Striking Planes Research Articles

New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore

On 23 March 2018, an event of magnitude ML 3.9 occurred about 10 km from the town of Ostuni, in the Adriatic offshore. It was the most energetic earthquake in South–Central Apulia ever recorded instrumentally. On 13 February 2019, in the same area, a second ML 3.3 event was recorded. The analysis of the 2018 event shows that the ambiguity of the solution of the fault plane reported by INGV (Istituto Nazionale di Geofisica e Vulcanologia) on the Italian National Earthquake Centre website can be solved considering existing seismic profiles, exploration well logs and the Quaternary activity of faults in the epicentral area. A seismogenic source was identified in the rupture of a small portion of a 40 km length structure with strike NW-SE, dipping at a high angle toward the south. In this work, we have relocated the recent earthquakes by using the seismic stations managed by the University of Bari (UniBa), one of which is quite close to the event’s epicenter (about 20 km), together with data coming from the RSN (Rete Sismica Nazionale). Furthermore, we have determined the focal mechanism of some events, with implications on stress field of the area. Our results show right-lateral transtensional kinematics of the seismogenic faults along approximately E-W striking planes, with a tension, T, with a trend of about 60° (NE-SW direction) and a plunge of 20°. Finally, we have tried to correlate the location of the four best constrained earthquakes with their seismogenic structures.

Stability Analysis on the Steeply-Inclined Small-Angle Layered Surrounding Rock Masses at the Upstream High Sidewall of a Large-Span Cavern

The underground powerhouse of China’s Wudongde hydropower station is one of the world’s underground powerhouses with the largest span and height. Because the underground powerhouse is all located in steeply-inclined layered rock masses, and the bedding plane strike of the rock mass has a small angle with the axis of the main caverns, there is a serious risk of high sidewall instability. According to the basic principles of the structurally-controlled concept, the mechanical properties of bedding planes and other discontinuities are largely attributed to this problem. A thorough investigation was conducted into the Wudongde underground powerhouse throughout the excavation process to clarify the possible mechanisms behind these failures and to realize the dynamic adjustment of construction. First, the geological conditions and mechanical properties of the rock masses in the study area were introduced in detail. The stability of the right bank underground powerhouse’s surrounding layered rock mass was then investigated using a discontinuum-based numerical method. Special attention was paid to the excavation-induced failure mechanisms of the steeply-inclined small-angle layered rock masses at the upstream high sidewall. The results show that the failure of steeply-inclined small-angle layered surrounding rock masses presents significant three-stage characteristics, i.e., 1) rock plate forms due to the unloading opening of the bedding plane; 2) rock plate cracks due to bending; and 3) rock plate buckles or slides due to crack penetration. Based on the understanding of the deformation and failure mechanisms of steeply-inclined small-angle layered rock masses, the excavation sequence needs to be optimized according to local geological conditions. In practice, it is vital to insist on the principles of excavation by thin layers and finish all the supports before the next-step excavation.

Transtensional coseismic fault slip of the 2021 Mw 6.7 Turt Earthquake and heterogeneous tectonic stress surrounding the Hovsgol Basin, Northwest Mongolia

Knowledge of the regional crustal deformation and stress field is fundamental to understanding and constrain the ongoing evolution of Hovsgol basin, northwest Mongolia. The 2021 Mw 6.7 Turt earthquake provides an unprecedented opportunity to probe the local tectonic stress field and upper crust deformation. We investigate the coseismic surface displacements and invert fault slip models using Interferometric Synthetic Aperture Radar observations and teleseismic data. The mainshock occurred as a result of normal faulting with a right-lateral strike-slip component on an NW striking plane, which is consistent with the transtensive local stress field inverted from regional focal mechanisms. It is surrounded by transpressive and strike-slip stress fields proposed by previous studies indicating regional stress heterogeneity probably controlled by the distinct structure geometries in and around Hovsgol basin. Our results also suggest that the current deformation of the Hovsgol basin is dominated by half-graben forming. Seven historical strong earthquakes (M 7) may advance the 2021 Turt earthquake by ~12% recurrence interval, based on the assumption of equal stress release from recurrence earthquakes, meanwhile, the 2021 Turt earthquake may increase the potential seismic hazard on the neighbor Mondy and South Hovsgol faults, which deserves more attention.

