Quantification of Long-Range Persistence in Geophysical Time Series: Conventional and Benchmark-Based Improvement Techniques

Time series in the Earth Sciences are often characterized as self-affine long-range persistent, where the power spectral density, S, exhibits a power-law dependence on frequency, f, S(f) ~ f ^?β , with β the persistence strength. For modelling purposes, it is important to determine the strength of self-affine long-range persistence β as precisely as possible and to quantify the uncertainty of this estimate. After an extensive review and discussion of asymptotic and the more specific case of self-affine long-range persistence, we compare four common analysis techniques for quantifying self-affine long-range persistence: (a) rescaled range (R/S) analysis, (b) semivariogram analysis, (c) detrended fluctuation analysis, and (d) power spectral analysis. To evaluate these methods, we construct ensembles of synthetic self-affine noises and motions with different (1) time series lengths N = 64, 128, 256, …, 131,072, (2) modelled persistence strengths β _model = ?1.0, ?0.8, ?0.6, …, 4.0, and (3) one-point probability distributions (Gaussian, log-normal: coefficient of variation c _v = 0.0 to 2.0, Levy: tail parameter a = 1.0 to 2.0) and evaluate the four techniques by statistically comparing their performance. Over 17,000 sets of parameters are produced, each characterizing a given process; for each process type, 100 realizations are created. The four techniques give the following results in terms of systematic error (bias = average performance test results for β over 100 realizations minus modelled β) and random error (standard deviation of measured β over 100 realizations): (1) Hurst rescaled range (R/S) analysis is not recommended to use due to large systematic errors. (2) Semivariogram analysis shows no systematic errors but large random errors for self-affine noises with 1.2 ≤ β ≤ 2.8. (3) Detrended fluctuation analysis is well suited for time series with thin-tailed probability distributions and for persistence strengths of β ≥ 0.0. (4) Spectral techniques perform the best of all four techniques: for self-affine noises with positive persistence (β ≥ 0.0) and symmetric one-point distributions, they have no systematic errors and, compared to the other three techniques, small random errors; for anti-persistent self-affine noises (β < 0.0) and asymmetric one-point probability distributions, spectral techniques have small systematic and random errors. For quantifying the strength of long-range persistence of a time series, benchmark-based improvements to the estimator predicated on the performance for self-affine noises with the same time series length and one-point probability distribution are proposed. This scheme adjusts for the systematic errors of the considered technique and results in realistic 95 % confidence intervals for the estimated strength of persistence. We finish this paper by quantifying long-range persistence (and corresponding uncertainties) of three geophysical time series—palaeotemperature, river discharge, and Auroral electrojet index—with the three representing three different types of probability distribution—Gaussian, log-normal, and Levy, respectively.     

Earthquake Source Zones in Northeast India: Seismic Tomography, Fractal Dimension and b Value Mapping

We have imaged earthquake source zones beneath the northeast India region by seismic tomography, fractal dimension and b value mapping. 3D P-wave velocity (Vp) structure is imaged by the Local Earthquake Tomography (LET) method. High precision P-wave (3,494) and S-wave (3,064) travel times of 980 selected earthquakes, m _d ≥ 2.5, are used. The events were recorded by 77 temporary/permanent seismic stations in the region during 1993–1999. By the LET method simultaneous inversion is made for precise location of the events as well as for 3D seismic imaging of the velocity structure. Fractal dimension and seismic b value has been estimated using the 980 LET relocated epicenters. A prominent northwest–southeast low Vp structure is imaged between the Shillong Plateau and Mikir hills; that reflects the Kopili fault. At the fault end, a high-Vp structure is imaged at a depth of 40 km; this is inferred to be the source zone for high seismic activity along this fault. A similar high Vp seismic source zone is imaged beneath the Shillong Plateau at 30 km depth. Both of the source zones have high fractal dimension, from 1.80 to 1.90, indicating that most of the earthquake associated fractures are approaching a 2D space. The spatial fractal dimension variation map has revealed the seismogenic structures and the crustal heterogeneities in the region. The seismic b value in northeast India is found to vary from 0.6 to 1.0. Higher b value contours are obtained along the Kopili fault (~1.0), and in the Shillong Plateau (~0.9) The correlation coefficient between the fractal dimension and b value is found to be 0.79, indicating that the correlation is positive and significant. To the south of Shillong Plateau, a low Vp structure is interpreted as thick (~20 km) sediments in the Bengal basin, with almost no seismic activity in the basin.     

