Horizontal Particle Motion Research Articles

Oil Particle-Induced Erosion Wear on the Deflector Jet Servo Valve Prestage

Severe oil particle-induced erosion to the prestage component progressively degrades the overall performance of the deflector jet servo valve (DJSV), even leading to valve failure. Herein, we present an approach for evaluating degradation in performance and predicting the erosion lifespan of the DJSV on different levels of oil pollution. Specifically, a mathematical model of the whole valve was built based on a previously established working principle and physical mechanism. In addition, considering the horizontal and rotational particle motions, combined with impact of particle size distributions under different oil contamination degrees, an erosion model was constructed. Then, after simulating and analyzing the pressure characteristics before and after the erosion of prestage, the performance degradation of the whole valve was examined, thereby predicting the erosion life of the valve. Investigations revealed that the maximum erosion rate occurred at the shunt wedge of the receiving holes, which increased with the contamination degree and accelerated after level 7. After erosion, however, the control pressure difference decreased significantly, and erosion life followed exponential distribution corresponding to the distribution of particles under different pollution levels. The aforementioned investigation can thus help diagnose faults and optimize the design of the servo valves in service.

A comparative evaluation of snowflake particle shape estimation techniques used by the Precipitation Imaging Package (PIP), Multi-Angle Snowflake Camera (MASC), and Two-Dimensional Video Disdrometer (2DVD)

Abstract. Measurements of snowflake particle shape are important for studying snow microphysics. While a number of instruments exist that are capable of measuring particle shape, this study focuses on the measurement techniques of three digital video disdrometers: the Precipitation Imaging Package (PIP), the Multi-Angle Snowflake Camera (MASC), and the Two-Dimensional Video Disdrometer (2DVD). To gain a better understanding of the relative strengths and weaknesses of these instruments and to provide a foundation upon which comparisons can be made between studies using data from different instruments, we perform a comparative analysis of the shape measurement algorithms employed by each of the three instruments by applying the algorithms to snowflake images captured by PIP during the ICE-POP 2018 field campaign. Our analysis primarily focuses on the measurement of the aspect ratio of either the particle itself, in the case of PIP and MASC, or of the particle bounding box, in the case of PIP and 2DVD. Both PIP and MASC use shape-fitting algorithms to measure aspect ratio. While our analysis of the MASC aspect ratio suggests that the measurements are reliable, our findings indicate that both the ellipse and rectangle aspect ratios produced by PIP underperformed considerably due to the shortcomings of the PIP shape-fitting techniques. We also demonstrate that reliable measurements of aspect ratio can be retrieved from PIP by reprocessing the raw PIP images using either the MASC ellipse-fitting algorithm or a tensor-based ellipse-fitting algorithm. Because of differences in instrument design, 2DVD produces measurements of particle horizontal and vertical extent rather than length and width. Furthermore, the 2DVD measurements of particle horizontal extent can be contaminated by horizontal particle motion. Our findings indicate that, although the correction technique used to remove the horizontal motion contamination performs remarkably well with snowflakes despite being designed for use with raindrops, the 2DVD measurements of particle horizontal extent are less reliable than those measured by PIP.

Velocity Contrast across the Zhaotong-Ludian Fault in Southwest China from the Analysis of Fault Zone Head Waves and Teleseismic P-Wave Arrivals

Abstract We image the Zhaotong-Ludian fault (ZLF) in the southeastern margin of the Tibetan plateau (SE Tibetan plateau) using waveforms from local and teleseismic earthquakes recorded by 14 seismic stations. We identify two types of fault zone head wave (FZHW) from two clusters of earthquakes by applying an automatic picking algorithm and a horizontal particle motion analysis. The first type of FZHWs shows a linear time–distance moveout and is only observed at stations on the southeast side of the fault in the northeastern section of the ZLF. The moveout slope suggests an average cross-fault velocity contrast of ∼2.5%. The second type of FZHWs exhibits a constant moveout and is recorded by stations on both sides of the ZLF in the southwestern section from a cluster of earthquakes located in a low-velocity zone. The difference in cross-fault velocity contrast between the northeast and southwest segments of the ZLF is also confirmed by teleseismic P-wave travel-time data. We attribute the prominent velocity contrast in the northeast section to a lithological difference between the South China block in the southeast and the Daliangshan subblock on the northwest side of the fault. The striking difference between the northeast and southwest sections also implies that earthquakes nucleating in one segment would hardly rupture through the entire fault, which can significantly affect our estimates of the maximum magnitude of future earthquakes occurring on the fault.

