Extensive Dilatancy Anisotropy Research Articles

S wave Splitting in Central Apennines (Italy): anisotropic parameters in the crust during seismic sequences

In this work, we reviewed the main anisotropic results obtained in the last two decades along the Central Apennines. Moreover, we improved this database, with new results coming from the seismicity that occurred in the Montereale area, between 2009 and 2017, which corresponds to a spatio-temporal gap in the previously analyzed datasets. The examined papers concerned both seismic sequences (as Colfiorito in 1997, Pietralunga in 2010, L’Aquila in 2009, Amatrice in 2016) and background seismicity (as the 2000-2001 Città di Castello experiment). The whole of the collected results shows a general NW-SE fast shear wave direction consistent with both the orientation of the extensional active Quaternary and inherited compressive fault systems, focal mechanisms and local stress field. Also, we observed a more intense anisotropy strength (normalized delay time > 0.006 s/km) nearby the strongest events (M > 5), all concentrated in the hanging-wall of the activated fault systems. In fact, this area is deeply affected by the surrounding rock volume perturbations that, in turn, have altered both the local stress field and crustal fracturing network. The most common anisotropic interpretative models that could explain our results are 1) the stress-induced anisotropy according to the Extensive-Dilatancy Anisotropy (EDA) model where the anisotropic pattern is related to the local stress variation and most of the variability is visible in time; 2) the tectonic-controlled anisotropy according to the Structural-Induced Anisotropy (SIA) model where the anisotropic pattern is related to the major structural features and most of the variability is visible only in space. As reported by the examined studies in Central Apennines the possibility to discriminate between stress and structural anisotropy is quite complex in a region where the directions of the extensional regime, the in situ horizontal maximum stress, the strike of major faults, both active and inherited coincide. Generally, in this review, we noted an overlap and mixture of the two aforementioned mechanisms and, just through a temporal analysis, made in the Montereale area, we supposed a predominant stressinduced anisotropy only in rock volumes where anisotropic parameter variations have been detected.

Shear Wave Splitting Evidence and Relations With Stress Field and Major Faults From the “Amatrice‐Visso‐Norcia Seismic Sequence”

AbstractWe present the largest dataset of anisotropic high‐quality parameters (~12,000) for the Amatrice‐Visso‐Norcia seismic sequence and investigate the physical mechanisms causing crustal anisotropy and its relation with crustal deformation, stress field, fluids, and earthquake generation. We performed shear wave splitting analysis on ~40,000 aftershocks recorded at 31 seismic stations during the first six months of the sequence following the 24 August 2016 Mw 6.0, Amatrice mainshock. Automatic and manual‐revised P‐ and S‐picking and high‐precision locations are used to delineate the fracturing pattern and spatio‐temporal variations in the anisotropic parameters: fast direction polarization (φ) and delay time (δt). The mean φ strikes N146°, parallel to the extensional Quaternary fault systems, and to the NW‐SE local active SHmax as proposed by the extensive dilatancy anisotropy model. Locally, φ directions outline the pattern of microscale and mesoscale structures that we relate to structural‐controlled anisotropy. Temporal variations of anisotropic parameters allowed us to deduce stress‐induced and pervasive fluid‐filled stress‐aligned crack systems as the prevalent anisotropic mechanisms in some sectors. Higher δt (>0.072 s) and higher anisotropy percentage (>1.5–2.0%) are found at the boundaries and in the western side of the activated fault systems, where heavily fractured carbonates are present. The fault network along with the lithological heterogeneities present in the area may act as a structural barrier along which fluids are channeled or trapped, thus causing overpressured fluid zones. Observations of shear‐wave splitting parameters during a seismic sequence can monitor the buildup of stress before earthquakes and the stress release as earthquakes occur.

