Stress accumulation between volcanoes: an explanation for intra-arc earthquakes in Nicaragua?

Destructive upper crustal earthquakes in Central America are often located between active volcanic centres—a geometric relationship that we study using finite element Coulomb failure stress (CFS) models that incorporate the rheologically heterogeneous nature of the volcanic arc. Volcanoes are simulated as mechanically weak zones within a stronger crust. We find that deformation of the volcanic centres within a regional stress field dominated by dextral shear causes stress increases in surrounding crust, with a maximum CFS change between neighbouring volcanoes. This increase in CFS enhances the probability of fault slip on arc-normal faults that are located between volcanic centres; for example, the Tiscapa fault, which ruptured during the 1972 December 13,   M _s   6.2 Managua earthquake. The amount of stress increase due to long-term (100 yr) volcano shearing is on the order of 0.1–0.6 bars, similar to values estimated for subduction zone earthquakes.     

Smaller source earthquakes and improved measuring techniques allow the largest earthquakes in Iceland to be stress forecast (with hindsight)

A group of three earthquakes in 2000 June in SW Iceland included the two largest earthquakes in Iceland in the past 30 yr. Previously, temporal changes in shear-wave splitting had not been recognized before these earthquakes as there were too few small earthquakes to provide adequate shear-wave data, and they were not stress forecast, even with hindsight. These large earthquakes were subject to a special investigation by the European Community funded PREPARED Project during which the seismic catalogue was extended to include smaller magnitude earthquakes. This more detailed data set, together with a semi-automatic programme for measuring the parameters of shear-wave splitting greatly increased the number of time-delay measurements.
The new measurements displayed the typical temporal variations before larger earthquakes as seen elsewhere: a long-term increase in time delays, interpreted as stress accumulation before the earthquake; and a decrease, interpreted as crack coalescence, immediately prior to the earthquake. The logarithms of the durations of both the implied accumulation of stress and the crack coalescence have the same self-similar relationships to earthquake magnitude as found elsewhere in Iceland. This means that, in principle, the time and magnitude of the larger earthquakes could have been stress forecast in real time had the smaller source earthquakes of the extended catalogue and the improved measuring procedures been available at the time.     

A microearthquake survey at the junction of the East Pacific Rise and the Wilkes (9°S) fracture zone

Summary. An ocean bottom seismograph survey of the junction of the East Pacific Rise and the Wilkes fracture zone detected only three microearthquakes beneath the rise crest during seven days of recording. In contrast, during the same period 41 events were detected on the fracture zone, all at distances greater than 10 km from the junction. These results suggest that near the rise crest the thin crust can support sufficient stress only to generate infrequent small earthquakes and that most faulting may take place by aseismic slip. At 10 km from the rise axis part of the crest has become competent enough to support stress, resulting in earthquakes probably at depths of up to 5 km below the sea-bed. Gear 5-waves on the seismometer records indicate that a magma chamber, if it exists near the junction, is less than 10 km across.     

Locally triggered seismicity in the central Swiss Alps following the large rainfall event of August 2005

In 2005 August, an unusual series of 47 earthquakes occurred over a 12-hr period in central Switzerland. The earthquakes occurred at the end of 3-d period of intensive rainfall, with over 300 mm of precipitation. The highest seismicity occurred as two distinct clusters in the region of Muotatal and Riemenstalden, Switzerland, a well-known Karst area that received a particularly large amount of rainfall. The large increase in seismicity, compared to the background, and the short time delay between the onset of the intense rainfall and the seismicity strongly suggest that earthquakes were triggered by rainfall. In our preferred model, an increase in fluid pressure at the surface due to a large amount of rain leads to a local increase in pore fluid pressure at depth. The increase in pore fluid pressure will reduce the shear strength of a porous medium by counteracting normal stress and, at the end, provoke failure. The series of triggered earthquakes in central Switzerland occurred in regions that have been seismically active in the past, showing similar hypocentre locations and magnitudes. This suggests that these earthquakes occurred on existing faults that were critically stressed. We modelled the intense rainfall as a step increase in fluid pressure at the surface that migrates to greater depths following the solution of the one-dimensional diffusion equation in a homogeneous half space. This allowed us to estimate the hydraulic diffusivity by plotting triggered seismicity in a time–depth plot. We found values of hydraulic diffusivity in the range from 0.01 to 0.5 m² s⁻¹ for our study area. These values are in good agreement with previous studies on earthquakes that were triggered by fluids, supporting the idea that the observed earthquake series was triggered by the large amount of rainfall.     