Tsunami-generated magnetic fields have primary and secondary arrivals like seismic waves

A seafloor geomagnetic observatory in the northwest Pacific has provided very long vector geomagnetic time-series. It was found that the time-series include significant magnetic signals generated by a few giant tsunami events including the 2011 Tohoku Tsunami. Here we report that the tsunami-generated magnetic fields consist of the weak but first arriving field, and the strong but second arriving field—similar to the P- and S-waves in seismology. The latter field is a result of coupling between horizontal particle motions of the conductive seawater and the vertical component of the background geomagnetic main field, which have been studied well so far. On the other hand, the former field stems from coupling between vertical particle motions and the horizontal component of the geomagnetic main field parallel to tsunami propagation direction. The former field has been paid less attention because horizontal particle motions are dominant in the Earth’s oceans. It, however, was shown that not only the latter but also the former field is significant especially around the magnetic equator where the vertical component of the background magnetic field vanishes. This implies that global tsunami early warning using tsunami-generated magnetic fields is possible even in the absence of the background vertical geomagnetic component.

Hydraulic Injection‐Induced Velocity Changes Revealed by Surface Wave Coda and Polarization Data at a Shale Play Site in Southwest China

AbstractWe investigated temporal variations of seismic wave velocity associated with hydraulic fracturing using Green's functions computed from ambient noise data. In October and November of 2014, we set up a broadband array at a shale play site inside the Sichuan basin where a pilot horizontal drilling and hydraulic injections were conducted. We first computed cross‐correlation functions using continuous data recorded by 21 three‐component broadband sensors deployed around the treatment well. We then employed a running window correlation‐based coda wave interferometry technique to measure apparent velocity changes from the daily Green's functions of all the station pairs in the frequency range of 1 to 3 Hz. We found significant velocity changes right after the hydraulic fracturing, which exhibited a clear direction‐dependent pattern. S wave velocity along raypaths parallel to the well trajectory showed a clear increase while those perpendicular exhibited a small decrease. The anisotropic changes in seismic velocity observed here were also confirmed from surface wave horizontal particle motion data. By comparing our observations with normal stress changes calculated with a half‐space elastic model, we speculate that stress changes induced by the hydraulic fracturing were likely to be responsible for the observed anisotropic changes in seismic velocity. Our results suggest that time‐lapse seismic imaging with ambient noise data provides a promising probe for monitoring geomechanical changes related to exploitation of unconventional oil and gas resources.

Parametric Resonance and Energy Transfer in Dusty Plasma

Model allowing analytical and numerical studies of a dusty plasma system is used to describe the dynamics of a monolayer of dusty particles. The mechanism of energy transfer between horizontal and vertical particle motion based on parametric resonance is described by the extended Mathieu equation. The resonance regions and growth rates of dust particles energy are obtained. The conditions for the occurrence of resonance and the initial stage of energy transfer are described more precisely based on the analysis of the obtained data. It is shown that a wide spectrum of dust particle oscillations participates in the system heating. The harmonics contributing the most to this process are determined.

Tunable interactions between particles in conically rotating electric fields.

Tunable interactions between colloidal particles in external conically rotating electric fields are calculated, while the (vertical) axis of the field rotation is normal to the (horizontal) particle motion plane. The comparison of different approaches, including the methods of noninteracting, self-consistent dipoles, and the boundary element method, indicates that the last method is the most suitable for tunable interaction analysis. Thorough analysis, performed for interactions in pairs and clusters of colloidal particles, indicate that two- and three-body interactions make the main contributions in the interaction energy, while the effect of high-order terms is negligible. The tunable interactions are determined by the dielectric properties of the particles and solvent and can be changed in a wide range, providing a rich variety for the experimental "design" of different interactions, including repulsion, attraction, combination of short-range repulsion with long-range attraction, barrier-type interactions with short-range attraction and long-range repulsion, and double-scale repulsive (core-shoulder) interactions. These conclusions can be generalized for magnetically induced tunable interactions. The results indicate that tunable interactions can be widely applied in self-assembly and particle-resolved studies of generic phenomena in fluids and crystals, and, therefore, are of broad interest in the fields of chemical physics, physical chemistry, materials science, and soft matter.