On the Causes of Frequency-Dependent Apparent Seismological Q

Variability of the Earth’s structure makes a first-order impact on attenuation measurements which often does not receive adequate attention. Geometrical spreading (GS) can be used as a simple measure of the effects of such structure. The traditional simplified GS compensation is insufficiently accurate for attenuation measurements, and the residual GS appears as biases in both Q ₀ and η parameters in the frequency-dependent attenuation law Q(f) = Q ₀ f ^η . A new interpretation approach bypassing Q(f) and using the attenuation coefficient χ(f) = γ + πf/Q _e(f) resolves this problem by directly measuring the residual GS, denoted γ, and effective attenuation, Q _e. The approach is illustrated by re-interpreting several published datasets, including nuclear-explosion and local-earthquake codas, Pn, and synthetic 50–300-s surface waves. Some of these examples were key to establishing the Q(f) concept. In all examples considered, χ(f) shows a linear dependence on the frequency, γ ≠ 0, and Q _e can be considered frequency-independent. Short-period crustal body waves are characterized by positive γ _SP values of (0.6–2.0) × 10^?2 s^?1 interpreted as related to the downward upper-crustal reflectivity. Long-period surface waves show negative γ _LP ≈ ?1.9 × 10^?5 s^?1, which could be caused by insufficient modeling accuracy at long periods. The above γ values also provide a simple explanation for the absorption band observed within the Earth. The band is interpreted as apparent and formed by levels of Q _e ≈ 1,100 within the crust decreasing to Q _e ≈ 120 within the uppermost mantle, with frequencies of its flanks corresponding to γ _LP and γ _SP. Therefore, the observed absorption band could be purely geometrical in nature, and relaxation or scattering models may not be necessary for explaining the observed apparent Q(f). Linearity of the attenuation coefficient suggests that at all periods, the attenuation of both Rayleigh and Love waves should be principally accumulated at the sub-crustal depths (~38–100 km).     

Earthquake Rupturing in Fluid-Overpressured Crust: How Common?

Whether or not ruptures nucleate in fluid-overpressured crust (λ _v = P _f/σ _v > 0.4) is important because pore-fluids overpressured above hydrostatic lower fault frictional strength and may also vary through the earthquake cycle, acting as an independent variable affecting fault failure. Containment of fluid overpressure is precarious because pressure-dependent activation of faults and fractures allows drainage from overpressured portions of the crust. Discharge of fluids through activated fault-fracture permeability (fault-valve action) decreases overpressure so that subsequent failure depends on the cycling of both overpressure and frictional strength as well as tectonic stress. Geometric and mechanical considerations suggest that fluid overpressures are more likely to develop and be sustained in compressional/transpressional regimes as opposed to extensional/transtensional tectonic settings. On the basis of geophysical observations and force-balance analyses, subduction interface shear zones appear to be strongly but variably overpressured to near-lithostatic levels (λ _v > 0.9) over the full depth range of seismogenic megathrusts. Strong overpressuring at seismogenic depths is also documented in active fold-thrust belts and in areas of ongoing compressional inversion (e.g., northern Honshu) where inherited normal faults are reactivated as steep reverse faults, requiring near-lithostatic overpressures (λ _v → 1.0) at depths of rupture initiation. Evidence for overpressuring around strike-slip faults is less clear but tends to be strongest in areas of transpression. In areas of extensional tectonics coincident with particularly high fluid discharge, there is some evidence of overpressuring concentrated towards the base of the seismogenic zone. In general, because of the limited resolution of geophysical techniques, it is easier to make the case for rupture propagation through overpressured crust than to make a definitive case for the direct involvement of overpressured fluids in rupture nucleation, though in some instances the circumstantial evidence is compelling. An unresolved related issue is the heterogeneity of overpressuring. Do the active fault zones themselves serve as fluid conduits that are locally overpressured with respect to the surrounding crust?     

f max and fault zone property of Lushan earthquake of 20 April 2013, Sichuan,China

In this study, we determined f _max from near-field accelerograms of the Lushan earthquake of April 20, 2013 through spectra analysis. The result shows that the values of f _max derived from five different seismography stations are very close though these stations roughly span about 100 km along the strike. This implies that the cause of f _max is mainly the seismic source process rather than the site effect. Moreover, according to the source–cause model of Papageorgiou and Aki (Bull Seism Soc Am 73:693–722, 1983), we infer that the cohesive zone width of the rupture of the Lushan earthquake is about 204 with an uncertainty of 13 m. We also find that there is a significant bulge between 30 and 45 Hz in the amplitude spectra of accelerograms of stations 51YAL and 51QLY, and we confirm that it is due to seismic waves’ reverberation of the sedimentary soil layer beneath these stations.     