The influence of water on the stress supported by experimentally faulted Westerly granite

Summary. Fault zones in wet Westerly granite deformed at temperatures of 300° and 400°C require markedly lower shear stresses for sliding than when dry, even when the effective confining pressure is held constant between the wet and dry tests, provided that the strain rate is lower than 10⁻⁷s⁻¹. The rate of strength reduction is enhanced by increasing the pore water pressure. The deformation rate is a power function of the applied stress where the stress exponent is approximately 7 for pore water pressure of 100 MPa and 21 for pore water pressure of 20 MPa.
The experimental results are extrapolated to conditions believed to occur at depths of 10 km along the San Andreas Fault Zone. It is suggested that for slow tectonic deformation at strain rates of 10⁻¹¹ and 10⁻¹⁴s⁻¹ the shear stress for sliding on faults in granite would be approximately 60 and 20 MPa, respectively, at pore water pressures equal to the hydrostatic head. Fluid overpressures of c. 0.8 lithostatic pressure are required to lower the shear stress for sliding into the 10 MPa range at the slower strain rate.     

Implications of stress concentration on a strike-slip fault in an elastic plate subject to basal shear stress

Summary We consider a long strike-slip fault in a lithosphere modelled as an elastic slab. To the base of the slab a shear stress distribution is applied which simulates the viscous drag exerted by the asthenosphere. The resulant stress on the fault plane may directly fracture the lithosphere in its brittle upper portion; alternatively it may give rise at first to a stable aseismic sliding in the lower portion. In the latter case, stress concentration due to the deep aseismic slip is the relevant feature of the pre-seismic stress acting on the upper section of the lithosphere. The two cases are examined by use of dislocation theory and their observable effects compared. Different depths of the aseismic slip zone and the presence or absence of a uniform friction on the seismic fault are allowed for. If the model is applied to the San Andreas fault region, where a steady sliding condition actually seems to be present at shallow depth, it turns out that the slip amplitudes commonly associated with large earthquakes are consistent with average basal stress values which can be substantially lower than a few bars, a value often quoted as the steady state basal stress due to a velocity gradient in the upper asthenosphere.     

Late Quaternary activity of faults and recurrence interval of earthquakes in the eastern Hokuriku region, northern central Japan, on the basis of precise cryptotephra analysis of fluvial terrace sequences

About 2000 active faults are known to exist within the land area of Japan. Most of these active faults have deformed the topographic surfaces which were formed in the late Quaternary, including fluvial terraces; and the formative ages of these terraces are estimated mainly by tephrochronology. Fluvial terraces in the eastern Hokuriku region, comprising the Toyama, Tonami, and Kanazawa Plains, northern central Japan, are widely distributed and have been deformed by reverse active faults. The formative age of terraces in this area has not been reported, as volcanic ash deposits are rarely visible within terrace deposits and the overlying loamy soil, and outcrops of fluvial terraces are quite scarce in this area. In the present study, we carried out a drilling survey on these terraces to obtain samples of the overlying loamy soil and upper part of terrace deposits. From these samples, we extracted some well-known widespread volcanic ash, from which we were able to estimate the approximate age of the terraces and the vertical slip rate of the active faults. Late Quaternary fluvial terraces in eastern Hokuriku are divided into 12 levels: Terraces 1 to 12 in descending order. Widespread tephras such as the Kikai-Tozurahara Tephra (K-Tz: 95 ka) are contained in the lowest part of the loamy soil in Terrace 4 and the Daisen-Kurayoshi Pumice (DKP: 55 ka) is present in the lowest part of the loamy soil in Terrace 6. From the ages and the vertical displacements of the fluvial terraces, the late Quaternary average vertical slip rates of active faults in eastern Hokuriku are estimated to be 0.2–0.9 mm/year (Uozu fault), 0.1–0.4 mm/year (Kurehayama fault), 0.1–0.3 mm/year (Takashozu fault), 0.1–0.4 mm/year (Hohrinji fault), and 0.5–0.8 mm/year (Morimoto-Togashi fault). We also estimated the recurrence interval of earthquakes related to active faults from displacement per event and ages of terraces and no significant difference in vertical displacement per single earthquake for different active faults, and recurrence intervals tend to be inversely proportional to vertical displacement rates. This study demonstrates that a combination of drilling of loamy soil and precise cryptotephra analysis of fluvial terraces can be used to estimate the formative age of the terraces and the average slip rate of active faults in areas where volcanic ash deposits are rare.     

