Coseismic Surface Rupture Research Articles

Gempa Nusantara: A Database of 7380 Macroseismic Observations for 1200 Historical Earthquakes in Indonesia from 1546 to 1950

ABSTRACT We present a new database called Gempa Nusantara, which is a collection of 7380 macroseismic observations for 1200 historical earthquakes in Indonesia between 1546 and 1950 C.E. using the European Macroseismic Scale (1998). Scrutinizing preserved original, first-hand, private, and official documentation from the colonial period in Indonesia, we could examine the completeness of this written record based on the gradual expansion of European influence in the Indonesian Archipelago. As the largest database of uniformly assessed macroseismic intensities ever assembled for Indonesia, our database can correct errors and fill gaps in other contemporary studies of historical Indonesian earthquakes, as well as paleoseismic studies such as the coral paleogeodetic record from Sumatra. Remarkably, given the presence of several major active faults, conclusive evidence of coseismic surface ruptures during the colonial period was limited to just two events in 1909 and 1933. Our reliance on original materials also allowed us to document extreme coseismic ground failure in Sumatra in 1936 with striking similarities to those observed on Sulawesi in 2018. From the perspective of seismic hazard in a rapidly urbanizing nation, we show that the frequencies of observed intensities over the duration of our database correspond with modern seismic hazard curves from recent publications by other authors for 12 Indonesian cities, including Jakarta, with some notable exceptions such as Ambon and Yogyakarta. In summary, our work on Gempa Nusantara demonstrates how a carefully vetted and well-documented historical record not only compliments studies of seismic hazard but is also itself an important standalone tool for the study of earthquake hazards in Indonesia.

Localization of large intraplate earthquakes along faulted density-contrast boundaries: Insights from the 2017 Mw6.5 Botswana earthquake

The major controls on the localization of deep crustal intraplate earthquakes remain enigmatic due to their deep hypocentral depths and rarity of coseismic surface ruptures. Here, we investigate the 3-D crustal density structure of the 2017 Mw 6.5 Botswana earthquake epicentral region, a strong lower-crustal (∼24–29 km) event which is suspected to have reactivated a Precambrian structure via normal faulting. We perform a 3-D inversion of the gravity data using published geological constraints, then integrate the resulting density model with aftershock hypocenter distribution and use as constraints in a 3-D thermo-mechanical geodynamic model implemented in ASPECT. Our results reveal crustal blocks of density anomalies, with the aftershocks clustering along a prominent NW-trending, NE-dipping density contrast separating a high-density (>2708 kg/m3) footwall and lower-density (2670–2700 kg/m3) hanging wall blocks. Additionally, a secondary density contrast boundary in the hanging wall coincides with a splay of aftershock clusters at depth. Our observations suggest that the 2017 Mw6.5 Botswana earthquake nucleated near a fault intersection in the lower crust and is associated with brittle normal faulting reactivation of a long-lived basement fault that follows a prominent deep-reaching density contrast boundary. Further, as demonstrated by geodynamic modeling results, we argue that in regions of low tectonic loading rates, where stress perturbations are high enough, faulted crustal-scale density contrast boundaries are preferential concentrators of strain that may localize intraplate earthquakes.

Long‐Term Fluid Injection Can Expedite Fault Reactivation and Development: Riedel Shear Structures Illuminated by Induced Earthquakes in Alberta, Canada

AbstractRiedel shear structures (RSS) are often observed in the embryonic stage of strike‐slip fault development, which can be depicted in the field through outcrops and coseismic surface ruptures. It is a critical concept linking the geomechanical behavior of individual earthquakes to structural geology at both local and regional scales. However, the influence of long‐term fluid injections on the developing process of RSS, as manifested by the common occurrences of injection‐induced earthquakes, has been rarely addressed. Here we document for the first‐time subsurface RSS expedited by long‐term wastewater disposal injections in western Canada. We study an earthquake sequence consisting of 187 events (ML ranging 1.3–3.9) between 1 January 2018 and 15 July 2021 in an area without any previous seismic history. According to 31 well‐constrained focal mechanism solutions, the injection‐related earthquake sequence exhibits various faulting types with the vast majority (87%) being compatible with the background stress regime (SHmax azimuth = N38°E). The orientation of derived nodal planes collectively indicates a model of RSS that consists of four primary strike‐slip structures striking 19° (R′), 79° (R), 94° (PDZ), and 109° (P), respectively. Moreover, six fault segments delineated from the relocated local seismicity are parallel to the substructures of RSS. Mohr‐Coulomb failure analysis further suggests a cumulative stress perturbation of up to 10.0 MPa. Our observations suggest that long‐term fluid injection can expedite the development of local fault systems. Therefore, it is probably important to consider the dimension of local/regional RSS in the assessment of the overall seismic hazard due to fluid injections.