Earthquake precursory signatures in electromagnetic radiation measurements in terms of day-to-day fractal spectral exponent variation: analysis of the eastern Aegean 13/04/2017–20/07/2017 seismic activity

The recent seismic activity in the eastern Aegean Sea, foregrounded by two major (M_L?=?6.1 and M_L?=?6.2) earthquakes occurring near Lesvos and Kos islands, respectively, is investigated in this work. Electromagnetic radiation measurements across different frequency bands and antenna orientations from a monitoring station in Agia Paraskevi, Lesvos Island, are analysed in order to reveal earthquake precursory signatures hidden in the electromagnetic data sequence. A straightforward, data-driven approach is employed in which day-to-day variations of the fractal characteristics of the measurements are adaptively monitored via a fractal spectral exponent similarity measure. The evolution of the fractal day-to-day variation in a 99-day period, which includes the two major earthquakes, shows a sustained, sudden increase lasting 1 to 4 days before every earthquake cluster of M_L?=?4.0 and above. Most importantly, this day-to-day variation subsides shortly after and remains relatively low during the absence of earthquakes, thus alleviating the emergence of false alarms.     

The b-value and fractal dimension of local seismicity around Koyna Dam (India)

Earthquakes began to occur in Koyna region (India) soon after the filling of Koyna Dam in 1962. In the present study, three datasets 1964–1993, 1993–1995, and 1996–1997 are analyzed to study the b-value and fractal dimension. The b-value is calculated using the Gutenberg–Richter relationship and fractal dimension D _corr. using correlation integral method. The estimated b-value and D _corr. of this region before 1993 are found to be in good agreement with previously reported studies. In the subsequent years after 1995, the b-value shows an increase. The estimated b-values of this region are found within the limits of global average. Also, the pattern of spatial clustering of earthquakes show increase in clustering and migration along the three zones called North-East Zone, South-East Zone (SEZ), and Warna Seismic Zone. The earthquake events having depth ≤5 km are largely confined to SEZ. After 1993, the D _corr. shows decrease, implying that earthquake activity gets clustered. This seismic clustering could be helpful for earthquake forecasting.     

Attenuation of High-Frequency Seismic Waves in Eastern Iran

We investigated the frequency-dependent attenuation of the crust in Eastern Iran by analysis data from 132 local earthquakes having focal depths in the range of 5–25 km. We estimated the quality factor of coda waves (Q _c) and body waves (Q _p and Q _s) in the frequency band of 1.5–24 Hz by applying the single backscattering theory of S-coda envelopes and the extended coda-normalization method, respectively. Considering records from recent earthquakes (Rigan M _w 6.5, 2010/12/20, Goharan M _w 6.2, 2013/5/11 and Sirch M _w 5.5, 2013/1/21), the estimated values of Q _c, Q _p and Q _s vary from 151 ± 49, 63 ± 6, and 93 ± 14 at 1.5 Hz to 1,994 ± 124, 945 ± 84 and 1,520 ± 123 at 24 Hz, respectively. The average frequency-dependent relationships (Q = Q _o f ⁿ ) estimated for the region are Q _c = (108 ± 10)f ^(0.96±0.01), Q _p = (50 ± 5)f ^(1.01±0.04), and Q _s = (75 ± 6)f ^(1.03±0.06). These results evidenced a frequency dependence of the quality factors Q _c, Q _p, and Q _s, as commonly observed in tectonically active zones characterized by a high degree of heterogeneity, and the low value of Q indicated an attenuative crust beneath the entire region.     

