Quaternary Faults Research Articles

Laterally Varying Crustal and Quaternary Fault-Zone Structures in the Seoul Metropolitan Area, South Korea, from a Joint Inversion Analysis Based on Dense Seismic Arrays

ABSTRACTThe crustal structure and Quaternary fault zone in the Seoul metropolitan area are investigated using a joint inversion that combines the receiver functions with surface wave dispersion based on seismic data from densely deployed seismic arrays. The uncertainty in inverted models is examined through a set of synthetic experiments. Synthetic tests and comparisons with other methods verify the observed crustal features. The Moho depths range between 28.7 and 32.8 km with relatively shallow depths in the central Seoul metropolitan area. The VP/VS ratio varies between 1.67 and 1.85. A Conrad discontinuity with a weak velocity increase is present at a depth of ∼20 km. Low-velocity anomalies appear at depths of ∼3 km and 6–11 km along the Chugaryeong fault zone and fault junction area. Apparent shear wave velocity contrasts appear across the fault zone where earthquakes are clustered. A high-velocity block in the western Seoul metropolitan area lies along the fault zone that runs across western Seoul. The shear wave velocity models and seismicity suggest that the fault zone may extend to a depth of 11 km at least.

Fault Zone Imaging With Distributed Acoustic Sensing: Surface‐To‐Surface Wave Scattering

AbstractFault zone complexities contain important information about factors controlling earthquake dynamic rupture. High‐resolution fault zone imaging requires high‐quality data from dense arrays and new seismic imaging techniques that can utilize large portions of recorded waveforms. Recently, the emerging Distributed Acoustic Sensing (DAS) technique has enabled near‐surface imaging by utilizing existing telecommunication infrastructure and anthropogenic noise sources. With dense sensors at several meters' spacing, the unaliased wavefield can provide unprecedented details for fault zones. In this work, we use a DAS array converted from a 10‐km underground fiber‐optic cable across Ridgecrest City, California. We report clear acausal and coda signals in ambient noise cross‐correlations caused by surface‐to‐surface wave scattering. We use these scattering‐related waves to locate and characterize potential faults. The mapped fault locations are generally consistent with those in the United States Geological Survey Quaternary Fault database of the United States but are more accurate than the extrapolated ones. We also use waveform modeling to infer that a 35 m wide, 90 m deep fault with 30% velocity reduction can best fit the observed scattered coda waves for one of the identified fault zones. These findings demonstrate the potential of DAS for passive imaging of fine‐scale faults in an urban environment.

QUaternary fault strain INdicators database - QUIN 1.0 - first release from the Apennines of central Italy

We present QUIN, a “QUaternary fault strain INdicators database”, designed to integrate and unify published and unpublished local-scale geological information and derive strain parameters for structural and seismotectonic analyses. It provides data on 3339 Fault Striation Pairs (FSP; fault plane and slickenline), distributed within 455 survey sites. These are exposed along the intra-Apennine Quaternary extensional faults of Central Italy. The area covers an extent of ~550 km in a NW-SE direction. We give information on FSP location, attitude and kinematics, and deformation axes. We also provide an original shapefile of the faults hosting the FSP. A large amount of homogeneously distributed Quaternary fault/slip data help to clarify and implement the contemporary geometric and kinematic deformation pattern of Central Italy that appears scattered and incomplete whenever exclusively derived from earthquake data. The high-density of structural data can help investigate stress pattern heterogeneities at local scales, with relevance for new generations of hazard assessment evaluation and a better understanding of rupture propagation and related barriers.

