Projecting seismicity induced by complex alterations of underground stresses with applications to geothermal systems

Seismicity associated with subsurface operations is a major societal concern. It is therefore critical to improve predictions of the induced seismic hazard. Current statistical approaches account for the physics of pore pressure increase only. Here, we present a novel mathematical model that generalises adopted statistics for use in arbitrary injection/production protocols and applies to arbitrary physical processes. In our model, seismicity is driven by a normalised integral over the spatial reservoir volume of induced variations in frictional Coulomb stress, which—combined with the seismogenic index—provides a dimensionless proxy of the induced seismic hazard. Our model incorporates the classical pressure diffusion based and poroelastic seismogenic index models as special cases. Applying our approach to modeling geothermal systems, we find that seismicity rates are sensitive to imposed fluid-pressure rates, temperature variations, and tectonic conditions. We further demonstrate that a controlled injection protocol can decrease the induced seismic risk and that thermo-poroelastic stress transfer results in a larger spatial seismic footprint and in higher-magnitude events than does direct pore pressure impact for the same amount of injected volume and hydraulic energy. Our results, validated against field observations, showcase the relevance of the novel approach to forecast seismic hazards induced by subsurface activities.

Geodetic, hydrologic and seismological signals associated with precipitation and infiltration in the central Southern Alps, New Zealand

<p>The Southern Alps of New Zealand is an actively deforming mountain range, along which collision between the Pacific and Australian plates is manifest as elevated topography, orographic weather, active contemporary deformation, and earthquakes. This thesis examines interactions between surface processes of meteorological and hydrological origin, the ground surface deformation, and processes within the seismogenic zone at depth. The two main objectives of the thesis are a better understanding of the reversible repetitive ground surface deformation in the central Southern Alps and the analysis of the evolution of the rate of microseismicity in the area to explore relationships between seismicity rates and the hydrologic cycle. Surface deformation in the central Southern Alps is characterised by a network of 19 continuous GPS stations located between the West Coast (west) and the Mackenzie Basin (east), and between Hokitika (north) to Haast (south). These show repetitive and reversible movements of up to ∼55mm on annual scales, on top of long-term plate motion, during a 17 year-long period. Stations in the high central Southern Alps exhibit the greatest annual variations, whereas others are more sensitive to changes following significant rain events. Data from 22 climate stations (including three measuring the snowpack), lake water levels and borehole pressure measurements, and numerical models of solid Earth tides and groundwater levels in bedrock fractures, are compared against geodetic data to examine whether these environmental factors can explain observed patterns in annual ground deformation. Reversible ground deformation in the central Southern Alps appears strongly correlated with shallow groundwater levels. Observed seasonal fluctuation and deformation after storm events can be explained by simple mathematical models of groundwater levels. As a corollary, local hydrological effects can be accounted for and ameliorated during preprocessing to reduce noise in geodetic data sets being analysed for tectonic purposes. Two catalogues of earthquakes (containing 38 909 and 89 474 events) in the area spanning the period 2008–2017 were built using a matched-filtered detection technique. The smaller catalogue is based on 211 template events, each of known focal mechanism, while the latter is based on 902 templates, not all of which have focal mechanisms, providing greater temporal resolution. Microseismicity data were examined in both time and frequency domains to explore relationships between seismicity rates and the hydrologic cycle. Microseismicity shows a pronounced seasonality in the central Southern Alps, with significantly more events detected during winter than during summer. These changes cannot be easily accounted for by either acquisition or analysis parameters. Two models of hydrologically-induced seasonal seismicity variations have been considered — surface water loading and deep groundwater circulation of meteoric fluids — but neither model fully explains the observations, and further work is required to explain them fully. An observed diurnal variation in earthquake detection rate is believed to originate mostly from instrumental effects, which should be accounted for in future seismological studies of earthquake occurrence in the central Southern Alps. Relationships and correlations observed between hydrological, geodetic, and seismological data from the central Southern Alps provide clear indications that surface processes exert at least some degree of influence on upper-crustal seismicity adjacent to the Alpine Fault.</p>

Geodetic, hydrologic and seismological signals associated with precipitation and infiltration in the central Southern Alps, New Zealand

