Regional Seismic Network Research Articles

A broad 660 km discontinuity beneath northeast China revealed by dense regional seismic networks in China

We examined the P wave velocity structure around the 660 km discontinuity at the tip of the subducting Pacific slab beneath northeastern China by forward modeling waveform triplication data. A total of 742 broadband seismograms were recorded by dense regional seismic networks in China from a deep earthquake that occurred near the border of east Russia and northeast China, providing an unprecedented density of ray coverage near the front edge of the subducting Pacific slab. Multiple P waves were observed on single seismograms in the distance range of ∼14°–29°. The P wave triplication shows the following two features: (1) the direct arrival traveling above the 660 km discontinuity (AB branch) extends as far as ∼29°, approximately 6° further than the prediction from velocity model of International Association of Seismology and the Physics of the earth's interior (iasp91); (2) the refracted wave propagating through the lower mantle (CD branch) appears at a distance a few degrees greater than that of the iasp91 synthetics. Forward waveform modeling suggests that they are best explained by a high‐velocity transition zone underlain by a ∼50–70 km thick 660 km discontinuity. The broadened discontinuity is likely caused by multiple phase transitions associated with the dissolutions of olivine and garnet components.

Detecting seasonal variations in seismic velocities within Los Angeles basin from correlations of ambient seismic noise

We analyse 3 yr of continuous seismic records from broad-band stations of the Caltech Regional Seismic Network (CI) in vicinity of the Los Angeles basin. Using correlations of ambient seismic noise, relative velocity variations in the order of 0.1 per cent can be measured between all interstation pairs. It is the first time that such an extensive study between 861 interstation pairs over such a large area has been carried out. We perform these measurements using the ‘stretching’ technique, assuming that one of the two waveforms is merely a stretched version of the other. Obviously this assumption is always violated and the two waveforms are generally decorrelated due to temporal changes in the Earth crust, due to different sources or simply because the cross-correlations are not fully converged. We investigate the stability of these measurements by repeating each measurement over various time-windows of equal length. On average between all interstation pairs in the Los Angeles basin a seasonal signal in the relative velocity variation is observed, with peaks and troughs during winter and summer time, respectively. Generally the observed signal decreases with increasing interstation distance and relative traveltime perturbations can only be measured up to an interstation distance of 60 km. Furthermore, the traveltime perturbations do not depend on azimuth of station pairs, suggesting that they are not related to seasonal variations of the noise sources. Performing a simple regionalization by laterally averaging measurements over a subset of stations we found the sedimentary basin showing the most consistent signal and conclude that the observed seasonality might be induced either by changes in the ground-water aquifer or thermo-elastic strain variations that persist down to a depth of 15–22 km.

Automated determination ofP-phase arrival times at regional and local distances using higher order statistics

P>We present an algorithm for automatic P-phase arrival time determination for local and regional seismic events based on higher order statistics (HOS). Using skewness or kurtosis a characteristic function is determined to which a new iterative picking algorithm is applied. For P-phase identification we apply the Akaike Information Criterion to the characteristic function, while for a precise determination of the P-phase arrival time a pragmatic picking algorithm is applied to a recalculated characteristic function. In addition, an automatic quality estimate is obtained, based on the slope and the signal-to-noise ratio, both calculated from the characteristic function. To get rid of erroneous picks, a Jackknife procedure and an envelope function analysis is used. The algorithm is applied to a large data set with very heterogeneous qualities of P-onsets acquired by a temporary, regional seismic network of the EGELADOS-project in the southern Aegean. The reliability and robustness of the proposed algorithm is tested by comparing more than 3000 manually derived P readings, serving as reference picks, with the corresponding automatically estimated P-wave arrival times. We find an average deviation from the reference picks of 0.26 +/- 0.64 s when using kurtosis and 0.38 +/- 0.75 s when using skewness. If automatically as excellent classified picks are considered only, the average difference from the reference picks is 0.07 +/- 0.31 s and 0.07 +/- 0.41 s, respectively. However, substantially more P-arrival times are determined when using kurtosis, indicating that the characteristic function derived from kurtosis estimation is to be preferred. Since the characteristic function is calculated recursively, the algorithm is very fast and hence suited for earthquake early warning purposes. Furthermore, a comparative study with automatically derived P-readings using Allen's and Baer & Kradolfer's picking algorithms applied to the same data set demonstrates better quantitative and qualitative performance of the HOS approach. This study shows, that precise automatic P-onset determination is feasible, even when using data sets with very heterogeneous signal-to-noise ratio.

