Local Seismic Events Research Articles

Definition of the death of seismic events by using digital channel data of the Main center of special control and foreign seismic stations

Among the main parameters of the source of the seismic event, the depth of the hypocentre is of greatest interest, which, as a rule, is determined with the least accuracy. The knowledge of the exact spatial position of the earthquake foci makes it possible to reveal their connection with the features of the structure of the earth’s crust, primarily with active faults. The solution of these problems is primarily ensured by the use of observational data obtained with the help of local, regional or global networks of digital seismic stations, as well as by improving computer technologies and methods of processing digital seismic data. To determine the parameters of low-energy seismic events, the epicenters of which are located within the platform part of the territory of Ukraine, it is expedient to use the data of instrumental observations obtained with the help of the regional network of digital seismic stations of the Main Center for Special Control (MCSK) of the State Space Agency of Ukraine. To increase the accuracy of localization of seismic events, it is necessary, on the one hand, to significantly expand the network of seismic observations, and on the other, to improve the methods for determining the basic parameters of earthquakes. The article analyzes the basic methods for determining the depths of the hypocenters of seismic events. The program-mathematical support allowing to make the depths of the earthquake foci by records of reflected waves and from the data of macroseismic observations is considered. To estimate the depth of seismic sources from the data of digital seismic channels of the MCSK and foreign seismic stations, the amplitude-frequency characteristics of which are available in the «Geotool», a specialized program «Assessment of depth» has been developed. A number of foreign and domestic seismic stations, most suitable for clarifying the depths of hypocenters of earthquakes from the area of the Vranch Mountains, has been determined. A method for determining the depth of an earthquake source from the area of the Vranch Mountains using data from digital seismic stations located at distances of 20—40 km from the center of a seismically active zone is proposed.

Study of Geo-Electric Data Collected by the Joint EMSEV-Bishkek RS-RAS Cooperation: Possible Earthquake Precursors

By employing the cross-correlogram method, in geo-electric data from the area of Kyrgyzstan for the period 30 June 2014–10 June 2015, we identified Anomalous Telluric Currents (ATC). From a total of 32 ATC after taking into consideration the electric current source properties, we found that three of them are possible Seismic Electric Signal (SES) activities. These three SES activities are likely to be linked with three local seismic events. Finally, by studying the corresponding recordings when a DC alternating source injects current into the Earth, we found that the subsurface resistivity seems to be reduced before one of these three earthquakes, but a similar analysis for the other two cannot be done due to their large epicentral distance and the lack of data.

Identification of Local Seismic Events Using the Mikhnevo Seismic Array

A large number of events with sources in the immediate vicinity of an array are usually detected during seismological observations with seismic arrays. These events should be detected and correctly interpreted during processing of seismic array records in order to avoid clogging up the event catalog. This problem can be solved by classifying records of local events by genetic features and creating a databank with the most representative samples. The present paper considers local events recorded using a unique scientific setup, the Mikhnevo small aperture seismic array. Epicenters of local seismic events are located less than 5 km from the center of the array. Seismic responses of acoustic shock waves are also examined. Seismic events caused by anthropogenic sources are identified and classified using cluster, cross-correlation, and wavelet analysis. Events accompanied only by the arrival of surface waves, as well as events represented by body, surface, and acoustic waves, are identified. Shock wave events are classified as a separate category. A small group of supposedly natural weak events is also found. As a result, a databank of waveforms of local seismic events for the Mikhnevo seismic array is established. In the future, this will make it possible to automate their identification when investigating the seismicity of the East European Platform.

New information about the seismicity of the Russian Arctic based on the work of the seismic station “Severnaya Zemlya”

