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Eggertsson, G., Lund, B., Roth, M. & Schmidt, P. (2024). Earthquake or blast?: Classification of local-distance seismic events in Sweden using fully connected neural networks. Geophysical Journal International, 236(3), 1728-1742
Open this publication in new window or tab >>Earthquake or blast?: Classification of local-distance seismic events in Sweden using fully connected neural networks
2024 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 236, no 3, p. 1728-1742Article in journal (Refereed) Published
Abstract [en]

Distinguishing between different types of seismic events is a task typically performed manually by expert analysts and can thus be both time and resource expensive. Analysts at the Swedish National Seismic Network (SNSN) use four different event types in the routine analysis: natural (tectonic) earthquakes, blasts (e.g. from mines, quarries and construction) and two different types of mining-induced events associated with large, underground mines. In order to aid manual event classification and to classify automatic event definitions, we have used fully connected neural networks to implement classification models which distinguish between the four event types. For each event, we bandpass filter the waveform data in 20 narrow-frequency bands before dividing each component into four non-overlapping time windows, corresponding to the P phase, P coda, S phase and S coda. In each window, we compute the root-mean-square amplitude and the resulting array of amplitudes is then used as the neural network inputs. We compare results achieved using a station-specific approach, where individual models are trained for each seismic station, to a regional approach where a single model is trained for the whole study area. An extension of the models, which distinguishes spurious phase associations from real seismic events in automatic event definitions, has also been implemented. When applying our models to evaluation data distinguishing between earthquakes and blasts, we achieve an accuracy of about 98 per cent for automatic events and 99 per cent for manually analysed events. In areas located close to large underground mines, where all four event types are observed, the corresponding accuracy is about 90 and 96 per cent, respectively. The accuracy when distinguishing spurious events from real seismic events is about 95 per cent. We find that the majority of erroneous classifications can be traced back to uncertainties in automatic phase picks and location estimates. The models are already in use at the SNSN, both for preliminary type predictions of automatic events and for reviewing manually analysed events.

Place, publisher, year, edition, pages
Oxford University Press, 2024
Keywords
Machine learning, Neural networks, fuzzy logic, Statistical methods, Time-series analysis, Seismicity and tectonics
National Category
Other Civil Engineering Geophysics
Identifiers
urn:nbn:se:uu:diva-523722 (URN)10.1093/gji/ggae018 (DOI)001150224200003 ()
Available from: 2024-02-27 Created: 2024-02-27 Last updated: 2024-02-27Bibliographically approved
Beckel, R. A., Lund, B., Eggertsson, G. A. & Juhlin, C. (2022). Comparing the performance of stacking-based methods for microearthquake location: a case study from the Burträsk fault, northern Sweden. Geophysical Journal International, 228(3), 1918-1934
Open this publication in new window or tab >>Comparing the performance of stacking-based methods for microearthquake location: a case study from the Burträsk fault, northern Sweden
2022 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 228, no 3, p. 1918-1934Article in journal (Refereed) Published
Abstract [en]

Traditional earthquake location relying on first arrival picking is challenging for microseismic events with low signal-to-noise ratio. Over the past years, alternative procedures have been explored based on the idea of migrating the energy of an earthquake back into its source position by stacking along theoretical traveltime curves. To avoid destructive interference of signals with opposite polarity, it is common to transform the input signals into positive time-series. Stacking-based source location has been successfully applied at various scales, but existing studies differ considerably in the choice of characteristic function, the amount of pre-processing and the phases used in the analysis. We use a data set of 62 natural microearthquakes recorded on a 2-D seismic array of 145 vertical geophones across the glacially triggered Burtrask fault to compare the performance of five commonly used characteristic functions: the noise filtered seismograms and the semblance, the envelope, the short-term average/long-term average ratio and the kurtosis gradient of the seismograms. We obtain the best results for a combined P- and S-wave location using a polarity-sensitive characteristic function, that is the filtered seismograms or the semblance. In contrast, the absolute functions often fail to align the signals properly, yielding biased location estimates. Moreover, we observe that the success of the procedure is very sensitive to noise suppression and signal shaping prior to stacking. Our study demonstrates the usefulness of including lower quality S-wave data to improve the location estimates. Furthermore, our results illustrate the benefits of retaining the phase information for location accuracy and noise suppression. To ensure optimal location results, we recommend carefully pre-processing the data and test different characteristic functions for each new data set. Despite the suboptimal array geometry, we obtain good locations for most events within similar to 30-40 km of the survey and the locations are consistent with an image of the fault trace from an earlier reflection seismic survey.