Reliability of the automatic procedures for locating earthquakes in southwestern Alps and northern Apennines (Italy)

Reliable automatic procedure for locating earthquake in quasi-real time is strongly needed for seismic warning system, earthquake preparedness, and producing shaking maps. The reliability of an automatic location algorithm is influenced by several factors such as errors in picking seismic phases, network geometry, and velocity model uncertainties. The main purpose of this work is to investigate the performances of different automatic procedures to choose the most suitable one to be applied for the quasi-real-time earthquake locations in northwestern Italy. The reliability of two automatic-picking algorithms (one based on the Characteristic Function (CF) analysis, CF picker, and the other one based on the Akaike’s information criterion (AIC), AIC picker) and two location methods (“Hypoellipse” and “NonLinLoc” codes) is analysed by comparing the automatically determined hypocentral coordinates with reference ones. Reference locations are computed by the “Hypoellipse” code considering manually revised data and tested using quarry blasts. The comparison is made on a dataset composed by 575 seismic events for the period 2000–2007 as recorded by the Regional Seismic network of Northwestern Italy. For P phases, similar results, in terms of both amount of detected picks and magnitude of travel time differences with respect to manual picks, are obtained applying the AIC and the CF picker; on the contrary, for S phases, the AIC picker seems to provide a significant greater number of readings than the CF picker. Furthermore, the “NonLinLoc” software (applied to a 3D velocity model) is proved to be more reliable than the “Hypoellipse” code (applied to layered 1D velocity models), leading to more reliable automatic locations also when outliers (wrong picks) are present.

Probing two low-velocity regions withPKPb-caustic amplitudes and scattering

SUMMARY We probe low-velocity regions of the lower mantle with PKP waves using South Pacific earthquakes recorded at stations from the temporary Ethiopia Afar Geoscientific Lithospheric Experiment array in Ethiopia and the German Regional Seismic Network (GRSN; Germany). The Ethiopian stations show strong scattering of PKPab waves, which arrive as precursors to PKPdf . They also show anomalously large amplitude diffractions from the PKP b-caustic. In contrast, the events recorded at the GRSN show scattering of PKPab as precursors to PKPdf but more normal amplitudes in the PKP b-caustic diffractions. To understand the cause for the observed amplitudes, we test a large set of 1-D models with anomalies in P- and S-wave velocities and density in the lowermost mantle and outer core. Structures that can cause the observed large PKP b-caustic amplitudes are low-velocity regions just above the core–mantle boundary, of the order of 50 km thick for the South Pacific to Ethiopia path, with velocity reductions of around 5 per cent for P- and 25 per cent for the S-wave velocity. Alternatively a 30-km-thick lamella, starting at 50 km above the core–mantle boundary with 10 per cent P-wave velocity reduction and 25 per cent S-wave velocity reduction, also produces a good fit to the data. We find that the low-velocity regions act as waveguides, enhancing the amplitude of the PKP b-caustic diffracted waves and causing superposition of PKP b-diffractions with PKPdf . The low-velocity regions cannot be smooth layers but must show a certain degree of small-scale heterogeneity to produce the high-frequency scattering observed in the data. The path to Germany needs small scatterers in the lowermost mantle, but the low-velocity region is less strong compared with the South Pacific-Ethiopia path. A plausible explanation for these low-velocity regions is the presence of melt inclusions, perhaps entrained in small-scale convective systems that act as high-frequency wavefield scatterers. In regions where these inclusions or convective systems are abundant, they appear as a large-scale low-velocity zone to lower-frequency waves. These observations support ideas that ultra-low-velocity zones at the base of the mantle may be heterogeneous regions of melt-rich material.