Investigation of quartz luminescence properties in bedrock faults: Fault slip processes reduce trap depths, lifetimes, and sensitivity

Quantitative constraints on the timing and temperatures associated with Quaternary fault slip inform earthquake mechanics and seismic hazard analyses. Optically stimulated luminesce (OSL) and thermoluminescence (TL) are tools that can provide these constraints from fault gouge and localized slip surfaces. This study investigates the quartz luminescence properties of five 2-mm-thick slices of rock as a function of distance perpendicularly from a discrete, m-scale mirrored fault surface that cuts quartz-rich conglomerate along the Hurricane fault, UT, USA. We use pulsed annealing linearly modulated OSL experiments to determine the response of OSL signals to annealing temperatures. Results were used to estimate trap depths and trap lifetimes. We also calculated changes in OSL and TL sensitivity across the fault-perpendicular transect. All five subsamples show a strong initial fast component peak following annealing steps of 200–300 °C, which is absent following higher pre-heat steps of 320–420 °C. The fast component trap lifetimes and depths indicate they are stable over the Quaternary and suitable for OSL dating. Data exhibit increasing trap depth, trap lifetime, and sensitivity with distance from the fault surface. We suggest mechanical processes, fluids, and/or elevated temperatures during seismicity work constructively to transform fault materials and affect the quartz luminescence properties at a mm-scale from this fault surface. Results highlight the importance of assessing the scale of fault-related impacts on host rock and luminescence properties when applying trapped-charge techniques to recover fault-slip chronologies and/or paleotemperature information.

Fault intersections control short period intraplate start-stop seismicity in the Korean Peninsula

After the devastating Tohoku-Oki earthquake (Mw 9.0, 2011) in Japan, the Korean Peninsula has experienced a higher number of large, plus Mw 5.0, earthquakes than recorded in the preceding half century of modern monitoring. In addition, seismicity has dramatically increased along with seismic waves arriving later than prior to the 2011 Tohoku-Oki earthquake, suggesting that the Korean crust has notably been perturbed. South Korea is densely populated, hence knowledge about active faults and earthquake mechanisms is of great relevance for public safety and risk mitigations. Quaternary faults, including the Chugaryeong crustal-scale fault, run through the Seoul metropolitan area and recent seismicity studies suggest that these faults are active. Based on two reflection seismic profiles, we provide compelling evidence that the depth clustered seismicity along the Chugaryeong fault is associated with the intersections of other fault systems. The two seismicity clusters, observed at two depth intervals of approximately 4.5–5 and 8–9 km, can be linked with two moderately-to-steeply-dipping bands of reflectivity interpreted to be splay faults and terminating at the Chugaryeong sub-vertical fault. We suggest that stress builds up at these fault intersections and is then released via strike-slip ruptures along the Chugaryeong fault. Time-clustered seismic events at the fault intersections support this hypothesis, indicating a start-stop mechanism is controlling the seismicity in the region at least based on nearly one decade of seismicity observations. The start-stop seismicity behaviour can possibly be used for forecasting earthquakes and their switching depth along the Chugaryeong fault.

Looking Into the Entanglement Between Karst Landforms and Fault Activity in Carbonate Ridges: The Fibreno Fault System (Central Italy)

The entanglement between active tectonics and karst systems is well-known in the literature. Karst systems are sound recorders of continental deformation in terms of brittle structures and seismic features and have been successfully used as markers for reconstructing tectonic stresses and assessing preferential directions of increased permeability in oil and gas fields. Karst systems could also be exploited to evaluate the past activity of faults bounding karst hydrostructures, thus providing useful data for the assessment of the seismic hazard of a specific area. In this work, we look into the complex relationship among karst development, recent tectonics and groundwater flow, which appear to be strongly interconnected with each other, to assess the activity of faults bounding karst hydrostructures. We focused our attention on an active karst area located in the Mesozoic and Cenozoic carbonate reliefs of the Italian central Apennines. In this context, the morphological and morphometric features of the karst landforms (dolines, dry valleys, and cave entrances), identified with geomorphological surveys, and their mutual relationship with fractures and fault segments, identified employing geostructural analysis, document stasis and deepening events in karst evolution. Such events are related to changes in the groundwater table and the consequent variation of the paleokarst base level associated with the Quaternary fault activity. A comprehensive evaluation of the evolution of karst systems at local and regional scales, considering the hydrogeological influence on base levels, allows us to use karst landforms as a proxy to unravel fault activity and evolution in Italy and in other similar karst environments.

