Fluid Injection Experiments Research Articles

Changes in effective stress during the 2003–2004 Ubaye seismic swarm, France

[1] We study changes in effective stress (normal stress minus pore pressure) that occurred in the French Alps during the 2003–2004 Ubaye earthquake swarm. Two complementary data sets are used. First, a set of 974 relocated events allows us to finely characterize the shape of the seismogenic area and the spatial migration of seismicity during the crisis. Relocations are performed by a double‐difference algorithm. We compute differences in travel times at stations both from absolute picking times and from cross‐correlation delays of multiplets. The resulting catalog reveals a swarm alignment along a single planar structure striking N130°E and dipping 80°W. This relocated activity displays migration properties consistent with a triggering by a diffusive fluid overpressure front. This observation argues in favor of a deep‐seated fluid circulation responsible for a significant part of the seismic activity in Ubaye. Second, we analyze time series of earthquake detections at a single seismological station located just above the swarm. This time series forms a dense chronicle of +16,000 events. We use it to estimate the history of effective stress changes during this sequence. For this purpose we model the rate of events by a stochastic epidemic‐type aftershock sequence model with a nonstationary background seismic rate l0(t). This background rate is estimated in discrete time windows. Window lengths are determined optimally according to a new change‐point method on the basis of the interevent times distribution. We propose that background events are triggered directly by a transient fluid circulation at depth. Then, using rate‐and‐state constitutive friction laws, we estimate changes in effective stress for the observed rate of background events. We assume that changes in effective stress occurred under constant shear stressing rate conditions. We finally obtain a maximum change in effective stress close to −8 MPa, which corresponds to a maximum fluid overpressure of about 8 MPa under constant normal stress conditions. This estimate is in good agreement with values obtained from numerical modeling of fluid flow at depth, or with direct measurements reported from fluid injection experiments.

Source mechanisms of micro-earthquakes induced in a fluid injection experiment at the HDR site Soultz-sous-Forêts (Alsace) in 2003 and their temporal and spatial variations

We have inverted the peak amplitudes of direct P waves of 45 micro-earthquakes with magnitudes between M = 1.4 and 2.9, which occurred during and after the 2003 massive fluid injection in the GPK3 borehole of the Soultz-sous-Forˆets Hot Dry Rock facility. These events were recorded by a surface seismic network of 15 stations operated by the Ecole et Observatoire des Sciences de la Terre, University of Strasbourg. The unconstrained moment tensor (MT) expression of the mechanism was applied, allowing the description of a general system of dipoles, that is, both double-couple (DC) and non-DC sources, as tensile fractures. The mechanisms of all but one event are dominantly DCs with a few per cent additional components at the most. We have checked carefully the reliability of the MT retrieval in bootstrap trials eliminating some data, by simulating the mislocation of the hypocentre and by applying simplified velocity models of the area in constructing Green’s functions. In some of the trials non-DC components amounting to several tens of per cent appear, but the F-test classifies them as insignificant. Even the only micro-earthquake with an exceptionally high non-DC mechanism cannot be classified unambiguously—the F-test assigns similar significance to the pure DC solution. The massive dominance of the DC indicates the shear-slip as the mechanism of the micro-earthquakes investigated. The mechanisms display large variability and are of normal dip-slip, oblique normal to strike-slip types. The T-axes are fairly stable, being concentrated subhorizontally roughly in the E–W direction. On the contrary, the P-axes are ill constrained varying in the N–S direction from nearly vertical to nearly horizontal, which points to heterogeneous stress in the Soultz injected volume. This is in agreement with the stress pattern from in situ measurements: the minimum stress axis is well constrained to E–W, whereas the maximum and intermediate stress values are close to one another, enabling the ambiguity of the P-axis direction. We found no significant dependence of source mechanisms either on magnitudes or depths. The time–space distribution of the events analysed suggests that the injection activated two segments of the natural faults existing in the area (I and II in our notation) showing different source mechanism patterns. The dip-slip regime is characteristic of fault segment I where the seismicity occurred during and also after injection, whereas the strike-slip regime prevails in segment II where the seismicity was triggered only after the injection shut in. This indicates that the tensile fractures, which are assumed to be created during injection, may have occurred on a smaller scale than the pure shear micro-earthquakes investigated.

