Near-lithostatic Pore Pressure Research Articles

Drought-induced seismicity modulation in the New Madrid Seismic Zone, central United States

The New Madrid Seismic Zone (NMSZ) in the central United States is a seismically active intraplate region composed of two fault segments: the Reelfoot and Cottonwood Grove fault segments. It has witnessed several major earthquakes, including a devastating 1811–1812 earthquake sequence of three ~M7 events. Nearly 200 years later, earthquakes still continue today in this complex seismic zone. This seismic zone is located in a domain with higher hydrological load than surrounding regions, which may play a crucial role in seismicity modulation. However, the hydrological unloading or loading-induced seismicity modulation and the underlying earthquake dynamics on this stable plate interior remain equivocal. Our study demonstrates that increased climate variability and drought-induced hydrological unloading can be potential drivers for the crustal stress change in the upper Mississippi embayment and seismicity modulation in the NMSZ. The seismicity rate associated with the Reelfoot fault segment shows ~60% increase during drought-induced prolonged drier periods, linked to La Niña cycles, than during the relatively wetter periods. However, such a feature is lacking for the seismicity associated with the Cottonwood Grove fault segment. We argue that the near-lithostatic pore pressure condition explicitly on the Reelfoot fault segment leads to an increase in the amplitude of the velocity perturbation, making this fault segment more susceptible to seismicity modulation on a multi-annual or annual time scale by the resonance destabilization process.

Episodic, compression-driven fluid venting in layered sedimentary basins

Fluid venting phenomena are prevalent in sedimentary basins globally. Offshore, these localized fluid-expulsion events are archived in the geologic record by the resulting pockmarks at the sea-floor. Venting is widely interpreted to occur via hydraulic fracturing, which requires near-lithostatic pore pressures for initiation. One common driver for these extreme pressures is horizontal tectonic compression, which pressurizes the entire sedimentary column over a wide region. Fluid expulsion leads to a sudden, local relief of this pressure, which then gradually recharges through continued compression, leading to episodic venting. Pressure recharge will also occur through pressure diffusion from neighbouring regions that remain pressurized, but the combined role of compression and pressure diffusion in episodic venting has not previously been considered. Here, we develop a novel poroelastic model for episodic, compression-driven venting. We show that compression and pressure diffusion together set the resulting venting period. We derive a simple analytical expression for this venting period, demonstrating that pressure diffusion can significantly reduce the venting period associated with a given rate of compression. Our expression allows this rate of compression to be inferred from observations of episodic venting. We conclude that pressure diffusion is a major contributor to episodic fluid venting in mudstone-dominated basins.

Cascadia low frequency earthquakes at the base of an overpressured subduction shear zone

In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures.

Along-fault pore-pressure evolution during a slow-slip event in Guerrero, Mexico

Slow earthquakes are important constituents of the seismic cycle and are involved in the stress transfer between the viscously slipping portion of the plate interface and the seismogenic zone. Their occurrence is likely associated with the near-lithostatic pore pressure in the slow earthquake source region, where fluids might modify fault friction and whose presence is indicated by high ratios of compressional (P)–wave velocity to shear (S)–wave velocity observed at the interface between the subducting plate and the overlying crust. Here we compare two slow earthquake phenomena observed in the Guerrero region of the Mexican subduction zone: low-frequency earthquakes (LFEs) and a slow-slip event (SSE) recorded by GPS. We observe variations of the LFE occurrence rates over month-long time scales during a large SSE that we interpret as a manifestation of transient changes in the fault shear strength. We argue that these transient changes are caused by a pore pressure fluctuation that migrates updip along the subduction interface. This mechanism suggests that fluids do not only passively weaken the plate interface but also play an active role in slow earthquake source regions.

Tremor-tide correlations and near-lithostatic pore pressure on the deep San Andreas fault

Since its initial discovery nearly a decade ago, non-volcanic tremor has provided information about a region of the Earth that was previously thought incapable of generating seismic radiation. A thorough explanation of the geologic process responsible for tremor generation has, however, yet to be determined. Owing to their location at the plate interface, temporal correlation with geodetically measured slow-slip events and dominant shear wave energy, tremor observations in southwest Japan have been interpreted as a superposition of many low-frequency earthquakes that represent slip on a fault surface. Fluids may also be fundamental to the failure process in subduction zone environments, as teleseismic and tidal modulation of tremor in Cascadia and Japan and high Poisson ratios in both source regions are indicative of pressurized pore fluids. Here we identify a robust correlation between extremely small, tidally induced shear stress parallel to the San Andreas fault and non-volcanic tremor activity near Parkfield, California. We suggest that this tremor represents shear failure on a critically stressed fault in the presence of near-lithostatic pore pressure. There are a number of similarities between tremor in subduction zone environments, such as Cascadia and Japan, and tremor on the deep San Andreas transform, suggesting that the results presented here may also be applicable in other tectonic settings.