Observation and analyses of shear wave splitting at the Larderello-Travale geothermal field, Italy

We measured shear wave splitting (SWS) parameters from a large dataset of local microearthquakes recorded at the Larderello-Travale geothermal field (LTGF; Tuscany, Italy). For that geothermal area, seismic anisotropy is distributed in the upper crust following a complex pattern. Although the overall trend reflects the strike of the normal faults dominating the region, measurements at the southern and central part of the LTGF show large (up to 90°) deviations from the dominant polarization direction. This anomalous pattern suggests that besides the extensive dilatancy anisotropy, the fast wave polarization direction is also likely affected by the presence of over-pressurized geothermal fluids, by local rearrangement of the regional stress, and by the presence of non-vertical cracks. We found large differences in normalised delay times between sparse and clustered seismicity. While the average anisotropy percentage is on the order of 1.7%, a significant amount of our measurements exceeds the 4.5%, reaching values as high as 16%. The highest anisotropy percentages are associated with earthquakes located at the center and at the SE margin of the geothermal area, at depths lower than 5 km and in the 5–10 km range, respectively. This latter occurrence may be interpreted in terms of cracks filled with fluids which, given the expected pressure and temperature conditions, are likely in supercritical conditions. Shear-wave splitting thus confirms to be a powerful tool for better constraining location and extent of those deep fractured rock portions possibly hosting supercritical fluids, that represent the next frontier of geothermal exploitation due to their enhanced heat capacity.

SHEAR WAVE ANISOTROPY MEASUREMENTS ABOVE SMALL EARTHQUAKES IN TRICHONIS LAKE, GREECE

On April 2007, an earthquake swarm occurred in the vicinity of Trichonis Lake, western Greece. The seismic activity started on April 10th, 2007 after the occurrence of three moderate size earthquakes MW 5.0-5.2. We performed shear wave splitting measurements on seismic events recorded during the first week of the seismic activity by a portable network of 8 stations that was installed in the area by the University of Patras Seismological Laboratory. We were able to take measurements from 5 stations as the seismicity was concentrated mostly at the eastern part of the lake. Two splitting parameters were measured through the data processing, the time-delays between the fast and slow shear wave components and the polarization directions of the fast components. In general, the data analysis revealed the presence of shear wave anisotropy in the study area. The average value of normalized time-delays was calculated at 6.9 ±1.1 ms/km while the fast polarization directions had an average of 130o ±14o. The mean fast polarization directions were consistent with what is expected concerning the local stress field, as it was sub-parallel to the strike of the major faults at the eastern part of the lake and almost perpendicular to the direction of extension. Therefore, the findings can be interpreted by an anisotropic volume of stress-aligned micro-cracks within the upper crust according to the extensive dilatancy anisotropy model.

ANISOTROPY STUDY OF THE FEBRUARY 4TH 2008 SWARM IN NW PELOPONNESUS (GREECE)

The Gulf of Corinth, located in central Greece, is characterized by normal faulting and by high seismicity since the antiquity. On 4 February 2008 a seismic swarm burst in NW Peloponnesus characterized by the occurrence of two moderate-size earthquakes of moment magnitudes 4.7 and 4.5, respectively. Analysis of the data revealed the existence of shear-wave splitting. The events that were chosen for the anisotropy study fit the selection criteria, having clear and impulsive S wave arrival phases on the horizontal components. In addition, the amplitude of the S wave phase on the vertical component was smaller than on the horizontal ones. The representations that were used to determine the polarization direction of the fast split shear wave, the time delay between the two split shear waves and the polarization direction of the source were the polarigram and the hodogram. The uniform fast shear wave polarizations, irrespective of the azimuth of each event, are consistent with the general NNE-SSW direction of extension in the Gulf and, therefore, in agreement with the extensive dilatancy anisotropy (EDA) model. Finally, a decrease of the time delay values was observed after the occurrence of the first moderate earthquake, implying changes of the medium’s properties.