Geodetic evidence for conjugate faulting during the 1988 Tennant Creek, Australia earthquake sequence

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Repeat levelling measurements and detailed topographic profiles from the epicentral area of the 1988 January 22 Tennant Creek, Australia earthquakes are used to constrain the geometry of faulting associated with three M 6+ earthquakes. The observed elevation changes are modelled assuming elastic deformation and uniform slip on several faults. The vertical deformation data are poorly fit by a single-fault model, and require at least three distinct faults. In the preferred model, two faults on either end of the zone of surface rupture have similar orientations, but the central fault has an orientation conjugate to the flanking faults. This interpretation is consistent with the identification of the fault planes with well-defined, dipping zones of aftershock hypocentres determined with data from portable seismograph arrays. It is also consistent with the sense of surficial deformation documented by 75 topographic profiles across the scarps. However, a fourth fault associated with possible conjugate faulting in the central fault segment at the time of the second main shock is not required by the levelling data.     

Stress and temperature in subduction shear zones: Tonga and Mariana

Summary. Because there is secondary sea-floor spreading in the Tonga and Mariana subduction systems, the island arcs are separate plates. Horizontal forces on the two sides of the arc must balance, and the maximum force on the back-arc side can be calculated from a lithostatic ridge model. This, in combination with gravity data, allows calculation of the average shear stress in the top 100 km of the subduction shear zone. Stress in Tonga is 220±100 bar, and in the Mariana it is 165±75 bar. These low stresses are probably made possible by a fluid pore pressure almost equal to the least compressive stress.
Knowledge of stress allows approximate calculation of temperature in the shear zone by integration of a single differential equation. These temperatures are too low to activate most dehydration reactions in the subducted crust. As it approaches the volcanic line, this crust is at 150–350°C in Tonga and 150–300°C in the Mariana. Shear melting of the crust is ruled out, and conductive melting of the slab by contact with the asthenosphere meets with geochemical objections. Magmas in these systems are probably produced by partial melting of asthenosphere, triggered by a sudden release of water from the slab.     

Comparison of frequency of occurrence of earthquakes with slip rates from long-term seismicity data: the cases of Gulf of Corinth, Sea of Marmara and Dead Sea Fault Zone

In this article, through the comparison of knowledge relating to historical earthquakes with the understanding of present-day earthquake mechanics and overall GPS slip rates in the eastern Mediterranean region, it has been possible to obtain an idea of how frequently large earthquakes may be expected in some parts of the region. It has also been possible to make an assessment from these early events of slip rates over a long period of time for the Gulf of Corinth in Greece, the Marmara Sea in Turkey and the Dead Sea Fault System, as well as deriving long-term magnitude–frequency relations for these same regions.
It has been demonstrated that slip rates calculated from historical data are in general comparable to those calculated from GPS measurements and field observations, while the size of historical earthquakes and their uncertainty can be quantified. This permits a more reliable estimation of the long-term hazard, the calculation of which is the concern of the engineering seismologist. It has also been shown that in most cases large earthquakes are less frequent when they are estimated from long-term data sets rather than from the instrumental period making the notion of recurrence time and of hazard assessment, questionable.
This study focuses on some of the few areas in the world for which long-term macroseism information exists and which facilitate this kind of analysis.     

