Intraplate Regions Research Articles

Cenozoic Reactivation of the Penacova-Régua-Verin and Manteigas-Vilariça-Bragança Fault Systems (Iberian Peninsula): Implication in Their Seismogenic Potential

The Penacova-Régua-Verin (PRV) and the Manteigas-Vilariça-Bragança (MVB) are two of the longest faults of the Iberian Peninsula. These faults striking NNE–SSW, over lengths of >200 km, were developed during late-Variscan Orogeny and reactivated in response to the Alpine Cycle tectonics. Their tectonic evolution during Alpine compression (Cenozoic) and their implication in the active tectonic activity of Iberia are under discussion. Their recent tectonic activity is recorded in the vertical offset of geomorphological surfaces, in the associated pull-apart basins, and in M > 7 paleoseismic events. Based on the vertical surface offset of Pliocene surfaces (140–300 m for the MVB fault and 150–200 m for the PRV), together with the horizontal offset (1300–1600 m for MVBF fault and 600–1400 m for PRVF), we can conclude that they were reactivated as left-lateral strike-slip faults with a reverse component during the Pliocene (3.6 Ma)–present. These results indicate that these faults are not related to the strain transmission during the collision with Eurasia (Eocene–Oligocene). However, they are related to the intraplate strain of the southern collision with the African plate during the Upper Neogene. The estimated slip-rate is 0.2–0.5 mm/a for both faults. These slip-rates evidence important implications for the seismic hazard of this intraplate region.

Drought-induced seismicity modulation in the New Madrid Seismic Zone, central United States

The New Madrid Seismic Zone (NMSZ) in the central United States is a seismically active intraplate region composed of two fault segments: the Reelfoot and Cottonwood Grove fault segments. It has witnessed several major earthquakes, including a devastating 1811–1812 earthquake sequence of three ~M7 events. Nearly 200 years later, earthquakes still continue today in this complex seismic zone. This seismic zone is located in a domain with higher hydrological load than surrounding regions, which may play a crucial role in seismicity modulation. However, the hydrological unloading or loading-induced seismicity modulation and the underlying earthquake dynamics on this stable plate interior remain equivocal. Our study demonstrates that increased climate variability and drought-induced hydrological unloading can be potential drivers for the crustal stress change in the upper Mississippi embayment and seismicity modulation in the NMSZ. The seismicity rate associated with the Reelfoot fault segment shows ~60% increase during drought-induced prolonged drier periods, linked to La Niña cycles, than during the relatively wetter periods. However, such a feature is lacking for the seismicity associated with the Cottonwood Grove fault segment. We argue that the near-lithostatic pore pressure condition explicitly on the Reelfoot fault segment leads to an increase in the amplitude of the velocity perturbation, making this fault segment more susceptible to seismicity modulation on a multi-annual or annual time scale by the resonance destabilization process.

Structural inheritance, morphotectonics and Holocene activity of the Cucalón-Pancrudo extensional fault (Iberian Chain, Spain)

A large active extensional fault is characterized, which will contribute to improve seismic hazard assessment in an intraplate region of the Mediterranean domain whose seismic potential until today has been underestimated. The Cucalón-Pancrudo fault (CPF) is a NNW-SSE striking extensional fault that represents the negative (extensional) inversion of a previous, Late Variscan and Paleogene, transpressional fault. It is part of the Río Grío-Pancrudo fault zone, the largest active structure in the intraplate central-eastern Iberian Chain. This study focuses on characterizing the CPF structure, morphotectonics and paleoseismology. The fault offsets a well-known regional planation surface (lower level of the Fundamental Erosion Surface, FES3, 3.5 Ma), providing a basis for calculating the maximum fault throw (c. 280–300 m, including the contribution of a minor synthetic fault and a gentle accommodation monocline), and hence estimating the net slip (c. 305–325 m) and the long-term slip rate (0.09 mm/a). A trench dug in a Holocene fluvial terrace reveals an ensemble of listric, domino-style ruptures synthetic to the main fault. They were activated during at least two paleoseismic events (X, Z), 14.9 ± 1.4 ka and 6.9 ± 0.4 ka in age. An intermediate event (Y), dated to 11.0 ± 1.0 ka, as an uncertainty because the attribution of their ruptures to event Z cannot be ruled out. The total net slip accumulated during these events (1.15–1.25 m) provides an apparent short-term slip rate of 0.07–0.09 mm/a, very similar to the long-term slip rate. Nevertheless, it is very likely that the faults exposed in the surveyed trench do not represent a complete paleoseismic record. Therefore, the actual slip rate during Late Pleistocene-Holocene times could be significantly higher, approaching that reported for most of the recent extensional faults in the central Iberian Chain. The inclusion of CPF in the map of Active Faults of Iberia (QAFI database) will result in improving seismic hazard assessment in Spain.