Wave Propagation Modeling and Amplitude-Variation-with-Offset Response in a Fractured Coalbed

Coalbed methane can be detected employing the amplitudevariation-with-offset technique. However, there are two issues in applying this technique to a coalbed: strong azimuthal anisotropy resulting from high-density fractures, and the seismic response being composed of many or several individual reflections within the coalbed. To overcome these difficulties, we present an exact solution for reflections in extensive dilatancy anisotropy media. First, we build a three-layer model and simulate the wave propagation in this model. Then we derive an exact P- and converted S-wave reflection coefficient equation based on boundary conditions. Finally, substituting given model parameters into the exact equation, we obtain the variation in the reflection coefficient with incidence angle. The results show that the fracture factors, wavelet frequency and thickness of the coalbed have different effects on the reflection coefficient. Furthermore, we create a synthetic seismogram by forward calculation, and the result fits well with results of the exact equation.

Structurally controlled seismic anisotropy above small earthquakes in crustal rocks beneath the Rwenzori region, Albertine Rift, Uganda

We investigated shear-wave splitting of local earthquakes recorded by a temporary network of stations deployed in the Rwenzori region (Albertine Rift) from February 2006 to September 2007. Over 21,700 local earthquakes with magnitudes ranging between −0.5 and 5.1 were recorded. Shear-wave splitting analyses were made on only local crustal earthquakes with incidence angles of less than 34°. About 7000 earthquakes were available for analysis of their particle motions. The fast shear-wave polarization directions (φ) and the delay times (δt) between the fast and the slow shear-wave were calculated using the covariance and cross-correlation methods respectively. Exactly 309 shear-wave splitting pairs showed evidence of shear-wave splitting due to the abrupt changes in their shear-wave polarization directions as shown on their polarization diagrams and were available for interpretation. Visual inspection of our results revealed a heterogeneous pattern of φ directions throughout the entire study area, although a more-or-less rift parallel pattern of φ directions dominated at stations in the southern part of our study area. The orientation patterns of fault planes, foliation planes and the directions of maximum compressional stress were observed to be aligned with the φ directions in the study area. The extensive dilatancy anisotropy hypothesis was considered to be the most likely explanation for the observed φ directions. In general, the results do not show a correlation of δt with the ray-path length. We therefore argued that the observed seismic anisotropy probably has its origin at shallow crustal depths. An average δt of about 0.04s was observed, however, relatively large values of δt, ranging between 0.1s and 0.26s, occur in the region northwest of the Rwenzori Mountains. We interpreted these large values to be caused by stress-aligned pockets of melt within the uppermost crust.

Crustal anisotropy in the forearc of the Northern Cascadia Subduction Zone, British Columbia

SUMMARY This paper aims to identify sources and variations of crustal anisotropy from shear-wave splitting measurements in the forearc of the Northern Cascadia Subduction Zone of southwest British Columbia. Over 20 permanent stations and 15 temporary stations were available for shear-wave splitting analysis on ∼4500 event-station pairs for local crustal earthquakes. Results from 1100 useable shear-wave splitting measurements show spatial variations in fast directions, with margin-parallel fast directions at most stations and margin-perpendicular fast directions at stations in the northeast of the region. Crustal anisotropy is often attributed to stress and has been interpreted as the fast direction being related to the orientation of the maximum horizontal compressive stress. However, studies have also shown anisotropy can be complicated by crustal structure. Southwest British Columbia is a complex region of crustal deformation and some of the stations are located near large ancient faults. To use seismic anisotropy as a stress indicator requires identifying which stations are influenced by stress and which by structure. We determine the source of anisotropy at each station by comparing fast directions from shear-wave splitting results to the maximum horizontal compressive stress orientation determined from earthquake focal mechanism inversion. Most stations show agreement between the fast direction and the maximum horizontal compressive stress. This suggests that anisotropy is related to stress-aligned fluid-filled microcracks based on extensive dilatancy anisotropy. These stations are further analysed for temporal variations to lay groundwork for monitoring temporal changes in the stress over extended time periods. Determining the sources of variability in anisotropy can lead to a better understanding of the crustal structure and stress, and in the future may be used as a monitoring and mapping tool.

