Broadband Seismographs Research Articles

Distribution of seismic wave speed changes associated with the 12 May 2008 Mw 7.9 Wenchuan earthquake

We used continuous recordings in Sichuan, China to track the temporal change of the seismic wave speed at a regional scale, for 2 years including the Wenchuan Mw 7.9 earthquake. The data are recorded by a temporary network of 156 broad‐band seismographs in a region that covers the southern 2/3 of the fault activated during the earthquake. A doublet analysis applied on the codas of seismic noise cross correlation functions is used to detect temporal velocity changes. We found clear evidence that the seismic velocity drops by up to 0.08% in the fault region just after the earthquake with fluctuations within 0.02% before the earthquake. We compared the measurements in different sub‐arrays to get a spatial distribution of the velocity changes. This distribution is consistent with the volumetric strain change during the Wenchuan earthquake and shows that the co‐seismic velocity change is not controlled by the response of sediments.

The coalescence and organization of lahars at Semeru volcano, Indonesia

We present multi-parameter geophysical measurements of rainfall-induced lahars at Semeru Volcano, East Java, using two observation sites 510 m apart, 11.5 km from the summit. Our study site in the Curah Lengkong channel is composed of a 30-m wide box-valley, with a base of gravel and lava bedrock, representing an ideal geometry for high density measurements of active lahars. Instrumentation included pore-pressure sensors (stage), a broad-band seismograph (arrival times, vibrational energy, and turbulence), video footage, and direct bucket sampling. A total of 8 rainfall-induced lahars were recorded, with durations of 1–3 h, heights 0.5–2 m, and peak velocities 3–6 m/s. Flow types ranged from dilute to dense hyperconcentrated flows. These recorded flows were commonly composed of partly coalesced, discrete and unsteady gravity current packets, represented by multiple peaks within each lahar. These packets most likely originate from multiple lahar sources, and can be traced between instrument sites. Those with the highest concentrations and greatest wetted areas were often located mid-lahar at our measured reach, accelerating towards the flow front. As these lahars travel downstream, the individual packets thus coalesce and the flow develops a more organised structure. Observations of different degrees of coalescence between these discrete flow packets illustrate that a single mature debris flow may have formed from multiple dynamically independent lahars, each with different origins.

The statistics of the time segments of low-frequency microseisms: Trends and synchronization

The problem of identifying the effects of synchronization in the parameters of lowfrequency microseismic noise from the data of 77 stations belonging to the Fnet broadband seismograph network in Japan for the period from the beginning of 1997 through August, 2009 is considered. The vertical components measured initially with a sampling rate of one second and subsequently converted into the signals sampled at 1 minute intervals by means of averaging and decimation are used in the analysis. Six statistics are taken as the parameters: the support width of the multifractal singularity spectrum; the generalized Hurst exponent; the asymmetry coefficient of the spectrum of singularity; the logarithmic variance; the spectral exponent; and the linear predictability index. These parameters are calculated from the realizations contained within con� secutive daily time intervals. When using the moving time window with a width of one year for evaluating the multiple correlation, the daily variations in the median values of the statistics of the noise measured at five spatial clusters of stations exhibit a stable increase in the synchronization not long before the Hokkaido earth� quake (September 25, 2003; M = 8.3), subsequently passing to the new level of high synchronization. Based on the analysis of the trends in the index of linear predictability it turned out possible to estimate the begin� ning of the enhancement in the synchronization with rather high accuracy as the middle of 2002. The effect revealed for the variations in the different parameters of microseisms is an independent argument for the ear� lier conclusion about the synchronization in the field of the microseismic noise on the Japan Islands.

Attenuation of High Frequency P and S Waves in the Gujarat Region, India

The local earthquake waveforms recorded on broadband seismograph network of Institute of Seismological Research in Gujarat, India have been analyzed to understand the attenuation of high frequency (2–25 Hz) P and S waves in the region. The frequency dependent relationships for quality factors for P (QP) and S (QS) waves have been obtained using the spectral ratio method for three regions namely, Kachchh, Saurashtra and Mainland Gujarat. The earthquakes recorded at nine stations of Kachchh, five stations of Saurashtra and one station in mainland Gujarat have been used for this analysis. The estimated relations for average QP and QS are: QP = (105 ± 2) f0.82 ± 0.01, QS = (74 ± 2) f1.06 ± 0.01 for Kachchh region; QP = (148 ± 2) f0.92 ± 0.01, QS = (149 ± 14) f1.43 ± 0.05 for Saurashtra region and QP = (163 ± 7) f0.77 ± 0.03, QS = (118 ± 34) f0.65 ± 0.14 for mainland Gujarat region. The low Q ( 0.5) as obtained from present analysis indicate the predominant seismic activities in the region. The lowest Q values obtained for the Kachchh region implies that the area is relatively more attenuative and heterogeneous than other two regions. A comparison between QS estimated in this study and coda Q (Qc) previously reported by others for Kachchh region shows that QC > QS for the frequency range of interest showing the enrichment of coda waves and the importance of scattering attenuation to the attenuation of S waves in the Kachchh region infested with faults and fractures. The QS/QP ratio is found to be less than 1 for Kachchh and Mainland Gujarat regions and close to unity for Saurashtra region. This reflects the difference in the geological composition of rocks in the regions. The frequency dependent relations developed in this study could be used for the estimation of earthquake source parameters as well as for simulating the strong earthquake ground motions in the region.

