High-precision earthquake locations in Switzerland using regional secondary arrivals in a 3-D velocity model

We present a new approach to relocate earthquakes in the greater western Alpine region using main crustal phases (Pg, Pn, PmP) that takes advantage of recent developments in P-wave velocity models and modelling of the Moho topography in the region, as well as the ability to track reflected and refracted phases in three-dimensional (3-D) heterogeneous media. Our approach includes a new 3-D P-wave velocity model for Switzerland and surrounding regions that combines a first-order Moho discontinuity based on local earthquake tomography (LET) and controlled-source seismology (CSS) information and 3-D seismic velocity information based on LET. Traveltimes for the main crustal phases (Pg, Pn, PmP) are computed using a fast marching method. We use a non-linear, probabilistic approach to relocate earthquakes that has been extended to include the use of secondary phases. We validate our approach using synthetic data, which was computed for a real earthquake and different combinations of available phases (Pg, Pn, PmP). We also applied our approach to relocate four selected earthquakes, two shallow and two deep crustal events in the northern Alpine foreland, for which independent information (ground truth information) on their focal depths exist. Our results demonstrate that the precision and accuracy of focal depth estimates can be greatly improved if secondary phases are used. This gain is a combined effect of an improved range of take-off angles and the use of differential traveltimes between first and secondary arriving phases. Our results also show that reliable information on the Moho depth is crucial to obtain accurate focal depths, if Pn or PmP phases are used in the relocation process. Finally, our approach demonstrates that proper identification of the main crustal phases in combination with an appropriate model parametrization in the forward solver will significantly improve earthquake location

Closing crack earthquakes within the Krafla caldera, North Iceland

Moment tensor analysis with a Bayesian approach was used to analyse a non-double-couple (non-DC) earthquake ($M_w$ ~ 1) with a high isotropic (implosive) component within the Krafla caldera, Iceland. We deduce that the earthquake was generated by a closing crack at depth. The event is well located, with high signal-to-noise ratio and shows dilatational $P$-wave first arrivals at all stations where the first arrival can be picked with confidence. Coverage of the focal sphere is comprehensive and the source mechanism stable across the full range of uncertainties. The non-DC event lies within a cluster of microseismic activity including many DC events. Hence, we conclude that it is a true non-DC closing crack earthquake as a result of geothermal utilization and observed magma chamber deflation in the region at present.Natural Environment Research Council (Grant ID: NE/H025006/1

Seismotectonic study of the Fergana region (Southern Kyrgyzstan): distribution and kinematics of local seismicity

We present new seismicity and focal-mechanism data for the Fergana basin and surrounding mountain belts in western Kyrgyzstan from a temporary local seismic network. A total of 210 crustal earthquakes with hypocentral depths shallower than 25 km were observed during a 12-month period in 2009/2010. The hypocenter distribution indicates a complex net of seismically active structures. The seismicity derived in this study is mainly concentrated at the edges of the Fergana basin, whereas the observed rate of seismicity within the basin is low. The seismicity at the dominant tectonic feature of the region, the Talas-Fergana fault, is likewise low, so the fault seems to be inactive or locked. To estimate the uncertainties of earthquake locations derived in this study, a strong explosion with known origin time and location is used as a ground truth calibration event which suggests a horizontal and vertical accuracy of about 1 km for our relocations. We derived 35 focal mechanisms using first motion polarities and retrieved a set of nine moment tensor solutions for earthquakes with moment magnitude (Mw) ranging from 3.3 to 4.9 by waveform inversion. The solutions reveal both thrust and strike-slip mechanisms compatible with a NW-SE direction of compression for the Fergana region. Two previously unknown tectonic structures in the Fergana region could be identified, both featuring strike-slip kinematics. The combined analysis of the results derived in this study allowed a detailed insight into the currently active tectonic structures and their kinematics where little information had previously been available

Swiss AlpArray temporary broad-band seismic stations deployment and noise characterization

Abstract. AlpArray is a large collaborative seismological project in Europe that includes more than 50 research institutes and seismological observatories. At the heart of the project is the collection of top-quality seismological data from a dense network of broadband temporary seismic stations, in compliment to the existing permanent networks, that ensures a homogeneous station coverage of the greater Alpine region. This Alp Array Seismic Network (AASN) began operation in January 2016 and will have a duration of at least 2 years. In this work we report the Swiss contribution to the AASN, we concentrate on the site selection process, our methods for stations installation, data quality and data management. We deployed 27 temporary broadband stations equipped with STS-2 and Trillium Compact 120 s sensors. The deployment and maintenance of the temporary stations across 5 countries is managed by ETH Zurich and it is the result of a fruitful collaboration between five institutes in Europe.Published15–291IT. Reti di monitoraggioJCR Journa