Seismic Monitoring near Ithaca, New York, Reveals Nonuniform Distribution of Microseismicity in an Intraplate Region

Abstract Cornell University intends to use a deep direct-use geothermal system to heat its Ithaca, New York, campus. In preparation for this project, the Cornell Seismic Network has been monitoring the background seismicity in this intraplate region since 2019. From January 2020 to June 2023, 95 events were detected within 20 km of the proposed geothermal well site, with local magnitudes ranging from −1.02 to 0.56. None of these events appear in regional or national catalogs. Events locate in a narrow geographic band, with one-fourth exhibiting multimodal hypocentral probability peaks both near the surface and at 1–4 km depth. We relocate events with a joint hypocenter and 1D velocity model inversion, in addition to a fully nonlinear method, and then compare observations with synthetic waveforms. Together, these approaches provide strong evidence for >95% of events locating at the surface or within the 3-km-thick sedimentary sequence. We explore how anthropogenic activity and regional topographic stress may contribute to frequent surficial events. This information is critical for characterizing the background microseismicity for comparison during future geothermal operations. Ithaca’s geology of Paleozoic sediments overlying Precambrian crystalline basement is typical of many continental interiors, so these results also provide insight into intraplate microseismicity patterns.

Interactions between Cenozoic tectonics and landscape dynamics in Londrina, Paraná, Brazil

The Londrina City, located in southern Brazil, is situated on the basalt of the Paraná Igneous Province (PIP), near tectonic structures such as the Guaxupé faults and the São Jerônimo-Curiúva and São Sebastião lineaments, with a history of reactivation in the Cenozoic. Between 2015 and 2018, low-magnitude earthquakes hit the municipality, suggesting a possible connection with these structures. This study investigates the relationship between Cenozoic tectonics and geomorphological dynamics, combining quantitative geomorphological analysis with field analysis of faults. The results reveal signs of tectonic control on the landscape, with two pulses of regional uplift/base level drop affecting the drainage network. Reactivated segments, especially at intersections of NE and NW structures, influence dynamics, with variations in uplift rates between basins and block rotation under a transtensive regime. Recent earthquakes are related to the reactivation of an N50-60E fault scarp, possibly the Guaxupé Fault, indicating recurrent tectonic movement. Paleostress analyses identify three deformation pulses, highlighting the complexity of landscape evolution in intraplate regions. These findings enhance understanding of regional tectonics, emphasizing the need to comprehend landscape evolution in interior plate areas, such as the South American Plate.

Archeoseismic Study of Damage in Medieval Monuments around New Delhi, India: An Approach to Understanding Paleoseismicity in an Intraplate Region

