Seismicity and source parameters of moderate earthquakes in Sikkim Himalaya

In this study, we accurately relocate 360 earthquakes in the Sikkim Himalaya through the application of the double-difference algorithm to 4?years of data accrued from a eleven-station broadband seismic network. The analysis brings out two major clusters of seismicity??one located in between the main central thrust (MCT) and the main boundary thrust (MBT) and the other in the northwest region of Sikkim that is site to the devastating Mw6.9 earthquake of September 18, 2011. Keeping in view the limitations imposed by the Nyquist frequency of our data (10?Hz), we select 9 moderate size earthquakes (5.3????Ml????4) for the estimation of source parameters. Analysis of shear wave spectra of these earthquakes yields seismic moments in the range of 7.95?×?10²¹ dyne-cm to 6.31?×?10²³ dyne-cm and corner frequencies in the range of 1.8?C6.25?Hz. Smaller seismic moments obtained in Sikkim when compared with the rest of the Himalaya vindicates the lower seismicity levels in the region. Interestingly, it is observed that most of the events having larger seismic moment occur between MBT and MCT lending credence to our observation that this is the most active portion of Sikkim Himalaya. The estimates of stress drop and source radius range from 48 to 389?bar and 0.225 to 0.781?km, respectively. Stress drops do not seem to correlate with the scalar seismic moments affirming the view that stress drop is independent over a wide moment range. While the continental collision scenario can be invoked as a reason to explain a predominance of low stress drops in the Himalayan region, those with relatively higher stress drops in Sikkim Himalaya could be attributed to their affinity with strike-slip source mechanisms. Least square regression of the scalar seismic moment (M ₀) and local magnitude (Ml) results in a relation LogM ₀?=?(1.56?±?0.05)Ml?+?(8.55?±?0.12) while that between moment magnitude (M _w ) and local magnitude as M _w ?=?(0.92?±?0.04)Ml?+?(0.14?±?0.06). These relations could serve as useful inputs for the assessment of earthquake hazard in this seismically active region of Himalaya.     

Seismicity of Gujarat

Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M _w7.8), 1956 (M _w6.0) and 2001 (M _w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.     

Source parameters and scaling relations for small earthquakes in the Kachchh region of Gujarat,India

The scaling relationships for stress drop and corner frequency with respect to magnitude have been worked out using 159 accelerograms from 34 small earthquakes (M _w 3.3–4.9) in the Kachchh region of Gujarat. The 318 spectra of P and S waves have been analyzed for this purpose. The average ratio of P- to S-wave corner frequency is found to be 1.19 suggestive of higher corner frequency for P wave as compared to that for S wave. The seismic moments estimated from P waves, M ₀(P), range from 1.98 × 10¹⁴ N m to 1.60 × 10¹⁶ N m and those from S waves, M ₀(S), range from 1.02 × 10¹⁴ N m to 3.4 × 10¹⁶ N m with an average ratio, M ₀(P)/M ₀(S), of 1.11. The total seismic energy varies from 1.83 × 10¹⁰ J to 2.84 × 10¹³ J. The estimated stress drop values do not depend on earthquake size significantly and lie in the range 30–120 bars for most of the events. A linear regression analysis between the estimated seismic moment (M ₀) and corner frequency (f _c) gives the scaling relation M ₀ f _c ³  = 7.6 × 10¹⁶ N m/s³. The proposed scaling laws are found to be consistent with similar scaling relations obtained in other seismically active regions of the world. Such an investigation should prove useful in seismic hazard and risk-related studies of the region. The relations developed in this study may be useful for the seismic hazard studies in the region.     

Source parameters of earthquakes in the reservoir-triggered seismic (RTS) zone of Koyna–Warna, Western India

New empirical relations are derived for source parameters of the Koyna–Warna reservoir-triggered seismic zone in Western India using spectral analysis of 38 local earthquakes in the magnitude range M _L 3.5–5.2. The data come from a seismic network operated by the CSIR-National Geophysical Research Institute, India, during March 2005 to April 2012 in this region. The source parameters viz. seismic moment, source radius, corner frequency and stress drop for the various events lie in the range of 10¹³–10¹⁶ Nm, 0.1–0.4 km, 2.9–9.4 Hz and 3–26 MPa, respectively. Linear relationships are obtained among the seismic moment (M ₀), local magnitude (M _L), moment magnitude (M _w), corner frequency (fc) and stress drop (?σ). The stress drops in the Koyna–Warna region are found to increase with magnitude as well as focal depths of earthquakes. Interestingly, accurate depths derived from moment tensor inversion of earthquake waveforms show a strong correlation with the stress drops, seemingly characteristic of the Koyna–Warna region.     

