Temporary Seismic Network Research Articles

Reflection imaging with earthquake sources and dense arrays

AbstractThe Mw 5.8 earthquake that occurred in Louisa County, Virginia, on 23 August 2011 provided an opportunity to record with several “high density” seismic arrays, in addition to traditional, sparse temporary seismic networks. Traditional aftershock networks consist of a few dozen stations spread over tens of kilometers. As a result, the recorded seismic waveforms suffer from spatial aliasing that is so severe that many types of waveform processing are not applicable. Here we report the results of recording with a large number of oil industry‐type instruments deployed at a spacing that is an order of magnitude closer than in traditional deployments. The objective was to image subsurface structure with array methods, using the aftershocks as sources. The dense array recorded continuously for 12 days and consisted of 172 vertical component seismometers that were placed at 200–400 m and a 60 km long three‐component regional profile with stations every 2 km. We demonstrate how processing techniques from Vertical Seismic Profiling can produce high‐resolution 3‐D reflection images of structure beneath the array. These images display reflectivity that correlates with that observed on a nearby deep reflection survey collected by the U.S. Geological Survey. Of particular interest is a strong reflector imaged across multiple profiles. Our analysis demonstrates how a surface array of seismometers can provide 3‐D images of structure using microearthquake sources when wavefields are sampled sufficiently densely.

Seismicity of the rocky mountains and Rio Grande Rift from the EarthScope Transportable Array and CREST temporary seismic networks, 2008–2010

AbstractWe developed a catalog of small magnitude (ML −0.1 to 4.7) seismicity across Colorado and New Mexico from the EarthScope USArray Transportable Array and CREST (Colorado Rocky Mountains Experiment and Seismic Transects) seismic networks from 2008 to 2010 to characterize active deformation in the Rio Grande Rift. We recorded over 900 earthquakes in the Rio Grande Rift region, not including induced earthquakes and mine blasts, and find that the rift is actively deforming both broadly and in distinct regions. Seismic events that are likely induced, mostly in the Raton Basin, make up 66% of the catalog (1837 earthquakes). Neogene faults in the northern rift in north central Colorado are seismically active in the North Park Basin and northwestern Colorado. The central rift from the San Luis Basin (southern Colorado) to south of the Socorro Magma Body is the most seismically active rift region, and seismicity delineates the deformation in the Colorado Plateau transition zone, which is spatially correlated with volcanic vents, dikes, and faults within the western Jemez Lineament. The eastern Jemez Lineament is nearly aseismic and surrounded by a halo of seismicity culminating in boundaries defined by recent moderate (Mw 3.9 and Mw 3.3) earthquakes. The southern rift is characterized by diffuse seismicity in Texas and Mexico. This study provides an updated seismic catalog built with uniformity in seismometer coverage and low epicentral uncertainties (~2 km) that allows for regional evaluation of seismicity. During this time period, clusters of seismicity and moderate magnitude earthquakes characterize deformation in a low‐strain rate extensional environment.

Estimation of the crustal structure in Central Anatolia (Turkey) using receiver functions

The receiver function analysis method was used to determine the crustal structure in Central Anatolia, Turkey, by using teleseismic earthquake records. The method is based on the conversion of incident P waves into S waves reaching an interface in the crust or upper mantle and arrival of the converted wave to the station just after the direct P waves. A temporary seismic network, the Ankara Earthquake Monitoring Network (AnkNET), consisting of six broadband seismograph stations, has been deployed in order to monitor the seismicity in Ankara and its surroundings during the period of 2007?2010. In this study, the crustal structures beneath AnkNET stations were investigated. The stations were located between latitudes 39°N and 41°N and longitudes 32°E and 34°E about 100 km apart from each other around a circle with a radius of about 100 km, including one in Ankara approximately in the center. Hypocentral parameters of 43 teleseismic earthquakes equal to or greater than an instrumental magnitude Mw of 6.5 that occurred at a distance between 30° and 100° far from the central coordinates of AnkNET (40°N, 33°E) were retrieved from the Incorporated Research Institutions for Seismology earthquake catalogue. According to the results of this study, the crustal thickness of the region is between 34.5 km and 40.5 km and the S-wave velocity varies between 3.3 km/s and 4.1 km/s. It is expected that the calculated crustal thickness and velocity values will contribute to future studies in the region.