The seismic shaking observed around Delhi and the surrounding region due to near-field and far-field earthquakes is a matter of concern for the seismic safety of the national capital of India, as well as the historical monuments of the region. Historical seismicity indicates that the Delhi region has been affected by several damaging earthquakes originating from the Himalayan region as far-field events, as well as due to near-field earthquakes with epicenters close to Delhi. The historical records, along with recent archeoseismological studies, suggest that Qutab Minar, a UNESCO World Heritage Site, was damaged by the earthquake of 1803 CE. This event represents the only evidence of seismic damage from the region, as there has been no detailed study of other historical monuments in the area or earthquakes that have caused damage. In this context, the earthquake damage to other monuments might have been overlooked to some extent around the Qutab Minar due to the lack of proper earthquake damage surveys and documentation in historical times. The main goal of this study is to identify evidence of earthquake archeological effects around the Qutab Minar and to shed new light on the occurrence and characteristics of ancient earthquakes while providing data to inform seismic risk assessment programs. With this aim, we describe different earthquake-related damage (EAE, earthquake archeological effects) at the Isa Khan Tomb and Humayun’s Tomb, built between 1548 CE and 1570 CE, respectively, as well as the older Tomb of Iltutmish (built in 1235 CE) along with the Qutab Minar, which was built between 1199 CE and 1220 CE. The damage was probably caused by seismic events with intensities between VIII and IX on the European Macroseismic Scale (EMS). Based on the methodology of paleo ShakeMaps, it is most likely that the 1803 CE earthquake was the causative earthquake for the observed deformation in the Isa Khan Tomb, Tomb of Iltutmish, and Humayun’s Tomb. More detailed regional paleoseismological studies are required to identify the responsible fault. In conclusion, the impressive cultural heritage of Delhi city and the intraplate region is constantly under seismic threats from near-field earthquakes and far-field Himalayan earthquakes.

Unraveling the Preparatory Processes of the 2023 Mw 7.8–7.6 Kahramanmaraş Earthquake Doublet

Abstract Within a span of 9 hr on 6 February 2023, two significant earthquakes, with magnitudes of Mw 7.8 and 7.6, struck the southeastern part of Türkiye and the northern region of Syria, resulting in significant casualties and widespread economic losses. The occurrence of such intense earthquakes in rapid succession on adjacent faults, especially within a highly complex intraplate region with a multifault network, poses a rare phenomenon, presenting new challenges for seismic hazard analysis in such areas. To investigate whether the preparatory processes for the Mw 7.8–7.6 earthquake doublet could be identified on a large spatial scale prior to the seismic events, we employed a data-driven approach for b-value calculation. The difference in b-values from the background values (Δb) in a reference period were used as inputs, and the cumulative migration pattern (CMP) method, quantitatively describing the migration of seismic activity, was utilized to calculate the corresponding probability distributions. The results indicate a widespread phenomenon of decreasing b-values in the study area over a decade before the occurrence of the earthquake doublet, revealing a significant enhancement of differential crustal stress over a large region. In addition, despite not being the region with the most pronounced decrease in b-values, there is a distinct high probability distribution of CMP near the nucleation points of the earthquake doublet, indicating a spatial and temporal “focus” of increased crustal differential stress in the study area, unveiling the preparatory process of the earthquake doublet. This study reveals quantifiable migration patterns over a long time scale and a large spatial extent, providing new insights into the evolution and occurrence processes of the 2023 Mw 7.8–7.6 Kahramanmaraş earthquake doublet. Moreover, it offers potential clues for seismic hazard analysis in such intraplate regions with multiple fault systems.

Stratigraphic and structural evidence of transpressional Quaternary fault reactivation along the SE Korean Peninsula

Comprehending the nature, extent, and distribution of seismogenic structures, along with their seismogenic potential to induce destructive earthquakes in low seismic strain regions, holds significant socio-economic significance. Analyzing historical and paleoearthquakes becomes essential to understanding the influencing factors controlling the origin of earthquakes in intraplate regions, such as the Korean Peninsula. Based on current knowledge, the seismic pattern within the intraplate region, particularly along the Korean Peninsula, exhibits greater heterogeneity. This observation is drawn from historical seismic records, instrumentally detected seismic patterns, and paleoseismological analyses. Comprehensive paleoseismological studies play a crucial role in providing new evidence for fault activity, and hence they are an important contribution to the understanding of the seismic and structural evolution of faults. Existing paleoseismic investigations have predominantly focused on southeastern Korea, primarily along the Yangsan or Ulsan faults, with less attention given to other faults in the southeastern part of the Korean Peninsula. In the ongoing Korean Active Fault Mapping Project, this study specifically identifies a fault trace approximately 5.5–7 km long along the Ulsan-Dongnae Fault system named as Seokgye fault, named after the locality. Detailed paleoseismic investigations were carried out along the Seokgye fault. The fault presents a clear example of fault reactivation under the present compressional tectonic setting with a paleoearthquake during the late Pleistocene (∼121 ± 6 ka). Based on the length of the fault traced and observed displacement in the trench, Seokgye fault is capable of generating an earthquake of magnitude similar to the 2016 Gyeongju earthquake (Mw 5.8). This research enhances our understanding of the ongoing tectonic processes and Quaternary activities of fault systems in the southeastern Korean peninsula. Additionally, it contributes valuable insights for local-scale seismic hazard analysis in the area.

