Seismological Investigations Research Articles

A multi-technology analysis of the 2017 North Korean nuclear test

Abstract. On 3 September 2017 official channels of the Democratic People's Republic of Korea announced the successful test of a thermonuclear device. Only seconds to minutes after the alleged nuclear explosion at the Punggye-ri nuclear test site in the mountainous region in the country's northeast at 03:30:02 (UTC), hundreds of seismic stations distributed all around the globe picked up strong and distinct signals associated with an explosion. Different seismological agencies reported body wave magnitudes of well above 6.0, consequently estimating the explosive yield of the device on the order of hundreds of kT TNT equivalent. The 2017 event can therefore be assessed as being multiple times larger in energy than the two preceding North Korean events in January and September 2016. This study provides a multi-technology analysis of the 2017 North Korean event and its aftermath using a wide array of geophysical methods. Seismological investigations locate the event within the test site at a depth of approximately 0.6 km below the surface. The radiation and generation of P- and S-wave energy in the source region are significantly influenced by the topography of the Mt. Mantap massif. Inversions for the full moment tensor of the main event reveal a dominant isotropic component accompanied by significant amounts of double couple and compensated linear vector dipole terms, confirming the explosive character of the event. The analysis of the source mechanism of an aftershock that occurred around 8 min after the test in the direct vicinity suggest a cavity collapse. Measurements at seismic stations of the International Monitoring System result in a body wave magnitude of 6.2, which translates to an yield estimate of around 400 kT TNT equivalent. The explosive yield is possibly overestimated, since topography and depth phases both tend to enhance the peak amplitudes of teleseismic P waves. Interferometric synthetic aperture radar analysis using data from the ALOS-2 satellite reveal strong surface deformations in the epicenter region. Additional multispectral optical data from the Pleiades satellite show clear landslide activity at the test site. The strong surface deformations generated large acoustic pressure peaks, which were observed as infrasound signals with distinctive waveforms even at distances of 401 km. In the aftermath of the 2017 event, atmospheric traces of the fission product 133Xe were detected at various locations in the wider region. While for 133Xe measurements in September 2017, the Punggye-ri test site is disfavored as a source by means of atmospheric transport modeling, detections in October 2017 at the International Monitoring System station RN58 in Russia indicate a potential delayed leakage of 133Xe at the test site from the 2017 North Korean nuclear test.

Detection of seismic events on Mars: a lunar perspective

The interior structures of planets are attracting more and more detailed attention; these studies could be of great value in improving our understanding of the early evolution of Earth. Seismological investigations of planet interiors rely primarily on seismic waves excited by seismic events. Since tectonic activities are much weaker on other planets, e.g. Mars, the magnitudes of their seismic events are much smaller than those on Earth. It is therefore a challenge to detect seismic events on planets using such conventional techniques as short-time average/long-time average (STA/LTA) triggers. In pursuit of an effective and robust scheme to detect small-magnitude events on Mars in the near future, we have taken Apollo lunar seismic observations as an example of weak-activity data and developed an event-detection scheme. The scheme reported here is actually a two-step processing approach: the first step involves a despike filter to remove large-amplitude impulses arising from large temperature variations; the second step employs a matched filter to unmask the seismic signals from a weak event hidden in the ambient and scattering noise. The proposed scheme has been used successfully to detect a moonquake that was not in the known moonquake catalogue, demonstrating that the two-step strategy is a feasible method for detecting seismic events on planets. Our scheme will provide a powerful tool for seismic data analysis of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission, and China’s future lunar missions.

