Towards a poroelastodynamics framework for induced earthquakes

Earthquakes can be triggered after pore pressure perturbations activate critically stressed seismogenic faults, where the perturbations can originate from natural causes like earth tides, rainfall, snowfall or anthropogenic causes like wastewater disposal, CO$_2$ injection, oil production, or groundwater extraction. As the faults slip under the action of the induced stress field, seismic waves are spawned from the hypocenter location. The waves propagate through the domain with a velocity that evolves with the evolving pressure and stress fields. The effect of these waves on the surrounding rock and the seismic velocity recorded on the seismograph can be modeled accurately only by incorporating elastodynamics in the deformation model coupled with flow-induced pressure perturbations. Hitherto, most of the literature in the realm has been limited to elastostatics coupled with flow within a prescribed/kinematic or quasi-dynamic fault slip framework. In this work, we provide a framework for coupling of wave propagation with pore pressure perturbations using one-way coupled poroelastodynamics in the presence of faults in which the pore pressure is specified apriori as a spatiotemporal function.We present results from analysis of displacement and velocity fields in the domain and tractions and slip evolution on the fault. The rendition of two-way coupled poroelastodynamics in which the flow problem is also solved is proposed as future work.

­Gas Fields and Large Shallow Seismogenic Reverse Faults Are Anticorrelated

We investigated the spatial relationships among 18 known seismogenic faults and 1,612 wells drilled for gas exploitation in the main hydrocarbon province of northern-central Italy, a unique dataset worldwide. We adopted a GIS approach and a robust statistical technique, and found a significant anticorrelation between the location of productive wells and of the considered seismogenic faults, which are generally overlain or encircled by sterile wells.Our observations suggest that (a) earthquake ruptures encompassing much of the upper crust may cause gas to be lost to the atmosphere over geological time, and that (b) reservoirs underlain by smaller or aseismic faults are more likely to be intact.These findings, which are of inherently global relevance, have crucial implications for future hydrocarbon exploitation, for assessing the seismic-aseismic behavior of large reverse faults, and for the public acceptance of underground energy storage facilities in tectonically active areas, a pillar of future low carbon energy systems.

Seismogenic Faults of the Changning Earthquake Sequence Constrained by High-Resolution Seismic Profiles in the Southwestern Sichuan Basin, China

The seismicity rate in the southwestern Sichuan basin, China, dramatically increased after 2014. The associated moderate earthquakes may have been induced by salt mining or shale gas exploration. The location of the seismogenic faults causing these moderate earthquakes has not been confirmed, resulting in a lack of understanding of the earthquake mechanisms in the study area. The detailed structural characteristics of pre-existing faults, which are typically responsible for induced seismicity, are unclear. In this study, we used high-resolution seismic reflection profiles in conjunction with geological, seismologic, and geodetic data to reveal the 3D distributions of the seismogenic faults. Basement thrust faults in the Changning anticline were identified using seismic interpretations and are associated with the 2019 Changning earthquake sequence. The geometry and location of these pre-existing faults are consistent with previous studies of the seismology and structural geology in the area. The well-developed pre-existing fault system in the sedimentary cover and basement makes the Changning area vulnerable to induced earthquakes. Present-day reactivation of the basement fault system reveals the unstable state of the local tectonic stress field. It is possible that the potential seismic risk in this region could be increased by industrial activity in the southwestern Sichuan basin.