The seismicity and tectonic stress field characteristics of the Longmenshan fault zone before the Wenchuan MS8.0 earthquake

The seismicity of Longmenshan fault zone and its vicinities before the 12 May 2008 Wenchuan MS8.0 earthquake is studied. Based on the digital seismic waveform data observed from regional seismic networks and mobile stations, the focal mechanism solutions are determined. Our analysis results show that the seismicities of Longmenshan fault zone before the 12 May 2008 Wenchuan earthquake were in stable state. No obvious phenomena of seismic activity intensifying appeared. According to focal mechanism solutions of some small earthquakes before the 12 May 2008 Wenchuan earthquake, the direction of principal compressive stress P-axis is WNW-ESE. The two hypocenter fault planes are NE-striking and NW-striking. The plane of NE direction is among N50°–70°E, the dip angles of fault planes are 60°–70° and it is very steep. The faultings of most earthquakes are dominantly characterized by dip-slip reverse and small part of faultings present strike-slip. The azimuths of principal compressive stress, the strikes of source fault planes and the dislocation types calculated from some small earthquakes before the 12 May 2008 Wenchuan earthquake are in accordance with that of the main shock. The average stress field of micro-rupture along the Longmenshan fault zone before the great earthquake is also consistent with that calculated from main shock. Zipingpu dam is located in the east side 20 km from the initial rupture area of the 12 May 2008 Wenchuan earthquake. The activity increment of small earthquakes in the Zipingpu dam is in the period of water discharging. The source parameter results of the small earthquakes which occurred near the initial rupture area of the 12 May 2008 Wenchuan earthquake indicate that the focal depths are 5 to 14 km and the source parameters are identical with that of earthquake.

Seismicity in Central and Northern Apennines integrating the Italian national and regional networks

In this paper we present and discuss an improved picture of the seismicity distribution of the Umbria–Marche–Abruzzi Apennines as obtained through the integration of the national and the regional seismic networks operating from 2002 to 2006. During this period, both the Istituto Nazionale di Geofisica e Vulcanologia (INGV) National Seismic Network and the regional networks have been greatly improved. We compare the results of the integrated catalogue obtained in this study with the Catalogue of the Italian Seismicity between 1981 and 2001 [Castello, B., Selvaggi, G., Chiarabba, C., Amato, A., 2006. CSI Catalogo della sismicità italiana 1981–2002, versione 1.1. INGV-CNT, Roma.http://legacy.ingv.it/CSI )], confirming the basic known features of the seismic activity in the region, but also evidencing some original and interesting results. In particular, the new data set allows us to better define the geometry and kinematics of the crustal seismicity, which is confined to the upper 20 km and shows a clear general deepening from west to east. In the crust, we find additional evidence of extensional seismicity below the central portion of the belt and thrust/reverse faulting mechanisms at the outer fronts of the Apennines. Looking at the seismicity along the belt, it is also possible to observe aseismic regions, which could be due to either locked or creeping portions of the Apenninic fault system. At greater depth, the west-dipping seismicity distribution down to about 70 km confirms the hypothesis of a slab of Adriatic lithosphere subducted below the Apennines, but also suggests that there are strong lateral heterogeneities and possibly tears in the slab.

Regional Seismic Characteristics of the 9 October 2006 North Korean Nuclear Test

We investigate the regional seismic signature of the 9 October 2006 North Korean nuclear test. Broadband regional data for the nuclear test and a group of earthquakes close to the test site were obtained between December 2000 and No- vember 2006. Epicentral distances from the stations to the test site are between 371 and 1153 km. We first use these regional events to calibrate the Lg-wave magnitude in the network. Then the network is used to calculate mbLg �� 3:93 for the North Ko- rean nuclear explosion. Using a modified fully coupled magnitude-yield relation, the yield of the North Korean nuclear test is estimated to be 0.48 kt. Because of large uncertainties in the source depth, the estimate is preliminary. The P=S-type spectral ratios Pg=Lg, Pn=Lg, and Pn=Sn are calculated for the nuclear explosion and a group of earthquakes close to the test site. At frequencies above 2 Hz, the network-averaged P=S spectral ratios clearly separate the 9 October 2006 explosion from the regional earthquakes. Our result indicates that a single-blast explosion in the North Korea re- gion shows different seismic characteristics from an earthquake. Any well-coupled single-blast explosion detonated in this region with yield similar to that for the North Korean nuclear test has a large probability of being identified by a regional seismic network such as the one adopted in this study.

Focal depths for moderate-sized aftershocks of the Wenchuan M S8.0 earthquake and their implications

Sliding-window cross-correlation method is firstly adopted to identify sPn phase, and to constrain focal depth from regional seismograms, by measuring the time separation between sPn and Pn phases. We present the focal depths of the 17 moderate-sized aftershocks (MS⩾5.0) of the Wenchuan MS8.0 earthquake, using the data recorded by the regional seismic broadband networks of Shaanxi, Qinghai, Gansu, Yunnan and Sichuan. Our results show focal depths of aftershocks range from 8 to 20 km, and tend to cluster at two average depths, separate at 32.5°N, i.e., 11 km to the south and 17 km to the north, indicating that these aftershocks are origin of upper-to-middle crust. Combined with other results, we suggest that the Longmenshan fault is not a through-going crustal fault and the Pingwu-Qingchuan fault may be not the northward extension of the Longmenshan thrust fault.