A possible rupture process of slow earthquakes on a frictional fault

Summary. A possible mechanism for the occurrence of slow earthquakes is investigated by calculating numerical solutions for the dynamical rupture process on a quasi-three-dimensional fault with heterogeneous frictional strengths. Experimental friction laws for the dependence of sliding frictional stress on slip velocity, which are based on the cohesive properties of fault asperities, are taken into considerations.
It is found that the applied stress does not drop very rapidly with time and the rupture velocity remarkably decreases as the dependence on slip-velocity becomes smaller. These deceleration effects for the rupture propagation are greatly enhanced with increasing heterogeneities in the distribution of frictional strength and as the initial shear stress has lower levels with respect to the average strength. For these cases, the growth of rupture is extremely slow in a nucleus region with the dimension as large as 10 times the initial rupture length, and gains a terminal velocity dependent on the above factors. The displacement-time function becomes noticeably extended in these cases, and indicates a stick—slip-like phenomena in the extended time interval for a strongly heterogeneous fault.
It seems that these results could explain the characteristic features of slow earthquakes.     

Basin and petroleum system modelling of the East Coast Basin,New Zealand: a test of overpressure scenarios in a convergent margin

In the East Coast Basin (ECB), an active convergent margin of the North Island, New Zealand, the smectite‐rich Eocene Wanstead Formation forms an effective regional seal, creating high overpressure in the underlying Cretaceous through Palaeocene units due to disequilibrium compaction. This study examines the evolution of pore pressure and porosity in Hawke Bay of the ECB based on stepwise structural reconstruction of a stratigraphic and structural framework derived from interpretation of a regional two‐dimensional seismic line. This framework is incorporated into a basin and petroleum system model to predict the generation, distribution, and dissipation of overpressure, and examine the influence of faults, erosion, structural thickening, and seal effectiveness of the Wanstead Formation on pore pressure evolution. We find that natural hydraulic fracturing is likely occurring in sub‐Wanstead source rocks, which makes it a favourable setting for potential shale gas plays. We use poroelastic modelling to investigate the impact of horizontal bulk shortening due to tectonic compression on pore pressure and the relative order of principal stresses. We find that shortening modestly increases pore pressure. When 5% or greater shortening occurs, the horizontal stress may approach and exceed vertical stress in the last 4 Myr of the basin's history. Shortening impacts both the magnitude and relative order of principal stresses through geological time. Due to the overpressured nature of the basin, we suggest that subtle changes in stress regime are responsible for the significant changes in structural deformational styles observed, enabling compressional, extensional, and strike‐slip fault regimes to all occur during the tectonic history and, at times, simultaneously.     

Modelling of short-interval silent slip events in deeper subduction interfaces considering the frictional properties at the unstable–stable transition regime

Recent high-resolution observations of crustal movements have revealed silent slip events (SSEs) with propagation velocities of around 10–15 km d⁻¹ and with intervals of 3–14 months along the deeper parts of the Cascadia and Nankai subduction zones. This study develops 2-D and 3-D models of these short-interval SSEs considering the frictional behaviour that was confirmed experimentally by Shimamoto for the unstable–stable transition regime. To represent this frictional behaviour, a small cut-off velocity to an evolution effect is introduced in a rate- and state-dependent friction law. When the cut-off velocity to the evolution effect is significantly smaller than that to a direct effect, steady-state friction exhibits velocity weakening at low slip velocities and velocity strengthening at high slip velocities. At the deeper Cascadia and Nankai subduction interfaces, the pore pressure is inferred to be high because of the dehydration of materials in the descending plate. Under conditions where the pore-fluid pressure is nearly equal to the lithostatic pressure and the critical weakening displacement is very small, short-interval SSEs with propagation velocities and slip velocities of 4–8 km d⁻¹ and  2 − 4 × 10⁻⁷  m s⁻¹, respectively, can be reproduced. The propagation velocity of short-interval SSEs is in proportion to the slip velocity. The results also show that during the nucleation process of large earthquakes, the occurrence of short-interval SSEs becomes irregular because of the accelerated slips that occur at the bottom of the seismogenic zone. Our results suggest that monitoring of short-interval SSEs might be useful for forecasting the main earthquakes.     

