Right-lateral Strike-slip Motion Research Articles

Estimating the slip rate in the North Tabriz Fault using focal mechanism data and GPS velocity field

Abstract This study delves into slip distribution on the North Tabriz Fault (NTF), a critical aspect of seismic hazard analysis due to its proximity to the Tabriz metropolis. The study operates within a uniform elastic half-space, maintaining constant values for fault geometry and regional rheological parameters throughout the research. To calculate strain boundary conditions, permanent and periodic global positioning system (GPS) data from the northwest region were utilized. The fault was constrained perpendicularly while allowing tangential movement, facilitating the determination of its annual slip rate using the boundary element method, with the Okada analytical model serving as the fundamental solution. The findings underscore the intricate relationship between the fault’s slip rate and boundary conditions, revealing a predominant right-lateral strike-slip motion. The study offers two slip rate assessments, obtained through earthquake focal mechanisms and GPS velocity data, yielding values of 5 and 5.5 mm / year {\rm{mm}}/{\rm{year}} , respectively. Importantly, the alignment of these calculated slip rates with paleo-seismological data underscores the credibility of the results generated via the boundary element method, distinguishing it as a reliable approach when compared to other numerical and analytical techniques. This research provides valuable insights into the behavior and slip dynamics of the NTF, which is pivotal for assessing seismic risks.

Tectonic evolution of the Ailaoshan–Red River shear zone and its relationship with the growth of the southeastern Tibetan Plateau

The Ailaoshan–Red River shear zone (ARSZ), which is ∼ 1000 km long and one of the largest boundary strike-slip fault zones in the southeastern margin of the Tibetan Plateau, provides valuable insights into the growth process of the southeastern Tibetan Plateau since the India-Asia collision. During the Late Miocene to Early Pliocene, the ARSZ underwent a kinematic transition from left-lateral strike-slip to right-lateral strike-slip motion. To reveal the mechanism of this kinematic transition and its relationship with the growth process of the southeastern Tibetan Plateau during the Cenozoic, we conducted paleomagnetic and tectonic deformation analyses on both sides of the ARSZ. The results show that the formation of the S-shaped bending and kinematic transition of the ARSZ were controlled by the changes in clockwise rotation rates of the Chuandian Fragment and the Indochina Block. This suggests that since the Oligocene, the growth of the southeastern Tibetan Plateau progressively extended outward from the southern margin of the Lhasa Terrane to the southeastern margin of the Tibetan Plateau, resulting in the sequential initiation of the clockwise rotation of the Shan-Thai Block, Indochina Block, and Chuandian Fragment since the Late Oligocene, and in variations in the rotational rates of these blocks. The different rotational movement of these blocks controlled the tectonic evolution of strike-slip fault systems on the southeastern margin of the Tibetan Plateau.

Neogene faulting, basin development, and relief generation in the southern Klamath Mountains (USA)