Ground motion models for Fourier amplitude spectra and response spectra using Machine learning techniques

AbstractOne of the main objectives in engineering seismology or in seismic hazard studies is to estimate the possible ground motion for a given earthquake scenario. In the sparse data regions mostly the ground motion models (GMM) are developed using either seismological models or hybrid empirical approaches. However, if these GMMs do not accommodate the regional seismological attributes, a large uncertainty in ground motion estimates is possible. To overcome this concern, scaling 5% damped Pseudo Spectral Acceleration (PSA) from Fourier amplitude spectra (FAS) proves to be physically consistent as it can capture both spatial and temporal characteristics of the ground motion. Hence, the present study aims to develop a GMM for PSA using FAS and significant duration as the predictor variables. However, since there are few GMMs available for FAS, the current study also aims to develop a GMM for FAS using the earthquake parameters as the predictor variables for intraplate regions. This article employs an Artificial Neural Network (ANN) to develop both GMMs using the Next Generation Attenuation (NGA)‐East database for both horizontal (Effective Amplitude spectra for FAS and RotD50 component for PSA) and vertical components. To verify the performance of the developed models, the residuals analysis and parametric studies have been performed. The parametric study shows that the GMMs can capture the magnitude and distance scaling consistent with the observations. Further, the PSA GMM compared with the global GMMs and it is observed that the predictions lie well within the median of all the available models, proving the models’ effectiveness in estimating the ground motion predictions for future data. The developed model can be used only within the considered ranges of the predicted variables such as rupture distances between [19.05–1000] km, the Mw ranges from [3.12–5.74] with Vs30 in the range of [209–2000] m/s.

Understanding seismicity and seismotectonics in a stable continental region (NW Iberian Peninsula): Implications for the nature of intraplate seismicity

Despite that earthquakes in stable continental regions (SCR) often cause more damage than interplate seismicity, they remain poorly understood. This is mainly because of the lower rate of intraplate seismicity and because of its different behaviour compared to the better-known seismicity at the plate boundary. Understand the characteristics of the intraplate seismicity is a challenge for the seismic risk studies. We study and characterise an SCR (NW Iberian Peninsula), which not only registers moderate instrumental intraplate seismicity, but also important historic seismicity and paleoseismic activity. To tackle some of the difficulties posed by intraplate seismicity, we analyse a wide and multidisciplinary data set (e.g., geological structures, seismicity, focal mechanisms, and geophysical data). Seismicity in this region is not associated with an old rift, but with inherited faults widely distributed throughout the region with a great variety of orientations. The reactivation kinematics of these faults are coherent with the current regional stresses. Instrumental seismicity is not associated with the large active faults nor with crustal limits. Seismicity is mainly clustered in swarms and sequences. Although seismic swarms present lower magnitudes, they are the most common. Based on swarms' characteristics (high b-values, upward spatiotemporal migration), reported mantellic CO2 in some thermal springs, and the reactivation of inherited steeply-dipping faults, we propose the migration of deep fluids through steeply-dipping fractured areas as the cause of the intraplate seismicity. These processes could increase the pore pressure and decrease the stresses necessary for the fault rupture in a fault-valve behaviour. In general, in intraplate context, the important control in the seismicity of the inherited fault systems favourable oriented under the current stress tensor is observed, and also the need for mechanisms that can decrease the effective stress for the fault ruptures. Mechanisms as hydrothermal fluids in arterial faults with fault-valve processes has been identified as an effective driver of intraplate seismicity, playing an important role in stability of tectonic faults. The large number and variety of these faults, that share the low strain rates in intraplate polyorogenic context, may explain the different characteristics of these intraplate regions compared with the interplate regions, as the “unanticipated” behaviour, variety of kinematics, the long quiescence periods without seismicity associated and erosion obliterating their morphotectonic expression.