A numerical model for fluid injection induced seismicity at Soultz-sous-Forêts

During fluid injection experiments at the geothermal site of Soultz-sous-Forêts (France), more than 114,000 induced seismic events with magnitudes between −2.0 and +2.9 were detected by a local downhole monitoring network. Of these, 35,039 events are sufficiently constrained to be located. Hypocenters align along a sub-vertical, planar structure with the apparent width being dominated by data scattering indicating that seismic activity predominantly occurs along a (pre-existing) larger scale fault structure. For this scenario, we present a numerical model to simulate hydraulic overpressures and induced seismicity during hydraulic injection. The numerical model is based on the physical processes of fluid pressure and stress diffusion with triggering of the induced seismicity being controlled by Coulomb friction. Even in its simplest form of a fault zone without any structural heterogeneity, the numerical model reproduces typical observations at Soultz-sous-Forêts, such as number and magnitude of induced events, hypocenter locations (including the Kaiser effect), occurrence of post-injection seismicity, and the largest magnitude event occurring several days after shut-in.

Microseismic signatures of non-linear pore-fluid pressure diffusion

SUMMARY Most fractured porous rocks show a strong dependence of their fluid transport properties on the effective pressure and thus on the pore-fluid pressure. We investigate the case where the hydraulic transport properties are exponentially dependent on pore-fluid pressure. As a consequence, the process of pore-fluid pressure diffusion is governed by a non-linear diffusion equation. Solutions of this equation that describe fluid injection experiments are analysed and the associated microseismic signatures are explored. In particular, it is shown that this process of non-linear pore-fluid pressure diffusion allows us to define two signatures. One which is the triggering front corresponding to the triggering of microseismic events without changing the fluid transport properties significantly. The other signature is the so-called fracturing domain. It triggers the majority of microseismic events and alters the fluid transport properties significantly. Evidence for the microseismic signature of a fracturing domain at the Fenton Hill hydrofrac experiment is given.

Investigation of Fault Mechanisms during Geothermal Reservoir Stimulation Experiments in the Cooper Basin, Australia

Abstract Subsequent to an initial reservoir stimulation in 2003, a second long-term fluid-injection experiment was conducted in the Cooper Basin, Australia, in 2005 to further enhance and enlarge the existent geothermal reservoir. Approximately 25,000 m 3 of water were injected into the granitic crust at 4250 m depth, thereby inducing about 16,000 seismic events, which were recorded by a local eight-station network. Event magnitudes calibrated by recordings of the permanent network of Geosciences Australia are in the range between M L -1.2 and 2.9 with seismic moments between 10 9 and 10 13 N m. Hypocenter locations could be determined for 8886 events. The spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified for the initial reservoir stimulation, which is further extended in the course of restimulation. Early seismicity is located near the outer rim of the previous zone of seismic activity and subsequently migrates both toward and away from the injection well. The immediate vicinity of the injection well remains seismically quiet during restimulation. These effects can be explained by a simple Kaiser effect model, where the induced seismicity is controlled by the in situ fluid pressure increase. Compound fault-plane solutions based on P -phase polarities indicate a similar fault mechanism for nearly all events. The dominating fault mechanism is consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution. A small number of events with reverse mechanisms might indicate the existence of conjugated fractures locally intersecting the main fracture zone. Cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly, accumulating up to several centimeters of shear displacement.

Earthquake Cluster: What Can We Learn from Waveform Similarity?