<p>The Southern Alps of New Zealand is an actively deforming mountain range, along which collision between the Pacific and Australian plates is manifest as elevated topography, orographic weather, active contemporary deformation, and earthquakes. This thesis examines interactions between surface processes of meteorological and hydrological origin, the ground surface deformation, and processes within the seismogenic zone at depth. The two main objectives of the thesis are a better understanding of the reversible repetitive ground surface deformation in the central Southern Alps and the analysis of the evolution of the rate of microseismicity in the area to explore relationships between seismicity rates and the hydrologic cycle. Surface deformation in the central Southern Alps is characterised by a network of 19 continuous GPS stations located between the West Coast (west) and the Mackenzie Basin (east), and between Hokitika (north) to Haast (south). These show repetitive and reversible movements of up to ∼55mm on annual scales, on top of long-term plate motion, during a 17 year-long period. Stations in the high central Southern Alps exhibit the greatest annual variations, whereas others are more sensitive to changes following significant rain events. Data from 22 climate stations (including three measuring the snowpack), lake water levels and borehole pressure measurements, and numerical models of solid Earth tides and groundwater levels in bedrock fractures, are compared against geodetic data to examine whether these environmental factors can explain observed patterns in annual ground deformation. Reversible ground deformation in the central Southern Alps appears strongly correlated with shallow groundwater levels. Observed seasonal fluctuation and deformation after storm events can be explained by simple mathematical models of groundwater levels. As a corollary, local hydrological effects can be accounted for and ameliorated during preprocessing to reduce noise in geodetic data sets being analysed for tectonic purposes. Two catalogues of earthquakes (containing 38 909 and 89 474 events) in the area spanning the period 2008–2017 were built using a matched-filtered detection technique. The smaller catalogue is based on 211 template events, each of known focal mechanism, while the latter is based on 902 templates, not all of which have focal mechanisms, providing greater temporal resolution. Microseismicity data were examined in both time and frequency domains to explore relationships between seismicity rates and the hydrologic cycle. Microseismicity shows a pronounced seasonality in the central Southern Alps, with significantly more events detected during winter than during summer. These changes cannot be easily accounted for by either acquisition or analysis parameters. Two models of hydrologically-induced seasonal seismicity variations have been considered — surface water loading and deep groundwater circulation of meteoric fluids — but neither model fully explains the observations, and further work is required to explain them fully. An observed diurnal variation in earthquake detection rate is believed to originate mostly from instrumental effects, which should be accounted for in future seismological studies of earthquake occurrence in the central Southern Alps. Relationships and correlations observed between hydrological, geodetic, and seismological data from the central Southern Alps provide clear indications that surface processes exert at least some degree of influence on upper-crustal seismicity adjacent to the Alpine Fault.</p>

Solid as a rock: Tectonic control of graben extension and dike propagation

The 2014–2015 CE rift event associated with the Bárðarbunga eruption at Holuhraun, Iceland, offers a unique opportunity to study the spatial and temporal evolution of a rift graben. We present the first four-dimensional (three-dimensional plus time) monitoring of the formation and evolution of a graben during active magma transport using a suite of digital elevation models spanning from shortly before the eruption throughout 6 months of magma transport and up to 4.5 years after the eruption. This multiscale data set enables investigations of how magma supply and eruption dynamics affect tectonic structures that feed eruptions. After formation (time scale of a few days), the graben is remarkably stable throughout the eruption and for years beyond. It is unaffected by large changes in eruptive activity and effusion and seismicity rates within the plumbing system. These data document that (1) there was no direct feedback between eruptive dynamics and graben topography, and (2) graben formation is near instantaneous on tectonic time scales. These results challenge the overarching role ascribed to magma transport in recent studies of tectonomagmatic relationships in rift events, favoring regional tectonics as the fundamental driving force.