THE EMPIRICAL GREEN’S FUNCTION TECHNIQUE MODIFICATION FOR ACCELEROGRAM SYNTHESIS IN LOWSEISMICITY REGIONS

In the absence of strong motion records at the future construction sites, different theoretical and semi-empirical approaches are used to estimate the initial seismic vibrations of the soil. If there are records of weak earthquakes on the site and the parameters of the fault that generates the calculated earthquake are known, then the empirical Green’s function can be used. Initially, the empirical Green’s function method in the formulation of Irikura was applied for main shock record modelling using its aftershocks under the following conditions: the magnitude of the weak event is only 1–2 units smaller than the magnitude of the main shock; the focus of the weak event is localized in the focal region of a strong event, hearth, and it should be the same for both events. However, short-termed local instrumental seismological investigation, especially on seafloor, results usually with weak microearthquakes recordings. The magnitude of the observed micro-earthquakes is much lower than of the modeling event (more than 2). To test whether the method of the empirical Green’s function can be applied under these conditions, the accelerograms of the main shock of the earthquake in L'Aquila (6.04.09) with a magnitude Mw = 6.3 were modelled. The microearthquake with ML = 3,3 (21.05.2011) and unknown origin mechanism located in mainshock’s epicentral zone was used as the empirical Green’s function. It was concluded that the empirical Green’s function is to be preprocessed. The complex Fourier spectrum smoothing by moving average was suggested. After the smoothing the inverses Fourier transform results with new Green’s function. Thus, not only the amplitude spectrum is smoothed out, but also the phase spectrum. After such preliminary processing, the spectra of the calculated accelerograms and recorded correspond to each other much better. The modelling demonstrate good results within frequency range 0,1–10 Hz, considered usually for engineering seismological studies.

Deep deformation of the Longmenshan fault zone related to the 2008 Wenchuan earthquake

This paper reviews studies of the past ten year related to the crustal deformation of the Longmenshan fault zone (LMSFZ) which was struck by the M 8.0 Wenchuan earthquake on May 12, 2008. The tectonics of the LMSFZ located at the eastern margin of the Tibetan Plateau is complex, with complicated fault geometry and a heterogeneous velocity structure. The Wenchuan earthquake rupture extended for about 300 km along the middle and northern segments of the LMSFZ, with a very complicated rupture process involving multiple sub-events. The Wenchuan slip distribution is characterized by two major slip patches near Hongkou/Yingxiu and Beichuan with peak slip of 5.0–15.5 m and extended from near the surface to below 20 km depth. Prior to the Wenchuan earthquake, the LMSFZ had been seismically quiet for several centuries and there were no hints that suggested that such an M 8.0 earthquake might strike the area. The long-term geological investigations and short-term geodetic measurements before the Wenchuan earthquake generally agree that the horizontal slip rate along the LMSFZ is no more than 3 mm per year. The low slip rate observed at the surface around the LMSFZ may not reveal the real state of accumulated strain at depth where the devastating Wenchuan earthquake nucleated. Rates of aseismic slip at depth derived from seismological investigation of repeating microearthquakes were found to be approximately twice as large as the interseismic rates inferred from surface GPS and geological data. Most of the clusters of repeating microearthquakes are located at the edge of locked areas where large coseismic slips were observed during the Wenchuan earthquake, suggesting a close relationship between microearthquakes and impending large earthquakes. A two-dimensional viscoelastic finite-element model produces a depth-related slip rate pattern around the LMSFZ that is consistent with that revealed by the seismological observation of repeating earthquakes. The measured in situ deep slip rates increase with depth and vary from 3.5 to 9.6 mm/a over a depth range of 4–18 km.The seismological observations of deep slip rates and microseismicity in the three decades before the Wenchuan earthquake reveal that the LMSFZ is indeed not as “quiet” as traditionally assumed in comparison with its neighboring fault systems. Considering the deep slip rates (3.5−9.6 mm/a) from the repeating microearthquakes and coseismic peak offsets of 5.0–15.5 m, the recurrence interval of Wenchuan-like events is estimated to be about 500−4500 years. The estimated recurrence interval based on the deep slip rates is much smaller than those estimates using the same coseismic displacements divided by GPS-derived or geological slip rates. Slip rate increases with depth were also recognized in the Parkfield section of the San Andreas fault zone and in the northeastern Japan subduction zone before the ruptures of the 2004 M w6.0 Parkfield earthquake and the 2011 M w9.0 Tohoku-oki earthquake, respectively. Accelerated slip is thought to have preceded a number of recent large subduction zone earthquakes and the 2008 Wenchuan earthquake, and repeating earthquakes may document short-term precursory slip at depth. Alternatively, the rapid slip rates indicated by the repeating microearthquakes may represent transiently accelerated slip preceding the Wenchuan mainshock. We suggest that slip rates at seismogenic depths are of critical importance in seismic hazard analysis. Repeating earthquakes can be regarded as “deep creepmeters” that measure the in - situ deep slip rate on otherwise aseismically slipping faults. For less well defined and widespread faults within the continents, it is essential to reveal a fault’s or region’s seismic history over different time scale. Combining a better understanding of earthquake diversity with modern technology is the key to effective and comprehensive hazard mitigation practices. The potential earthquake hazard of locked faults with unusually high inferred deep slow slip rates should be paid more attention. The 2008 Wenchuan earthquake is the best-studied continental earthquake to date with a large number of scientific publications enabled by the vast collected data sets. The research community efforts have provided first-order information about this unexpected event regarding its coseismic slip distribution and fault geometry. However, there are many remaining questions about the basic nature of this earthquake to be further constrained with multiple data sets, including the relationship of its rupture with prior coupling and interseismic creep, the varying dynamical rupture processes with depth, the frequency dependence of seismic radiation across the fault zone, and the role of multiple postseismic deformation processes.