Abstract Development and evaluation of models for tectonic evolution in the Cascadia forearc require understanding of along-strike heterogeneity of strain distribution, uplift, and upper-plate characteristics. Here, we investigated the Neogene geologic record of the Klamath Mountains province in southernmost Cascadia and obtained apatite (U-Th)/He (AHe) thermochronology of Mesozoic plutons, Neogene graben sediment thickness, detrital zircon records from Neogene grabens, gravity and magnetic data, and kinematic analysis of faults. We documented three aspects of Neogene tectonics: early Miocene and younger rock exhumation, development of topographic relief sufficient to isolate Neogene graben-filling sediments from sources outside of the Klamath Mountains, and initiation of mid-Miocene or younger right-lateral and reverse faulting. Key findings are: (1) 10 new apatite AHe mean cooling ages from the Canyon Creek and Granite Peak plutons in the Trinity Alps range from 24.7 ± 2.1 Ma to 15.7 ± 2.1 Ma. Inverse thermal modeling of these data and published apatite fission-track ages indicate the most rapid rock cooling between ca. 25 and 15 Ma. One new AHe mean cooling age (26.7 ± 3.2 Ma) from the Ironside Mountain batholith 40 km west of the Trinity Alps, combined with previously published AHe ages, suggests geographically widespread latest Oligocene to Miocene cooling in the southern Klamath Mountains province. (2) AHe ages of 39.4 ± 5.1 Ma on the downthrown side and 22.7 ± 3.0 Ma on the upthrown side of the Browns Meadow fault suggest early Miocene to younger fault activity. (3) U-Pb detrital zircon ages (n = 862) and Lu-Hf isotope geochemistry from Miocene Weaverville Formation sediments in the Weaverville, Lowden Ranch, Hayfork, and Hyampom grabens south and southwest of the Trinity Alps can be traced to entirely Klamath Mountains sources; they suggest the south-central Klamath Mountains had, by the middle Miocene, sufficient relief to isolate these grabens from more distal sediment sources. (4) Two Miocene detrital zircon U-Pb ages of 10.6 ± 0.4 Ma and 16.7 ± 0.2 Ma from the Lowden Ranch graben show that the maximum depositional age of the upper Weaverville Formation here is younger than previously recognized. (5) A prominent steep-sided negative gravity anomaly associated with the Hayfork graben shows that both the north and south margins are fault-controlled, and inversion of gravity data suggests basin fill is between 1 km and 1.9 km thick. Abrupt elevation changes of basin fill-to-bedrock contacts reported in well logs record E-side-up and right-lateral faulting at the eastern end of the Hayfork graben. A NE-striking gravity gradient separates the main graben on the west from a narrower, thinner basin to the east, supporting this interpretation. (6) Of fset of both the base of the Weaverville Formation and the cataclasite-capped La Grange fault surface by a fault on the southwest margin of the Weaverville basin documents 200 m of reverse and 1500 m of right-lateral strike-slip motion on this structure, here named the Democrat Gulch fault; folded and steeply dipping strata adjacent to the fault confirm that faulting postdated deposition of the Weaverville Formation. Based on these findings, we suggest that Miocene rock cooling recorded by AHe ages, accompanying graben formation, and development of topographic relief record early to middle Miocene initiation of underplating or “subcretion” in the southern Cascadia subduction zone beneath the southern Klamath Mountains.

SLIP SOURCE MODEL OF THE 1995 NEFTEGORSK EARTHQUAKE (NORTH SAKHALIN) FROM GEODETIC DATA

The May 27, 1995 Mw=7.0 Neftegorsk earthquake occurred in the north of Sakhalin Island, rupturing the Upper Piltun fault, a secondary feature of the main Hokkaido-Sakhalin regional fault zone. The fault geometry, coseismic slip model, and Coulomb stress changes in the earthquake focal area were calculated based on a finite fault modeling. We used near-field coseismic offsets at 24 points obtained by comparison between predating triangulation and GPS observations, which were collected before and after the earthquake. Our slip distribution model shows two major slip patches. Larger slip asperity (amplitude up to 6.36 m) was characterized by right-lateral strike-slip movements, which correspond to focal mechanism of the earthquake, whereas the northern segment has reverse fault mechanism with maximum slip of 2.64 m. The fault length and width, average slip and stress drop values are estimated at 78 km, 28 km, 1.91 m and 11.3 MPa, respectively. The estimated release moment is approximately 7.49×1019 N∙m equal to Mw=7.2, which is larger than that reported by the USGS and GCMT but consistent with the values reported by other researchers. The coseismic Coulomb stress changes enhanced the stress by more than 10 MPa on the southern segment of the Gyrgylaninsky fault and middle section of the Hokkaido–Sakhalin fault. Seismic risks on the nearest faults cannot be ignored in the future despite the fact that the earthquake with a magnitude of 5.8 occurred in 2010 near the Gyrgylaninsky fault. The recent GPS rates in the surroundings of the Neftegorsk surface rupture mean that the recurrence interval for similar earthquakes may be more than a thousand years.

