Apparent stress and stress drop for intraplate earthquakes and tectonic stress in the plates

The magnitude of shear stress in the lithosphere is bounded from below by the apparent stress and stress drop during intraplate earthquakes. Apparent stresses and stress drops for a number of mid-plate earthquakes are calculated from the earthquake magnitude, SH wave amplitude spectra, and estimates of the length of the fault zone. Apparent stresses vary between 0.1 and 2 bars, ifm _b is used as a measure of seismic energy, and stress drops lie between 2 and 70 bars. There is no systematic difference in either apparent stress or stress drop between these intraplate events and typical plate boundary earthquakes. These bounds on intraplate shear stresses are consistent with the inference from current models of plate tectonic driving forces that regional stress differences in the plates are typically on the order of 100 bars. The highest stress drops measured for midplate earthquakes under this model represent nearly total release of local tectonic stress.     

Source parameters and scaling relations for small earthquakes in Kumaon Himalaya, India

We investigate the scaling relationships among earthquake source parameters using more than 300 good quality broad band seismograms from 30 small earthquakes in the Kumaon Himalaya from the spectral analysis of P and S waves. The average ratio of P/S wave corner frequency is found to be 1.13, which is suggestive of shift in the corner frequency. The estimated seismic moment range from 1.6?×?10¹³–5.8?×?10¹⁵ N?m, while the stress drop varies from 0.6 to 16 bars with 80 % of the events below 10 bars. An analysis of stress drop and apparent stress drop indicates the partial stress drop mechanism in the region. The source radii are between 0.17 and 0.88 km. The total seismic energy varies from 1.79?×?10⁸ to 7.30?×?10¹¹ J. We also observe the variation in seismic energy for a given seismic moment. The scaling relation between the seismic moment and stress drop suggests the breakdown of constant stress drop scaling for the range of seismic moments obtained here for the region. This shows the anomalous behavior of small earthquakes in the region. The study indicates that the stress drop is the dominant scaling factor for the moments studied here.     

Site response and source spectra of S waves in the Zagros region, Iran

S wave amplitude spectra from shallow earthquakes with magnitudes ranging between 4.2 and 6.2 in the Zagros region of Iran that occurred between 1998 and 2008 are used to examine source parameters and site response of S waves. A generalized inversion scheme has been used to separate the source, propagation path, and local site effects from S wave spectra. For removing the trade-off between source and site terms and propagation effects (including geometric and anelastic attenuation), the spectral amplitudes of the records used were corrected for attenuation and geometrical spreading function using a path model proposed by Zafarani and Soghrat (Bull Seism Soc Am 102:2031–2045, 2012) for the region. We assume a Brune’s point source model to retrieve source parameters like corner frequency, moment magnitude, and high-frequency fall off coefficient, for each event. When the source spectra are interpreted in terms of Brune’s model, the average stress drops obtained are about 7.1 and 5.9 MPa (71 and 59 bars), respectively for the eastern and western Zagros regions. Stress drops range from 1.4 to 35.0 MPa (14 to 350 bars), with no clear dependence on magnitude. The results in terms of stress drop and S wave seismic energy indicate that the Zagros events are more similar to interplate earthquakes of western North America than to intraplate events of eastern North America. The method also provides us with site responses for all 40 stations individually and is an interesting alternative to other methods, such as the H/V method. A new empirical relationship between body-wave magnitudes and moment magnitude has been proposed for the Iranian plateau using derived seismic moment from the inversion.     

Stress Drops of the 1997–1998 Colfiorito,Central Italy Earthquakes: Hints for a Common Behaviour of Normal Faults in the Apennines