Tutorial review of seismic surface waves’ phenomenology

In recent years, surface wave seismology has become one of the leading directions in seismological investigations of the Earth’s structure and seismic sources. Various applications cover a wide spectrum of goals, dealing with differences in sources of seismic excitation, penetration depths, frequency ranges, and interpretation techniques. Observed seismic data demonstrates the great variability of phenomenology which can produce difficulties in interpretation for beginners. This tutorial review is based on the many years’ experience of authors in processing and interpretation of seismic surface wave observations and the lectures of one of the authors (ALL) at Workshops on Seismic Wave Excitation, Propagation and Interpretation held at the Abdus Salam International Center for Theoretical Physics (Trieste, Italy) in 1990–2012. We present some typical examples of wave patterns which could be encountered in different applications and which can serve as a guide to analysis of observed seismograms.

Seismological investigation of September 09 2016, North Korea underground nuclear test

On Sep. 9, 2016, a seismic event of mb 5.3 took place in North Korea. This event was reported asa nuclear test. In this study, we applied a number of discriminant techniques that facilitate the ability to distinguish between explosions and earthquakes on the Korean Peninsula. The differences between explosions and earthquakes are due to variation in source dimension, epicenter depth and source mechanism, or a collection of them. There are many seismological differences between nuclear explosions and earthquakes, but not all of them are detectable at large distances or are appropriate to each earthquake and explosion. The discrimination methods used in the current study include the seismic source location, source depth, the differences in the frequency contents, complexity versus spectral ratio and Ms-mb differences for both earthquakes and explosions. Sep. 9, 2016, event is located in the region of North Korea nuclear test site at a zero depth, which is likely to be a nuclear explosion. Comparison between the P wave spectra of the nuclear test and the Sep. 8, 2000, North Korea earthquake, mb 4.9 shows that the spectrum of both events is nearly the same. The results of applying the theoretical model of Brune to P wave spectra of both explosion and earthquake show that the explosion manifests larger corner frequency than the earthquake, reflecting the nature of the different sources. The complexity and spectral ratio were also calculated from the waveform data recorded at a number of stations in order to investigate the relation between them. The observed classification percentage of this method is about 81%. Finally, the mb:Ms method is also investigated. We calculate mb and Ms for the Sep. 9, 2016, explosion and compare the result with the mb: Ms chart obtained from the previous studies. This method is working well with the explosion.

Subducting continental lower crust and crustal thickness variations in the intermediate seismic zone of Pamir–Hindu Kush inferred from Moho underside reflection pmP

The Pamir–Hindu Kush region is an orogenic belt which formed as a result of recent continental collision between the Indian and Eurasian Plates. A comprehensive understanding of the tectonic history of this region has been hampered due to limited seismological investigations. In this study, we use the Moho underside reflection pmP phase to constrain crustal thickness variations in the intermediate-depth seismic zone (36–37°N, 69–72°E). The seismic events characterized by focal depth deeper than 100km and magnitude>5.8 (Mw) were used. The crustal thickness was determined by identifying the depth phase pP and the Moho underside reflection pmP. The measured thickness varies spatially from 58.1 to 76.2km, with uncertainties most likely resulting from deviation of the average P-wave velocities (~6.21km/s with a deviation of 0.22km/s) in the crust. The strong Moho depth variations imply a large structural deformation of the crust, which reflects a complex collision-related mountain building history. We also detected two strong reflections from deep interfaces down to ~97km underneath the southernmost Pamir. Based on our direct observations and waveform modeling, we interpret that the two reflections are possibly the manifestations of the underplating subducted Eurasian lower crust in this region. Our observations complement those of other seismic results, including receiver functions from previous studies.

