A study of Guptkashi,Uttarakhand earthquake of 6 February 2017 (<Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript>5.3) in the Himalayan arc and implications for ground motion estimation

The 2017 Guptkashi earthquake occurred in a segment of the Himalayan arc with high potential for a strong earthquake in the near future. In this context, a careful analysis of the earthquake is important as it may shed light on source and ground motion characteristics during future earthquakes. Using the earthquake recording on a single broadband strong-motion seismograph installed at the epicenter, we estimate the earthquake’s location (30.546° N, 79.063° E), depth (H?=?19 km), the seismic moment (M₀?=?1.12×10¹⁷ Nm, M _w 5.3), the focal mechanism (φ?=?280°, δ?=?14°, λ?=?84°), the source radius (a?=?1.3 km), and the static stress drop (Δσ _s ~22 MPa). The event occurred just above the Main Himalayan Thrust. S-wave spectra of the earthquake at hard sites in the arc are well approximated (assuming ω^?2 source model) by attenuation parameters Q(f)?=?500f^0.9, κ?=?0.04 s, and f_max?=?infinite, and a stress drop of Δσ?=?70 MPa. Observed and computed peak ground motions, using stochastic method along with parameters inferred from spectral analysis, agree well with each other. These attenuation parameters are also reasonable for the observed spectra and/or peak ground motion parameters in the arc at distances ≤?200 km during five other earthquakes in the region (4.6?≤?M _w ?≤?6.9). The estimated stress drop of the six events ranges from 20 to 120 MPa. Our analysis suggests that attenuation parameters given above may be used for ground motion estimation at hard sites in the Himalayan arc via the stochastic method.     

Gravity variation before Kunlun mountain pass western <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>=8.1 earthquake

The relation between the gravity variation features and M _S=8.1 earthquake in Qinghai-Xizang monitoring area is analyzed preliminarily, by using spatial dynamic variation results of regional gravity field from absolute gravity and relative gravity observation in 1998 and 2000. The results show that: 1) M _S=8.1 earthquake in Kulun mountain pass western occurred in the gravity variation high gradient near gravity’s high negative variation; 2) The main tectonic deformation and energy accumulation before M _S=8.1 earthquake are distributed at south side of the epicenter; 3) The range of gravity’s high negative variation at east of the M _S=8.1 earthquake epicenter relatively coincides with that rupture region according to field geology investigation; 4) Gravity variation distribution in high negative value region is just consistent with the second shear strain’s high value region of strain field obtained from GPS observation.     

A new <Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript> estimation parameter for use in earthquake early warning systems

We propose a method that employs the squared displacement integral (ID2) to estimate earthquake magnitudes in real time for use in earthquake early warning (EEW) systems. Moreover, using τ _c and P _d for comparison, we establish formulas for estimating the moment magnitudes of these three parameters based on the selected aftershocks (4.0 ≤ M _s  ≤ 6.5) of the 2008 Wenchuan earthquake. In this comparison, the proposed ID2 method displays the highest accuracy. Furthermore, we investigate the applicability of the initial parameters to large earthquakes by estimating the magnitude of the Wenchuan M _s 8.0 mainshock using a 3-s time window. Although these three parameters all display problems with saturation, the proposed ID2 parameter is relatively accurate. The evolutionary estimation of ID2 as a function of the time window shows that the estimation equation established with ID2 ^Ref determined from the first 8-s of P wave data can be directly applicable to predicate the magnitudes of 8.0. Therefore, the proposed ID2 parameter provides a robust estimator of earthquake moment magnitudes and can be used for EEW purposes.     

Nature of spatiotemporal variations in the rigidity of rock masses in seismically active zones of the San Andreas fault (Northern California)

The paper presents results of analysis of spatiotemporal variations in the rigidity of seismically active rock masses obtained from California Integrated Seismic Network data on first arrivals of P waves from local background (M < 5.0) earthquakes. The main goal was to determine sizes of zones of an anomalous response to the nucleation of strong earthquakes and to reveal specific features of dynamic manifestations of anomalies in crustal fault zones. As a result, conclusions are drawn on the nature of the variations, their implications for the development of strong earthquake sources, and their suitability for earthquake prediction.     