Stress drop estimates of moderate-magnitude earthquakes in the Umbria–Marche region, in the northern Apennines, exhibit a large scatter. For the two M _w 5.7 and 6.0 main shocks of 26 September 1997 near Colfiorito, several papers resulted in stress drop estimates of 20 MPa, but values as low as 2–3 MPa were proposed as well. Also for the largest aftershocks (M _w > 4), estimates spread from < 1 MPa up to values ten times larger. We have critically revisited methods and data used in the literature. We have specifically faced the trade-off between source and propagation effects, as we believe that it is responsible for a part of the large scatter. To keep this trade-off under control, we have applied a methodology that combines the best fit of both source spectra after Empirical Green’s Function (EGF) deconvolution and observed ground motion spectra, finding that the results of the two different data sets converge independently at the same solution. We have used ground motions observed in the Colfiorito basin, where an accelerograph and a co-located seismological broad-band station recorded three clusters of earthquakes in a broad magnitude interval (1.7 ≤ M _w ≤ 6.0). We have found that the mainshock–aftershock sequences result in stress drops of 2–5 MPa at M _w ≥ 5.6, with an average tendency to decrease at smaller magnitudes where stress drop variability increases. These findings confirm the source scaling recently assessed through Empirical Green’s Function deconvolution for another well-monitored seismic sequence of normal-faulting earthquakes, which struck the city of L’Aquila in the central Apennines in April 2009. The similar scaling law of the two areas suggests common mechanisms of stress release for the shallow normal faults in the Apennines. The propensity of smaller earthquakes to increase in variability, with a tendency toward smaller stress drops, may reflect an effect of fault strength heterogeneities for smaller size ruptures.     

Stress Release and Source Scaling of the 2010–2011 Canterbury,New Zealand Earthquake Sequence from Spectral Inversion of Ground Motion Data

The Canterbury earthquake sequence beginning with the 2010 M _W 7.2 Darfield earthquake is one of the most notable and well-recorded crustal earthquake sequences in a low-strain-rate region worldwide and as such provides a unique opportunity to better understand earthquake source physics and ground motion generation in such a tectonic setting. Ground motions during this sequence ranged up to extreme values of 2.2 g, recorded during the February 2011 M _W 6.2 event beneath the city of Christchurch. A better understanding of the seismic source signature of this sequence, in particular the stress release and its scaling with earthquake size, is crucial for future ground motion prediction and hazard assessment in Canterbury, but also of high interest for other low-to-moderate seismicity regions where high-quality records of large earthquakes are lacking. Here we present a source parameter study of more than 200 events of the Canterbury sequence, covering the magnitude range M _W 3–7.2. Source spectra were derived using a generalized spectral inversion technique and found to be well characterized by the ω ^?2 source model. We find that stress drops range between 1 and 20 MPa with a median value of 5 MPa, which is a factor of 5 larger than the median stress drop previously estimated with the same method for crustal earthquakes in much more seismically active Japan. Stress drop scaling with earthquake size is nearly self-similar, and we identify lateral variations throughout Canterbury, in particular high stress drops at the fault edges of the two major events, the M _W 7.2 Darfield and M _W 6.2 Christchurch earthquakes.     

Iterative deconvolution of complex body waves from great earthquakes—the Tokachi-Oki earthquake of 1968

A method of body-wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone. A sequence of point sources, each being characterized with the seismic moment, the onset time and the location, are iteratively derived from observed records at multi-stations, where the two-dimensional extent of the source location is taken into account. A modification is made of the iterative method of Kikuchi and Kanamori on the formulation of inversion procedure to facilitate the computation.Using this method, we analyse long period P waves of the Tokachi-Oki earthquake of 1968 (M_w = 8.2) and obtain several significant subevents with time durations of ～ 10 s. Their spatio-temporal distribution shows that the rupture process consists of three characteristic stages: (A) a stage of introductory rupture, (B) a stage of main rupture and (C) a stage of aftershocks. The main rupture takes place in the form of clustering around a few sites of the fault plane. The largest subevent occurs in the northwestern corner. The stress drop associated with this event is estimated to be ～ 200 bars, one order of magnitude higher than the stress drop averaged over the entire fault plane. The sum of the seismic moments of the individual subevents amounts to 2.3 × 10²⁸ dyn. cm which approximately coincides with the one estimated from the analysis of long-period surface waves. This implies that the source of the Tokachi-Oki earthquake consists of several major subevents with time durations of ～ 10 s in addition to other minor subevents.     

Stochastic Finite Fault Modeling of Subduction Zone Earthquakes in Northeastern India

In this article, a stochastic finite fault source model is calibrated to estimate ground motion in northeastern India for intermediate depth events originating in the Indo-Burmese tectonic domain. A total of 47 three-component accelerograms from eight events with magnitudes ranging from M _w 4.8–6.4 are used to estimate the input source and site parameters of the finite fault source model. Key seismic parameters such as stress drop (Δσ) and site amplification function are determined from the recorded strong motion data. The obtained stress drop of the eight recorded events lies in between 105 and 165 bars.     