Geological, Geophysical and Seismological Investigations for Siting of Seismic Stations in Minna and Abakaliki, Nigeria for Data Reliability

Geological, Geophysical and Seismological Investigations for Siting of Seismic Stations in Minna and Abakaliki, Nigeria for Data Reliability

Seismological investigation of the 2016 January 6 North Korean underground nuclear test

AbstractSeismology plays an important role in characterizing potential underground nuclear tests. Using broad-band digital seismic data from Northeast China, South Korea and Japan, we investigated the properties of the recent seismic event occurred in North Korea on 2016 January 6. Using a relative location method and choosing the previous 2006 explosion as the master event, the 2016 event was located within the North Korean nuclear test site, with its epicentre at latitude 41.3003°N and longitude 129.0678°E, approximately 900 m north and 500 m west of the previous event on 2013 February 12. Based on the error ellipse, the relocation uncertainty was approximately 70 m. Using the P/S spectral ratios, including Pg/Lg, Pn/Lg and Pn/Sn, as the discriminants, we identify the 2016 event as an explosion rather than an earthquake. The body-wave magnitude calculated from regional wave Lg is mb(Lg) equal to 4.7 ± 0.2. Adopting an empirical magnitude–yield relation, and assuming that the explosion is fully coupled and detonated at a normally scaled depth, we find that the seismic yield is about 4 kt, with the uncertainties allowing a range from 2 to 8 kt.

Seismological Investigations of Two Massive Explosions in Tianjin, China

In this study, we analyze available weak‐ and strong‐motion seismograms to discern the main source characteristics of the 12 August 2015 Tianjin explosions. The disaster involved two large explosions, which can be clearly identified in the near‐field seismic recordings. The first explosion (event 1) took place at 15:34:04.68 (UTC) and the second at 15:34:36.94 (UTC). The average time interval between the two events was about 32.3 s, which is consistent with the result determined by cross correlation of the whispering gallery wave ( WG phase). The two events did not occur at the same site, and the second explosion was located 65±10 m northwest (N352°W) of the first one. The local magnitudes M L of the two explosions were further measured as 2.4 and 2.9, respectively. The Fourier spectra from the two closest stations, DAG and BET, show that the predominant frequency of the waveforms ranged from 0.7 to 0.9 Hz after the long‐period wavetrains had arrived. According to the distribution of the three‐component peak ground acceleration, the area with a shaking intensity of modified Mercalli intensity VI spanned up to 131 km2. However, the peak intensity of this explosion did not reach an intensity of VII, based on the ground‐motion attenuation relation.

Seismicity of the Central Kuril Islands before and after the catastrophic M = 8.3 (November 15, 2006) and M = 8.1 (January 13, 2007) earthquakes

The data on foreshocks and aftershocks of the strongest earthquakes occurred in the area of Kuril island arc on November 15, 2006 (Mw = 8.3) and January 13, 2007 (Mw = 8.1) are compared. In 2006, two months before the catastrophic earthquake in November, the detailed seismological investigations were carried out with the use of bottom stations in the area of seismic gap in the Central Kuril arc. The maps of epicenters and vertical cross sections for the equal time intervals are compared. The clear difference between distributions of hypocenters in the periods of foreshocks and aftershocks is revealed. Earthquakes were not distributed within the limits of the seismoactive block in the Kuril arc during the foreshocks period; instead, boundaries of seismoactive zone abruptly narrowed and hypocenters concentrated largely within the Benioff zone.