Effect of confining pressures on transverse isotropy of Maha Sarakham salt

Uniaxial and triaxial compression tests have been performed on prismatic specimens of rock salt under confining pressures (σ3) up to 40 MPa. The specimens contain different bedding plane orientations with respect to the major axis. Results indicate that transverse isotropic effects occur under low confinement where the salt strength is lowest when the normal to bedding planes makes an angle (β) of 60° with the major principal axis. The lowest intrinsic elastic modulus is obtained at β = 0°, and the highest is at β = 90°. The apparent elastic parameters measured for 0° < β < 90° agree well with those predicted by Amadei's solutions. The confining pressures rapidly increase the elastic and shear moduli normal to bedding plane strike, and eventually they reach those parallel to the bedding planes under σ3 about 30 MPa. Loading the salt under high confinement induces some plastic deformation, as evidenced by the evolution of Ramberg and Osgood's parameters with the confining pressures. The confining pressures gradually tighten the inter-crystalline boundaries along bedding planes, and hence the applied differential stress can no longer recognize their transverse isotropic textures. The salt strengths for all bedding orientations become equal under σ3 above 30 MPa. The proposed exponential equation is capable of describing the salt dilations and strengths under unconfined to highly confined conditions. Distortional strain energy (Wd) induced at dilation linearly increases with mean strain energy (Wm). The Wd-Wm relation for β = 60° can represent the dilation of the salt from low to high confinements where the mechanical responses of the rock transitionally change from transverse isotropic to isotropic behavior. The Wd-Wm relations for β ≠ 60° are applicable only for confining pressures less than 30 MPa.

The M 6.5 Ambon earthquake 26 September 2019: the source mechanism and the aftershock sequence characteristics

The area of Ambon, Maluku is located in the subduction zone in bands where the Australian plate meets the Eurasian plate, thus causing tectonic activities. The Ambon earthquake on 26th September 2019 with 6.5 Magnitude, while the Epicentral coordinates of the earthquake were determined as 3,53° S and 128,39° E and a focal depth of 10 km, according to the Agency for Meteorology Climatology and Geophysics, Indonesia. This earthquake was strongly felt at the biggest shock was felt with intensity VI-VII as unified in Ambon City, while several other areas are reported to have experienced small shaking, such as Intensity V in Masohi, and Intensity IV in Namlea and Namrole. We used a dataset of 24 waveforms of seven sensors, we determine a tabular solution, which have a large moment of 0.4573 x 1019 N-m, the depth is 6 km by minimizing the inversion residual. The method resulting strike and rake fault, with strike: 341.8°; dip; 81.5°; rake: 158.4°, and second nodal plane strike: 75.1°; dip; 68.6°; rake: 9.14°. The mechanisms were compared with those from other agency in agreement. The time decay intervals between mainshocks and significant aftershocks follow Mogi and Utsu’s Law but with a relatively faster rate of decay than that of aftershocks in general.

Integrative structural study of the Kibuye-Gitarama-Gatumba area (West Rwanda): A contribution to reconstruct the Meso- and Neoproterozoic tectonic framework of the Karagwe-Ankole Belt

The geodynamic evolution of the Mesoproterozoic Karagwe-Ankole Belt, situated in Central Africa, is largely unconstrained and topic of discussion. To overcome this knowledge gap, the tectonic history of the Western Domain has been studied, focusing on bedding, tectonic foliations and lineations, and quartz vein generations from the Kibuye-Gitarama-Gatumba area (West Rwanda). The results of our study reveal two fold generations, that are interpreted as the consequence of two separate compressional deformation events. The first fold generation (F1) consists of regional-scale symmetrical folds, with a N30W to NS axial plane strike and variable fold-cylindricity. The second generation (F2) is expressed by local small-scale folds with a generally steep axial plane, superposed on the regional fold structure. The strike of the F2 axial planes varies over the study area (N60W vs N20W), possibly linked to a buttressing effect from the rigid Gitarama batholith. Three quartz vein categories were determined: early quartz veins, strongly shortened or emplaced during F1 (A-category), bedding-parallel, boudinaged quartz veins, deformed pre- to syn-F2 (B-category) and undeformed, syn- to post-F2 quartz veins (C-category). Based on geochronological data from literature, the first major compressional event can be connected to the Late Mesoproterozoic Rodinia assembly during the Stenian-Tonian, while the youngest tectono-metamorphic event may represent the influence of the Late Neoproterozoic Gondwana assembly in the study area. This study furthermore demonstrates that the tectonic history of the Western Domain differs significantly from that of the Eastern Domain.