Analysis of the 2012–2013 Torreperogil-Sabiote seismic swarm

This study analyses the temporal clustering, spatial clustering, and statistics of the 2012–2013 Torreperogil-Sabiote (southern Spain) seismic swarm. During the swarm, more than 2200 events were located, mostly at depths of 2–5 km, with magnitude event up to m_bLg 3.9 (M_w 3.7). On the basis of daily activity rate, three main temporal phases are identified and analysed. The analysis combines different seismological relationships to improve our understanding of the physical processes related to the swarm's occurrence. Each temporal phase is characterized by its cumulative seismic moment. Using several different approaches, we estimate a catalog completeness magnitude of m_c≅ 1.5. The maximum likelihood b-value estimates for each swarm phase are 1.11 ± 0.09, 1.04 ± 0.04, and 0.90 ± 0.04, respectively. To test the hypothesis that a b-value decrease is a precursor to a large event, we study temporal variations in b-value using overlapping moving windows. A relationship can be inferred between change in b-value and the regime style of the rupture. b-values are indicators of the stress regime, and influence the size of ruptures. The fractal dimension D₂ is used to perform spatial analysis. Cumulative gamma and beta functions are used to analyse the behaviour of inter-event distances during the earthquake sequence.     

Body wave attenuation characteristics in the crust of Alborz region and North Central Iran

Attenuation of P and S waves has been investigated in Alborz and north central part of Iran using the data recorded by two permanent and one temporary networks during October 20, 2009, to December 22, 2010. The dataset consists of 14,000 waveforms from 380 local earthquakes (2 < M _L < 5.6). The extended coda normalization method (CNM) was used to estimate quality factor of P (Q _P) and S waves (Q _S) at seven frequency bands (0.375, 0.75, 1.5, 3, 6, 12, 24 Hz). The Q _P and Q _S values have been estimated at lapse times from 40 to 100 s. It has been observed that the estimated values of Q _P and Q _S are time independent; therefore, the mean values of Q _P and Q _S at different lapse times have been considered. The frequency dependence of quality factor was determined by using a power-law relationship. The frequency-dependent relationship for Q _P was estimated in the form of (62 ± 7)f ^{(1.03 ± 0.07)} and (48 ± 5)f ^{(0.95 ± 0.07)} in Alborz region and North Central Iran, respectively. These relations for Q _S for Alborz region and North Central Iran have estimated as (83 ± 8)f ^{(0.99 ± 0.07)} and (68 ± 5)f ^{(0.96 ± 0.05)}, respectively. The observed low Q values could be the results of thermoelastic effects and/or existing fracture. The estimated frequency-dependent relationships are comparable with tectonically active regions.     

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.     

Characteristics of Soil Response in Near-Fault Zones During the 1999 Chi-Chi, Taiwan, Earthquake

Distribution of parameters characterizing soil response during the 1999 Chi-Chi, Taiwan, earthquake (M _w = 7.6) around the fault plane is studied. The results of stochastic finite-fault simulations performed in Pavlenko and Wen (2008) and constructed models of soil behavior at 31 soil sites were used for the estimation of amplification of seismic waves in soil layers, average stresses, strains, and shear moduli reduction in the upper 30 m of soil, as well as nonlinear components of soil response during the Chi-Chi earthquake. Amplification factors were found to increase with increasing distance from the fault (or, with decreasing the level of “input” motion to soil layers), whereas average stresses and strains, shear moduli reduction, and nonlinear components of soil response decrease with distance as ~ r ^?1 . The area of strong nonlinearity, where soil behavior is substantially nonlinear (the content of nonlinear components in soil response is more than ~40–50% of the intensity of the response), and spectra of oscillations on the surface take the smoothed form close to E(f) ~ f ^?n , is located within ~20–25 km from the fault plane (~ 1/4 of its length). Nonlinearity decreases with increasing distance from the fault, and at ~40–50 km from the fault (~ 1/2 of the fault length), soil response becomes virtually linear. Comparing soil behavior in near-fault zones during the 1999 Chi-Chi, the 1995 Kobe (M _w = 6.8), and the 2000 Tottori (Japan) (M _w = 6.7) earthquakes, we found similarity in the behavior of similar soils and predominance of the hard type of soil behavior. Resonant phenomena in upper soil layers were observed at many studied sites; however, during the Chi-Chi earthquake they involved deeper layers (down to ~ 40–60 m) than during lesser-magnitude Kobe and Tottori earthquakes.     