Crustal structure of the Pannonian Basin: The AlCaPa and Tisza Terrains and the Mid-Hungarian Zone

The Pannonian Basin of Central Europe is one of the key examples of Miocene continental extension that is easily accessible to surface seismological investigation. It comprises two major crustal blocks: AlCaPa and Tisza which abut along a poorly understood structure referred to as the Mid-Hungarian Zone (MHZ), the whole being surrounded by the arc of the Carpathian Mountains, the Alps and the Dinarides. Using data from the CBP (Carpathian Basins Project) temporary broadband seismic array of 46 stations deployed across the western Pannonian Basin in 2006–2007, we calculated receiver functions that constrain the variation of crustal thickness across the basin and derive a map of Moho depth across a NW–SE oriented swath about 450km long and 75km wide. The measured Moho depths show no significant change in crustal thickness between AlCaPa and Tisza terrains, but the Moho is not or very weakly imaged along a ca. 40km wide strip centred on the MHZ. Moho depths within the Pannonian Basin are typically in the range 25–30km, and increase toward the periphery of the basin. Our measurements are generally consistent with earlier VP models from controlled-source seismic surveys and recent VS models determined by tomographic analysis of ambient noise signals. The lack of a sharp Moho image beneath the MHZ suggests that the crust–mantle boundary in that zone may consist of a gradual increase in velocity with depth. The relatively constant crustal thickness across the two domains of the Pannonian Basin suggests that thinning to the same final state is controlled thermally. This structural characteristic seems to be governed by a large-scale balance of gravitational potential energy that is insensitive to the separate prior histories of the two regions.

Crustal rheology control on earthquake activity across the eastern margin of the Tibetan Plateau: Insights from numerical modelling

The devastating 2008Mw7.9 Wenchuan and 2013 Mw6.6 Lushan earthquakes occurred in the Longmen Shan (LMS) fault zone, the eastern margin of the Tibetan Plateau. Seismology investigation reveals that earthquakes of Ms>5.0 are frequently recorded in the eastern Tibetan Plateau, however rare earthquake of Ms>5.0 and only a few earthquakes of 3.0<Ms<5.0 have been recorded in the Sichuan Basin. This study investigates the relation between the crustal rheology and the earthquake activity across the eastern margin of the Tibetan Plateau through 3D numerical experiments assuming visco-elasticity rheology. We setup several finite element lithospheric models based on different rheological structure. The interseismic stress accumulation processes in the models are simulated with boundary conditions of steady compressional deformation rate constrained by GPS observations. The results show that the crustal stress accumulation process across the eastern margin of the Tibetan Plateau is significantly controlled by the regional crustal rheology, which presents large horizontal viscosity variation in the lower crusts between the Tibetan Plateau and the Sichuan Basin. The stress accumulation rate in the upper crust of the eastern Tibetan Plateau is much higher than the Sichuan Basin. This stress spatial distribution explains the seismology observation that earthquakes are densely recorded in the eastern Tibetan Plateau, whereas rarely in the Sichuan Basin. The stress concentration nearly at the bottom of the entire LMS fault zone is thought to be responsible for the generation of the 2008Mw7.9 Wenchuan and 2013Mw6.6 Lushan earthquakes. There is a high possibility of future earthquakes in the seismic gaps at the southwest segment of the LMS fault zone, where it was stress accumulated but did not rupture during the last two events of the 2008 Wenchuan and 2013 Lushan earthquakes.

Effect of azimuthal coverage of an earthquake on moment tensor solutions estimated by waveform inversion

The purposes of this seismological investigation are to understand and describe the effect of decrease in the azimuthal coverage of an earthquake on moment tensor solution estimated by waveform inversion. It will be very useful and worthwhile as mostly seismological networks are sparse and in case when only one or two station data are available. In this work, we have used two moderate earthquakes, 21 September 2009 (near Uttarakashi) and 3 May 2010 (near Ghansali). The waveform inversion has been carried using ISOLA software. The moment tensor is first estimated by using all station data and then by removing the stations so that the azimuthal coverage changes. The results show that strike of both nodal planes is varying with the change in azimuthal coverage. However, the slip and dip of both nodal planes remain quite stable against the variation in azimuthal coverage for these two earthquakes analyzed. The effect of decrease in the azimuthal coverage showed increase in double-couple percentage (DC %) and decrease in compensated linear vector decomposition (CLVD %). The other focal parameters such as T-axis azimuth, P-axis azimuth, T-axis plunge, and P-axis plunge have been found stable against the variation in azimuth coverage. The study also demonstrates that the moment tensor solutions obtained from waveform inversion using single station are almost similar to those estimated using maximum azimuthal coverage data and by polarity inversion.