A New Smoothed Seismicity Approach to Include Aftershocks and Foreshocks in Spatial Earthquake Forecasting: Application to the Global Mw ≥ 5.5 Seismicity

Seismicity-based earthquake forecasting models have been primarily studied and developed over the past twenty years. These models mainly rely on seismicity catalogs as their data source and provide forecasts in time, space, and magnitude in a quantifiable manner. In this study, we presented a technique to better determine future earthquakes in space based on spatially smoothed seismicity. The improvement’s main objective is to use foreshock and aftershock events together with their mainshocks. Time-independent earthquake forecast models are often developed using declustered catalogs, where smaller-magnitude events regarding their mainshocks are removed from the catalog. Declustered catalogs are required in the probabilistic seismic hazard analysis (PSHA) to hold the Poisson assumption that the events are independent in time and space. However, as highlighted and presented by many recent studies, removing such events from seismic catalogs may lead to underestimating seismicity rates and, consequently, the final seismic hazard in terms of ground shaking. Our study also demonstrated that considering the complete catalog may improve future earthquakes’ spatial forecast. To do so, we adopted two different smoothed seismicity methods: (1) the fixed smoothing method, which uses spatially uniform smoothing parameters, and (2) the adaptive smoothing method, which relates an individual smoothing distance for each earthquake. The smoothed seismicity models are constructed by using the global earthquake catalog with Mw ≥ 5.5 events. We reported progress on comparing smoothed seismicity models developed by calculating and evaluating the joint log-likelihoods. Our resulting forecast shows a significant information gain concerning both fixed and adaptive smoothing model forecasts. Our findings indicate that complete catalogs are a notable feature for increasing the spatial variation skill of seismicity forecasts.

Modulation of Ground Deformation and Earthquakes by Rainfall at Vesuvius and Campi Flegrei (Italy)

Volcanoes are complex systems whose dynamics is the result of the interplay between endogenous and exogenous processes. External forcing on volcanic activity by seasonal hydrological variations can influence the evolution of a volcanic system; yet the underlying mechanisms remain poorly understood. In the present study, we analyse ground tilt, seismicity rates and rainfall amount recorded over 6 years (2015–2021) at Vesuvius and Campi Flegrei, two volcanic areas located in the south of Italy. The results indicate that at both volcanoes the ground deformation reflects the seasonality of the hydrological cycles, whereas seismicity shows a seasonal pattern only at Campi Flegrei. A correlation analysis on shorter time scales (days) indicates that at Vesuvius rain and ground tilt are poorly correlated, whereas rain and earthquakes are almost uncorrelated. Instead, at Campi Flegrei precipitations can affect not only ground deformation but also earthquake rate, through the combined action of water loading and diffusion processes in a fractured medium, likely fostered by the interaction with the shallow hydrothermal fluids. Our observations indicate a different behavior between the two volcanic systems: at Vesuvius, rain-induced hydrological variations poorly affect the normal background activity. On the contrary, such variations play a role in modulating the dynamics of those metastable volcanoes with significant hydrothermal system experiencing unrest, like Campi Flegrei.

Ground motions variability in Israel from 3-D simulations of M 6 and M 7 earthquakes

In Israel, due to low seismicity rates and sparse seismic network, the temporal and spatial coverage of ground motion data is insufficient to estimate the variability of moderate-strong (M > 6) ground motions required to construct a local ground motion model (GMM). To fill this data gap and to study the ground motions variability of M > 6 events, we performed a series of 3-D numerical simulations of M 6 and M 7 earthquakes. Based on the results of the simulations, we developed a statistical attenuation model (AM) and studied the residuals between simulated and AM PGVs and the single station variability. We also compared the simulated ground motions with a global GMM in terms of peak ground velocity (PGV) and significant duration (Ds 595). Our results suggest that the AM was unable to fully capture the simulated ground motions variability, mainly due to the incorporation of super-shear rupture and effects of local sedimentary structures. We also show that an imported GMM considerably deviates from simulated ground motions. This work sets the basis for future development of a comprehensive GMM for Israel, accounting for local sources, path, and site effects.

Upper Plate Structure and Tsunamigenic Faults near the Kodiak Islands

The Kodiak Islands lie near the southern terminus of the 1964 Great Alaska earthquake rupture area and within the Kodiak subduction zone segment. Both local and trans-Pacific tsunamis were generated during this devastating megathrust event, but the local tsunami source region and the causative faults are poorly understood. We provide an updated view of the tsunami and earthquake hazard for the Kodiak Islands region through tsunami modelling and geophysical data analysis. Through seismic and bathymetric data, we characterize a regionally extensive sea floor lineament related to the Kodiak shelf fault zone, with focused uplift along a 50-km long portion of the newly named Ugak fault as the most likely source of the local Kodiak Islands tsunami in 1964. We present evidence of Holocene motion along the Albatross Banks fault zone, but suggest that this fault did not produce a tsunami in 1964. We relate major structural boundaries to active forearc splay faults, where tectonic uplift is collocated with gravity lineations. Differences in interseismic locking, seismicity-rates, and potential field signatures argue for different stress conditions at depth near presumed segment boundaries. We find that the Kodiak segment boundaries have a clear geophysical expression and are linked to upper plate structure and splay faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf fault zone when compared to the nearby Albatross Banks fault zone, suggesting short travel paths and little tsunami warning time for nearby communities.

