Small Dip Angle Research Articles

Characteristics of subevents and three-stage rupture processes of the 2015 Mw 7.8 Gorkha Nepal earthquake from multiple-array back projection

On 25 April 2015 an Mw 7.8 earthquake occurred in Nepal and caused about 9000 casualties. This earthquake ruptured part of the Main Himalaya Thrust fault, which is due to the convergence of the subducting Indian plate to the overriding Eurasian plate, and showed thrust mechanism with a very small fault dip angle (about 7–10°). We apply teleseismic multiple-array back projection analysis to study the rupture process of this earthquake and find 6 clear high frequency radiation sources (subevents). Our results illustrate a simple unilateral eastward rupture of ∼160km with relative stable rupture speed of ∼2.8km/s and a duration of 56s. The entire rupture processes can be divided into 3 stages. The high frequency radiation appears to be mainly located at the edge of the large slip area, but the subevents have different characteristics in the western and eastern rupture areas. For this 2015 Nepal earthquake, the scales of asperities appear to be mainly controlled by depth, which dominates the overall patterns of slip and high frequency radiation. We finally propose a multiple-scale asperity model with stress and structural heterogeneities along the rupture direction to explain the distribution of high frequency subevents, co-seismic slip, and aftershocks.

Impacts of injection pressure of a dip‐angle sloping strata reservoir with low porosity and permeability on CO2 injection amount

AbstractSloping strata reservoirs with low porosity and permeability are widely distributed in China. The first CO2 capture and geological storage (CCS) demonstration project in China that has been constructed in the Ordos Basin is a representative. In this paper, a 3D numerical model was constructed to evaluate the influence of injection pressure and strata dip angle on the CO2 injection amount in a sloping strata reservoir during CO2 injection period. We reached the following conclusions: (i) There is a ‘balance time point (BTP)’ for the total CO2 injection amount between a horizontal reservoir and a sloping one, and the BTP will decrease with the injection pressure increase. (ii) The influence of the dip angle of a sloping reservoir on the total injection amount is non‐monotonicity. For a fixed injection pressure, before the BTP, the greater the dip angle, the greater the CO2 injection amount, but after the BTP, the change rule is reversed. (iii) The injection pressure significantly affected the total injected CO2 amount, but the influence of the dip angle (in the range of 0° to 15°) on the total injection amount was not significant during injection. (iv) To a sloping reservoir, for long time injection (i.e., injection time is longer than the BTP), the bigger the dip angle, the greater the distance of the CO2 upward migration, and the smaller the total injected amount compared to the horizontal reservoir. So, in actual site selection of a CCS project, the small dip angle reservoir was beneficial. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd.

Numerical simulations of failure behavior around a circular opening in a non-persistently jointed rock mass under biaxial compression

Pre-existing discontinuities change the mechanical properties of rock masses, and further influence failure behavior around an underground opening. In present study, the failure behavior in both Inner and Outer zones around a circular opening in a non-persistently jointed rock mass under biaxial compression was investigated through numerical simulations. First, the micro parameters of the PFC3D model were carefully calibrated using the macro mechanical properties determined in physical experiments implemented on jointed rock models. Then, a parametrical study was undertaken of the effect of stress condition, joint dip angle and joint persistency. Under low initial stress, the confining stress improves the mechanical behavior of the surrounding rock masses; while under high initial stress, the surrounding rock mass failed immediately following excavation. At small dip angles the cracks around the circular opening developed generally outwards in a step-path failure pattern; whereas, at high dip angles the surrounding rock mass failed in an instantaneous intact rock failure pattern. Moreover, the stability of the rock mass around the circular opening deteriorated significantly with increasing joint persistency.