A κ Model for Mainland France

An important parameter for the characterization of strong ground motion at high-frequencies (>1 Hz) is kappa, κ, which models a linear decay of the acceleration spectrum, a(f), in log-linear space (i.e. a(f) = A ₀ exp(? π κ f) for f > f _E where f is frequency, f _E is a low frequency limit and A ₀ controls the amplitude of the spectrum). κ is a key input parameter in the stochastic method for the simulation of strong ground motion, which is particularly useful for areas with insufficient strong-motion data to enable the derivation of robust empirical ground motion prediction equations, such as mainland France. Numerous studies using strong-motion data from western North America (WNA) (an active tectonic region where surface rock is predominantly soft) and eastern North America (ENA) (a stable continental region where surface rock is predominantly very hard) have demonstrated that κ varies with region and surface geology, with WNA rock sites having a κ of about 0.04 s and ENA rock sites having a κ of about 0.006 s. Lower κs are one reason why high-frequency strong ground motions in stable regions are generally higher than in active regions for the same magnitude and distance. Few, if any, estimates of κs for French sites have been published. Therefore, the purpose of this study is to estimate κ using data recorded by the French national strong-motion network (RAP) for various sites in different regions of mainland France. For each record, a value of κ is estimated by following the procedure developed by Anderson and Hough (Bull Seismol Soc Am 74:1969–1993, 1984): this method is based on the analysis of the S-wave spectrum, which has to be performed manually, thus leading to some uncertainties. For the three French regions where most records are available (the Pyrenees, the Alps and the Côtes-d’Azur), a regional κ model is developed using weighted regression on the local geology (soil or rock) and source-to-site distance. It is found that the studied regions have a mean κ between the values found for WNA and ENA. For example, for the Alps region a κ value of 0.0254 s is found for rock sites, an estimate reasonably consistent with previous studies.     

Approximate Stochastic Self-Similarity of Envelopes of High-Frequency Teleseismic P-Waves from Large Earthquakes

A wavetrain of high-frequency (HF) P waves from a large earthquake, when recorded at a distant station, looks like a segment of modulated noise, with its duration close to the duration of rupture. These wavetrains, with their bursts and fadings, look much more intermittent than a segment of common stationary random noise. We try to describe quantitatively this bursty behavior. To this end, variogram and spectral analyses are applied to time histories of P-wave envelopes (squared-amplitude or instant-power signals) in six HF bands of 1-Hz width. Nine M _w = 7.6–9.2 earthquakes were examined, using, in total, 232 records and 992 single-band traces. Variograms of integrated instant power are approximately linear on a log–log scale, indicating that the correlation structure of the instant-power signal is approximately self-similar. Also, estimates of the power spectrum of the instant-power signal look approximately linear on a log–log scale. Log–log slopes of the variograms and spectra deliver estimates of the Hurst exponent H that are mostly in the range 0.6–0.9, markedly above the value H = 0.5 of uncorrelated (white-noise) signals. The preferred estimate over the entire data set is H = 0.83, still, this estimate may include some bias, and must be treated as preliminary. The inter-event scatter of H estimates is about 0.04, reflecting individual event-to-event variations of H. Many of the average log–log spectral plots show slight concavity that perturbs the approximately linear slope; this is a secondary effect that seems to be mostly related to the limited bandwidth of the data. Evidence is given in support of the idea that the observed approximately self-similar correlation structure of the P-wave envelope originates in a similar structure of the body wave instant-power signal radiated by the source, so that the propagation-related distortions can be regarded as limited. The facts presented suggest that the space–time organization of the earthquake rupture process is multiscaled and bears significant fractal features; it deviates from the brittle-crack model with its two well-separated characteristic scales. Phenomenologically, the high-frequency body-wave radiation from an earthquake source can be thought of as a product of stationary noise and the square root of a positive random envelope function with a power-law spectrum. From the viewpoint of applications, the self-similarity of body wave envelopes provides a useful constraint for earthquake source models used to simulate strong ground motions.     