SISMIKO: emergency network deployment and data sharing for the 2016 central Italy seismic sequence

<p>At 01:36 UTC (03:36 local time) on August 24th 2016, an earthquake Mw 6.0 struck an extensive sector of the central Apennines (coordinates: latitude 42.70° N, longitude 13.23° E, 8.0 km depth). The earthquake caused about 300 casualties and severe damage to the historical buildings and economic activity in an area located near the borders of the Umbria, Lazio, Abruzzo and Marche regions. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) located in few minutes the hypocenter near Accumoli, a small town in the province of Rieti. In the hours after the quake, dozens of events were recorded by the National Seismic Network (Rete Sismica Nazionale, RSN) of the INGV, many of which had a ML > 3.0. The density and coverage of the RSN in the epicentral area meant the epicenter and magnitude of the main event and subsequent shocks that followed it in the early hours of the seismic sequence were well constrained. However, in order to better constrain the localizations of the aftershock hypocenters, especially the depths, a denser seismic monitoring network was needed. Just after the mainshock, SISMIKO, the coordinating body of the emergency seismic network at INGV, was activated in order to install a temporary seismic network integrated with the existing permanent network in the epicentral area. From August the 24th to the 30th, SISMIKO deployed eighteen seismic stations, generally six components (equipped with both velocimeter and accelerometer), with thirteen of the seismic station transmitting in real-time to the INGV seismic monitoring room in Rome. The design and geometry of the temporary network was decided in consolation with other groups who were deploying seismic stations in the region, namely EMERSITO (a group studying site-effects), and the emergency Italian strong motion network (RAN) managed by the National Civil Protection Department (DPC). Further 25 BB temporary seismic stations were deployed by colleagues of the British Geological Survey (BGS) and the School of Geosciences, University of Edinburgh in collaboration with INGV. All data acquired from SISMIKO stations, are quickly available at the European Integrated Data Archive (EIDA). The data acquired by the SISMIKO stations were included in the preliminary analysis that was performed by the Bollettino Sismico Italiano (BSI), the Centro Nazionale Terremoti (CNT) staff working in Ancona, and the INGV-MI, described below.</p>

Site effect studies following the 2016 Mw 6.0 Amatrice Earthquake (Italy): the Emersito Task Force activities

<em>On August 24, 2016, at 01:36 UTC a M<sub>W </sub>6.0 earthquake struck an extensive area of the Central Apennines (Italy). It was followed by a large aftershock (M<sub>W </sub>5.3, August 24, 02:33 UTC) and about 20 earthquakes with magnitude greater than 4.0, located between the towns of Norcia and Amatrice. Due to the mainshock magnitude and the widespread damaging level of buildings in the epicentral area, the Emersito task force has been mobilized by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The aim of Emersito is to carry out and coordinate the monitoring of local site effects, caused by geological and geomorphological settings. During the first days of the seismic emergency, Emersito installed a temporary seismic network for site effect studies at 4 municipalities close to the epicentral area (Amandola, Civitella del Tronto, Montereale and Capitignano), using 22 stations equipped with both velocimetric and accelerometric sensors. The selection of the sites where stations have been installed was mainly driven by the proximity to the epicentral area (without interfere with the rescue operation) and by peculiar geologic and geomorphologic settings (topographic irregularities, fault zones, alluvial plains).</em>

The 12 September 2016 Gyeongju earthquakes: 2. Temporary seismic network for monitoring aftershocks

The ML 5.8 earthquake in Gyeongju, southeastern Korea, on September 12, 2016 11:32:54 (UTC) was the largest earthquake on the Korean Peninsula since instrumental monitoring began in 1978. It was preceded by an ML 5.1 foreshock and is being followed by numerous aftershocks. Within an hour of the mainshock, the first temporary seismic station to monitor aftershocks was installed at about 1.5 km east of the announced epicenter. The current temporary seismic network consists of 27 stations equipped with broadband sensors covering an area of ∼38 × 32 km in the mainshock region. This is the first high-density aftershock monitoring array in the Korean Peninsula. Initial results, using data from both the regional seismic networks and the aftershock monitoring array, indicate that earthquakes during the first 10 days following the mainshock are related to the Yangsan Fault System. Establishment of an official rapid-response team to monitor aftershocks of major earthquakes is advised.