In November 2016 a seismic station “Severnaya Zemlya“ with code SVZ was installed on the Severnaya Zemlya archipelago (Bolshevik island, research facility “Ice base “Baranov’s cape”) by joint efforts N. Laverov Federal Center for Integrated Arctic Research (Arkhangelsk) and Arctic and Antarctic Research Institute (St. Petersburg). This station was included in the Arkhangelsk seismic network and a broadband set of CMG-6TD devices (Great Britain) was installed there. The microseismic background is characterized by a low level of industrial noise which allows to obtain the high-quality seismic information. Over 400 regional earthquakes and 25 local seismic events were recorded by seismic station SVZ within period from November 2016 to December 2017. Processing of regional earthquakes was carried out by us according to the data of at least three stations. We used wave forms from the Arctic stations of the Arkhangelsk seismic network and/or other seismological networks, whose stations are installed in the Arctic. In general, the majority of seismic events occur at epicentral distances of 3.5°–4° (i.e., they are regional) and associated with Gakkel Ridge. The strongest earthquakes are recorded from the areas of the Knipovich Ridge and Spitsbergen. Regional earthquakes magnitude range recorded by the SVZ is from 3.1 to 5.4. ML = 2.5 — it is the value of the magnitude from which the station registers earthquakes in full range. Local earthquakes magnitude range is from 1.2 to 4.9. Weak earthquakes with МL < 2.5 predominate. The difficulty is that out of the total number of regional and local events registered by SVZ only part of them could be processed according to the standard seismic rule (at least 3 stations). These events were included in the Seismic Catalogue. The predominant numbers of earthquakes are recorded only by SVZ station and they can not be included in the Seismic Catalogue due to the low quality data processing seeing that only one station was used. However, even in the first approximation these epicenters are confined to Eastern part of Gakkel Ridge, the boundary of the Kara plate and fall into the zone of the North Taimyr deformation associated with tectonic fault. Seismicity around Severnaya Zemlya to all appearance associated with an indirect reflection of rifting processes emerging in the central seismically active zone of the Laptev Sea. The calculation of earthquakes focal mechanisms recorded by SVZ is currently not possible due to the practical absence of seismic stations in the region. The task of expanding seismic observation networks in the Russian Arctic is urgent.

Automated seismic waveform location using multichannel coherency migration (MCM)–I: theory

With the proliferation of dense seismic networks sampling the full seismic wavefield, recorded seismic data volumes are getting bigger and automated analysis tools to locate seismic events are essential. Here, we propose a novel Multichannel Coherency Migration (MCM) method to locate earthquakes in continuous seismic data and reveal the location and origin time of seismic events directly from recorded waveforms. By continuously calculating the coherency between waveforms from different receiver pairs, MCM greatly expands the available information which can be used for event location. MCM does not require phase picking or phase identification, which allows fully automated waveform analysis. By migrating the coherency between waveforms, MCM leads to improved source energy focusing. We have tested and compared MCM to other migration-based methods in noise-free and noisy synthetic data. The tests and analysis show that MCM is noise resistant and can achieve more accurate results compared with other migration-based methods. MCM is able to suppress strong interference from other seismic sources occurring at a similar time and location. It can be used with arbitrary 3D velocity models and is able to obtain reasonable location results with smooth but inaccurate velocity models. MCM exhibits excellent location performance and can be easily parallelized giving it large potential to be developed as a real-time location method for very large datasets.

Tetra‐stage cluster identification model to analyse the seismic activities of Japan, Himalaya and Taiwan

From the decades, due to the independent and Poisson nature of background seismicity, they are extensively used for hazard analysis, modelling of prediction phenomenon and also used for earthquake simulations. In this study, a tetra-stage cluster identification model is proposed for accurate estimation of background seismicity and triggered seismicity. The proposed method considers a seismic event's occurrence time, location, magnitude and depth information available in the given catalogue to classify the event as a background or aftershock. The model has flexible threshold parameters which can be tuned to a proper value according to the specific seismic zone to be analysed. It exploits the current seismic activities of the region by taking care the past samples of the region over last 25 years. The analyses of Japan, Himalaya and Taiwan catalogues are carried out using the proposed model. Superior results with the proposed model are achieved, compared with benchmark models by Nanda et al., Gardner–Knopoff and Uhrhammer et al. in terms of percentage of background seismicity, lambda plot and cumulative plot. The ergodicity present in the original seismic catalogue and catalogue after de-clustering are compared using Thirumalai-Mountain metric to justify the stationary and linearity.