Characteristics of Co-Seismic Surface Rupture of the 2021 Maduo Mw 7.4 Earthquake and Its Tectonic Implications for Northern Qinghai–Tibet Plateau

A magnitude (Mw) 7.4 Maduo earthquake occurred on 22 May 2021 in the northern Qinghai-Tibet Plateau, with predominantly left-lateral strike-slip faulting and a component of normal faulting within the Bayan Har Block. The co-seismic surface rupture extended in a NWW direction for ~160 km with a complicated geometry along a poorly known young fault: the Jiangcuo Fault. The main surface rupture propagated bilaterally from the epicenter and terminated eastward in horsetail splays. The main rupture can be divided into five segments with two rupture gaps. Field surveys and detailed mapping revealed that the co-seismic surface ruptures were characterized by a series of left-lateral offsets, en echelon tensional cracks and fissures, compressional mole tracks, and widespread sand liquefication. The observed co-seismic left-lateral displacements ranged from 0.2 m to ~2.6 m, while the vertical displacements ranged from 0.1 m to ~1.5 m, much lower than the InSAR inverse slip maximum of 2–6 m. Based on the comprehensive analysis of the causative fault geometry and the tectonic structure of the northern Bayan Har Block, this study suggests that the multiple NWW trending sub-faults, including the Jiangcuo Fault, developed from the East Kunlun fault northeast of the Bayan Har Block could be regarded as the sub-faults of the East Kunlun Fault system, constituting a broad and dispersive northern boundary of the Block, controlling the inner strain distribution and deformation.

High‐Resolution Seismic Imaging of Fault‐Controlled Basins: A Case Study From the 2009 Mw 6.1 Central Italy Earthquake

AbstractWe present the first seismic reflection images of the Paganica and Bazzano basins, two tectonic basins developed in the hanging wall of the Paganica‐San Demetrio Fault System, the causative fault of the 2009 Mw 6.1 L'Aquila earthquake, Italy. Five high‐resolution seismic profiles were acquired along a main, 7 km long transect cutting across the strands of an active fault system in urbanized areas with widespread sources of seismic noise. Three processing approaches were chosen to tackle a variable and site‐dependent data quality . To aid interpretation of this complex setting, we complemented seismic amplitude images with energy and similarity attributes as well with post‐stack acoustic impedance inversion. The final seismic sections expose, with unprecedented resolution, the basins' structure and the uppermost splays of the 2009 earthquake. The seismic data show fine details of the subsurface stratigraphic setting, revealing continental depocenters carved in the marine Meso‐Cenozoic substratum and displaced by a series of conjugate normal faults, mostly unknown before this study. Several of the imaged fault strands connect to the 2009 coseismic surface ruptures. Matching the seismic interpretation with constraints from surface geology and shallow boreholes, published data from field surveys and scientific drilling, we present a structural map of the Bazzano and Paganica basins with an estimation of the depth of the Meso‐Cenozoic substratum. This map highlights a different structure, evolution, and age of the two basins, with the older Bazzano basin that likely began to form in late Pliocene.