Attenuation of coda waves in the Northeastern Region of India

Coda wave attenuation quality factor Qc is estimated in the northeastern region of India using 45 local earthquakes recorded by regional seismic network. The quality factor Qc was estimated using the single backscattering model modified by Sato (J Phys Earth 25:27–41, 1977), in the frequency range 1–18 Hz. The attenuation and frequency dependence for different paths and the correlation of the results with geotectonics of the region are described in this paper. A total of 3,890 Qc measurements covering 187 varying paths are made for different lapse time window of 20, 30, 40, 50, 60, 70, 80, and 90 s in coda wave. The magnitudes of the analyzed events range from 1.2 to 3.9 and focal depths range between 7 and 38 km. The source–receiver distances of the selected events range between 16 and 270 km. For 30-s duration, the mean values of the estimated Qc vary from 50 ± 12 (at 1 Hz) to 2,078 ± 211(at 18 Hz) for the Arunachal Himalaya, 49 ± 14 (at 1 Hz) to 2,466 ± 197 (at 18 Hz) for the Indo-Burman, and 45 ± 13 (at 1 Hz) to 2,069 ± 198 (at 18 Hz) for Shillong group of earthquakes. It is observed that Qc increases with frequency portraying an average attenuation relation \(Qc=52.315\pm 1.07f\,^{\left( {1.32\,\pm \,0.036} \right)}\) for the region. Moreover, the pattern of Qc − 1 with frequency is analogous to the estimates obtained in other tectonic areas in the world, except with the observation that the Qc − 1 is much higher at 1 Hz for the northeastern region. The Qc − 1 is about 10 − 1.8 at 1 Hz and decreases to about 10 − 3.6 at 18 Hz indicating clear frequency dependence. Pertaining to the spatial distribution of Qc values, Mikir Hills and western part of Shillong Plateau are characterized by lower attenuation.

Seismological Report on the 6 August 2007 Crandall Canyon Mine Collapse in Utah

At 2:48 a.m. local time (MDT; 08:48 UTC) on 6 August 2007, a major collapse occurred in the Crandall Canyon coal mine in east-central Utah. This collapse resulted in the deaths of six miners who were working underground in the area of the collapse. Ten days later, a much smaller collapse in this mine killed three rescue workers and injured six others (Stricklin 2007). A local magnitude ( ML ) 3.9 seismic event occurred at approximately the same time and place as the Crandall Canyon Mine collapse. Seismological evidence summarized in this paper and in a companion paper by Ford et al. (2008, this issue of SRL ) indicates that most of the seismic wave energy in this event was generated by the mine collapse and not by a naturally occurring earthquake. The ML 3.9 event was preceded and followed by numerous smaller ( ML ≤ 2.5) seismic events in the same area, most of which are probably mining-related as well. These events include an ML 1.6 shock associated with the coal burst that killed the three rescuers. For convenience, we will refer to the ML 3.9 event as the Crandall Canyon Mine “main shock” and to the events that followed it as “aftershocks,” even though these terms are normally used for naturally occurring earthquakes. There is a high-quality data set for the Crandall Canyon Mine seismic events from surrounding stations of the University of Utah regional seismic network, a five-station temporary network that we deployed in the mine area after the 6 August collapse, the National Science Foundation EarthScope Transportable Array, and other networks. In this paper, we analyze the seismological data for the Crandall Canyon Mine main shock and for preceding events and aftershocks in order to gain a better understanding of these events and …

Using an airgun array in a land reservoir as the seismic source for seismotectonic studies in northern China: experiments and preliminary results