Geodynamic and Seismotectonic Activity in Eastern Tibet in the 21st Century

At the beginning of the 21st century, a series of great earthquakes were recorded in northeastern Tibet, along the periphery of the Bayan Hara lithospheric block. An earthquake with M_S = 8.1 occurred within the East Kunlun fault zone in the Kunlun Mountains, which caused an extended surface rupture with left-lateral strike slip. An earthquake with M_S = 8 occurred in Wenchuan (China) on May 12, 2008, giving rise to an extended overthrust along the Lunmanshan fault zone. An earthquake with M_S = 7.1 occurred in Yushu (China) on April 14, 2010; its epicenter was on the Grazze–Yushu–Funchuoshan fault; a left-lateral strikeslip offset was observed on the surface. An earthquake with M_S = 7 occurred in the vicinity of Lushan on April 20, 2013; its epicenter was within the Lunmanshan fault zone, 103 km southwest of the zone of the catastrophic Wenchuan earthquake. An earthquake with M_S = 8.2 occurred in Nepal on April 25, 2015. Based on the CSN seismic catalog, the energy of all earthquakes in eastern Tibet at the end of the 20th and beginning of the 21st centuries was estimated. It was found that Tibet was seismically quiet from 1980 to 2000. The beginning of the 21st century has been marked by seismic activation with earthquake sources migrating southward to surround the Bayan Hara lithospheric block from every quarter. Therefore, this block can be regarded as one of the most seismically active regions of China.     

Seismic source zoning and maximum credible earthquake prognosis of the Greater Kashmir Territory,NW Himalaya

We present the seismic source zoning of the tectonically active Greater Kashmir territory of the Northwestern Himalaya and seismicity analysis (Gutenberg-Richter parameters) and maximum credible earthquake (m _max) estimation of each zone. The earthquake catalogue used in the analysis is an extensive one compiled from various sources which spans from 1907 to 2012. Five seismogenic zones were delineated, viz. Hazara-Kashmir Syntaxis, Karakorum Seismic Zone, Kohistan Seismic Zone, Nanga Parbat Syntaxis, and SE-Kashmir Seismic Zone. Then, the seismicity analysis and maximum credible earthquake estimation were carried out for each zone. The low b value (<1.0) indicates a higher stress regime in all the zones except Nanga Parbat Syntaxis Seismic Zone and SE-Kashmir Seismic Zone. The m _max was estimated following three different methodologies, the fault parameter approach, convergence rates using geodetic measurements, and the probabilistic approach using the earthquake catalogue and is estimated to be M _w 7.7, M _w 8.5, and M _w 8.1, respectively. The maximum credible earthquake (m _max) estimated for each zone shows that Hazara Kashmir Syntaxis Seismic Zone has the highest m _max of M _w 8.1 (±0.36), which is espoused by the historical 1555 Kashmir earthquake of M _w 7.6 as well as the recent 8 October 2005 Kashmir earthquake of M _w 7.6. The variation in the estimated m _max by the above discussed methodologies is obvious, as the definition and interpretation of the m _max change with the method. Interestingly, historical archives (～900 years) do not speak of a great earthquake in this region, which is attributed to the complex and unique tectonic and geologic setup of the Kashmir Himalaya. The convergence is this part of the Himalaya is distributed not only along the main boundary faults but also along the various active out-of-sequence faults as compared to the Central Himalaya, where it is mainly adjusted along the main boundary fault.     

Spatial and time variations in critical frequencies of the ionospheric <Emphasis Type="Italic">F</Emphasis> region above the zone of equatorial earthquake preparation

Spatial and time variations in the critical frequencies foF2 before the strong earthquake of August 15, 1963, with the magnitude M = 7.75 are analyzed. The epicenter of the earthquake was located in the vicinity of the magnetic equator in the American longitudinal sector. The data of the topside ionosonde on board Alouette-1 and of the series of ground-based ionosondes has been used for this purpose. The ground-based ionosondes made it possible to detect an insignificant anomalous decrease in foF2 within the zone of earthquake preparation a few days prior to the earthquake. This result confirms the conclusion drawn earlier on the basis of the satellite data. The modification of the ionosphere at the F-region level is more evident in the satellite than in the ground-based data. It is also noted that the character of the time variations in foF2 a day before the earthquake is similar to the so called “quiet time” Q-disturbances in the ionosphere, when the electron concentration at the F region maximum differs from the median values by more than 20% under undisturbed geophysical conditions.     