Near-lithostatic pore pressure at seismogenic depths: a thermoporoelastic model

SUMMARY A model is presented for pore pressure migration through a transition layer separating a meteoric aquifer at hydrostatic pressure from a deeper reservoir at lithostatic pressure. This configuration is thought to be pertinent to the South Iceland seismic zone (SISZ) and to other tectonically active regions of recent volcanism, where volatiles are continuously released by ascending magma below the brittle‐ductile transition. Poroelastic parameters are computed for basaltic rock. The model is 1-D, the fluid viscosity is temperature dependent and rock permeability is assumed to be pressure dependent according to a dislocation model of a fractured medium. Environment conditions are considered, pertinent to basalt saturated with water at shallow depth (case I) and at mid-crustal depth (case II). If the intrinsic permeability of the rock is high, no significant effects are observed in the pressure field but advective heat transfer shifts the brittle‐ ductile transition to shallower depths. If the intrinsic permeability is low, the pressure-dependent permeability can propagate near-lithostatic pore pressures throughout most of the transition layer, while the temperature is practically unaffected by advective contributions so that the rock in the transition layer remains in brittle condition. Geometrical parameters characterizing the fracture distribution are important in determining the effective permeability: in particular, if an interconnected system of fractures develops within the transition layer, the effective permeability may increase by several orders of magnitude and near-lithostatic pore pressure propagates upwards. These modelling results have important bearings on our understanding of seismogenic processes in geothermal areas and are consistent with several geophysical observations in the SISZ, in connection with the two 2000 June M = 6.5 earthquakes, including: (i) fluid pressure pulses in deep wells, (ii) low resistivity at the base of the seismogenic layer, (iii) low VP/VS ratio and time-dependent seismic tomography, (iv) heterogeneity of focal mechanisms, (v) shear wave splitting, (vi) high b-value of deep foreshocks, (vii) triggered seismicity and (viii) Radon anomalies.

Inferring fault strength from earthquake rupture properties and the tectonic implications of high pore pressure faulting

The strength of seismogenic faults is fundamental to earthquake mechanics and plate tectonics, affecting many subsidiary processes in the solid earth. The key to understanding fault strength is determining the pore pressures and hydraulic properties within the faults and surrounding crust. The debate has lasted over decades, with evidence provided for both strong fault and weak fault scenarios. A recently proposed hypothesis for quantifying the strength at which faults fail uses earthquake scaling arguments to show that earthquakes fail over a range of strengths in the upper 15–20 km, and at near-lithostatic pore pressure below this depth. This observation, if correct, has important implications for crustal hydraulics, plate tectonics, and earthquake hazard assessment. This paper summarizes the arguments for high pore pressure faulting, and explores its implications for earthquake stress drops, strength of the lithosphere, and earthquake scaling. The hope is to establish a general framework for quantifying the role of fluids in the earthquake process, and to demonstrate that high fluid pressure may dominate failure of the brittle crust.

Implications of hydrostatic pore pressures and high crustal strength for the deformation of intraplate lithosphere

Observations from deep boreholes at several locations worldwide indicate that (i) hydrostatic pore pressures persist to depths of as much as 12km in the upper crust, (ii) the brittle crust is in a state of failure equilibrium according to Coulomb frictional-failure theory, and (iii) bulk permeability is high — 10−17–10−16m2 — apparently due to fluid flow along critically stressed faults. As a result of these factors, the brittle crust is stronger than it would be under near-lithostatic pore pressure conditions.This result provides a constraint on models of lithospheric deformation. Postulating that the upper and lower crust and lithospheric mantle are totally coupled and that the total strength of the lithosphere is equal to the magnitude of tectonic driving forces (∼3×1012Nm−1), we have calculated lithospheric strain rates under representative thermal and rheological conditions such that the integrated differential stress over the entire thickness of the lithosphere equals the plate driving force. For a strike-slip stress state and surface heat flow of 60±6mWm−2, average strain rates are approximately 10−18s−1 under hydrostatic upper crustal pore pressure conditions, and approximately 10−15s−1 under near-lithostatic pore pressures. The latter strain rates are higher than either observed geodetically using very long baseline interferometry (VLBI), or estimated on the basis of plate tectonic reconstructions. Hence we argue that hydrostatic upper crustal pore pressures enable lithospheric plates to behave rigidly over time scales of tens to hundreds of millions of years.

Evidence for near-frictionless faulting in the 1989 (M 6.9) Loma Prieta, California, earthquake and its aftershocks

Analysis of the unusually diverse right-lateral, left-lateral, reverse, and normal faulting aftershocks of the Loma Prieta earthquake reveals a systematic relation between fault-plane orientation and slip direction; right-lateral aftershock planes strike slightly more north-ward (10°-15°) than the main shock, left-lateral planes strike slightly more westward, reverse planes dip slightly less steeply, and normal planes dip slightly more steeply than the main shock. We demonstrate that these slip patterns are consistent with an approximately uniaxial stress field acting nearly perpendicular to the main-shock fault plane. These results imply an extremely weak fault zone, possibly resulting from near-lithostatic pore pressure.