Keywords
Geochemistry and Petrology, Geophysics
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-461334 (URN)10.1093/gji/ggab437 (DOI)000728171300027 ()
Available from: 2021-12-13 Created: 2021-12-13 Last updated: 2022-07-04Bibliographically approved
Vachon, R., Schmidt, P., Lund, B., Plaza-Faverola, A., Patton, H. & Hubbard, A. (2022). Glacially Induced Stress Across the Arctic From the Eemian Interglacial to the Present-Implications for Faulting and Methane Seepage. Journal of Geophysical Research - Solid Earth, 127(7), Article ID e2022JB024272.
Open this publication in new window or tab >>Glacially Induced Stress Across the Arctic From the Eemian Interglacial to the Present-Implications for Faulting and Methane Seepage
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2022 (English)In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 127, no 7, article id e2022JB024272Article in journal (Refereed) Published
Abstract [en]

Strong compressive and shear stresses generated by glacial loading and unloading have a direct impact on near-surface geological processes. Glacial stresses are constantly evolving, creating stress perturbations in the lithosphere that extend significant distances away from the ice. In the Arctic, periodic methane seepage and faulting have been recurrently associated with glacial cycles. However, the evolution of the Arctic glacial stress field and its impact on the upper lithosphere have not been investigated. Here, we compute the evolution in space and time of the glacial stresses induced in the Arctic lithosphere by the North American, Eurasian and Greenland ice sheets during the latest glaciation. We use glacial isostatic adjustment (GIA) methodology to investigate the response of spherical, viscoelastic Earth models with varying lithospheric thickness to the ice loads. We find that the GIA-induced maximum horizontal stress (sigma(H)) is compressive in regions characterized by thick ice cover, with magnitudes of 20-25 MPa in Fennoscandia and 35-40 MPa in Greenland at the last glacial maximum. Simultaneously, a tensile regime with sigma(H) magnitude down to -16 MPa dominates across the forebulges with a mean of -4 MPa in the Fram Strait. At present time, sigma(H) in the Fram Strait remains tensile with an East-West orientation. The evolution of GIA-induced stresses from the last glaciation to present could destabilize faults along tensile forebulges, for example, the west-coast of Svalbard. A more tensile stress regime as during the Last Glacial Maximum would have more impact on pre-existing faults that favor gas seepage from gas reservoirs.

Place, publisher, year, edition, pages
American Geophysical Union (AGU)American Geophysical Union (AGU), 2022
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-481677 (URN)10.1029/2022JB024272 (DOI)000823302600001 ()
Funder
The Research Council of Norway, 287865The Research Council of Norway, 223259
Available from: 2022-08-15 Created: 2022-08-15 Last updated: 2024-01-15Bibliographically approved
Juhlin, C., Erlstrom, M., Lund, B. & Rosberg, J.-E. (2022). Seismic reflectivity, fracturing and stress field data from the FFC-1 exploratory geothermal project in SW Skåne, Sweden. Geothermics, 105, Article ID 102521.
Open this publication in new window or tab >>Seismic reflectivity, fracturing and stress field data from the FFC-1 exploratory geothermal project in SW Skåne, Sweden
2022 (English)In: Geothermics, ISSN 0375-6505, E-ISSN 1879-3576, Vol. 105, article id 102521Article in journal (Refereed) Published
Abstract [en]

Enhanced geothermal systems (EGS) are a potential heat source in many parts of the world, even in locations where the temperature gradient is relatively low. We present here an integrated study of reflection seismic data, borehole logs and seismicity analysis performed in conjunction with a geothermal exploratory project operated by E.ON in Malmo center dot, Sweden. In 2020, the pre-existing 2.1 km deep FFC-1 borehole through the sedimentary cover was deepened into the crystalline basement to about 3.1 km vertical depth. Combined interpretation of the reflection seismic data and geophysical wireline logs show that most of the reflectivity in the Precambrian basement is likely generated by lenses of mafic amphibolite embedded in a felsic gneissic matrix. The general structural bedding and foliation is gently dipping to sub-horizontal, similar to other locations in southwest Sweden. Fracture frequency is relatively high in the crystalline rock mass, with heavy fracturing in the uppermost part of the crystalline basement, obscuring a clear reflection from the top of the Precambrian. Highly fractured and hydraulically conductive intervals are also found between 2,562 and 2,695 m based on a temperature drop and the interpretation of the geophysical data. Open fractures, both natural and induced, have a clear N-S orientation, contrasting with the expected NW-SE direction based on the orientation of the SorgenfreiTornquist Zone and earthquake fault plane solutions to the north. This difference may be partly explained by local variations in the stress field near the FFC-1 borehole and vairations in the stress field with depth. Despite this, the data from the FFC-1 well provide novel and unique information on the complex physical state of the crystalline basement on the margin of the Fennoscandian Shield, which further addresses the need for obtaining in-situ stress data to fully understand the local stress field prior to any stimulation. A temperature of 84 degrees C measured at 3 km depth indicates that a desired EGS temperature of 120-140 degrees C may be reached at 5-6 km depth, assuming a temperature gradient of about 20 degrees C. If the relatively high fracture frequency and occurrence of fracture zones down to 3.1 km are also present at these target depths, then the FFC-1 location may be suitable for heat extraction if the rock mass is properly characterized before stimulation.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Geothermal, EGS, Fennoscandian shield, Seismicity, Earthquakes, Seismics, Geophysical logs, Fracturing, Stress field
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-523221 (URN)10.1016/j.geothermics.2022.102521 (DOI)000865317400002 ()
Funder
Swedish Energy Agency, 49110-1
Available from: 2024-02-26 Created: 2024-02-26 Last updated: 2024-05-20Bibliographically approved
Jussila, V., Fälth, B., Mäntyniemi, P., Voss, P. H., Lund, B. & Fülöp, L. (2021). Application of a Hybrid Modeling Method for Generating Synthetic Ground Motions in Fennoscandia, Northern Europe. Bulletin of The Seismological Society of America (BSSA), 111(5), 2507-2526
Open this publication in new window or tab >>Application of a Hybrid Modeling Method for Generating Synthetic Ground Motions in Fennoscandia, Northern Europe
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2021 (English)In: Bulletin of The Seismological Society of America (BSSA), ISSN 0037-1106, E-ISSN 1943-3573, Vol. 111, no 5, p. 2507-2526Article in journal (Refereed) Published
Abstract [en]