Disturbances in the <Emphasis Type="Italic">F</Emphasis>2 region critical frequency before the earthquake of September 11, 2008 off the coast of Hokkaido,Japan, and during a moderate magnetic storm based on data from ground-based vertical ionosphere sounding stations

Based on data from ground-based vertical sounding stations, the behaviors of the ionosphere F region before a strong M 6.8 earthquake off the coast of Hokkaido, Japan, and during the moderate magnetic storm before this earthquake are compared. It was found that the critical frequency of the ionosphere F region (foF2) above the Wakkanai ground-based ionosphere vertical sounding station, which was located in the preparation zone of this earthquake, suffered a long-term disturbance of slightly more than an hour nearly half a day before the earthquake. The magnitude of earthquake-induced disturbance is comparable to that caused by a magnetic storm.     

Variation of <Emphasis Type="Italic">Q</Emphasis> value before and after the 1999 Xiuyan,Liaoning Province, <Emphasis Type="Italic">M</Emphasis>=5.4 earthquake on the basis of analysis on attenuation dispersion of P waves

A method for determining medium quality factor is developed on the basis of analyzing the attenuation dispersion of the arrived first period P wave. In order to enhance signal to noise ratio, improve the resolution in measurement and reduce systematic error we applied the data resampling technique. The group velocity delay of P wave was derived by using an improved multi-filtering method. Based on a linear viscoelastic relaxation model we deduced the medium quality factor Q _m, and associated error with 95% confidence level. Applying the method to the seismic record of the Xiuyan M=5.4 earthquake sequences we obtained the following result: (1) High Q _m started to appear from Nov. 9, 1999. The events giving the deduced high Q _m value clustered in a region with their epicenter distances being between 32 and 46 km to the Yingkou station. This Q _m versus distance observation obviously deviates from the normal trend of Q _m linearly increasing with distance. (2) The average Q _m before the 29 Dec. 1999 M=5.4 earthquake is 460, while the average Q _m between the M=5.4 event and the 12 Jan. 2000 M=5.1 earthquake is 391, and the average Q _m after the M=5.1 event is 204.     

Regeneration and utilization of nutrients during collapse of a <Emphasis Type="Italic">Mesodinium rubrum</Emphasis> red tide and its influence on phytoplankton species composition

High-biomass red tides occur frequently in some semi-enclosed bays of Hong Kong where ambient nutrients are not high enough to support such a high phytoplankton biomass. These high-biomass red tides release massive inorganic nutrients into local waters during their collapse. We hypothesized that the massive inorganic nutrients released from the collapse of red tides would fuel growth of other phytoplankton species. This could influence phytoplankton species composition. We tested the hypothesis using a red tide event caused by Mesodinium rubrum (M. rubrum) in a semi-enclosed bay, Port Shelter. The red tide patch had a cell density as high as 5.0×10⁵ cells L^?1, and high chlorophyll a (63.71 μg L^?1). Ambient inorganic nutrients (nitrate: \(\rm{NO}_3^-\), ammonium: \(\rm{NH}_4^+\), phosphate: \(\rm{PO}_4^{3-}\), silicate: \(\rm{SiO}_4^{3-}\)) were low both in the red tide patch and the non-red-tide patch (clear waters outside the red tide patch). Nutrient addition experiments were conducted by adding all the inorganic nutrients to water samples from the two patches followed by incubation for 9 days. The results showed that the addition of inorganic nutrients did not sustain high M. rubrum cell density, which collapsed after day 1, and did not drive M. rubrum in the non-red-tide patch sample to the same high-cell density in the red tide patch sample. This confirmed that nutrients were not the driving factor for the formation of this red tide event, or for its collapse. The death of M. rubrum after day 1 released high concentrations of \(\rm{NO}_3^-\), \(\rm{PO}_4^{3-}\), \(\rm{SiO}_4^{3-}\), \(\rm{NH}_4^+\), and urea. Bacterial abundance and heterotrophic activity increased, reaching the highest on day 3 or 4, and decreased as cell density of M. rubrum declined. The released nutrients stimulated growth of diatoms, such as Chaetoceros affinis var. circinalis, Thalassiothrix frauenfeldii, and Nitzschia sp., particularly with additions of \(\rm{SiO}_4^{3-}\) treatments, and other species. These results demonstrated that initiation of M. rubrum red tides in the bay was not directly driven by nutrients. However, the massive inorganic nutrients released from the collapse of the red tide could induce a second bloom in low-ambient nutrient water, influencing phytoplankton species composition.     

