Hindu Kush Earthquake Research Articles

SEISMICITY of TAJIKISTAN and ADJACENT TERRITORIES in 2018–2019

In 2018–2019, seismic monitoring of the territory of Tajikistan with adjacent areas of several states was carried out by a network of 7 digital seismic stations of the Geophysical Servey of the National Academy of Sciences of Tajikistan. For two years, 17195 earthquakes with energy classes KR=8.6–14.9 were registered. Among them 2870 were small–focus (h<70 km), scattered throughout the Southwestern Tien Shan (zone I), Southern Tien Shan (II), the crustal Pamir-Hindu Kush (III), and 14325 – with intermediate depths h=70–310 km, localized in the Pamir-Hindu Kush zone IV. 112 earthquakes were tangible, two of them, Sarikhosor earthquakes of March 29, 2018 with KR=13.1, I0=6 and of March 7, 2019 with KR=12.1, I0=5–6, were surveyed with the construction of isoseist maps and described in a separate article of this issue of the journal. The strongest (Mw>6) earthquakes were grouped in zone IV of Hindu Kush earthquakes with intermediate depths both in 2018 (January 31 with KR=14.8, Mw=6.2, h=200 km and May 9 with KR=14.9, Mw=6.3, h=105 km) and in 2019 (December 20 with KR=14.8, Mw=6.2, h=209 km). All notable earthquakes with foci in the Earth's crust occurred in the Southern Tien Shan zone (II) and associated with a system of near-latitudinal South-Hissar and Ilak-Vakhsh deep faults. Among them the earthquakes occurred in 2018 (March 29 with KR =13.1, N,афт=521; September 17 and December 5 with KR=12.0, N,aft=59) and in 2019 (March 7, KR=12.1, N, aft=53), which were accompanied by large series of aftershocks.

THE HINDU KUSH EARTHQUAKE on October 26, 2015 with Mw=7.5, I0~7: PRELIMINARY SEISMICITY and AFTERSHOCK SEQUENCE

On October 26, 2015, a strong Hindu Kush earthquake with KR=17.0, Mw=7.5 occurred in the Afghan Pamir-Hindu Kush subzone at a depth of hpP=217 km. Shakes of varying intensity caused by this earthquake were recorded in settlements of 14 states: Afghanistan, Tajikistan, Pakistan, Turkmenistan, India, Kyrgyzstan, Uzbekistan, Kazakhstan, China, Iran, Nepal, United Arab Emirates, Russia, Qatar and Bangladesh with a total area of S=14106 km2. The earthquake was preceded by three large (KR=12.5, 12.1, 14.0) foreshocks and was accompanied by a series of more than 1400 aftershocks unprecedented for aftershocks of deep earthquakes with KR=9–13. The energy step between the mainshock and the maximum foreshock is Kfor=3.0, between the mainshock and the maximum (KR=12.8) aftershock – Kaft=4.2. The aftershock recurrence graph has a slope =–0.67, which in absolute value is higher than the average value in the region =0.50. The attenuation para-meter  of the Omori law in the initial phase of attenuation, =–1.26, in absolute value is also higher than the average =1.0 for strong earthquakes in the World. Based on the results of a joint analysis of the focal me-chanism solutions of different agencies and vertical sections along and across the aftershock cloud, it was con-cluded that an upthrust movement occurred in the source along a steep east-south-east nodal plane, dipping to the south. The reason for the activity at the site of the earthquake is the movement of the Indian continent to the north and its collision with Eurasia, as a result of which the separation and subduction of the Hindu Kush plate continue. The Hindu Kush earthquake on October 26, 2015, and its aftershocks are just one of the events of successive deformation and stress relief in the latitudinal zone, marked in 2015 by the migration of earthquake epicenters with KR=13–17 from east to west.

