Regional Seismic Network Research Articles

Evolution and strengthening of the Calabrian Regional Seismic Network

Abstract. The Calabrian Arc is an area of high seismic hazard, in the past often affected by destructive earthquakes. The seismicity of the Calabrian region is monitored by the Italian National Seismic Network integrated by the Calabrian Regional one and, in the last three years, by the Pollino temporary array. We have applied the Seismic Network Evaluation through Simulation to assess the individual contribution of each network in locating earthquakes with epicentres in the Calabrian region and surrounding. We shows that the Calabrian Regional Seismic Network greatly improves the quality of the coverage in almost the Calabria territory except in the Crotone Basin, in the Serre and in the offshore areas. We show that the contribution of the Pollino temporary array is instead restricted to a very small area centred on the Pollino Chain. Due to the presence in the Serre of important seismogenic volumes, which in the past have generated destructive earthquakes, it would be opportune to add at least several seismic stations in this area and surrounding to improve the seismic monitoring.

Interaction of microseisms with crustal heterogeneity: A case study from the San Jacinto fault zone area

We perform a multicomponent analysis to evaluate the validity and limits of noise imaging in the San Jacinto fault zone (SJFZ) area. Estimates of noise propagation and scattering length scales in the area are combined with a noise correlation‐based analysis of variability of noise constituents, excitation regions, and propagation patterns. We evaluate the quality of correlation‐phase and ‐amplitude imaging of tectonic features in the context of observed noise properties. Statistical properties of a regional high‐resolution 3‐D velocity model indicate that propagation of double‐frequency microseism Rayleigh waves is sensitive to medium heterogeneity in the southern California plate boundary area. The analysis of noise correlation functions constructed from records of a regional seismic network suggests stable excitation of microseisms along the southern California coastline. The proximity to the source region together with randomization properties of the heterogeneous medium govern the scattered yet anisotropic character of the wave field. Insignificant travel time errors resulting from the associated imperfect reconstruction of interstation Green's function estimates allow the resolution of a velocity contrast across the SJFZ from noise correlations. However, attenuation estimates are biased by the anisotropic propagation directions. The interaction of the ambient surface wave field with medium heterogeneity facilitates imaging of the velocity structure, but the inversion of the amplitude pattern is limited since it is dominated by wave field instead of medium properties.

Seismotectonic Model of the Kraljevo 3 November 2010 Mw 5.4 Earthquake Sequence

Online Material: Additional Kraljevo sequence data, details. An M w 5.4 earthquake occurred on 3 November 2010 (Fig. 1) near the city of Kraljevo, Serbia, the population of which is more than 100,000 people. By the end of March 2011, the earthquake had been followed by a sequence of more than 650 aftershocks of magnitude greater than 1.0. Despite the moderate magnitude of the event, two people were killed, many others were injured, and the total damage to the city has been evaluated as more than 100 million EUR (United Nations, Kraljevo Earthquakes Report, 2010). Liquefaction and rockslides occurred in several places (Serbian Seismological Survey (SSS) Report, 2011). The earthquake also caused changes in groundwater circulation: some existing wells have totally dried out and new geothermal springs have appeared. Figure 1. Geodynamics of the Central Mediterranean region: Major plates (Africa, Eurasia, and Anatolia) and their boundaries, important fault systems, and orogenic belts (after Dilek, 2010). Thick black arrows, plate convergence directions; white star, the location of the 3 November 2010 Kraljevo earthquake. Symbols, seismological station networks from which data were used in this study. Virtual Regional Seismic Network includes data from the following networks: Italy, Romania, Croatia, Former Yugoslav Republic of Macedonia, Republic of Serbia, and Albania. The Kraljevo 3 November 2010 earthquake was recorded by seismological stations worldwide. Although different seismological agencies show consistency in determination of magnitude ( M w 5.3 or 5.4), proposed locations for the mainshock vary greatly. In particular, proposed depths vary from 2 to 30 km. In this article, we jointly determine the location of the mainshock and aftershocks that occurred between 3 November and 31 December 2010, along with a new P ‐ and S ‐wave 1D velocity model for that area. Focal mechanism solutions are obtained for selected events using both local and regional data. The …