Slow‐Growing and Extended‐Duration Seismicity Swarms: Reactivating Joints or Foliations in the Cahuilla Valley Pluton, Central Peninsular Ranges, Southern California

AbstractThree prolific earthquake swarms and numerous smaller ones have occurred since 1980 in the Mesozoic igneous plutonic rocks of the Perris block of the Peninsular Ranges, Southern California. The major swarms occurred in 1980–1981, 1983–1984, and 2016–2018, with the latest swarm still ongoing. These swarms have no clear mainshock, with the largest events of ML 3.6, ML 3.7, and Mw 4.4. Each successive swarm had larger cumulative seismic moment release with about 314 and 411 events of M ≥ 1.5, while the third swarm has produced about 451 events of M ≥ 1.5 (as of September 2018). The concurrent strike‐slip faulting occurred on north to northwest striking planes but with no orthogonal northeast trending seismicity alignments. These shallow swarms are probably driven by intrablock Pacific‐North America plate boundary stress loading of the two bounding major late Quaternary strike‐slip faults, the Elsinore and San Jacinto faults. The state of stress within the Cahuilla Valley pluton has a ~40° angle between the maximum principal stress and the average trend of the swarms, suggesting that migrating pore fluid pressures aid in the formation and growth of zones of weakness. These swarms, which last more than 600 days each, exhibit clear bilateral spatial migration for distances of up to ~7–8 km and reach their full length in about 20 months. The slow spatial‐temporal development of the swarms corresponds to a fluid diffusivity of 0.006 to 0.01 m2/s, consistent with very low permeability rocks as expected for this block. There is no geodetic or other evidence for a slow slip event driving the swarms.

Source characteristics of the 18 September 2011 Sikkim earthquake and zoning

Using detailed waveform analysis of Pn, sPn depth phases from the nearby seismic stations, attempts were made to resolve the focal depth of the 2011 Sikkim earthquake. Focal depth of 46.8 km thus determined was found closer to the GCMT source mechanism solution as compared to ISC solution (29.6 km). Re-examination of its source mechanism with the spatial/ depth distribution of its aftershocks sug- gested that both the nodal planes oriented WNW or ENE, initially ruptured but the orientation of meizoseismal area and larger concen- tration of its aftershocks parallel to the Tista lineament conformed WNW striking nodal plane relatively more active. The large value of the stress drop derived from S-wave spectra based on the data of Indian stations was attributed to its strike-slip mechanism and deeper focal depth. The stress drop of its foreshock was much lower than that the main shock. The b-value also showed a decrease during the last decade prior to the main earthquake.The recent Sikkim earthquake (Mw 6.9) has brought out the limitations of the microzoning of the Sikkim region attempted earlier.

The 23 January 2014 Jizan earthquake and its tectonic implications in southwestern Saudi Arabia

A moderate-sized earthquake of Mw 4.4 occurred in southwestern Saudi Arabia on 23 January 2014. The event is considered as the largest observed earthquake that has occurred in the epicentral area. To examine the seismogenic stress regime and resolve the fault plane ambiguity, the hypocenter locations and focal mechanisms were determined using well recorded waveforms of the broadband stations operated by the Saudi Geological Survey. The current analysis included dataset consisting of mainshock and a total number of 113 well-located aftershocks. Focal mechanism solutions of the mainshock and fifteen aftershocks were determined. The focal mechanism solutions were inverted using stress tensor inversion. It indicates that the maximum compressive stress, σ1, has a nearly shallow plunge (11.8°) of ESE orientation and minimum compressive stress, σ3, has a shallow plunge (0.3°) toward NNE. Our results obtained from precise earthquake locations, focal mechanism solutions and stress tensor inversion reveal dextral strike-slip faulting over the ENE-WSW striking plane. The analysis of Coulomb failure stress emphasized the causative fault of the 2014 Jizan earthquake sequence. The active fault implies a reactivation of a high-angle fault, buried in the Precambrian basement, which is conjugate to the Red Sea spreading axis and NNW-SSE Najd fault system. The present study provided an impetus toward understanding the seismogenic stress regime in a virgin area.