Seismic anisotropy of the shallow crust in the Umbria–Marche (Italy) region

Small magnitude earthquake data, recorded during the 1997 Umbria–Marche (Italy) seismic crisis, have been used to study the shear wave splitting in the central Apennine. Using the method of diagonalization of the covariance matrix, a set of 1234 S waves impinging the Earth at an angle less than the critical angle has been selected. The cross-correlation method was used to estimate the splitting time between the two horizontal S components and the direction of the symmetry axis of the anisotropic structure. A further analysis, based on visual inspection of corrected traces, allowed us to estimate the anisotropic parameters for 884 S waves. An average splitting time 〈 τ 〉 = 32.3 ∓ 4 ms has been estimated. Splitting times do not generally show a clear pattern with distance, which is an indication of possible spatial variations of seismic anisotropy. A minimum 2 km depth of the anisotropic layer is estimated. The trend of normalized splitting time vs. depth roughly indicates a maximum 4–5 km depth of the anisotropic layer. The distribution of the fast symmetry axis indicates a main symmetry axis oriented along the direction of the maximum compressive stress in the area and a secondary symmetry axis almost orthogonal to the first one. The stereographic plots of fast axis direction seem exclude a depth dependence of anisotropic parameters. A clear spatial correlation between the directions of the fast symmetry axes at the considered stations and the orientations of the nearest fault systems and thrust fronts is found, which confirms the spatial variability in the anisotropic properties of the area, as inferred by the analysis of splitting times. Based also on previous studies, results are interpreted in terms of the extensive dilatancy anisotropy (EDA) model. The estimated crack density is always smaller than the critical coalescence value, enforcing the hypothesis that fluid-filled separated crack systems control the polarization of S waves.

Theoretical and experimental study of 3-D initial fracture and its significance to faulting

The experimental results of 3-D fracture under compression are introduced in brief and the theory of stress criterion of 3-D fracture is studied. Methods to imitate initial fractures are developed. It is pointed that there are important defects in the extreme value (EV) method ever proposed by Palaniswamy and Knauss. The major defect lies in that only two Euler angles (2EA) are considered, but another one is neglected. If the variation of all the three Euler angles (3EA) are considered, one can get better result which is consistent with the observation of faulting that extends on curved surfaces but not on planes. The method of evaluating maximal normal stress direction vector (NSDV) is proposed and further proved to be equivalent to the 3EA method. It is proved that the NSDV method can be further optimized to the method of composition of the first principal differential plane (CFPDP). The results from CFPDP method can fit the curved surfaces of initial growth observed in the experiments of 3-D fracture. The CFPDP method can also be used to interpret the 3-D fractures of the slipping section between the asperities in the buried fault plane that is modeled as ellipse crack. The results of 3-D fracture can be applied to interpreting the related problems of faulting including the mechanism of a lot of shatter rocks with different dimensions, the cause of earthquakes occurred at the edge of plate under low shear stress, and the mechanism of anisotropy caused by the extensive dilatancy anisotropy (EDA) cracks.

Stress aligned cracks in the upper crust of the Val d'Agri region as revealed by shear wave splitting

SUMMARY Shear wave splitting is measured at 19 seismic stations of a temporary network deployed in the Val d’Agri area to record low-magnitude seismic activity. The splitting results suggest the presence of an anisotropic layer between the surface and 15 km depth (i.e. above the hypocentres). The dominant fast polarization direction strikes NW–SE parallel to the Apennines orogen and is approximately parallel to the maximum horizontal stress in the region, as well as to major normal faults bordering the Val d’Agri basin. The size of the normalized delay times in the study region is about 0.01 s km −1 , suggesting 4.5 per cent shear wave velocity anisotropy (SWVA). On the south-western flank of the basin, where most of the seismicity occurs, we found larger values of normalized delay times, between 0.017 and 0.02 s km −1 . These high values suggest a 10 per cent of SWVA. These parameters agree with an interpretation of seismic anisotropy in terms of the Extensive-Dilatancy Anisotropy (EDA) model that considers the rock volume pervaded by fluid-saturated microcracks aligned by the active stress field. Anisotropic parameters are consistent with borehole image logs from deep exploration wells in the Val d’Agri oil field that detect pervasive fluid saturated microcracks striking NW–SE parallel to the maximum horizontal stress in the carbonatic reservoir. However, we cannot rule out the contribution of aligned macroscopic fractures because the main Quaternary normal faults are parallel to the maximum horizontal stress. The strong anisotropy and the seismicity concentration testify for active deformation along the SW flank of the basin.

