pore fluid pressure
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2022 ◽  
Author(s):  
Miles P. Wilson ◽  
Gillian R. Foulger ◽  
Christopher Saville ◽  
Samuel P. Graham ◽  
Bruce R. Julian

ABSTRACT Relationships between the weather and earthquakes have been suspected for over 2400 yr. However, scientific evidence to support such relationships has grown only since the 1980s. Because faults in Earth’s crust are generally regarded as critically stressed, small changes in stress and pore-fluid pressure brought about by rainfall, snow, and atmospheric pressure and temperature variations have all been proposed to modulate seismicity at local and regional scales. Elastic static stress changes as low as 0.07 kPa and pore-fluid pressure changes as low as 0.5 kPa have been proposed to naturally trigger earthquakes. In the UK, the spatial distributions of onshore earthquakes and rainfall are highly nonuniform and may be related; the wetter and most naturally seismically active areas occur on the west side of the country. We found significant spatial and temporal relationships between rainfall amount and the number of earthquakes for 1980–2012, suggesting larger volumes of rainfall promote earthquake nucleation. Such relationships occur when human-induced seismicity is included or excluded, indicating that meteorological conditions can also modulate seismicity induced by subsurface anthropogenic activities such as coal mining. No significant relationships were observed for monthly time lags, suggesting that the triggering effect of rainfall in the UK is near-instantaneous or occurs within 1 mo. With global climate changing rapidly and extreme weather events occurring more frequently, it is possible that some global regions may also experience changes in the spatial and temporal occurrence of earthquakes in response to changes in meteorologically induced stress perturbations.


2021 ◽  
Author(s):  
◽  
Anna Karen Pulford

<p>Lithospheric deformation along and adjacent to the Pacific-Australian Plate boundary through New Zealand has resulted in different expressions in North and South Islands. This thesis investigates some aspects of crustal and upper mantle structure in New Zealand and is divided into two distinct parts. The first examines the structure of the obliquely compressional crustal plate boundary in South Island using seismic techniques; the second focuses on the domed topography of central North Island and its relationship to mantle processes. High density active source, one and three-component, seismic data from a transect across the Southern Alps provides information on the deformation of the crust across the Australia-Pacific plate boundary of South Island. These data show 0-0.08 s ([approximately] 0.25 %) delay times between the radial and transverse directions for shear waves (Sg and SmS phases), with maximum possible delays of 140 ms and the fast direction aligned with the transverse direction (approximately parallel to the plate boundary). The transect is perpendicular to the Alpine Fault, which is slightly oblique to the fast mantle directions determined from SKS phases. The small values of crustal splitting may result from the oblique angle of the ray paths to the actual crustal structure at depth, or the complex nature of the deformation as observed at the surface, which though on a small scale can be strongly anisotropic, may not add constructively over a large region. Poisson's ratio, determined from forward modelling of both P and S phases, shows low values of 0.21 - 0.24 for the crust of South Island. A broad region of low values ([sigma]=0.15) exists at 10-20 km depth under the Southern Alps, which corresponds to a previously identified body of low Vp and high resistivity. The low [sigma] is interpreted as low pore fluid pressure and high silica composition rocks. This contrasts with previous interpretations of iii iv high pore fluid pressure at this depth. The topography of central North Island, New Zealand, describes a 250 km wide and [approximately] 500 m high dome. Exhumation estimates from mudstone porosity measurement indicate an increase in exhumation from [approximately] 500 m at the coast to 2 km in the region of the present topographic high. Combining these values gives an estimate of rock uplift of over 2.5 km for central North Island, since 4 Ma, a rate of 0.6 mm/yr. Tectonic uplift of 1.25 km indicates that [approximately] 50 % of the rock uplift occurs in response to exhumation. An independent local estimation of differential erosion in central North Island gave 300 m of exhumation since at least 500 ka, a rate of [greater than or equal to] 0.6 mm/yr. Using a digital elevation model of New Zealand the fluvial incision of the landscape was calculated and [approximately]169 m of rebound can be attributed to incision. Contouring maximum incision elucidates a region of high incision [approximately] 50 km south of the present centre of domed rock uplift. Using incision as a proxy for rock uplift, it is hypothesised that the incision signal is recent and demonstrates the southward migration of the centre of rock uplift. Rebound of sedimentary basins due to a reduction in plate coupling forces can also account for some of the observed rock uplift. Buoyancy forces required to create the pattern and magnitude of rock uplift are investigated using a 3 D loading model of the lithosphere. Strong upward forces (65 MPa) are required under the Central Volcanic Region, combined with broad uplift (36 MPa) over western North Island, to fit the observed rock uplift. Low Pn velocities under the Central Volcanic Region indicate temperatures 500 [degrees] C hotter than that of normal mantle. This temperature anomaly corresponds to 60 kg/[cubic metre] less dense than normal mantle, which is consistent with the change in density of 66 kg/[cubic metre] estimated from the loading model and aassuming the density change occurs over a 100 km depth range. The southern extent of buoyancy forces does not correspond well to regions of high seismic attenuation in the lithosphere but instead with the region of high incision.</p>