The 2013 Seismic Swarm in Chihuahua, Mexico: Evidence of Active Extensional Deformation in the Southern Basin and Range

Abstract An earthquake swarm took place from 27 August 2013 to December 2013 in the southern Basin and Range province of Chihuahua, Mexico. During this period, the Seismological Service of Mexico located approximately 60 earthquakes in the region. The largest earthquake took place on 21 September 2013 ( M w 5.3). A temporary seismic network was installed in the area for a period of two weeks. The hypocenters were relocated using the double‐difference algorithm. The results show a cluster of seismicity along the Vallecillo fault, where most of the seismicity took place, and on the Penasco fault. These two faults lie within the foothills of the sierras of the same name that bound the El Caballo desert basin. The seismicity suggests that both faults bounding the El Caballo basin slipped during the swarm. The main event of 27 August 2013 apparently took place on the Vallecillo fault. A third cluster of seismicity was located on Sierra La Mezcalera, about 20 km to the south of the Vallecillo fault, at the western end of the La Boquilla dam. Focal mechanisms of the larger events indicate right‐lateral slip with extensional deformation on the Vallecillo fault, with T axes oriented in a northeast–southwest direction. These source mechanisms reflect a regime of extensional tectonics in the southern Basin and Range province. The Vallecillo and Penasco fault system is exposed for approximately 12 km along strike. Scaling relations suggest that earthquakes as large as M w 6.4 may take place on them. This magnitude is similar to that of the 1928 Parral earthquake that occurred about 125 km to the south of the swarm. However, segmented faults may rupture sequentially and generate a larger earthquake than the one predicted on the basis of the length of individual mapped faults.

The Katla volcanic system imaged using local earthquakes recorded with a temporary seismic network

AbstractKatla is one of the most active subglacial volcanoes in Iceland. A temporary seismic network was operated on and around Katla for 2.5 years. A subset of ~800 analyzed local earthquakes clustered geographically in four regions: (1) the caldera, (2) the western region, (3) the southern rim, and (4) the eastern rim of the glacier. Based on the frequency content of recorded seismograms, each event was labeled as volcano tectonic (VT), long period (LP), or ‘Mixed’. The southern cluster consists of LP events only, and the eastern cluster consists of VT events, while the western cluster is ‘Mixed’ although primarily LP. The caldera seismicity is confined to a subregion centered in the northeastern part of the caldera above 1 km below sea level (bsl) and gradually deepens away from its center to about 4 km depth. Deeper events are almost all VT, whereas LP events in the center of caldera locate at shallow depths. This is also where the velocities are lowest in the top 3 km of the crust of our 3‐D tomographic model. A high‐velocity core (~6.5 km/s) is found at 4 km bsl beneath this low‐velocity zone. We propose that a “subcaldera” may be developing within the present caldera and suggest a conceptual model for Katla volcano with a thin volume (~1 km thick) that may host hot rhyolitic material in the shallow crust below the relocated seismic activity and above the high‐velocity core. We interpret this core to consist of mafic cumulates resulting from fractionation of mafic intrusions and partial melting of subsiding hydrothermally altered rocks.

