Focal Mechanism Parameters Research Articles

SEISMICITY of the ARCTIC in 2018–2019

The article provides an overview and analysis of seismicity within the boundaries of the Arctic region for 2018–2019, a description of seismic station networks, and processing methods. The consolidated catalog of earthquakes in the Arctic region was compiled on the basis of catalogs of several organizations and seismological centers. In total, 1177 earthquakes for 2018–2019 are included in the consolidated catalog. Most of the earthquakes that occurred in 2018–2019, including all the strongest earthquakes, were located within the mid-ocean ridges of Mon, Knipovich. and Gakkel. In the offshore territories, most of the earthquakes were confined to the Svalbard archipelago, in particular, to the seismically active zone in the Sturfjord strait. Within the shelf areas, seismicity is also characteristic of the "continent-ocean" transition zone of the Barents-Kara region, Bely (Kvitøya) Island and Novaya Zemlya archipelago. For 28 earthquakes, the focal mechanism parameters are presented according to the Global CMT catalog.

Evaluating a Prospective Fault-Based Stress-Transfer Forecast for the M 7.9 Wenchuan Earthquake Region, 15 Years Later

Abstract Four days after the 12 May 2008 M 7.9 Wenchuan earthquake struck the Sichuan region of China, we submitted a prospective earthquake forecast based on transfer of stress from the mainshock onto significant faults crossing through populated areas. We identified where the largest aftershocks were likely to occur that could cause loss of life. We returned the revised article to the journal on 5 June 2008, marking the last day of our observation period. The primary testable features are locations and focal mechanisms of larger (M ≥ 4.5) earthquakes; did these events happen on or very near the faults we said they would? Did they have the same strikes, dips, and rakes as the faults we modeled? In retrospect, is the stress transfer method consistent with all M ≥ 4.5 earthquakes that occurred? We find all but one M ≥ 4.5 aftershock with known focal mechanisms located on stress-increased faults, and their focal mechanism parameters overlap with geological characteristics we used in making calculations. Six of the seven lethal M > 4.5 earthquakes that occurred in the region since 5 June 2008 were located on stress-increased faults, with the lone exception triggered by hydraulic fracturing.

Normative expression and accurate understanding of focal mechanism parameters

This paper aims at the understanding of the relationship between the upper and lower hemisphere projection maps of the focal sphere, the correct reading of the focal mechanism parameters on the projection diagram, and to investigate if there is uncertainty about whether the slip angle has a unique solution when there are only two nodal strikes and dip angles. Based on the principle of stereographic projection, the following four expressions and understanding skills about focal mechanism parameters are refined: (1) according to the principle of stereographic projection, we analyzed the difference of the presentation of focal mechanism parameters on different projection maps. There is a 180° rotation relationship between the upper and the lower hemisphere projection map. (2) The method of determining the nodal strike on the upper and lower hemisphere projections: for the former, the observer faces the convex direction of the nodal arc, and the direction to the right of the nodal plane is the stike of the fault. The angle turned clockwise in the north direction is the strike angle of the nodal plane; For the latter, the observer faces the convex direction of the arc of the nodal plane, and the point corresponding to the point on the right side of the nodal plane plus 180° is the strike of the fault. (3) The relationship between the azimuth angles of the P, T and N-axis on upper and lower hemisphere projections: the azimuth angles of the P, T, and N-axis on the upper hemisphere projection map = the azimuth angles of the P, T, and N-axis on the lower hemisphere projection map + 180°. (4) Only the strike and dip angles of two nodal planes are used to derive the rake: according to the slip angle formula, two groups of rake solution can be calculated, indicating that the solution are not unique. The actual slip angle can be determined by combining the hanging wall and footwall and strike of fault, or by combining the actual P-axis and T-axis data, or the first motion of P-waves. Aims to aid accurate understanding and long-term memory of focal mechanism parameters with consistent and simple rules.