Ground Motion Relations for Active Regions in India

In this article, a study on development of ground motion prediction equations (GMPEs) is undertaken for seismically active regions in India. To derive the equations, the seismically active regions are divided into four units based on seismotectonic setting and geology. Due to lack of strong motion data, a stochastic finite-fault simulation method is used for generating a complete synthetic database with respect to magnitude and distance. The input parameters in the stochastic seismological model, such as site amplification and stress drop, are first derived from the past strong-motion data. A total of 236 three-component records from 62 earthquakes with magnitudes ranging from M _w 3.4 to 7.8 are used to calibrate the seismological model. The obtained stress drops of these 62 events lie in between 60 and 165 bars. With the help of a large synthetic database generated from the calibrated seismological model, ground motion relations for 5 % damped spectral acceleration are obtained by regression analysis. The developed ground motion relations are compared with the existing GMPEs of the other active regions in the world. Although the proposed equations have trends similar to those of the existing relations, there are some differences attributed to stress drop and the quality factor of active regions in India. These relations will be useful to prepare spectral acceleration hazard maps of India for a given annual probability of exceedance.     

Spectral analysis of body waves for earthquakes and their source parameters in the Himalaya and nearby regions

The source characteristics of 33 earthquakes with magnitude m_b between 4.4 and 6.0, which occurred in the Himalayan and nearby regions, are investigated using the records of the Hyderabad seismograph station. The P- and S-wave spectra of these events are interpreted in terms of Brune's seismic source model for estimating the source parameters, i.e., seismic moment, source dimension, stress drop, average dislocation, apparent stress and the radiated energy. Seismic moments, M₀, vary between 0.3 × 10²⁴ and 9.0 × 10²⁶ dyne cm; source dimensions, r, between 4.3 and 18.6 km; stress-drops, Δσ between 0.3 and 151.6 bar; average dislocations, $\text{u}$ between 0.6 and 381 cm; apparent stresses, $\text{η}\text{σ}$ between 0.1 and 73.2 bar. The radiated energy, E_R is estimated by the spectrum integration method and is found to vary between 0.2 × 10¹⁸ and 9.3 × 10²² erg. In general, the stress drop and apparent stress are found to be high, indicating high stresses in these regions.     

Seismic sources and attenuation properties at the Campi Flegrei volcanic area

Microearthquake digital data collected at Campi Flegrei during the recent (1982–1985) ground uplift episode have been analyzed in order to infer source and medium seismic properties. The main results obtained from these analyses are:

1. Hypocenter distribution and the size of the seismic zone do not change with time and do not depend on the ground uplift rate. Events occurred clustered in time with no simple causal relations between the cluster occurrences and their energy.
2. Anelastic attenuation does not depend strongly on frequency, showing a constant pattern at high frequencies. The observed values of low and high frequency attenuation, due to the short source receiver distances, do not seriously affect the spectral content of signals radiated by the sources.
3. A constant Brune stress drop pattern (～4–5 bars) as a function of seismic moment is observed. This indicates that the manner of fracturing is almost independent on magnitude of earthquakes (hypothesis of self-similarity (Aki, 1967)). Seismic processes in a prefractured medium can explain the observed small stress drop values.
4. Focal mechanisms from moment tensor estimates show that radiation patterns are mostly well interpreted in terms of double couple source models.
5. The scaling of peak ground motion parameters (A _max andV _max vs seismic moment) can be explained by an ω² source model (constant stress drop) multiplied by an exponential function with a small decay parameter, which takes into account the measured attenuation.

These results support the hypothesis of earthquakes generated by simple shear fractures along prefractured structures as a response to changes in the stress field due to the ground deformation.     