Mass estimates of the scalar seismic moments of small earthquake foci on southern Sakhalin

The frequency dependence of the function of the seismic wave attenuation was determined for the first time for southern Sakhalin on the basis of seismic coda of local earthquakes using the model of single scattering. The algorithm of the automated definition of the scalar seismic moments was realized for small earthquake foci. Mass estimates of the scalar seismic moments were obtained as exemplified by the after-shocks of the August 17, 2006, Gornozavodsk earthquake (M_W 5.6) and the May–June 2004 Kostromskoe earthquake swarm events, which occurred in South Sakhalin. The dynamic parameters of the earthquake foci were determined from the SH-wave spectra adjusted for absorption and geometrical spreading. The loglinear relationship determined between the seismic moment and the local magnitude is in good agreement with the estimates obtained for other regions and, in a certain sense, does not contradict the average world dependence.     

Estimation of maximum magnitude (M max ): Impending large earthquakes in northeast region,India

In the present study, the cumulative seismic energy released by earthquakes (M _w ≥ 5) for a period of 1897 to 2009 is analyzed for northeast (NE) India. For this purpose, a homogenized earthquake catalogue in moment magnitude (M _w ) has been prepared. Based on the geology, tectonics and seismicity, the study region is divided into three source zones namely, 1: Arakan-Yoma Zone (AYZ), 2: Himalayan Zone (HZ) and 3: Shillong Plateau Zone (SPZ). The maximum magnitude (M _max ) for each source zone is estimated using Tsuboi’s energy blocked model. As per the energy blocked model, the supply of energy for potential earthquakes in an area is remarkably uniform with respect to time and the difference between the supply energy and cumulative energy released for a span of time, is a good indicator of energy blocked and can be utilized for the estimation of maximum magnitude (M _max ) earthquakes. The proposed process provides a more consistent model of gradual accumulation of strain and non-uniform release through large earthquakes can be applied in the assessment of seismic hazard. Energy blocked for source zone 1, zone 2 and zone 3 regions is 1.35×10¹⁷ Joules, 4.25×10¹⁷ Joules and 7.25×10¹⁷ Joules respectively and will act as a supply for potential earthquakes in due course of time. The estimated M _max for each source zone AYZ, HZ, and SPZ are 8.2, 8.6, and 8.7 respectively. M _max obtained from this model is well comparable with the results of previous workers from NE region.     

Scaling properties of corner frequencies of Kamchatka earthquakes

Scaling properties of earthquake populations bear the major information on the physics of the source process of an earthquake. To determine scaling properties, source spectra of more than 400 earthquakes of Kamchatka were determined in a frequency range 0.1–30 Hz using materials of digital registration of PET station, and characteristic frequencies of earthquakes were estimated. The range of magnitudes is 4–6.5, the range of distances is 80–220 km. To enable reduction of a spectrum to the source, attenuation properties of the medium around PET were determined beforehand. It is revealed that source spectra show several corner (characteristic) frequencies: f _c1, f _c2 and f _c3; where the spectral trend changes: from f ⁰ to f ^?1, from f ^?1 to f ^?2, and from f ^?2 to f ^?3, respectively. Although in some cases f _c1 ≈ f _c2 in agreement with the usual ω^?2 spectral model, the main part of spectra has more complicated character. For a large part of the studied earthquakes a source-controlled upper cutoff of acceleration spectrum, or corner frequency f _c3, is observed. This is an important fact, as the existence of f _c3 (source-controlled f _max) is not recognized in the bulk of the seismological literature. For f _c1, the observed scaling agrees with the usual hypothesis of similarity of the earthquake sources of different size (magnitude); it is close to f _c1 ～ M ₀ ^?1/3 , where M ₀ is seismic moment. For f _c2, scaling is close to f _c2 ～ M ₀ ^?0.17 ～ f _c1 ^0.5 , that indicates an expressed violation of similarity. For f _c3, scaling is close to f _c2 ～ M ₀ ^?0.08 ～ f _c1 ^0.25 , so that similarity is broken even sharper in this case. Hypotheses about possible causes of the observed scaling are discussed.     