Seismic and volcanic activity during 2014 in the region involved by TOMO-ETNA seismic active experiment

<p>This paper presents an overview of the seismic and volcanic activity occurred during 2014 in the region involved by the TOMO-ETNA seismic active experiment (Mt. Etna, Aeolian Islands and Peloritani-Messina Strait areas). To better characterize the seismicity over the year, three-dimensional hypocenter locations and focal mechanism solutions of a dataset of 678 selected small-to-moderate magnitude earthquakes (0.5 ≤ M<sub>L</sub> ≤ 4.3) were analyzed. In the framework of the TOMO-ETNA experiment, a temporary seismic network was installed on-land from June to November 2014, both to acquire seismic signals produced by shots <span>and </span>to record the local seismicity. Data collected by the temporary network were used to integrate those deriving from the permanent seismic network operated by the Istituto Nazionale di Geofisica e Vulcanologia (INGV)-Osservatorio Etneo (Etna Observatory), thus obtaining a numerically more robust dataset. In agreement with previous analysis and studies, the distribution of the hypocentral locations is well representative of the seismicity that typically characterizes this area. The selected well-constrained 42 fault plane solutions evidence two domains characterized by different motions and style of deformation. In particular, an extensional domain in the northeastern Sicily and a strike-slip regime in the northernmost part of the studied region have been observed.</p>

TOMO-ETNA experiment at Etna volcano: activities on land

<p>In the present paper we describe the on-land field operations integrated in the TOMO-ETNA experiment carried out in June-November 2014 at Mt. Etna volcano and surrounding areas. This terrestrial campaign consists in the deployment of 90 short-period portable three-component seismic stations, 17 Broadband seismometers and the coordination with 133 permanent seismic station belonging to Italy’s Istituto Nazionale di Geofisica e Vulcanologia (INGV). This temporary seismic network recorded active and passive seismic sources. Active seismic sources were generated by an array of air-guns mounted in the Spanish oceanographic vessel “Sarmiento de Gamboa” with a power capacity of up to 5200 cubic inches. In total more than 26,000 shots were fired and more than 450 local and regional earthquakes were recorded. We describe the whole technical procedure followed to guarantee the success of this complex seismic experiment. We started with the description of the location of the potential safety places to deploy the portable network and the products derived from this search (a large document including full characterization of the sites, owners and indication of how to arrive to them). A full technical description of the seismometers and seismic sources is presented. We show how the portable seismic network was deployed, maintained and recovered in different stages. The large international collaboration of this experiment is reflected in the participation of more than 75 researchers, technicians and students from different institutions and countries in the on-land activities. The main objectives of the experiment were achieved with great success.</p>

Dense network, intense seismicity and tectonics of Taiwan

A temporary seismic network consisting of 295 land stations was deployed in 2009 in Taiwan to monitor airgun sources during an onshore-offshore experiment in Taiwan. We exploit the continuously recorded dataset to detect and map seismicity in the seismically very active Taiwan. By combining recent automatic detection and phase picking techniques, we successfully generate an initial earthquake catalog of over 8400 events. The hypoDD algorithm is used to relocate and filter these events. This network recorded smaller events than the permanent regional network because of the many stations around the high mountain ranges and the generally high station density along 6 lines. The results based on the 2009 data generally reproduce the dominant seismicity features from many years of earthquake monitoring in Taiwan. In addition, we map hitherto unknown dipping zones under the Foothills and the Central Range that may correspond to seismogenic structures.

“Time-building specific” seismic vulnerability assessment of a hospital RC building using field monitoring data

In the context of seismic vulnerability assessment of reinforced concrete (RC) buildings, the use of field monitoring data constitutes a significant tool for the representation of the actual structural state, reducing uncertainties associated with the building configuration properties as well as many non-physical parameters (age, maintenance, etc.), enhancing thus the reliability in the risk assessment procedure. In this study, the seismic vulnerability of existing RC buildings is evaluated, combining through a comprehensive methodology, the numerical analysis and field monitoring data. The proposed methodology is highlighted through the derivation of “time-building specific” fragility curves for an eight-storey RC structure (hospital building), built almost five decades ago, that is composed by two adjacent units connected with a structural joint. The assessment of the dynamic characteristics is performed using ambient noise measurements recorded by a temporary seismic network which was deployed inside the hospital. The modal identification results are used to update and better constrain the initial finite element model of the building, which is based on the available design and construction documentation plans. Three-dimensional incremental dynamic analysis is performed to derive the fragility curves for the initial as built model (“building-specific”) and for the real structures as they are nowadays (“time-building specific”). The initial “building specific” curves are evaluated through their comparison with conventional generic curves that are commonly used in risk assessment studies. Moreover, in order to enhance the reliability of the obtained results, the “time-building specific” fragility curves, are compared to time-dependent curves derived for the hospital units adopting an appropriate for the specific case study corrosion scenario. Results derived from both approaches indicate that the consideration of the actual state of structures may significantly alter their expected seismic performance leading to higher vulnerability values.