State of stress in the Long Valley caldera, California

Research Article| September 01, 1993 State of stress in the Long Valley caldera, California Daniel Moos; Daniel Moos 1Department of Geophysics, Stanford University, Stanford, California 94305-2215 Search for other works by this author on: GSW Google Scholar Mark D. Zoback Mark D. Zoback 1Department of Geophysics, Stanford University, Stanford, California 94305-2215 Search for other works by this author on: GSW Google Scholar Author and Article Information Daniel Moos 1Department of Geophysics, Stanford University, Stanford, California 94305-2215 Mark D. Zoback 1Department of Geophysics, Stanford University, Stanford, California 94305-2215 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (9): 837–840. https://doi.org/10.1130/0091-7613(1993)021<0837:SOSITL>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Daniel Moos, Mark D. Zoback; State of stress in the Long Valley caldera, California. Geology 1993;; 21 (9): 837–840. doi: https://doi.org/10.1130/0091-7613(1993)021<0837:SOSITL>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Earthquake focal-plane mechanisms and well-bore breakouts in the Long Valley caldera, California, indicate that the resurgent dome and caldera south moat are characterized by a northeast extensional stress field, consistent with geodetically determined extensional strain within the caldera. Similar data from the western caldera indicate that it is characterized by a markedly different, northwest-trending, extensional stress field. We hypothesize that this localized rotation of the stress field is possible because of near-lithostatic pore pressure at depth. Because an east-west extensional stress field appears to have existed in the western caldera at the time of emplacement of the Inyo volcanic deposits (500-1000 yr ago), the state of stress in the Long Valley caldera appears to be both spatially and temporally heterogeneous, most likely as a consequence of intracaldera processes related to magmatic resurgence. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A stress measurement profile to mid-crustal depth in KTB scientific drilling project, southeastern Germany: scientific questions and technological challenges

Although many estimates of crustal stress levels at seismogenic depth have been made on the basis of laboratory data and simple faulting theory, the Kontinentales Tiefbohr (KTB) deep drilling project offers the first opportunity to directly measure at mid-crustal depth the absolute magnitudes of crustal stresses, the uniformity of stress changes with depth, the influence of pore pressure on stress magnitude and the magnitude and orientation of tectonic stresses in the vicinity of the brittle-ductile transition. The planned 10-12 km depth of the KTB Hauptbohrung (main borehole) will penetrate those depths where the shear stresses in the crust are highest and where the greatest discrepancy will exist between observations and predictions if either ‘Byerlee’s law' is not applicable to the mid-crust or near-lithostatic pore pressure exists which substantially weakens the crust.

Accretion and recent deformation of sediments along the northern Cascadia subduction zone

New multichannel seismic reflection profiles, acoustic imagery, and swath bathymetric data from the Vancouver Island continental margin reveal the surface morphology, cross-sectional geometry, and deformation style of this part of the Cascadia accretionary prism. The incoming 3- to 5-km-thick sediment section fails most commonly along landward-dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust. The faults are listric with approximately circular cross sections, and hanging-wall sediments are back-tilted in a planar manner. High, near-lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement, and from the long distances over which the decollement has propagated seaward. The faults dip more steeply at 40°-45° near the surface where Pleistocene turbidites are displaced. Similar folds have developed where the turbidites are particularly thick (>4 km), and near-hydrostatic pore pressures are inferred to occur in the turbidite part of the section. An increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading, particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites. The top of the downgoing Juan de Fuca plate, which defines the base of the accreted sedimentary wedge beneath the margin, is well imaged in the new seismic reflection profiles and is further defined by land reflection data, seismic refraction, and earthquake hypocenters. The landward-dipping continental backstop to the prism is also well located. If these constraints are used for the geometry of the accretionary prism, a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42-m.y. accretion history. This result, along with the observation that deformation-front faults penetrate most or all of the sediment section, suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the Eocene. Application of the critically tapered wedge model of Davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism, particularly the seawardmost part beneath the lower slope. Beneath the continental shelf, the dip of the subducting slab increases to greater than 11°, a dip that according to the critically tapered wedge model, is too steep to allow upward growth of an accretionary prism. This provides a simple explanation for ongoing subsidence of the Tofino sedimentary basin over the older part of the accretionary complex. To the southeast, the Juan de Fuca plate dips much more gently beneath the Olympic Peninsula of northern Washington, allowing the accretionary prism to grow upward well above sea level to form the Olympic Mountains.