Abstract The seismograms of earthquakes, which have closely spaced hypocenters, tend to be similar due to the similarity of the Green’s functions characterizing the source-receiver paths. Based on the λ /4 criterion, it is frequently argued that similar earthquakes may represent repeated slip of the same patch of a fault. Because of the phenomenological nature of waveform similarity, such interpretations are strongly dependent on seismic signal characteristics and on the way, the waveform similarity is defined. In this article, we use two-dimensional synthetic wave-field simulations in lateral heterogeneous media to investigate how the waveform similarity of closely spaced hypocenters changes with interevent separation. We analyze the influence of correlation window length, signal frequency bandwidth, and source-receiver geometry on the waveform similarity and discuss under which conditions the λ /4 criterion can be applied to the synthetic data set. With the correlation window length defined as 2.8 times the travel-time difference between the S - and P -phase onsets, we find a correlation threshold value of 0.95 independent of the signal frequency bandwidth. We use the same threshold value for two field data examples that are similar to the synthetic data in frequency content and waveform complexity, and we discuss the implications of the λ /4 criterion. For three microearthquakes occurring during a fluid-injection experiment at the German deep drilling site (Kontinentale Tiefbohrung [KTB]), the interevent separation constrained by the λ /4 criterion is sufficient to identify these events as a sequence of repeating earthquakes in the sense that at least a fraction of the source area experienced repeated slip. For a second data example of four natural (micro-) earthquakes occurring near the island of Crete, the λ /4 criterion does not sufficiently constrain the hypocenter location to identify these events as repeating earthquakes due to the lack of high-frequency information.

Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)

We summarise the results of seismological studies related to triggering mechanisms, driving forces and source processes of the West Bohemia/Vogtland earthquake swarms with the aim to disclose the role of crustal fluids in the preparation, triggering and governing of the swarms. We present basic characteristics distinguishing earthquake swarms from tectonic mainshock-aftershock sequences and introduce existing earthquakes swarm models. From the statistical characteristics and time-space distribution of the foci we infer that self-organization is a peculiarity of West Bohemia/Vogtland swarms. We discuss possible causes of the foci migration in these swarms from the viewpoint of co-seismic and/or post-seismic stress changes and diffusion of the pressurized fluids, and we summarize hitherto published models of triggering the 2000-swarm. Attention is paid to the source mechanisms, particularly to their non-shear components. We consider possible causes of different source mechanisms of the 1997-and 2000-swarms and infer that pure shear processes controlled solely by the regional tectonic stress prevail in them, and that additional tensile forces may appear only at unfavourably oriented faults. On data from the fluid injection experiment at the HDR site Soultz (Alsace), we also show that earthquakes triggered by fluids can represent purely shear processes. Thus we conclude that increased pore pressure of crustal fluids in the region plays a key role in bringing the faults from the subcritical to critical state. The swarm activities are mainly driven by stress changes due to co-seismic and post-seismic slips, which considerably depend on the frictional conditions at the fault; crustal fluids keep the fault in a critical state. An open question still remains the cause of the repeatedly observed almost simultaneous occurrence of seismic activity in different focal zones in a wider area of West Bohemia/Vogtland. The analysis of the space-time relations of seismicity in the area between 1991 and 2007 revealed that during a significant part of this time span the seismicity was switching among distant focal zones. This indicates a common triggering force which might be the effect of an increase of crustal-fluid pore-pressure affecting a wider epicentral region.