Spectral source parameters for weak local earthquakes in the Pannonian basin

Dynamic source parameters are estimated from P-wave displacement spectra for 18 local earthquakes (1.2 < M_L < 3.7) that occurred in two seismically active regions of Hungary between 1995 and 2004. Although the geological setting of the two areas is quite different, their source parameters cannot be distinguished. The source dimensions range from 200 to 900 m, the seismic moment from 6.3x10¹¹ to 3.48×10¹⁴ Nm, the stress drop from 0.13 to 6.86 bar, and the average displacement is less than 1 cm for all events. The scaling relationship between seismic moment and stress drop indicates a decrease in stress drop with decreasing seismic moment. A linear relationship of M _w = 0.71 M _L + 0.92 is obtained between local magnitude and moment magnitude.     

Technogenic tectonic seismicity in Kuzbass

This paper presents an analysis of the development of the current seismic state of the Kuznetsk coal basin, which is characterized by an increase in technogenic seismicity of different types under the influence of prolonged intensive mining operations. The development of technogenesis led to a significant increase in technogenic seismicity in the Kuznetsk Basin in the 1970-1980s, when the number of technogenic earthquakes began to exceed the number of natural earthquakes. Among the various types of induced seismicity, special attention is paid to strong technogenic tectonic earthquakes with a regional magnitude M_b ≥ 3 and, accordingly, a seismic energy release of more than 10⁹ J, i.e., earthquakes of energy class K > 9. These small-focus earthquakes are often accompanied by destruction of underground mines, collapse of quarries and pits, damage to surface facilities and equipment, and other adverse effects. In this paper, such earthquakes are defined as technogenic tectonic to emphasize their dual origin: technogenic impacts and the subsequent relaxation of tectonic stresses. It is also noted that the Earth’s interior in Kuzbass initially had its own natural seismicity and a developed system of tectonic faults. Natural seismotectonic activity combined with constantly increasing scales of mining and explosive consumption has led to an increase in the number of technogenic seismic events and their intensity. A striking example of such an event was the 18 June, 2013 Bachat earthquake with a regional magnitude M_b= 5.8 and a seismic intensity of 7 in the epicentral zone. It was the world’s largest man-made earthquake induced by the mining of solid minerals. We consider the possible causes of this catastrophic earthquake and discuss the conditions favoring the formation of foci of such technogenic tectonic earthquakes resulting from changes in the geodynamic and hydrogeological conditions in the Earth’s crust under man-caused impacts. These induced changes in natural processes are accompanied by a change in the stress-strain state, resulting in the concentration of tectonic stresses at heterogeneities and in fault zones, which become sources of induced technogenic seismicity.The paper discusses the current period of the occurrence and increase in such anthropogenic seismicity in the Kuzbass region with increasing scales of coal mining and blasting. Over the last 20 years, the consumption of explosives at Kuzbass enterprises increased from 100-200 to 500-600 thousand tons per year, and, accordingly, the amounts of broken and transported rock increased from several million tons per year to a billion tons per year, which disturbed the dynamic equilibrium in the Earth’s crust and changed the existing field of tectonic stresses. Moreover, the continuously increasing consumption of explosives has also increased the technogenic impact on the crust structures. The location of the epicenters of large-scale blasts inducing seismic events with regional magnitudes M_b= 3.0-4.5 has made it possible to identify regions with the greatest technogenic impact in Kuzbass. Using the data of the ISC seismological catalog, we separated seismic events with the above magnitudes into day and night ones. Since blasting work is forbidden at night, night seismic events are referred to as technogenic tectonic earthquakes (night event criterion). The maximum magnitude of seismic events induced by blasting operations in the Kuznetsk Basin was estimated at M_b ≤ 4.4. The annual number of technogenic tectonic earthquakes with 3.0 ≤ M_b ≤ 3.4, 3.5 ≤ M_b ≤ 3.9, 4.0 ≤ M_b ≤ 4.4, and M_b ≥ 4.5 was determined based on the night event criterion. The regions of their occurrence were identified from the location of the epicenters of technogenic tectonic earthquakes.     