2021 ◽  
Author(s):  
◽  
Anna Karen Pulford

<p>Lithospheric deformation along and adjacent to the Pacific-Australian Plate boundary through New Zealand has resulted in different expressions in North and South Islands. This thesis investigates some aspects of crustal and upper mantle structure in New Zealand and is divided into two distinct parts. The first examines the structure of the obliquely compressional crustal plate boundary in South Island using seismic techniques; the second focuses on the domed topography of central North Island and its relationship to mantle processes. High density active source, one and three-component, seismic data from a transect across the Southern Alps provides information on the deformation of the crust across the Australia-Pacific plate boundary of South Island. These data show 0-0.08 s ([approximately] 0.25 %) delay times between the radial and transverse directions for shear waves (Sg and SmS phases), with maximum possible delays of 140 ms and the fast direction aligned with the transverse direction (approximately parallel to the plate boundary). The transect is perpendicular to the Alpine Fault, which is slightly oblique to the fast mantle directions determined from SKS phases. The small values of crustal splitting may result from the oblique angle of the ray paths to the actual crustal structure at depth, or the complex nature of the deformation as observed at the surface, which though on a small scale can be strongly anisotropic, may not add constructively over a large region. Poisson's ratio, determined from forward modelling of both P and S phases, shows low values of 0.21 - 0.24 for the crust of South Island. A broad region of low values ([sigma]=0.15) exists at 10-20 km depth under the Southern Alps, which corresponds to a previously identified body of low Vp and high resistivity. The low [sigma] is interpreted as low pore fluid pressure and high silica composition rocks. This contrasts with previous interpretations of iii iv high pore fluid pressure at this depth. The topography of central North Island, New Zealand, describes a 250 km wide and [approximately] 500 m high dome. Exhumation estimates from mudstone porosity measurement indicate an increase in exhumation from [approximately] 500 m at the coast to 2 km in the region of the present topographic high. Combining these values gives an estimate of rock uplift of over 2.5 km for central North Island, since 4 Ma, a rate of 0.6 mm/yr. Tectonic uplift of 1.25 km indicates that [approximately] 50 % of the rock uplift occurs in response to exhumation. An independent local estimation of differential erosion in central North Island gave 300 m of exhumation since at least 500 ka, a rate of [greater than or equal to] 0.6 mm/yr. Using a digital elevation model of New Zealand the fluvial incision of the landscape was calculated and [approximately]169 m of rebound can be attributed to incision. Contouring maximum incision elucidates a region of high incision [approximately] 50 km south of the present centre of domed rock uplift. Using incision as a proxy for rock uplift, it is hypothesised that the incision signal is recent and demonstrates the southward migration of the centre of rock uplift. Rebound of sedimentary basins due to a reduction in plate coupling forces can also account for some of the observed rock uplift. Buoyancy forces required to create the pattern and magnitude of rock uplift are investigated using a 3 D loading model of the lithosphere. Strong upward forces (65 MPa) are required under the Central Volcanic Region, combined with broad uplift (36 MPa) over western North Island, to fit the observed rock uplift. Low Pn velocities under the Central Volcanic Region indicate temperatures 500 [degrees] C hotter than that of normal mantle. This temperature anomaly corresponds to 60 kg/[cubic metre] less dense than normal mantle, which is consistent with the change in density of 66 kg/[cubic metre] estimated from the loading model and aassuming the density change occurs over a 100 km depth range. The southern extent of buoyancy forces does not correspond well to regions of high seismic attenuation in the lithosphere but instead with the region of high incision.</p>


SPE Journal ◽  
2021 ◽  
pp. 1-23 ◽  
Author(s):  
Kien Nguyen ◽  
Amin Mehrabian ◽  
Ashok Santra ◽  
Dung Phan