Induced Seismicity during the Stimulation of a Geothermal HFR Reservoir in the Cooper Basin, Australia

A long-term fluid-injection experiment was performed in the Cooper Basin (Australia) in 2003 to stimulate a geothermal reservoir. More than 20,000 m 3 of water were injected into the granitic crust at 4250 m depth. During reservoir stimulation about 27,000 induced seismic events were detected by a local, eight- station seismic monitoring system deployed in nearby boreholes. Hypocenter locations for 11,068 events were determined by using an averaged velocity model that was calibrated by associating early events with the injection point. The spatial hypocenter distribution forms a nearly subhorizontal structure with a lateral extension of 2 km × 1.5 km and an apparent thickness of approximately 150–200 m, which is in the order of the hypocenter location confidence limits. The hypocenter distribution exhibits a high degree of spatiotemporal ordering with the seismic activity systematically migrating away from the injection well with increasing time. Previously activated regions become seismically quiet indicating relaxation processes. High-resolution relative hypocenter locations determined for clusters of “similar” events locally reduce the apparent thickness of the structure to the level of a few tens of meters indicating that the reservoir is dominated by a single fracture zone only. Consistent with these findings, a subsequently drilled well intersects a dominating, high-permeable fracture within 15 m of the predicted intersection depth. Based on drilling and logging information, the fracture zone is interpreted as a preexisting (possibly tectonically formed) feature that (partly) sheared during stimulation. Triggering of the induced seismicity is found to be predominantly controlled by the increase of fluid pressure implicating a (local) reduction of the effective normal stress resolved on the fracture plane. Additionally, perturbations of the stress field caused by the largest-magnitude events may trigger seismicity (“aftershocks”) on a local, short-ranging scale.

Spatiotemporal pore pressure evolution due to fluid-mass point sources in dynamic poroelasticity

SUMMARY The spatiotemporal behaviours of pore pressure induced by a fluid mass point source in quasi-static and dynamic poroelasticity are compared and contrasted. Explicit results for the pore pressure in time domain are derived using the known fundamental solution of dynamic poroelasticity in frequency domain. Depending on the frequency content of the source function, different regimes can be identified. First, the diffusive regime where only the diffusive slow wave exists; the latter is equivalent to the process of pore pressure diffusion. Second, there is the propagation–diffusion regime where slow and fast wave modes coexist. It is shown that the amount of induced pore pressure due to the diffusive slow wave is different for quasi-static loadings (e.g. fluid injection experiments) and transient loads (e.g. propagating low-frequency seismic waves). The results are discussed in the context of fluid injection experiments where short-term fluctuations of the injection rate can create both, the fast compressional wave and the slow diffusive wave. The results can be used in order to generate synthetic seismograms and validate numerical schemes for pressure disturbances in dynamic poroelasticity.

Fluid induced seismicity guided by a continental fault: Injection experiment of 2004/2005 at the German Deep Drilling Site (KTB)

Recent hydraulic experiments at the KTB site have shown that seismicity induced by long‐term fluid injection directly into a continental crustal fault remains guided by this fault. The seismicity is triggered by pressure perturbations as low as 0.01–1 bars at the hypocenters. A combination of sequential one‐year fluid extraction (2002/2003) and one‐year fluid injection (2004/2005) experiments has shown that only positive pore pressure perturbation (i.e., injections) was able to induce the seismicity. Moreover, the onset of seismicity roughly coincides with the time of compensation of the extracted fluid volume by the following injection. This confirms that the pressure diffusion is a dominant mechanism of seismicity triggering by fluid injections. The probed fault shows a significant anisotropy and non‐linearity of its hydraulic behaviour. Its hydraulic diffusivity is up to one order of magnitude larger than that of surrounding rocks.

Fault mechanisms of induced seismicity at the superdeep German Continental Deep Drilling Program (KTB) borehole and their relation to fault structure and stress field