ABSTRACTThis paper reports the field setup and preliminary results of experiments utilizing an airgun array in a reservoir in north China for a seismotectonic study. Commonly used in offshore petroleum resource exploration, the airgun source was found to be more useful than a traditional explosive source for large‐scale and long offset land seismic surveys. The airgun array, formed by four 1,500 in3 airguns (a total of 6,000 in3 in volume) was placed at a depth of 6–9 m into the reservoir to generate the pressure impulse. No direct evidence was found that the airgun source adversely affected the fish in the reservoir. The peak ground acceleration recorded on the top of the reservoir dam 100 m away was 17.8 gal in the horizontal direction; this is much less than the designed earthquake‐resistance threshold of 125 gal for this dam. The energy for one shot of this airgun array is about 6.68 MJ, equivalent to firing a 1.7 kg explosive. The seismic waves generated by the airgun source were recorded by receivers of the regional seismic networks and a temporary wide‐angle reflection and refraction profile formed by 100 short‐period seismometers with the maximum source‐receiver offset of 206 km. The seismic wave signature at these long‐offset stations is equivalent to that generated by a traditional blast source in a borehole with a 1,000–2,000 kg explosive. Preliminary results showed clear seismic phases from refractions from the multi‐layer crustal structures in the north China region. Forward modelling using numerical simulation confirms that the seismic arrivals are indeed from lower crustal interfaces. The airgun source is efficient, economical, environmentally friendly and suitable for being used in urbanized areas. It has many advantages over an explosive source for seismotectonic studies such as the high repeatability that is supreme for stacking to improve signal qualities. The disadvantage is that the source is limited to existing lakes or reservoirs, which may restrict experimental geometry.

Empirical Ground-Motion Prediction Equations for Northern Italy Using Weak- and Strong-Motion Amplitudes, Frequency Content, and Duration Parameters

Abstract The goals of this work are to review the Northern-Italy ground-motion prediction equations (GMPEs) for amplitude parameters and to propose new GMPEs for frequency content and duration parameters. Approximately 10,000 weak and strong waveforms have been collected merging information from different neighboring regional seismic networks operating in the last 30 yr throughout Northern Italy. New ground-motion models, calibrated for epicentral distances ≤100 km and for both local ( M L ) and moment magnitude ( M w ), have been developed starting from a high quality dataset (624 waveforms) that consists of 82 selected earthquakes with M L and M w up to 6.3 and 6.5, respectively. The vertical component and the maximum of the two horizontal components of motion have been considered, for both acceleration (peak ground horizontal acceleration [PGHA] and peak ground vertical acceleration [PGVA]) and velocity (peak ground horizontal velocity [PGHV] and peak ground vertical velocity [PGVV]) data. In order to make comparisons with the most commonly used prediction equations for the Italian territory (Sabetta and Pugliese, 1996 [hereafter, SP96] and Ambraseys et al. 2005a,b [hereafter, AM05]) the coefficients for acceleration response spectra (spectral horizontal acceleration [SHA] and spectral vertical acceleration [SVA]) and for pseudovelocity response spectra (pseudospectral horizontal velocity [PSHV] and pseudospectral vertical velocity [PSVV]) have been calculated for 12 periods ranging between 0.04 and 2 sec and for 14 periods ranging between 0.04 and 4 sec, respectively. Finally, empirical relations for Arias intensities (IA), Housner intensities (IH), and strong motion duration (DV) have also been calibrated. The site classification based on Eurocode (hereafter, EC8) classes has been used (ENV, 1998, 2002). The coefficients of the models have been determined using functional forms with an independent magnitude decay rate and applying the random effects model (Abrahamson and Youngs, 1992; Joyner and Boore, 1993) that allow the determination of the interevent, interstation, and record-to-record components of variance. The goodness of fit between observed and predicted values has been evaluated using the maximum likelihood approach as in Spudich et al. (1999). Comparing the proposed GMPEs with SP96 and AM05, it is possible to observe a faster decay of predicted ground motion, in particular for distances greater than 25 km and magnitudes higher than 5.0. The result is an improvement in fit of about one order of size for magnitudes spanning from 3.5 to 4.5.