Body-wave wave-form modelling and source parameters for the indochina border earthquakes

Wave-form modelling of body waves has been done to study the seismic source parameters of three earthquakes which occurred on October 21, 1964 (M _b =5.9), September 26, 1966 (M _b =5.8) and March 14, 1967 (M _b =5.8). These events occurred in the Indochina border region where a low-angle thrust fault accommodates motion between the underthrusting Indian plate and overlying Himalaya. The focal depths of all these earthquakes are between 12–37 km. The total range in dip for the three events is 5°–20°. TheT axes are NE-SW directed whereas the strikes of the northward dipping nodal planes are generally parallel to the local structural trend. The total source durations have been found to vary between 5–6 seconds. The average values of seismic moment, fault radius and dislocation are 1.0–11.0×10²⁵ dyne-cm, 7.7–8.4km and 9.4–47.4 cm, respectively whereas stress drop, apparent stress and strain energy are found to be 16–76 bars, 8.2–37.9 bars and 0.1–1.7×10²¹ ergs, respectively. These earthquakes possibly resulted due to the tension caused by the bending of the lithospheric plate into a region of former subduction which is now a zone of thrusting and crustal shortening.     

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.     

Did the M S7.0 Lushan earthquake dynamically trigger earthquakes in the Datong volcanic region (Shanxi Province)?

Immediately following the M _S7.0 Lushan earthquake on April 20, 2013, using high-pass and low-pass filtering on the digital seismic stations in the Shanxi Province, located about 870–1,452 km from the earthquake epicenter, we detected some earthquakes at a time corresponding to the first arrival of surface waves in high-pass filtering waveform. The earthquakes were especially noticed at stations in Youyu (YUY), Shanzizao (SZZ), Shanghuangzhuang (SHZ), and Zhenchuan (ZCH), which are located in a volcanic region in the Shanxi Province,but they were not listed in the Shanxi seismic observation report. These earthquakes occurred 4–50 min after the passage of the maximum amplitude Rayleigh wave, and the periods of the surface waves were mainly between 15 and 20 s following. The Coulomb stresses caused by the Rayleigh waves that acted on the four stations was about 0.001 MPa, which is a little lower than the threshold value of dynamic triggering, therefore, we may conclude that the Datong volcanic region is more sensitive to the Coulomb stress change. To verify, if the similar phenomena are widespread, we used the same filtering to observe contrastively continuous waveform data before, and 5 h after, the M _S7.0 Lushan earthquake and M _S9.0 Tohoku earthquake in 2011. The results show that the similar phenomena occur before the earthquakes, but the seismicity rates after the earthquakes are remarkably increased. Since these weak earthquakes are quite small, it is hard to get clear phase arrival time from three or more stations to locate them. In addition, the travel time differences between P waves and S waves (S–P) are all less than 4 s, that means the events should occur in 34 km around the stations in the volcanic region. The stress of initial dynamic triggering of the M _S9.0 Tohoku earthquake was about 0.09 MPa, which is much higher than the threshold value of dynamic triggering stress. The earthquakes after the M _S9.0 Tohoku earthquake are related to dynamic triggering stress, but the events before the earthquake cannot be linked to seismic events, but may be related to the background seismicity or from other kinds of local sources, such as anthropogenic sources (i.e., explosions). Using two teleseismic filtering, the small background earthquakes in the Datong volcanic region occur frequently, thus we postulate that previous catalog does not apply bandpass filter to pick out the weak earthquakes, and some of the observed weak events were not triggered by changes in the dynamic stress field.     

Seismic moment of great deep shocks

The long-period Rayleigh waves were investigated for the largest four deep shocks in 1963–1973 to determine the seismic moment by the same technique as used for shallow earthquakes. The results could be used for a quantitative comparison of source parameters between shallow and deep events. Three of the four shocks occurred beneath the South American continent (the Colombia earthquake, 1970; the western Brazil earthquake, 1963; the Peru—Bolivia border earthquake, 1963) and the other beneath the Japan Sea (1973). The focal depths are 653, 576, 593 and 575 km, respectively. The largest value of seismic moment was obtained as 2.1 · 10²⁸ dyncm for the Colombia earthquake. This value is still about forty times smaller than that for the great Alaskan earthquake. A slight inconsistency was found between the first-motion diagram and the Rayleigh wave radiation pattern for the Colombia earthquake and the Peru—Bolivia border earthquake.     