Variation of Moho depth in the central part of the Alborz Mountains, northern Iran

The Alborz Mountains of northern Iran form a belt of active crustal deformation along the southern side of the Caspian Sea within the broad Arabian–Eurasia continental collision zone. Although the range has an average elevation of about 3000 m with the volcanic peak Damavand reaching an elevations of 5671 m, early gravity studies found that the crust beneath the range is no thicker than that beneath the surrounding region suggesting the range is not supported by a crustal root. We determine a model for the crust of the central Alborz Mountains using teleseismic receiver functions from data recorded on a network of broad-band seismographs temporarily deployed across the central part of the range. The receiver functions from these recordings have been inverted simultaneously with fundamental-mode Rayleigh wave group velocity measurements in the 10–100 s period range. Our analysis shows a thickening of the crust from ∼48 km beneath the northern part of the Central Iranian Plateau to 55–58 km below the central part of the Alborz Mountains, then a thinning of the crust to ∼46 km north of the Alborz Mountains beneath the coastal region of the South Caspian Sea. Our seismological results show that the central Alborz Mountains have a moderate crustal root but of insufficient thickness to compensate the elevation of the range. The analysis of free-air gravity shows that the elevation of the Alborz Mountains is largely supported by the elastic strength of the Iranian Plate, the South Caspian Plate, or both.

Teleseismic magnitude relations

Using available sets of magnitude determinations, primarily from Uppsala seismological bulletin, various extensions are made of the Zurich magnitude recommendations of 1967. Thus, body-wave magnitude (m) and surface-wave magnitudes (M) are related to each other for 12 different earthquake regions as well as world-wide. Depth corrections for M are derived for all focal depths. Formulas are developed which permit calculation of M also from vertical component long-period seismographs. Body-wave magnitudes from broad-band and narrow-band short-period seismographs are compared and relations deduced. Applications are made both to underground nuclear explosions and to earthquakes. The possibilities of explosion-earthquake discrimination on the basis of magnitudes are examined, as well as the determination of explosive yield from magnitudes. For earthquakes, relations between magnitudes of main earthquakes and largest aftershocks are investigated. A world-wide station network for more homogeneous magnitude determinations is suggested in order to provide the necessary reference system.

Source model of an earthquake doublet that occurred in a pull-apart basin along the Sumatran fault, Indonesia

SUMMARY On 2007 March 6, an earthquake doublet occurred along the Sumatran fault, Indonesia. The epicentres were located near Padang Panjang, central Sumatra, Indonesia. The first earthquake, with a moment magnitude (M w) of 6.4, occurred at 03:49 UTC and was followed two hours later (05:49 UTC) by an earthquake of similar size (M w = 6.3). We studied the earthquake doublet by a waveform inversion analysis using data from a broadband seismograph network in Indonesia (JISNET). The focal mechanisms of the two earthquakes indicate almost identical rightlateral strike-slip faults, consistent with the geometry of the Sumatran fault. Both earthquakes nucleated below the northern end of Lake Singkarak, which is in a pull-apart basin between the Sumani and Sianok segments of the Sumatran fault system, but the earthquakes ruptured different fault segments. The first earthquake occurred along the southern Sumani segment and its rupture propagated southeastward, whereas the second one ruptured the northern Sianok segment northwestward. Along these fault segments, earthquake doublets, in which the two adjacent fault segments rupture one after the other, have occurred repeatedly. We investigated the state of stress at a segment boundary of a fault system based on the Coulomb stress changes. The stress on faults increases during interseismic periods and is released by faulting. At a segment boundary, on the other hand, the stress increases both interseismically and coseismically, and may not be released unless new fractures are created. Accordingly, ruptures may tend to initiate at a pull-apart basin. When an earthquake occurs on one of the fault segments, the stress increases coseismically around the basin. The stress changes caused by that earthquake may trigger a rupture on the other segment after a short time interval. We also examined the mechanism of the delayed rupture based on a theory of a fluidsaturated poroelastic medium and dynamic rupture simulations incorporating a rheological velocity hardening effect. These models of the delayed rupture can qualitatively explain the observations, but further studies, especially based on the rheological effect, are required for quantitative studies.