Tomography of the Chukou Fault Zone, Southwest Taiwan: Insights from Microearthquake Data

The vigorous collision between the Eurasian plate and Philippine Sea plate in Taiwan causes a series of imbricate fold and thrust belts to develop at the deformation front. The Chukou Fault (CKF), characterized by a thrust type fault, located in Chiayi County, southwest (SW) Taiwan, is a prominent boundary between the fold-thrust belts and the Western Coastal Plain. Most of the seismicity in SW Taiwan is associated with this fault and its neighboring fault systems. The seismotectonic structures in the CKF zone, especially in the east, are complex due to the interactions among fault systems with distinct slip motions. To gain better insights into the seismogenic characteristics in the CKF zone, we used 1661 microearthquakes recorded by a temporary dense broadband seismic network and the Central Weather Bureau Seismic Network (CWBSN) between 2003 and 2004 to investigate the physical properties of the crust in the CKF zone. A waveform cross-correlation technique was applied to 143086 pairs of waveform data to determine the relative differential travel time between the P- and S-waves. By combining both the absolute and relative differential travel time data, we were able to obtain a new 3-D crustal P-wave velocity structure and Vp/Vs ratios. This study suggests that by using both absolute and relative differential travel time data in tomographic inversion can obtain precise 3-D velocity images and also gain better correlation between seismic events and fault structures, which is crucial for understanding the seismogenic process in our study area.

Characterization of the hydrothermal system of the Tinguiririca Volcanic Complex, Central Chile, using structural geology and passive seismic tomography

A structural characterization of the hydrothermal–volcanic field associated with the Tinguiririca Volcanic Complex had been performed by combining passive seismic tomography and structural geology. This complex corresponds to a 20km long succession of N25°E oriented of eruptive centers, currently showing several thermal manifestations distributed throughout the area. The structural behavior of this zone is controlled by the El Fierro–El Diablo fault system, corresponding to a high angle reverse faults of Oligocene–Miocene age. In this area, a temporary seismic network with 16 short-period stations was setup from January to April of 2010, in the context of the MSc thesis of Lira (2010), covering an area of 200km2 that corresponds with the hydrothermal field of Tinguiririca Volcanic Complex (TVC), Central Chile, Southern Central Andes. Using P- and S- wave arrival times, a 3D seismic velocity tomography was performed. The preliminary locations of 2270 earthquakes have first been determined using an a priori 1D velocity model. Afterwards, a joint inversion of both, the 3D velocity model and final locations have been obtained. High Vp/Vs ratios are interpreted as zones with high hot fluid content and high fracturing. Meanwhile, low Vp/Vs anomalies could represent the magmatic reservoir and the conduit network associated to the fluid mobility. Based on structural information and thermal manifestations, these anomalies have been interpreted. In order to visualize the relation between local geology and the velocity model, the volume associated with the magma reservoir and the fluid circulation network has been delimited using an iso-value contour of Vp/Vs equal to 1.70. The most prominent observed feature in the obtained model is a large “V” shaped low-velocity anomaly extending along the entire study region and having the same vergency and orientation as the existing high-angle inverse faults, which corroborates that El Fierro–El Diablo fault system represents the local control for fluid mobility. This geometry coincides with surface hydrothermal manifestations and with available geochemical information of the area, which allowed us to generate a conceptual model of fluid circulation in the volcanic–hydrothermal system as well as define the location of the magmatic reservoir.