Time-dependent Seismic Footprint of Thermal Loading for Geothermal Activities in Fractured Carbonate Reservoirs

This paper describes and deploys a workflow to assess the evolution of seismicity associated to injection of cold fluids close to a fault. We employ a coupled numerical thermo-hydro-mechanical simulator to simulate the evolution of pressures, temperatures and stress on the fault. Adopting rate-and-state seismicity theory we assess induced seismicity rates from stressing rates at the fault. Seismicity rates are then used to derive the time-dependent frequency-magnitude distribution of seismic events. We model the seismic response of a fault in a highly fractured and a sparsely fractured carbonate reservoir. Injection of fluids into the reservoir causes cooling of the reservoir, thermal compaction and thermal stresses. The evolution of seismicity during injection is non-stationary: we observe an ongoing increase of the fault area that is critically stressed as the cooling front propagates from the injection well into the reservoir. During later stages, models show the development of an aseismic area surrounded by an expanding ring of high seismicity rates at the edge of the cooling zone. This ring can be related to the “passage” of the cooling front. We show the seismic response of the fault, in terms of the timing of elevated seismicity and seismic moment release, depends on the fracture density, as it affects the temperature decrease in the rock volume and thermo-elastic stress change on the fault. The dense fracture network results in a steeper thermal front which promotes stress arching, and leads to locally and temporarily high Coulomb stressing and seismicity rates. We derive frequency-magnitude distributions and seismic moment release for a low-stress subsurface and a tectonically active area with initially critically stressed faults. The evolution of seismicity in the low-stress environment depends on the dimensions of the fault area that is perturbed by the stress changes. The probability of larger earthquakes and the associated seismic risk are thus reduced in low-stress environments. For both stress environments, the total seismic moment release is largest for the densely spaced fracture network. Also, it occurs at an earlier stage of the injection period: the release is more gradually spread in time and space for the widely spaced fracture network.

A Big Problem for Small Earthquakes: Benchmarking Routine Magnitudes and Conversion Relationships with Coda Envelope-Derived Mw in Southern Kansas and Northern Oklahoma

ABSTRACT Earthquake magnitudes are widely relied upon measures of earthquake size. Although moment magnitude (Mw) has become the established standard for moderate and large earthquakes, difficulty in reliably measuring seismic moments for small (generally Mw&lt;4) earthquakes has meant that magnitudes for these events remain plagued by a patchwork of inconsistent measurement scales. Because of this, magnitudes of small earthquakes and statistics derived from them can be biased. Furthermore, because small earthquakes are much more numerous than large ones, many applications, such as seismic hazard modeling, depend critically on analysis of events characterized by magnitudes other than Mw. To assess this problem, we apply coda envelope analysis to reliably determine moment magnitudes for a case study of small earthquakes from northern Oklahoma and southern Kansas. Not surprisingly, we find significant differences among ML, mbLg, and Mw for M ∼2–4 earthquakes examined here. More troublingly, we find that relations designed to convert other magnitudes to Mw, which are relied upon for important applications such as seismic hazard analysis, often increase rather than decrease this bias for our dataset. In our case study, we find that converted magnitudes can result in a systematic bias sometimes exceeding 0.5 magnitude units, a difference that typically corresponds to a factor of ∼3 in seismicity rate. Moreover, we find a correspondingly large bias in Gutenberg–Richter b-values, controlled primarily by inaccurate magnitude scaling in the conversion relationships. Although this study focuses on a relatively small geographic area, we can expect that similar issues exist with varying severity in other regions. Therefore, magnitudes of small earthquakes and their associated statistics, including seismicity rates and b-values, should be treated with caution.