Seismicity and Subterranean Sounds in the Northwest Deccan Volcanic Province of India

Abstract The northwestern Deccan volcanic province in India is one of the most seismically active intraplate regions of the world. In addition, the region is associated with episodic swarm activity and reports of sounds, whose linkage hitherto remains elusive. During the month of January 2016, a swarm activity occurred in the Kachchh and Saurashtra regions and continued for about two months. Many of the events were accompanied by audible sounds, like blasting, that caused severe panic among local residents, prompting us to investigate the causative mechanism. The events were recorded by our seismic stations and an additional five stations that we were able to deploy at the onset of the swarm. The activity produced sounds with good energy in the audible frequency range of humans. Spectrogram analysis of the events with associated sounds revealed frequencies ≥20 Hz, in contrast to the lower frequencies for those that did not generate the sounds. In addition to the higher frequencies, we observed horizontal particle motion that was dominated by retrograde elliptical motion consistent with Rayleigh waves. These observations were not recorded from any of the events that did not generate sounds. Audible sounds generated by earthquakes are consistently reported from shallow earthquakes that generate high‐frequency surface waves.

Intrinsic Directional Information of Ground Roll Waves.

Ground roll waves traveling across the seabed provide extra information, their direction of rotation, compared with plane waves in fluids or solids. Idealized Rayleigh waves are "retrograde" in that their horizontal particle motion opposes the direction of travel of the wave when the interface is raised. A single point measurement near the seabed can determine this rotation. In water, there are associated evanescent pressure waves that are largely confined to the bottom, likely to affect fish and other creatures near the seabed during pile driving. The directional information may prove key to the lifestyle of such creatures.

Upper mantle seismic anisotropy beneath the West Antarctic Rift System and surrounding region from shear wave splitting analysis

SUMMARY We constrain azimuthal anisotropy in the West Antarctic upper mantle using shear wave splitting parameters obtained from teleseismic SKS, SKKS and PKS phases recorded at 37 broad-band seismometres deployed by the POLENET/ANET project. We use an eigenvalue technique to linearize the rotated and shifted shear wave horizontal particle motions and determine the fast direction and delay time for each arrival. High-quality measurements are stacked to determine the best fitting splitting parameters for each station. Overall, fast anisotropic directions are oriented at large angles to the direction of Antarctic absolute plate motion in both hotspot and no-net-rotation frameworks, showing that the anisotropy does not result from shear due to plate motion over the mantle. Further, the West Antarctic directions are substantially different from those of East Antarctica, indicating that anisotropy across the continent reflects multiple mantle regimes. We suggest that the observed anisotropy along the central Transantarctic Mountains (TAM) and adjacent West Antarctic Rift System (WARS), one of the largest zones of extended continental crust on Earth, results from asthenospheric mantle strain associated with the final pulse of western WARS extension in the late Miocene. Strong and consistent anisotropy throughout the WARS indicate fast axes subparallel to the inferred extension direction, a result unlike reports from the East African rift system and rifts within the Basin and Range, which show much greater variation. We contend that ductile shearing rather than magmatic intrusion may have been the controlling mechanism for accumulation and retention of such coherent, widespread anisotropic fabric. Splitting beneath the Marie Byrd Land Dome (MBL) is weaker than that observed elsewhere within the WARS, but shows a consistent fast direction, possibly representative of anisotropy that has been ‘frozenin’ to remnant thicker lithosphere. Fast directions observed inland from the Amundsen Sea appear to be radial to the dome and may indicate radial horizontal mantle flow associated with an MBL plume head and low upper mantle velocities in this region, or alternatively to lithospheric features associated with the complex Cenozoic tectonics at the far-eastern end of the WARS.

New Step for Broadband Seismic Observation on the Seafloor: BBOBS-NX

Essential information for clarifying geodynamic processes is obtained by imaging the Earth's interior through geophysical observations. Huge oceanic areas are important locations for conducting such observations. The broadband ocean-bottom seismometer (BBOBS) that we developed has been used since 1999 in several array observations, which gave us new information. But, the BBOBS's noise model in periods longer than 10 s indicates the high noise level in horizontal components above the new high noise model (NHNM), although the vertical one is between the NHNM and the new low noise model (NLNM). It makes it difficult to apply modern analysis methods using horizontal component waveforms even from the data of the one-year-long observation at a single station. Recently, we have developed a geophysical instrument to investigate the oceanic mantle, namely the next-generation broadband ocean-bottom seismometer (BBOBS-NX), operated by a remotely operated underwater vehicle (ROV). The BBOBS-NX provides data of much higher quality than the conventional BBOBS, because it shows a comparable noise model in horizontal components with that of land seismic stations in periods longer than 10 s. Comparison between bottom currents and horizontal particle motions of the BBOBS-NX and conventional BBOBSs clearly indicates the effective reduction of the noise due to the bottom current in this period range. A preliminary receiver function analysis also shows the advantage of the BBOBS-NX to the conventional BBOBS, even if the observation period of the former was shorter than a quarter of that of the latter.

