S-wave Spectra Research Articles

Path Attenuation and Source Characteristics during the 2019 Sichuan Changning Earthquake Sequence in China

To give an insight into the path attenuation and source characteristics of the 2019 Sichuan Changning earthquake sequence, 99 strong‐motion recordings at 33 stations were studied. The S‐wave Fourier amplitude spectra were used to determine the path‐attenuation function and key parameters of the seismic source characteristics of this sequence. The results show the following: (1) The geometric spreading effect presents hinged‐trilinear characteristics, and the corresponding geometric spreading rates were −0.97, 0.29, and −0.73 with the crossover distances 90 km and 183 km. (2) The quality factor Q is correlated with frequency and expressed as 217 × f0.82 for the range from 0.1 to 10.0 Hz. (3) The seismic moment was inversely proportional to the cube of the corner frequency, and their product was 2.37 × 1016 N. m/s3. (4) The stress drop varied from 1.18 to 12.44 MPa. Both the attenuation effect and source parameters of this sequence are significantly different from those of the Lushan and the Wenchuan aftershock sequences in the Sichuan region of China.

Robust Bayesian estimator for S-wave spectra, using a combined empirical Green’s function

Summary We propose a new fully automatic and robust Bayesian method to estimate precise and reliable model parameters describing the observed S-wave spectra. All the spectra associated with each event are modelled jointly, using a t-distribution as likelihood function together with informative prior distributions for increased robustness against outliers and extreme values. The model includes the observed noise and a combined empirical Green’s function. It captures source-, receiver-, and path-dependent terms in the description of the observed spectra by combining a physical source and attenuation model with a spatially and event-size dependent empirical compensation. The proposed method propagates estimation uncertainties along the entire processing chain starting from the hypocentre location and delivers reliable uncertainty description of the estimands. The objective is to automatically provide robust and valid descriptions of the observed S-wave spectra generated from an earthquake source in a noisy and heterogeneous environment. The efficiency of the method is tested with synthetic seismograms, and the model is calibrated and cross-validated using 31 640 mining induced seismic events in a iron ore mine (in north of Sweden) with an comprehensive seismic network. The model is evaluated using both posterior predictive checks and residual analysis and we found no evidence that indicates any model deficiencies with respect to central tendency, dispersion, and residual trends.

Source Spectral Properties of Earthquakes in the Delaware Basin of West Texas

AbstractIn recent years, the Delaware basin of west Texas has seen a sharp rise in earthquake occurrence, driven in large part by increases in unconventional hydrocarbon production. The advent of Texas Seismological Network in 2017 has allowed for the characterization of these events in greater detail. We exploit the recent densification in seismic station coverage to study the spectral properties of earthquakes in this region. We show that the low-frequency moments of S-wave spectra, when corrected for site and distance, can be used to calibrate a consistent moment magnitude scale for small and moderate earthquakes. For a subset of >3000 well-recorded events, we compute earthquake stress drop under the assumption of Brune spectral model. Earthquakes in the Delaware basin show coherent spatial patterns in stress drop across the region. Through a reanalysis of independently collated data from the oil and gas industry, we find that earthquakes that are likely associated with hydraulic-fracturing operations have slightly different spectral characteristics than earthquakes that are likely associated with saltwater disposal. In particular, events associated with hydraulic fracturing show greater variability in the statistical distribution of stress drop and have higher median stress-drop values. Although the differences are subtle, they suggest that there may be important distinctions in the underlying physical mechanisms and resulting hazards of distinct classes of induced events, differences that may be unraveled with more detailed joint analyses of industrial and seismic datasets.