昌马断裂带活动构造地貌之研究

本文根据野外地质调查资料和人工探槽的剖折,并结合¹⁴C年龄数据,讨论了昌马活动断裂带晚更新世晚期以来形成的构造地貌的类型、特征及分期问题.     

Lyttelton Volcano, Banks Peninsula, New Zealand: Primary volcanic landforms and eruptive centre identification

Lyttelton Volcano, Banks Peninsula, New Zealand, has historically been viewed as a simple volcanic cone. This paper uses digital terrain models (DTM) and primary volcanic landforms to reinterpret Lyttelton Volcano as having multiple eruptive centres. Primary volcanic landforms are features produced during active volcanism, classified as constructional, hypabyssal, and erosional volcanic features. Constructional volcanic features are lava flows, scoria cones and domes; hypabyssal volcanic features are dykes and sills; and erosional volcanic features are valley and ridge patterns and orientations. Lava flow trends are recognised from aerial photograph analysis and supported by field observations, highlighting radiating lava trends around specific locations within Lyttelton Harbour. Scoria cones and domes occur on the outer flanks of volcanic cones, and are used as such in the identification of remnant cone surfaces. Dyke orientations are plotted and then projected to the interior of the volcano, defining 13 zones of convergence. The projected arrays of these orientations highlight defined regions along the erosional crater rim, each indicating a radial dyke swarm, from which the projected trends of the associated dykes indicate an eruptive centre. Valley and ridge orientations are projected from the longest valley or ridge segment, towards the inner harbour. Radiating erosive patterns are incepted during the growth and degradation of a volcanic cone, with the resulting trends orienting to the summit. Zones of convergence/eruptive centres are identified from lava flow orientations, onlapping lava sequences, scoria cones, and intrusive locations. The summit of a volcanic edifice can be identified from the orientations of valleys and ridges, while radial dyke systems determine whether this summit was a volcanic centre or simply a local topographic high. Volcanic landforms are used to identify cone sectors, the preserved sector associated with a particular eruptive centre. Cone sector limits are defined by a basal footprint and an erosional crater rim, with similar arcuate features (remnant cone features) being exposed in the interior of the volcano. Lyttelton Volcano comprises fifteen volcanic cones, with vent locations controlled by underlying fault lineaments. The growth and erosion of each volcanic cone is reflected in primary volcanic landforms, with the preserved features of cones confined to cone sectors and cone artefacts.     

Aeolian shear stress ratio measurements within mesquite-dominated landscapes of the Chihuahuan Desert, New Mexico, USA

A field study was conducted to ascertain the amount of protection that mesquite-dominated communities provide to the surface from wind erosion. The dynamics of the locally accelerated evolution of a mesquite/coppice dune landscape and the undetermined spatial dependence of potential erosion by wind from a shear stress partition model were investigated. Sediment transport and dust emission processes are governed by the amount of protection that can be provided by roughness elements. Although shear stress partition models exist that can describe this, their accuracy has only been tested against a limited dataset because instrumentation has previously been unable to provide the necessary measurements. This study combines the use of meteorological towers and surface shear stress measurements with Irwin sensors to measure the partition of shear stress in situ. The surface shear stress within preferentially aligned vegetation (within coppice dune development) exhibited highly skewed distributions, while a more homogenous surface stress was recorded at a site with less developed coppice dunes. Above the vegetation, the logarithmic velocity profile deduced roughness length (based on 10-min averages) exhibited a distinct correlation with compass direction for the site with vegetation preferentially aligned, while the site with more homogenously distributed vegetation showed very little variation in the roughness length. This distribution in roughness length within an area, defines a distribution of a resolved shear stress partitioning model based on these measurements, ultimately providing potential closure to a previously uncorrelated model parameter.     