Activation of seismicity in Central and South Asia after the Makran earthquakes: Possible acceleration of preparation of large seismic events in the Tien Shan region

Two zones of seismicity (ten events with M _w = 7.0–7.7) stretching from Makran and the Eastern Himalaya to the Central and EasternTien Shan, respectively, formed over 11 years after the great Makran earthquake of 1945 (M _w = 8.1). Two large earthquakes (M _w = 7.7) hit theMakran area in 2013. In addition, two zones of seismicity (M ≥ 5.0) occurred 1–2 years after theMakran earthquake in September 24, 2013, stretching in the north-northeastern and north-northwestern directions. Two large Nepal earthquakes struck the southern extremity of the “eastern” zone (April 25, 2015, M _w = 7.8 and May 12, 2015, M _w = 7.3), and the Pamir earthquake (December 7, 2015, M _w = 7.2) occurred near Sarez Lake eastw of the “western” zone. The available data indicate an increase in subhorizontal stresses in the region under study, which should accelerate the possible preparation of a series of large earthquakes, primarily in the area of the Central Tien Shan, between 70° and 79° E, where no large earthquakes (M _w ≥ 7.0) have occurred since 1992.     

Seismicity anomalies before the great earthquake of <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>=8.1 in the Kunlun Pass and its significance to earthquake prediction

A great earthquake of M _S=8.1 took place in the west of Kunlun Pass on November 14, 2001. The epicenter is located at 36.2°N and 90.9°E. The analysis shows that some main precursory seismic patterns appear before the great earthquake, e.g., seismic gap, seismic band, increased activity, seismicity quiet and swarm activity. The evolution of the seismic patterns before the earthquake of M _S=8.1 exhibits a course very similar to that found for earthquake cases with M _S≥7. The difference is that anomalous seismicity before the earthquake of M _S=8.1 involves in the larger area coverage and higher seismic magnitude. This provides an evidence for recognizing precursor and forecasting of very large earthquake. Finally, we review the rough prediction of the great earthquake and discuss some problems related to the prediction of great earthquakes.     

The coda attenuation of the Yao’an area in Yunnan Province

An earthquake withM=6.5 happened on January 15, 2000 in Yao’an of Yunnan Province. After the earthquake, a temporary digital network with 6 detectors around the epicenter area was set up. 402 aftershocks were located more precisely. According to coda short recording observed, the coda averaging quality factor has been acquired via Sato’s single scattering model analyses,Q _c(f)=49f ^0.95,f=1.5～20.0 Hz, which has the attenuation characteristics of high structural active region.     

Probabilistic aftershock hazard analysis,two case studies in West and Northwest Iran