Earth crust vertical movements according to ITRF2000, ITRF2005, ITRF2008 and ITRF2014 coordinate systems and their comparing analysis

The analysis of Earth’s crust movement vertical velocities was made both for separate regions, and averaged on regions. As input data coordinates and velocities of earth crust points, obtained in International Coordinate Systems ITRF2000, ITRF2005, ITRF2008 on the base of processing radio interferometric (VLBI), laser (SLR), Doppler (DORIS) and GPS observations was used. For the purpose of global analysis all input velocity values were averaged in the х trapezoids. For filling trapezoids the spherical function expansion to N=36 was made. Expansion harmonic coefficients allowed determine the global characteristics of earth crust movements both for all Earth, and for separate hemispheres, polar and equatorial regions, continents and oceans. It appears, that polar regions were risen, and equatorial ones were lowered, that can indicate the modern Earth oblateness reduction. The constructed maps of vertical velocities were compared with obtained by us map of earth crust strain distribution. It is appeared, that regions of modern earth surface rising (Fennoscandia, Canada, Antarctica) coincide with regions of vertical extension strain, and the lowering regions – with compression regions. Simultaneously with the determination of harmonious coefficients the mean-square error of approximation for expansion of power n=1 N and power dispersion were determined. Whereas the results converge badly and dispersion increased with the n grows, hence we can concludes, that vertical movements do not characterized global earth crust movement and denotes the unrelated character of these movement. Seemingly, the main cause of such movement origin is the local seismic events (earthquake, volcanic eruption and so on). It confirms by our conclusions about correspondence of vertical strain maximal gradient (and therefore vertical velocities) map of the earthquake distribution. In work the detailed analysis of obtained results for separate Earth’s regions was made.

Local seismicity alterations in the South Yakutia mining region due to the technogenic impact on its geological environment

Impulse loads, arising due to the high natural seismicity of the South Yakutia region, exercise both direct and indirect effects on the upper part of the Earth's crust during industrial explosions. The direct effects result from nonlinear displacements caused by the blast wave and the subsequent formation of new disturbances. The indirect effects arise due to the activation of structural elements along geological contacts, leading to the emergence of technogenic seismicity foci. The foci of induced seismicity are either confined to the blast points, or located along the tectonic structures crossing quarry fields. The technogenic impact on the geological environment transforms the independent local seismic process, since explosions trigger a chain of local seismic events. The near-surface layers of the Earth's crust become activated in the area of dynamic influence of active faults. Under the influence of explosions, both the number of seismic events and the average level of released energy alter. Impulse loads on the geological environment lead to a spatial redistribution of the foci of low-energy (K &lt;7) earthquakes. The main form of the geodynamic development of seismogenic faults is the movement of their sides in the form of mutual “slippage”. Seismic events are manifested only when the aforementioned form of deformation is impossible or difficult to develop, in other words, when the stress-state areas of the Earth's crust develop. Therefore, the shaking impact of blasts can be considered as a factor contributing to the predominance of aseismic forms of fault motion in the form of smooth slippage of their sides. In conclusion, the impact of industrial blasts can not only activate faults around the mining area, but also have an unloading effect on the foci of seismic hazard forming in the interior, i.e. the redistribution of earthquake energy in terms of reducing earthquake energy class.

CO2 Sequestration Safety Monitoring by using Seismic Imaging and Double Difference Tomography

Shale gas has become an increasingly important clean energy, which has been explored worldwide in recent decades. In the shale gas production, supercritical CO 2 acts as a fracturing fluid. For preventing any kinds of leakage of the injected supercritical CO 2 , it is essential to monitor the stability of its storage hundreds of kilometres beneath the Earth’s surface. Seismic tomography is an imaging technique that uses induced seismic waves to create three dimensional images of the subsurface. It is an effective monitoring method to evaluate the caprock integrity in the CO 2 sequestration storage (CCS). In this experimental research, a simulated uniaxial compressive load was applied on a granite sample to analyze the stress redistribution for long-term in-situ caprock integrity during CO 2 injection. The induced seismic waves were recorded, and seismic events were located according to the Geiger algorithm. The frequency of seismic events correlates with the caprock failure evolution. Based on the frequency of seismic events and the failure process, the seismic data is divided into four regimes to examine the failure evolution. Finally, the double difference tomography (TomoDD) algorithm using arrival time was adopted to recalculate to modify the locations of seismic events and velocity structure in each regime. The results indicate that the passive seismic system can map the caprock stress distribution and allow for imaging of the caprock integrity. TomoDD exhibits sound improvements to relocate seismic events both in relative and absolute locations as well as to characterize the local velocity structure. The study further reveals that seismic monitoring along with TomoDD could evaluate the caprock failure accurately in the CCS.