Seismic hazard across Bulgaria and neighbouring areas: regional and site-specific maximum credible magnitudes and earthquake perceptibility

A probabilistic seismic hazard assessment is developed here using maximum credible earthquake magnitude statistics and earthquake perceptibility hazard. Earthquake perceptibility hazard is defined as the probability a site perceives ground shaking equal to or greater than a selected ground motion level X, resulting from an earthquake of magnitude M, and develops estimates for the most perceptible earthquake magnitude, M _P(max). Realistic and usable maximum magnitude statistics are obtained from both whole process and part process statistical recurrence models. These approaches are extended to develop relationships between perceptible earthquake magnitude hazard and maximum magnitude recurrence models that are governed by asymptotic and finite return period properties, respectively. Integrated perceptibility curves illustrating the probability of a specific level of perceptible ground motion due to all earthquakes over the magnitude range extending from ?∞ to a magnitude M _i are then developed from reviewing site-specific magnitude perceptibility. These lead on to achieving site-specific annual probability of exceedance hazard curves for the example cities of Sofia and Thessaloniki for both horizontal ground acceleration and ground velocity. Both the maximum credible earthquake magnitude M ₃ and the most perceptible earthquake magnitude M _P(max) are of importance to the earthquake engineer when approaching anti-seismic building design. Both forms of hazard are illustrated using contoured hazard maps for the region bounded by 39°–45°N, 19°–29°E. Patterns are observed for these magnitude hazard estimates—especially M _P(max) specific to horizontal ground acceleration and horizontal ground velocity—and compared to inferred patterns of crustal deformation across the region. The full geographic region considered is estimated to be subject to a maximum credible earthquake magnitude M ₃—estimated using cumulative seismic moment release statistics—of 7.53 M _w, calculated from the full content of the adopted earthquake catalogue, while Bulgaria’s capital, Sofia, is estimated a comparable value of 7.36 M _w. Sofia is also forecast most perceptible earthquake magnitudes for the lowest levels considered for horizontal ground acceleration of M _PA(50) = 7.20 M _w and horizontal ground velocity of M _PV(5) = 7.23 M _w for a specimen focal depth of 15 km.     

Andean earthquakes triggered by the 2010 Maule,Chile (Mw 8.8) earthquake: Comparisons of geodetic,seismic and geologic constraints

The Maule, Chile, (M_w 8.8) earthquake on 27 February 2010 triggered deformation events over a broad area, allowing investigation of stress redistribution within the upper crust following a mega-thrust subduction event. We explore the role that the Maule earthquake may have played in triggering shallow earthquakes in northwestern Argentina and Chile. We investigate observed ground deformation associated with the M_w 6.2 (GCMT) Salta (1450 km from the Maule hypocenter, 9 h after the Maule earthquake), M_w 5.8 Catamarca (1400 km; nine days), M_w 5.1 Mendoza (350 km; between one to five days) earthquakes, as well as eight additional earthquakes without an observed geodetic signal. We use seismic and Interferometric Synthetic Aperture Radar (InSAR) observations to characterize earthquake location, magnitude and focal mechanism, and characterize how the non-stationary, spatially correlated noise present in the geodetic imagery affects the accuracy of our parameter estimates. The focal mechanisms for the far-field Salta and Catamarca earthquakes are broadly consistent with regional late Cenozoic fault kinematics. We infer that dynamic stresses due to the passage of seismic waves associated with the Maule earthquake likely brought the Salta and Catamarca regions closer to failure but that the involved faults may have already been at a relatively advanced stage of their seismic cycle. The near-field Mendoza earthquake geometry is consistent with triggering related to positive static Coulomb stress changes due to the Maule earthquake but is also aligned with the South America-Nazca shortening direction. None of the earthquakes considered in this study require that the Maule earthquake reactivated faults in a sense that is inconsistent with their long-term behavior.     