Broadband waveform inversion of moderate earthquakes in the Messina Straits, southern Italy

We report the first application of the Cut and Paste (CAP) method to compute earthquake focal mechanisms in the Messina Straits area by waveform inversion of Pnl and surface wave segments. This application of CAP has furnished new knowledge about low-magnitude earthquake mechanics that will be useful for improved understanding of the local geodynamics. This is possible because the CAP inversion technique can be applied to small earthquakes, for which traditional moment tensor inversion methods are not appropriate and P-onset focal mechanisms in the study area fail because of a lack of sufficient observations. We estimate the focal mechanisms of 23 earthquakes with local magnitudes in the range of 3–4 occurring in the 2004–2008 time period, and recorded by the broadband stations of the Italian National Seismic Network and the Mediterranean Very Broadband Seismographic Network (MedNet) run by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The solutions show that normal faulting is the prevailing style of seismic deformation in the northern part of the study area while co-existence of normal faulting and strike-slip has been detected in the southern part. In the whole area of investigation the T-axes of focal mechanisms display a preferential northwest-southeast direction of extension. Combined with the findings of previous investigations, this improved database of focal mechanisms allows us to better detail the transitional area between the extensional domain related to subduction trench retreat (southern Calabria) and the compressional one associated with continental collision (western-central Sicily). The observed spatial change of seismic deformation regime offers new data to current seismotectonic and seismic hazard investigations in the area of Messina Straits where a magnitude 7.2 earthquake caused more than 60,000 casualties on 28 December 1908.

Surface wave dispersion measurements from ambient seismic noise analysis in Italy

SUMMARY We present the surface wave dispersion results of the application of the ambient noise method to broad-band data recorded at 114 stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) national broad-band network, some stations of the Mediterranean Very Broadband Seismographic Network (MedNet) and of the Austrian Central Institute for Meteorology and Geodynamics (ZAMG). Vertical-component ambient noise data from 2005 October to 2007 March have been cross-correlated for station-pairs to estimate fundamental mode Rayleigh wave Green's functions. Cross-correlations are calculated in 1-hr segments, stacked over periods varying between 3 months and 1.5 yr. Rayleigh wave group dispersion curves at periods from 8 to 44 s were determined using the multiple-filter analysis technique. The study region was divided into a 0.2°× 0.2° grid to invert for group velocity distributions. Checkerboard tests were first carried out, and the lateral resolution was estimated to be about 0.6°. The resulting group velocity maps from 8 to 36 s show the significant difference of the crustal structure and good correlations with known geological and tectonic features in the study region. The Po Plain and the Southern Alps evidence lower group velocities due to soft alluvial deposits, and thick terrigenous sediments. Our results also clearly showed that the Tyrrhenian Sea is characterized with much higher velocities below 8 km than the Italian peninsula and the Adriatic Sea which indicates a thin oceanic crust beneath the Tyrrhenian Sea.

Estimation of Sedimentary Thickness in Kachchh Basin, Gujarat Using SP Converted Phase

An inexpensive method using natural earthquake data is utilized for determining the sedimentary thickness in Kachchh. The Institute of Seismological Research (ISR) is operating a network of broadband seismographs and strong motion accelerographs in Gujarat. We used data from 13 broadband seismographs and two strong motion accelerographs in the study. The stations are within 5 to 80 km from the epicenters. In this study the S-to-P converted phase, SP, is used. This phase is generated due to large impedance contrast between sediments and basement. This phase is clear in the vertical component. The difference in the travel times of S and SP phases and velocities of P and S waves is used for determining the sedimentary layer thickness. The thickness of sediments beneath each of these 15 stations was determined covering an area of 23,500 sq km.