Bulk crustal properties in NE Tibet and their implications for deformation model

The crust beneath the northeastern (NE) Tibetan Plateau records far field effects of collision and convergence occurring between the Indian and Eurasian plates. A better structural understanding of the crust beneath NE Tibet can improve our understanding of Cenozoic deformation resulting from the India–Eurasia collision. Taking advantage of the relatively dense coverage in most areas in NE Tibet except for the Qaidam basin by regional seismic networks of Gansu and Qinghai Provinces, we isolate receiver functions from the teleseismic P wave data recorded from 2007 to 2009 and resolve the spatial distribution of crustal thickness and Vp/Vs ratio beneath NE Tibet from H–κ scanning. Our results can be summarized as: (1) NE Tibet is characterized by ~60-km-thick crust beneath the Nan Shan, Qilian Shan thrust belts and the Anyemaqen Shan, and 45–50km-thick crust beneath the Tarim basin, the Alashan depression and the Ordos basin; the crust thins gradually from west to east in addition to the previously observed pronounced thinning from south to north; (2) the crust of NE Tibet exhibits a relatively lower Vp/Vs ratio of 1.72 than the north China block and a decrease in average crustal Vp/Vs ratio with increasing crustal thickness; and (3) the crustal thicknesses are less than the values predicted by the simple isostatic model of the whole Tibetan plateau wherein the elevation is larger than 3.0km. Our observations can be explained by the hypothesis that deformation occurring in NE Tibet is predominated by upper-crustal thickening or lower-crust extrusion.

Multi‐fault rupture and successive triggering during the 2012 Mw 8.6 Sumatra offshore earthquake

On April 11, 2012 a Mw 8.6 strike‐slip earthquake struck the northern Sumatra offshore region, and became the largest intraplate event ever recorded there. Preliminary study results show a remarkably complex rupture pattern, on a fault system manifested by north‐south trending paleo‐transform faults and east‐west striking abyssal hill fabric. To image the rupture pattern of the earthquake, we back‐projected teleseismic P wave recordings observed at three regional seismic networks (EU, AU, and F‐net). Our results indicate that the earthquake ruptured on a conjugate fault system, composed of two subparallel WNW‐ESE trending faults, and a NNE‐SSW striking fault in between, for a duration of at least 120 s. The rupture started at the WNW end of the first WNW‐ESE trending fault located at the east of the conjugate fault system, and propagated toward ESE. It then jumped to the second NNE‐SSW trending fault located west of the first fault, and first extended SSWward and 15 s later NNEward. After its end on the second fault, the rupture jumped onto the third WNW‐ESE trending fault and continued WNWward. The static Coulomb stress changes calculated from the rupture of the first and then the second faults, based on simplified uniform slip models, suggest that initiation of the rupture on the second fault could be statically and dynamically triggered by the first fault. However, the third fault was primarily dynamically triggered by S or even Love waves radiated from the southern branch of the second fault.

Mantle Transition Zone Structure Beneath Hainan and Adjacent Areas Derived from Receiver Function Analysis

AbstractWe have determined the upper mantle 410 km discontinuity and 660 km discontinuity structure and transition zone thickness beneath Hainan and adjacent areas by using receiver function method. The data used in this study are teleseismic waveforms recorded by 88 seismic stations in Hainan, Guangdong and Guangxi Regional Seismic Network and National Seismic Network during August, 2007 to March, 2010. The results show that the structure of 410 km discontinuity is complex below this region, which is depressed to 447 km locally. The 660 km discontinuity is simple relatively, with the depth around 670 km. The transition zone is thinned anomalously by 25±5 km within an area approximately 200 km in diameter centered northeast of the Hainan island, consistent with an excess temperature of ~180°C. The locally thinned transition zone implies that the Hainan plume originates from lower mantle.