The effect of slope angle on the development of type a and type b channels of rinnenkarren with field and laboratory measurements

The relation between the inclination and the quality of the bearing slope, and the channel type and channel density of various rinnenkarren (type A and type B channels) was investigated. On the bare karren slopes of Totes Gebirge, the connection between the densities of type A and type B (type A channels have no tributary channels, and they are smaller; type B channels have tributary channels, and they are larger) channels and the slope angle was studied, while in laboratory the relation between the number of rivulets forming the channels and the slope angle and the quality of slope was analysed. The density of type A channels increases with the increase in the slope angle, while that of type B channels decreases. In the laboratory the number of rivulets on the various parts of the slope was studied on glass, gypsum and metal plates between 5 and 50° (as well as on metal plate between 1 and 70°). The dip angle of the bearing slope was modified every 5°. The number of rivulets increases with the slope angle mainly on the upper part of the slope and on the metal plate. The number of rivulets decreases towards the lower part of the slope in the case of the same slope and the same slope angle. The number of rivulets (and thus the number of channels) depends on the speed of water flow and the roughness of the surface. On a slope with a small dip angle (smaller than 20°), few rivulets and channels develop and, thus, rinnenkarren systems with large catchment areas may form whose main channels develop into large, type B channels. On slopes with a large dip angle (greater than 30°), numerous rivulets develop even at the beginning of channel development. Because of large rivulet density, channels with small catchment area and thus small-sized (type A) channels may develop. On the karren slopes, two models of channel development are possible: according to model 1, on slopes with a continuously smaller dip angle, the beginning of channel development is increasingly different, thus rinnenkarren systems form which are built by main and tributary channels. In the case of an increasing slope angle, this tendency is less and less valid. On slopes with an increasingly larger slope angle, the beginning of channel development will not be different. According to model 2 (based on the laboratory model), the rivulets that developed on the upper part of the slope join on the lower part of the slope. In this case the initiation of channel formation of the main and tributary channels of the rinnenkarren systems is similar. On the bare slopes of Totes Gebirge, model 1 can be regarded as the dominant one.

The 20 April 2013 Lushan, Sichuan, mainshock, and its aftershock sequence: tectonic implications

Using the double-difference relocation algorithm, we relocated the 20 April 2013 Lushan, Sichuan, earthquake (MS 7.0), and its 4,567 aftershocks recorded during the period between 20 April and May 3, 2013. Our results showed that most aftershocks are relocated between 10 and 20 km depths, but some large aftershocks were relocated around 30 km depth and small events extended upward near the surface. Vertical cross sections illustrate a shovel-shaped fault plane with a variable dip angle from the southwest to northeast along the fault. Furthermore, the dip angle of the fault plane is smaller around the mainshock than that in the surrounding areas along the fault. These results suggest that it may be easy to generate the strong earthquake in the place having a small dip angle of the fault, which is somewhat similar to the genesis of the 2008 Wenchuan earthquake. The Lushan mainshock is underlain by the seismically anomalous layers with low-VP, low-VS, and high-Poisson’s ratio anomalies, possibly suggesting that the fluid-filled fractured rock matrices might significantly reduce the effective normal stress on the fault plane to bring the brittle failure. The seismic gap between Lushan and Wenchuan aftershocks is suspected to be vulnerable to future seismic risks at greater depths, if any.

Analysis of Tight Gas Reservoir Forming Condition in Gulong-Changjiaweizi Region, Northern Songliao Basin

This paper makes systematic analysis of geological factors of natural gas accumulation in Denglouku formation of Gulong-Changjiaweizi region, including reservoir characteristics, gas source condition, source-reservoir relationship, structural condition, etc. It turned out that K1d2 in Gulong-Changjiaweizi region is generally typical tight sandstone reservoir with low porosity and permeability due to the poor physical properties. The gas source rock of K1d2 formation has larger gas producing capacity.The relationship between source rock and reservoir shows as interbed interfinger or directly contiguity contact, which is beneficial for large-area gas accumulation. The gas generation area of source rock in this region is always in the center and slow downdip direction of Gulong depression with a smaller dip angle on the adjacent tight sandstone reservoir, where faults are rare. The result of comprehensive analysis shows that K1d2 formation in Nothern Songliao Basin and its neighboring layers could be considered as a favorable target of the tight gas reservoir study in Northern Songliao Basin due to its favorable geological conditions of deep basin tight gas reservoir generation, such as tight reservoir, sufficient gas source, communicating source-reservoir relationship and constant flattened structure.