Site selection for a countrywide temporary network in Austria: noise analysis and preliminary performance

Abstract. Site selection is a crucial part of the work flow for installing seismic stations. Here, we report the preparations for a countrywide temporary seismic network in Austria. We describe the specific requirements for a multi-purpose seismic array with 40 km station spacing that will be operative approximately three years. Reftek 151 60 s sensors and Reftek 130/130S digitizers form the core instrumentation of our seismic stations which are mostly installed inside abandoned or occasionally used basements or cellars. We present probabilistic power spectral density analysis to assess noise conditions at selected sites and show exemplary seismic events that were recorded by the preliminary network by the end of July 2015.

A 3DQPAttenuation Model for All of New Zealand

Online Material: Table of New Zealand-wide Q P and V P with resolution estimate; figure showing nonparametric curve fit Q P ( V P ). Over the last 13 years, we have developed 3D seismic attenuation models for many parts of New Zealand through individual regional studies using local earthquake data, often collected using regional temporary seismic networks. Most of New Zealand is seismically active, as illustrated by Figure 1a, which shows the distribution of earthquakes of magnitude≥4.5 for the 2001–2011 period. These earthquakes illustrate how convergence between the Pacific and Australian plates is accommodated in very different ways along the New Zealand plate boundary, from subduction of the Pacific plate under the Australian plate at the Hikurangi subduction zone in the North Island and northern South Island, to continental convergence in the central South Island, to steep subduction of the Australian plate beneath the Pacific plate in Fiordland in southwestern South Island. Figure 1. (a) Tectonic setting of the New Zealand region, showing earthquake epicenters for events of magnitude≥4.5 from the period 2001–2011; the 2 km bathymetry contour is shown offshore. The dashed box shows the extent of the defining New Zealand-wide grid, and the narrower box (solid lines) indicates the area shown in Figure 3. (b) Stations used in the QP inversions and areas of individual studies: A, northeastern North Island (Eberhart-Phillips and Chadwick, 2002); B, southern North Island (Eberhart-Phillips et al. , 2005); C, central–southern South Island (Eberhart-Phillips, Chadwick, et al. , 2008); D, central North Island (Eberhart-Phillips, Reyners, et al. , 2008); E, northern South Island (Eberhart-Phillips et al. , 2014). The locations of previous regional studies of 3D seismic attenuation are shown in Figure 1b. From north to south, these are Eberhart-Phillips …

Increasing background seismicity and dynamic triggering behaviors with nearby mining activities around Fangshan Pluton in Beijing, China

AbstractDynamic triggering in western Fangshan Pluton, Beijing, China, has been repeatedly identified, but previous studies are limited by sparse seismic station coverage. Here we systematically analyze continuous waveforms recorded by both permanent stations and a temporary seismic network 40 days before and after the 11 March 2011 Mw 9.1 Tohoku‐Oki and the 14 April 2012 Mw 8.6 Indian Ocean earthquakes. We first build a template database using a short‐term average to long‐term average method. Next, we apply the matched filter technique that cross correlates the template waveforms with continuous data to detect additional seismic events. Overall, we detect 1956 and 950 seismic events around the Tohoku‐Oki and Indian Ocean main shocks, respectively. Most detected events are shallow (<5 km) and clustered at Beiling Syncline in western Fangshan Pluton, which is adjacent to a running coalmine. Seven and 10 events are detected during the large‐amplitude surface waves of the two main shocks, respectively, but no similar burst is detected following their major foreshock and aftershocks. Multiple statistical tests indicate that the short‐term bursts after the two main shocks are dynamically triggered. We suggest that mining‐related activities may perturb the subsurface stress conditions and hence make the region more susceptible for dynamic triggering than other places.