Analysis of Seismicity in Elbasani Geothermal Zone during Five Last Decades

During the last five decades since 1968, the Elbasani zone in Albania is dominated by low seismicity. This fault zone has evidenced some seismological phenomena. It is a transversal active fault zone with normal and strike-slip faults characterized by low-velocity layers. From instrumental seismicity in this area have been located 1831 seismic events, with magnitude ML > 1.1 Richter and 3 of them with magnitude ML > 5.0. Recently on May 19, 2014, a moderate earthquake ML5.2 occurred in the Elbasani seismogenic zone. This earthquake was located 6 km southeast of Belshi town and 18 km south of Elbasani town and was felt over Albania. The analysis of seismicity during the period of time 968-2022 years indicates that most earthquakes were located in the upper and middle earth crust and very few in the uppermost mantle. The focal mechanisms solution of some earthquakes indicates an extensional regime in this fault zone. The parameters of focal mechanisms indicate the predominance of normal fault motion and strike-slip fault motion is compatible with the present-day extension regime. The Elbasan zone has a complex geological structure with a significant number of geothermal water resources and some features of seismicity. The analysis of the features of seismicity during the five last decades provides us with correct interpretations of seismotectonic, low velocity lowers and understanding of geodynamic phenomena of the Earth's crust. The area of active faults of Elbasan presents a threat to the city of Elbasan and to the curative and tourist area of Llinxha-Hidrat.

SEISMICITY of the ARCTIC in 2016–2017

The article provides an overview and analysis of seismicity within the boundaries of the Arctic region for 2016–2017, a description of seismic station networks, and processing methods. The catalog of earth-quakes in the Arctic region was compiled on the basis of catalogs of several organizations and seismological centers. In total, 875 earthquakes are included in the earthquake catalog. Most of the earthquakes that occurred in 2016–2017, including all the strongest earthquakes, were located within the mid-ocean ridges of Mon, Knipovich. and Gakkel. In the offshore territories, most of the earthquakes were confined to the Svalbard archipelago, in particular, to the seismically active zone in the Sturfjord strait. Within the shelf areas, seismicity is also character-istic of the "continent-ocean" transition zone of the Barents-Kara region. Most earthquakes within the transition zone are confined of the trenches. For 39 earthquakes, the focal mechanism parameters are presented according to the Global CMT catalog.

FELT EARTHQUAKE of JULY 4, 2017 with ML=3.7, I0=5–6 to NORTHWEST of the 1988 SPITAK EARTHQUAKE SOURCE ZONE

The instrumental data and results of macroseismic survey of the July 4, 2017 earthquake with ML=3.7, I0=5–6, occurred in the source zone of the devastating Spitak earthquake of 1988 with I0=10, are given. The earthquake was accompanied by a sound like an explosion. According to the focal mechanism parameters, a reverse faulting occurred along the NP1 plane of northwest strike and northeast dipping, or a thrust along the near-meridional NP2 plane dipping west. The migration of aftershocks to the north and east in October-December 2017 testifies in favor of choosing the NP1 nodal plane as the main one. An isoseismal map of the earthquake is constructed, according to which the isoseists are oriented to the northeast. The discrepancy between their orientation and the strike of the nodal planes is explained by the predominant influence on the macroseismic field of northeast oriented Akhourian fault, in the zone of which most of the surveyed settlements are located.

Focal Mechanism and Source Parameters Analysis of Mining-Induced Earthquakes Based on Relative Moment Tensor Inversion.

Mining-induced earthquakes (MIEs) in underground coal mines have been a common phenomenon that easily triggers rock bursts, but the mechanism is not understood clearly. This research investigates the laws of focal mechanism and source parameters based on focal mechanism and source parameters analysis of MIEs in three frequent rock burst areas. The relative moment tensor inversion (MTI) method was introduced, and the way to construct the inversion matrix was modified. The minimum ray and source number conditions were calculated, and an optimized identification criterion for source rupture type was proposed. Results show that the geological structure, stress environment, and source horizon influence the focal mechanism. The tensile type sources can distribute in the roof and coal seam, while the shear types are primarily located in the coal seam. In the typical fold structure area, the difference in source rupture strength and stress adjustment between tensile and shear types is negligible, while the disturbance scale of tensile types is distinct. The shear types have higher apparent volume and seismic moment in the deep buried fault area but lower source energy. The apparent stress of the tensile types is higher than that of the shear types, representing that the stress concentration still exists in the roof after the MIEs, but the stress near the faults could be effectively released. In the high-stress roadway pillar area, the primary fracture of the coal pillar easily produces a continuous shear rupture along the dominant stress direction under the extrusion of the roof and floor. The source parameters (except apparent stress) of shear types are higher than tensile types and have higher dynamic risk. The results contribute to expanding the understanding of rock burst mechanisms and guide MIEs’ prevention.