Source analysis of the March 7, 2019 mathbf {M_L=4.0} Somogyszob, Hungary earthquake sequence

Between February 16 and April 5, 2019, a series of earthquakes took place around the village of Somogyszob, Somogy county, Hungary. The mainshock occurred on March 7 with a local magnitude M_L=4.0 and epicentral intensity of 5 on the EMS scale. The main event was preceded by four foreshocks and followed by four aftershocks. The largest foreshock (M_L=2.6) was also felt with maximum intensity of 4 EMS. This earthquake sequence is the first remarkable one in the region that was recorded by a significant number of high-quality broadband digital seismographs. We have estimated the hypocenters of the 9 earthquakes using the hypoDD multiple-event location algorithm. The events occurred in a tight region around the mainshock at around 13–14 km depth. For the main event, we obtained an average moment magnitude of M_w=3.75, source radii of r^P=509 m and r^S=400 m, and static stress drops of varDelta sigma ^P=1.19times 10^6 Pa and varDelta sigma ^S=4.00times 10^6 Pa from the analysis of displacement P- and S-wave spectra, respectively. The resulting spectral source parameters for the investigated events agree well with the results of earlier research. We have also shown that our recently developed probabilistic waveform inversion techniques applied in this study are suitable to retrieve the source mechanism for weak local earthquakes. We have successfully estimated the focal mechanism for the mainshock, a foreshock and an aftershock. Each earthquake was a thrust faulting event with a sub-horizontal P-axis pointing towards N-NE, coinciding with the general trend of the compressional stress field in the epicentral region.

Source parameters of the moderate Mozambique – Zimbabwe border earthquake on 22 December 2018

An earthquake of moment magnitude (Mw = 5.6) occurred in central Mozambique close to the Zimbabwean border on 22 December 2018 at 05:37:4 GMT. The event was widely felt in Mozambique, Zimbabwe and some parts of South Africa. Buildings were damaged near the epicentre but without loss of human life. An intensity survey was conducted by manually distributing questionnaires in the affected areas as well as from online submitted questionnaires, giving a total of 215 replies. We obtained the highest intensity of between VII and VIII in Espungabera, a town approximately 25 km from the epicentre, and in Chipinge about 50 km from the epicentre. A source depth of 21.2 ± 3.8 km in the lower crust was obtained. The moment release, measured from S-wave spectra, was 3.16 × 1017 Nm yielding a moment magnitude (Mw) of 5.6. Stress drop and source radius of the mainshock were estimated to be 11.7 ± 2.3 MPa and 2.3 ± 0.3 km respectively. The focal mechanism of the event shows normal faulting with nodal planes oriented in NNE-SSW direction. The first nodal plane was at a strike of 209°, dip of 54°, and had a rake of −78°. The second nodal plane was at a strike of 9°, dip of 38° with a rake of −106°. These results are consistent with most fault plane solutions previously obtained for earthquakes in Central Mozambique.

Modelling of earthquake locations and source parameters in Kachchh region to understand genesis of earthquakes

Modelling of earthquake source locations and parameters infers seismogenesis of earthquakes. In this study, we modelled the earthquake source locations through hypocenter location algorithm using the difference in arrival time of P and S waves and source parameters through the Levenberg–Marquardt non-linear inversion method using S-wave spectra. A total of 340 aftershocks of 2001 Bhuj mainshock ( $$ 1.8 \le M_{w} < 4.3 $$ ), which have occurred in Kachchh, Gujarat, India from January 2014 to January 2015, are located in this study. Out of 340 aftershocks, digital waveforms of 78 aftershocks ( $$ 2.2 \le M_{w} < 3.9 $$ ) are used for estimation of the earthquake source parameters. The results obtained from earthquake locations show two clusters of seismicity along the Kachchh Mainland Fault (KMF) and North Wagad Fault (NWF) and three felt events ( $$ M_{w} \ge 3.0 $$ ); one along the Katrol Hill Fault (KHF) ( $$ M_{w} = 3.3 $$ ), two along the Banni Fault (BF) ( $$ M_{w} = 3.0,3.2 $$ ). The generation of these three felt events is attributed to the triggering mechanisms caused by the migration of fluids or the stress pulse generated by the 20 MPa stress drop of the Mw 7.7 Bhuj earthquake. A marked concentration of events is noticed in 15–30 km depth range, which could be attributed to the presence of a mafic intrusive body, resulting in stress build-up for earthquake generation in this region. The results of source parameters; seismic moment (M0), source radius (r) and stress drop (Δσ) vary from 1.86 × 1012 to 3.2 × 1015 N m, 146–262 m and 0.04–5.73 MPa, respectively. The maximum stress drop value is estimated to be 5.73 MPa at 24 km depth for the largest studied event of $$ M_{w} $$ 3.9. Large stress drops are confined to the 8–33 km depth range, which indicates the probable existence of the base of the seismogenic layer in this depth range. This observed large stress drops could be attributed to stresses induced by crustal mafic intrusive bodies and the presence of aqueous fluids in the lower crust below the region.