Produire des lames de hache en dolérite au Néolithique en Mayenne. La carrière-atelier de Beulin à Saint-Germain-le-Guillaume

Produire des lames de hache en dolérite au Néolithique en Mayenne. La carrière-atelier de Beulin à Saint-Germain-le-Guillaume

New constraints shed light on strike-slip faulting beneath the southern Apennines (Italy): The 21 August 1962 Irpinia multiple earthquake

On 21 August 1962 an earthquake sequence set off near the city of Benevento, in Italy's southern Apennines. Three earthquakes, the largest having Mw 6.1, struck virtually the same area in less than 40min (at 18:09, 18:19 and 18:44 UTC, respectively). Several historical earthquakes hit this region, and its seismic hazard is accordingly among the highest countrywide. Although poorly understood in the past, the seismotectonics of this region can be revealed by the 1962 sequence, being the only significant earthquake in the area for which modern seismograms are available.We determine location, magnitude, and nodal planes of the first event (18:09 UTC) of the sequence. The focal mechanism exhibits dominant strike-slip rupture along a north-dipping, E-W striking plane or along a west-dipping, N-S striking plane. Either of these solutions is significantly different from the kinematics of the typical large earthquakes occurring along the crest of the Southern Apennines, such as the 23 November 1980 Irpinia earthquake (Mw 6.9), caused by predominant normal faulting along NW-SE-striking planes.The epicentre of the 21 August 1962, 18:09 event is located immediately east of the chain axis, near one of the three north-dipping, E-W striking oblique-slip sources thought to have caused one of the three main events of the December 1456 sequence (Io XI MCS), the most destructive events in the southern Apennines known to date.We maintain that the 21 August 1962, 18:09 earthquake occurred along the E-W striking fault system responsible for the southernmost event of the 1456 sequence and for two smaller but instrumentally documented events that occurred on 6 May 1971 (Mw 5.0) and 27 September 2012 (Mw 4.6), further suggesting that normal faulting is not the dominant tectonic style in this portion of the Italian peninsula.

Location and source mechanism of the Karlsruhe earthquake of 24 September 2014

On 24 September 2014, a ML 2.3 earthquake occurred southwest of the urban area of Karlsruhe, Germany, which was felt by a few people (maximum intensity I 0 = III). It was the first seismic event in this highly populated area since an I 0 = VII earthquake in 1948. Data of 35 permanent and temporary seismometers were analysed to localise the event and to determine the focal mechanism to compare it to previous seismicity. Restricting the data to P- and S-phases from 18 nearby stations and optimising the local earth model result in an epicentre in the southwest of the city at 48.986°N/8.302°E and in a hypocentral depth of 10 km. To calculate the focal mechanism, 22 P- and 5 SH-polarities were determined that constrain a stable left lateral strike-slip focal mechanism with a minor thrusting component and nodal planes striking NE-SW and NW-SE. The epicentre lies in the vicinity of the I 0 = VII earthquake of 1948. Both events are part of the graben-parallel flower structure beneath the Upper Rhine Graben, parallel to the active Rastatt source zone, which runs 5 km further east and included the epicentre of the 1933 Rastatt I 0 = VII earthquake. The focal mechanisms of the 2014 and 1948 earthquakes show NE-SW striking nodal planes that dip to the southeast. However, for the 1948 event, a normal faulting mechanism was determined earlier. Taking the uncertainty of the epicentre and focal mechanism in 1948 and its fault dimensions into account, both events might have happened on the same fault plane.

Subduction tractions and vertical axis rotations in the Zagros–Makran transition zone, SE Iran: the 2013 May 11Mw6.1 Minab earthquake

The source parameters and slip distribution of the 2013 May 11 Mw 6.1 Minab earthquake are studied using seismology, geodesy and field observations. We observe left-lateral strike-slip motion on a fault striking ENE–WSW; approximately perpendicular to previously studied faults in the Minab–Zendan–Palami fault zone. The fault that ruptured in 2013 is one of a series of ∼E–W striking left-lateral faults visible in the geology and geomorphology. These accommodate a velocity field equivalent to right-lateral shear on ∼N–S striking planes by clockwise rotations about vertical axes. The presence of these faults can reconcile differences in estimates of fault slip rates in the western Makran from GPS and Quaternary dating. The longitudinal range of shear in the western Makran is likely to be controlled by the distance over which the underthrusting Arabian lithosphere deepens in the transition from continent–continent collision in the Zagros to oceanic subduction in the Makran.