New constraints on micro-seismicity and stress state in the western part of the North Anatolian Fault Zone: Observations from a dense seismic array

With the aim of extensively investigating the crustal structure beneath the western segment of the North Anatolian Fault Zone where it splays into northern and southern branches, a temporary seismic network (dense array for North Anatolia-DANA) consisting of 70 stations was deployed in early May 2012 and operated for 18months in the Sakarya region during the FaultLab experiment. Out of 2437 events contaminated by explosions, we extracted 1371 well located earthquakes. The enhanced station coverage having a nominal station spacing of 7km, lead to a minimum magnitude calculation of 0.1. Horizontal and vertical location uncertainties within the array do not exceed 0.8km and 0.9km, respectively. We observe considerable seismic activity along both branches of the fault where the depth of the seismogenic zone was mostly confined to 15km. Using our current earthquake catalog we obtained a b-value of 1. We also mapped the b-value variation with depth and observed a gradual decrease. Furthermore, we determined the source parameters of 41 earthquakes with magnitudes greater than 1.8 using P-wave first motion polarity method. Regional Moment Tensor Inversion method was also applied to earthquakes with magnitudes greater than 3.0. Focal mechanism solutions confirm that Sakarya and its vicinity is stressed by a compressional regime showing a primarily oblique–slip motion character. Stress tensor analysis indicates that the maximum principal stress is aligned in WNW–ESE direction and the tensional axis is aligned in NNE–SSW direction.

The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China

In order to infer the distribution of local stress and the deep geodynamic process in North China, this study detects seismic anisotropy in the crust and upper mantle beneath the Bohai Sea area. A total of 535 local shear-wave and 721 XKS (including SKS, PKS and SKKS phases) splitting measurements were obtained from stations in permanent regional seismograph networks and a temporary seismic network called ZBnet-E. The dominant fast polarization orientation of local shear-waves in the crust is nearly East-West, suggesting an East-West direction of local maximum compressive stress in the area. Nearly North-South fast orientation was obtained at some stations in the Tan-Lu fault belt and the Zhang-Bo seismic belt. The average fast orientation from XKS splitting analysis is 87.4° measured clockwise from the North. The average time-delays of XKS splitting are range from 0.54s to 1.92s, corresponding to a 60–210km thick layer of anisotropy. The measured results indicate that upper mantle anisotropy beneath Bohai Sea area, even the eastern part of North China, is mainly from asthenospheric mantle flow from the subduction of the Pacific plate. From the complicated anisotropic characteristics in this study, we infer that there might be multiple mechanisms in the crust and upper mantle around the Bohai Sea area that led to the observed anisotropy.

Seismotectonic significance of the 2008–2010 Walloon Brabant seismic swarm in the Brabant Massif, Belgium

Between 12 July 2008 and 18 January 2010 a seismic swarm occurred close to the town of Court-Saint-Etienne, 20km SE of Brussels (Belgium). The Belgian network and a temporary seismic network covering the epicentral area established a seismic catalogue in which magnitude varies between ML -0.7 and ML 3.2. Based on waveform cross-correlation of co-located earthquakes, the spatial distribution of the hypocentre locations was improved considerably and shows a dense cluster displaying a 200m-wide, 1.5-km long, NW-SE oriented fault structure at a depth range between 5 and 7km, located in the Cambrian basement rocks of the Lower Palaeozoic Anglo-Brabant Massif. Waveform comparison of the largest events of the 2008–2010 swarm with an ML 4.0 event that occurred during swarm activity between 1953 and 1957 in the same region shows similar P- and S-wave arrivals at the Belgian Uccle seismic station. The geometry depicted by the hypocentral distribution is consistent with a nearly vertical, left-lateral strike-slip fault taking place in a current local WNW–ESE oriented local maximum horizontal stress field. To determine a relevant tectonic structure, a systematic matched filtering approach of aeromagnetic data, which can approximately locate isolated anomalies associated with hypocentral depths, has been applied. Matched filtering shows that the 2008–2010 seismic swarm occurred along a limited-sized fault which is situated in slaty, low-magnetic rocks of the Mousty Formation. The fault is bordered at both ends with obliquely oriented magnetic gradients. Whereas the NW end of the fault is structurally controlled, its SE end is controlled by a magnetic gradient representing an early-orogenic detachment fault separating the low-magnetic slaty Mousty Formation from the high-magnetic Tubize Formation. The seismic swarm is therefore interpreted as a sinistral reactivation of an inherited NW–SE oriented isolated fault in a weakened crust within the Cambrian core of the Brabant Massif.