Source characteristics of the 18 September 2011 Sikkim earthquake and zoning

Using detailed waveform analysis of Pn, sPn depth phases from the nearby seismic stations, attempts were made to resolve the focal depth of the 2011 Sikkim earthquake. Focal depth of 46.8 km thus determined was found closer to the GCMT source mechanism solution as compared to ISC solution (29.6 km). Re-examination of its source mechanism with the spatial/ depth distribution of its aftershocks sug- gested that both the nodal planes oriented WNW or ENE, initially ruptured but the orientation of meizoseismal area and larger concen- tration of its aftershocks parallel to the Tista lineament conformed WNW striking nodal plane relatively more active. The large value of the stress drop derived from S-wave spectra based on the data of Indian stations was attributed to its strike-slip mechanism and deeper focal depth. The stress drop of its foreshock was much lower than that the main shock. The b-value also showed a decrease during the last decade prior to the main earthquake.The recent Sikkim earthquake (Mw 6.9) has brought out the limitations of the microzoning of the Sikkim region attempted earlier.

A study of Guptkashi, Uttarakhand earthquake of 6 February 2017 (M w 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) = 500f0.9, κ = 0.04 s, and fmax = 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.

Determination of corner frequencies of source spectra for subduction earthquakes in Avacha Gulf (Kamchatka)

The source spectra of M = 4.0-6.5 subduction earthquakes of 2011-2014 in Kamchatka are studied. The dataset comprises 1272 source spectra recovered from S waves of 372 earthquakes recorded by six digital rock-ground stations. The structure of the spectra is examined on the basis of a spectral model with three corner frequencies fc1, fc2, and fc3. It was assumed that the spectra behave as f−2 between fc2 and fc3, where fc3 denotes “source-controlled fmax” after Aki and Gusev. To determine the corner frequencies, we extracted the source spectra from S-wave spectra using a previously developed attenuation model for the study area. The spectra were first reduced to the reference hard-rock station, employing a specially determined set of spectral amplifications of stations. We approximated the recovered source spectrum by a piecewise power-law function, estimated fc1, fc2, and fc3, and examined their dependence on the seismic moment M0 (i.e., scaling). The dependence fc1(M0) does not contradict the hypothesis of source similarity when one expects fc1∝M0−1/3. For fc2 and fc3, the scaling is close to fc2∝M0−0.23 and fc3∝M0−0.13, respectively, indicating a clear violation of the similarity, especially prominent for fc3. Systematic identification of the frequency fc3, its determination, and analysis of its scaling are the main results of the study, important for understanding the physics of earthquake source processes. The use of fc3 as a source parameter in strong ground motion simulations will eliminate biases in estimating attenuation parameters, in particular, the spectral decay parameter “kappa”.

Estimation of earthquake source parameters in the Kachchh seismic zone, Gujarat, India, using three component S-wave spectra