A novel high-selective bandpass frequency selective surface with multiple transmission zeros

The design, analysis, and measurement of a novel frequency selective surface (FSS) are proposed, which works with high selectivity in X-band and wide out of band rejection in C-band and Ku-band. The proposed FSS is achieved by cascading two-layer periodic arrays of unit cells based on square loop structure. Multiple transmission zeros are obtained for broadening the stopband on the both sides of the passband. The presented double-layer FSS is insensitive to the variation of incident angle and polarization of a striking plane electromagnetic wave. By studying the electric fields and currents excited in the proposed structure, it explains the passband characteristics and the multiple transmission zeros in the two stopbands. Experimental verifications are carried out, which are consistent with the simulated results.

Earthquake Focal Mechanisms in the New Madrid Seismic Zone

Online Material: Table of focal mechanisms; color versions of figures. The occurrence of intraplate earthquakes is largely an unexplained phenomenon. The New Madrid Seismic Zone (NMSZ) lies within the interior of the North American plate and is approximately 2000 km from the nearest active plate boundary. Yet, it has produced sequences of large earthquakes with widespread and coincident areas of paleoliquefaction features: A.D. 900±100, A.D. 1450±150, and 1811, 1812 (Tuttle et al. , 2002). Several theoretical driving mechanisms of NMSZ intraplate earthquakes have been proposed (e.g., Grana and Richardson, 1996; Liu and Zoback, 1997; Kenner and Segall, 2000; Grollimund and Zoback, 2001; Pollitz et al. , 2001; Forte et al. , 2007; Tavakoli et al. , 2010; Calais et al. , 2010). A short summary of these driving mechanisms is available in Powell and Horton (2009). Our aim is to contribute earthquake focal mechanism solutions for the study of seismotectonics in this region. The dominant trends in strike of the nodal planes are compared with the orientations of major structures in the region. We also estimate the variation in crustal stress via stress inversion in the northern Mississippi embayment. The focal mechanism results from this study (Johnson, 2008) show a possible rotation of the regional horizontal compressive stress. For future studies, these results may serve as a constraint to physical models of deformation. Focal mechanisms in this study are grouped into four areas, herein referred to as segments, which relate dominant trends of focal mechanism nodal plane strike to localized areas of seismicity (Fig. 1). Two of these segments strike northeast–southwest and lie at the northern and southern ends of the NMSZ, having nearly vertical dip. These will be referred to as the northeastern segment (northern end) and southwestern segment. They are characterized by mainly strike‐slip events and …

The 2012 August 11 Ahar earthquakes: consequences for tectonics and earthquake hazard in the Turkish–Iranian Plateau

We have examined the faulting in the 2012 August 11 Mw 6.4 and 6.3 Ahar (NW Iran) earthquakes using a combination of field mapping, remote-sensing observations of tectonic geomorphology, the cross-correlation of optical satellite images and the inversion of seismic waveforms. The first event was close to pure strike-slip, and the second was an oblique combination of thrust and strike-slip motion. Mapped surface ruptures indicate at least one of these events accommodated mostly right-lateral strike-slip motion on an ∼E–W striking plane. The occurrence of these earthquakes highlights the spatially distributed deformation in NW Iran, which has implications for both hazard assessment (the Ahar events killed over 300 people and injured over 3000), and also tectonic models of the region. Furthermore, these earthquakes demonstrate that the tectonics of the Ahar area is characterized by strike-slip faulting and a component of shortening, and not the previously suggested extension.

Angular-stable and polarization-independent frequency selective structure with high selectivity

We present a three-dimensional bandpass frequency selective structure (FSS) with high selectivity and wide out-of-band rejection. Furthermore, the structure is insensitive to the variation of incident angle and polarization of a striking plane electromagnetic wave. By studying the electric fields and currents excited in the proposed structure, it is revealed that four resonators are constructed to produce the desired filtering response. Experiments show that the fabricated FSS has a 3 dB fractional bandwidth of 18.75% for the passband and a bandwidth of 268.25% for the upper out-of-band rejection better than 20 dB under the normal incidence.