Correlation Dimension in Sumatra Island Based on Active Fault, Earthquake Data, and Estimated Horizontal Crustal Strain to Evaluate Seismic Hazard Functions (SHF)

This study intends to evaluate the possible correlation between the correlation dimension (DC) and the seismic moment rate for different late Quaternary active fault data, shallow crustal earthquakes, and GPS on the island of Sumatra Probabilistic Seismic Hazard Analysis (PSHA). The seismicity smoothing was applied to estimate the DC of active faults (DF) and earthquake data (DE) and then to correlate that with the b-value, which will be used to identify seismic hazard functions (SHF) along with the Sumatra Fault Zone (SFZ). The seismicity based on GPS data was calculated by the seismic moment rate that is estimated based on pre-seismic horizontal surface displacement data. The correlation between DF, DE, and the b-value was analyzed, and a reasonable correlation between the two seismotectonic parameters, DF-b, and DE-b, respectively, could be found. The relatively high DC coincides with the high seismic moment rate model derived from the pre-seismic GPS data. Furthermore, the SHF curve of total probability of exceedance versus the mean of each observation point’s peak ground acceleration (PGA) shows that the relatively high correlation dimension coincides with the high SHF. The results of this study might be very beneficial for seismic mitigation in the future.

Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

AbstractThorough understanding of seabed fluid flow system is of great significance to geohazard identification and hydrocarbon exploration as it can reshape the seabed and act as an indicator of subsurface hydrocarbon resources. For the first time, an integrated study of side‐scan sonar, single‐ and multi‐channel seismic data and magnetic data reveals a complex fluid flow system composed of various seafloor expressions (i.e. pockmarks and mounds) and shallow fluid migration pathways in the central‐west Bohai Sea off northeast China. Gas chimneys, mud diapirs and a dense network of Quaternary faults are the main fluid migration pathways in the shallow subsurface. The gas chimneys can be classified into three categories (Type A formed by relatively rapid gas escape, Type B formed by episodic fluid expulsion and Type C formed by fluid escape from mud diapirs), based on their distribution and seismic character, implying variability in the formation processes. Sediment remobilization and basement‐involved faults contribute to deep fluid migration into shallow depths. As a seal for up‐moving fluids, the nature and thickness of Holocene marine sediments generally decide the permeability and overburden pressure that may control the distribution of pockmarks and mounds since they are almost distributed above relatively thin Holocene deposits (thickness <20 m) and localized coarse surface sediments. The results of the interpretation gain an improved understanding of the geological processes controlling the genesis and spatial distribution of gas chimney formation and show the significance of gas chimney classification. The distribution pattern of different types of gas chimneys may signify the difference of geological background and fluid flow process, like fluid migration through faults or flow of mobilized sediments, that is crucial for the evaluation of global petroleum systems and Carbon Capture and Storage studies.

Structure of the upper crust at the axis segmentation stage of rift evolution as revealed by gravity data: Case study of the Gedemsa magmatic segment, Main Ethiopian Rift

The upper crustal structure of the individual tectono-magmatic segments within the Main Ethiopian Rift is poorly understood. The Gedemsa tectono-magmatic segment (GTMS) is considered to be within an intermediate stage of rift development with magma-assisted rifting contributing to overall extension. The interpretation of gravity data from Global Gravity Model plus2013 has provided an insight on the nature and structure of the shallow upper crust beneath the GTMS within the Central Main Ethiopian Rift. A detailed investigation of the structure of the shallow upper crust based on 2D gravity models, 3D geologic interpretation maps, and residual and upward continued gravity anomaly maps indicate the occurrence of dense, mafic intrusions at depths between 10-20 km and 5–10 km beneath the Tullu Moye-Gedemsa and Boku volcanic centers, respectively. The gravity data indicate at deeper depths, the GTMS (25 km wide and 60 km long) is elongated parallel to the Main Ethiopian Rift axis. Our 2D gravity modeling indicates that the Quaternary faults within the Wonji Fault Belt are closely associated with the dense magmatic intrusions as the faults aid in transporting melt from the deeper crustal melt areas to the shallow magma chambers.

