. American Geophysical Union. All Rights Reserved. Fault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging-wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP-2). We present observational evidence for extensive fracturing and high hanging-wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP-2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging-wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off-fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation.

Sensitivity Analysis of the Interfrequency Correlation of Synthetic Ground Motions to Pseudodynamic Source Models

Given the deficiency of recorded strong ground-motion data, it is important to understand the effects of earthquake rupture processes on near-source ground-motion characteristics and to develop physics-based ground-motion simulation methods for advanced seismic hazard assessments. Recently, the interfrequency correlation of ground motions has become an important element of ground-motion predictions. We investigate the effect of pseudodynamic source models on the interfrequency correlation of ground motions by simulating a number of ground-motion waveforms for the 1994 Northridge, California, earthquake, using the Southern California Earthquake Center Broadband Platform. We find that the cross correlation between earthquake source parameters in pseudodynamic source models significantly affects the interfrequency correlation of ground motions in the frequency around 0.5 Hz, whereas its effect is not visible in the other frequency ranges. Our understanding of the effects of earthquake sources on the characteristics of near-source ground motions, particularly the interfrequency correlation, may help develop advanced physics-based ground-motion simulation methods for advanced seismic hazard and risk assessments.

A stochastic rupture earthquake code based on the fiber bundle model (TREMOL v0.1): application to Mexican subduction earthquakes

In general terms, earthquakes are the result of brittle failure within the heterogeneous crust of the Earth. However, the rupture process of a heterogeneous material is a complex physical problem that is difficult to model deterministically due to numerous parameters and physical conditions, which are largely unknown. Considering the variability within the parameterization, it is necessary to analyze earthquakes by means of different approaches. Computational physics may offer alternative ways to study brittle rock failure by generating synthetic seismic data based on physical and statistical models and through the use of only few free parameters. The fiber bundle model (FBM) is a stochastic discrete model of material failure, which is able to describe complex rupture processes in heterogeneous materials. In this article, we present a computer code called the stochasTic Rupture Earthquake MOdeL, TREMOL. This code is based on the principle of the FBM to investigate the rupture process of asperities on the earthquake rupture surface. In order to validate TREMOL, we carried out a parametric study to identify the best parameter configuration while minimizing computational efforts. As test cases, we applied the final configuration to 10 Mexican subduction zone earthquakes in order to compare the synthetic results by TREMOL with seismological observations. According to our results, TREMOL is able to model the rupture of an asperity that is essentially defined by two basic dimensions: (1) the size of the fault plane and (2) the size of the maximum asperity within the fault plane. Based on these data and few additional parameters, TREMOL is able to generate numerous earthquakes as well as a maximum magnitude for different scenarios within a reasonable error range. The simulated earthquake magnitudes are of the same order as the real earthquakes. Thus, TREMOL can be used to analyze the behavior of a single asperity or a group of asperities since TREMOL considers the maximum magnitude occurring on a fault plane as a function of the size of the asperity. TREMOL is a simple and flexible model that allows its users to investigate the role of the initial stress configuration and the dimensions and material properties of seismic asperities. Although various assumptions and simplifications are included in the model, we show that TREMOL can be a powerful tool to deliver promising new insights into earthquake rupture processes.

Paleoseismological uncertainty estimation in the Acambay region, Central Mexico

Paleseismological studies provide valuable information of the earthquake rupture processes such as fault dimensions, average and maximum displacements, as well as recurrence times and magnitudes of events which took place in the geologic past. This information is based on observations of the geological record. Interpretation of geological observa-tions has a source of uncertainties inherent to the large number of hypothesis that explain the observed geological features. Information obtained from paleoseismic studies is im-portant in seismic hazard analyses, and particularly crucial for regions of low seismic activity where the recurrence period of major earthquakes reaches several thousand years. However, using this information in hazard analysis requires the systematic treatment of uncertainties. We estimated uncertainties of four paleoseismological studies conducted at three different faults of the Acambay graben region in Central Mexico. The method used is based on a logic-tree formalism that quantifies the cumulative uncertainties associated with the different stages of the paleoseismic studies together with a quantification of the entropy at each step and at the end of the process. The final uncertainty and its relative importance in seismic hazard analysis is expressed as the paleoseismic quality factor, which indicate 0.14, 0.40-0.50, and 0.41 for the Acambay-Tixmadejé, Pastores and San Mateo faults, respectively. These values can be incorporated in seismic hazard analyses for the region.