Evaluation of Location Performance and Magnitude of Completeness of the Alaska Regional Seismic Network by the SNES Method

Abstract We apply the Seismic Network Evaluation through Simulation (SNES) method to evaluate the performance of the Alaska Regional Seismic Network (ARSN) in locating regional earthquakes in terms of validity of the velocity models, hypocentral errors, and magnitude of completeness. We find that background noise levels at the sites are highest in the Aleutian region and lowest in mainland Alaska. We demonstrate that the ARSN provides the best monitoring in the south‐central region, with horizontal and vertical errors of less than 1.5 and 3 km, respectively, at the completeness level of M L 2.0. At the completeness level of M L 3.0, the network is capable of locating earthquakes for the whole region down to depths of about 250 km with satisfactory errors. The lowest magnitude completeness levels, of about 1.4 and even less, are achieved with satisfactory location errors in limited areas of the south‐central region that have the densest station coverage. The network does not monitor the seismogenic areas in northern, western, southeastern, and offshore Alaska at an adequate level in terms of earthquake location errors. Network upgrades in these areas could be optimized using the SNES results.

Real-time risk assessment in seismic early warning and rapid response: a feasibility study in Bishkek (Kyrgyzstan)

Earthquake early warning systems (EEWS) are considered to be an effective, pragmatic, and viable tool for seismic risk reduction in cities. While standard EEWS approaches focus on the real-time estimation of an earthquake’s location and magnitude, innovative developments in EEWS include the capacity for the rapid assessment of damage. Clearly, for all public authorities that are engaged in coordinating emergency activities during and soon after earthquakes, real-time information about the potential damage distribution within a city is invaluable. In this work, we present a first attempt to design an early warning and rapid response procedure for real-time risk assessment. In particular, the procedure uses typical real-time information (i.e., P-wave arrival times and early waveforms) derived from a regional seismic network for locating and evaluating the size of an earthquake, information which in turn is exploited for extracting a risk map representing the potential distribution of damage from a dataset of predicted scenarios compiled for the target city. A feasibility study of the procedure is presented for the city of Bishkek, the capital of Kyrgyzstan, which is surrounded by the Kyrgyz seismic network by mimicking the ground motion associated with two historical events that occurred close to Bishkek, namely the 1911 Kemin (M = 8.2; ±0.2) and the 1885 Belovodsk (M = 6.9; ±0.5) earthquakes. Various methodologies from previous studies were considered when planning the implementation of the early warning and rapid response procedure for real-time risk assessment: the Satriano et al. (Bull Seismol Soc Am 98(3):1482–1494, 2008) approach to real-time earthquake location; the Caprio et al. (Geophys Res Lett 38:L02301, 2011) approach for estimating moment magnitude in real time; the EXSIM method for ground motion simulation (Motazedian and Atkinson, Bull Seismol Soc Am 95:995–1010, 2005); the Sokolov (Earthquake Spectra 161: 679–694, 2002) approach for estimating intensity from Fourier amplitude spectra; and the Tyagunov et al. (Nat Hazard Earth Syst Sci 6:573–586, 2006) approach for risk computation. Innovatively, all these methods are jointly applied to assess in real time the seismic risk of a particular target site, namely the city of Bishkek. Finally, the site amplification and vulnerability datasets considered in the proposed methodology are taken from previous studies, i.e., Parolai et al. (Bull Seismol Soc Am, 2010) and Bindi et al. (Soil Dyn Earthq Eng, 2011), respectively.

Research & Development of E-Map Service System Based on Baidu Map API for Regional Seismic Network

In this paper, new B/S management information software developed for the regional seismic network is introduced. The main function of the software is to provide e-map service for two types of clients: administrators and guests. The application of the software will change the traditional ways and improve the working efficiency of the maintenance engineers. It was developed with Eclipse and MySQL database, following MVC framework specification. It should be deployed in the center of the regional seismic network, providing e-map services over Internet for clients. It can also provide certain dedicated functions for users according to their privileges. It has been proved that the software has both useful functions and reliable performance. The e-map service is not only helpful for the maintenance of regional seismic network, but also beneficial to the emergency relief after strong earthquake.