Late Quaternary faulting in the southern Matese (Italy): implications for earthquake potential and slip rate variability in the southern Apennines

Abstract. We studied the Gioia Sannitica active normal fault (GF) along the southern Matese fault (SMF) system in the southern Apennines of Italy in detail. The current activity of the fault system and its potential to produce strong earthquakes have been underestimated so far and are now defined here. Precise mapping of the GF fault trace on a 1:20 000 geological map and point and line data on the geometry, kinematics, and slip rate of the faults forming the SMF system are made available in electronic format. The GF, and the entire fault system along the southern Matese mountain front in general, is made of slowly slipping faults with a long active history revealed by the large geologic offsets, mature geomorphology, and complex fault patterns and kinematics. Present activity has resulted in late Quaternary fault scarps resurrecting the foot of the mountain front and Holocene surface faulting. The resurrected mountain front indicates variation in slip rate through time. The slip rate varies along-strike, with a maximum Upper Pleistocene–Holocene slip rate of ∼ 0.5 mm yr−1. Activation of the 11.5 km long GF can produce up to M 6.2 earthquakes. If activated together with the 18.5 km long Ailano–Piedimonte Matese fault (APMF), the seismogenic potential would be M 6.8. The slip history of the two faults is compatible with a contemporaneous rupture. The observed Holocene displacements on the GF and APMF are compatible with activations during some poorly constrained historical earthquakes, such as the 1293 (M 5.8), 1349 (M 6.8; possibly a southern prolongation of the rupture on the Aquae Iuliae fault), and 346 CE earthquakes. A fault rupture during the poorly constrained 847 CE earthquake is also chronologically compatible with the dated displacements.

Geologic map of Slate Range Crossing area, California, USA

This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex Neogene–Quaternary deformation in the Slate Range Crossing area (California, USA) of the active dextral transtension of the Death Valley region and Walker Lane belt. This map integrates the late Cenozoic structures and geologic units with the Mesozoic geologic units and deformation as a data set to examine the controls on reactivation of older structures. These geologic data were collected to study pre-, syn-, and post-kinematic rocks to examine the deformation history of the area and to find palinspastic markers to examine the late Cenozoic fault displacement and displacement history across Panamint Valley to the east, as reported in Andrew and Walker (2009). The study focused on defining the Miocene and Pliocene rocks and deposits and examining lateral changes and depositional sources of clasts. There are two different volcanic-sedimentary sequences in this area. A Miocene section contains mafic to felsic volcanic units, numerous debris-flow to laharic deposits, and several associated conglomerates and breccias containing exotic clasts. The exotic clasts are matched to rocks in the Panamint Range on the east side of Panamint Valley as reported in Andrew and Walker (2009) as displacement vectors for palinspastic reconstructions. These Miocene strata ubiquitously dip eastward 20–40º. A younger volcanic-sedimentary sequence contains relatively thin mafic lava flows and associated locally derived, coarse-grained mass wasting deposits. These younger basaltic lavas generally have gentle dipping lava flow features and foliation. Numerous faults cut the different age deposits allowing a chronology of Neogene to Quaternary faulting; additionally, there are numerous fabrics associated with Jurassic contraction and Cretaceous(?) dextral shear. The area near Slate Range Crossing has a conspicuous zone of earthquake foci; this study found that some of this seismic activity coincides with a zone of southwest-striking, moderately dipping to the north, sinistral-oblique normal faults, which cut across the northernmost Slate Range. These faults form a structural boundary between the Argus and Slate Ranges and link the fault networks in Panamint Valley with those in Searles Valley. This mapping and structural data demonstrate the two-stage Neogene fault history of the Walker Lane belt deformation in this area and show that regional tilting of rocks occurred after ca. 13 Ma and before ca. 4 Ma; this eastward down-tilting appears to be a discrete event and may mark the change from extension to transtension. This detailed geologic mapping and collection of structural data for the rocks in the eastern Argus and northern Slate Ranges and Panamint Valley were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer. This map is a simplification of detailed geologic mapping data collected at 1:6000 scale and reduced to 1:20000 scale. Structural data includes kinematic and relative timing of deformation information.