Numerical modeling of the productivity of vertical to shallowly dipping fractured zones in crystalline rocks

Groundwater resources in crystalline rock are typically associated with the weathered zone and regional sub-vertical faults that are well connected to the surface. However, some sub-horizontal and shallowly dipping fractured zones can also be highly-productive aquifers. In this paper, numerical simulations of a conceptual hydrogeological model show that the flow to such strongly transmissive fractured zones is controlled by their transmissivity or by their deepening structure. While leakage through the overlying rock units is generally the limiting factor, recharge always occurs at least close to the outcrop of the fractured zone where the overlying rock is thinner and guarantees the availability of some groundwater. At small dip angles, recharge extends spatially and the flow within the fractured zone may even become the limiting factor when the hydraulic conductivity of the overlying rock is not less than two orders of magnitude smaller than the fractured zone transmissivity. This is precisely the case of the Ploemeur aquifer (Brittany, France) located in a crystalline rock geologic setting, where groundwater in a shallowly dipping fractured zone is used as the source of water supply for a nearby city of 20,000 people. Simulation results show that the fractured zones may represent potential aquifers under a large variety of hydrogeological conditions. Aquifers in shallowly dipping structures differ strongly from those located in regional subvertical fault zones in terms of flow patterns, and thus supposedly in terms of management of the groundwater resource. They are more local than regional in scale, and consequently do not require regional fracture connectivity. The leakage through the overlying rock unit enhances water quality. Finally, we argue that the potential widespread occurrence of these alternative and possibly less accessible resources should promote the development of appropriate identification methods.

Reply to "Comment on 'P-Wave Back-Azimuth and Slowness Anomalies Observed by an IMS Seismic Array LZDM' by Chunyue Hao and Zhong Zheng" by Lei Li

The author of a comment (Li, 2011) on our 2010 paper (Hao and Zheng, 2010) said that our dip angle seems too small to produce the observed large deviations in back-azimuth and slowness at LZDM International Monitoring System (IMS) Seismic Array. He used forward modeling to show that a large dip angle on the Moho is needed to match the large observed deviations. In this reply, we recalculated our forward model based on the equations given by Niazi (1966) and found that the small dip angle in our original paper resulted from using a negative sign in equation (7) of Niazi (1966) for the calculation of the n value; using a positive sign in the expression of the n value gave rise to a large dip angle of about 53° in accordance with Li (2011). We also verified that only the positive sign can lead to the results given in the tables (Fig. 1) supplied by Niazi (1966) and the tables (Fig. 2) given by Ram and Yadav (1984). Thus, …

Slow slip events controlled by the slab dip and its lateral change along a trench

Several slow slip events (SSEs) have recently been globally detected at subduction zones by analyses of high precision GPS data. They occur at the transition zone from seismogenic to stable sliding ones, but seem to occur at restricted areas. Though researchers have proposed some generation mechanisms of SSE, they made no mention of such restricted occurrence areas of SSE. To explain why SSEs occur in some restricted area, we compile the features of globally detected SSEs and we find that the slab dip and its lateral change are important factors for controlling their occurrences. Through simulations of earthquake cycle including SSEs using a simplified cell model, we examine the effect of dip angle at the transition zone on the interseismic SSEs. The remarkable SSEs occur several times in a seismic cycle at the cell with a smaller dip, while at the cell with a larger dip, few small SSEs occur and attenuate in the interseismic term. Actually, SSEs occur only at the area with the smallest dip at the transition zone. Moreover in the case where cells are placed at the neighbor of the existing cell in the lateral direction, SSEs are restrained to become smaller if the dips of cells are the same. To the contrary, the cells with different dips do not restrain SSEs. This result well explains the observed feature that SSEs occur at the area where the dip of the plate is laterally changing. In addition, it is also reproduced that a cell with a smaller dip angle has SSEs with a longer interval and longer duration. These simulation results are understood in terms of the frictional condition related to the dip-dependent stiffness of plate interface, that is, the stress accumulation due to the plate subduction, relative to the critical stiffness determined by frictional parameters and the stress interaction which is also dependent on the dip angle.