Earthquake source parameters and crustal $$Q_{0}$$ values for the 138 selected local events of ( $$\hbox {M}_{\mathrm{w}}{:}2.5{-}4.4$$ ) the 2001 Bhuj earthquake sequence have been computed through inversion modelling of S-waves from three-component broadband seismometer data. SEISAN software has been used to locate the identified local earthquakes, which were recorded at least three or more stations of the Kachchh seismological network. Three component spectra of S-wave are being inverted by using the Levenberg–Marquardt non-linear inversion technique, wherein the inversion scheme is formulated based on $$\omega ^{2}$$ source model. SAC Software (seismic analysis code) is being utilized for calculating three-component displacement and velocity spectra of S-wave. The displacement spectra are used for estimating corner frequency (in Hz) and long period spectral level (in nm-s). These two parameters play a key role in estimating earthquake source parameters. The crustal $${Q}_{0}$$ values have been computed simultaneously for each component of three-component broadband seismograph. The estimated seismic moment ( $$M_{0}$$ ) and source radius (r) using S-wave spectra range from 7.03E+12 to 5.36E+15 N-m and 178.56 to 565.21 m, respectively. The corner frequencies for S-wave vary from 3.025 to 7.425 Hz. We also estimated the radiated energy ( $$E_{S}$$ ) using velocity spectra, which is varying from 2.76E+06 to 4.07E+11 Joules. The estimated apparent stress drop and static stress drop values range from 0.01 to 2.56 and 0.53 to 36.79 MPa, respectively. Our study also reveals that estimated $$Q_{0}$$ values vary from 119.0 to 7229.5, with an average $$Q_{0}$$ value of 701. Another important parameter, by which the earthquake rupture process can be recognized, is Zuniga parameter. It suggests that most of the Kachchh events follow the frictional overshoot model. Our estimated static stress drop values are higher than the apparent stress drop values. And the stress drop values are quite larger for intraplate earthquakes than the interplate earthquakes.

Analysis of earthquake body wave spectra for potency and magnitude values: implications for magnitude scaling relations

We develop a simple methodology for reliable automated estimation of the low-frequency asymptote in seismic body wave spectra of small to moderate local earthquakes. The procedure corrects individual P- and S-wave spectra for propagation and site effects and estimates the seismic potency from a stacked spectrum. The method is applied to >11 000 earthquakes with local magnitudes 0 < M_L < 4 that occurred in the Southern California plate-boundary region around the San Jacinto fault zone during 2013. Moment magnitude M_w values, derived from the spectra and the scaling relation of Hanks & Kanamori, follow a Gutenberg–Richter distribution with a larger b-value (1.22) from that associated with the M_L values (0.93) for the same earthquakes. The completeness magnitude for the M_w values is 1.6 while for M_L it is 1.0. The quantity (M_w – M_L) linearly increases in the analysed magnitude range as M_L decreases. An average earthquake with M_L = 0 in the study area has an M_w of about 0.9. The developed methodology and results have important implications for earthquake source studies and statistical seismology.

Estimation of site effects using strong motion data of BYTNet array in Turkey

Simultaneous estimation of effects of source, propagation path, and local site amplification was carried out using observed strong motion records in a frequency range from 0.8 to 20 Hz for the purpose of empirical evaluation of the local site effects in different geological conditions in the northwestern part of Turkey. The analyzed data are S-wave portions of 162 accelerograms from 39 shallow events observed at 14 sites of BYTNet array. A spectral separation method was applied to the observed S-wave spectra. The solutions for source spectra, inelasticity factor of propagation path for S-waves (Q s-value), and factor of site amplification at each site were obtained in a least squares sense. In the analysis, we assumed that the factor of the site amplification at a reference site is the same as that of theoretical amplification of S-waves to the soil model whose bottom layer has an S-wave velocity around 2.15 km/s. The estimated Q s-value of the propagation path is modeled as Q s(f) = 87.4f0.78. The estimated site amplifications are characterized into three groups. The sites in the first group belong to rock site with no dominant peaks at a frequency range of 2 to 10 Hz. The second group of hard soil sites is characterized with moderately dominant peaks at a frequency of 5 Hz. The last group for soft soil sites has common peaks at a frequency of 4 Hz with larger amplitudes than those in the hard soil group. We, then, compare the amplifications with average S-wave velocity in top 30 m of the shallow S-wave profiles and proposed linear empirical formula between them at each frequency. We, furthermore, inverted the observed amplification factors into S-wave velocity and Q s-value profiles of the deep soil over the basement.