Environmental effects and seismogenic source characterization of the December 2020 earthquake sequence near Petrinja, Croatia

SUMMARYOn 29 December 2020, a shallow earthquake of magnitude Mw 6.4 struck northern Croatia, near the town of Petrinja, more than 24 hr after a strong foreshock (ML 5). We formed a reconnaissance team of European geologists and engineers, from Croatia, Slovenia, France, Italy and Greece, rapidly deployed in the field to map the evidence of coseismic environmental effects. In the epicentral area, we recognized surface deformation, such as tectonic breaks along the earthquake source at the surface, liquefaction features (scattered in the fluvial plains of Kupa, Glina and Sava rivers), and slope failures, both caused by strong motion. Thanks to this concerted, collective and meticulous work, we were able to document and map a clear and unambiguous coseismic surface rupture associated with the main shock. The surface rupture appears discontinuous, consisting of multi-kilometre en échelon right stepping sections, along a NW–SE striking fault that we call the Petrinja-Pokupsko Fault. The observed deformation features, in terms of kinematics and trace alignments, are consistent with slip on a right lateral fault, in agreement with the focal solution of the main shock. We found mole tracks, displacement on faults affecting natural features (e.g. drainage channels), scarplets and more frequently breaks of anthropogenic markers (roads, fences). The surface rupture is observed over a length of ∼13 km from end-to-end, with a maximum displacement of 38 cm, and an average displacement of ∼10 cm. Moreover, the liquefaction extends over an area of nearly 600 km2 around the epicentre. Typology of liquefaction features include sand blows, lateral spreading phenomenon along the road and river embankments, as well as sand ejecta of different grain size and matrix. Development of large and long fissures along the fluvial landforms, current or ancient, with massive ejections of sediments is pervasive. These features are sometimes accompanied by small horizontal displacements. Finally, the environmental effects of the earthquake appear to be reasonably consistent with the usual scaling relationships, in particular the surface faulting. This rupture of the ground occurred on or near traces of a fault that shows clear evidence of Quaternary activity. Further and detailed studies will be carried out to characterize this source and related faults in terms of future large earthquakes potential, for their integration into seismic hazard models.

Retraction Note to: Structural features and seismotectonic implications of coseismic surface ruptures produced by the 2016 Mw 7.1 Kumamoto earthquake

Retraction Note to: Structural features and seismotectonic implications of coseismic surface ruptures produced by the 2016 Mw 7.1 Kumamoto earthquake

High-Detail Fault Segmentation: Deep Insight into the Anatomy of the 1983 Borah Peak Earthquake Rupture Zone (Mw 6.9, Idaho, USA)

Abstract Following observations made in a survey campaign along the Lost River Fault (Idaho, USA) in 2019, we integrate both original and previously published data to obtain a detailed segmentation of the fault sections that failed in the 1983 Borah Peak earthquake (Mw 6.9). The earthquake ruptured the topographic surface with an oblique-normal faulting mechanism, activating two SW-dipping fault segments (Thousand Springs and Warm Springs) and a branching SSW-dipping fault (Arentson Gulch Fault) and producing coseismic surface ruptures with up to 3 m of vertical separation. We augment the 1983 earthquake description by interpreting high-resolution topography and fault mapping. We use quality vertical separation data, rupture zone width measurements, and fault slip data to analyze major and minor structural-geometric complexities, highlighting a partition of the deformation and a fault segmentation into four detail levels (i.e., segments, sections, subsections, and sectors). Our work provides new details of the 1983 Borah Peak earthquake, constraints for paleoseismic and seismotectonic studies, and a methodological approach applicable in other areas of the world. Our fault-slip data show variations along fault-strike that we interpret as kinematic partitioning. In 1983, the main southern segment had a large rupture zone width, while the northern segment localized the deformation. The distributed ruptures accommodate a large portion of the rupture length (~19.5 km versus 31 km for the main rupture) and displacement (~66%). 83% of the surface faulting and 80% of the displacement are located at the hanging wall of the main rupture. There is a strong correlation between vertical separation, rupture zone width, rupture position (footwall or hanging wall), and fault geometry. We highlight the control of the obliquity and kinematic partitioning in the surface expression of the earthquake propagation. We interpret the coseismic (i.e., 1983) and long-term (i.e., Quaternary) behavior, showing that the two activated segments had similar cumulated behaviors in distributing the deformation between synthetic and antithetic ruptures, despite the different geometries. Our results have implications for fault rupture behavior with application to rupture hazard.