Enhancing micro-seismic P-phase arrival picking: EMD-cosine function-based denoising with an application to the AIC picker

Micro-seismic P-phase arrival picking is an elementary step into seismic event location, source mechanism analysis, and seismic tomography. However, a micro-seismic signal is often mixed with high frequency noises and power frequency noises (50Hz), which could considerably reduce P-phase picking accuracy. To solve this problem, an Empirical Mode Decomposition (EMD)-cosine function denoising-based Akaike Information Criterion (AIC) picker (ECD-AIC picker) is proposed for picking the P-phase arrival time. Unlike traditional low pass filters which are ineffective when seismic data and noise bandwidths overlap, the EMD adaptively separates the seismic data and the noise into different Intrinsic Mode Functions (IMFs). Furthermore, the EMD-cosine function-based denoising retains the P-phase arrival amplitude and phase spectrum more reliably than any traditional low pass filter. The ECD-AIC picker was tested on 1938 sets of micro-seismic waveforms randomly selected from the Institute of Mine Seismology (IMS) database of the Chinese Yongshaba mine. The results have shown that the EMD-cosine function denoising can effectively estimate high frequency and power frequency noises and can be easily adapted to perform on signals with different shapes and forms. Qualitative and quantitative comparisons show that the combined ECD-AIC picker provides better picking results than both the ED-AIC picker and the AIC picker, and the comparisons also show more reliable source localization results when the ECD-AIC picker is applied, thus showing the potential of this combined P-phase picking technique.

Magmatic plumbing systems of the Koryakskii–Avacha Volcanic Cluster as inferred from observations of local seismicity and from the regime of adjacent thermal springs

An analysis of local seismicity within the Avacha–Koryakskii Volcanic Cluster during the 2000–2016 period revealed a sequence of plane-oriented earthquake clusters that we interpret as a process of dike and sill emplacement. The highest magmatic activity occurred in timing with the 2008–2009 steam–gas eruption of Koryakskii Volcano, with magma injection moving afterwards into the cone of Avacha Volcano (2010–2016). The geometry of the magma bodies reflects the NF geomechanical conditions (tension and normal faults, $$S_V>S_{H_{\text{max}}}>S_{h_{\text{min}}}$$ ) at the basement of Koryakskii Volcano dominated by vertical stresses S v , with the maximum horizontal stress $$S_{h_{\text{max}}}$$ pointing north. A CFRAC simulation of magma injection into a fissure under conditions that are typical of those in the basement of Koryakskii Volcano (the angle of dip is 60°, the size is 2 × 2 km2, and the depth is –4 km abs.) showed that when the magma discharge is maintained at the level of 20000 kg/s during 24 hours the fissure separation increases to reach 0.3 m and the magma injection is accompanied by shear movements that occur at a rate as high as 2 × 10–3 m/s, thus corresponding to the conditions of local seismic events with Mw below 4.5. We are thus able to conclude that the use of planeoriented clusters of earthquakes for identification of magma emplacement events is a physically sound procedure. The August 2, 2011 seismicity increase in the area of the Izotovskii hot spring (7 km from the summit of Koryakskii Volcano), which is interpreted as the emplacement of a dike, has been confirmed by an increase in the spring temperature by 10–12°C during the period from October 2011 to July 2012.

BARENTS16: a 1-D velocity model for the western Barents Sea

A minimum 1-D seismic velocity model for routine seismic event location purposes was determined for the area of the western Barents Sea, using a modified version of the VELEST code. The resulting model, BARENTS16, and corresponding station corrections were produced using data from stations at regional distances, the vast majority located in the periphery of the recorded seismic activity, due to the unfavorable land–sea distribution. Recorded seismicity is approached through the listings of a joint bulletin, resulting from the merging of several international and regional bulletins for the region, as well as additional parametric data from temporary deployments. We discuss the challenges posed by this extreme network-seismicity geometry in terms of velocity estimation resolution and result stability. Although the conditions do not facilitate the estimation of meaningful station corrections at the farthermost stations, and even well-resolved corrections do not have a convincing contribution, we show that the process can still converge to a stable velocity average for the crust and upper mantle, in good agreement with a priori information about the regional structure and geology, which reduces adequately errors in event location estimates.