The 2006-2007 Earthquake Sequence at Bar Harbor, Maine

ABSTRACT One purpose of monitoring earthquake activity in northeastern North America is to discover which geologic structures are seismically active in this region. If seismically active structures can be found, they can be studied to decipher their seismic history and their potential for strong earthquakes. No seismically active geologic structures have yet been confirmed in the northeastern United States (Ebel and Kafka 1991). The only earthquake with observed surface faulting in northeastern North America took place in the Ungava Peninsula of northern Quebec in 1989 (Adams et al. 1991). Other than some minor offsets of glacial striations (Oliver et al. 1970), no geologic evidence of Holocene surface faulting in the northeastern United States has been reported in the literature. Furthermore, the seismicity that has been detected by modern regional seismic networks in the northeastern United States does not align convincingly along known or suspected geologic structures. Nevertheless, the persistence of small-earthquake activity over time and the historic occurrences of past damaging earthquakes ( e.g. , Ebel 1996; Ebel 2000; Ebel et al. 2000; Ebel 2006) indicate that there must be some seismically active structures in the region that are capable of hosting earthquakes above magnitude 6.0. Because such earthquakes are capable of causing significant damage, there is great incentive to learn which structures are seismically active in this heavily populated region. Between fall 2006 and spring 2007, a sequence of earthquakes took place near the town of Bar Harbor on Mount Desert Island on the coast of Maine ([figure 1][1]). The largest earthquake in the sequence was Lg-magnitude (MLg) 4.2. It caused several rock falls in Acadia National Park near Bar Harbor, forcing the closure of several hiking trails and one road (figures [2][2] and [3][3]; ). Acadia National Park has many steep (almost vertical) rock faces of jointed granite, and some rockslides occur annually due to weathering effects. In this earthquake sequence, only the largest of the events generated sufficiently strong ground motions to generate rock falls of the unstable slopes. A water well at McFarland Hill near Bar Harbor that was being monitored by the U.S. Geological Survey showed an unusual drop in water level of about 2 m immediately following this event (the water level data can be accessed through the Web site ). The MLg 4.2 earthquake was felt over the southern two-thirds of Maine with a few felt reports from New Hampshire (see the community Internet intensity map at [http://pasadena.wr.usgs.gov/shake/ne/STORE/Xtib1\_06/ciim\_display.html][4]) and was the largest event centered in Maine since 1988. A total of 38 earthquakes were detected by seismic stations at regional distances from the Bar Harbor area from the start of the sequence on 22 September 2006 until the end of 2006. Two more events were detected in the spring of 2007. The purpose of this paper is to report on an analysis of the relative locations of the Bar Harbor earthquakes detected by the regional seismic network and to use the results of that analysis to assess what geologic structure might have been seismically active in this earthquake sequence. [1]: #F1 [2]: #F2 [3]: #F3 [4]: http://pasadena.wr.usgs.gov/shake/ne/STORE/Xtib1_06/ciim_display.html

Shear-wave splitting of Sichuan Regional Seismic Network

Using seismic data recorded by the Chengdu Digital Seismic Network from May 1, 2000 to December 31, 2006, we obtain the dominant polarization directions of fast shear-waves at eight digital seismic stations adopting the SAM technique. The results show that the dominant directions of polarizations of fast shear-waves at most of stations are mainly in nearly NE-SW or NW-SE direction in Sichuan. The dominant polarization directions of the fast shear-waves at stations located at the active faults or intersection of several active faults are consistent with the strikes of active faults which control the earthquakes used in the analysis, and are basically consistent with the directions of regional compression axis. However, several stations show that the fast shear-waves are not consistent with the strikes of active faults and the directions of regional compression axis, due to the influence of local complicated crustal structure.

Shear wave splitting and mantle flow associated with the deflected Pacific slab beneath northeast Asia

A total of 361 SKS and five local S wave splitting measurements obtained at global and regional seismic network stations in NE China and Mongolia are used to infer the characteristics of mantle fabrics beneath northeast Asia. Fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features. The dominant fast directions at the eastern part, beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone (MTZ), are about 100° from north, which are almost exactly the same as the motion direction of the Eurasian plate relative to the Pacific plate, and are independent of the direction of local geological features. The splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3% anisotropy. The shear wave splitting observations, complemented by the well‐established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the Paleozoic era, can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected Pacific slab in the MTZ, or by back‐arc asthenospheric flow in the mantle wedge above the slab.

Focal Mechanisms and Seismogenic Structures of the Ms5.7 and Ms4.8 Jiujiang‐Ruichang Earthquakes of Nov.26, 2005