Coseismic surface rupture characteristics and earthquake damage analysis of the eastern end of the 2021 MS 7.4 Madoi (Qinghai) earthquake

At 02:04 on May 22, 2021, an MS 7.4 earthquake occurred in Madoi County in Qinghai Province, China. This earthquake is the largest seismic event in China since the 2008 MS 8.0 Wenchuan earthquake. Thus, it is critical to investigate surface deformation and damage in time to accurately understand the seismogenic structure of the Madoi earthquake and the seismogenic capacity of the blocks in this region. This study focuses on the Xuema Village, located at the eastern end of the coseismic surface ruptures produced by the event, and assesses the deformation and seismic damage in this area based on field surveys, UAV photogrammetry, and ground penetrating radar (GPR). The results indicate that the rupture scale is substantially smaller at the eastern end of the rupture zone compared to other segments. En echelon type shear tensile fractures are concentrated in a width range of 50–100 m, and the width of single fractures ranges from 20 to 30 cm. In contrast, the degree of seismic damage significantly increases at this site. All of the brick and timber houses are damaged or collapsed, while the steel frame structures and the color steel houses are slightly damaged. More than 80% of the bridge decks on the Changma River Bridge collapse, similar to the terraces along the Youerqu and Changma Rivers and the cut slopes of Provincial Highway S205. We infer that the seismogenic fault of the Madoi earthquake exerts a tail effect in this segment. The tension zone has led to a reduction at the eastern end of the rupture zone, causing shaking damage. Local topography and buildings without earthquake-resistant construction along the strike of the rupture zone have undergone different levels of seismic damage.

Coseismic surface ruptures, slip distribution, and 3D seismogenic fault for the 2021 Mw 7.3 Maduo earthquake, central Tibetan Plateau, and its tectonic implications

The 2021 Mw 7.3 Maduo earthquake, which hit the high mountain area of the internal Bayan Har Block in the central Tibetan Plateau, generated a ~ 154 km-long, nearly E-W-striking surface rupture zone. Field observations indicate prominent surface breaks characterized by discontinuous shear faults, en echelon tensional fissures, and mole track structures, revealing a dominantly left-lateral strike-slip motion with a maximum sinistral offset of ~2.7–2.8 m and general offset of ~1.0–1.5 m. The west part of the Maduo surface rupture zone occurred along the pre-existing Jiangcuo fault. Its east portion cut through some WNW-trending tectonics, indicating a previously unknown fault. The epicenter and slip distribution show a bilateral rupture for the Maduo quake. The complex 3D geometry of the Maduo seismogenic fault has controlled the occurrence of aftershocks and the multimodal distribution of coseismic displacements. The abnormally long Maduo surface rupture zone suggests that the rupture area-magnitude relationship might be more applicable than the rupture length-magnitude relationship for the magnitude estimation of a paleo-event. The occurrence of the Maduo earthquake challenges the active block model which used to predict little or no slip partitioning and large earthquakes in the block interior. A reappraisal of seismic hazard along intra-block faults with low strain rates is needed, especially across densely populated regions.

Co-Seismic Surface Ruptures of the CE 1738 M 7.6 Dangjiang Earthquake Along the NW Continuation of the Xianshuihe Fault Zone and Tectonic Implications for the Central Tibetan Plateau

The role of large-scale strike-slip faults in high-elevation areas in absorbing the strain resulting from plate convergence has yet to be scientifically understood. The Dang Jiang Fault (DJF), as the NW continuation of the Xianshuihe Fault Zone (XFZ) in the central Qinghai-Tibetan Plateau, may provide an excellent testing ground for this question, given its high slip rate, sparse vegetative cover, minimal modification, and possible relationship with the CE 1738 Dangjiang destructive earthquake. However, co-seismic surface ruptures and seismotectonics remain in dispute because of inconvenient transportation and lack of oxygen at high altitudes. Thus, field investigations are conducted here to determine co-seismic surface ruptures. The newly synthesized data from geologic observations, historical record reviews, geomorphic mapping, trench logging, and sample dating indicate that the CE 1738 Dangjiang earthquake produced an ∼100 km-long surface rupture that includes offsets of gullies, linear scarps and troughs, sag ponds,en echelonfractures, and pressure ridges. The magnitude is re-estimated asM7.6, with average and maximum strike-slip displacements of ∼2.1 ± 0.1 m and ∼3.5 ± 0.1 m, respectively. The DJF has undergone multiple seismic faulting events, and the linear fitting surface displacement rate in the Holocene is ∼6.3 ± 1.9 mm/yr with a 95% confidence interval. This study implies that the seismic hazard of the DJF cannot be underestimated given that its elapsed time is close to or beyond the recurrence interval of major earthquakes and that the oblique convergence of the Qiangtang Block might be accommodated by the clockwise rotation of the block through repeated left-lateral strike-slip movements along the southern boundary of the Bayan Har Block.