Numerical simulations of passing seismic waves at the Larderello‐Travale Geothermal Field, Italy

AbstractPassing seismic waves released by large‐magnitude earthquakes may affect geological systems located thousands of miles far from the epicenter. The M9.0 Tohoku earthquake struck on 11 March 2011 in Japan. We detected local seismic activity at the Larderello‐Travale geothermal field, Italy, coinciding with the maximum amplitudes of the Rayleigh waves generated by the Tohoku earthquake. We suggest that the earthquakes were triggered by passing Rayleigh waves that induced locally a maximum vertical displacement of approximately 7.5 mm (for waves with period of 100 s). The estimated dynamic stress was about 8 kPa for a measured peak ground velocity of 0.8 mm/s. Previous similar observations pointed out local seismicity at the Larderello‐Travale Geothermal Field triggered by the 2012 Mw5.9 Po Plain earthquake. We conducted forward numerical modeling to investigate the effects caused by passing P, S, Love, and Rayleigh waves through the known velocity structure of the geothermal field. Results indicate that maximum displacements focus differently when considering body or surface waves, with displacement values being higher within the first 2 km of depth. The focusing of the displacement below 3 km seems to be strongly controlled by the velocity structure of the Larderello‐Travale geothermal field. We propose that seismic activity triggered by passing seismic waves may be related to a clock‐advancing mechanism for local seismic events that may have occurred in any case. Furthermore, our analysis shows that local anisotropies in the velocity structure of the Larderello‐Travale geothermal field (possibly linked to compartments of elevated pore pressures) strongly control the reactivation of regions of the geothermal field affected by passing seismic waves.

Structure of the Suasselkä postglacial fault in northern Finland obtained by analysis of local events and ambient seismic noise

Abstract. Understanding the inner structure of seismogenic faults and their ability to reactivate is particularly important in investigating the continental intraplate seismicity regime. In our study we address this problem using analysis of local seismic events and ambient seismic noise recorded by the temporary DAFNE array in the northern Fennoscandian Shield. The main purpose of the DAFNE/FINLAND passive seismic array experiment was to characterize the present-day seismicity of the Suasselkä postglacial fault (SPGF), which was proposed as one potential target for the DAFNE (Drilling Active Faults in Northern Europe) project. The DAFNE/FINLAND array comprised an area of about 20 to 100 km and consisted of eight short-period and four broadband three-component autonomous seismic stations installed in the close vicinity of the fault area. The array recorded continuous seismic data during September 2011–May 2013. Recordings of the array have being analysed in order to identify and locate natural earthquakes from the fault area and to discriminate them from the blasts in the Kittilä gold mine. As a result, we found a number of natural seismic events originating from the fault area, which proves that the fault is still seismically active. In order to study the inner structure of the SPGF we use cross-correlation of ambient seismic noise recorded by the array. Analysis of azimuthal distribution of noise sources demonstrated that during the time interval under consideration the distribution of noise sources is close to the uniform one. The continuous data were processed in several steps including single-station data analysis, instrument response removal and time-domain stacking. The data were used to estimate empirical Green's functions between pairs of stations in the frequency band of 0.1–1 Hz and to calculate corresponding surface wave dispersion curves. The S-wave velocity models were obtained as a result of dispersion curve inversion. The results suggest that the area of the SPGF corresponds to a narrow region of low S-wave velocities surrounded by rocks with high S-wave velocities. We interpret this low-velocity region as a non-healed mechanically weak fault damage zone (FDZ) formed due to the last major earthquake that occurred after the last glaciation.