Global Positioning System (GPS) Measurements of Crustal Deformation across the Frontal Eastern Himalayan Syntaxis and Seismic-Hazard Assessment

Abstract Tectonics and earthquake occurrence processes of the two Himalayan syntaxes are complex. Their limited accessibility has restricted seismological investigations in these regions, resulting in poor knowledge about the geodynamic processes. Here, we provide Global Positioning System (GPS) measurements of crustal deformation across the frontal eastern Himalayan syntaxis (EHS). In this region, the Indian plate obliquely underthrusts the Eurasian plate, and the eastward‐extruding Tibetan plateau moves around the EHS due to India–Eurasia convergence. Our GPS measurements in the region suggest that the frontal EHS accommodates at least ∼20 mm/yr of oblique convergence between the Indian and Eurasian plates. Under the frontal EHS, on which this convergence is accommodated through a stick‐and‐slip process, the 65 km wide main thrust zone of the plate boundary interface currently is locked. These measurements and the limited information about the great 1950 Assam earthquake ( M w 8.6) suggest that this great earthquake probably occurred in the EHS.

MOZART: A Seismological Investigation of the East African Rift in Central Mozambique

Online Material: Table of station coordinates; preliminary seismicity catalog. From Afar to the Malawi–Mozambique border, the East African Rift System (EARS; Fig. 1) is well described on the basis of geological and geophysical evidence (Ebinger et al. , 1987; Ring et al. , 1992; Simiyu and Keller, 1997; Chorowicz, 2005; Roberts et al. , 2012), seismicity (Nyblade and Langston, 1995; Yang and Chen, 2008, 2010; Kim et al. , 2009; Delvaux and Barth, 2010), and Global Positioning System (GPS) data (Fernandes et al. , 2004, 2013; Calais et al. , 2006; Stamps et al. , 2008; Saria et al. , 2013). However, its extent south of the Malawi rift is unclear. Various trajectories have been proposed: a split near Lake Tanganyika continuing southwest through the Okavango delta in Botswana (Scholz et al. , 1975); offshore through the Mozambique Channel (Mougenot et al. , 1986); through the Urema Rift in Central Mozambique (Ebinger et al. , 1987; Hartnady, 1990; Steinbruch, 2010); distributed between the Okavango rift and the offshore Davie Ridge (Grimison and Chen, 1988). The M w 7 Machaze earthquake (Fenton and Bommer, 2006; Yang and Chen, 2008; Raucoules et al. , 2010; Copley et al. , 2012) highlighted the importance of the Central Mozambique sector of the EARS. Figure 1. Project MOZAmbique Rift Tomography (MOZART) study area (white rectangle) in relation to the main surface features of the East African Rift System (EARS; dark gray; after Chorowicz, 2005 and Kinabo et al. , 2007). Project MOZAmbique Rift Tomography (MOZART) set out to investigate the seismicity and the crustal structure of central Mozambique with four main goals: 1. to delineate the active structures of the EARS in this region; 2. to characterize the imprint left in the crust and upper …

Geodetic and seismological investigation of crustal deformation near Izmir (Western Anatolia)

The Aegean region including western Turkey, mainland Greece, and the Hellenic Arc is the most seismological and geodynamical active domain in the Alpine Himalayan Belt. In this study, we processed 3years of survey-mode GPS data and present the analysis of a combination of geodetic and seismological data around Izmir, which is the third most populated city in Turkey. The velocities obtained from 15 sites vary between 25mm/yr and 28mm/yr relative to the Eurasian plate. The power law exponent of earthquake size distribution (b-value) ranges from 0.8 to 2.8 in the Izmir region between 26.2°E and 27.2°E. The lowest b-value zones are found along Karaburun Fault (b=0.8) and, between Seferihisar and Tuzla Faults (b=0.8). A localized stress concentration is expected from numerical models of seismicity along geometrical locked fault patches. Therefore, areas with lowest b-values are considered to be the most likely location for a strong earthquake, a prediction that is confirmed by the 2005 Mw=5.9 Seferihisar earthquake sequences, with epicentres located to the south of the Karaburun Fault. The north–south extension of the Izmir area is corroborated by extension rates up to 140 nanostrain/yr as obtained from our GPS data. We combined the 3-year GPS velocity field with the published velocity field to determine the strain rate pattern in the area. The spatial distribution of b-value reflects the normal background due to the tectonic framework and is corroborated by the geodetic data. b-Values correlate with strain pattern. This relationship suggests that decrease of b-values signifies accumulating strain.