The 2011–2012 unrest at Santorini rift: Stress interaction between active faulting and volcanism

AbstractAt Santorini, active normal faulting controls the emission of volcanic products. Such geometry has implication on seismic activity around the plumbing system during unrest. Static Coulomb stress changes induced by the 2011–2012 inflation within a preexisting NW‐SE extensional regional stress field, compatible with fault geometry, increased by more than 0.5 MPa in an ellipsoid‐shaped zone beneath the Minoan caldera where almost all earthquakes (96%) have occurred since beginning of unrest. Magmatic processes perturb the regional stress in the caldera where strike‐slip rather than normal faulting along NE‐SW striking planes are expected. The inflation may have also promoted more distant moderate earthquakes on neighboring faults as the M &gt; 5 January 2012, south of Christiania. Santorini belongs to a set of en echelon NE‐SW striking rifts (Milos, Nysiros) oblique to the Aegean arc that may have initiated in the Quaternary due to propagation of the North Anatolian fault into the Southern Aegean Sea.

Zero separation results for solutions of second order linear differential equations

The oscillation of solutions of f″+Af=0 is discussed by focusing on four separate situations. In the complex case A is assumed to be either analytic in the unit disc D or entire, while in the real case A is continuous either on (−1,1) or on (0,∞). In all situations A is expected to grow beyond bounds that ensure finite oscillation for all (non-trivial) solutions, and the separation between distinct zeros of solutions is considered.In the complex case, it is shown that the growth of the maximum modulus of A determines the minimal separation of zeros of all solutions, and vice versa. This gives rise to new concepts called zero separation exponents, which measure the separation of zeros of either all solutions or of individual analytic functions. In D these quantities are defined in terms of the hyperbolic distance, while in the complex plane the Euclidean distance is used. As a by-product of these findings, the 1955-result of B. Schwarz, which asserts that supz∈D|A(z)|(1−|z|2)2<∞ if and only if the zero-sequences of all solutions are separated in the hyperbolic sense, is rediscovered. The striking plane analogue established reveals that the Euclidean distance between all distinct zeros of every solution is uniformly bounded away from zero if and only if A is a constant. As an outgrowth of the results, new information on the zero distribution of solutions in the classical polynomial coefficient case is also obtained. The main results are proved by using a method of localization, which naturally induces characterizations of certain subclasses of locally univalent functions in terms of the growth of their pre-Schwarzian and Schwarzian derivatives.In the real case, it is shown that the separation of zeros of non-trivial solutions is restricted according to the growth of A, but not conversely.

Relative location and source mechanism of inland earthquakes in Northern Egypt

We have relocated 259 inland earthquakes in northern Egypt using the double-difference hypocenter technique. Among this dataset we are able to determine source mechanisms of 200 events using P-wave polarities and amplitude ratios as well. The studied earthquakes have been recorded by the Egyptian National Seismological Network from October 1997 to December 2006 with local magnitude (ML) varies between 1.5 and 5.0. Three earthquake dislocations have been defined namely: Dahshour, southeast Beni-Suef, and Cairo-Suez district. Earthquake activities tend to occur in clusters along the first dislocation (Dahshour) however, relatively scattered along the second (southeast Beni-Suef) and the third (Cairo-Suez district) dislocations. At Dahshour dislocation three distinct clusters have been distinguished. Source mechanism solutions of Dahshour earthquakes displayed normal faulting with a strike-slip component to strike-slip faulting with a minor normal dip-slip component. Most of earthquake focal mechanism orientations are varying from NE-SW to NW-SE. The fault plane solutions of Beni-Suef earthquakes represented normal faulting with a strike-slip component. If the NNW-SSE striking plane has been chosen to be the actual fault plane, some solutions would indicated normal faulting with a sinistral strike-slip motion and other reflect normal faulting with a dextral strike-slip component. The fault plane solutions of Cairo-Suez district earthquakes are compatible with E-W to ENE-WSW striking normal fault with a dextral strike-slip motion.