Estimates of fracture parameters of earthquakes

A new estimate of the fracture parameters of earthquakes is provided in this paper. By theMuskhelishvili method (1953) a number of basic relations among fracture-mechanics parameters are derived. A scheme is proposed to evaluate the slip weakening parameters in terms of fault dimension, average slip, and rise time, and the new results are applied to 49 events compiled in the earthquake catalogue ofPurcaru andBerckhemer (1982). The following empirical relations are found in the paper: $$\begin{gathered} \frac{{\tau _B - \tau _f }}{{\tau _\infty - \tau _f }} = 2.339 \hfill \\ {{\omega _c } \mathord{\left/ {\vphantom {{\omega _c } {W = 0.113}}} \right. \kern-\nulldelimiterspace} {W = 0.113}} \hfill \\ \log G_c \left( {{{dyne} \mathord{\left/ {\vphantom {{dyne} {cm}}} \right. \kern-\nulldelimiterspace} {cm}}} \right) = 2 \log L (km) + 6.167 \hfill \\ \log \delta _c (cm) = 2 \log L (km) - 1.652 \hfill \\ \end{gathered} $$ whereG _c is the specific fracture energy,ω _c the size of the slip weakening zone,δ _c the slip weakening displacement,τ _B ?τ _f the drop in strength in the slip weakening zone,τ _∞?τ _f the stress drop,L the fault length, andW the fault width. The investigation of 49 shocks shows that the range of strength dropτ _B ?τ _f is from several doze to several hundred bars at depthh<400 km, but it can be more than 10³ bars ath>500 km; besides, the range of the sizeω _c of the strength degradation zone is from a few tenths of a kilometer to several dozen kilometers, and the range of the slip weakening displacementδ _c is from several to several hundred centimeters. The specific fracture energyG _c is of the order of 10⁸ to 10¹¹ erg cm^?2 when the momentM ₀ is of the order of 10²³ to 10²⁹ dyne cm.     

Mechanism and tectonic implications of the great Banda Sea earthquake of November 4, 1963

The Banda Sea earthquake of November 4, 1963 (h = 100 km, m_B = 7.8) is probably one of the largest intermediate-depth shocks to have occurred worldwide this century. The mechanism of this earthquake is studied in detail on the basis of P-wave first motion, surface wave and aftershock data. From the analysis of long-period multiple surface waves, a seismic moment of 3.1 × 10²⁸ dyn-cm is obtained, which is the largest reported so far for any intermediate or deep focus shock. This value, together with the estimated fault area of 90 × 70 km², gives an average dislocation of 7.0 m and a stress drop of 120 bar. This event represents an oblique thrust movement on a plane with dip direction N170°E, dip 48° and rake 52°. A geometrical consideration for the fault plane and the configuration of the inclined seismic zone beneath the Banda arc suggests, almost definitely, that the large-scale faulting took place within the subducted plate and offset it. Further repetition of such large-scale faulting might eventually break the subducted plate. The 1963 Banda Sea earthquake thus represents a seismological manifestation of the large-scale deformation of the subducted plate in the mantle.     

Seismotectonic study of Kishtwar region of Jammu Province using local broadband seismic data

Two moderate earthquakes of Mw 5.7 on the first of May and Mw 5.2 on the second of August occurred in the Kishtwar region in the year 2013. Our broadband seismic observatories located in the region recorded these events and the aftershocks. We analyzed these data to understand the seismotectonics of this region. Most of the events were located between 33.03° to 33.29° N latitude and 75.40° to 76.07° E longitude. Focal depths of these shallow earthquakes range from 7 to 12 km and are confined between Panjal Thrust (PT) and Kishtwar Window (KW). Spectral analysis of these events reveals that stress drop, source radius, corner frequency, and moment magnitude varied between 3.3 and 70.1 bars, 0.121 and 3.55 km, 0.397 and 6.06 Hz, and Mw 2.2 and Mw 5.7, respectively. The low stress drop of small-magnitude earthquakes reveals the brittle nature of the upper crust which is coincident with the field observations. The variation of stress drop with magnitude shows positive correlation whereas no such relation was observed between stress drop and depth of focus. The b value calculated (0.83) for the area reveals high stress accumulation within the incompetent rock zones in the area.     