A Catalog of Regional Moment Tensors in Utah from 1998 to 2011

Online Material: The electronic supplement includes details for all 48 moment tensor solutions in this study, including waveform fits and a table of event parameters. The University of Utah Seismograph Stations (UUSS) operates regional seismic networks in and around Utah and Yellowstone National Park and provides routine earthquake locations and earthquake catalogs for these two areas (Pechmann et al. , 2007). In this study, we expand the reporting to include moment tensors for seismic events occurring in the Utah region (Fig. 1). This region is a complex seismic environment with diverse source types: tectonic earthquakes, mining‐induced seismicity (MIS) including mine collapses, and man‐made sources such as explosions. Most tectonic earthquakes in the region occur within the Intermountain Seismic Belt (ISB; Smith and Arabasz, 1991). In northern Utah, the ISB roughly delineates the boundary between the Basin and Range and the Middle Rocky Mountains, whereas in central and southern Utah, the ISB separates the actively extending Basin and Range from the more stable Colorado Plateau. The MIS is mostly associated with long‐wall coal mining in central Utah (Fig. 1; Arabasz et al. , 2007). The man‐made blasts occur throughout the state and are related to both surface mining operations and military munitions detonations (Arrowsmith et al. , 2008). Figure 1. UUSS Utah monitoring area (dark gray box) and the surrounding region. Solid black line separates the UUmod velocity model regions between Basin and Range to the northwest and Colorado Plateau to the southeast. Dashed line separates physiographic provinces: Basin and Range (BR), Colorado Plateau (CP), and Middle Rocky Mountains (MR). The transition zone is roughly the overlap between the physiographic and velocity model boundaries between the Basin and Range and Colorado Plateau in Central Utah. Crescent‐shaped and rectangular polygons in central‐eastern Utah are coal mining areas. ECSZ is the Eastern California …

East Asia mantle tomography: New insight into plate subduction and intraplate volcanism

We present a new P-wave tomographic model beneath East Asia by inverting 1,401,797 arrival-time data recorded by the regional seismic networks in East Asia and temporary seismic arrays deployed on the Tibetan Plateau. Our high-resolution tomography shows that the subducted Pacific slab is revealed clearly as a high-velocity (high-V) zone and it becomes stagnant in the mantle transition zone beneath eastern China. Prominent high-V anomalies atop the 410km discontinuity are detected beneath the eastern North China Craton and Northeast China, suggesting that lithospheric delamination may have occurred and greatly affected the thermal state, intraplate magmatism and surface topography of this region. The Indian lithosphere is characterized by a high-V anomaly and it is subducting nearly horizontally beneath the entire or most parts of western Tibet and with a small dipping angle to the southernmost part of eastern Tibet. The intraplate magmatism in different parts of East Asia has different origins. The active Tengchong volcano is underlain by a prominent low-velocity (low-V) anomaly in the shallow mantle, which may be caused by the subduction and dehydration of the Burma microplate plate (or Indian plate). The Hainan volcano is underlain by a plume-like low-V anomaly that extends down to at least 1000km depth and seems to be related to the deep subductions of the Burma microplate (or Indian plate) in the west and the Philippine Sea plate in the east. Prominent low-V anomalies are detected beneath the Changbai, Longgang and Xianjindao volcanoes in Northeast Asia, which may be related to the rollback of the subducting Pacific slab as well as the convective circulation process in the big mantle wedge above the stagnant slab. The origin of the Wudalianchi volcano in Northeast China seems associated with the upwelling of asthenospheric materials caused by the subduction-induced lithospheric delamination.

An iterative algorithm for separation ofSandScSwaves of great earthquakes

Teleseismic SH waves are essential for imaging the rupture processes of large earthquakes. However, for great earthquakes (M8+) such as the 2004 Sumatra earthquake, the 2008 Wenchuan earthquake and the recent Tohoku-Oki earthquake, the source duration is very long (>100 s). Thus the direct SH waves are overlapped with ScS waves for epicentral distances larger than 60°, leaving contaminated S waves for source processes modelling. Therefore artefacts in finite fault models of large earthquake could be produced with such contaminated body waves. We propose an iterative algorithm based on the slowness information of S and ScS waves and stacking technique, to separate S and ScS waves with records from a regional seismic network. Tests on various synthetic data sets show that the algorithm is effective in retrieving teleseismic SH waveforms from complicated wave trains containing both S and ScS. Separation of waveforms for the 2008 Wenchuan earthquake with our algorithm clearly demonstrates the influence of ScS energy, suggesting necessity of recovering S waves.