Active faults studies in Delhi and national capital region (NCR): Inferences from satellite data and field investigations

In recent years, the National Capital Region (NCR) of Delhi has experienced several earthquakes ranging in magnitude from 1.0 to 6.7. According to the last 50 years of earthquake data, the majority of earthquakes in the NCR have occurred near the Mahendragarh Dehradun Fault (MDF) and the Sohna Fault (SF). The region is bounded by a number of subsurface Ridges, Faults, and Lineaments, which are also influenced by the active plate boundary of the Indian and Eurasian plates. Active fault mapping is critical for the precise identification and marking of active fault traces in the NCR area for a precise seismic hazard assessment. We used high resolution Cartosat-1 stereopair data obtained from NRSC, Hyderabad, and Anaglyph (A 3D representation of the surface) and DEM prepared with ENVI software to map the active faults. We identified 12 sites in the NCR region based on satellite data interpretation, primarily along the MDF and Sohna Fault and their extensions. The presence of tectono-geomorphic markers along the MDF and Sohna Fault, such as warped surfaces indicative of fault scarps, stream offsets, gully erosion, and sag ponds, suggests active tectonic movement along these faults, most likely in the recent geological past. We believe the MDF is a right-lateral strike-slip fault with a compressional component on the western side and an extensional component on the eastern side. It acts as a segment boundary between compressional and extensional boundaries. We also identified the right lateral Nuh-Jhirka fault (NJF), which can be the Sohna Fault’s southern extension from Nuh to Jhirka. The western limb of the Delhi Mega fold has also seen a few right-lateral strike-slip movements that have extended up to the eastern bank of the Yamuna River, where the river reflects the base-level change and tight meandering on its upward side and a straight pattern on its downward side. This fault is known as the Delhi Fault (DF). The findings are preliminary, and further research would be required to create a detailed active fault map of the Delhi-NCR region to conduct a precise Seismic Hazard Assessment (SHA) of the region.

Underground Water Level, Strain, and Tilt Changes Observed Around the Fault Near Tono Research Institute of Earthquake Science

Near the Tono Research Institute of Earthquake Science (TRIES), Mizunami City, Gifu, two 500 m- deep shafts with diameters of 4.5 m and 6.5 m were excavated by the Japan Atomic Energy Agency (JAEA). The 6.5 m shaft is crossed by an NNW-trending geological fault (NNW fault), which has a right-lateral strike-slip component and a dip-slip component. Since before the excavation TRIES has been performing observations by a comprehensive set of borehole crustal activity observation devices in deep boreholes and extensometers in a vault near the shafts to observe variations in, for example, strain, tilt, and groundwater level continuously. During the shaft excavation, groundwater inflow to the shafts was encountered. The water was twice pumped out. During the pumping and non-pumping periods, TRIES’s observation data such as the groundwater level, the strain, and the tilt showed anomalous variations. The analysis of them revealed relationships among the groundwater level, the fault, the strain and the tilt. Strain analysis of TGR350 borehole station showed that the direction of the maximum shear strain was parallel to strike of the NNW fault. In addition, when the water in the shafts was being pumped out, the groundwater level at TGR350 declined and the maximum shear strain showed a right-lateral strike-slip movement in the NNW direction. When pumping was paused, the groundwater level increased, and the maximum shear strain showed a left-lateral strike-slip movement in the NNW direction. On the other hand, strain analysis of TGR165 borehole station showed different behavior from that of TGR350 borehole station deeper than TGR165. Tilt analysis of TGR165 borehole station showed tilt changes nearly perpendicular to strike of the NNW fault corresponding to the pumping and the non-pumping periods. Tilt analysis of TGR350 borehole station showed tilt changes of E-W direction with much smaller amplitude than that of TGR165. Behavior of the strain and the tilt during the pumping and non-pumping periods can be attributed in part to differences in the response of the medium to groundwater fluctuations, due to differences in the number and distribution characteristics of the fractures in the medium where the observation instruments are installed. We also discussed the observation results from the viewpoint of poroelasticity.