Image the Zhefang‐Binchuan and Monglian‐Malong Wide‐Angle Seismic Profiles in Yunnan Province

AbstractBased on the finite difference inversion and ray inversion method, the crust and upper mantle structure along the Zhefang–Binchuan and Menglian–Malong wide–angle seismic profiles, both of which are located in Yunnan province, are imaged using the geophysical data of travel‐time, amplitude ratio and Bouguer gravity anomalies. Thus some new recognitions about the geodynamics and the seismotectonic environment are derived. The crust thickness along the Zhefang–Binchuan profile is 35~46km, and that of the Menglian–Malong profile is 33~44.5km. The geometry of the Moho interface looks like that given by Hu H X et al. The P wave velocity on the top of the mantle beneath some places is relatively low and the variation range of the velocity is very large, which may indicates that the Yunnan region is a typical area of active tectonics. Moreover, there exits a prominent consistency between the shallow and deep structures, which implies that the activity of shallow material always has a deep background. The velocity structure of Zhefang–Binchuan profile indicates that there exists a large low velocity anomaly which penetrates the whole crust to the east of the Nujiang fault exposed on the ground, and this possibly associated with the upwelling of the deep material . The characteristic of the large faults which are the borders between some first order tectonic units can also be derived from the both velocity structure profiles: the Red River fault is a hyper‐fault that cuts the lithosphere, the Nujiang fault cuts deeply in the crust and even stretches downward into the top of the upper mantle, yet the Changning–Shuangjiang fault has a listric shape with a small dip angle, which may suggest that its incision depth is not very deep. The large earthquakes took place in Yunnan province always have a relationship with the large and deep faults that stretches into the upper mantle. Some earthquakes with shallow focal depths are generally located at the converge positions of different faults within the upper‐middle crust or the places where the velocity contours on the belt between the high and low velocity anomaly blocks are crooked. It is estimated that these locations are favorable to the accumulation of energy and regional stress.

Numerical analysis of the azimuth moveout operator for vertically inhomogeneous media

The azimuth‐moveout (AMO) operator, unlike the dip‐moveout (DMO) operator, has a 3D structure in homogeneous isotropic media, with an out‐of‐plane (crossline) component. In general, this component is concave downward, giving the operator an overall skewed‐saddle shape. The AMO operator, necessary for azimuth correction only, is typically smaller in size than conventional DMO operators, which corresponds to purely offset correction to zero offset. When velocity varies vertically, the operator shape changes depending on the degree of velocity variation. The general shape of the operator, however, remains saddlelike. In fact, for smooth velocity increases with depth, similar to those found in the Gulf of Mexico, the v(z) AMO operator does not differ much from its homogeneous counterpart. In this case, the residual AMO operator, constructed by cascading a forward homogeneous AMO operator with an inverse v(z) one, is extremely small, which suggests that the impact of such v(z) variations on the AMO operator is generally small. Complex vertical velocity variations, on the other hand, result in more complicated AMO operators that include, among other things, triplications at moderate angles. Regardless of the complexity of the model, the v(z) operator has the same first‐order behavior as its homogeneous counterpart. As a result, for small dip angles the homogeneous AMO, as a tool for partial stacking, often enhances the image. Moderate to steep dips in complex v(z) media requires the application of an algorithm that honors such velocity variations.

Microearthquake seismicity in relation to double convergence around the Solomon Islands arc by ocean-bottom seismometer observation