Analysis of Petrinja 2020 Earthquake (Croatia) and First 500 Aftershocks >M1.0, Identification of Their Sources and Seismogenic Faults

Strong earthquake of M6.4 stroke Petrinja and neighbouring cities of Sisak and Glina in Croatia on December 29th 2020. It was preceded by two foreshocks of M5.2 and M5.0, and followed by a series of aftershocks of various magnitudes and intensities. We have analysed first 500 earthquakes and aftershocks of > M1.0 which occurred from December 28th 2020 to January 19th 2021, their frequency, focal depths, and coseismic surface phenomena. Correlation of focal depths revealed the source of earthquakes was faulting of hanging wall of a listric normal fault with NW-SE strike and dip towards NE. Major fault seems to have caused earthquakes with only minor magnitudes. The strongest two earthquakes of M6.4 and M5.2 were initiated on synthetic fault, whereas M5.0 earthquake was initiated on an antithetic fault. Almost 50% of all seismic energy of the first 500 analysed seismic events over M1.0 was released on 1 km and 10 km deep hypocentres. Focal mechanisms of major earthquakes and strong fore- and aftershocks indicate dextral-slip mechanism, which is also in accordance with the orientation of surface cracks, land faulting and sand volcano trains. Co-seismic surface phenomena are land cracks and fissures, land faults, sand volcanoes, eruptive springing of ground water, activation of landslides, and formation of dozens of collapse sinkholes which continued to form and grow for about a month following the major earthquake.

Quaternary Deposits Affect Coseismic Offset on Thrust Faults: Evidence from the 2008 Mw 7.9 Wenchuan, China, Earthquake

Coseismic offset is an important parameter to determine the characteristics of surface ruptures produced by large earthquakes and has significant implications for understanding fault-zone mechanics. To date, most studies have focused on broad-wavelength variations in coseismic offset and their related mechanisms. However, high-frequency variations in coseismic offset have been less commonly reported due to difficulty in field identification. Here, we show that three sites have typical abrupt changes in coseismic offset within short distances along surface ruptures produced by the 2008 Mw 7.9 Wenchuan earthquake. The Bailu, Qingping, and Xiaoyudong sites on different segments of the seismogenic faults show that coseismic vertical offsets can vary from ∼0.6 to ∼1.7 m at neighboring locations. Moreover, the offset gradients at the three sites are estimated from ∼2.5 to ∼30.9 m/km. Based on geologic and geophysical investigations at the three sites, we suggest that Quaternary deposits are the primary factor affecting coseismic offset. Specifically, thick and loosely packed deposits are more likely to yield smaller coseismic offsets than thin and densely packed deposits. Finally, through a compilation of recent thrust-type earthquakes, we suggest that the coseismic vertical offset gradient for thrust faults can vary greatly, which requires caution in seismic hazard assessments when designing linear infrastructure projects and constraining slip rates at specific sites.

Active Fault Systems in the Inner Northwest Apennines, Italy: A Reappraisal One Century after the 1920 Mw ~6.5 Fivizzano Earthquake

Based on the review of the available stratigraphic, tectonic, morphological, geodetic, and seismological data, along with new structural observations, we present a reappraisal of the potential seismogenic faults and fault systems in the inner northwest Apennines, Italy, which was the site, one century ago, of the devastating Mw ~6.5, 1920 Fivizzano earthquake. Our updated fault catalog provides the fault locations, as well as the description of their architecture, large-scale segmentation, cumulative displacements, evidence for recent to present activity, and long-term slip rates. Our work documents that a dense network of active faults, and thus potential earthquake fault sources, exists in the region. We discuss the seismogenic potential of these faults, and propose a general tectonic scenario that might account for their development.

A new 3D velocity model of Central Apennines: insights from 2D/3D geological modelling and earthquake relocation.