Topographic evidence of long wavelength active folding in the New Madrid seismic zone, intraplate North America

The northeast-striking Axial and Bootheel faults (AF and BHF) in the central USA are components of the New Madrid seismic zone (NMSZ) fault complex, source of M > 7 earthquakes in 1811–1812. Previous work shows Holocene displacement of the AF and BHF to be primarily right-lateral strike-slip movement west and south of the NMSZ restraining bend. In this study, treads of two Late Pleistocene alluvial terraces of the Mississippi River, the 14–16 ka Kennett and the 12 ka Morehouse terraces, were used as structural datums to assess any long-wavelength vertical component of deformation across these two faults. Using 10 m-resolution LiDAR, we constructed a best-fit polynomial trend surface of these terrace treads. Our results reveal a subtle ≤4 m-deep, 40 km-wide, 190 km-long, topographic trough. In the south, it corresponds to the southern segment of the AF and parallels the BHF for 50 km. Farther north it corresponds to a syncline in subsurface Paleozoic strata 10 to 20 km northwest of and parallel to the BHF.The trough axis crosses obliquely down-valley from the younger Morehouse terrace to the older Kennett terrace and does not correspond to major drainages. Significantly, the Malden-Bernie fluvial scarp separating the two terraces continues into the trough but is down-warped, inconsistent with an erosional origin of the trough. Along the southern segment of the trough, the older Kennett tread is topographically lower than the Morehouse tread, but relict Kennett braided channels were not buried during deposition of younger Morehouse alluvium. This suggests the Kennett terrace tectonically subsided along the trough after Morehouse terrace deposition. We interpret this trough as a subtle syncline along the western margin of the NMSZ restraining bend that developed in response to down-on-the-northwest fault movement after deposition of Morehouse alluvium.

The effects of pre-stress assumptions on dynamic rupture with complex fault geometry in the San Gorgonio Pass, California, region

Abstract We use three-dimensional (3-D) dynamic finite-element models to investigate potential rupture paths of earthquakes propagating along faults through the western San Gorgonio Pass, a structurally complex region along the San Andreas fault system in southern California (USA). We focus on the right-lateral San Bernardino strand of the San Andreas fault system, the oblique thrust–right-lateral San Gorgonio Pass fault zone, and a portion of the right-lateral Garnet Hill strand of the San Andreas fault system. We use the 3-D finite-element method to model rupture propagation along a fault geometry that reflects current understanding of the local geometrical complexity and is consistent with long-term loading and observed surface deformation. We test three different types of pre-stress assumptions: (1) constant tractions (assuming pure right-lateral strike-slip motion on the San Bernardino and Garnet Hill strands and oblique thrust–right-lateral strike-slip motion on the San Gorgonio Pass fault zone), (2) a uniform regional stress regime, and (3) long-term (evolved) stress from quasi-static crustal deformation modeling. Our results imply that under the more realistic regional stress and evolved stress assumptions, throughgoing rupture propagation from the southeast to northwest (i.e., from the Garnet Hill to the San Bernardino strand) may be more likely than throughgoing rupture in the reverse direction (from the San Bernardino to the Garnet Hill strand). The results may have implications for the earthquake potential in the region as well as for ground motion in the Los Angeles Basin. The results also emphasize how fault geometry and stress patterns combine to influence rupture propagation on complex fault systems.

Tectonic geomorphology and prehistoric earthquakes of the West Helanshan fault, West Ordos, and its implications for regional tectonics and seismic hazard

The West Helanshan fault is situated at the junction of the northeastern Tibetan Plateau, Alxa, and Ordos blocks, providing an opportunity to investigate the deformation in the transitional zone between the contractional northeastern Tibetan Plateau and extensional North China. Using field investigations and satellite imagery interpretations, we determined detailed geometry and kinematics of the West Helanshan fault. The West Helanshan fault is divided into three segments, including the northern, middle, and southern segments, based on varying fault strike, slip sense, and geomorphic expression. The northern and middle segments are dominated by right-lateral strike-slip motion as indicated by linear fault traces, horizontally offset streams and terrace risers, relatively insignificant vertical offset, and frequently changing facing direction of fault scarps. The late Quaternary right-slip rate is estimated to be 0.2–0.4 mm/yr, which is relatively low comparing with other strike-slip faults in the northeastern Tibetan Plateau, based on 10Be-dated offset fluvial/alluvial surfaces. The southern segment shows conspicuous fault scarps with significant systematic vertical offset, which has a reverse component as revealed in the trench. The timing of the most recent earthquake revealed in the trench is constrained to be between 6.2 ± 0.7 ka and 5.9 ± 0.4 ka. Notably, the West Helanshan fault cuts through the city center of Azuoqi town, which is the capital of Azuoqi county and has about 100,000 residents, carrying high potential earthquake hazard in the future. Finally, combining the regional fault geometry and kinematics with GPS observations, we propose a limited north-northeastward extrusion model for the southern Alxa Block.