SEISMICITY of TAJIKISTAN and ADJACENT TERRITORIES in 2015

The seismic monitoring system consisting of seven digital stations continued to operate in Tajikistan in 2015. This network has registered 9071 earthquakes with KR=8.6–17.0, 6427 of which were Pamir-Hindu Kush earthquakes with intermediate depths (h=70–300 km), and 2644 were shallow events. The total seismic energy released was E=1.8151017 J. The strongest for 2015 was the Hindu Kush earthquake on Octo-ber 26 with Mw=7.5, h=230 km (hpP=217 km) that occurred near the southern borders of the Republic. This earthquake caused significant damage and the death of at least 115 people. It was felt on the territory of 14 states, with a total shaking area of more than 14106 km2. A detailed isoseismal map of this earthquake is given for the Tajikistan territory only. The earthquake was accompanied by a series of over 1400 aftershocks with KR=8.6–12.8, unexpectedly numerous for a deep earthquake. Within the borders of the Republic, the Sarez-II earthquake occurred near the Lake of Sarez on December 7 with Mw=7.2, h=20 km, I0=8, was the strongest one. Undoubtedly, it was triggered by the Hindu Kush earthquake on October 26. In total, more than 500 houses were damaged, dozens of people were injured, and there were human casualties. A detailed isoseismal map of this earthquake was made for four levels of intensity – I=7, 6, 5 and 4. The number of its aftershocks for 24 days only was 1342, with KR=8.6–13.9. As a result the level of seismicity in Tajikistan in 2015, both in terms of the number of earthquakes and the level of released seismic energy, was the highest during the period of instrumental observations.

SEISMICITY of NORTHERN EURASIA in 2015

The review of the Northern Eurasia seismicity for 2015 includes a description of seismic networks, the results of analysis of the seismic regime and individual noticeable earthquakes in 16 regions of Russia and neighbouring countries. Seismic monitoring was carried out by the networks of seismic station of Russia, Azerbaijan, Armenia, Belarus, Kazakhstan, Kyrgyzstan, Latvia, Moldova, Turkmenistan, Tajikistan, Uzbekistan, Ukraine, including 599 digital, 7 analogue stations and eight seismic groups. In 2015, these networks registered about 27 thousand tectonic earthquakes, over 6 thousand volcanic earthquakes, 599 explosions, 23 mountain-tectonic shocks and induced earthquakes. Focal mechanisms of 592 earthquakes were determined, the information on manifestations of 449 perceptible earthquakes was collected. 26 shocks were felt in settlements of Northern Eurasia with an intensity Ii≥5. According to estimates of the annual number and released seismic energy in 2015 in comparison with the long-term characteristics of the seismic regime, the seismic process in most regions of Northern Eurasia proceeded in the “background” regime. An exception is Tajikistan and adjacent territories, where two strong earthquakes occurred – the Hindu Kush earthquake on October 26 with Mw=7.5, h=230 km in northern Afghanistan, near the border with Tajikistan, and the Sarez earthquake on December 7 with Mw=7.2, Ms=7.6, h=20 km in Tajikistan. Both earthquakes were accompanied by numerous aftershocks and were felt in Tajikistan with intensities Imax=7 and Imax=7–8 respectively, on the MSK-64 scale. Notable event on the territory of Northern Eurasia in 2015 is the emergence of the Muyakan sequence of earthquakes, the largest for the period of instrumental observations in the region "Baikal and Transbaikalia", as a result of which the number of recorded earthquakes in the region quadrupled concerning 2014. The other interesting fact is occurrence of tangible earthquakes in the regions, traditionally considered weakly seismic – near the Semipalatinsk test area in Eastern Kazakhstan (Chingiz earthquake on January 20, Ms=4.1, I0=5–6), in the Middle Urals (Middle Ural earthquake on October 18 with ML=4.7, I0=6) and in the southwest of East -European platform (Poltava earthquake on February 3 with KR=10.7, I0=6).

Calibration of local magnitude scale for Hindukush continental subduction zone

Hindukush is an active subduction zone where at least one earthquake occurs on daily basis. For seismic hazard studies, it is important to develop a local magnitude scale using the data of local seismic network. We have computed local magnitude scale for Hindukush earthquakes using data from local network belonging to Center for Earthquake Studies (CES) for a period of three years, i.e. 2015–2017. A total of 26,365 seismic records pertaining to 2,683 earthquakes with magnitude 2.0 and greater, was used with hypocentral distance less than 600 km. Magnitude scale developed by using this data comes to be ML = logA + 0.929logr + 0.00298r – 1.84. The magnitude determined through formulated relation was compared with that of standard relation for Southern California and relation developed by the same authors for local network for Northern Punjab. It was observed that Hindukush region has high attenuation as compared to that of Southern California and Northern Punjab which implies that Hindukush is tectonically more disturbed as compared to the said regions, hence, seismically more active as well. We have calculated station correction factors for our network. Station correction factors do not show any pattern which probably owes to the geological and tectonic complexity of this structure. Standard deviation and variance of magnitude residuals for CES network determined using Hutton and Boore scale and scale developed in this study were compared, it showed that a variance reduction of 44.1% was achieved. Average of magnitude residuals for different distance ranges was almost zero which showed that our magnitude scale was stable for all distances up to 600 km. Newly developed magnitude scale will help in homogenization of earthquake catalog. It has been observed that b-value of CES catalog decreases when magnitude is calculated by using newly developed magnitude scale.