One hundred twenty‐five fault plane solutions for microearthquakes induced during a long‐term fluid injection experiment at the German Continental Deep Drilling Program (KTB) boreholes (Germany) in 2000 are investigated. A predominant strike‐slip mechanism is observed, partly with components of normal but also with reverse faulting. Adding 54 fault plane solutions of an earlier injection experiment at the KTB, we determine the local stress field and find a subhorizontal north‐south orientation for the maximum principal stress and a near‐vertical orientation for the intermediate principal stress. The stress field exhibits no temporal or spatial variations within the resolved accuracy of ±15°. However, the results of the stress tensor inversion point to heterogeneities of second order. On the basis of the hypocentral distribution of the induced microearthquakes and the similarity of fault mechanisms, we relate our data to the fault structure at the KTB. We find that the larger faults act as pathways for the injected fluid, whereas the brittle failure occurs on fault asperities of the larger mapped faults and nearby smaller faults, both in agreement with the local stress field. Applying a thorough error analysis of the individual fault plane solutions, we correlate the diversity of mechanisms with their strength and find that the strongest events tend to a representative mechanism that is in good correspondence with the stress field. In contrast, the diversity of fault mechanisms is larger for the smaller events, indicating local stress perturbations.

Mutual relationship between microseismicity and seismic reflectivity: Case study at the German Continental Deep Drilling Site (KTB)

Two fluid injection experiments were conducted at the German Continental Deep Drilling Site (KTB) in 1994 and 2000. Microseismicity occurred at different depth intervals. Hypocenter locations were precisely localized. Here, microseismicity is analyzed in terms of its spatio‐temporal evolution characteristics. An approach is applied which assumes microseismicity to be triggered by a diffusive process of pore pressure relaxation. The method yields estimates of hydraulic parameters of rocks on large spatial scales. At the KTB site the method enables us to study hydraulic diffusivity at two different depth intervals. We observe significant variations in the evolution characteristics of the seismic activity at different depths. Estimates of hydraulic diffusivity for shallower parts of the crust seem to be much smaller than for deeper regions. To understand reasons for this, we have analyzed spatial relations of hypocenter locations to the distribution of intensities of seismic reflections. Low values of hydraulic diffusivity correlate with low reflection intensities and high diffusivities with large intensities, respectively. The analysis confirms the hypothesis that the process of pore pressure relaxation along pre‐existing and critically stressed natural fractures is an important triggering factor for induced microseismicity.

A model for fluid-injection-induced seismicity at the KTB, Germany

SUMMARY The 9.1 km deep KTB (Kontinentale Tiefbohrung, Germany) drilling hole is one of the best investigated deep-drilling sites in the world. Among other parameters, in situ measurements revealed continuous profiles of principal stresses, pore fluid pressure and fracture geometry in the vicinity of the borehole. The present study combines these parameters with hydraulic and seismicity data obtained during fluid-injection experiments conducted at the KTB to derive a conceptual model for fluid-injection-induced seismicity at the KTB. This model rests on the well constrained assumptions that (1) the crust is highly fractured with a permeable fracture network between 9 km depth and the Earth's surface and (2) the crust is in near-failure equilibrium, whereby a large number of fracture planes are under near-critical condition. During the injection experiment, the elevated pore fluid pressure remained well below the least principal stress and thus was too small to cause hydraulic opening of existing fractures. Consequently, the geometry of the fracture network was assumed to have not changed during fluid injection with induced seismicity occurring solely as a result of lowering of the effective normal stress, consistent with observed source mechanisms. The key parameter in the present model is the fracture permeability, which exhibits large spatial and directional variations. These variations are proposed to primarily control fluid migration paths and associated propagation of elevated fluid pressure during fluid injection. In contrast with common models based on isotropic fluid diffusion or spatially averaged permeability, highly permeable branches of the fracture network strongly affect the propagation of fluid pressure and prohibit the concept of a smooth ‘pressure front’. We find evidence that major fluid flow exists at comparatively low fluid pressure (below the critical pressure required to cause seismic failure) without being detected seismically. This might also explain the difference between 1011 J of hydraulic energy inserted into the system during fluid injection and ∼108 J of seismic energy: a major part of the hydraulic energy might be converted to potential energy of the ground water level caused by upward migrating fluid. From the fluid level response to changes of injection rate observed in a second borehole we estimate fluid signal velocities to be as large as 300 m d−1. Importantly, the suggested model also accounts for the occurrence of repeating earthquakes (multiplets), a large number of which were observed during the injection experiment. The present model also suggests that coseismic changes of the stress field caused by tectonic shear stress release are very local and of small magnitude. This is consistent with the observation that none of the larger induced events is followed by aftershock series that would be expected if coseismic processes had noticeably perturbed the local stress field.