Tectonics in a very slowly deforming region in an orogenic belt

We use GPS and InSAR data to examine the present-day kinematics of the Tuz Gölü Fault Zone with a view to documenting an ‘intra-plate-like’ behaviour within a highly active plate boundary zone. In order to generate the strain rate field of the region, we utilize two different approaches. Both of the approaches reveal that the area has a shear-dominated deformation. Furthermore, we design a simple block model to understand better especially the slip on the Tuz Gölü Fault. The results indicate that the fault behaviour can be explained by right-lateral strike-slip motion. This is in contradiction with the previous interpretations of it displaying normal fault behaviour based on geomorphological observations of limited spatial extent. On the other hand, the present-day kinematics of the fault is not an agreement with the thrust features that are observed around it. We think that the propagation of the rupture of the North Anatolian Fault Zone may have put an end to the thrust regime along the Tuz Gölü Fault at the end of Pliocene. We think that this study may provide guidelines for understanding the origin and behaviour of slowly deforming ‘germanotype’ structures within zones of rapidly deforming ‘alpinotype’ regions.

Stress changes associated with the significant first subevent of the 2008 Wenchuan earthquake and implications for the rupture behavior transition

The rupture process of the Wenchuan earthquake demonstrated a transition from thrust-dominated slip to northeastward strike-slip motion along the Longmen Shan Fault Zone. The initial stress has been reported as playing a critical role in this process; however, the stress changes, especially those caused by the significant first subevent of the Wenchuan earthquake are not well understood. Here, we employ a three-dimensional finite element model of the Sichuan-Yunnan region to analyze the stress change caused by the significant first subevent and explore the possible influence on the following ruptures. The results indicate that the auxiliary maximum principal compressive stress (SH) associated with the significant first subevent was horizontal and that the auxiliary stress regime was SH>Sh>Sv, supporting the ongoing regional thrust motion near the southwestern segment of the rupture plane. However, in the northeastern segment, the auxiliary stress regime transitioned to SH>Sv>Sh, demonstrating that the stress changes promoted the transition of the rupture behavior from predominantly thrust motion in the southwest to right-lateral strike-slip motion in the northeastern segment, which was also supported by the dominant shear stress change and the subtle normal stress change along the fault plane in the northeastern segment. In addition, our modeled results also indicate that the orientation of the maximum principal compressive stress changed from SEE to northeastward NEE along the strike of Longmen Shan Fault Zone. This anticlockwise rotation hastened the rupture behavior transition, suggesting that both the initial stress and the stress changes associated with the first subevent jointly controlled the following northeastward rupture of the Mw7.9 Wenchuan earthquake.

Using multispectral and radar remote sensing data for geological investigation, Qena-Safaga Shear Zone, Eastern Desert, Egypt

In the present study, Qena-Safaga Shear Zone (QSSZ) has been the subject matter of detailed remote sensing investigation using Landsat-8, ASTER, and SRTM DEM data. Gathering information from these sensors allowed revealing the different lithological, geological, and structural features. Field observations were carried out to verify the remotely sensed data analysis and interpretation. A fused Landsat-8 OLI and ALOS/PALSAR data clearly improved the view of structural elements that are difficult to be revealed by optical images. The results showed that the area is controlled by the dextral ENE-WSW shearing trend which extends across the Nile at Qena Bend. Lineament analysis and lineament density map demonstrated that the area was subjected to different phases of stress; accordingly, conjugate faults were formed; most of these trends were rejuvenated. The ENE- WSW, NE-SW, and NW-SE are the major tectonic trends affecting the area but the N-S direction is less dominant. WNW, NW, NE, E, and N directions are indicated by sinistral strike-slip movements. The ENE, NE, and WNW are governed by right-lateral strike-slip movements and most of these trends controlled the emplacement of granitoids. The image transformation technique also allowed revealing hydrothermal alteration effects related to the NW-SE and NE-SW fault systems. Overall, combining of multispectral and radar data and field observations is significant for characterizing the geological features in arid regions.