K-Means Cluster for Seismicity Partitioning and Geological Structure Interpretation, with Application to the Yongshaba Mine (China)

Seismicity partitioning is an important step in geological structure interpretation and seismic hazard assessment. In this paper, seismic event location (X,Y,Z) and Euclidean distance were selected as the K-Means cluster, the Gaussian mixture model (GMM), and the self-organizing maps (SOM) input features and cluster determination measurement, respectively, and 1516 seismic events (M&gt;-1.5) obtained from the Yongshaba mine (China) were chosen for the cluster analysis. In addition, a Silhouette and Krzanowski-Lai- (KL-) combined S-KL index was proposed to obtain the possible optimum cluster number and to compare the cluster methods. Results show that the K-Means cluster obtains the best cluster “quality” with higher S-KL indexes on the whole and meaningful clusters. Furthermore, the optimal number for detailed geological structure interpretation is confirmed as eleven clusters, and we found that two areas probably have faults or caves, and two faults may be falsely inferred by mine geologists. Seismic hazard assessment shows that C5 and C7 (K=11) have a high mean moment magnitude (mM) and C1, C2, C3, and C4 (K=11) have a relatively high mM, where special attention is needed when mining. In addition, C7 (K=11) is the most shear-related area with a mean S-wave to P-wave energy ratio (mEs/Ep) of 41.21. In conclusion, the K-Means cluster provides an effective way for mine seismicity partitioning, geological structure interpretation, and seismic hazard assessment.

Monitoring of Rock Stress Redistribution in Geological CO2 Sequestration

Shale gas has become an increasingly important clean energy, which has been explored worldwide in recent decades. Supercritical CO 2 acts as fracturing fluid for shale gas production. The safety monitoring is essential to prevent any kinds of leakage from the reservoir as the supercritical CO 2 physically stored hundred kilometres underground. Seismic tomography is an imaging technique that uses induced seismic waves to create three dimensional images of the subsurface. It is an effective monitoring method to evaluate the caprock integrity in the carbon dioxide sequestration storage (CCS). In this experimental research, a simulated uniaxial compressive load is applied on a granite sample to analyze the stress redistribution for long-term in-situ caprock integrity during CO 2 injection. The induced seismic waves are recorded and seismic events are traced based on the Geiger algorithm. The frequency of seismic events correlates with the caprock failure evolution. The acquired seismic data is divided into four regimes based on the frequency of seismic events and the failure process to examine the failure evolution. Furthermore, the travel time and distance is plotted to analyze the variation of velocity. Finally, the double difference tomography (TomoDD) algorithm using arrival time is adopted to recalculate the locations of seismic events and velocity structure in each regime. The results indicate that the passive seismic system can map the caprock stress distribution and allow for imaging of the caprock integrity. TomoDD exhibits sound improvements to relocate seismic events both in relative and absolute locations as well as to characterize the local velocity structure. The study further reveals that seismic monitoring along with TomoDD could evaluate the caprock failure accurately in the CCS.

Using fluid-induced seismicity to infer permeability

For engineering applications involving fluid flow through porous rocks underground, the permeability of a rock mass is an important parameter. Imaging of the 3D permeability distribution is generally done by a history-matching approach: fluid pressure due to injection in a well with known pressure and flow rates is numerically simulated, and the rock permeability is adjusted so as to match predicted pressure values to measured observations. These pressure observations are made at producing wells. Because there will normally be few wells, the observations, while being dense in time, are spatially sparse. The idea of this paper is to augment these downhole pressure measurements by using microseismic data to infer pressure at seismic event locations.The mechanism of pressure-induced seismicity is the reduction of effective normal stress across a plane of weakness. The rock strength can be represented as a critical pressure - the pore pressure above which the rock will fail. The rock strength is highly heterogeneous, because of the existence of weaknesses such as joints, bedding planes, and clay bands, and stronger regions such as sandstone channels. So the rock strength will be random. We model the rock strength as a Weibull-distributed critical field. A microseismic event occurs where pressure exceeds this critical field, and so is effectively a point pressure measurement, with an uncertainty given by this Weibull distribution. The idea is to augment the well-pressure observations with these “virtual” pressure measurements at seismic event locations.We model pressure diffusion using a finite volume approach, and examine the inversion, for permeability, of two different types of data, (1) pressure measurements in boreholes, and (2) virtual pressure measurements at seismic event locations. We show that the two types of data provide complementary information.