Economic Competition and Civilian Support for Rebel Reintegration

Economic considerations play a critical role in combatants' participation in civil conflict. We bring novel evidence to bear on a related, but under-studied question: do economic considerations impact civilian support for conflict termination? Reconciliation depends on successful reintegration of ex-combatants into the peace-time economy. However, competitive considerations may undermine civilian support for reintegration. We provide evidence for this claim employing a quasi-experimental approach, leveraging localized effects of the 2015 Hindu Kush earthquake and individual-level survey data on support for Taliban reintegration. The earthquake reduced support for reintegration into disproportionately impacted economic sectors. We find no change in support for reintegration into unaffected sectors. The results are robust to a battery of tests including a novel spatial randomization with geocoded fault line segments representing the universe of counterfactual earthquakes. Our findings provide new insight into the economics of conflict resolution: economic considerations may undermine civilian support for rebel reintegration.

Observed failure modes of unreinforced masonry buildings during the 2015 Hindu Kush earthquake

On 26th October 2015, an Mw 7.5 earthquake struck northern Pakistan, with its epicenter located 45 km southwest of Jarm in the Hindu Kush region of Afghanistan. The earthquake resulted from reverse faulting at a depth of 210 km, resulting in 280 fatalities and substantial damage to some 109,123 buildings. Regional seismicity, characteristics of recorded strong motions, damage statistics, and building performance observations are presented. Earthquake damage was mostly constrained to seismic-deficient unreinforced masonry (URM) buildings. Typical failure modes included toppled minarets, partial or complete out of plane collapse of URM walls, diagonal shear cracking in piers, flexural cracking in spandrels, corner damage, pounding damage, and damage due to ground settlement. The majority of human loss resulted due to failure of URM walls and subsequent roof collapse. URM buildings located in rural hilly areas closer to the epicenter suffered more intense and frequent damage than urban URM buildings located farther away in larger cities.

Little Geodetic Evidence for Localized Indian Subduction in the Pamir‐Hindu Kush of Central Asia

AbstractGeodetically derived velocities from Central Asia show that Northern Afghanistan, the Tajik Pamir, and northwestern Pakistan all move northward with comparable large velocities toward Eurasia. Steep velocity gradients, hence high strain rates, occur only across the Main Pamir Fault zone and with lesser magnitude between the northernmost Hindu Kush and the south and southeast margins of the Tajik Depression. Localized shortening is not apparent on any active India‐Hindu Kush crustal boundary; hence, crustal convergence between India and Eurasia in Central Asia is absorbed primarily on the northern and western margins of the Pamir. This concentrated strain on the Pamir margins is consistent with one, geometrically complex, interface between subducting Asian lithosphere and the Pamir. That interface might curve westward such that the Hindu Kush seismic zone is a continuation of the Pamir seismic zone, or alternatively, Hindu Kush earthquakes might occur in convectively unstable mantle lithosphere mechanically detached from surface faults.

Earthquake-Induced Injuries: Retrospective Epidemiological Analysis of the 2015 Hindu Kush Earthquake in Pakistan.