The Storfjorden, Svalbard, Earthquake Sequence 2008–2020: Transtensional Tectonics in an Arctic Intraplate Region

AbstractAn earthquake sequence in the Storfjorden offshore area southwest of Spitsbergen in the Svalbard archipelago initiated with a 21 February 2008 magnitude Mw 6.1 event. This area had previously not produced any significant earthquakes, but between 2008 and 2020, a total of ∼2800 earthquakes were detected, with ∼16 of them being of moderate size (ML≥4.0). Applying double-difference relocation to improve relative locations reveals that the activity is linked to several subparallel faults striking southwest–northeast that extend across the entire crust. The southwest–northeast trend is also found as a possible fault plane from regional moment tensor inversion. The solutions range from oblique normal in the center of the cluster to pure strike slip farther away and are consistent with the compressional σ1 axis roughly in the east–west direction and plunging 57°, and the extensional σ3 axis subhorizontal trending north–south. The mainshock fault is steeply dipping to the southeast, but several other faults appear to be near vertical. The existence of oblique, right-lateral strike-slip motion on southwest–northeast-trending faults with a normal component and pure normal faulting events in between suggests transtensional tectonics that in and around Storfjorden result in activation of a complex fault system.

Coseismic deformation and multi-fault slip model of the 2019 Mindanao earthquake sequence derived from Sentinel-1 and ALOS-2 data

From October to December 2019, a sequence of four earthquakes of Mw > 6.0 occurred in southern Mindanao, the Philippines, and revealed complex tectonics in this region. We used Advanced Land Observing Satellite (ALOS)-2 and Sentinel-1A/B Synthetic Aperture Radar (SAR) data to map the coseismic deformation and to constrain both the fault geometry and kinematics of coseismic slip distributions during the four earthquakes. Our observed coseismic deformation field shows that these four earthquakes ruptured two intersected conjugate faults with NE and NW strikes. Based on ascending and descending Interferometric SAR (InSAR) data, we constructed elastic half-space models to search for the optimal fault dip angle and inverted the slip distributions of the four earthquakes. The results show that these four earthquakes were all dominated by left- and right-lateral strike-slip motion. The two conjugate faults clearly had significant differences in terms of slip behaviour and fault kinematics. The static Coulomb stress change indicates that earlier earthquakes had a significant triggering effect on subsequent earthquakes. The static Coulomb stress change induced by the four earthquakes suggests a significant stress loading effect on NW-trending faults. The results of this study show that the potential for future earthquakes on NW-trending faults cannot be ignored; however the magnitudes of future earthquakes should be limited by complex fault geometry and segmentation.

Strike-Slip Earthquakes at the Northern Edge of the Calabrian Arc Subduction Zone

AbstractWe analyzed earthquakes of a swarm started in October 2019 in the Tyrrhenian Sea, at the northern border of the Calabrian arc subduction zone. The swarm is located in the same area where a subduction-transform edge propagator (STEP) shear- zone -oriented east–west is recognized from ocean floor morphology and submarine volcanoes. We computed focal mechanism, relative location, stress drop, corner frequency, and source directivity of the mainshock Mw 4.4 and of some aftershocks in the local magnitude range 2.3–3.7. Results indicate clearly that the mainshock occurred on a northwest–southeast-oriented fault, with right-lateral strike-slip motion, and it was characterized by a strong directivity of the rupture propagation from northwest to southeast. On the contrary, most of aftershocks were located on another strike-slip fault oriented northeast–southwest and had left-lateral kinematics. The kinematic features of these earthquakes indicate a strain field with the P-axis oriented north–south and the T-axis oriented east–west. Fault directions and stress field are in good agreement with the theoretical fracture model of shear zones associated with a STEP.