Earthquake-induced landslides in Norway

Abstract. Norway is located in an intraplate setting with low-to-moderate seismicity. The mountainous landscape leads to a high level of landside activity throughout the country. Earthquake-induced landslides (EQILs) are common in seismically active areas, but there are only a few studies of EQILs in intraplate regions. We systematically analyse all earthquakes in Norway with magnitudes ≥ 4.5 in the time period 1800–2021 CE. For each event we search for reports of EQILs in the available macroseismic data and in the Norwegian landslide database. We furthermore consider precipitation data from the Norwegian Centre for Climate Services to evaluate the role of precipitation in the triggering of the identified potential EQILs. Through this approach, we identify 22 EQILs that have been triggered by eight earthquakes in the magnitude range 4.5–5.9. The events are widely distributed in northern and southern Norway. The maximum landslide distance limits and landslide-affected areas are much larger than those found in empirical studies of global datasets and are in agreement with data from other intraplate regions. For four of the earthquakes, it seems that landslide triggering was due to a combined effect of precipitation and earthquake ground shaking. Our observations confirm that intraplate earthquakes have the potential to trigger EQILs over large distances, most likely due to the low ground motion attenuation in such regions. Slope susceptibility seems to be another important factor in the triggering. Our conclusions demonstrate the importance of considering EQIL potential in earthquake risk management in intraplate regions.

Review of tectonic inversion of sedimentary basins in NE and N Brazil: Analysis of mechanisms, timing and effects on structures and relief

Most intraplate areas worldwide are under compression characterized by thrust and strike-slip deformations. The sources of the stresses in these regions are a combination of far-field (regional) stresses (e.g., ridge push, slab pull, lithospheric drag) and near-field (local) stresses (e.g., topography, lateral and vertical anisotropy of strength or elastic properties, erosion, flexural bending, overloading, buoyancy of low-density regions). These intraplate regions are sites favorable for the tectonic inversion of sedimentary basins. Here, we review the tectonic inversions of the sedimentary basins in NE and N Brazil, which comprise a significant part of the South American intraplate (the South American Platform). These regions have been under compression since the Late Cretaceous. Reports of post-rift basin inversions derived from seismic profiles, fault-slip data, and topography have been described in the Sergipe-Alagoas, Paraíba, Potiguar, Ceará, Rio do Peixe, Araripe (NE Brazil), and Solimões (N Brazil) basins. Reports of inversions in the rift phase have also been described in the Araripe Basin. The present-day stress field indicates that inversions in NE Brazil can mainly be characterized as strike-slip and thrust fault regimes, whereas in N Brazil, inversions have occurred under reverse stress regimes. The most frequent inversion structures are reactivated faults, folds, and dome structures affecting the topography and the consequent uplift of rift and post-rift sedimentary units. Inversions have led to the reactivation of continental-scale structures, such as the strike-slip dextral shearing of the Transbrasiliano lineament, which affected rift and post-rift units in the Equatorial Margin of Brazil. The small crustal thickness of these areas favors inversion, as indicated by the most recent Moho depth map of South America. The interplay between far-field and near-field stresses explains the stress pattern in NE and N Brazil, where inversions have been observed.