The aim of this study was to analyze retrospectively the earthquake-induced injuries caused by the October 2015 Hindu Kush earthquake in Pakistan. This is the first population-based study to assess epidemiologically earthquake-induced injuries in the Hindu Kush region, one of the world's most mountainous and seismically active regions. Unfortunately, only limited studies have investigated the earthquake-induced injuries and deaths in the region epidemiologically. The 5 worst affected districts were selected according to the highest number of deaths and injuries recorded. A total of 1,790 injuries and 232 deaths were reported after the 2015 earthquake. In our study area, 391 persons were recorded and verified to have been injured as a result of the earthquake. We attempted to investigate all of the 391 injured people, but the final study looked at 346 subjects because the remaining 45 subjects could not be traced because of the non-availability of their complete records and their refusal to participate in the study. Using the International Statistical Classification of Diseases and Related Health Problems, 10th revision (ICD 10), we found that the highest number - 20.23% (70 of 346) - of injuries in the earthquake fall in the class of "Injuries to an unspecified part of trunk, limb, or body region (T08-T14)." The class of "Injuries to knee and lower leg (S80-S89)," which count 15.61% (54 out of 346), followed it, and "Injuries involving multiple body regions (T00-T07)" were making 14.74% of total injuries (51 out of 346). In times of natural disasters like earthquakes, collecting and analyzing real-time data can be challenging. Therefore, a retrospective data analysis of deaths and injuries induced by the earthquake is of high importance. Studies in these emerging domains will be crucial to initiate health policy debates and to prevent and mitigate future injuries and deaths. (Disaster Med Public Health Preparedness. 2018;13:732-739).

Abundant aftershock sequence of the 2015 Mw7.5 Hindu Kush intermediate-depth earthquake

The 2015 M_w7.5 Hindu Kush earthquake occurred at a depth of 213 km beneath the Hindu Kush region of Afghanistan. While many early aftershocks were missing from the global earthquake catalogues, this sequence was recorded continuously by eight broad-band stations within 500 km. Here we use a waveform matching technique to systematically detect earthquakes around the main shock. More than 3000 events are detected within 35 d after the main shock, as compared with 42 listed in the Advanced National Seismic System catalogue (or 196 in the International Seismological Centre catalogue). The aftershock sequence generally follows the Omori's law with a decay constant p = 0.92. We also apply the recently developed double-pair double-difference technique to relocate all detected aftershocks. Most of them are located to the west of the hypocentre of the main shock, consistent with the westward propagation of the main-shock rupture. The aftershocks outline a nearly vertical southward dipping plane, which matches well with one of the nodal planes of the main shock. We conclude that the aftershock sequence of this intermediate-depth earthquake shares many similarities with those for shallow earthquakes and infer that there are some common mechanisms responsible for shallow and intermediate-depth earthquakes.

Assessing the public health impacts of disasters: A retrospective study of the October 2015 Hindu Kush earthquake in Pakistan

It is well-established that the rate of natural disasters has increased in the last three decades, with millions of people affected. The direct impacts of disasters, including earthquakes, have been monitored and documented; however, only limited information is available on the impact of natural disasters on healthcare institutions and communities. Even less information is available on the different ways in which aftereffects of these events are reflected in global and public health indicators. This study aims to assess and analyze earthquake-related public health impacts on communities and their health service-providing institutions. To this end, we conducted a case study of ten districts hit by the October 2015 Hindu Kush earthquake in Pakistan. We grouped these districts into two groups of five each: Group I include the districts most seriously affected by the earthquake in terms of death toll, number of injured and structural collapses; Group II includes the less-affected districts, which experienced lower mortality and injury rates and less structural damage than those in Group I. We retrieved six years of monthly data from the District Health Information System of the selected districts and analysed it to examine trends of ten indicators in pre- and post-earthquake contexts. Findings from both groups revealed that districts in Group I showed significant variation in trends relating to rates of communicable disease, e.g. tuberculosis and hepatitis C; of vector-borne diseases, e.g. malaria; and of psychological problems such as depression and anxiety. On the other hand, trends in indicators relating to health care services including antenatal care, immunization coverage, and postnatal care remained stable. It can be concluded that earthquakes does impact the health status and utilization of health service services within affected communities.

Natural disasters and public health in the era of Sustainable Development Goals: a retrospective study of the October 2015 Hindu Kush earthquake in Pakistan

Abstract Over the last two decades, earthquakes across the world have taken hundreds of thousands of human lives and caused countless injuries. However, limited information is available on how these natural disasters, such as earthquakes, impact healthcare institutions and communities and how this is reflected in global health indicators. We aim to assess how earthquakes impact communities’ public health and healthcare service providers by using select global health indicators. We used retrospective cohort study design, and retrieved ten districts’ monthly data. We determine, uniquely, that the geographic area most affected by the earthquake in 2015 is the area with the highest tuberculosis prevalence rate. The study found that, in the area severely affected by the earthquake, the baseline number of women attending the antenatal clinics in January 2011 was 3,718, and, throughout that year to 2016, there is a slow, but steady, increase in the number of women visiting the antenatal clinics. The same month of earthquake, number of fully immunized children drops significantly, likely due to the interruption of regular health services provided in the aftermath of earthquake. This is the first comparisons of pre and post-earthquake health indicators of two region most and least affected by the earthquake. By analyzing selected health indicators before and after the earthquake, it is clear that earthquakes impact public health and cause various vulnerabilities.