The early Paleozoic structures and its influence on the Permian strata, Midland Basin: Insights from multi-attribute seismic analysis

The Permian Basin is one of the most important petroleum-producing regions in the United States. The structural history of the Permian Basin is extremely complex. Most of the structural traps in the Permian Basin are associated with the strike-slip fault system, which presented challenges for past structural interpretation completed primarily using well log and 2D seismic data. We utilized a 3D seismic survey, a part of the Midland Basin and eastern margin of the Central Basin Platform (CBP), to identify the Early Paleozoic structural elements to study its influence on the HC production from the Permian strata. We computed volumetric seismic attributes including, coherence which can identify faults with a significant amount of throw, along with curvature which can detect flexure and folds. We also used, recently developed aberrancy attribute, which illuminates subtle faults and flexures. We observed several strong lineations on seismic attributes extracted along the Woodford surface. Lineations have three different trends. Trend 1 lineations are drag folds associated with the Andrews shear zone (ASZ). Trend 2 lineations, which show a left-lateral strike-slip motion, are R’ Riedel Shear, are oriented N 65° W truncating against the CBP. The trend 2 lineations continue upward to the shallower surfaces, as well as they are parallel to the surface lineations identified on the satellite images, which indicates the continued movement of the pre-existing faults. The trend 3 lineations, which show a right-lateral strike-slip motion, are R Riedel Shear and align with the ASZ. The formation micro image (FMI) shows the fracture orientations in trend 2 and trend 3 directions. The aberrancy lineation anomalies are indicative of highly fractured areas; wells drilled in these areas display lower oil-water ratio, as well as a higher amount of production, which may be due to the increase in permeability due to the fractures.

Late Quaternary paleoseismology and faulting behavior of the Yilan-Yitong fault zone and implications for seismic hazards of the Tanlu fault zone, eastern China

Abstract Along the north branch of the Tanlu fault zone (TLFZ), the Yilan-Yitong fault zone (YYFZ) displaces Quaternary geomorphic features, forming a ~70-km-long fresh surface rupture. In this study, we investigated offset landforms and paleoearthquake history by excavating four trenches along the YYFZ within the Fangzheng graben combined with Quaternary dating of landforms and trench horizons. Four paleoearthquakes are identified with their timings constrained at 1730 ± 40 a BP to 770 ± 30 a BP, 28.5 ± 1.2 ka BP to 21.7 ± 0.9 ka BP, 59.4 ± 2.4 ka BP to 33.1 ± 1.1 ka BP, and before 59.4 ± 2.4 ka BP, respectively. Seismic recurrence behavior of the YYFZ follows a quasi-periodic pattern with a very long interval of 10–20 ka. Geological and geomorphic features show that the YYFZ is dominated by right-lateral strike-slip movement with a low dextral slip rate of ~0.2–0.3 mm/a in late Quaternary, in accord with present-day geodetic observations. As with the YYFZ, paleoearthquakes along the TLFZ commonly share a recurrence interval of ~10–20 ka. We suggest that active segments of the TLFZ that have been dormant for thousands of years may pose significant earthquake hazards in the future. Moreover, strain transfer from the bounding faults to those more central in the graben may be significant on the TLFZ and should be given consideration when assessing regional seismic hazards. Insight into past earthquakes and faulting behavior in a low-strain environment is crucial for earthquake-hazard analysis and often requires multidisciplinary approaches.