Probing the Crust to 9-km Depth: Fluid-Injection Experiments and Induced Seismicity at the KTB Superdeep Drilling Hole, Germany

A 60-day, long-term fluid-injection experiment was performed at the 9.1-km-deep Kontinentale Tiefbohrung, Germany (KTB), borehole. About 4000 m3 of water were injected into the well head to induce seismicity near the open-hole section at 9-km depth. Because of several leaks in the borehole casing (unknown before), seismicity occurred at distinct depth levels between 3-km and 9-km depth. Two events occurred at 10-km and 15-km depth. The combination of a temporary, 40-element, three-component surface network of seismometers and a three-component downhole sonde at 3.8-km depth in the nearby pilot hole enabled us to determine absolute hypocenter locations by using a velocity model that was calibrated from several downhole shots at depths of 5.4 km and 8.5 km. Of a total of 2799 induced events, hypocenter locations were obtained for 237 events having good signal-to-noise ratio at surface stations. The spatiotemporal distribution of hypocenters at each depth level exhibits complex structures extending several hundred meters from the injection points, with strong spatial and temporal clustering. Regions that were seismically active at a certain time often showed reduced or no activity at later times, indicating local shear-stress relaxation. A similar “memory” effect (Kaiser effect) is observed by comparing hypocenter locations of the present experiment with those obtained for a previous injection experiment at KTB. The limitation of hypocentral depths to 9.1 km for events near the borehole suggests changes in rheological properties of the upper crust and thus supports a transition from the regime of brittle failure to ductile deformation at this depth. Large fluid-level changes observed in the nearby pilot hole demonstrate that fluid flow occurs over distances greater than 1.5 km and that major flow zones are not mapped by the induced seismicity. This might also explain the occurence of isolated events at greater distances and depths. Brittle failure at depths greater than 10 km indicates the existence of critically stressed fractures even at temperature over 300°C.

Magma Intrusions and Earthquake Swarm Occurrence in the Western Part of the Bohemian Massif

We are proposing a hypothesis that earthquake swarms in the West Bohemia/Vogtland seismoactive region are generated by magmatic activity currently transported to the upper crustal layers. We assume that the injection of magma and/or related fluids and gases causes hydraulic fracturing which is manifested as an earthquake swarm at the surface. Our statements are supported by three spheres of evidence coming from the western part of the Bohemian Massif: characteristic manifestations of recent geodynamic activity, the information from the neighbouring KTB deep drilling project and from the 9HR seismic reflection profile, and the detailed analysis of local seismological data. (1) Recent manifestations of geodynamic activity include Quaternary volcanism, rich CO2emissions, anomalies of mantle-derived3He, mineral springs, moffets, etc. (2) The fluid injection experiment in the neighbouring KTB deep borehole at a depth of 9 km induced hundreds of micro-earthquakes. This indicates that the Earth's crust is near frictional failure in the western part of the Bohemian Massif and an addition of a small amount of energy to the tectonic stress is enough to induce an earthquake. Some pronounced reflections in the closely passing 9HR seismic reflection profile are interpreted as being caused by recent magmatic sills in the crust. (3) The local broadband seismological network WEBNET provides high quality data that enable precise localization of seismic events. The events of the January 1997 earthquake swarm are confined to an extremely narrow volume at depths of about 9 km. Their seismograms display pronounced reflections of P- and S-waves in the upper crust. The analysis of the process of faulting has disclosed a considerable variability of the source mechanism during the swarm.