Source Functions and Path Effects from Earthquakes in the Farallon Transform Fault Region, Gulf of California, Mexico that Occurred on October 2013

We determined source spectral functions, Q and site effects using regional records of body waves from the October 19, 2013 (Mw = 6.6) earthquake and eight aftershocks located 90 km east of Loreto, Baja California Sur, Mexico. We also analyzed records from a foreshock with magnitude 3.3 that occurred 47 days before the mainshock. The epicenters of this sequence are located in the south-central region of the Gulf of California (GoC) near and on the Farallon transform fault. This is one of the most active regions of the GoC, where most of the large earthquakes have strike–slip mechanisms. Based on the distribution of the aftershocks, the rupture propagated northwest with a rupture length of approximately 27 km. We calculated 3-component P- and S-wave spectra from ten events recorded by eleven stations of the Broadband Seismological Network of the GoC (RESBAN). These stations are located around the GoC and provide good azimuthal coverage (the average station gap is 39◦). The spectral records were corrected for site effects, which were estimated calculating average spectral ratios between horizontal and vertical components (HVSR method). The site-corrected spectra were then inverted to determine the source functions and to estimate the attenuation quality factor Q. The values of Q resulting from the spectral inversion can be approximated by the relations Q P = 48.1 1±1f 0:880:04 and QS = 135:4 1:1f ±0:580:03 and are consistent with previous estimates reported by Vidales-Basurto et al. (Bull Seism Soc Am 104:2027–2042, 2014) for the south-central GoC. The stress drop estimates, obtained using the ω2 model, are below 1.7 MPa, with the highest stress drops determined for the mainshock and the aftershocks located in the ridge zone. We used the values of Q obtained to recalculate source and site effects with a different spectral inversion scheme. We found that sites with low S-wave amplification also tend to have low P-wave amplification, except for stations BAHB, GUYB and SFQB, located on igneous rocks, where the P-wave site amplification is higher.

The seismic wave absorption in the crust and upper mantle in the vicinity of the Kislovodsk seismic station

The Q-factor estimates of the Earth’s crust and upper mantle as the functions of frequency (Q(f)) are obtained for the seismic S-waves at frequencies up to ~35 Hz. The estimates are based on the data for ~40 earthquakes recorded by the Kislovodsk seismic station since 2000. The magnitudes of these events are M W > 3.8, the sources are located in the depth interval from 1 to 165 km, and the epicentral distances range from ~100 to 300 km. The Q-factor estimates are obtained by the methods developed by Aki and Rautian et al., which employ the suppression of the effects of the source radiation spectrum and local site responses in the S-wave spectra by the coda waves measured at a fixed lapse time (time from the first arrival). The radiation pattern effects are cancelled by averaging over many events whose sources are distributed in a wide azimuthal sector centered at the receiving site. The geometrical spreading was specified in the form of a piecewise-continuous function of distance which behaves as 1/R at the distances from 1 to 50 km from the source, has a plateau at 1/50 in the interval from 50–70 km to 130–150 km, and decays as \({\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {\sqrt R }}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${\sqrt R }$}}\) beyond 130–150 km. For this geometrical spreading model and some of its modifications, the following Q-factor estimates are obtained: Q(f) ~ 85f 0.9 at the frequencies ranging from ~1 to 20 Hz and Q(f) ~ 75f 1.0 at the frequencies ranging from ~1 to 35 Hz.

The Q-factor estimates for the crust and upper mantle in the vicinity of Sochi and Anapa (North Caucasus)

The regularities in the radiation and propagation of seismic waves in the regions of the North Caucasus are analyzed for estimating the ground motion parameters during the probable future strong earthquakes. Based on the records of the regional earthquakes with magnitudes MW ~ 3.9–5.6 within epicentral distances up to ~300 km obtained during the period of digital measurements at the Sochi and Anapa seismic stations, the Q-factors in the vicinities of these sites are estimated at ~55 f0.9 and ~90f0.7, respectively. The estimates were obtained by the coda normalization method developed by Aki, Rautian, and other authors. This method is based on the phenomenon of suppression of the earthquake (source) effects and local (site) responses by coda waves in the S-wave spectra. The obtained Q-factor estimates can be used for forecasting the ground shaking parameters for the future probable strong earthquakes in the North Caucasus in the vicinities of Sochi and Anapa.