Anisotropy, range dependence and seismo-acoustic propagation in shallow water.

The shallow water environment may be highly variable, with both range dependence and anisotropy almost ubiquitous in the seafloor bottom/sub-bottom regions. Some common causes of range dependence include marine-sediment composition, non-planar boundaries, rough surfaces, strong density, or velocity contrasts, and variation in water-column depth and/or sediment-cover thickness. Common causes of elastic anisotropy are compositional layering or vertically aligned cracks. There is an apparent trade-off between anisotropy and range dependence, and difficult to separate the two effects in a propagating signal. If the symmetry axis of a compositionally layered sediment is not exactly normal to the seafloor, the seismo-acoustic wave field has particle-motion polarizations in all three coordinate directions. Even in a one-dimensional medium, an explosion source excites sediment particle motion with all three polarizations. Assuming sediment isotropy when it is not justified can cause errors in layer-thickness computations, elastic property-gradient determinations, acoustic attenuation, and predictions of long-range sound propagation. Employing a modal representation of the seismo-acoustic wavefield, we illustrate the effects of anisotropy and range dependence on the modes, such as mode-identity switching due to angular dispersion and coupling of longitudinal, vertical, and horizontal particle motion polarization. [Work supported by ONR.]

Ultra-long period seismic signals and cyclic deflation coincident with eruptions at Santiaguito volcano, Guatemala

Broadband seismic observations made near the active vent at Santiaguito provide constraints on changing pressures inside the edifice. During January 2009 we recorded episodic ultra-long period signals (ULPs, > 30 s) that were coincident with eruptive activity centered on the active El Caliente vent. These signals lasted 2–5 min and corresponded to depressurization of a volume beneath the dome. Coincident shorter-period seismic signals (> 0.5 Hz) appeared to be associated with shallow level pyroclastic emissions and dome surface motions. These shorter period signals varied significantly from event to event, unlike the ULP waveforms, which were self-similar. As opposed to the short period signals, which were recorded across the network, the ULPs were only recorded on the horizontal channels of 3 stations that were within 1.1 km of the vent. This and other factors suggest that they were a result of long period rotational motion rather than horizontal translation. ULP source locations derived from horizontal particle motions point to the northwestern part of El Caliente 150 m from the vent axis, the general locus of activity from 1967 until flank collapses in 1989/1990. We modeled the ULP signal as a Mogi source and solved for three dimensional location and equivalent source strength. Through analysis of 78 events, we identify a mean Mogi source that is 200 m west of and 250 m beneath the centre of the vent with the peak volume changes accompanying eruptions ranging from 125 to 1825 m 3. These inferred volume losses coincide with the onset of gas venting and/or explosive eruptions at the surface of Caliente. Based upon these observations we develop a model to describe the pre-eruptive, eruptive and inter-eruptive physical processes occurring at the surface and inside Santiaguito.

Precise location of the fault plane and the onset of the main rupture of the 2005 West Off Fukuoka Prefecture earthquake

AbstractNear-source strong-motion records of the 2005 West Off Fukuoka Prefecture earthquake have two remarkable features. One is the presence of a relatively long period pulse with large amplitude on the fault-normal component of the velocity and displacement records, which is the result of the forward rupture directivity. The other is that the records show several seconds of small but increasing amplitude arrival (“initial rupture phase”) followed by the onset of the main energy release (“main rupture phase”). We first determined the precise geometry of the fault plane of this earthquake by examining the horizontal particle motion of the mainS-wave portion on the records, as follows: the strike is N304°E, and the dip angle is 87°. The dip direction of the fault plane is northeast, and the surface intersection of the fault plane passes by the eastern coast of Genkai Island. We also obtained the relative location of the onset of the main rupture with respect to the hypocenter, and the mean rupture velocity between them. The distance between them is 5.1 km, the onset of the main rupture is located southeast above the hypocenter. The mean rupture velocity along the straight path is 1.4 km/s. It is found that the main rupture began 3.6 sec later from the origin time, at the central point between the hypocenter and Genkai Island. Our results suggest that Genkai Island directly suffered the strong effects of the forward rupture directivity during the earthquake.