Towards ground truthing exploration in the central Arctic Ocean: a Cenozoic compaction history from the Lomonosov Ridge

The Integrated Ocean Drilling Program's Expedition 302, the Arctic Coring Expedition (ACEX), recovered the first Cenozoic sedimentary sequence from the central Arctic Ocean. ACEX provided ground truth for basin scale geophysical interpretations and for guiding future exploration targets in this largely unexplored ocean basin. Here, we present results from a series of consolidation tests used to characterize sediment compressibility and permeability and integrate these with high‐resolution measurements of bulk density, porosity and shear strength to investigate the stress history and the nature of prominent lithostratigraphic and seismostratigraphic boundaries in the ACEX record. Despite moderate sedimentation rates (10–30 m Myr^?1) and high permeability values (10^?15–10^?18 m²), consolidation and shear strength measurements both suggest an overall state of underconsolidation or overpressure. One‐dimensional compaction modelling shows that to maintain such excess pore pressures, an in situ fluid source is required that exceeds the rate of fluid expulsion generated by mechanical compaction alone. Geochemical and sedimentological evidence is presented that identifies the Opal A–C/T transformation of biosiliceous rich sediments as a potential additional in situ fluid source. However, the combined rate of chemical and mechanical compaction remain too low to fully account for the observed pore pressure gradients, implying an additional diagenetic fluid source from within or below the recovered Cenozoic sediments from ACEX. Recognition of the Opal A–C/T reaction front in the ACEX record has broad reaching regional implications on slope stability and subsurface pressure evolution, and provides an important consideration for interpreting and correlating the spatially limited seismic data from the Arctic Ocean.     

A resampling approach to test stress-field uniformity from fault data

Several methods have been proposed to constrain the stress field from fault plane orientations and slip directions within a crustal volume characterized by brittle deformation. All the methods are based on the assumption that the stress field is uniform in the volume considered. If this hypothesis is not checked in advance, however, the methodology may lead to misleading conclusions. In this work, a procedure is defined to check stress-field uniformity by a statistical analysis of the available fault data. Since, in most cases, the statistical features of the uncertainties that affect such data are not well known, a distribution-free approach is proposed. It is based on a simple search algorithm, devoted to selecting stress configurations compatible with available data, combined with a bootstrap resampling approach. The test results are more conservative than the ones so far proposed in the literature. When the test allows stress heterogeneities to be safely excluded, approximate confidence intervals for the principal stress directions can be obtained; otherwise, the level of stress heterogeneity present in the volume under study can be assessed. An application of the proposed procedure to a sample of fault data deduced from seismological data is presented.     

Electrical conductivity models for the continental crust based on laboratory measurements on high-grade metamorphic rocks

Summary. Deep electrical conductivity soundings are increasingly being used as an additional source of information regarding the nature of the lower continental crust. However, a lack of relevant laboratory-based conductivity measurements makes interpretation of such soundings difficult. Laboratory measurements have been made on saturated and unsaturated samples of possible lower crustal rock types subjected to confining pressures up to 0.4 GPa, temperatures up to 300°C, and with variable pore fluid pressure up to the confining pressure. Extrapolation of these results suggests that the surprisingly high conductivities deduced for depths of approximately 20 km in certain stable continental areas may result from a combination of basic rock type and high pore fluid pressures, for whereas the conductivities measured in acid rock types can be explained in terms of conduction through the pore fluid alone, the conductivities measured in basic rock types imply enhanced conduction presumably through the matrix or along grain boundaries. The lower conductivities deduced from field experiments for the upper crust may be due to more acid rock types and/or lower pore fluid pressures, perhaps due to hydration reactions. In areas where the high conductivity layer is coincident with a low velocity layer an explanation in terms of changing pore fluid pressure, i.e. low to high, with increasing depth seems more likely.     

Application of some dynamic properties of stick—slip to earthquakes

Summary . Quantitative comparison of dynamic parameters is made between stick—slip in the laboratory and earthquakes in the Earth. It is shown that the same theory is successfully applied for both earthquakes and stick—slip for rock samples for which slip occurs over the entire pre-existing fault, in so far as the physical processes of earthquakes are explained by the elastic rebound theory.