Analysis of the shallow crustal structure of the Bam Fault Zone, Iran, using the surface wave and first-arrival P-wave tomography approaches

The crustal structure around the Bam fault located southeast of Iran was investigated by applying surface and P-wave tomography to locally recorded aftershocks of the destructive Bam earthquake (Mw 6.6) on December 26, 2003. The aftershock dataset was recorded for 1 month after the main shock by 23 temporary seismic stations operated by the International Institute of Earthquake Engineering and Seismology (IIEES). After applying the selection criteria, 331 well-located events, including more than 1700 waveforms, remained. By calculating the surface wave dispersion measurements in the period range of 0.6–5 s, a tomographic procedure was then performed, and the local dispersion curves were extracted for each geographic grid point (1.8 km × 1.8 km). We applied an iterative damped least-squares inversion procedure to obtain the VSV and VSH models at depths of up to 6.5 km. The discrepancy between these VSV and VSH models also resulted in the radial anisotropy model. Moreover, the P-wave first-arrival times were inverted by the computer program SIMULPS to calculate the VP model for a depth range of 6.5–14 km, while the calculated VP model does not have sufficient resolution at depths of up to 6 km. The tomographic results indicate that a ~ 2 km thick upper sedimentary layer with a shear wave velocity of less than 2.4 km/s and relatively negative radial anisotropy covers the study area (except Bam City, with a thin 500 m positive radial anisotropy layer), which overlay on relatively positive radial anisotropy. Our results indicate that the main shock occurred on the Bam fault escarpment; then the rupturing was shifted to the co-seismic surface rupture (approximately 5 km west of the escarpment) at a depth of ~8–10 km. This dividing of energy on the fault plane (an oblique Y shape) leads to damping most of the energy as a deformation in the lower sedimentary layer.

Slip rate deficit partitioned by fault-fold system on the active Haiyuan fault zone, Northeastern Tibetan Plateau

This study focuses on the complexity of fault-fold system developed in the Hasishan section of the Haiyuan fault zone in the northeastern margin of the Tibetan Plateau, along which a 237-km-long coseismic surface rupture zone formed during the 1920 M 8.5 Haiyuan earthquake. Interpretations of satellite imageries combined with field investigations and structural analyses of fault geometry and kinematics reveal that the fault-fold system developed in the Hasishan section are characterized by four main elements (i) the left-lateral strike-slip Haiyuan primary fault F1; (ii) a discrete branch fault F2 with an identical sense of movement; (iii) a tension structure, characterized by a NE-trending normal fault F3 developed in piedmont terrace deposits; and (iv) a compression structure P, characterized by a NW-trending en echelon folds within the Cretaceous–Cenozoic sedimentary sequences including the most recent late Pleistocene–Holocene unconsolidated and/or weakly-consolidated deposits. Previous geologic and geophysical observations show that the slip rate along the whole Haiyuan fault zone is non-uniform, indicating an eastward decrease with a gradient of ∼2.65 mm/100 km. In contrast, the slip rate for the primary fault F1 in the Hasishan section was reported to be 3.2 ± 0.2 mm/yr, ∼3 mm/yr less than the result of 6.2 mm/yr inferred from the slip rate gradient. The finding in this study demonstrates that the fault-fold system consisting of faults F2, F3, and fold P has partitioned ∼3 mm/yr slip within the Hasishan section of the Haiyuan fault zone. This study documents that the slip rates at different sections along the primary strike-slip faults are partially dependent on local geometric structure and fault complexity, and the recognition and analyses of related fault-fold system within large strike-slip fault zone are important for better characterizing the total slip rate budget across primary strike-slip faults, which will contribute to improve seismic hazard assessments.