New evidence for a low-velocity layer on the subducting Philippine Sea plate in southwest Japan

A distinct precursory phase is sometimes found prior to S arrival, when deep-focus and intermediate-depth earthquakes are observed at seismic stations in the Chugoku district, southwest Japan. The phase from intermediate-depth earthquakes has apparent velocity close to that of S wave in the Philippine Sea (PHS) plate, but at epicentral distance of 100–250km it arrives earlier by about 2.5s than the S wave propagated as a head wave along the upper boundary of the HPS plate. Thus we interpreted the phase as Sp wave into which the S wave refracted upward after traveling in the high-velocity PHS plate was converted at a boundary while propagating on a ray path between the plate and seismic station. The precursory phase to S wave from deep-focus earthquakes was also identified as Sp wave that was caused by conversion of the direct S wave into P wave, because the S–Sp times were independent of epicentral distance and focal depth and the direction of horizontal particle motion of the phase almost agreed with the azimuth direction of the epicenter. The Sp conversion interface was located at the upper boundary of the PHS plate. Further, it was demonstrated that a thin low-velocity layer should overly just above the PHS plate in order to explain both the observed difference in polarity of the S and Sp waves and the amplitude ratio of the Sp wave to the direct S wave.

ESTIMATION METHOD AND CHARACTERISTICS OF WAVE FORCE ON A SMALL-DIAMETER CYLINDER IN DIRECTIONAL WAVES

Wave forces on a vertical cylinder in directional waves have been investigated. Morison equation has been utilized to express the wave force. In directional waves, the effect of the lift force may dominates over the drag and inertia forces when one component of the horizontal water particle motion dominates over the other one. Under such conditions, the Morisone quationc onfirmedt o be less accuratei n estimatingt he wave force. The effect of the lift force, however, becomes less important in estimating the wave force acting on a whole cylinder. This paper shows that the wave force on the cylinder can be evaluated with the drag and inertia coefficients obtained in this study. Moreover, it is confirmed that the wave forces in directional waves, for a given magnitude of the velocity, is larger than that of the uni-directional waves. Thus, effect of the directional spreading should be properly consideredi n the estimatingt he wavef orce.

From the Tropics to the Poles in Forty Days

Field experiments show that the poleward velocity of high-altitude weather balloons may, on rare occasions, be much higher than the observed poleward winds, while their eastward velocity is much slower than the observed eastward winds. Considering a simple physical model of horizontal particle's motion in the atmosphere, which includes a realistic model of the pressure field, it is shown that the existence of a nearly flat parabolic resonance in the model gives rise to such flights of balloons on the observed timescales even though the associated atmospheric pressure field does not support large poleward velocities.

Attenuation anisotropy beneath the subduction zones in Japan

We analyze high quality digital waveform data at stations in Japan from the POSEIDON and IRIS broad‐band seismograph networks. The analysis of multiple ScS waves of the deep event beneath Sakhalin on 12 May 1990 shows that the orientation of the principal axis of horizontal ScSn particle motion is rotated clockwise from NWN‐SES to N‐S as the number of reflections increases. This suggests that the attenuation of the E‐W component is higher than that of the N‐S component of ScS waves. The spectral ratio ScS2/ScS1 differs between the two horizontal components. We estimate the quality factor QScS of the whole mantle for each horizontal component using the decay rate of spectral ratio of band‐passed ScS1 and ScS2 phases in the frequency range from 0.01 to 0.03 Hz. We obtain a higher average QScS for the N‐S component (141±8) than that for the E‐W component (91±5). That is, the E‐W component attenuates more than the N‐S component. For upcoming ScS waves the N‐S direction is nearly parallel to the strike of the subducting slab beneath the mantle wedge in the Japanese islands, and the E‐W is perpendicular to the strike. The up‐welling and downgoing flow in the mantle wedge due to drag caused by the subduction is likely to cause the difference in attenuation between the two directions, in other words, azimuthal attenuation anisotropy.