Aftershock hazard maps contain the essential information for search and rescue process, and re-occupation after a main-shock. Accordingly, the main purposes of this article are to study the aftershock decay parameters and to estimate the expected high-frequency ground motions (i.e., Peak Ground Acceleration (PGA)) for recent large earthquakes in the Iranian plateau. For this aim, the Ahar-Varzaghan doublet earthquake (August 11, 2012; M _N =6.5, M _N =6.3), and the Ilam (Murmuri) earthquake (August 18, 2014 ; M _N =6.2) have been selected. The earthquake catalogue has been collected based on the Gardner and Knopoff (Bull Seismol Soc Am 64(5), 1363-1367, 1974) temporal and spatial windowing technique. The magnitude of completeness and the seismicity parameters (a,??b) and the modified Omori law parameters (P,??K,??C) have been determined for these two earthquakes in the 14, 30, and 60 days after the mainshocks. Also, the temporal changes of parameters (a,??b,??P,??K,??C) have been studied. The aftershock hazard maps for the probability of exceedance (33%) have been computed in the time periods of 14, 30, and 60 days after the Ahar-Varzaghan and Ilam (Murmuri) earthquakes. For calculating the expected PGA of aftershocks, the regional and global ground motion prediction equations have been utilized. Amplification factor based on the site classes has also been implied in the calculation of PGA. These aftershock hazard maps show an agreement between the PGAs of large aftershocks and the forecasted PGAs. Also, the significant role of b parameter in the Ilam (Murmuri) probabilistic aftershock hazard maps has been investigated.     

Creepex of Kamchatka shallow earthquakes

Results of investigation of the lithosphere in the Kamchatka seismic focal zone from dynamic characteristics of earthquake records obtained at regional stations are presented. It is assumed that the specificity of the source zone can be estimated by the relation Cr = K _P ? bK _S ? c characterizing relative energies (energy classes, according to [Fedotov, 1972]) of short period transverse and longitudinal waves in the source. Azimuthal, spatial, and temporal variations in Cr and their relation to focal mechanisms are examined. Spatiotemporal variations in this parameter are shown to be caused by the influence of variations in the conditions in the source zone (its substance or process) on the radiation of P and S waves.     

An evaluation of earthquake hazard parameters in the Iranian Plateau based on the Gumbel III distribution

The Gumbel’s third asymptotic distribution (GIII) of the extreme value method is employed to evaluate the earthquake hazard parameters in the Iranian Plateau. This research quantifies spatial mapping of earthquake hazard parameters like annual and 100-year mode beside their 90 % probability of not being exceeded (NBE) in the Iranian Plateau. Therefore, we used a homogeneous and complete earthquake catalogue during the period 1900–2013 with magnitude M _w ? ≥?4.0, and the Iranian Plateau is separated into equal area mesh of 1° late?×?1° long. The estimated result of annual mode with 90 % probability of NBE is expected to exceed the values of M _w 6.0 in the Eastern part of Makran, most parts of Central and East Iran, Kopeh Dagh, Alborz, Azerbaijan, and SE Zagros. The 100-year mode with 90 % probability of NBE is expected to overpass the value of M _w 7.0 in the Eastern part of Makran, Central and East Iran, Alborz, Kopeh Dagh, and Azerbaijan. The spatial distribution of 100-year mode with 90 % probability of NBE uncovers the high values of earthquake hazard parameters which are frequently connected with the main tectonic regimes of the studied area. It appears that there is a close communication among the seismicity and the tectonics of the region.     

Comment on Pisarenko et al., “Characterization of the Tail of the Distribution of Earthquake Magnitudes by Combining the GEV and GPD Descriptions of Extreme Value Theory”

In this short note, I comment on the research of Pisarenko et al. (Pure Appl. Geophys 171:1599–1624, 2014) regarding the extreme value theory and statistics in the case of earthquake magnitudes. The link between the generalized extreme value distribution (GEVD) as an asymptotic model for the block maxima of a random variable and the generalized Pareto distribution (GPD) as a model for the peaks over threshold (POT) of the same random variable is presented more clearly. Inappropriately, Pisarenkoet al. (Pure Appl. Geophys 171:1599–1624, 2014) have neglected to note that the approximations by GEVD and GPD work only asymptotically in most cases. This is particularly the case with truncated exponential distribution (TED), a popular distribution model for earthquake magnitudes. I explain why the classical models and methods of the extreme value theory and statistics do not work well for truncated exponential distributions. Consequently, these classical methods should be used for the estimation of the upper bound magnitude and corresponding parameters. Furthermore, I comment on various issues of statistical inference in Pisarenkoet al. and propose alternatives. I argue why GPD and GEVD would work for various types of stochastic earthquake processes in time, and not only for the homogeneous (stationary) Poisson process as assumed by Pisarenko et al. (Pure Appl. Geophys 171:1599–1624, 2014). The crucial point of earthquake magnitudes is the poor convergence of their tail distribution to the GPD, and not the earthquake process over time.     