Temporal variations of seismicity parameters in the Central Alborz,Iran

Temporal changes of b-value, fractal (correlation) dimensions of epicenters (D _e ) and occurrence time of earthquakes (D _t ) and relations between these parameters were calculated to investigate precursory changes before 28 May 2004, Baladeh-Kojour earthquake (M _w = 6.3) of Central Alborz, Iran. 2086 events with M _N ≥ 1.7 were selected for our analyses. A wide range of variation was seen in these parameters: b-value ～ 0.6–1.11, D _e ～ 0.97–1.64, and D _t ～ 0.13–0.93. The results showed decreases in all fractal parameters several months before the main shock. This decrease, which might have arisen due to clusters of events occurred between 2002–2003, was followed by a systematic increase, corresponding to the increased level of low-magnitude seismicity. It seems that changes in fractal parameters may be precursors of Baladeh-Kojour earthquake which was caused by seismic activation and quiescence. Furthermore, a positive correlation between b-value and D _e was detected before the main shock (D _e = 0.87 + 0.7b) and during aftershock sequences (D _e = 2b ± 0.09), which was further on changed to a negative one (D _e = 2.56–1.32b).     

Retrospective Study on the Predictability of Pattern Informatics to the Wenchuan M8.0 and Yutian M7.3 Earthquakes

Two large earthquakes occurred in the western part of China in 2008, one of them being the Yutian (35.6°N, 81.6°E) M7.3 earthquake that occurred on March 21 (BJT) and the other the Wenchuan (31.0°N, 103.4°E) M8.0 earthquake that occurred on May 12 (BJT). In this paper, the West Continental China (included in 20.0°–50.0°N, 70.0°–110.0°E region) was the study region for verifyong the predictability of the pattern informatics (PI) method using the receiver-operating characteristic curve (ROC) test and R score test. Different forecasting maps with different calculating parameters were obtained. The calculating parameters were the grid size Δx, base time t _b, reference interval t _b to t ₁, change interval t ₁ to t ₂, and forecasting interval t ₂ to t ₃. In this paper, the base time t _b fixed to June 1, 1971, the ending forecast time t ₃ fixed to June 1, 2008, and the forecasting interval t ₂ to t ₃ changed from 1 to 10 years, and the grid sizes were chosen as 1° × 1° and 2° × 2°, respectively. The results show that the PI method could forecast the Yutian M7.3 and Wenchuan M8.0 earthquakes only using suitable parameters. Comparing the forecast results of grid sizes 1° × 1° and 2° × 2°, the models with 2° × 2° grids were better. Comparing the forecast results with different forecasting windows from 1 to 10 years, the models with forecasting windows of 4–8 years were better using the ROC test, and the models with forecasting windows of 7–10 years were better using the R score test. The forecast efficiency of the model with a grid size of 2° × 2° and forecast window of 8 years was the best one using either the ROC test or the R score test.     

Rupture process of the Lorca (southeast Spain) 11 May 2011 (M w = 5.1) earthquake

On 11 May 2011, a M _w ?=?5.1 earthquake shook the town of Lorca (SE Spain) causing a disproportionately large damage for its magnitude. In order to contribute to knowledge of the behavior of the active faults present in the region and define the parameters which control their motion, we made a detailed study of the rupture process of this earthquake from inversion of body waves at regional and teleseismic distances. Ground motion displacements obtained in this way are in agreement with near-field strong motion data and GPS observations recorded in Lorca. We have obtained a partly bilateral rupture propagating to WSW (238°, 54°, 59°) with 27 cm of maximum slip and shallow focus (4 km). The fault plane orientation corresponds to that of the Cejo de los Enamorados Fault located NE of the Lorca town and parallel to the Alhama de Murcia Fault. The distribution of slip on the fault plane can explain the lack of any observed surface rupture as we found that the rupture started at 4-km depth along a plane dipping at 54°, with motion propagating upward to stop at 1.5 km below the surface. The strong motion and GPS data recorded near the epicenter are in agreement with the maximum slip on the fault. Directivity effects and the extreme shallowness of the rupture could explain the considerable damage that the earthquake caused in the town of Lorca.     