Application of tectonophysical approaches to the solution of seismotectonic problems by the example of the Simushir earthquakes of November 15, 2006 and January 13, 2007 in Central Kuriles

The seismotectonic, seismological, macroseismic, and geodetic investigations carried out by different authors provided the idea about the size and structure of the sources of the largest Simushir earthquakes that occurred on November 15, 2006 and January 13, 2007. Sound conclusions concerning the character of dislocations arising in the epicentral area of strong submarine seismic events are drawn from the detailed analysis of GPS-based geodetic observations of surface deformations on the islands and in the southern Kamchatka. The focal mechanism solutions for two of the strongest quakes, together with the hypocentral positions of their aftershocks, are used for identifying the fault plane and reconstructing the pattern of the slip on it.

Precise relative earthquake location using surface waves

AbstractEarthquake locations provide a fundamental tool for seismological investigations. While dense seismic networks can provide robust locations, accuracy and precision of these locations suffer outside dense networks. This is particularly true in offshore areas, where location analysis relies heavily on distant seismic observations. We present a method for estimating precise relative seismic source epicentroid locations using surface waves. Several reasons, including lower velocities and strength of the signal at distance, make use of surface waves for event location appealing. We focus on the Panama Fracture Zone region and relocate 81 strike‐slip earthquakes to produce tectonically consistent epicentroid locations. The resulting pattern of earthquakes more clearly delineates recently active regional structures than original body‐wave locations. The mean shift between the US Geological Survey National Earthquake Information Center epicenter and our epicentroids is about 14 km (the median is about 11 km), and typical origin time changes are generally less than ±2 s. We find that north of 6.5°N, the plate boundary motion is split across two roughly north‐south striking structures, the Panama and Balboa Fracture zones. For the last 36 years, slip along these two structures roughly matches slip along the Panama Fracture Zone to the south (from 4.5°N to 6.25°N), but the Balboa Fracture zone has roughly three times the moment than the northern Panama Fracture Zone. Our analyses show that observed Rayleigh‐wave signal‐to‐noise ratios for moderate‐to‐large shallow earthquakes are suitable for applying the procedure and that Rayleigh‐wave observations form a self‐consistent set of constraints on the relative location of earthquake centroids.

Geomorphological and seismological investigations in a part of western Kumaun Himalaya, Uttarakhand, India

The Kumaun Himalaya in India is seismotectonically one of the most responsive sectors of the Himalayan arc. The region falls in the seismic gap and is frequently rocked by the earthquakes of low to medium magnitude. In the present investigation, the seismic data of the area were obtained by deploying a seismic network, and an attempt is made to interpret the seismological data of the local events and their relation with the geomorphology and thrusts/faults rejuvenation. The epicentral distribution of the earthquakes defines a relatively narrow zone of seismicity. A majority of earthquakes are confined to shallow depth with low stress drop and higher peak ground velocity (PGV). Further, the neotectonic investigations along the major river valleys indicate the neotectonic activity in the form of paired and unpaired terraces, shifting of river courses, fault scarps, development of palaeolakes, straightening of river channels, and deep cut V-shaped valleys. From the geomorphological and seismological data, the thrust and fault systems in the area are seismotectonically quite active even today. Some of the transverse faults — viz. Dwarahat-Chaukhutia fault, Takula-Basauli fault (Sim Gad valley), Tilora-Jyala fault (Kosi valley) — show spectacular tectonic landforms and seismicity. As a matter of fact, the NNW/NW–SSE/SE trending faults are active in the region.