Source parameters of the Bay of Bengal earthquake of 21 May 2014 and related seismotectonics of 85°E and 90°E ridges

Source parameters of the Bay of Bengal earthquake of 21 May 2014 have been studied using full waveform inversion. Its source mechanism thus determined the orientation of the strike slip faulting as NW-SE/NE-SW. The occurrence of past earthquakes along the NE-SW nodal plane suggested its preference as the main fault which could result from the transmission of stresses from the Indian plate boundary. High stress drop of this earthquake (216 bar) is attributed to its location in the intraplate region, strike slip faulting and focus in the colder upper mantle. Comparison of the stress drop of deeper focus Hindukush earthquakes with that of the Bay of Bengal earthquake showed a smaller felt radius due to fractured lithosphere in the Himalayas vis-a-vis more efficient propagation of seismic waves in the peninsular region from the source region of this recent earthquake. The seismological evidence presented for the 85°E and 90°E ridges shows the predominance of strike slip faulting with thrusting on both the ridges. Integrating their source mechanism with that of the May 2014 earthquake, it could be inferred that the Bay of Bengal region (excluding Andaman Sumatra subduction zone) is characterised predominantly by strike slip faulting in the region north of latitude 20°N and strike slip with thrusting in the remaining portion.

Variable anelastic attenuation and site effect in estimating source parameters of various major earthquakes including M w 7.8 Nepal and M w 7.5 Hindu kush earthquake by using far-field strong-motion data

Strong-motion records of recent Gorkha Nepal earthquake (Mw 7.8), its strong aftershocks and seismic events of Hindu kush region have been analysed for estimation of source parameters. The Mw 7.8 Gorkha Nepal earthquake of 25 April 2015 and its six aftershocks of magnitude range 5.3–7.3 are recorded at Multi-Parametric Geophysical Observatory, Ghuttu, Garhwal Himalaya (India) >600 km west from the epicentre of main shock of Gorkha earthquake. The acceleration data of eight earthquakes occurred in the Hindu kush region also recorded at this observatory which is located >1000 km east from the epicentre of Mw 7.5 Hindu kush earthquake on 26 October 2015. The shear wave spectra of acceleration record are corrected for the possible effects of anelastic attenuation at both source and recording site as well as for site amplification. The strong-motion data of six local earthquakes are used to estimate the site amplification and the shear wave quality factor (Qβ) at recording site. The frequency-dependent Qβ(f) = 124f0.98 is computed at Ghuttu station by using inversion technique. The corrected spectrum is compared with theoretical spectrum obtained from Brune’s circular model for the horizontal components using grid search algorithm. Computed seismic moment, stress drop and source radius of the earthquakes used in this work range 8.20 × 1016–5.72 × 1020 Nm, 7.1–50.6 bars and 3.55–36.70 km, respectively. The results match with the available values obtained by other agencies.

Building Typologies Prevalent in Northern Pakistan and Their Performance during the 2015 Hindu Kush Earthquake

The M7.5 earthquake of 26 October 2015 resulted due to reverse faulting at an intermediate depth of 210 km within the northeast-trending tabular zone underneath the Hindu Kush region, with its epicenter located 45 km southwest of Jarm in Afghanistan. In Pakistan alone, the earthquake and subsequent aftershock swarm resulted in 280 fatalities, injuries to 1,770 persons, and notable damage to 109,123 buildings. A synopsis of observations is presented herein, covering details about seismotectonics, strong motion characteristics, damage statistics, and typical building failure modes. Building damage was observed to mostly concentrate in vulnerable rural and old unreinforced masonry buildings, with aspects such as complete or partial out of plane collapse of walls, collapse of roofs due to loss of seating, shear cracking in masonry walls/panels, shear and flexural damage in masonry spandrels, cracking at infill-frame interface, damage at building corners, pounding damage, toppled minarets, and damage due to ground settlement.