<p>The Central Apennines fold-and-thrust belt (Central Italy) is characterized by the presence of several active faults, potentially capable of generating damaging earthquakes. To support seismic hazard studies over the area, a new 3D velocity model was built, integrating a wide range of surface and subsurface data.</p><p>The tectonic framework of the area (from Sulmona plain to Maiella Mt), is still debated in literature, also due to the lack of both an adequate geophysical data set and a reliable velocity model at the crustal scale.</p><p>In addition, the low number of seismic stations available for the acquisition of Vp/Vs arrival times, and the very low seismicity detected in the study area (the Sulmona and Caramanico Apennine valleys are considered as “seismic gaps”), lead to a difficult interpretation of the subsurface tectonic structures.</p><p>3D velocity modelling could well represent an important tool to support these deep crustal reconstructions as well earthquake relocation studies and could enhance the definition of seismogenic faults deep geometries, hence supporting a better risk assessment over the area of these potential locked faults.</p><p>Using the knowledge developed within the oil&gas industry as well in gas/CO<sub>2</sub> storage projects for the construction of 3D velocity models, extensively used to obtain subsurface imaging and define the geometry of the reservoirs and traps in the depth domain, a similar methodological approach was implemented over the study area.</p><p>The subsurface dataset was partially inherited by the past hydrocarbon exploration activities (e.g. seismic lines, exploration wells and sonic logs) and by the literature (e.g. time/depth regional models). Tomographic sections and relocated earthquake hypocentres were also integrated form geophysical studies. Geological maps (1:50.000 & 1:100.000 scale) represent the surface dataset that we used to create the surface interpretation of the regional geology.</p><p>As a first step, 18 2D balanced regional geological cross-sections, dip-oriented (W-E) across the Central Apennine, were built define the structural picture at regional scale. The cross-sections were built using MOVE (Petroleum Experts) and Petrel (Schlumberger) software. The following modelling step was the 3D model construction, in which the surface/subsurface data as well as all the geological sections were integrated in the final 3D structural and geological model.</p><p>The main geological layers reconstructed in the 3D model were than populated using the appropriated interval velocity values, building the final 3D velocity model in which the lateral velocity variation due to the presence of different facies/geological domains were considered.</p><p>As one of the results, we defined several 1D-velocity models coherent with the regional 3D velocity model, in which the key seismic stations and the earthquakes hypocentres dataset for the most potential seismogenic faults were included. 1D models were characterized by different degree of simplification, in order to test diverse approaches for the earthquake relocation. For this exercise, we used public dataset extracted by the analysis of microseismicity of the Sulmona basin.</p><p>We believe that the proposed approach can represents an effective method for combining geological and geophysical data to improve the subsurface and seismogenic faults interpretation, contributing to the seismic hazard assessment.</p>

A European Fault Database as a stepping stone towards improved subsurface evaluation of hazards and resources

<p>Faults are prominent features in the subsurface that define the geological development and distribution of geological formations and resources therein. Faults can define resources themselves (e.g. minerals, thermal conduits), but more often they can pose a hazard to subsurface drilling, injection and extraction activities . Well-known examples are Basel – Switzerland (geothermal stimulation), Oklahoma – US (waste water injection) and Groningen – The Netherlands (conventional hydrocarbon extraction).</p><p>Despite that faults are a typical product of geological mapping, there was, until now, no consistent insight in these structures in a pan-European context. There are some examples focusing on the publication of seismogenic faults (e.g. GEM Global Active Faults Database, SHARE  European Database of Seismogenic Faults, USGS Quaternary faults database), yet deeply buried faults are under-represented here. With the European fault database, the GeoERA-HIKE project addresses the following objectives: i) develop a consistent and uniform repository for fault data and characteristics across Europe, ii) Implement an associated tectonic vocabulary which provides a framework for future interpretation, modelling and application of fault data, and iii) assess the applicability of fault data in case studies.</p><p>The current fault database is envisioned to be a major stepping stone for a sustained and uniform development and dissemination of tectonic data and knowledge which will be applicable to a broad spectrum of subsurface research challenges. The database contains data from Geological Survey Organizations and partners in the Netherlands, Germany, Austria, Belgium, Iceland, Denmark, Poland, Lithuania, Italy, France, Ukraine, Portugal, Slovenia, Albania and various countries in the Pannonian Basin Area.</p><p>The GeoERA-HIKE project has received funding from the European Union’s Horizon 2020 research and innovation programme under agreement No. 731166</p>