Using InSAR for evaluating the accuracy of locations and focal mechanism solutions of local earthquake catalogues

SUMMARYEarthquake source parameters play a fundamental role in evaluating seismic hazard. In many countries, the relatively low accuracy of the earthquake source parameters owing to sparse seismic monitoring networks prevents accurate seismic hazard evaluation. Interferometric Synthetic Aperture Radar (InSAR) is a useful tool for accurate fault parameter determination of shallow inland earthquakes. We compared the fault parameters (location and rupture geometry) derived from InSAR with those derived from local and global seismic data to assess the credibility of the earthquake catalogues. We set our targets to Iran and Japan, where seismic networks are relatively sparse and dense, respectively, and analysed a total of 10 shallow inland earthquakes. To compare the fault parameters for each studied earthquake, we first created interferograms using InSAR data from multiple satellites and conducted fault model inversions assuming uniform slip on a rectangular finite fault. For the studied Iranian earthquakes, the epicentres of the United States Geological Survey (USGS) catalogue were more consistent with the InSAR-derived fault model compared to those of the local catalogue of the Iranian Seismological Center (IRSC). For the studied Japanese earthquakes, the epicentres of the Japan Meteorological Agency (JMA) catalogue were more consistent with the InSAR-derived fault model compared to those of the USGS. Assuming that the accuracy of the USGS locations is comparable for the two countries, the JMA catalogue has a higher accuracy than the IRSC catalogue. The difference in the accuracy for the two local catalogues can be explained by the larger azimuthal gaps of the IRSC seismic network. We also showed that the IRSC- and JMA-derived centroids were more consistent with the InSAR-derived fault model compared to those of the Global Centroid Moment Tensor (GCMT) catalogue. The focal mechanism solutions derived from InSAR and seismic CMT solutions were significantly different, highlighting the difficulties in determining the focal mechanism parameters from seismic data, even when the seismic station coverage is dense. As InSAR data can be easily accessed, this study presents a cost-effective method to assess and improve the accuracy of the local earthquake catalogues anywhere in the world.

SEISMICITY of the ARCTIC in 2015

The article provides an overview and analysis of seismicity within the boundaries of the Arctic region for 2015, a description of seismic station networks, and processing methods. The catalog of earthquakes in the Arctic region was compiled on the basis of catalogs of several organizations and seismological centers. In total, 334 earthquakes are included in the earthquake catalog. Most of the earthquakes that occurred in 2015, including all the strongest earthquakes, were located within the mid-ocean ridges of Mon, Knipovich and Gakkel. In the offshore territories, most of the earthquakes were confined to the Svalbard archipelago, in particular, to the seismically active zone in the Sturfjord strait. The renewal of instrumental seismological observations in 2011 (station ZFI) on Alexandra Land Island in the Franz Josef Land archipelago made it possible to record weak earthquakes in the north of the shelf of the Barents and Kara Seas. For twelve earthquakes, the focal mechanism parameters are presented according to the Global CMT catalog.

THE CURRENT STRESS OF EARTH'S CRUST IN THE TERRITORY OF UZBEKISTAN ACCORDING TO FOCAL EARTHQUAKE MECHANISMS

The current stress of Earth's crust in the territory of Uzbekistan has been studied using a focal earthquake mechanisms catalogue that includes the data provided by many authors. Stress reconstructions are based on the cataclastic analysis of displacements along fractures. For reconstructing the stress state at different depths of the crust in several seismically active regions of the study area, we consider a minimum number of earthquakes in a homogeneous sample equal to 6 and an averaging radius of 10 to 30 km within a single domain. The azimuths and dip angles of the principal stress axes, Lode – Nadai coefficients, geodynamic types of stress modes, relative (normalized to rock strength) values of maximum shear stresses, and effective pressure values are determined. Maps showing the spatial distribution of the studied parameters are constructed for both the entire seismically active layer and the depth layers. Stress fields are reconstructed and compared at two hierarchical levels based on the parameters of focal mechanisms of weak and moderate earthquakes (М≤4.5) and those of strong (М≥5.0) earthquakes. "Tectonic Stresses of Eurasia", the Internet resource created by IPE RAS, is used to visualize the stress field reconstructed from the data on strong (М≥5.0) earthquakes.