On the possibility of anisotropy in the D″ layer as inferred from the polarization of diffracted S waves

S waves diffracted along the core-mantle boundary (Sd) show in a few cases anomalously large SV components. To test if azimuthal anisotropy in D″ can be a cause for these anomalies, a method based on the Langer approximation and on perturbation theory is developed to model Sd waves in the presence of anisotropy in D″. By comparing the synthetics in azimuthally anisotropic models with those in isotropic or transversely isotropic models, we search for the characteristic features in the data which would discriminate between the different models. The SVd/SHd amplitude ratio turns out not to be a discriminating feature. A ratio of 30% at an epicentral distance of 115° can, for example, be explained by 1% of azimuthal anisotropy at the core-mantle boundary (CMB), decreasing linearly to zero 150 km above, by a mean of 0.5% azimuthal anisotropy through D″, by a realistic negative velocity gradient in an isotropic model of D″, or by transverse isotropy in D″. The phase difference between the SH and SV components and their frequency content are not discriminating either. Thus, with a single recording, azimuthal anisotropy cannot be discriminated from isotropy or transverse isotropy in D″. The only effect characteristic of azimuthal anisotropy is that the horizontal particle motion of Sd hardly depends on the focal mechanism, as opposed to the linear dependence which exists in isotropic or transversely isotropic models. Also, it becomes invariant with epicentral distance beyond 115°. To discriminate between different kinds of models, the particle motion of Sd waves from several events with known focal mechanisms therefore needs to be analysed. To test the hypothesis of Vinnik et al. (Geophys. Res. Lett., 16: 519–522, 1989) concerning the origin of the anomalous SVd they observed, we analyse three of their data. These are Fiji events recorded at the North American Geoscope station WFM and at the North Atlantic World Wide Standardized Seismograph Network station BEC, all showing large SVd waves. The Sd waveforms of the two events recorded at the smallest epicentral distance cannot be explained simultaneously by a laterally homogeneous isotropic or a transversely isotropic model for D″. An azimuthally anisotropic model with 1% azimuthal anisotropy at the CMB explains them better. However, neither an isotropic nor an anisotropic model could be found which explains the waveforms observed at the furthest station. The hypothesis of azimuthal anisotropy in D″ under the Pacific Ocean to explain these clearly anomalous waveforms is not confirmed. A much larger data set should, however, be analysed before drawing a conclusion for that region. The example of the data of Vinnik et al. shows that the clear predictions which are made here as to which waveforms are expected if D″ is anisotropic allow discriminating studies of D″.

Seismic anisotropy in the shallow crust of the Loma Prieta segment of the San Andreas Fault System

We study seismic anisotropy beneath the Santa Cruz Mountains using digital recordings of S waves from 60 aftershocks of the 1989 Loma Prieta earthquake (Mω = 7.0) recorded at 18 local stations. Three‐component particle motion covariance matrix decomposition combined with a cross‐correlation method is used to estimate the polarization direction of the fast shear wave arrival and the delay time between the split shear waves. Results from this procedure are compared with those obtained through visual inspection of horizontal particle motions. Inclusion of the vertical component in the particle motion analysis allows the recognition and elimination of S to P converted phases from our study. Uncertainties in the polarization of the fast shear wave and in the delay time between arrival of the fast and slow waves are estimated to be about 20° and 10 ms, respectively. About half of the S wave records examined show clear evidence of shear wave splitting. Two persistent polarization directions of fast shear wave arrivals are observed regardless of the focal mechanism and hypocenter location of the individual events; 52% of the polarizations are parallel to the strike of the San Andreas fault (NW‐SE), and 24% are parallel to the probable direction of local maximum horizontal compressive stress (NE‐SW). The fault‐parallel fast polarization direction dominates at 10 stations and may result from mineral or fracture alignment caused by shearing along the plate boundary. The fast polarization direction perpendicular to the local axes of minimum horizontal compressive stress dominates at one station farthest from the San Andreas fault and is consistent with the hypothesis of extensive dilatancy anisotropy (EDA), where parallel alignment of fluid‐filled fractures produces the anisotropy. The predominance of fault‐parallel fast shear wave polarizations indicates that the use of shear wave splitting results to deduce the orientation of local axes of principal stress may not always yield accurate results. The splitting time between fast and slow shear waves varies from 10 to 125 ms and shows no systematic relationship with either hypocentral distance or focal depth. This suggests that the seismic anisotropy is no deeper than 2 km, the depth of the shallowest earthquakes analyzed. For an anisotropic zone 2 km thick, the average velocity anisotropy is about 5% in our study region.