Precursory seismic quiescence along the Sumatra-Andaman subduction zone: past and present

In this study, the seismic quiescence prior to hazardous earthquakes was analyzed along the Sumatra-Andaman subduction zone (SASZ). The seismicity data were screened statistically with mainshock earthquakes of M _w?≥?4.4 reported during 1980–2015 being defined as the completeness database. In order to examine the possibility of using the seismic quiescence stage as a marker of subsequent earthquakes, the seismicity data reported prior to the eight major earthquakes along the SASZ were analyzed for changes in their seismicity rate using the statistical Z test. Iterative tests revealed that Z factors of N?=?50 events and T?=?2?years were optimal for detecting sudden rate changes such as quiescence and to map these spatially. The observed quiescence periods conformed to the subsequent major earthquake occurrences both spatially and temporally. Using suitable conditions obtained from successive retrospective tests, the seismicity rate changes were then mapped from the most up-to-date seismicity data available. This revealed three areas along the SASZ that might generate a major earthquake in the future: (i) Nicobar Islands (Z?=?6.7), (ii) the western offshore side of Sumatra Island (Z?=?7.1), and (iii) western Myanmar (Z?=?6.7). The performance of a stochastic test using a number of synthetic randomized catalogues indicated these levels of anomalous Z value showed the above anomaly is unlikely due to chance or random fluctuations of the earthquake. Thus, these three areas have a high possibility of generating a strong-to-major earthquake in the future.     

Temporal and spatial distributions of precursory seismicity rate changes in the Thailand-Laos-Myanmar border region: implication for upcoming hazardous earthquakes

To study the prospective areas of upcoming strong-to-major earthquakes, i.e., M _w  ≥ 6.0, a catalog of seismicity in the vicinity of the Thailand-Laos-Myanmar border region was generated and then investigated statistically. Based on the successful investigations of previous works, the seismicity rate change (Z value) technique was applied in this study. According to the completeness earthquake dataset, eight available case studies of strong-to-major earthquakes were investigated retrospectively. After iterative tests of the characteristic parameters concerning the number of earthquakes (N) and time window (T _w ), the values of 50 and 1.2 years, respectively, were found to reveal an anomalous high Z-value peak (seismic quiescence) prior to the occurrence of six out of the eight major earthquake events studied. In addition, the location of the Z-value anomalies conformed fairly well to the epicenters of those earthquakes. Based on the investigation of correlation coefficient and the stochastic test of the Z values, the parameters used here (N = 50 events and T _w  = 1.2 years) were suitable to determine the precursory Z value and not random phenomena. The Z values of this study and the frequency-magnitude distribution b values of a previous work both highlighted the same prospective areas that might generate an upcoming major earthquake: (i) some areas in the northern part of Laos and (ii) the eastern part of Myanmar.     

Frictional and viscous effects on the nucleation phase of an earthquake

We study the frictional and viscous effects on earthquake nucleation, especially for the nucleation phase, based on a one-degree-of-freedom spring-slider model with friction and viscosity. The frictional and viscous effects are specified by the characteristic displacement, U _c, and viscosity coefficient, η, respectively. Simulation results show that friction and viscosity can both lengthen the natural period of the system and viscosity increases the duration time of motion of the slider. Higher viscosity causes a smaller amplitude of lower velocity motion than lower viscosity. A change of either U _c (under large η) or η (under large U _c) from a large value (U _ch for U _c and η _h for η) to a small one (U _cl for U _c and η _l for η) in two stages during sliding can result in a clear nucleation phase prior to the P-wave. The differences δU _c = U _ch ? U _cl and δη = η _h ? η _l are two important factors in producing a nucleation phase. The difference between the nucleation phase and the P-wave increases with either δU _c or δη. Like seismic observations, the peak amplitude of P-wave, which is associated with the earthquake magnitude, is independent upon the duration time of nucleation phase. A mechanism specified with a change of either η or U _c from a larger value to a smaller one due to temporal variations in pore fluid pressure and temperature in the fault zone based on radiation efficiency is proposed to explain the simulation results and observations.