Geomechanical conditions of causes of high-energy rock mass tremors determined based on the analysis of parameters of focal mechanisms

Geomechanical conditions of causes of high-energy rock mass tremors determined based on the analysis of parameters of focal mechanisms

ARCTIC

The article provides an overview and analysis of seismicity within the boundaries of the Arctic region for 2014, a description of seismic station networks and processing methods. The catalog of earthquakes in the Arctic region was compiled on the basis of catalogs of several organizations and seismological centers. In total, 452 earthquakes with ML≥1.5 are included in the earthquake catalog. Most of the earthquakes occurred in 2014, including all the strongest earthquakes, werelocated within the mid-ocean ridges of Mon, Knipovich and Gakkel. In the offshore territories, most of the earthquakes were confined to the Svalbard archipelago, in particular, to the seismically active zone in the Sturfjord strait. The renewal of instrumental seismological observations in 2011 (station ZFI) on Alexandra Land Island in the Franz Josef Land archipelago made it possible to record weak earthquakes in the north of the shelf of the Barents and Kara Seas. For seven earthquakes, the focal me-chanism parameters are presented according to Global CMT catalog.

SPITAK-V EARTHQUAKE on July 6, 2014 with МL=4.0, I0=5–6 (Armenia)

The work presents the results of macroseismic survey of the Spitak-V earthquake which occurred on July 6, 2014 with МL=4.0, I0=5–6 in the focal zone of the destructive Spitak earthquake 1988 with I0=10. The isoseismal map of the July 6, 2014 earthquake was made and the focal mechanism parameters were determined. According to the focal mechanism decision, the movement in the source was a strike-slip with minor uplift components.

Characterization of Major Fault Systems in the Kachchh Intraplate Region, Gujarat, India, by Focal Mechanism and Source Parameters

AbstractThe focal mechanism and source parameters of 41 local earthquakes (Mw 4.0–5.1) that occurred in the Kachchh rift basin, which is seismically one of India’s most active intraplate regions, are determined to characterize various active fault systems in that region. The tectonics in the rift basin are heterogeneous and complex. In the present study, it was found that one-third of the earthquakes exhibit reverse mechanism and three-fourth are either strike slip or have some components of strike slip. Thus, we conclude that transverse tectonics are currently dominant in the Kachchh rift. These transverse faults are preferably oriented in the northeast–southwest and northwest–southeast directions in the eastern and western parts of the rift, respectively. The movement is sinistral and dextral on faults that are oriented in the northeast–southwest and northwest–southeast directions, respectively. These transverse faults are almost vertical (dip>70°) and mostly blind with no surface expressions. Most of the significant faults that strike east–west dip toward the south and are listric. The stress drop of these 41 earthquakes ranges between 2.3 and 10.39 MPa. It was found that the stress drop of earthquakes may depend on the focal mechanism and is independent of focal depths. The average stress drop is found to be the highest (7.3 MPa) for the earthquakes that show a dominant normal mechanism accompanied by strike slip (5.4 MPa) and reverse (4.7 MPa). The average stress drop of the Kachchh intraplate region is 5.3 MPa, which is consistent with other intraplate regions of the world. A conceptual model of the fault system in the Kachchh region is proposed, based on the results obtained in the present study.

Relocation, Focal Mechanisms and Stress Field Characteristics of Earthquake Swarms in the Songyuan Area of Jilin Province, China