SUMMARY The Solomon Islands arc area is a complex plate convergence zone. At the North Solomon Trench on the northern side of the arc, it is believed that the Pacific Plate was subducting before coming into collision with the Ontong Java Plateau, the world's largest oceanic plateau. After the collision about 5 Ma, northeastward subduction initiated along the southern side of the arc at the San Cristobal Trench, another trench on the south side. GPS observations and crustal seismic structure surveys confirm that convergence occurs at both trenches. Without detailed and accurate seismicity, it is difficult to characterize the plate subduction to reveal the tectonics of such a complex zone where a key mechanism of continental growth may also exist. In 1994, an ocean-bottom seismometer (OBS) experiment was carried out for the first time in the area around the Solomon Islands arc to locate microearthquakes. Observations started in late August and continued until early September. Five digital recording OBSs were deployed around the Russell Islands west of Guadalcanal Island. OBS spacing was about 20 km. All the OBSs were recovered and yielded data with a good signal-to-noise ratio. 40 earthquakes, with magnitudes in the range 1.5–4.4 were located over 8 days. The seismicity clearly images the two subducting plates. Though the seismicity beneath the arc side slope of the San Cristobal Trench is relatively high, we can see the seismicity which is related to the subducting Pacific Plate beneath Santa Isabel Island. In addition, earthquakes occur within the crust beneath the southern part of the New Georgia Basin and the Russell Islands. An aseismic area extending 40 km inward from the San Cristobal trench axis implies initial aseismic slip of the India–Australia Plate at a small dip angle.

Synfolding remagnetization and deformation: results from Palaeozoic sedimentary rocks in West Virginia

SUMMARY Palaeomagnetic analysis was conducted at 27 sites from two stratigraphically adjacent Palaeozoic units along two transects (Meldey and Rig) of the Patterson Creek anticline in the Appalachian orogen of West Virginia to better understand the relationship between deformation and remagnetization. While no primary directions were observed, we isolated identical, well-defined, secondary Late Palaeozoic magnetizations in carbonates from the Late Silurian Tonoloway Formation and the Early Devonian Helderberg Group. Standard application of conventional palaeomagnetic fold tests suggests that the magnetizations are synfolding in the Helderberg Group and pre-folding to early synfolding in the Tonoloway Formation. A fold test on all data combined indicates a synfolding magnetization (best grouping at 60–65 per cent unfolding), but the resulting distributions are not circular. An alternative interpretation, based on optimal differential unfolding at the site mean level yields one group with a nearly pre-folding, or early synfolding, magnetization (>70 per cent unfolding) that is both circular and concordant with North America's apparent polar wander path. The second group has a ‘variable synfolding’ magnetization that is elliptical at the optimum unfolding level and falls off the path. Differences in either the age of remagnetization or viscous partial thermoremanent magnetization contamination cannot account for this result. The sites falling into the pre-folding group are biased towards the Tonoloway Formation, the Medley Transect, the northwest limb, away from small folds and lower dip angle relative to the sites in the synfolding group. The two groups defined by the fold test results also have a relationship with both total strain and strain partitioning. The rocks containing the early synfolding remanence exhibit less total strain and, in particular, less pressure solution strain relative to grain boundary sliding and calcite twinning. It is likely that the remagnetization in the entire area was coincident and occurred prior to folding even though some sites reveal synfolding directions. The variation in the behaviour of the remagnetization relative to folding is unclear at present but it indicates that pressure solution and stylolite formation are important processes in remagnetization. In any event, the relationship between deformation and magnetization is not simple. Apparently identical magnetizations from adjacent sites may have very different histories and/or responses to deformation. While the exact relationship between remagnetization and deformation remains elusive this study demonstrates that significant variation in behaviour can exist at a very local scale.