Pliocene–Quaternary seismogenic faults in the inner Northern Apennines (Valdelsa Basin, southern Tuscany) and their role in controlling the local seismicity

AbstractPliocene–Quaternary faults are relevant structures with which to constrain the seismotectonic context and contribute to the evaluation of the seismic hazard of a region. Many of these faults, however, do not show clear surface evidence even when releasing earthquakes. For these reasons they can be extremely dangerous as they receive relatively little attention and can be difficult to identify. From among the various surface geology studies and/or palaeoseismological investigations, we focus our attention on the integration of different datasets such as seismic reflection profiles, surface kinematic data and the relocation of seismological data, which make it possible to identify and characterize active faults whose dimension and earthquake potential would otherwise not be large enough to make them identifiable. We take as an example the Montespertoli NE-trending fault in southern Tuscany (central Italy) with which we associate the 2016 M=3.9 Castelfiorentino earthquake. This structure is part of a wider (in the order of 15–20 km) crustal-scale shear zone, which may be responsible for strong historical earthquakes in the area.

Active Faulting of Landforms along the Tuosuhu-Maoniushan Fault and Its Seismotectonic Implications in Eastern Qaidam Basin, China

Abstract The fold-and-thrust belt along the northern margin of the Qaidam basin is a typical active tectonic belt located in the northeast Tibetan Plateau. This belt is at a high risk of strong earthquakes with magnitudes larger than 6, as shown by multiple recorded events during 1962–2009. The lack of detailed late Quaternary surficial faulting data and systematic seismotectonic studies has posed difficulties in properly assessing the seismic risks and understanding the ongoing geodynamics in this region. In this study, we mapped the geomorphic features and fault traces from high-resolution satellite images and field investigations of the Tuosuhu-Maoniushan fault (TMF). Field photogrammetry was conducted to obtain deformation measurements using a DJI M300 real-time kinematic (RTK) drone. The TMF displaces the Holocene and late Pleistocene alluvial terraces in the eastern Qaidam basin. This fault dips to the south in the west and central segments (as a boundary of the Denan depression) and to the north in the eastern segment along the piedmont of the Maoniushan Mountains. The vertical slip rate is estimated to be 0.37 ± 0.08 mm/yr, which is similar to that of the active southern Zongwulongshan fault. By integrating our investigations with the previously published studies on deep structures and Cenozoic geology of the region, we propose a deep-seated thrust model for the seismotectonics of the northern margin of the Qaidam basin. The Aimunike, Tuosuhu-Maoniushan, southern Zongwulongshan, and Zongwulong faults, along with many folds, form an active compressional zone. The complex across-strike structures and along-strike segmentation could facilitate the release of strain through earthquakes of magnitude 6–7 in this broad seismotectonics belt, rather than through strong surface-rupturing events resulting from a single mature large fault.

2021 US National Seismic Hazard Model for the State of Hawaii

The 2021 US National Seismic Hazard Model (NSHM) for the State of Hawaii updates the previous two-decade-old assessment by incorporating new data and modeling techniques to improve the underlying ground shaking forecasts of tectonic-fault, tectonic-flexure, volcanic, and caldera collapse earthquakes. Two earthquake ground shaking hazard forecasts (public policy and research) are produced that differ in how they account for declustered catalogs. The earthquake source model is based on (1) declustered earthquake catalogs smoothed with adaptive methods, (2) earthquake rate forecasts based on three temporally varying 60-year time periods, (3) maximum magnitude criteria that extend to larger earthquakes than previously considered, (4) a separate Kīlauea-specific seismogenic caldera collapse model that accounts for clustered event behavior observed during the 2018 eruption, and (5) fault ruptures that consider historical seismicity, GPS-based strain rates, and a new Quaternary fault database. Two new Hawaii-specific ground motion models (GMMs) and five additional global models consistent with Hawaii shaking data are used to forecast ground shaking at 23 spectral periods and peak parameters. Site effects are calculated using western US and Hawaii specific empirical equations and provide shaking forecasts for 8 site classes. For most sites the new analysis results in similar spectral accelerations as those in the 2001 NSHM, with a few exceptions caused mostly by GMM changes. Ground motions are the highest in the southern portion of the Island of Hawai’i due to high rates of forecasted earthquakes on décollement faults. Shaking decays to the northwest where lower earthquake rates result from flexure of the tectonic plate. Large epistemic uncertainties in source characterizations and GMMs lead to an overall high uncertainty (more than a factor of 3) in ground shaking at Honolulu and Hilo. The new shaking model indicates significant chances of slight or greater damaging ground motions across most of the island chain.