Crustal structure and deformation of the SE Tibetan plateau revealed by receiver function data

We analyze a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration to estimate crustal structure and deformation beneath the southeast margin of the Tibetan plateau and its surrounding areas. We develop a comprehensive analysis method that facilitates robust extraction of azimuthal seismic anisotropy from receiver function data. The method includes an estimate of fast polarization direction and splitting time by a joint analysis of radial and transverse receiver function data, and an evaluation of measurement reliability by statistical and harmonic analysis. We find significant seismic anisotropy with a splitting time of 0.5–0.9s beneath the SE margin of the Tibetan plateau. Both the splitting time and fast polarization direction are comparable to those estimated from SKS/SKKS data, suggesting that crustal anisotropy is the main cause of shear wave splitting of the SKS/SKKS wave. This also suggests that deformation in the upper mantle is either weak or predominantly vertical, and is obviously different from the one in the crust. A vertical flow in the upper mantle, combined with the observation of a thin lithosphere beneath the study area, leads to the inference that part of the mantle lithosphere may have been removed and is descending into deep mantle. Stations located in the surrounding areas, on the other hand, exhibit very little to no crustal anisotropy. The estimated Moho depth and Vp/Vs ratio also show a distinct difference between the SE Tibetan plateau and the surrounding regions. Stations on the Tibetan plateau have a Vp/Vs ratio of ∼1.79, which is substantially higher than those measured at the Yunnan–Guizhou (Yungui) plateau (∼1.69). Our observations here are consistent with the scenario that the SE Tibet has been built by lower crustal flow. They also suggest that the mantle lithosphere beneath the margin may have been mechanically decoupled from the upper crust.

Broadband Rayleigh-Wave Dispersion Curve and Shear-Wave Velocity Structure for Yucca Flat, Nevada

The geology near a seismic source has a major effect on seismic waves recorded at distance. This can be especially true in the case of man‐made explosions, due to increased geologic heterogeneity at shallow depths and interactions with the free surface. Yucca Flat (YF), a sedimentary basin on the Nevada National Security Site, has hosted hundreds of well‐recorded underground nuclear tests. As such, it should be an ideal natural laboratory for the study of shallow explosions. Unfortunately, basin‐wide models of such important physical properties as compressive‐ and shear‐wave velocity are not available with sufficient fidelity to maximize the potential of the studies. We attempt to remedy this situation by creating a new shear‐wave velocity model of YF. This model was generated by inverting Rayleigh‐wave phase‐velocity dispersion measurements. Because no single dataset provided a dispersion curve of the necessary frequency bandwidth for shallow, intermediate, and deep basin depths simultaneously, we combined three dispersion curves with complementary bandwidths from three data sources. The datasets, in order of low frequency to high, were (1) underground nuclear tests at YF, recorded on regional seismic networks (0.14–0.4 Hz); (2) a multimode spatially averaged coherency microtremor array located on YF (0.2–20 Hz); and (3) several refraction microtremor (ReMi) linear arrays, also on YF (2.5–50 Hz). Compared to previous work, our model is characterized by slower velocities. The known geologic boundaries such as the depth of the basin and water table are prominent at reasonable locations.

Lg Wave Attenuation in the Isparta Angle and Anatolian Plateau (Turkey)