Summary Estimation of near-wellbore fracture widths remains central to designing the particle size distribution (PSD) and composition of lost circulation material (LCM) blends. Although elastic rock models are often used for this purpose, they fall short in capturing the substantial effect of pore fluid pressure on the fracture width. The problem is addressed in this paper by incorporating the poroelastic back stress in width estimation of axial fractures nearby an inclined wellbore. The poroelastic back stress is caused by a nonideal drilling fluid filter cake allowing for fluid pressure communication between the wellbore and pore space of the rock surrounding the wellbore. In this aspect, a proper definition of the filter-cake efficiency is made in terms of the wellbore pressure, far-field pore fluid pressure, and pore fluid pressure of the rock surrounding the wellbore. The value of this parameter is estimated from the standard drilling fluid filtration test results, as well as the formation rock permeability. The filter-cake efficiency is next used to develop the long-time, asymptotic analytical solution for the poroelastic stress of an inclined wellbore. By accounting for the obtained poroelastic back stress, an improved estimation of the wellbore tensile limit that depends on the filter-cake efficiency parameter is developed. For wellbore pressures beyond the wellbore tensile limit, the width of the near-wellbore fractures is estimated. The fracture width estimation is made through an analytical, dislocation-based fracture mechanics solution to the integral equation describing the nonlocal stress equilibrium along the fracture faces. The commonly practiced scheme for geometric design of LCM blends is enhanced by using the presented improvement in estimation of the near-wellbore fracture width. A case study is used to demonstrate the substantial effect of drilling fluid filtration properties and the resulting poroelastic back stress on the wellbore tensile limit, estimated fracture width, and consequently, composition of the recommended LCM blend.


2021 ◽  
Author(s):  
Zoe Braden ◽  
Jonas B. Ruh ◽  
Whitney M. Behr

&lt;p&gt;Observations of several active shallow subduction megathrusts suggest that they are localized as d&amp;#233;collements within sedimentary sequences or at the contact between sedimentary layers and the underlying mafic oceanic crust.&amp;#160; Exhumed accretionary complexes from a range of subduction depths, however, preserve underplated mafic slivers, which indicate that megathrust faults can occasionally develop within the mafic oceanic crustal column. The incorporation of mafic rocks into the subduction interface shear zone has the potential to influence both long-term subduction dynamics and short-term seismic and transient slip behaviour, but the processes and conditions that favour localisation of the megathrust into deeper oceanic crustal levels are poorly understood.&lt;/p&gt;&lt;p&gt;In this work, we use visco-elasto-plastic numerical modelling to explore the long-term (million year) factors influencing the incorporation of mafic volcanic rocks into the subduction interface and accretionary wedge through underplating. We focus on the potential importance of oceanic seafloor alteration in facilitating oceanic crustal weakening, which is implemented through a temperature-dependent pore-fluid pressure ratio (lambda = 0.90-0.99 between 160 and 300oC). We then examine the underplating response to changes in sediment thickness, geothermal gradient, sediment fluid pressure, and surface erosion rates. Our results indicate that a thinner incoming sediment package and a lower geothermal gradient cause oceanic crustal underplating to initiate deeper beneath the backstop (overriding plate) compared to thicker incoming sediment and a higher geothermal gradient. Relative pore fluid pressure differences between sediments and altered oceanic crust control the amount of altered oceanic crust that is underplated, as well as the location of underplating beneath the backstop or accretionary wedge. When sediments on top of the altered oceanic crust have the same fluid pressure as the altered oceanic crust, no oceanic crustal underplating occurs. Modelling results are also compared to exhumed subduction complexes to examine the amount and distribution of underplated mafic rocks.&lt;/p&gt;


2021 ◽  
Author(s):  
Maurizio Battaglia ◽  
Carolina Pagli ◽  
Stefano Meuti

&lt;p&gt;Volcanoes commonly subside during eruptions as magma flows out of a chamber, but continued subsidence during non-eruptive episodes is not easy to explain. In this work, we use InSAR and source modelling to understand the causes of the continued subsidence of Dallol, a nascent volcano along the spreading Erta Ale ridge of Afar (Ethiopia). The Dallol volcano never erupted and no volcanic deposits originating from the volcano exists at the surface. Recent seismicity, diking and continuous deformation of a crustal magma chamber indicate the Dallol is a nascent central volcano with its own rift segment. An active magma plumbing exists and the injection of a dike beneath the volcano was imaged in 2004 from InSAR data. This unrest episode was followed by complete quiescence until subsidence started in 2008. We analysed InSAR data from 2004-2010 to create time-series of line-of-sight (LOS) surface deformation. Average velocity maps show that subsidence centred at Dallol initiated in October 2008 and continued as far as February 2010 at an approximately regular rate of up to 10 cm/yr. The inversion of InSAR average velocities found that a sill-like source, located a depth between 1.2 and 1.5 km under Dallol with a mean volume change of &amp;#160;-0.62 to -0.53 10&lt;sup&gt;6&lt;/sup&gt; km&lt;sup&gt;3&lt;/sup&gt;/yr and a radius of approximately 1.6 km, best fits the InSAR observations. The observed volume change could be explained by changes in pore fluid pressure in a confined hydrothermal aquifer or by thermoelastic deformation caused by changes in temperature in a volume of rock. Simple models of poro-elastic and thermo-elastic contraction indicates that the observed deformation would require either a decrease in pore fluid pressure of the order of 10&lt;sup&gt;-2&lt;/sup&gt;G, where G is the rock shear modulus, or a decrease in temperature between 60 &amp;#176;C and 80 &amp;#176;C. &amp;#160;&lt;/p&gt;


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