Slip, Stress Drop and Ground Motion of Earthquakes: A View from the Perspective of Fractional Brownian Motion

—The characteristics of slip and stress drop distributions accompanying earthquakes are explored from the perspective of fractional Brownian motion (fBm). Slip and stress drop distributions are assumed to be processes of fBm.The Hurst exponent (H), which reveals the roughness of a random process of fBm, is first estimated from ten inferred slip maps for six crustal earthquakes occurring in California. The relationships between the Hurst exponents with respect to static slip (H _u ),stress (H _τ ), static stress drop (H _δσ ) and slip velocity (H _ú ) are then established following Andrews (1980). They are found to be H _δσ = H _τ = H _u -1 = H _ú -0.5. Empirically, H _u is recognized as being about 1 which, according to the theory of fBm, implies that the static slip distribution of an earthquake is just on the margin between being and not being self-similar, depending on the individual case. Cases where H _u is less than 1 (i.e., self-similar) suggest that H _δσ < 0(i.e., the distribution of static stress drop diverges), which is, in light of fBm, invalid. One possible explanation for this paradox is that H _u is less than 1 in crustal earthquake phenomena only over a certain specific bandwidth of wavenumbers, or it could be that the relation H _δσ = H _u -1 is not valid, which implies that static stress drop in the wavenumber domain is not the product of stiffness and slip as described in Andrews (1980). It could be that some different physics apply over this particular bandwidth. In such cases, multi-fractals may be a better way to explore the characteristics of the Hurst exponents of slip. In general, static stress drop and stress distributions are more likely to be self-similar than static slip distribution. H _u ≌ 1 and H _δσ ≌ 0 are good first approxima tions for the slip and stress drop distributions. The spectrum of ground motion displacement falls off as ω ^{-(Hδσ + 2)} with H _δσ ≌ 0, consistent with an ω^-2 model of the earthquake source.     

The Gilbert Islands (Republic of Kiribati) earthquake swarm of 1981–1983

A major swarm of intraplate earthquakes at the southeastern end of the Gilbert Islands Chain (3.5°S, 177.5°E) commenced in December 1981 and lasted through March 1983. No seismicity had been reported in the historical record in this region prior to 1981, but during the swarm 217 events with m_b ? 4.0 were located by the NEIS, with 86 events having m_b ? 5.0. The source region is quite remote, and the uniform detection level for the NEIS is for m_b ? 4.8. A b-value of 1.35 is found for the swarm using the maximum likelihood method. Four events in the sequence were large enough (m_b = 5.6?5.9) to determine focal mechanisms teleseismically using body- and surface-wave analysis. These events are found to have a range of mechanisms, from predominantly thrust with a significant oblique component, to purely strike-slip. The compression axes are consistent for all four events, with horizontal orientation trending NNE-SSW. This orientation is perpendicular to the direction of plate motion. The events are located at depths between 15 and 20 km placing them deep in the oceanic crust or in the upper mantle. No obvious bathymetric feature can be related to the fault plane orientations, though there is an offset in the island chain near the epicenters. While some characteristics of the swarm suggest a magmatic origin, the nature of the focal mechanisms, the location of the swarm, and the large accumulated moment release of the sequence favor a tectonic origin.     

Seismological model parameters for northeastern and its surrounding region of India

A point source seismological model is used in this study to model the available strong motion accelerograms recorded by 17 events and to calculate three seismological model parameters, the source, path, and quality factor. Due to the paucity of recorded events, this is the first time these model parameters have been obtained for the northeastern and its surrounding region of India. The quality factors of the horizontal and vertical components of recorded events with corresponding standard deviations are Q_H(f) = 188.55f^0.94, σ₁ value (25, 0.025) and Q_V(f) = 169.76f^0.93, σ₁ value (20, 0.03), respectively. The source parameter stress drop values (μσ) vary within 124–180 bars for the subduction region and 80–169 bars for the active region. The Kappa factors for the horizontal and vertical components of recorded events on the soft rock site are 0.06 and 0.05, respectively. These seismological model parameters obtained in this study will be useful for future work deriving a ground motion attenuation relation based on a spectral model. Finally, these results are useful for seismic hazard assessment of a region having sparsely recorded events.