Corrigendum: Recency of Faulting and Subsurface Architecture of the San Diego Bay Pull-Apart Basin, California, USA

Figure 13. Regional fault orientations and plate boundary parameters used for kinematic analysis and interpretation. Solid orange arrows are maximum and minimum horizontal stress orientations, ! and " , respectively (Hardebeck and Hauksson, 2001), large black and white outlined arrows are plate motion vectors, and the average orientations for Group-1, Group-2, and La Nacion fault zones are solid black, toothed lines. Large black hatched lines are regional faults. G1, Group 1; G2, Group 2. Background fault traces (thin red lines) are from USGS Quaternary fault database (USGS, 2019). Background topography and bathymetry are from the National Centers for Environmental Information Southern California Coastal Relief Model (v2) (Calsbeek et al., 2013).

Low‐Rate Faulting on the Margin of the Colorado Plateau and Rio Grande Rift in North‐Central New Mexico

AbstractIn northern New Mexico, seismic hazard outside of the Rio Grande Rift (RGR) is generally considered low due to limited historical seismicity and few mapped Quaternary faults. Although the region has a complex tectonic history, nearly all known faults are considered inactive because of a lack of faulted Quaternary deposits. Analysis of lidar and existing geologic and geomorphic mapping permits identification and characterization of four faults on the Colorado Plateau/RGR margin. The Gallina, Willow Creek, and East and West Brazos Peak faults are normal or dextral‐normal faults that strike NW‐SE with lengths from 15 to 50 km. The Willow Creek fault has scarp heights <0.5–6.4 m on five Quaternary surfaces, with displacement increasing with relative surface age. An ∼5‐m‐high scarp on the ∼250 ka Brazos basalt yields an ∼0.02 mm/yr minimum dip‐slip rate. Oblique slip is recorded by dextral offset (∼40 m) of a middle Quaternary (?) terrace riser, a linear and en‐echelon stepping fault pattern, and a consistent down‐to‐the‐northeast fault expression. The West Brazos Peak fault displaces late Quaternary (?) surfaces by <4 m down‐to‐the‐southwest. We interpret these faults are Laramide‐aged faults reactivated as normal or dextral‐oblique faults in the Quaternary, consistent with regional stress and geodetic data. Although the faults have low slip rates (<0.1 mm/yr), the fault lengths suggest they may rupture in M6‐7 earthquakes and may pose a previously unrecognized seismic hazard to the region. Dextral transtension within the eastern Colorado Plateau is consistent with geodetic strain rates and earthquake focal mechanisms.

Ground-penetrating radar signature of Quaternary faulting: a study from the Mt. Pollino region, southern Apennines, Italy

Abstract. With the aim of unveiling evidence of Late Quaternary faulting, a series of ground-penetrating radar (GPR) profiles were acquired across the southern portion of the Fosso della Valle–Campotenese normal fault (VCT), located at the Campotenese continental basin (Mt. Pollino region) in the southern Apennines active extensional belt (Italy). A set of 49 GPR profiles, traced nearly perpendicular to this normal fault, was acquired using 300 and 500 MHz antennas and carefully processed through a customized workflow. The data interpretation allowed us to reconstruct a pseudo-3D model depicting the boundary between the Mesozoic bedrock and the sedimentary fill of the basin, which were in close proximity to the fault. Once the GPR signature of faulting was reviewed and defined, we interpret near-surface alluvial and colluvial sediments dislocated by a set of conjugate (W- and E-dipping) discontinuities that penetrate inside the underlying Triassic dolostones. Close to the contact between the continental deposits and the bedrock, some buried scarps which offset wedge-shaped deposits are interpreted as coseismic ruptures, subsequently sealed by later deposits. Our pseudo-3D GPR dataset represented a good trade-off between a dense 3D-GPR volume and conventional 2D data, which normally requires a higher degree of subjectivity during the interpretation. We have thus reconstructed a reliable subsurface fault pattern, discriminating master faults and a series of secondary splays. This contribution better characterizes active Quaternary faults in an area which falls within the Pollino seismic gap and is considered prone to severe surface faulting. Our results encourage further research at the study site, whilst we also recommend our workflow for similar regions characterized by high seismic hazard and scarcity of near-surface geophysical data.