Azimuthal anisotropy of the Pg-wave velocity in hypocentral volumes of NW Croatia

Based on a large set of arrival times of the Pg phase reported by local and regional stations, we estimate azimuthal anisotropy of the Pg-wave velocity in focal volumes of the upper crust in NW Croatia. The method is based on analyses of the azimuthal dependence of ratios of cumulative differences of arrival times and travel paths between foci of earthquake pairs, computed for rays propagating within narrow azimuthal windows. The results clearly indicate the presence of anisotropy of 3.3% with the direction of fast velocity (approximately NNE-SSW) coinciding with the direction of the maximum tectonic pressure as revealed by 23 available focal mechanisms and previous geological investigations. Although a large part of observed anisotropy can be explained assuming that focal volumes are pervaded by a system of vertical extensive-dilatancy anisotropy (EDA) cracks aligned under the influence of local tectonic stress field, there is indication that — to a smaller extent — some role was also played by alignment of structural features in the region.

Wave propagation through fluid-filled inclusions of various shapes: interpretation of extensive-dilatancy anisotropy

SUMMARY Previous examinations of the effects of extensive-dil atancy anisotropy (EDA) on seismic waves have been largely restricted to discussions of parallel fluid-filled cracks of small aspect ratio. It is recognized, however, that EDA may be caused by stress-aligned fluid-filled cracks, microcracks, and preferentially oriented porespace, in a variety of shapes, dimensions, and distributions, which may not be adequately modelled by the effects of uniform distributions of thin parallel cracks. The effects on seismic waves are examined for distributions of inclusions ranging from spherical pores and oblate spheroids (bubbles), to penny-shaped cracks. Unlike thin cracks, distributions of aligned oblate spheroids induce significant P-wave velocity anisotropy. In contrast, the parallel polarizations of the leading split shear waves within the shear wave window, which is one of the most distinctive features of shear waves in the crust, are preserved for all aspect ratios except spherical bubbles. The 3-D effects show minor variations that are most distinctive at small aspect ratios. Shear waves are very sensitive to changes in the geometry of such thin inclusions, and there is some observational evidence for temporal variations in splitting as the stress acting on the rockmass changes. If this sensitivity is confirmed it would suggest that the detailed geometry of a reservoir during production or enhanced oil recovery (EOR) might be monitored by repeated shearwave VSPs.

Seismic anisotropy and its relation with crust structure and stress field in the Reggio Emilia Region (Northern Italy)

SUMMARY Shear wave splitting is measured at 14 seismic stations in the Reggio Emilia region above local background seismicity and two sequences of seismic events. The good quality of the waveforms together with the favourable distribution of earthquake foci allows us to place strong constraints on the geometry and the depth of the anisotropic volume. It is about 60 km 2 wide and located between 6 and 11 km depth, inside Mesozoic age carbonate rocks. The splitting results suggest also the presence of a shallower anisotropic layer about 1 km thick and few km wide in the Pliocene‐Quaternary alluvium above the Mesozoic layer. The fast polarization directions (N30 ◦ E) are approximately parallel to the maximum horizontal stress (σ 1 is SSW‐NNE) in the region and also parallel to the strike of the main structural features in the Reggio Emilia area. The size of the delay times suggests about 4.5 per cent shear wave velocity anisotropy. These parameters agree with an interpretation of seismic anisotropy in terms of the extensivedilatancy anisotropy model which considers the rock volume to be pervaded by fluid-saturated microcracks aligned by the active stress field. We cannot completely rule out the contribution of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel shear wave polarizations we found are diagnostic of transverse isotropy with a horizontal axis of symmetry. This symmetry is commonly explained by parallel stress-aligned microcracks.