Stress Field Pattern in the Northeastern Part of Azerbaijan

Azerbaijan is characterized by a strong level of seismicity with earthquakes concentrated within several active areas. In this study, we calculated the source mechanism of earthquakes occurring in the northeastern part of Azerbaijan, by using the digital waveform data recorded by the Republican Seismic Survey Center (RSSC) at the Azerbaijan National Academy of Sciences (ANAS) during the period from 2003 to 2017. Polarity of the first motion of a P-wave was the main criteria for focal mechanism solutions with high reliability. In investigation of the different tectonic deformations and modern stress field pattern in the tectonic areas of the northeastern part of Azerbaijan, the focal mechanism solutions were applied. The results show mainly a thrust faulting mechanism with few events displaying also normal faulting or strike-slip faulting. The World Stress Map (WSM) Create A Stress Map Online (CASMO) and Cataclastic Analysis Method are used in the current research for defining the regional stress field orientations. Western and central parts of the Greater Caucasus ridge are mainly characterized by northeastern–southwestern (NE–SW) tension. In the eastern part, the tension reverses into intensive compression. Additionally, the transformation of sinistral strike-slip motion into predominant right-lateral strike-slip motion can be obviously traced southward from mountain ridges of the Greater Caucasus. The differences between these characteristics are considered for the large tectonic zones (Balaken–Zagatala, Sheki–Oguz–Gabala, Ismailli–Shamakhi). In the Zagatala region of the studied area, the NW–SE orientation of the compression axes is observed, while in the Sheki region, orientation stretches towards N–S and smoothly extends clockwise towards NE–SW orientation in the Caspian Sea. The tension axis is mostly directed towards NE–SW and N–S which is linked with the heat of the Kur depression under the zone of the Greater Caucasus. Significant spatial variations in orientations of the axes of principal stresses in the shear zone and their local weak gentle variations are evidence of a consistent general stress in the NE–SW direction.

Seismic Potential Around the Northeastern Edge of the Longmenshan Fault Zone as Inferred from Seismological Observations

We assessed the seismic potential around the northeastern edge of the Longmenshan fault zone (LFZ) by analyzing the aftershocks of the 2008 Mw 7.9 Wenchuan earthquake, focusing on the right-lateral strike-slip Qingchuan fault (QCF). The data used in the present study were collected from a dense network of broadband seismographs, installed after the Mw 7.9 mainshock by the China Earthquake Administration. We analyzed 440 aftershocks that occurred around the northeastern part of the aftershock area between December 2008 and June 2009, the local magnitudes (ML) of which ranged from 2.0 to 5.2. We determined 127 well-constrained focal mechanism solutions for these aftershocks down to ML 2.5 by using P-wave polarity data in conjunction with body-wave amplitudes. An analysis of the stress tensor inversion revealed that, upon approaching the QCF from the LFZ, the stress field changes from a reverse faulting regime to a mixed regime containing strike-slip components and the orientation of the maximum compressional stress changes from a NW–SE to a WNW–ESE direction. This stress field favors a right-lateral strike-slip movement along the fault. The slip tendency analysis revealed that the QCF is regarded as being favorably oriented to the present-day stress field. The coseismic Coulomb stress changes induced by the mainshock increased along the preferred fault plane in the northeastern half of the QCF, which is further loaded by viscoelastic relaxation. This evidence suggests that the potential for a future earthquake in the QCF is high.

Aftershock Sequences of the Recent Major Earthquakes in New Zealand

The three clusters of the epicenters of the nine recent (1993–2018) earthquakes of magnitude 7.0 or larger in New Zealand are located in three different tectonic environments of the Australia–Pacific Plate boundary, including the southern part of the Kermadec Trench (showing rapid westward subduction), the oblique collision zone between the Pacific Plate and Indo-Australian Plate with the dominant Alpine Fault (showing right-lateral strike-slip movement), and the Puysegur Trench (showing eastward oblique subduction). From the viewpoint of the unified scaling law for earthquakes (USLE), these regions are characterized by different levels of seismic rate (A), earthquake magnitude exponent (B), and fractal dimension of epicenter loci (C). The recent major earthquakes exemplify different scenarios of aftershock sequences in terms of either the dynamics of interevent time (τ) or the USLE control parameter (η = τ × 10B×(5−M) × LC), where τ is the time interval between two successive earthquakes, M is the magnitude of the second one, and L is the distance between them. We find the existence, in the long term, of different, intermittent levels of rather steady seismic activity characterized by near-constant values of mean η (〈η〉), which, in the mid-term, switch between one another at times of critical transitions, including those associated with all but one magnitude 7.0 or larger earthquake. At such a transition, seismic activity may follow different scenarios with interevent time scaling of different kinds. Evidently, although these results based on analysis of an individual series do not support the presence of universality in seismic energy release, they provide constraints on modeling realistic seismic sequences for earthquake physicists and supply decision-makers with information for improving local seismic hazard assessments.