Quaternary uplift and fault movement near Waitomo, North Island, New Zealand

ABSTRACT The hill country of west central North Island, west of Te Kuiti and Otorohanga, is recognised to be slowly uplifting and relatively stable. The New Zealand national seismic hazard model shows the area to be in a low-risk zone and the New Zealand active faults database recognises no active faults in the region. However, dated evidence provided by speleothems in a cave near Waitomo indicates the intermittent movement of the Okohua fault from before the Last Interglacial until around 38 kyrs ago. Evidence of older movement along the Waipa Fault near Te Kuiti is also apparent from the vertical displacement of the Ngaroma ignimbrite (1.55 Myrs) by about 150 m in the interval prior to the deposition of the Ongatiti ignimbrite at 1.21 Myrs. Net uplift in the West Waikato Hills and Ranges, by about 0.1 mm/yr in the Late Quaternary, is accommodated in a series of NNE-trending faults and transverse faults, resulting in a staircase of fault-bound blocks rising westwards from Waitomo to the western watershed of the Waipa basin. Geomorphologically fresh cliffed lineaments on the Okohua and Hikurangi faults convey evidence of mid to late Quaternary fault activity.

Ambient Noise Tomography of Upper Crustal Structures and Quaternary Faults in the Seoul Metropolitan Area and Its Geological Implications

AbstractWe investigate the upper‐crustal seismic velocity structure in the Seoul metropolitan area, where about 20 million people live. The Chugaryeong fault zone (CFZ) is placed in this area, but the seismic hazard potential remains unclear. We conducted ambient noise tomography to illuminate the high‐resolution upper‐crustal structure in the Seoul metropolitan area. We analyzed continuous vertical seismic records for ∼5 months from a dense seismic array with 77 broadband stations. Group velocity dispersion curves and tomographic maps were extracted between 0.5 and 10 s periods. We inverted 3‐D group velocity tomography models up to a depth of ∼10 km from the group velocity maps. The shear‐wave velocity model is consistent with the geological features. High‐velocity anomalies at shallow depths are correlated with the surface topography and geology. The CFZ is located at a low velocity below the 5 km depth and presented as the simplified model. The large VS contrast regions are located beneath NS‐trending faults. The cross‐sections coincide with the near‐vertical strike‐slip faults in this area. In the southern region of the Seoul metropolitan area, low‐velocity anomalies correlate with high heat flow regions. Our results effectively suggest high resolution upper‐crustal structures and subsurface hidden faults in the urban area.

Surface trace of the active Katrol Hill Fault and estimation of paleo-earthquake magnitude for seismic hazard, Western India

Estimation of seismogenic potential through geological studies of faults is an important step in seismic hazard evaluation and mitigation. We investigated the Katrol Hill Fault (KHF), an E-W trending and south dipping high angle reverse fault in the seismically active Kachchh Basin at the western continental margin of India. While several faults in the eastern part of the basin are presently seismically active, the KHF is the only fault in the entire basin that is shown to have produced three Late Quaternary surface faulting events. We illustrate here an integrated geological approach using field mapping, microscopic analysis (petrography and quartz surface textures using SEM) and shallow geophysical surveys using GPR for determining the precise surface trace of the active KHF. We also determined the displacement and slip-rate for each surface faulting event. Based on the Late Quaternary surface faulting parameters deduced, we calculated the moment magnitude (Mw) of the events using well established empirical relationships. These calculations yielded Mw values consistently ≥ 6.5. Our study shows that the KHF is capable of producing high magnitude earthquakes. We infer that these surface faulting events may be of relatively shallow focus as higher magnitude earthquakes in recent times along other faults in the basin, except the 1819 Allahbund earthquake, did not produce surface rupture. We recommend a revision of seismic hazard risk and mitigation strategies of the region as a consequence of our findings.