Contemporary Seismicity in and Around the Yakima Fold-and-Thrust Belt in Eastern Washington

We examined characteristics of routinely cataloged seismicity from 1970 to the present in and around the Yakima fold-and-thrust belt (YFTB) in eastern Washington to determine if the characteristics of contemporary seismicity provide clues about regional-scale active tectonics or about more localized, near-surface processes. We employed new structural and hydrologic models of the Columbia River basalts (CRB) and found that one-third to one-half of the cataloged earthquakes occur within the CRB and that these CRB earthquakes exhibit significantly more clustered, and swarmlike, behavior than those outside. These results and inferences from published studies led us to hypothesize that clustered seismicity is likely associated with hydrologic changes in the CRB, which hosts the regional aquifer system. While some general features of the regional groundwater system support this hypothesis, seismicity patterns and mapped long-term changes in groundwater levels and present-day irrigation neither support nor refute it. Regional tectonic processes and crustal-scale structures likely influence the distribution of earthquakes both outside and within the CRB as well. We based this inference on qualitatively assessed alignments between the dominant northwest trends in the geologic structure and the seismicity generally and between specific faults and characteristics of the 2009 Wooded Island swarm and aseismic slip, which is the only cluster studied in detail and the most vigorous since regional monitoring began.USGS-NAGTGeological Science

Seismology: neotectonics and structure of the Baltic Shield

Recent Danish seismological projects involving neotectonic investigations and structural studies have determined the edge of the Baltic Shield underlying Denmark. The most active earthquake zones in Denmark are located in northwestern Jylland and adjoining offshore areas, and in the region around Kattegat, Øresund and north-east Sjælland (Fig. 1). This pattern was originally recognised by Lehmann (1956) and has been confirmed by several later studies, e.g. Gregersen et al. (1998). Recent, more detailed investigations have documented that changes in the pattern of earthquake activity have occurred within a short time span. The most pronounced example of change – possibly related to exploitation of hydrocarbons – is an activity recorded in the Central Graben area of the North Sea that was first documented by Gregersen et al. (1998). The south-western margin of the Precambrian Baltic Shield separates areas of different earthquake activity (Fig. 1; Gregersen et al. 1991). Although lithospheric stresses are more or less uniform in northern Europe, there are pronounced differences in the behaviour of the lithosphere across Denmark. The north-eastern area underlain by the Baltic Shield experiences brittle failure as recorded by common earthquakes, whereas earthquakes are virtually absent in the region southwest of the shield (Fig. 1). The margin of the Baltic Shield as defined by earthquake activity is not identical with that distinguished structurally in sedimentary studies (EUGENO-S Working Group 1988; Vejbæk & Britze 1994), in crustal studies (Abramovitz & Thybo 2000), or by recent studies of the structure of the subcrustal lithosphere (Gregersen et al. 2002; Shomali et al. 2002). The physical edge of the Baltic Shield cannot be uniquely determined on the basis of seismological studies. The earthquakes recorded, although of low magnitude, do give information about the released stresses. The earthquakes seem to be a response to a dominant NW–SE compression, also apparent elsewhere in Scandinavia and northern Europe (Slunga et al. 1984; Slunga 1989; Gregersen 1992; Müller et al. 1992). These stresses are part of the large-scale stress systems associated with continued plate motion pattern (Gregersen & Basham 1989; Zoback et al. 1989). In contrast to present low-magnitude earthquakes, postglacial sediments in northern Scandinavia have preserved features interpreted as caused by earthquakes of magnitudes around 7; these major, c. 9000 years old earthquakes are believed to be related to the post-glacial uplift of Scandinavia (e.g. Arvidsson et al. 1991; Gregersen 2002). Earthquakes are always related to fault activity, but attempts to link recent earthquakes occurring in and around Denmark to geologically known faults have only been partly successful (Gregersen et al. 1996). The most significant fault zone in Denmark, the Sorgenfrei–Tornquist Zone, is only locally active. Recent geodetic and seismic investigations demonstrate that the two sides of the Sorgenfrei–Tornquist Zone are characterised by different patterns of deformation, but the zone itself is not defined by a present-day seismicity trend crossing the central parts of Denmark (Fig. 1)