Numerical simulations of deformation and dynamics of horizontally lying slabs

SUMMARY Recent results of seismic tomography have revealed that configurations of subducted slabs are varied; some lie horizontally on the boundary between the upper and the lower mantle, and others penetrate into the lower mantle. In this study, to understand the physical mechanism of deformation and dynamics of subducted slabs in the mantle, a 2-D numerical model was constructed. For this purpose, the slabs were treated so as to move freely in an incompressible fluid model by removing a guide right after its bending at a depth of 660 km and at a depth of 400 km in the upper mantle. Incorporating density and viscosity jumps and a Clapeyron slope for a γ → Pv + Mw phase transformation of olivine obtained from high-pressure and hightemperature experiments at a depth of 660 km into the model, we investigate temperature and velocity distributions, state of stress and viscosity distribution in relation to the formation of horizontally lying slabs at a depth of 660 km. When the slabs are treated to move freely just after their bending at the 660 km phase boundary, the slabs temporarily appear to lie horizontally on the phase boundary. However, the slabs penetrate the phase boundary and eventually sink into the lower mantle. The length of the horizontally lying portion of the slab becomes longer as descending velocity increases; the lengths are about 1500 km, 1200 km, and 800 km for subducting velocities of 8 cm yr −1 , 4 cm yr −1 ,a nd 2c m yr −1 , respectively. When a viscosity jump of 10 to 100 times the viscosity for the upper mantle is given at the 660 km phase boundary, the length of the horizontally lying portion of the slab becomes shorter because of friction between the slab and the underlying lower mantle. For all the models in which the guide was removed at a depth of 400 km, the slabs penetrate into the lower mantle without lying horizontally on the phase boundary. These results indicate that it is difficult for the slabs to bend at the γ → Pv + Mw phase boundary, and to form the configuration of the horizontally lying slabs even if the slabs descend with a small dip angle. The calculated maximum principal compressive stress axes are oriented horizontally within the horizontally lying portion of the slabs, which is inconsistent with seismologically observed vertical directions of the P-axes there. In order to explain the observed configurations and state of stress within the horizontally lying slabs, it would be necessary to investigate the effects of a complicated viscosity and composition structure in the mantle in more detail.

Morphology of the subducting slab boundary in the northeastern Japan arc

The detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate, and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones. It is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism. In this study we have estimated the morphology of the upper surface of the subducting Pacific plate under northeastern (NE) Japan by using 1372 SP converted waves at the slab boundary, and 666 earthquakes beneath the Pacific Ocean that are well located with sP depth phases. We found that the Pacific slab beneath NE Japan subducts at a small dip angle of less than 10° for the first descent to about 20 km depth and has a steeper dip angle of 15°–25° at 30–50 km depth and about 30° at deeper areas. The slab depth is approximately 50 km under the Pacific coast, 150 km under the Japan Sea coast, and nearly 100 km beneath the volcanic front. At 60–150 km depths, earthquakes in the lower plane of the double seismic zone occur in the central part of the slab, while the upper-plane events occur within 10–15 km beneath the slab boundary. There is a change in the slab surface geometry around 38°N which corresponds to the landward extension of an oceanic fracture zone. The strike of the slab is NNE in the north and becomes NE in the south. The southern part of the slab has a steeper dip angle than the northern part. We believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of NE Japan. Many great (M8.0–8.5) interplate earthquakes have occured in northern NE Japan during recorded history (830–1995), and microearthquakes show a wide distribution in the forearc region from the Japan Trench to the Pacific coast, suggesting strong interplate seismic coupling in that region. In contrast, only a few M7.0–7.5 class earthquakes have occurred in southern NE Japan in the same period, and microearthquakes tend to cluster in many small areas, suggesting weaker interplate coupling there. The subducted slab may be segmented by the oceanic fracture zones, and the degree of seismic coupling changes from segment to segment. The behavior of the segment in southern NE Japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle. This interpretation is supported by geomagnetic, topographic and geological studies which show that many small-sized seamounts were subducted in the past from the southern part of the Japan Trench, but almost no seamounts were subducted beneath the northern part.

Study of the characteristics of crustal stress field in East China by inversion of nodal-plane data

We combine a search procedure with an optimization procedure to invert for the average stress tensor and apply this method to the study of the crustal stress field using the data from nodal plane solutions of P wave first motion. From the analysis of data from Haicheng, Tangshan, Xingtai, Anyang, Liyang, Taiwan, Fujian and Guangdong, we have obtained the characteristics of the crustal stress field of East China: the directions of the principal pressure stress always possess a small dip angle but the azimuths vary from ENE (in northern East China) to ESE (in the southern part). This pattern is probably related to the effects of convergence of the northwestern Pacific plate from the east-northeast and the Philippine plate from the east-southeast.