AbstractBased on the records of 6 stations in Jiangxi regional seismic network and CSN, we obtained the focal mechanisms of the MS5.7 Jiujiang‐Ruichang earthquake and its MS4.8 aftershock of Nov.26, 2005 with the “Cut and Paste” (CAP) method, and analyzed the seismogenic structure by combining with relocation of the aftershock sequence and the geologic settings in the region. Our result shows that the best double couple solution of the MS5.7 event is 324°, 55° and 18° for strike, dip, and rake angles respectively, the other nodal plane is 223°, 75°, and 144°, for the MS4.8 aftershock the solution is 54°, 71°, and –160° with the other nodal plane of 317°, 71° and –20° respectively. Complexities can be found in the aftershock and source depth distributions of the sequence. After the occurrence of the MS5.7 main shock, aftershocks occurred and propagated from SE to NW and from shallower to deeper crust, then they probably encountered an asperity and triggered the MS4.8 strong aftershock. This complexity and the different focal mechanisms imply that the two earthquakes may not occur on the same fault. We infer that the MS5.7 main shock was caused by the NW striking Yangjishan‐Wushan‐Tongjiangling fault buried in Ruichang basin and the MS4.8 aftershock occurred on the NE striking Dingjiashan‐Guilinqiao‐Wuning fault in the northwest margin of the basin.

Continuous infrasonic recording within the Utah regional seismic network

In May 2006, the University of Utah Seismograph Stations, as part of a larger consortium, installed an infrasound array east of the Salt Lake Valley (NOQ). Infrasound can be thought of as seismology in the air focusing on subaudible acoustic signals in the frequency band from 0.01 to 20 Hz. Advantages of installing an infrasound array in northern Utah include a proximity to solid earth sources (mine blasts and earthquakes) and the possibility of incorporating the data into a regional seismic network. The primary application of infrasound is in identification of explosions detonated at or near the solid earth‐atmosphere boundary. However, to fully utilize the infrasonic data, better understanding of infrasonic source characteristics and propagation is needed. Goals of the infrasound installation at NOQ include (1) use of local ground truth information to assess the role of infrasound in source identification; (2) classifying properties of the infrasound signal by source type; (3) assessing the time varying...

Coupling characteristics of stress and strain at different layers of different sub-regions in Yunnan and its adjacent areas

In this paper, we collect 6 361 waveform data to calculate the shear wave splitting parameters from a regional seismic network of 22 digital stations in Yunnan and its adjacent area from July 1999 to June 2005. By using the cross-correlation method, 64 splitting events of 16 stations are processed. We also collect the splitting results of eight earthquake sequences to present the characteristics of shear wave splitting in Yunnan and its adjacent areas. The orientations of maximum principal compressive stress of three sub-regions in this area are derived from the CMT focal mechanism solutions of 43 moderate-strong earthquakes provided by Harvard University by the P axis azimuth-averaging method. The principal strain rate at each observatory is deduced from the observations of Crustal Movement Observation Network of China during the period from 1999 to 2004. In addition, the data of Pn aniso-tropy and SKS splitting of Yunnan and its adjacent areas are also collected. We have discovered from this study that the continental lithosphere, as a main seismogenic environment for strong earthquake, can be divided into blocks laterally; the mechanical behavior of lithosphere varies with depth and can be divided into different layers in the vertical orientation; the information of crustal deformation obtained from GPS might be affected by the type of blocks, since there are different types of active blocks in Yunnan and its adjacent areas; the shear wave splitting in this region might be affected mainly by the upper crust or even the surface tectonics.

Q0 Tomography of S Wave Attenuation in Sichuanyunnan and Adjacent Regions

AbstractWe have selected 27530 ML amplitude readings from 5897 events recorded by 149 stations, as reported in the Annual Bulletin of Chinese Earthquakes (ABCE) and regional seismic network of Yunnan province and Sichuan province, and have used tomographic imaging to estimate the lateral variations of the quality factor Q0 (Q at 1Hz) in the crust of Sichuan‐Yunnan and its adjacent regions. Estimated Q0 values vary from 200 to 600 with an average of 400. Q0 value is consistent with tectonic and topographic structure. Q0 is low in the active tectonic regions having many faults, such as the Haiyuan‐Qilian, Fenhe‐Weihe, East Kunlun zones, Western Sichuan‐Northwestern Yunnan‐Baoshan area, and Yunnan and Guizhou boundary‐Kunming‐Simao area. Q0 is high in the stable regions, such as Sichuan Basin, Markam block, Western Guangxi‐Eastern Yunnan block, Ordos Craton, and Qinling‐Dabie area. Additionally, the gains of most of the station terms in Sichuan‐Yunnan and its adjacent regions are near 1.0, only a few as large as 2.0, and the distribution is irregular. Most of the magnitude biases are less than 0.3, only a few as large as 0.4, and the distribution has relation with Q0 value variation, which means that the seismic attenuation has influence on the magnitude measurement.