Typical Riedel shear structures of the coseismic surface rupture zone produced by the 2021 Mw 7.3 Maduo earthquake, Qinghai, China, and the implications for seismic hazards in the block interior

Large earthquakes in the Tibetan Plateau generally occur on block-boundary faults and produce coseismic surface ruptures that indicate the sense of block movement. However, seismic hazard in the block is less concerned. The Mw 7.3 Maduo earthquake on 22 May 2021 hit the internal Bayan Har block, which provides a rare opportunity to study the deformation features of the intra-block tectonics in central Tibet and related kinematics implications. We developed the high-resolution survey via the DJI drone of Phantom 4 Pro RTK and detailed field observations, to sum up the typical characteristics of the Maduo coseismic surface rupture zone. Then we classified these surface ruptures based on rupture orientation, spatial distribution, and stress state and analyzed their kinematic mechanisms. Our results indicate that (1) the typical surface ruptures of the Maduo earthquake are shear fractures, tensional cracks, and mole tracks, which follow the Riedel shear model; (2) the Riedel shear structures reveal a sinistral strike-slip movement sense for the seismogenic fault, accordant with regional tectonics; (3) seismic potential on minor faults in the interior block should be emphasized highly. This study will not provide insights into the kinematics of surface ruptures on a strike-slip fault but will strengthen the understanding of seismic hazards on an intra-block active fault.

Holocene surface-rupturing earthquakes on the Dinaric Fault System, western Slovenia

Abstract. The Dinaric Fault System in western Slovenia, consisting of NW–SE-trending, right-lateral strike-slip faults, accommodates the northward motion of Adria with respect to Eurasia. These active faults show a clear imprint in the morphology, and some of them hosted moderate instrumental earthquakes. However, it is largely unknown if the faults also had strong earthquakes in the late Quaternary. This hampers our understanding of the regional tectonics and the seismic hazard. Geological evidence of co-seismic surface ruptures only exists for one historical event, the 1511 Idrija earthquake with a magnitude of ∼ M 6.8, but the causative fault is still disputed. Here we use geomorphological data, near-surface geophysical surveys, and paleoseismological trenching to study two of these faults: the Predjama Fault and the Idrija Fault. In a paleoseismological trench across the Predjama Fault we found deformation features that may have been caused by an earthquake between 13–0.7 ka, very likely not earlier than 8.4 ka. At the Idrija Fault, a surface-rupturing earthquake happened around 2.5 ka. We show that instrumental and historical seismicity data do not capture the strongest events in this area.

Co-Seismic Surface Rupture and Recurrence Interval of Large Earthquakes along Damaoyaba-Litang Segment of the Litang Fault on the Eastern Margin of the Tibetan Plateau in China

Co-Seismic Surface Rupture and Recurrence Interval of Large Earthquakes along Damaoyaba-Litang Segment of the Litang Fault on the Eastern Margin of the Tibetan Plateau in China

Characterizing the reactivation mechanisms of coseismic surface ruptures associated with the 2011 Mw 6.7 Fukushima-ken Hamadori earthquake in Japan through borehole hydromechanical testing

The reactivation mechanisms of coseismic surface ruptures associated with the 2011 Mw 6.7 Fukushima-ken Hamadori earthquake in Japan are investigated using in-situ controlled hydraulic injections in subsurface boreholes. Two fault segments were selected for reactivation studies, one across a coseismic rupture, the Shionohira site, and one across a non-coseismically ruptured segment, the Minakami-kita site. A series of water injections in sealed sections of boreholes set across the fault progressively bring the fault to rupture by a step-by-step decrease of the effective normal stress clamping the fault. While the fault is rupturing during these hydraulic stimulations, borehole displacements, fluid pressure and injection flowrate are continuously monitored. Then, the tests were analyzed using fully coupled hydromechanical modeling. The model was calibrated on field data, and a parametric study was conducted to examine the modes of fault reactivation. Coseismic surface rupture of the Shionohira fault showed a pure dilatant slip response to hydraulic tests, while the tectonically un-activated Itozawa fault (South) indicated a complex hybrid response to tests related to both a higher frictional and cohesive strengths of the fault. The analysis of the induced Shionohira slip event showed that it is reasonably modeled as a Coulomb rupture with an eventual dependency of friction on slip velocity, in good accordance with laboratory-derived rate-and-state friction data on the Shinohira gouge samples. In contrast, the Itozawa fault reactivation mechanism appears dominated by tensile failure with limited Coulomb shear failure. Thus, the applied protocol proves to be able to isolate significant differences in fault physical properties and rupture mechanisms between two segments of the same fault system, opening perspectives to better assess near-surface rupture effects, and therefore the safety of structures such as dams or nuclear waste repositories subject to large earthquakes.