Tectonic structures across the East African Rift based on the source parameters of the 20 May 1990 M7.2 Sudan earthquake

Earthquakes in Kenya are common along the Kenya Rift Valley because of the slow divergent movement of the rift and hydrothermal processes in the geothermal fields. This implies slow but continuous radiation of seismic energy, which relieves stress in the subsurface rocks. On the contrary, the NW-SE trending rift/fault zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift/fault zones have been the sites of a number of strong earthquakes in the past such as the M _w = 7.2 southern Sudan earthquake of 20 May 1990 and aftershocks of M _w = 6.5 and 7.1 on 24 May 1990, the 1937 M _s = 6.1 earthquake north of Lake Turkana close to the Kenya-Ethiopian border, and the 1913 M _s = 6.0 Turkana earthquake, among others. Source parameters of the 20 May 1990 southern Sudan earthquake show that this earthquake consists of only one event on a fault having strike, dip, and rake of 315°, 84°, and ?3°. The fault plane is characterized by a left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment M _o = 7.65 × 10¹⁹ Nm, and moment magnitude, M _w = 7.19 (?7.2). The fault rupture started 15 s earlier and lasted for 17 s along a fault plane having dimensions of ?60 km × 40 km. The average fault dislocation is 1.1 m, and the stress drop, ?σ, is 1.63 MPa. The distribution of historical earthquakes (M _w ≥ 5) from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW–SE alignment of epicenters is characterized by earthquakes of local magnitude M _l ≤ 4.0, except the 1928 Subukia earthquake (M _s = 6.9) in central Kenya. This NW–SE alignment of epicenters is consistent with the trend of the Aswa-Nyangia Fault Zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean. We therefore conclude that the NW–SE trending rift/fault zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general.     

Premonitory crustal deformations, strains and seismotectonic features (b-values) preceding Koyna earthquakes

There have been instances of premonitory variations in tilts, displacements, strains, telluric current, seismomagnetic effects, seismic velocities ( V_p, V_s) and their ratio (V_p/V_s), b-values, radon emission, etc. preceding large and moderate earthquakes, especially in areas near epicentres and along faults and other weak zones. Intensity and duration (T) of these premonitory quantities are very much dependent on magnitude (M) of the seismic event. Hence, these quantities may be utilised for prediction of an incoming seismic event well in advance of the actual earthquake. In the recent past, tilts, strain in deep underground rock and crustal displacements have been observed in the Koyna earthquake region over a decade covering pre- and postearthquake periods; and these observations confirm their reliability for qualitative as well as quantitative premonitory indices. Tilt began to change significantly one to two years before the Koyna earthquake of December 10, 1967, of magnitude 7.0. Sudden changes in ground tilt measured in a watertube tiltmeter accompanied an earthquake of magnitude 5.2 on October 17, 1973 and in other smaller earthquakes in the Koyna region, though premonitory changes in tilt preceding smaller earthquakes were not so much in evidence. However, changes in strains in deep underground rock were observed in smaller earthquakes of magnitude 4.0 and above. Furthermore, as a very large number of earthquakes (M = 1–7.0) were recorded in the extensive seismic net in the Koyna earthquake region during 1963–1975, precise b-value variations as computed from the above data, could reveal indirectly the state of crustal (tectonic) strain variations in the earthquake focal region and consequently act as a powerful premonitory index, especially for the significant Koyna earthquakes of December 10, 1967 (M = 7.0) and October 17, 1973 (M = 5.2). The widespread geodetic and magnetic levelling observations covering the pre- and postearthquake periods indicate significant vertical and horizontal crustal displacements, possibly accompanied by large-scale migration of underground magma during the large seismic event of December 10, 1967 in the Koyna region (M = 7.0). Duration (T) of premonitory changes in tilt, strains, etc., is generally governed by the equation of the type logT = A + BM (A and B are statistically determined coefficients). Similar other instances of premonitory evidences are also observed in micro-earthquakes (M = − 1 to 2) due to activation of a fault caused by nearby reservoir water-level fluctuations.     