We estimate Lg wave attenuation using local and regional seismic phases in the Isparta Angle and the Anatolian Plateau (Turkey). The Isparta Angle (IA) is a tectonically active zone forming the boundary between the African Plate and the Anatolian Plateau, and is currently undergoing N–S extensional deformation. The Anatolian Plateau contains many intra-continental faults including the North Anatolian Fault Zone and the East Anatolian Fault Zone as well as the Menderes Massif. A large waveform data set was compiled from a variety of local and regional seismic networks including 121 digital seismic stations (broad-band and short period) between 1999 and 2008 spanning the IA, the Anatolian Plateau and Azerbaijan. The data set was used to determine the nature of Lg wave propagation and characterize the nature of seismic attenuation within the crust of these regions. Lg waveforms were used to calculate the frequency-dependent Lg-Q o and Lg- $$ \eta $$ . A wide range of Lg-Q o values was obtained between ~52 ± 6 and 524 ± 227. Low Lg-Q o values (~90–155) are calculated towards the north of IA, Iskenderun Gulf and its vicinity, Bingol-Karliova, Izmit and its vicinity. Lg-Q o values are especially low (<90) along the Menderes Massif and the Aksehir-Simav Fault Zones. This may be due to intrinsic attenuation of Lg associated with the partially molten crust and young volcanism. The high Lg-Q o values (~350) are probably caused by the crust not being subject to large amounts of extensional deformation like the Antalya Gulf and apparently being thick enough to support Lg propagation. Relatively higher values along the border of this subduction zone and plate boundary might be related to the Taurus Mountain belts and Bitlis-Zagros Suture Zone. The lateral frequency dependency Lg- $$ \eta $$ is also consistent with high tectonic activity in this region.

The East Anatolian Fault Zone: Seismotectonic setting and spatiotemporal characteristics of seismicity based on precise earthquake locations

The East Anatolian Fault Zone (EAFZ) represents a plate boundary extending over ∼500 km between the Arabian and Anatolian plates. Relative plate motion occurs with slip rates ranging from 6 to 10 mm/yr and has resulted in destructive earthquakes in eastern Turkey as documented by historical records. In this study, we investigate the seismic activity along the EAFZ and fault kinematics based on recordings from a densified regional seismic network providing the best possible azimuthal coverage for the target region. We optimize a reference 1‐D velocity model using a grid‐search approach and re‐locate hypocenters using the Double‐Difference earthquake relocation technique. The refined hypocenter catalog provides insights into the kinematics and internal deformation of the fault zone down to a resolution ranging typically between 100 and 200 m. The distribution of hypocenters suggests that the EAFZ is characterized by NE‐SW and E‐W oriented sub‐segments that are sub‐parallel to the overall trend of the fault zone. Faulting mechanisms are predominantly left‐lateral strike‐slip and thus in good correlation with the deformation pattern derived from regional GPS data. However, we also observe local clusters of thrust and normal faulting events, respectively. While normal faulting events typically occur on NS‐trending subsidiary faults, thrust faulting is restricted to EW‐trending structures. This observation is in good accordance with kinematic models proposed for evolving shear zones. The observed spatiotemporal evolution of hypocenters indicates a systematic migration of micro‐ and moderate‐sized earthquakes from the main fault into adjacent fault segments within several days documenting progressive interaction between the major branch of the EAFZ and its secondary structures. Analyzing the pre versus post‐seismic phase for M &gt; 5 events we find that aftershock activities are initially spread to the entire source region for several months but start to cluster at the central part of the main shock rupture thereafter.

Neogene tectonic and climatic evolution of the Western Ross Sea, Antarctica — Chronology of events from the AND-1B drill hole

Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 1284.87m sedimentary succession from beneath the sea floor. Key age data for the core include magnetic polarity stratigraphy for the entire succession, diatom biostratigraphy for the upper 600m and 40Ar/39Ar ages for in-situ volcanic deposits as well as reworked volcanic clasts. A vertical seismic profile for the drill hole allows correlation between the drill hole and a regional seismic network and inference of age constraint by correlation with well‐dated regional volcanic events through direct recognition of interlayered volcanic deposits as well as by inference from flexural loading of pre‐existing strata. The combined age model implies relatively rapid (1m/2–5ky) accumulation of sediment punctuated by hiatuses, which account for approximately 50% of the record. Three of the longer hiatuses coincide with basin‐wide seismic reflectors and, along with two thick volcanic intervals, they subdivide the succession into seven chronostratigraphic intervals with characteristic facies:1.The base of the cored succession (1275–1220mbsf) comprises middle Miocene volcaniclastic sandstone dated at approx 13.5Ma by several reworked volcanic clasts;2.A late-Miocene sub-polar orbitally controlled glacial–interglacial succession (1220–760mbsf) bounded by two unconformities correlated with basin‐wide reflectors associated with early development of the terror rift;3.A late Miocene volcanigenic succession (760–596mbsf) terminating with a ~1my hiatus at 596.35mbsf which spans the Miocene–Pliocene boundary and is not recognised in regional seismic data;4.An early Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (590–440mbsf), separated from;5.A late Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (440–150mbsf) by a 750ky unconformity interpreted to represent a major sequence boundary at other locations;6.An early Pleistocene interbedded volcanic, diamictite and diatomite succession (150–80mbsf), and;7.A late Pleistocene glacigene succession (80–0mbsf) comprising diamictite dominated sedimentary cycles deposited in a polar environment.