The March 2021 Damasi Earthquake Sequence, Central Greece: Reactivation Evidence across the Westward Propagating Tyrnavos Graben

On 3 March 2021, a strong shallow earthquake affected northern Thessaly, Greece, with an epicenter close to Damasi village causing significant destruction of many stone houses. In this contribution, we provide fieldwork observations, satellite radar interferometry, mapping of the active faults exposed in the epicentral area, liquefactions and coseismic surface ruptures, and preliminary geomorphological analyses of the epicentral area. The geomorphological analysis is based on air photographs, digital surface models analysis, Real-Time Kinematik (RTK) measurements with Global Navigation Satellite System (GNSS) receivers, and data from UAV flight campaigns. Although the seismotectonic setting of the area is complex and there is an apparent mismatch between field and interferometric data, the results of our investigations suggest that at least three fault segments were reactivated by the major shocks of the March seismic sequence. These tectonic structureslikely represent the westward propagation of the Tyrnavos Graben, where newly formed and inherited low-angle faults interplay in a complex manner.

The 2021 M<sub>w</sub> 6.7 Khankh earthquake in the Khuvsgul rift, Mongolia

A Mw 6.7 occurred at Lake Khuvsgul in northwestern Mongolia at 05:32:56 AM Ulaanbaatar time on the 12th of January 2021. The epicenter of the event was offshore south of the Doloon Uul peninsula around 30 km SSW of Khankh village. Shaking was felt within most of central and western Mongolia, including the capital city Ulaanbaatar ~600 km from the epicenter. The earthquake appears to have ruptured the Khuvsgul fault along the western coast of Lake Khuvsgul. The earthquake is the largest in Mongolia since the Mw 6.3 Busiin Gol earthquake in 1991. Our research team from the Institute of Astronomy and Geophysics, Mongolian Academy of Sciences visited the epicenter area for 5 days soon after the earthquake and installed four broad-band seismic stations and searched the area for geological evidence of the earthquake. The location, azimuth, dip and depth of this earthquake defined by moment tensor solutions calculated by the international seismological centers and analysis of InSAR interferograms and field observations. The projected intersections of the east-dipping nodal planes with the surface for solutions of the international seismological centers and researchers correlate relatively well with the mapped strike and location of the old tectonic scarp of the Khuvsgul fault although we have not discovered any primary co-seismic surface rupture. The InSAR interferogram demonstrates the sharp discontinuity and fringes in the area between the Western Range and Doloon Uul peninsula which implies surface deformation. Aftershocks that have continued during the three months subsequent to the earthquake define overall strike of the mainshock rupture.

Primary and Secondary Environmental Effects Triggered by the 30 October 2020, Mw = 7.0, Samos (Eastern Aegean Sea, Greece) Earthquake Based on Post-Event Field Surveys and InSAR Analysis

On 30 October 2020, an Mw = 7.0 earthquake struck the eastern Aegean Sea. It triggered earthquake environmental effects (EEEs) on Samos Island detected by field surveys, relevant questionnaires, and Interferometric Synthetic Aperture Radar (InSAR) analysis. The primary EEEs detected in the field comprise coseismic uplift imprinted on rocky coasts and port facilities around Samos and coseismic surface ruptures in northern Samos. The secondary EEEs were mainly observed in northern Samos and include slope failures, liquefaction, hydrological anomalies, and ground cracks. With the contribution of the InSAR, subsidence was detected and slope movements were also identified in inaccessible areas. Moreover, the type of the surface deformation detected by InSAR is qualitatively identical to field observations. As regards the EEE distribution, effects were generated in all fault blocks. By applying the Environmental Seismic Intensity (ESI-07) scale, the maximum intensities were observed in northern Samos. Based on the results from the applied methods, it is suggested that the northern and northwestern parts of Samos constitute an almost 30-km-long coseismic deformation zone characterized by extensive primary and secondary EEEs. The surface projection of the causative offshore northern Samos fault points to this zone, indicating a depth–surface connection and revealing a significant role in the rupture propagation.