Crustal scattering at the KTB from a combined microearthquake and receiver analysis

SUMMARY In December of 1994 a fluid injection experiment which triggered several hundreds of microearthquakes was conducted at the KTB main borehole (Oberpfalz, Germany). These events were recorded with a temporal seismic network at the surface. Out of the complete data set, a cluster of five events recorded at four mini-arrays consisting of eight or nine stations was used to investigate the crustal scattering properties in the vicinity of the KTB. For this purpose, the 'Double Beam Method' (DBM; Kruger et al. 1993, 1995, 1996) and the 'Double Beam Imaging Method' (Scherbaum, Kruger & Weber 1997) were extended to curved wave fronts to drop the restriction of plane- wave propagation. This technique is used for imaging the crustal scattering strength using earthquake clusters recorded at close-by mini-arrays. The results of the array analysis show that the composition of the P coda is mainly aVected by the site location of the arrays. Near-surface and deeper crustal scattering contribute in a very complicated pattern. Furthermore, with the present data set it was possible to identify reflections from the top of the Erbendorf Body. This is a very pronounced arrival in most of the recorded traces. In one of the arrays its amplitudes are even greater than the direct P phases. Five to eight coherent phases could be identified by the mini-arrays. Using only these phases, synthetic P-coda traces were constructed, which only contain the coherent part of the observed wavefield. By subtracting the synthetic coherent wavefield from the original traces we achieve a variance reduction in the P coda of up to 37 per cent. This leads to the conclusion that a large amount of the P coda at the KTB can be modelled by a simple deterministic single-scattering model using a small number of individual scatterers.

Seismicity and stress field in the vicinity of the KTB location

A local, broadband, seismic network of four observatory‐quality stations (KTB‐NET) was operated during the drilling of the KTB hole, within the framework of the interdisciplinary German Continental Deep Drilling Program (KTB). The aim was to investigate the seismic activity with regard to the tectonic stress field and to compare it with data from in situ measurements in the 9.1‐km deep borehole sections (bottom temperature of 260°C and heat flow of 82–85 mW/m2). From October 1990 to November 1995, over 80 local microearthquakes with magnitudes from 0.2 to 2.8 ML were recorded: eight small events by the KTB‐NET only and four earthquake swarms with 73 events by the KTB‐NET and stations of the Vogtland/Western Bohemia networks. Six of the small events are located within or close to the KTB‐NET. The swarm events occurred at the southwestern extension of the Ohre rift, in an area 20 km north of the drill site, which is revealed to be part of the Vogtland/Western Bohemia seismotectonic unit, characterized by swarm activity. The hypocenters are limited to the upper 13 km of the crust, with a distinct concentration between 10 and 12 km. All types of fault plane solutions are found, but at depths greater than 8 km, reversed faulting mechanisms predominate. P axes are very uniformly oriented in a NNW‐SSE direction, corresponding to the well‐known regional stress orientation in central Europe and in agreement with the special in situ stress measurements of the KTB program. The focal mechanism of a ML=1.2 event induced by a fluid injection experiment fits into the results obtained from the natural events. Possible indications for the brittle‐ductile transition are discussed in view of the observed earthquake depth and focal mechanism distributions.