Research on anisotropic parameters by synthetic seismogram

Based on the extensive-dilatancy anisotropy theory, the method of synthetic seismogram is used to estimate the anisotropic parameters. The advantages of the method lie in that it avoids the singularity resolution and saves calculation time of computer by using the eigenvalue and eigenvector analytical expressions of Christoffel equation, at the same time, the result is tested by coherence function. The test result reveals there exists a fine linear relation between original records and synthetic records, indicating the anisotropic parameters estimated by synthetic seismogram can reflect and describe the anisotropic characteristics of the given region medium.

Ductile Precambrian fabric control of seismic anisotropy in the Açu dam area, northeastern Brazil

In the João Câmara area, NE Brazil, seismic anisotropy has been interpreted as the result of either Precambrian ductile fabric or stress‐aligned, fluid‐filled grain boundary caused by high pore fluid pressures. The first interpretation is based on the fact that the direction of the fastest S wave splitting agrees with the north‐south trending Precambrian fabric. The second interpretation, related to the extensive dilatancy anisotropy (EDA), is based on the stress field orientation and on the assumption that the pore fluid pressure in the region is high enough to modify microcrack distribution and cause S wave polarization to become parallel to the north‐south oriented minimum horizontal stress SHmin. Here we address this question by investigating this ambiguity. We have analyzed the seismicity in the Açu dam area, ∼150 km west of João Câmara, over a 3‐year period. In the Açu area, the northeast‐southwest trending ductile Precambrian fabric is not parallel or orthogonal to the east‐west trending, maximum horizontal stress SHmax. We combined the interpretation of fastest S wave polarizations, their time delays, and fabric orientation measured in the field with side‐looking airborne radar imagery taken before the lake impoundment. Thus we discriminate between (1) stress‐aligned fluid‐filled cracks, (2) high pore pressure fluids modifying the microcrack distribution and therefore causing 90° flips, and (3) fabric or “paleostrain” as causes for the observed S wave anisotropy. Our main result indicates that the direction of the fastest S wave polarization in the Açu dam area agrees with the northeast‐southwest trending fabric (∼N40°E). EDA or 90°Flips control of seismic anisotropy, if occurring in Açu, have minor effects.

Azimuthal anisotropy of the P-wave velocity in the hypocentral volume of the Krn Mt. (Slovenia) earthquake sequence

Azimuthal anisotropy of P-wave velocity in the hypocentral volume of the Krn Mt. (Slovenia) earthquake sequence is measured using the differences of travel times and of travel paths of the Pg-phase towards the recording stations of the local and regional networks. The observed velocity varies between 6.0 km/s in the ENE–WSW direction and 6.4 km/s for waves propagating NNW–SSE. These directions closely match those of the mean regional principal stress components obtained from individual fault-plane solutions for events from the Krn Mt. sequence. A large part of observed anisotropy may be explained if hypocentral volume is assumed to be pervaded by a system of vertical/subvertical extensive-dilatancy anisotropy (EDA) cracks aligned under the influence of local tectonic stress field.