Deep India meets deep Asia: Lithospheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia)

Subduction of buoyant continental lithosphere is one of the least understood plate-tectonic processes. Yet under the Pamir–Hindu Kush, at the northwestern margin of the India–Asia collision zone, unusual deep earthquakes and seismic velocity anomalies suggest subduction of Asian and Indian lithosphere. Here, we report new precise earthquake hypocenters, detailed tomographic images and earthquake source mechanisms, which allow distinguishing a narrow sliver of Indian lithosphere beneath the deepest Hindu Kush earthquakes and a broad, arcuate slab of Asian lithosphere beneath the Pamir. We suggest that this double subduction zone arises by contrasting modes of convergence under the Pamir and Hindu Kush, imposed by the different mechanical properties of the three types of lithosphere involved. While the buoyant northwestern salient of Cratonic India bulldozes into Cratonic Asia, forcing delamination and rollback of its lithosphere, India's thinned western continental margin separates from Cratonic India and subducts beneath Asia. This torn-off narrow plate sliver forms a prominent high-velocity anomaly down to the mantle transition zone. Our images show that its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab, providing a rare glimpse at the ephemeral process of slab break-off.

On the relation of variations in the travel times of seismic waves with the changes in the speed of the Earth’s rotation

The results of long-term sounding of the lithosphere by seismic waves from the deep-focus Hindu Kush earthquakes are presented. The travel time series of the first longitudinal wave on a fixed base are constructed for six seismic observation stations (SS) located on the Russian Platform (the Obninsk SS), on the Siberian Platform (the Eltsovka SS), on the Cis-Ural Trough (the Arti SS), in the Central-Ural Megazone (the Sverdlovsk SS), in the Transbaikalia (the Bodaibo SS), and in the Northern Tien Shan (the Przhevalsk SS). The time series duration in years for these stations ranged from 1964–1970 to 2007. The travel time series of seismic waves for the stations indicated are characterized by multi-slope negative linear trends caused by changes in the stress-strain state of rocks. From the comparison of the trend slopes at different stations it follows that the changes in the stress conditions within the lithosphere are relatively weak in the aseismic regions of the Russian and Siberian Platforms and in the Ural Megazone, whereas in the seismically active regions of Tien Shan, Transbaikalia and the Cis-Ural Trough they are more pronounced. The correlation has been observed between the time series trends of the average annual travel times of seismic waves and the time series of the Earth’s rotation speed. The strongest correlation between the series can be seen for the stations, located on the platforms with weak manifestations of both seismicity and active geodynamic processes. Within the long-term periods of deceleration and acceleration of the Earth’s rotation, travel times of seismic waves are decreased and increased, respectively.

Late Cretaceous-Paleogene deepwater basin of North Afghanistan and the Central Pamirs: Issue of Hindu Kush earthquakes

The within-Iranian backarc basins, including the largest Sebzawar Basin, opened in the Mid-Cretaceous. Spreading in this basin was completed by the end of the Cretaceous. The basin closed in the Eocene with the formation of subduction zones and volcanic-plutonic belts. Data on North Afghanistan and the Central Pamirs have allowed us to reconstruct the eastern continuation of the Sebzawar Basin up to the west of the Central Pamirs. No fragments of oceanic crust are retained in Afghanistan and the Pamirs, but by analogy with the Sebzawar Basin, thick Paleogene flysch sequences and volcanic-plutonic complexes indicate setting of the active margin and subduction. It is suggested that the belt of mantle seismicity that extends for 550 km to the south of the Central Pamirs is related to the plunging and deformation of the lithosphere once underlying the Cretaceous-Paleogene basin. The extremely vigorous seismicity of the Hindu Kush megasource at the western termination of the seismic belt is caused by a number of specific tectonic features that predetermined the early onset of plunging of the subducted sheet (slab). In the megasource, the slab sank to a depth of 300 km and became vertical; its active deformation has proceeded up to the present. In the eastern part of the seismic belt, the slab started to plunge much later and therefore has retained a gentle slope, so that the depth of the hypocenters is shallower (down to 200 km), and earthquakes are less strong.