A comparison of the features of windstorms Kyrill and Emma based on seismological and meteorological observations

In mid-January 2007 and at the turn of February and March 2008, two extremely strong windstorms, i.e. Kyrill and Emma, hit most of the central European countries. These windstorm hurricane-force winds caused serious damage to structures, to a large number of properties, cars, trees and, moreover, some fatalities were documented, as well. During the period mentioned, secondary and/or ocean-generated microseisms (T = 4-8 s) recorded by broad-band seismographs at the seismic station Ostrava-Krasne Pole (OKC) (Czech Republic) were analysed. These microseisms were caused by ocean surface oscillations related to the propagation of depression lows over the North Atlantic Ocean. While the peak-to-peak displacement amplitudes observed during windstorm Kyrill reached values of up to 2 μm, during the passage of Emma, the amplitudes were about 1.5 μm. Predominating periods of oscillations were observed within the interval T ≈ 5.4-5.7 s for both windstorms, while the FFT analysis documented two distinct peaks in calculated spectra, namely at f ≈ 0.16-0.18 Hz and/or T ≈ 5.5-6.2 s and at f ≈ 0.06-0.08 Hz and/or T ≈ 12.5-16.5 s, corresponding with secondary and primary microseisms, respectively. The second part of the input data represent some results of regular meteorological observations, i.e. wind speed, wind gusts and wind direction, which were completed with the description of the origin and development of both studied low-pressure systems. During the passage of Kyrill the maximum velocity of the wind gusts in northern Bohemia (at the top of Mt. Sněžka in the Giant /Krkonose/ Mts.) was estimated to be 60 m/s (218 km/h), while during Emma it reached 54 m/s (194 km/h) at the same observation site. In principle, it was found that the velocity of the windstorm Emma spreading was lower as compared with windstorm Kyrill. Moreover, a description of the origin and development of both low-pressure systems is attached. Their features were compared using all the data from the seismological observatory and meteorological stations related to both windstorms being available. The resulting values of the selected parameters, chosen for comparison, are given in Table 2.

South Pacific mantle plumes imaged by seismic observation on islands and seafloor

The South Pacific region is characterized by a broadly elevated seafloor known as the South Pacific superswell. This region has a concentration of midplate volcanoes that experienced massive eruptions in the mid‐Cretaceous period (90–120 Ma). These characteristics suggest the presence of a large‐scale mantle plume beneath the South Pacific, called the South Pacific superplume. The geometry, origin depth, temperature, and composition of the superplume remain controversial, however, mainly due to the lack of seismological data that documents the mantle structure beneath the South Pacific. Seismic stations are sparse in the area due to its remote ocean environment. To obtain a better seismic image of the superplume, we deployed temporary broadband seismographs on oceanic islands and the seafloor in the South Pacific, which made possible the highest spatial resolution that has ever been achieved for the mantle structure beneath the region. The seismic image obtained from this new seismic data indicates that large‐scale low‐velocity anomalies (on the order of 1000 km in diameter), indicative of the superplume, are located from the bottom of the mantle to a depth of 1000 km, and small‐scale low‐velocity anomalies (on the order of 100 km in diameter) are present above it. A comparison of the seismic image with recent mantle convection studies based upon laboratory and numerical experiments suggests that the superplume may be a hot and chemically distinct mantle dome, and that the small‐scale anomalies may be narrow plumes generated from the top of the dome. This model may explain various characteristics of hot spots in the South Pacific, such as the seafloor swell, short‐lived hot spot chains, and the periodicity of massive eruptions.

A Comparison of Short-Period and Broadband Seismograph Systems in the Context of the Seismology of the Eastern United States

The study compares the capabilities of modern short-period seismograph systems with those using broadband sensors. The results indicate that dual-gain recording using 1-Hz seismometers will capture essentially all of the available information in the seismic wavefield generated by moderate magnitude shocks ( M 4.0–6.0) at regional distances, with dynamic range sufficient to record on-scale a magnitude 6.0 event at approximately 55 km. Short-period instrument operation in seismically active areas remains an effective means of collecting weak-motion data for basic seismological research relevant to public safety in the eastern United States.

Lg Attenuation near the North Korean Border with China, Part II: Model Development from the 2006 Nuclear Explosion in North Korea

An underground nuclear explosion was set off on 9 October 2006 in North Korea. The event was recorded at near regional to regional distances over purely continental paths by a network of broadband seismographs located along the northern edge of the Korean Peninsula. The event provided the first window of opportunity to make a reliable measurement of Lg-wave attenuation in northernmost North Korea, where the regional phase propagation characteristics are poorly known. The results of our analysis show Q0 � 317 � 14 and η � 0:343 � 0:024, in close agreement with the corresponding values obtained of late using a completely different methodology and an independent set of seismic recordings in an adjacent region to the northwest, in southern Jilin, and in Liaoning provinces in China.