Comparing the Performance of Regional Earthquake Early Warning Algorithms in Europe

Several earthquake early warning (EEW) algorithms have been developed worldwide for rapidly estimating real-time information (i.e., location, magnitude, ground shaking, and/or potential consequences) about ongoing seismic events. This study quantitatively compares the operational performance of two popular regional EEW algorithms for European conditions of seismicity and network configurations. We specifically test PRobabilistic and Evolutionary early warning SysTem (PRESTo) and the implementation of the Virtual Seismologist magnitude component within SeisComP, VS(SC), which we use jointly with the SeisComP scanloc module for locating events. We first evaluate the timeliness and accuracy of the location and magnitude estimates computed by both algorithms in real-time simulation mode, accounting for the continuous streaming of data and effective processing times. Then, we focus on the alert-triggering (decision-making) phase of EEW and investigate both algorithms’ ability to yield accurate ground-motion predictions at the various temporal instances that provide a range of warning times at target sites. We find that the two algorithms show comparable performances in terms of source parameters. In addition, PRESTo produces better rapid estimates of ground motion (i.e., those that facilitate the largest lead times); therefore, we conclude that PRESTo may have a greater risk-mitigation potential than VS(SC) in general. However, VS(SC) is the optimal choice of EEW algorithm if shorter warning times are permissible. The findings of this study can be used to inform current and future implementations of EEW systems in Europe

Automatic data processing and analysis system for monitoring region around a planned nuclear power plant

The Institute of Seismology of University of Helsinki is building a new local seismic network, called OBF network, around planned nuclear power plant in Northern Ostrobothnia, Finland. The network will consist of nine new stations and one existing station. The network should be dense enough to provide azimuthal coverage better than 180° and automatic detection capability down to ML −0.1 within a radius of 25 km from the site.The network construction work began in 2012 and the first four stations started operation at the end of May 2013. We applied an automatic seismic signal detection and event location system to a network of 13 stations consisting of the four new stations and the nearest stations of Finnish and Swedish national seismic networks. Between the end of May and December 2013 the network detected 214 events inside the predefined area of 50 km radius surrounding the planned nuclear power plant site. Of those detections, 120 were identified as spurious events. A total of 74 events were associated with known quarries and mining areas. The average location error, calculated as a difference between the announced location from environment authorities and companies and the automatic location, was 2.9 km. During the same time period eight earthquakes between magnitude range 0.1–1.0 occurred within the area. Of these seven could be automatically detected. The results from the phase 1 stations of the OBF network indicates that the planned network can achieve its goals.Abstract. The Institute of Seismology of University of Helsinki is building a new local seismic network, called OBF network, around planned nuclear power plant in Northern Ostrobothnia, Finland. The network will consist of nine new stations and one existing station. The network should be dense enough to provide azimuthal coverage better than 180° and automatic detection capability down to ML −0.1 within a radius of 25 km from the site. The network construction work began in 2012 and the first four stations started operation at the end of May 2013. We applied an automatic seismic signal detection and event location system to a network of 13 stations consisting of the four new stations and the nearest stations of Finnish and Swedish national seismic networks. Between the end of May and December 2013 the network detected 214 events inside the predefined area of 50 km radius surrounding the planned nuclear power plant site. Of those detections, 120 were identified as spurious events. A total of 74 events were associated with known quarries and mining areas. The average location error, calculated as a difference between the announced location from environment authorities and companies and the automatic location, was 2.9 km. During the same time period eight earthquakes between magnitude range 0.1–1.0 occurred within the area. Of these seven could be automatically detected. The results from the phase 1 stations of the OBF network indicates that the planned network can achieve its goals.Peer reviewe

Evidence for both crustal and mantle earthquakes in the subducting Juan de Fuca plate

We investigate the relationship between high-precision hypocentres in the subducting Juan de Fuca plate beneath southwest British Columbia, and the velocity structure from receiver function analysis and reflection seismic data. 108 earthquakes at depths of 40 to 65 km were relocated using a double-difference algorithm to obtain precise earthquake locations. Correlation of the relocated seismicity with structural information shows a concentration of earthquakes near the top of the subducting oceanic crust, and a deeper layer of seismicity in the uppermost mantle of the subducting Juan de Fuca plate. The strong correlation of the seismicity with the structure of the subducting plate suggests that seismic failure is caused by changes in mechanical strength within the plate, supporting the hypotheses that phase transformations and thermal-petrological conditions play an important role in the seismogenesis of double-seismic zones