Hierarchy of earthquakes in the Baikal rift system: implications for lithospheric stress

We investigated space-time lithospheric stress patterns of the Baikal rift system according to the hierarchy of earthquakes using mechanisms of 265 K_P ≥ 10 events recorded from 1950 to 1998 and seismic moments of 802 K_P ≥ 11 events from 1968 to 1994. The lithosphere of the region was confirmed to undergo rifting with mostly normal-slip events, while local areas of frequent strike-slip and reverse motions may record stress heterogeneity. The dominance of rifting, although being evident in the stress dynamics, is unstable, which is indicated by increase in strike-slip and reverse motions to as many as normal slip events in the latest 1980s–earliest 1990s. The lithospheric stress patterns inferred from seismic-moment data are generally consistent with those derived from the classical focal mechanism method. The suggested approach of seismic zoning according to earthquake slip geometry may provide a more reliable background for successful mitigation of seismic hazard in the region.     

Bayesian probabilities of earthquake occurrences in Shanxi rift system (China)

China has a long history of earthquake records. The Shanxi rift system (SRS) is situated along the axial zone of the domal uplift of the Shanxi Highlands and is the boundary between the Ordos block and the North China Plain block. Strong earthquakes in the SRS have been recorded since the thirteenth century. In our work, we applied the Bayesian probability method using extreme value distribution of earthquake occurrences to estimate the seismic hazard in the SRS. The seismic moment, slip rate, earthquake recurrence rate, and magnitude were considered as the basic parameters for computing the Bayesian prior estimates of the seismicity. These estimates were then updated in terms of Bayes’ theorem and historical estimates of seismicity in the SRS. The probability of occurrence of $M_{\text{s}} = 5.0$ for Z1, Z2, and Z3 is less than 0.3, 0.1, and 0.6, respectively (T = 5 years). The probability of the occurrence of M  $\ge$  8.0 is small for the whole SRS. The selection of upper bound magnitude probably influences the result, and the upper bound magnitude of Z1, Z2, and Z3 may be 7.5, 7.0, and 8.5, respectively. We obtained the values of the magnitude of completeness M _c (3.2) and the Gutenberg–Richter b value before applying the Bayesian extreme value distribution of earthquake occurrences method.     

Seismicity assessment in and around Syria based on instrumental data: application of Gumbel distributions and Gutenberg-Richter relationship

An instrumental earthquake catalog covering the time span between 1903 and 2007 and for the area bounded by 32°N–38°N and 35°E–43°E has been compiled in this research. The catalog has a magnitude of completeness (M _c ) with 3.5. Least squares and statistical probability Gumbel’s techniques with different approaches have been applied on the instrumental events in order to assess the average recurrence time periods for different earthquake magnitudes. The constants a and b of Gutenberg-Richter and the average recurrence times have been computed firstly for the study area and secondly for the central and northern parts of Dead Sea fault system. The different statistical computations using Knopoff and Kagan formalism are generally in agreement and suggest an average recurrence time of 203 years for an earthquake of magnitude 7 for the region. The occurrence of large well-documented historical earthquakes in Lebanon and western Syria, the existence of active fault segments, the absence of large earthquakes during the study period, the increasing number of the low-magnitude earthquakes, and the continued accumulation of the strain since 1900 indicate therefore the probability of an earthquake occurrence of a large magnitude. This should be permanently taken into consideration in seismic hazard assessment on the local and regional scales.     

Magnitude conversion problem for the Turkish earthquake data

Earthquake catalogues which form the main input in seismic hazard analysis generally report earthquake magnitudes in different scales. Magnitudes reported in different scales have to be converted to a common scale while compiling a seismic data base to be utilized in seismic hazard analysis. This study aims at developing empirical relationships to convert earthquake magnitudes reported in different scales, namely, surface wave magnitude, M _S, local magnitude, M _L, body wave magnitude, m _b and duration magnitude, M _d, to the moment magnitude (M _w). For this purpose, an earthquake data catalogue is compiled from domestic and international data bases for the earthquakes occurred in Turkey. The earthquake reporting differences of various data sources are assessed. Conversion relationships are established between the same earthquake magnitude scale of different data sources and different earthquake magnitude scales. Appropriate statistical methods are employed iteratively, considering the random errors both in the independent and dependent variables. The results are found to be sensitive to the choice of the analysis methods.     