Segregated production method for oil wells with active water coning

This paper describes a simulation study of a novel method to control water coning in an oil producing well. The method uses a dual-completion configuration that is above and below OWC. In this configuration, the well section above OWC is completed in the oil zone and produces oil, while the well section completed below OWC in the water-saturated zone produces water both independently of and concurrently with oil production. Segregated water production creates a drain (rat hole water sink) to control the rise of the water cone. Coning control results from the effect of water sink on flow potential between the sink and oil-producing perforations. The study investigated simulated production performance of the well both with and without the rat hole water sink. At first, the water sink's position and flow rate were examined to determine the critical production rate of oil (i.e., maximum production rate without the water breakthrough). Next, the various amounts of water produced during oil production at rates greater than critical were compared. The comparison study used both lab data and field data from an actual well/reservoir system of known geometry and reservoir properties. The coning behavior of the production system with the rat hole water sink was mathematically modelled using the flow potential distribution generated by two constant-terminal-rate sinks located between the two linear boundaries and the constant-pressure outer radial boundary. The model was verified by using the existing and simulated data of water coning in conventional wells and by setting the water sink flow rate at zero, with oil being the only produced fluid. Also, verification tests were used to select the type of oil reservoir with the thick oil zone and the small dip angle for the comparison studies. The study demonstrates the existence of an active mechanism to control water coning with the rat hole water sink. It shows that a sink with low water flow rate may prevent the water cone from breaking through the oil zone and into the producing perforations so that the oil with no water cut can be produced above the critical flow rate. A simulation using the new method yielded less produced water and either the same or a higher oil rate than the conventional method. However, potential increase of oil production above its critical value is limited by the stability of the controlled water cone. Also, for high oil production rates, more precision is needed to control the water production rate.

A Note on the Effect of the Free Surface on the Elastic Deformation due to a Long Dip-Slip Fault.

To calculate the elastic deformation due to a thrust fault in the lithosphere-asthenosphere composite, Nur and Mavko (1974) used the exact solution of Mura (1968) for an edge dislocation parallel to the boundary of two elastic half-spaces in welded contact. They approximated the effect of the free surface by considering Mura's solution for two equal and opposite edge dislocations with the free surface midway between them. By considering a simpler problem of a long dip-slip fault in a uniform half-space, we show that the approximate method used by Nur and Mavko does not yield satisfactory results. For small dip angles and beyond a certain epicentral distance, while the exact solution predicts subsidence, the approximate solution predicts uplift. Moreover, the approximate solution yields non-zero values for the surface tractions at the free surface.

Crustal Deformation Associated with Two-Dimensional Thrust Faulting.

The effect of the source-depth and the dip of the fault on the surface displacement field due to a long thrust fault embedded in an elastic half-space is studied. It is found that, for small dip angles, the displacement field is highly asymmetric about the fault strike, the displacement of the footwall is very small, and the horizontal displacement of the hanging wall shows a wavy pattern. The vertical displacement of the hanging wall is an uplift near the fault. For small dip angles, this uplift is maximum at a point almost vertically above the upper edge of the fault. As we go away from the fault strike, the vertical displacement changes sign, i.e., it becomes subsidence. For small dip angles, this subsidence is maximum at a point roughly vertically above the lower edge of the fault. Steeper fault dips decrease the subsidence relative to the uplift. For a vertical dip-slip fault, the horizontal displacement is completely symmetric about the fault strike and the vertical displacement is completely antisymmetric. The variation of the coseismic and postseismic shear stresses and shear strains for a surface-breaking long thrust fault with depth is studied for a viscoelastic half-space. It is found that, while the coseismic and postseismic shear stresses are of opposite signs, the coseismic and postseismic shear strains are of the same sign.

Focal mechanism of small quakes before and after the Kaoiki, Hawaii, earthquake (M S =6.6) of November 16, 1983

The Kaoiki, Hawaii, earthquake with magnitude 6. 6 of November 16, 1983 was a strike-slip faulting event on a fault with large dip angle. The results for mechanisms of smaller events before and after the Kaoiki mainshock show that there were two kinds of mechanisms: (1) strike-slip on the fault with large dip angle; (2) slip on the crustal discontinuity plane with smaller dip angle, and systematic and alternative changes in the mechanisms were observed.