Source Spectra of Near Kamchatka Earthquakes: Recovering them from S-Wave Spectra, and Determination of Scaling for Three Corner Frequencies

We describe a procedure for mass determination of the “source-controlled fmax”—an important though not conventional parameter of earthquake source spectrum, relabeled here as “the third corner frequency,” fc3, and discuss the results of its application. fmax is the upper cutoff frequency of Fourier acceleration spectrum of a record of a local earthquake; both source and path attenuation contribute to fmax. Most researchers believe the role of attenuation (“κ” parameter) to be dominating or exclusive. Still, source effect on fmax is sometimes revealed. If real, it may be important for source physics. To understand better the fmax phenomena, the constituents of fmax must be accurately separated. With this goal, we process seismograms of moderate earthquakes from Kamchatka subduction zone. First, we need reliable estimates of attenuation to recover source spectra. To this goal, an iterative processing procedure is constructed, that adjusts the attenuation model until the recovered source acceleration spectra become, on the average, flat up either to fc3, or up to the high-frequency limit of the frequency range analyzed. The latter case occurs when fc3 is non-existent or unobservable. Below fc3, the double-corner source spectral model is thought to be valid, and the lower bound of acceleration spectral plateau is considered as the second corner frequency of earthquake source spectrum, fc2. The common corner frequency, fc1, is also estimated. Following this approach, more than 500 S-wave spectra of M = 4–6.5 Kamchatka earthquakes with hypocentral distances 80–220 km were analyzed. In about 80 % of the cases, fc3 is clearly manifested; the remaining cases show, at high frequency, flat source acceleration spectra. In addition, in about 2/3 of cases, fc2 is clearly above fc1, showing that double-corner spectra may dominate even at moderate magnitudes. Scaling behavior was examined for each of the corners. The fc1 vs. M0 trend is common and close to similarity (fc1 ∝ M0−1/3), whereas the trends for two other corners (fc2 ∝ M0−0.17; fc3 ∝ M0−0.11) dramatically contradict the concept of similarity. Physical interpretation of such a behavior is discussed. The origin of fc3 is ascribed to existence of the lowermost wavelength/size of fault heterogeneity. Its dependence on M0 may reflect evolution of maturity of a fault in geological time. The approximate scaling fc2 ∝ \(f_{c1}^{0.5}\) suggests that during propagation of slip pulse over a fault, its width, assumedly related to 1/fc2, grows in a stochastic manner; this reminds the random evolution of propagating boundary in the framework of the known Eden model of random growth.

Seismic attenuation tomography of the Southwest Japan arc: new insight into subduction dynamics

We determined the first high-resolution P- and S-wave attenuation (Qp and Qs) tomography of the crust and upper mantle under the entire Nankai subduction zone from the Nankai Trough to the Japan Sea using a large number of high-quality t* data measured from P- and S-wave spectra of local earthquakes. The suboceanic earthquakes used in this study were relocated precisely using sP depth phases and ocean-bottom-seismometer data. The overall pattern of the obtained Q models is similar to that of velocity models of the study region. Our present results show that high-Q (i.e. weak attenuation) anomalies in the upper crust generally correspond to plutonic rocks widely exposed in the Nankai arc. Some of the low-Q (i.e. strong attenuation) anomalies in the upper crust along the Pacific coast are associated with the Cretaceous–Cenozoic accretionary wedge. Obvious low-Q anomalies exist in the crust under the active arc volcanoes. Most of the large inland crustal earthquakes are located in or around the low-Q zones in the crust. The subducting Philippine Sea slab is imaged clearly as a landward dipping high-Q zone. Prominent low-Q anomalies are revealed in the mantle wedge under the volcanic front and backarc area, which reflect the source zone of arc magmatism caused by slab dehydration and corner flow in the mantle wedge. Significant low-Q anomalies exist in the forearc mantle wedge, which reflects a highly hydrated and serpentinized forearc mantle wedge due to abundant fluids released from dehydration of the young and warm Philippine Sea slab.