Local Seismic Events in the Area of Poland Based on Data from the PASSEQ 2006–2008 Experiment

PASSEQ 2006–2008 (Passive Seismic Experiment in TESZ; Wilde-Piorko et al. 2008) was the biggest passive seismic experiment carried out so far in the area of Central Europe (Poland, Germany, the Czech Republic and Lithuania). 196 seismic stations (including 49 broadband seismometers) worked simultaneously for over two years. During the experiment, multiple types of data recorders and seismometers were used, making the analysis more complex and time consuming. The dataset was unified and repaired to start the detection of local seismic events. Two different approaches for detection were applied for stations located in Poland. The first one used standard STA/LTA triggers (Carl Johnson’s STA/LTA algorithm) and grid search to classify and locate the events. The result was manually verified. The second approach used Real Time Recurrent Network (RTRN) detection (Wiszniowski et al. 2014). Both methods gave similar results, showing four previously unknown seismic events located in the Gulf of Gdansk area, situated in the southern Baltic Sea. In this paper we discuss both detection methods with their pros and cons (accuracy, efficiency, manual work required, scalability). We also show details of all detected and previously unknown events in the discussed area.

Identification of Stress Change Within a Rock Mass Through Apparent Stress of Local Seismic Events

Mine blasting produces excavation geometry changes which induce stress change that can be observed in the seismic source parameter apparent stress calculated for local seismic events. Using high apparent stress as a proxy for increasing stress within a rock mass, areas experiencing increases in the local stress conditions can be determined. This paper presents the use of apparent stress of seismic events to identify areas within a rock mass experiencing local stress change. Examples from a deep Canadian mine, operating in excess of 2900 m below surface, are provided.

Structural and Seismic Stratigrapic study in the Center of the Magdalena Shelf in the Western Margin of Baja California Based on Seismic Reflection Data

The Magdalena Shelf is a shallow, low-relief surface located along the Baja California Pacific margin. As part of a forearc basin, the shelf was a convergent margin setting before the oblique divergent plate boundary formed in the Gulf of California at 12 Ma. It is thought that since 12–8 Ma, this basin has been a transtensional or strike–slip basin. To constrain the geometry, structural characteristics and some stratigraphic relationships, an active-source, seismic-reflection study was carried out in the central part of the shelf. As a result, the analyzed data show faults, basins and unconformities. Two out of four observed basins are clearly controlled by the Santa Margarita and San Lazaro faults that dip ~40° NE; a third basin is controlled by the Tosco-Abreojos fault. These three basins are part of the deformation zone that is associated with the Tosco-Abreojos fault system. The Iray-Margarita basin, on the other hand, is a fourth basin located at the northeast sector of the study area. An additional feature observed is a stepover lying between the overlapping ends of the Santa Margarita and San Lazaro faults. Small faults oriented sub-parallel to the above major faults are present, mainly throughout the western sector of the study area. Some of those minor faults cut through the seafloor indicating recent tectonic activity. Santa Margarita, San Lazaro and Tosco-Abreojos are also the names given to half-grabens controlled by the active faults that have the same names. The first two basins are affected by many more small faults in comparison with what we see in the third basin. Tectonically, this means that those two basins are the more active in the area of study. In all four basins, the upper seismic sequence consists of sediments controlled by faults of Neogene age. We found that the Iray-Santa Margarita basin is the deepest of all four basins (beyond the resolution of the data, >5 km), and lack of minor faults there indicates that the basin is not tectonically active. Two unconformities are present in the region, unconformity-1 of Miocene age and unconformity-2 of Paleocene-Eocene and Cretaceous ages. Unconformity-1 is present in the entire region, while Unconformity-2 is present mainly in the Iray-Magdalena and San Lazaro basins. In the Iray-Magdalena basin the sedimentary sequences show an uplift that took place in late Jurassic time along its western portion. We also see that moderate compressive deformation increases gradually from west to east. A chaotic-reflector oriented NW–SE is observed with an irregular shape in all regions. It shows very superficial in some places, sub-parallel to the major faults and in association with the subduction complex. Seismic activity reported for the region showed a strong correlation with the fault plane of the Santa Margarita fault, indicating that the Tosco-Abreojos deformation zone is ~90 km wide in the area. Finally, the Santa Margarita fault should be considered in future hazard-risk assessments because of their proximity to Puerto San Carlos. As it has been reported, local seismic events (M < 4) have generated alarm in the local populace, as well as soil liquefaction and minor damage to structures in the region.