Re-evaluation of early instrumental earthquake locations: methodology and examples

The difficulties of locating earthquakes in the early instrumental period are not always fully appreciated. The networks were sparse, and the instruments themselves were of low gain, often had inappropriate frequency response and recording resolution, and their timing could be unreliable and inaccurate. Additionally, there was only limited knowledge of earth structure and consequent phase identification and propagation. The primitive Zöppritz tables for P and S, with no allowance for the core, did not come into use until 1907, and remained the main model until the adoption of the Jeffreys-Bullen tables in the mid-1930s. It was not until the early 1920s that studies of Hindu Kush earthquakes revealed that earthquake foci could have significant depth. Although many early locations are creditably accurate, others can be improved by use of more modern techniques. Early earthquakes in unusual places often repay closer investigation. Many events after about 1910 are well enough recorded to be re-located by computer techniques, but earlier locations can still be improved by using more recent knowledge and simpler techniques, such as phase re-identification and graphical re-location. One technique that helps with early events is to locate events using the time of the maximum phase of surface waves, which is often well reported. Macroseismic information is also valuable in giving confirmation of earthquake positions or helping to re-assess them, including giving indications of focal depth. For many events in the early instrumental period macroseismic locations are to be preferred to the poorly-controlled instrumental ones. Macroseismic locations can also make useful trial origins for computer re-location. Even more recent events, which appear to be well located, may be grossly in error due to mis-interpretation of phases and inadequate instrumental coverage. A well converging mathematical solution does not always put the earthquake in the right place, and computer location programs may give unrealistically small estimates of error. Examples are given of improvements in locations of particular earthquakes in various parts of the world and in different time periods.

Hindu Kush earthquakes: Relict seismicity in the zone of a closed Cretaceous-Paleogene basin

We consider different formation models of deep focus earthquakes at the boundary between the Pamir and Hindu Kush mountains including the Hindu Kush megasource. Reconstruction of the Late Cretaceous‐ Paleogene basin between the Northern and Central Pamirs allows us to connect anomalously high seismicity with the deformation of the subduction plate (slab) of the oceanic lithosphere in this basin during its submersion into the mantle. In this sense the Vrancea zone in the Carpathians is an analog of the Hindu Kush megasource. The concentration of mantle earthquakes of the intermediate type with hypocenter depths up to 300 km and magnitudes up to 8 and greater [1] within a small megasource region at the boundary of tectonic zones in the Hindu Kush, Central, and Southwestern Pamirs has long attracted the interest of researchers (Fig. 1). Here, 90% of strong earthquakes in the Pamir‐Hindu Kush zone and 95% of released energy are concentrated on a territory of 200 × 120 km [2]. In the western region, mantle earthquakes are sharply interrupted at 69 ° 30 � E. The zone of mantle earthquakes approximately 350 km long with hypocenter depths up to 200 km is extended to the northeast of the megasource already on the territory of the Pamirs. Thus, the belt of mantle seismicity with a total length up to 500 km is clearly pronounced. The Hindu Kush source is confined to its southwestern part. L.I. Lobkovskii suggested a model based on the concept of two-level plate tectonics to explain the nature of the Hindu Kush earthquakes [3‐5]. Calculations demonstrated that dissipative heating of the upper parts of the mantle up to 700‐750 ° C occurs during the underthrust of the continental lithosphere in the collision zones. At such temperatures, the mechanism of brittle deformation (earthquake) becomes impossible. At the same time, the mantle part of the lithosphere separated by the asteno-layer from the crust can submerge into the upper mantle. This process is generally aseismic. Deep-focus mantle earthquakes appear only in the anomalous regions either as a result of insufficient heating, or due to the penetration of water or other fluids. In the latter case, the rocks become brittle and capable of seismogenic deformations. According to the other version [2], mantle seismicity in the Hindu Kush is related to the submersion of rocks of the ancient oceanic crust (Paleozoic or Early Mesozoic ophiolites) to the depths of 40‐70 km under the weight of the tectonic nappes. In this case, the basalt-eclogite phase transition occurs and their submersion to the upper mantle of lower density. This is an earthquake-related process. We eventually admit that the local source of mantle seismicity can be related to submersion, deformation, and separation of a slab of oceanic lithosphere in the basin that had closed recently. For example, seismicity in the Vrancea zone (East Carpathians) is related to the residual phenomena in the subduction zone of a deep basin that completely closed in the Middle‐Late Miocene (see Fig. 4.11 in [5] and references in this publication). Such an explanation was not applied to the Hindu Kush earthquakes because the existence of a young basin and related subduction were not assumed in this region.