Correlation of paleoearthquake records at multiple sites along the southern Yangsan Fault, Korea: Insights into rupture scenarios of intraplate strike-slip earthquakes

The construction of spatiotemporal models of earthquake occurrence for intraplate areas is challenging due to the low deformation rates in these areas. In this study, we conducted paleoseismological investigations along the southern Yangsan Fault (SYF), a typical low-deformation-rate fault, on the Korean Peninsula. The SYF is distinct from the northern Yangsan Fault (NYF), and the boundary between them is located at the junction between the NNE-striking YF and another major structure, the NNW-striking Ulsan Fault (UF), which branches off from the YF. Paleoseismological trenches at four sites along the SYF indicate that this fault section has not ruptured during the Holocene, in contrast to the NYF and UF. In detail, surface ruptures along the studied section of the SYF occurred during three different time periods, as inferred from stratigraphy and radiocarbon dating: 74 to 49 ka at two sites, 39 to 35 ka at another site, and 28,000 cal yr BP (or 30 ka considering the OSL age) to 16 ka at all sites. These results suggest two alternative rupture scenarios for the timing of paleoearthquakes along the studied fault section during the Late Pleistocene: (1) full rupture along the entire studied section during each earthquake event, or (2) multiple partial ruptures along the two structurally distinguishable parts of the studied fault section, that is, the Wolsan–Miho and Inbo north–Inbo sections. We conclude that geometric discontinuities of the long-lived YF system in the Korean Peninsula intraplate region have played an important role in controlling recent spatiotemporal rupture behavior.

Site-Specific Response Spectra and Accelerograms on Bedrock and Soil Surface

This paper is aimed at serving the needs of structural engineering researchers who are seeking accelerograms that realistically represent the time histories of earthquake ground in support of their own investigations. Every record is identified with a specific earthquake scenario defined by the magnitude–distance combination and site conditions; the intensity of the presented records is consistent with ultimate limit state design requirements for important structures in an intraplate region. Presented in this article are accelerograms that were generated on the soil surface of two example class Ce sites and two example class De sites based on site response analyses of the respective soil column models utilizing bedrock excitations as derived from the conditional mean spectrum (CMS) methodology. The CMS that were developed on rock sites were based on matching with the code spectrum model stipulated by the Australian standard for seismic actions for class Be sites at reference periods of 0.2, 0.5, 1 and 2 s for return periods ranging from 500 to 2500 years. The reference to Australian regulatory documents does not preclude the adoption of the presented materials for engineering applications outside Australia. To reduce modeling uncertainties, the simulation of the soil surface ground motion is specific to the site of interest and is based on information provided by the borelogs. The site-specific simulation of the strong motion is separate to the CMS-based accelerogram selection–scaling for obtaining the bedrock accelerograms (utilizing strong motion data provided by the PEER). The decoupling of the two processes is a departure from the use of the code site response spectrum models and has the merit of reducing modeling uncertainties and achieving more realistic representation of the seismic actions.

Tectonic stress of northeastern Indian region derived from seismic focal mechanisms and the effect of focal mechanism on stress drop: a comparative analysis with Kachchh intraplate region of India

SUMMARY In this study, the focal mechanism solutions and source parameters of recent earthquakes that occurred in the northeastern region of India have been determined. The region has very complex tectonics as it is subjected to the compressional forces from all sides, due to the collision of the Indian Plate with the Eurasian, Burma and Tibetan plates. Waveform data from deployed broad-band seismographs (BBS) and strong motion accelerographs (SMA) in the northeastern India are used to determine the focal mechanism solutions and source parameters of moderate earthquakes, respectively. The estimated focal mechanisms are used to understand the existing stress field in the region. It is found that the Shillong-Plateau as well as the Indo-Burma subduction zone is dominated by the compressional tectonic regime, Mikir Hills and Bengal basin are dominated by the trans-tension tectonic regime, and the easternmost Himalayan region is dominated by the strike-slip tectonic regime. The maximum horizontal stress direction Shmax is also determined for above subregions. The direction of Shmax is southeast in the Bengal basin, northeast in Mikir Hills and Indo-Burma subduction zone whereas it is NNE in Shillong Plateau and SSW in the eastern Himalayas. The estimated stress drop value of the earthquakes in the region ranges from 2.11 to 23.89 MPa. The relationship between the source parameters and focal mechanisms is also explored. It is found that the earthquakes with a strike-slip mechanism have the highest average stress drop (7.05 MPa) followed by reverse (6.82 MPa) and normal (5.12 MPa) in the northeastern region of India. According to the examined data set, the stress drop is found to be dependent on the type of focal mechanism, seismic moment and hypocentral depths. The comparison of the results with the Kachchh intraplate region in western India shows earthquakes in Kachchh have larger mean stress drop for all types of mechanisms. In both intraplate and interplate regions of India, the stress drop of earthquakes depends on the type of focal mechanism solution.