Determination of source parameters for local and regional earthquakes in Israel

We have investigated earthquake source parameters and seismic moment-magnitude relations from 103 regional and local earthquakes with moment magnitude 2.6 to 7.2, which occurred in a distance range from 4.5 to 550 km during 1995–2012 by applying Brune’s seismic source model (J Geophys Res 75:4997–5009, 1970, J Geophys Res 76:5002, 1971) for P- and S/Lg-wave displacement spectra. Considering P- and S-wave data separately, we first studied the empirical dependence of the Fourier spectral amplitudes Ω due to the geometrical spreading and the inelastic attenuation and of the corner frequency, f 0, with the epicentral distances, R. We found the distance correction parameters, Re 0.0042R and R 0.8333 e 0.00365R for the low-frequency spectral amplitudes and f 0 = f 0 ′ e 0.00043R and f 0 = f 0 ′ e 0.00044R for the corner frequency at the source, f 0, and observed at the station, f 0 ′ , from P-wave and S-wave spectra, respectively. Applying the distance correction procedure, we determined the source displacement spectrum of P and S waves for each earthquake to estimate the seismic moment, M 0; the moment magnitude, M W; the source radius, r; and the stress drop, Δσ. The seismic moments range from 1.06 × 1013 to 7.67 × 1019 N m, and their corresponding moment magnitudes are in the range of 2.6–7.2. Values of stress drop Δσ vary from 0.1 to 44 MPa. It was found that the stress drop increases with the increasing seismic moment in the range of 1013–1016 N m and possibly becomes constant at higher magnitudes, reaching a maximum value of about 40–45 MPa. We demonstrate that the values of the M 0 and M W estimated from P-wave and S-wave analysis are consistent and confirmed by the results of waveform inversions, i.e., centroid moment tensor (CMT) solution.

Seismological aspects of the Varzeghan twin Earthquakes on 11 August 2012 (Mw 6.3 and Mw 6.1), in East Azerbaijan province, NW Iran

Two earthquakes occurred in the east Azerbaijan province of NW Iran with Mw 6.3 at 12:23:15.9 and Mw 6.1; 11 minutes after the first shock at 12:34:34.8 GMT time on 11 August 2012. The epicenters of the earthquakes were determined using seismograms recorded by the International Institute of Earthquake Engineering and Seismology (IIEES) broadband stations. We obtained their focal mechanisms and also calculated their seismic moments and moment magnitudes based on the source spectrum of these recordings. Other source parameters such as, stress drops, corner frequencies and Kappa (ave) were determined for earthquakes by the S-wave spectra using a model-fitting approach from the near stations. The maximum intensity of the earthquakes was determined as VIII+ on the EMS98 scale in Varzeghan and affected villages.

Global variation of body-wave attenuation in the upper mantle from teleseismic P wave and S wave spectra

[1] We constrain the spatial variation of P-wave (tP*) and S-wave (tS*) attenuation by inverting 190,000 teleseismic P- and S-wave spectra up to 0.8 Hz. These spectra are derived from 250 deep earthquakes recorded at 880 broadband global and regional network stations. The variance and ratios of tP* and tS* values are consistent with PREM's upper mantle velocity and Q structures and conventional tP* and tS* values. High attenuation is resolved beneath stations in tectonically active regions characterized by high heat flow. Low attenuation marks stable continental regions. The maps of tP* and tS* correlate well with the variations of tS* computed and inferred from (1) the most recent surface-wave Q model and (2) a thermal interpretation of shear-wave velocity tomography. This indicates that maps of body- and surface-wave attenuation reflect intrinsic attenuation and variable temperature in the mantle.