Himalayan-Tibetan Underplate

The Himalayas formed from the collision of India with Eurasia beginning about 50 million years ago, but the fate and position of the subducted Indian crust was not well defined until the Hi-CLIMB seismic experiment was initiated. The centerpiece of the project is an 800-kilometer-long, closely spaced, linear array of broadband seismographs, extending from the Ganges lowland, across the Himalayas, and onto the central Tibetan plateau. Nabělek et al. (p. [1371][1]) present images of the crust and upper mantle of the Southern Tibetan plateau underthrust northward by the Indian plate, in which they trace the base of the Indian plate to 31°N. The character of the crust-mantle interface in this region suggests that the Indian crust is at least partly decoupled from the mantle beneath. [1]: /lookup/doi/10.1126/science.1167719

Underplating in the Himalaya-Tibet Collision Zone Revealed by the Hi-CLIMB Experiment

We studied the formation of the Himalayan mountain range and the Tibetan Plateau by investigating their lithospheric structure. Using an 800-kilometer-long, densely spaced seismic array, we have constructed an image of the crust and upper mantle beneath the Himalayas and the southern Tibetan Plateau. The image reveals in a continuous fashion the Main Himalayan thrust fault as it extends from a shallow depth under Nepal to the mid-crust under southern Tibet. Indian crust can be traced to 31 degrees N. The crust/mantle interface beneath Tibet is anisotropic, indicating shearing during its formation. The dipping mantle fabric suggests that the Indian mantle is subducting in a diffuse fashion along several evolving subparallel structures.

Source parameters and focal mechanisms of local earthquakes: Single broadband observatory at ISM Dhanbad

A three-component broadband seismograph is in operation since January 2007 at the Indian School of Mines (ISM) campus. We have used the broadband seismograms of two local earthquakes M <3 recorded by this single station to illustrate its efficacy in understanding the source processes and tectonics in Dhanbad area. Source parameters and fault plane solutions are obtained through waveform inversion. It is observed that these two earthquakes occurred in the lower crust at a depth of 26 km by strike slip faulting. North-south compressional and east-west tensional stresses are dominant in the area, and the lower crust is the source area for the local earthquakes.

Estimation of surface wave Green's functions from correlation of direct waves, coda waves, and ambient noise in SE Tibet

Empirical Green's functions (EGFs) between receivers can be obtained from seismic interferometry through cross-correlation of pairs of ground motion records. Full reconstruction of the Green's function requires diffuse wavefields or a uniform distribution of (noise) sources. In practice, EGFs differ from actual Green's functions because wavefields are not diffuse and the source distribution not uniform. This difference, which may depend on medium heterogeneity, complicates (stochastic) medium characterization as well as imaging and tomographic velocity analysis with EGFs. We investigate how source distribution and scale lengths of medium heterogeneity influence surface wave Green's function reconstruction in the period band of primary microseisms ( T = 10–20 s). With data from a broad-band seismograph array in SE Tibet we analyze the symmetry and travel-time properties of surface wave EGFs from correlation of data in different windows: ambient noise, direct surface waves, and surface wave coda. The EGFs from these different windows show similar dispersion characteristics, which demonstrates that the Green's function can be recovered from direct wavefields (e.g., ambient noise or earthquakes) or from wavefields scattered by heterogeneity on a regional scale. Directional bias and signal-to-noise ratio of EGFs can be understood better with (plane wave) beamforming of the energy contributing to EGF construction. Beamforming also demonstrates that seasonal variations in cross-correlation functions correlate with changes in ocean activity.

Seismic anisotropy of upper mantle in eastern China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation.

Upper mantle anisotropy beneath Indochina block and adjacent regions from shear-wave splitting analysis of Vietnam broadband seismograph array data

The Indochina block is located at the junction of four major plates. It has a close relationship with the uplift of Tibet plateau and the sea-floor spreading of the South China Sea in the geological evolution histories. We deployed a temporary broadband seismograph array in Vietnam from February 2000 to October 2005, and measured shear-wave splitting of core-refracted phases to investigate the upper mantle anisotropy beneath the Indochina block and adjacent regions. The observed delay times of up to 1.5 s are large, indicating a mantle source for the anisotropy. Two distinct regimes of seismic anisotropy are revealed. The fast polarization directions in the northern Indochina block with a main trend of N100°E are almost consistent with the direction of both the surface displacement and the absolute plate motion. These observations suggest the anisotropy is possibly generated by the present-day asthenospheric flow caused by the India–Eurasia collision. The fast polarization directions in the southern Indochina block and the Hainan island are nearly N60°E. The anisotropy in this area may reflect a combination of the India–Eurasia collision and the past orogenies in the lithosphere, or an abrupt variation in the asthenospheric flow.