Delineation of Paleochannel and Groundwater Resources in the Khari River Basin, Kachchh using Transient Electromagnetics

Abstract The paleochannels are riverine geomorphic features, which are either buried or shifted due to tectonic, climate, and anthropogenic activities. In general, these channels serve as potential groundwater repositories, and their identification can help in developing potable groundwater sources. Here, the results of transient electromagnetic (TEM) study carried out near and around the Khari river basin of the tectonically active Kachchh intraplate region, western India are presented. The TEM investigations at 22 sites along three traverses have been carried out in the river basin that cut the present-day river channel and its presumed paleochannel. The geoelectric sections of the three profiles decipher information down to 300-350m and suggest a multilayer aquifer system. The sub-surface model infers a deeper confined aquifer at variable depths from 100-150m associated with the river as well as its paleochannel. The paleochannel aquifer is observed to be buried under 15-20m thick unconsolidated sediments that overlie a Mesozoic sandstone layer of the Bhuj Formation. The resistivity sections across the paleochannel suggest a relatively fresh aquifer layer with higher storage capacity. A vertical bedrock offset of 100-120m and the discontinuity of lithostratigraphic layers observed from the study suggest the dominant tectonic control in the formation of the paleochannel that correlated with the location of the reactivated splay of the Katrol Hill Fault.

Transition from tholeiitic to alkali basalts via interaction between decarbonated eclogite-derived melts and peridotite

Intraplate basalts generally show a geochemical continuum from alkali to tholeiitic basalts. However, the genetic link between these two types of rocks has remained controversial. The Early Jurassic Karamay basalts in the West Junggar terrane, southern Central Asian Orogenic Belt (CAOB), erupted to form a small-volume outcrop in the stable continental intraplate region. The basalts are characterized by aphyric textures without any visible phenocrysts. Thus, they are different from the ubiquitous porphyritic-textured intraplate basalts and have a composition close to that of the mantle-derived primary melt. In contrast to chemically and petrographically well-defined alkali and tholeiitic basalts, the Karamay basalts exhibit transitional compositions spanning from alkali (with normative olivine+nepheline and normative olivine+hypersthene) to tholeiitic (with normative quartz+hypersthene), providing an important case to address the geochemical continuum of intraplate basalts. Similar to the alkali basalts in eastern China, the Karamay basalts have isotopic imprints of sedimentary carbonates, i.e., significantly lighter Mg (δ26Mg = −0.54‰ to −0.34‰) and heavier Zn (δ66Zn = 0.36–0.46‰) isotopes than the normal mantle. However, they display initial ((87Sr/86Sr)t ratios of 0.7047–0.7051, positive εNd(t) values (3.3–4.2) and positive anomalies of Nb-Ta-Ti-Zr-Hf, which are not expected in the case of incorporation of recycled carbonates. This contradiction can be reconciled by considering a decarbonation reaction between carbonates (i.e., dolomite and magnesite) and co-existing eclogite in the subducted oceanic slab at pressure >5 GPa, leaving light Mg and heavy Zn isotope signatures in the stagnant eclogite residue in the deep mantle. Combining the geochemical compositions of our samples with the geologic evidence, and considering the previous results of melt-peridotite reaction experiments, we conclude that the Karamay basalts might have originated from the interaction of silica-rich tholeiitic melt derived from the recycled decarbonated eclogite with fertile peridotite during its ascent. Our study highlights that intraplate alkali basalts, especially silica-rich ones (e.g., with SiO2>45 wt.%), can be transformed from tholeiitic melts through reaction with peridotite mantle, and demonstrates that deeply recycled oceanic crust stagnated in mantle can serve as a main source for alkaline lavas.