Two earthquake swarms, the Qianguo swarm and Ningjiang swarm, have been observed since 2013 in the Songyuan area, Jilin Province, China. Using the seismic data of broadband permanent and temporal stations that recorded a larger number of earthquakes in the two swarms, we analyze the distribution of aftershocks and the focal mechanisms of these earthquakes and investigate the stress field and faulting properties of the Songyuan area. The relocated hypocenters of the Qianguo earthquakes are distributed along a NNW direction at focal depths of 4–13 km, while the dominant distribution of relocated Ningjiang earthquakes is along a NE direction at depths of 8–16 km. The focal mechanism solutions of 36 earthquakes with ML ≥ 3.6 demonstrate a combined strike-slip and thrust faulting mechanism with centroid depth concentrated at 3–7 km for the Qianguo swarm and a strike-slip mechanism with centroid depth at 5–7 km for the Ningjiang swarm. The full moment tensors of seven moderate–strong earthquakes show that these earthquakes contain a high percentage of non-double-couple components. The regional stress field is inverted using the focal mechanism parameters of the 36 earthquakes. In the Qianguo area, the horizontal maximum principal compressive stress ( $$\sigma_{1}$$ ) is in the E–W direction with a dip angle of 14.1°. In the Ningjiang area, the $$\sigma_{1}$$ is in the ENE–WSW direction with a dip angle of 9.2°. The $$\sigma_{1}$$ directions of the two areas are slightly different, but both approximately along an E–W direction. The comprehensive results of this study show that the Qianguo swarm is likely caused by the movement of a buried fault in the NNW direction oblique to the Changling–Dashanzi fault, whereas the Fuyu–Zhaodong fault in the NE direction is responsible for the Ningjiang swarm. Both the Qianguo and Ningjiang swarms are probably closely related to the thrusting effect of the subduction of the Pacific Plate into the Northeast Asian continental margin and the complex geological conditions in the Songyuan area.

Seismic Hazard Assessment of the Mid-Northern Segment of Xiaojiang Fault Zone in Southwestern China Using Scenario Earthquakes

ABSTRACTSeismic hazards in the vicinity of the mid-northern segment of the Xiaojiang fault zone were estimated from scenario earthquakes. Based on its history of earthquakes and activity data, the whole segment is considered capable of generating large earthquakes. The characteristics of previous ruptures and the geometric structures of the fault were used to establish seven scenario earthquakes with various strike and length properties. On the basis of focal mechanism parameters and the distribution of previous earthquakes, we determined the values of the source parameters for each scenario using empirical equations. We then used a stochastic finite-fault model to generate the ground motion from the targeted fault. The results show that the vicinity of the mid-northern segment of the Xiaojiang fault zone faces significant seismic hazards. The peak ground acceleration (PGA) values along the fault line are obviously larger than in other areas. Based on the recurrence interval theory, the Qiaojia–Menggu and Xiaoxinjie–Xujiadu areas have a high probability of being struck by large earthquakes in the near future. The results also show that the current design PGA for the near-fault area of the mid-northern segment of Xiaojiang fault zone might not be adequate.

Earthquake source dynamics and kinematics of the Eastern Indian Shield and adjoining regions

The Eastern Indian Shield (EIS) consists of two cratonic nuclei, namely Singhbhum craton and Chhotanagpur Granitic Gneissic terrain. This area contains several crisscross faults, lineaments, shear zones, numerous hot springs and three major rivers (e.g., Ganga, Brahmaputra and Damodar). The area is regionally covered by 7 seismic stations and jointly recorded 16 events from the study area, and less noisy waveforms of 4 events were used for focal mechanism analysis using the Cut and Paste method. The focal parameters of these 4 events were compiled with results of 8 events computed by different workers for the study area. To understand the detailed tectonics, focal mechanisms of 21 events for the Himalayan segment were taken from CMT Harvard catalog of duration 1976–2017. Spatial variations of operative stress fields for major tectonic domains were analyzed in the present study based on stress inversion of focal mechanism parameters. We observed strike-slip-dominated movements in the EIS, which changes partially into extension in the northeast part between the Ganga and Brahmaputra Rivers. Thrust-dominated movements evidenced by focal mechanisms and the pure compression in the western segment of the Himalaya in the north might be promoting shear movements in the EIS and adjoining regions. The pure strike-slip in the eastern segment of the Himalaya and its deeper level account for lateral shearing and eastward movements of different tectonic blocks. The normal faulting earthquakes in the northeast part might be indicating stretching in the basement because of convergence of Indian lithosphere beneath the Myanmar plate.

АРКТИЧЕСКИЙ БАССЕЙН

The review of the Arctic basin seismicity for 2013 is presented. It is based mainly on the data from the sta-tions of world network, located outside the basin. The data from a single local seismic station was used. It was "Franz Josef Land" station with two sets of seismic sensors, ZF1 and ZF2. The station is a part the Ar-khangelsk network, operating in the region since September 2011. There were 38 earthquakes with mb=3.0–4.6 in the Arctic basin in total. A map of their epicenters is provided. 28 of them are connected with the seismically active zone, which extends through the deep-water part of the Arctic basin to the shelf of the Laptev Sea. For two earthquakes, the parameters of focal mechanisms are given according to the GCMT da-ta. The magnitude frequency distribution for 2013 is compared with the distributions for 1990-2012. The level of seismic energy released in 2013 is 1.3 times lower than in 2012 and 1.6 times higher than its average one for 23 previous years. The distribution of earthquakes in the Arctic region in 2013 is presented.