Yield Estimation of the 25 May 2009 North Korean Nuclear Explosion

We collect nine vertical component broadband seismograms from the 25 May 2009 North Korean nuclear explosion for a regional seismic network in which eight stations also recorded the 9 October 2006 North Korean nuclear test. Comparing the observed waveforms and spectra from the two events, we estimate that the amplitudes of the records from the second event are approximately five times those from the first one. Additionally, we use 599 vertical broadband seismograms from 82 regional events recorded on the regional network between December 1995 and May 2009 to calibrate the network Lg ‐wave magnitude. The calibrated network is used to calculate the Lg ‐wave body‐wave magnitude m b( Lg )=4.53 for the 25 May 2009 North Korean nuclear explosion. Based upon 15 first arrivals from the two North Korea nuclear explosions, the regional Pn velocity in the northeast China–North Korea region is calculated to be 8.0 km/s. This result, along with the regional geological structures, suggests that the North Korean test site (NKTS) is located at a relatively stable continental region. We thus use a modified fully coupled magnitude–yield relation to estimate the explosion yield, and the result shows that the yield of the 25 May 2009 North Korean nuclear test is approximately 2.35 kt under the minimum burial depth assumption. Online Material: Epicentral parameters, computed m b( Lg ), and corrected m b( Lg ) for 82 events used in the study.

Crustal-Scale Weak Zone along a Collisional Suture Revealed by Spatial Variations in Velocity Structures and Seismicity

Abstract High‐quality first‐arrival data collected with a high‐density temporary seismic array and regional seismic network were used to construct a P ‐wave velocity model and identify the precise location of earthquakes in the active collision zone of southeastern Taiwan. A crustal‐scale weak zone, defined by high seismicity, is characterized by a steeply east‐dipping lower‐velocity anomaly and represents a boundary between an uplifted upper‐mantle and the Luzon volcanic arc. The main features of the weak zone, with its associated low‐velocity zone and seismicity pattern, vary significantly along the collisional boundary of southeastern Taiwan. Along the weak zone, unlithified sediments or highly fractured materials are compacted as the Philippine Sea plate moves toward the Eurasian plate. In areas of more advanced collision, soft material is more compacted and can therefore store significantly greater amounts of strain energy. This energy is released through episodic earthquakes.

Montana Seismic Network Performance: An Evaluation through the SNES Method

In this paper we analyze the location performance of the Montana Regional Seismic Network (MRSN) using the Seismic Network Evaluation through Simulation (SNES) method. Montana has a high level of seismicity that includes approximately 1500 locatable earthquakes annually. The MRSN comprises 38 stations deployed over an area of approximately 50,000 km2. The application of the SNES method permits us to evaluate the background noise levels of the network stations and estimate an empirical law that links the variance of P and S travel‐time residuals to hypocentral distance. This in turn permits us to assess the appropriateness of the velocity model used by the MRSN in the location routine. We constructed SNES maps for M L 1.4, 1.6, 1.8, 2.0, and 2.2, fixing the hypocentral depth at 10 km and at the 95% confidence level. Through application of the SNES method, we show that MRSN provides the best monitoring coverage in the Flathead Valley of northwestern Montana, with errors for M L 2 that are less than 2 and 6 km for epicenter and hypocentral depth, respectively. At magnitude 2.2, this seismic network is capable of locating earthquakes as deep as 150 km and provides a threshold of completeness down to magnitude 1.5 for most of western Montana. We delineate some seismogenic areas of western Montana, including the central portion of the centennial tectonic belt in extreme southwestern Montana, that are not adequately covered by the MRSN alone.