지형지표를 활용한 조구조활성도 평가방법 및 활용성

Geomorphic indices have been used to assess relative tectonic activity, quantify surface relief and identify geomorphic anomalies caused by blind fault and/or fold. These indices, especially related with drainage system and mountain front, can be easily recognized through aerial photo as well as high-resolution DEM. The tectonic activity even over large area is easily interpreted based on geomorphic indices in a relatively short time. Despite of these advantages there are several limitations to be considered: 1) Because this method is based on Davis’s cycle of normal erosion, effects of other factors such as rock type, climate change and anthropogenic influences are often ignored; 2) It has not been reviewed sufficiently whether it can be applied in areas with low tectonic rate such as intra-plate region or not. If the aforementioned constraints are resolved and the applicability in areas with low slip rate is confirmed, this morphometric approach can be applicable to Korean peninsula. Furthermore the results can be used to trace surface ruptures and fault segmentation along with the various data such as geophysical and geodesic data. Through the follow-up studies, we will evaluate the southeastern part of Korean peninsula where major seismogenic structures are developed.

Spatial change in differential stress magnitudes around the source fault before intraplate earthquakes

SUMMARYHow are the sizes of the earthquakes determined? To solve this important problem, we analysed the data from a dense temporary seismic observation network installed in the aftershock area of the 2016 Mw 6.2 Central Tottori earthquake, which occurred in an intraplate region in Japan. We compared the stress field estimated from approximately 10 000 accurate focal mechanisms of aftershocks with the calculated post-earthquake stress field and found that the differential stress before the earthquake was very small near both horizontal edges. These results did not depend significantly on the modeled slip distribution and the orientation of the principal stress before the earthquake. Similar results were obtained for the 2000 Mw 6.7 Western Tottori earthquake, which also occurred in the same intraplate region in Japan. These results suggest that the fault size of large intraplate earthquakes can be determined by the region of small differential stress surrounding future earthquake faults.

Source parameters and scaling relations for small earthquakes in mainland Gujarat region of Western Deccan Volcanic Province, India

The return period of large earthquakes in intraplate regions is more in comparison to the interplate regions. Also, the intraplate region is less instrumented and hence, these regions are devoid of acceleration time histories of large and great earthquakes. The mainland Gujarat region is one of such intraplate regions for which acceleration time histories are not available, however, is vulnerable for earthquakes of M ∼ 6.0. Simulation is performed in these scenarios and the prerequisite is the knowledge of source parameters and scaling laws. We analyzed the spectra of the P- and S-waves recorded by the stations of Gujarat Seismic Network for the mainland Gujarat region of western Deccan Volcanic Province, India. Additionally, we developed scaling laws based on the obtained source parameters. We analyze 177 digital records from 43 local earthquakes during the period 2007–2016 with a magnitude of Ml = 2.5–4.3, which are located in a distance range of 10–250 km, using Brune’s seismic source model (Brune 1970, 1971). According to our study, the calculated stress drop values do not vary considerably with seismic moment values and are generally between 1 and 100 bar for most of the events. Therefore, the earthquakes in this region adhere to the law of self-similarity. As widely observed for conventional earthquakes, we find an inverse relationship between seismic moment and corner frequency for these events. Based on a linear regression analysis between the assessed seismic moment (M 0) and corner frequency (fc), we developed the scaling relation M 0 f c 3 = 2.29 × 1015. Scaling relations developed here are in agreement with those developed for the nearby Kachchh region and various other seismically active areas around the world. Use of regional scaling is recommended instead of global scaling laws for better estimation of seismic hazard. The source parameters and scaling laws presented here can be used for a variety of applications in mainland Gujarat, including the assessment of seismic hazards and the development of earthquake scenarios.