Coda Q Estimates in the Andaman Islands Using Local Earthquakes

The attenuation property of Andaman Island has been investigated analyzing coda waves from 57 local earthquakes in the magnitude range of 2.0–4.9, using the single backscattering model. These earthquakes waveforms, recorded on five broadband seismographs sited over the island from north to south during Nov. 2003 to March 2004, have been used to calculate the frequency dependent Coda Q (Q _c ) applying the time domain coda-decay method. The Coda Q, computed at central frequencies from (0.5–12) Hz and five-lapse time windows from 40 to 80 s, progressively increases from 105 f ^0.88 in the north Andaman to 135 f ^0.79 in the south Andaman with an average of 119 f ^0.80. The average Q _c values vary from 75 ± 42 at 0.5 Hz to 697 ± 54 at 12 Hz central frequency for 40 s lapse time window, while for 80 s lapse time window its variation is from 117 ± 38 at 0.5 Hz to 1256 ± 115 at 12 Hz. The Q _c estimated at different lapse times manifests a significant variation from 122f ^0.75 to 174f ^0.73, corresponding to lapse time window lengths of 40 and 80 s, respectively. The variation of Q _c with frequency, lapse time and also with the location of seismograph reflects the marked structural and compositional inhomogeneity with depth along the Andaman Islands. These observations are well correlated with the seismicity pattern and distinct high angle subduction beneath the island.     

Estimation of Q p and Q s of Kinnaur Himalaya

The attenuation characteristics of the Kinnaur area of the North West Himalayas were studied using local earthquakes that occurred during 2008–2009. Most of the analyzed events are from the vicinity of the Panjal Thrust (PT) and South Tibetan Detachment Thrust, which are well-defined tectonic discontinuities in the Himalayas. The frequency-dependent attenuation of P and S waves was estimated using the extended coda normalization method. Data from 64 local earthquakes recorded at 10 broadband stations were used. The coda normalization of the spectral amplitudes of P and S waves was done at central frequencies of 1.5, 3, 6, 9, and 12 Hz. Q _p increases from about 58 at 1.5 Hz to 706 at 12 Hz, and Q _s increases from 105 at 1.5 Hz to 1,207 at 12 Hz. The results show that the quality factors for both P and S waves (Q _p and Q _s) increase as a function of frequency according to the relation Q?=?Q _o f ⁿ , where Q _o is the corresponding Q value at 1 Hz frequency and “n” is the frequency relation parameter. We obtained Q _p?=?(47?±?2)f ^(1.04±0.04) and Q _s?=?(86?±?4)f ^(0.96±0.03) by fitting power law dependency model for the estimated values of the entire study region. The Q ₀ and n values show that the region is seismically very active and the crust is highly heterogeneous. There was no systematic variation of values of Q _p and Q _s at different frequencies from one tectonic unit to another. As a consequence, average values of these parameters were obtained for each frequency for the entire region, and these were used for interpretation and for comparison with worldwide data. Q _p values lie within the range of values observed for some tectonically active regions of the world, whereas Q _s values were the lowest among the values compared for different parts of the world. Q _s/Q _p values were >1 for the entire range of frequencies studied. All these factors indicate that the crust is highly heterogeneous in the study region. The high Q _s/Q _p values also indicate that the region is partially saturated with fluids.     

Seismic source characterization for the Shillong Plateau in Northeast India

The present study aims at understanding the seismotectonic province of the Shillong Plateau (SP) by identifying the potential seismic source zones within a radius of 500 km from the centre of the SP. From existing literature and earthquake (EQ) data, the seismotectonic region is found to vary in terms of seismicity, tectonic features, geology, thickness of overburden, rupture characteristics and rate of movement. Thus, entire 500-km-radius seismotectonic region is divided into four seismic source zones: namely (1) the Shillong Plateau–Assam Valley Zone (SP-AVZ), (2) the Indo-Burma Ranges Zone (IBRZ), (3) the Bengal Basin Zone (BBZ) and (4) the Eastern Himalaya Zone (EHZ). EQ catalogues for each source zone is analysed for completeness of magnitude and time. Seismic parameter b estimated using a maximum likelihood method is found to be 0.91 ± 0.03, 0.94 ± 0.02, 0.80 ± 0.03 and 0.89 ± 0.03 for the SP-AVZ, IBRZ, BBZ and EHZ, respectively. In addition, the maximum likelihood method is used to estimate the mean annual activity rate, maximum possible magnitude (m _max), return period and probability of exceedance for the four zones. The b values estimated suggest that the BBZ is seismically more active; however, the rate of occurrence of EQs is highest in the IBRZ. Findings from this study are an indication of the relative contribution from each of the four seismic source zones towards a seismic hazard of the SP.