粘弾性体への破壊をともなった流体浸入

Fluid injection experiments into a Hele-Shaw cell filled with agar gels were carried out at a constant injection pressure. Agar gels of concentration of 0.1wt%, 0.2wt%, and 0.3wt% agar were used as a model material with visco-elastic properties. In the case of lower concentration of agar, agar gels behave as a viscous fluid. However, the property of agar gels as elastic solids increases with increasing concentration of agar. The ratio of relative tensile strength among agar gels of concentration of 0.1wt%, 0.2wt%, and 0.3wt% agar resulted from indentation experiments are 0.14:1.0:3.9. Growth patterns obtained by our Hele Shaw cell experiments are classified into three typical patterns. They are patterns generated by viscous fingering, visco-elastic fingering, and single plane cracking. A visco-elastic fingering is new pattern formation observed in the fluid injection experiments into visco-elastic materials. The characteristics of the visco-elastic fingering pattern are following. 1) The visco-elastic fingering pattern has a branched structure same as one of the viscous fingering pattern. 2) The visco-elastic fingering pattern has an acute growing tip similar to that of single plane cracking, though a shape of growing tip of viscous fingering is round. The phase diagram for the Hele-Shaw cell experiment filed with the agar gel is obtained by changing both the pressure of air injection and the concentration of agar systematically. Fluid intrusion phenomena into visco-elastic materials often occur in geophysical process: e. g., magma migration, core formation, fluid migration in accretionary prisms, etc. Our results will give us new implications for the geophysical process in which the earth or planet's materials behave as visco-elastic materials.

Strength changes due to reservoir‐induced pore pressure and stresses and application to Lake Oroville

The association of a significant increase in seismic activity with the filling of some large water reservoirs is well documented. One possible such case is the earthquake sequence at Oroville, California, in 1975. It has been suggested that this activity may be triggered on faults close to failure by the weight of the water, the increase in pore pressure, or both. Two‐dimensional half‐space models with surface loading are used to study the change in strength produced by these mechanisms for thrust, normal, and strike slip faulting. For the case of the water weight alone the strength increased across fault planes dipping as in the Oroville sequence. It is concluded that the water load alone is an unlikely mechanism. The inclusion of pore pressure effects in these models, however, is shown to be very encouraging. Using the Biot linearized quasi‐static elasticity theory for fluid‐infiltrated porous materials, two types of loaded half‐space models were studied. The first was homogeneous with respect to permeability and found to produce broad zones of weakening. The second type included a fault zone of different permeability. This model intensifies the magnitude and broadens the zone of strength drop if the fault zone is highly permeable in relation to the rest of the half space. For fault dips roughly corresponding to the Oroville situation it was found that although weakening occurs for ambient stresses favoring either thrust or normal faulting, the magnitude and breadth of strength drop were significantly greater for normal faulting: Similar effects were observed for strike slip faulting. It was also found that all types of models developed significant zones of weakening very rapidly, permitting reasonable values of permeability to be estimated. The case of the impervious reservoir bottom was examined, with significantly differing results. Just as in the permeable case, zones of weakening were initially developed. However, in the impermeable case these dissipated with time, eventually reaching the same strength distribution as that of the water weight alone. Thus with sufficiently slow impoundment the impervious reservoir may not weaken at all, whereas the permeable reservoir will continue to weaken in time. The results also show that rapid unloading may cause instantaneous crustal weakening owing to excess pore pressure. Although the models are very simplified, it is interesting to apply them to the case of Lake Oroville. Using reasonable parameters, strength drops for normal faulting of 8–14 bars were calculated, which compared favorably to the fluid injection experiments at Rangely and Matsushiro. The observed lag time between initial impoundment and seismicity yielded a permeability of 0.2 mdarcy for the homogeneous model and 20 mdarcy for the greater strength drops produced by the heterogeneous model, both of which are reasonable rock values. If the period of rapid filling is assumed to have triggered the activity, then the permeability was found to be 3.6 mdarcy for the homogeneous model and 360 mdarcy for the heterogeneous model, the latter of which is appropriate for fractured rock. It is concluded that the pore pressure trigger mechanism is very promising and that the methods of this paper may be extended to three dimensions to study reservoirs in more detail. The mechanism is limited, however, by the lack of knowledge of, first, the ambient pore pressure in situ and, second, the in situ permeability, coupling parameters, reservoir bottom permeability, and local tectonic stress. Perhaps the most important implications of the results is the presence of water to depths of 10 km or more in the crust. Thus at least in some places the crust may be weak, and tectonic stresses low. This also suggests an important potential for geothermal energy at great depth in the crust.