On the assessment of maximum earthquakes in the alps and adjacent areas

Strong tectonic earthquakes within the crust always occur on already existing faults, and they have the property of a shear rupture. Such earthquakes with surface-wave magnitudes M < 7 obviously have a geometric similarity. Because of this similarity and the validity of the Gutenberg and Richter's energy—magnitude relation, the expression M = 2 log₁₀ L + const., with L = focal length, is valid.The expression L_maxL^* for the maximum focal length, is also valid if L^* is the length of the rectilinear extent of the seismic line on which the maximum earthquake occurs. The bounds of L^* may be given by sharp bends and/or by traversing deep faults. Thus the maximum imaginable earthquake on a seismic line with the length L^* has the magnitude M_max = 2 log₁₀ L^* + const.For the investigated region — the Alps and adjacent areas — from the data of recent and historical strong earthquakes, it follows that M_max = 2 log₁₀ L^* + 1.7, if L^* is measured in kilometres. These limiting values lie in the centre-field of the magnitude range for maximum earthquakes, published by Shebalin in 1970. By the aid of this equation it is also possible to assess the upper limiting value of the accompanying maximum scale intensity.     

Seismic hazard monitoring in Kamchatka and its applications to the M = 6.6 earthquake of 6 October 1987

The earthquake of 6 October 1987 (M = 6.6), which occurred near the Shipunsky Cape, Kamchatka, was the largest crustal event in the vicinity of the main city of Kamchatka — Petropavlovsk-Kamchatsky — during the last three decades. It was followed by numerous aftershocks. This earthquake allowed us to test the effectiveness of the seismic hazard monitoring in Kamchatka, including the seismological, geodetic and hydrogeochemical surveys. The seismic survey provided the location and source nature of the main shock and aftershocks and the seismic environment of the main shock. The geodetic and hydrogeochemical surveys have yielded data on the response to earthquakes of the Earth's surface deformations, water level, and chemical elements concentration in the underground water. As a result, the following data were obtained:

u

The earthquake of 6 October had a seismic moment 4–10 E18 Nm, thrust type of faulting and the source volume of 20 × 20 × 10 km³. The maximum intensity was VI–VII (MSK-64 scale) and maximum acceleration 88 cm/s².

Before this event, a relative increase in the number of the upper mantle (depth more than 100 km) moderate magnitude earthquakes during 5 years and a one-year period of seismic quiescence for small shallow earthquakes, were recognized. Significant anomalies in HCO₃ and H₃BO₃ concentrations in the underground waters were observed in the wells a week before the main shock.

     

Evaluating of the earthquake hazard parameters with Bayesian method for the different seismic source regions of the North Anatolian Fault Zone

The earthquake hazard parameters and earthquake occurrence probabilities are computed for the different regions of the North Anatolia Fault Zone (NAFZ) using Bayesian method. A homogenous earthquake catalog for M _S magnitude which is equal or larger than 4.0 is used for a time period between 1900 and 2015. Only two historical earthquakes (1766, M _S = 7. 3 and 1897, M _S = 7. 0) are included in Region 2 (Marmara Region) where a large earthquake is expected in the near future since no large earthquake has been observed for the instrumental period. In order to evaluate earthquake hazard parameters for next 5, 10, 20, 50, 100 years, M _max (maximum regional magnitude), β value, λ (seismic activity or density) are computed for the different regions of NAFZ. The computed M _max values are changed between 7.11 and 7.89. While the highest magnitude value is calculated in the Region 9 related to Tokat-Erzincan, the lowest value in the Region 10 including the eastern of Erzincan. The “quantiles” of “apparent” and “true” magnitudes of future time intervals of 5, 10, 20, 50, and 100 years are calculated for confidence limits of probability levels of 50, 70 and 90 % of the 10 different seismic source regions. The region between Tokat and Erzincan has earthquake hazard level according to the determined parameters. In this region the expected maximum earthquake size is 7.8 with 90 % occurrence probability in next 100 years. While the regional M _max value of Marmara Region is computed as 7.61, expected maximum earthquake size is 7.37 with 90 % occurrence probability in next 100 years.