Local shear wave observations in João Câmara, northeast Brazil

We present shear wave splitting data from a 10 km aperture, eight‐station digital three‐component seismograph network, which operated from December 1992 to August 1994 near João Câmara (5°33′S, 35°51′W), to record shear waves from continuing microearthquake activity persisting 6 years after a main shock in November 1986. Previous aftershock recording had shown remarkably impulsive simple seismograms and hypocentral errors of typically 200 m in a structure of near‐uniform wave speed, but only one station was three component. The pattern of polarization directions shown is consistent with the trend of Precambrian foliation observed in the field. It is therefore concluded that these shear wave observations are controlled by the seismic anisotropy associated with the Precambrian foliation and hence bear the signature of the “paleo‐stress field,” i.e., the Precambrian deformation regime. This conclusion has important implications for the interpretation of shear wave splitting observations made in crustal crystalline rock, because other authors have asserted that shear wave splitting normally results from the effect of stress‐aligned fluid‐filled cracks (extensive dilatancy anisotropy or EDA). EDA is, by contrast, a signature of the present‐day stress field. In this region, there is firm evidence that the present‐day maximum compressive stress is E‐W. This evidence comes from earthquake focal mechanisms and borehole breakout data and is summarized in the World Stress Map. We therefore conclude that our present data are in maximum conflict with an interpretation in terms of EDA. Our interpretation is supported by the observation that delay times are consistent with those expected from modeling of elastic waves in the crust.

Biot/Squirt Model in Viscoelastic Porous Media

Viscoelastic wave equations are derived for both theBiot flow and squirt-flow involved. The relationship between phase velocityor attenuation of viscoelastic wave and macro parameters of the reservoiris clarified in a porous extensive dilatancy anisotropy medium.Numerical models clearly demonstrate that the viscoelasticproperty of rocks, not the squirt-flow, causes the dispersion andattenuation in the low-frequency range. The attenuation of the quasiSV-wave, SH-wave and the quasi P-wave depend strongly on the directionsof permeabilities. Furthermore, the trends of attenuation of the quasiP-wave fast and the quasi SV-wave are inversed in both the high-frequencyand low-frequency ranges. Our numerical result is in agreementwith the experimental result that was carried out for theBiot/Squirt model by Jorge [Geophys. 65(2000) 202]

Poroelastic wave equation including the Biot/squirt mechanism and the solid/fluid coupling anisotropy

There is relative motion and inertial coupling between solids and fluids during seismic and acoustic propagation in rocks with fluids. This inertial coupling is anisotropic because of the microvelocity anisotropy of the fluid relative to solid in an anisotropic medium. The Biot mechanism and the squirt-flow mechanism are the two most important mechanisms of solid/fluid interaction in rocks. We extend the Biot/squirt (BISQ) theory to include the solid/fluid coupling anisotropy and develop a general poroelastic wave equation including both mechanisms simultaneously. The new model estimates velocity/frequency dispersion and attenuation of waves propagating in the 2D PTL ( periodic thin layers )+ EDA (extensive dilatancy anisotropy) medium with fluids. The attenuation and dispersion of the two quasi-P-waves and the quasi-SV-wave, which are related to the solid/fluid coupling density and the permeability tensors, are anisotropic. The anisotropy are simultaneously affected by the anisotropies of the solid skeleton, the permeability, and the solid/fluid coupling effect of the formation. Numerical modeling suggests that variations of attenuation of both the fast quasi-P-wave and the quasi-SV-wave strongly depend on the permeability anisotropy. In the low-frequency range, the maximum attenuation is in the direction of the maximum permeability for the fast quasi-P-wave and in the direction of the minimum permeability for the quasi-SV-wave. The attenuation behaviors of the two waves in the high-frequency range, however, are opposite to those in the low-frequency range. This paper also presents numerically how the attenuation and velocity dispersion of both the fast quasi-P-wave and the quasi-SV-wave are influenced by the anisotropic solid/fluid coupling density. The model results demonstrate when the wave propagation is perpendicular to the direction of maximum solid/fluid coupling density, the wave motion exhibits maximum attenuation ( Q −1) and maximum velocity dispersion for the fast quasi-P-wave, and minimum attenuation ( Q −1) and minimum velocity dispersion for the quasi-SV-wave. These phenomena may be applied in extracting anisotropic permeability and further determining the preferential directions of fluid flow in a reservoir containing fluid-filled cracks from attenuation and dispersion data derived from sonic logs and crosswell seismics.