Казахстан

In 2013, seismic monitoring in Kazakhstan was conducted by two organizations: the Seismological Experience-Methodical expedition of the Ministry of Education and Science of the Republic of Kazakhstan (SEME), and the Institute of Geophysical Research of the Ministry of Energy of the Republic of Kazakhstan (IGR). The station network consists of three-component stations and seismic arrays. The work presents detailed information on seismic observation networks. The catalogue of seismic events of Kazakhstan is made by da-ta of above mentioned Organizations. It includes 1036 earthquakes with energy class КР=6.6–14.7. The catalog contains several additional sheets with Attachments: Microseismic data on felt earthquakes, parameters of focal mechanisms of 55 earthquakes, information on coordinates of 29 urban areas with felt shakes, parameters of 1123 aftershocks, and two forshocks of Saryzhas (Narynkol) earthquake in the range of energy classes КР=4.5–11.8. Two maps of earthquake epicenters are shown in the paper: for the whole Kazakhstan territory with КР≥8.6 and for Northern Tien Shan territory with КР≥6.6. The largest earthquake of the year was the Saryzhas earthquake that occurred in Northern Tien Shan on January 28, MS=6.0, КР=14.7. The earthquake was felt on the territory of Kazakhstan and China and was followed by numerous aftershocks. For the first day, only its number exceeded 500. It was noted that the total released seismic energy in 2013 exceeded significantly the same parameter of 2012.

УЛАХАН-ЧИСТАЙСКОЕ ЗЕМЛЕТРЯСЕНИЕ 20 января 2013 г. с КР=14.4, Mw=5.6, I0р=8 (Северо-восток Якутии)

The results of interpretation of seismological, geological–geophysical, seismotectonic, and macroseismic da-ta obtained for the focal zone of the strong Ulakhan-Chistay earthquake (Mw=5.6) that occurred on January 20, 2013, in the Chersky Ranges system, at the boundary of lithospheric plates of Eurasia and North Ameri-ca, are presented. The epicenter of the event is confined to Ulakhan-Chistay Ridge, close the Cenozoic-aged Bugchan basin. Comparison is made between the hypocentre parameters of this event determined on the basis of data from regional seismic stations and the solutions based on the data from other seismic agencies worldwide. The earthquake occurred in the zone of the major Ulakhan fault which is seen in space images and is traced in topography between the Indigirka and Kolyma Rivers over a distance of 1500 km. A characteristic feature of the fault is origination within its limits of quakes without aftershocks which was the case during the 2013 event too. Statistics of seismic manifestations observed in the past century and early this century show that over 14 thousand local event were noted in the zone influenced by the Ulakhan fault within the Earth’s crust (~35 km thick) at a depth of 7–30 km. The epicentral intensity of more than ten events reached 6–7 in the Russian scale (MSK-64). Apart from high seismicity, some additional geodynam-ic, indicators of Ulakhan fault reactivation were recognized, particularly a high heat flux, icings, paleo and modern seismodislocations, etc. Macroeffects of the Ulakhan–Chistay earthquake observed over an area of 300 thousand sq. km in three arctic districts of Yakutia and in the south of the Magadan region were studied. The epicentral intensity of this event is 8. This is supperted by close proximity of the regional epicenter to the well–preserved Tirekhtyakh paleoseismodislocation that originaled more than a thousand years ago after the strongest paleo earthquake and is clearly seen the Google search system. It is shown that the epicentral inten-sity of the earthquake corresponds to the regional macroseismic field equation. The comparison of focal mechanism parameters of the 2013 earthquake and morphokinematic characteristic of the Ulakhan fault, to which epicenter is confined, permitted us to confidently determine the crush plane has NW strike and is in-clined to SW at 74–76°. According to seismological data, motions in the earthquake source coincide with the modern type of tectonic motions on the Ulakhan fault and correspond left lateral strike slip. It is supposed that the Ulakhan possesses a higher seismic potential and can generate earthquakes MS ≥6.