We present a modeling technique for generating synthetic ground motions, aimed at earthquakes of design significance for critical structures and ground motions at distances corresponding to the engineering near field, in which real data are often missing. We use dynamic modeling based on the finite‐difference approach to simulate the rupture process within a fault, followed by kinematic modeling to generate the ground motions. The earthquake source ruptures were modeled using the 3D distinct element code (Itasca, 2013). We then used the complete synthetic program by Spudich and Xu (2002) to simulate the propagation of seismic waves and to obtain synthetic ground motions. In this work, we demonstrate the method covering the frequency ranges of engineering interests up to 25 Hz and quantify the differences in ground motion generated. We compare the synthetic ground motions for distances up to 30 km with a ground‐motion prediction equation, which synthesizes the expected ground motion and its randomness based on observations. The synthetic ground motions can be used to supplement observations in the near field for seismic hazard analysis. We demonstrate the hybrid approach to one critical site in the Fennoscandian Shield, northern Europe.

Place, publisher, year, edition, pages
Seismological Society of America (SSA), 2021
Keywords
Geochemistry and Petrology, Geophysics
National Category
Geophysics
Research subject
Geophysics with specialization in Seismology
Identifiers
urn:nbn:se:uu:diva-461333 (URN)10.1785/0120210081 (DOI)000704250000001 ()
Available from: 2021-12-13 Created: 2021-12-13 Last updated: 2023-05-25Bibliographically approved
Xu, Z., Zhang, F., Juhlin, C., Lund, B., Ask, M. & Han, L. (2021). Extrapolated supervirtual refraction interferometry. Geophysical Journal International, 227(2), 1439-1463
Open this publication in new window or tab >>Extrapolated supervirtual refraction interferometry
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2021 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 227, no 2, p. 1439-1463Article in journal (Refereed) Published
Abstract [en]

Accurate picking of head-wave arrival times is an important component of first-arrival traveltime tomography. Far-offset traces in particular have low signal-to-noise ratio (SNR), but picking on these traces is necessary in order to obtain velocity information at depth. Furthermore, there is often an insufficient number of far-offset traces for obtaining reliable models at depth. We present here an extrapolation method for increasing the number of first arrivals beyond the maximum recorded offset, thereby extending the supervirtual refraction interferometry (SVI) method. We refer to the method as extrapolated SVI (ESVI). It is a novel attempt to extrapolate first arrivals using a fully data-driven method. We first test the methodology on synthetic data sets, and we then apply ESVI to two published real data sets over the Parvie fault system in northern Sweden. These data sets were acquired along the same profile at different times with different acquisition parameters and noise levels. The results show that ESVI enhances the SNR of head waves when the noise level is high. That is the same as the conventional SVI. ESVI also increases the number of pickable first arrivals by extrapolating head waves past the original maximum offset of each shot. We also show that the significant increase in first-arrival traveltime picks is beneficial for improving resolution and penetration depth in the tomographic imaging and, consequently, better revealing the subsurface velocity distribution. The tomographic images show higher velocities in the hanging walls of the main Parvie fault and another subsidiary fault, as interpreted relative to migrated images from previous seismic reflection processing.

Place, publisher, year, edition, pages
Oxford University PressOxford University Press (OUP), 2021
Keywords
Interferometry, Tomography, Wave propagation
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-456826 (URN)10.1093/gji/ggab283 (DOI)000697754400024 ()
Funder
Swedish Research Council
Available from: 2021-11-01 Created: 2021-11-01 Last updated: 2024-01-15Bibliographically approved
Steffen, R., Wu, P. & Lund, B. (2021). Geomechanics of glacially triggered faulting. In: Holger Steffen; Odleiv Olesen; Raimo Sutinen (Ed.), Glacially-Triggered Faulting: (pp. 20-40). Cambridge: Cambridge University Press
Open this publication in new window or tab >>Geomechanics of glacially triggered faulting
2021 (English)In: Glacially-Triggered Faulting / [ed] Holger Steffen; Odleiv Olesen; Raimo Sutinen, Cambridge: Cambridge University Press, 2021, p. 20-40Chapter in book (Refereed)
Abstract [en]

The reactivation of glacially induced faults is linked to the increase and decrease of ice mass. But, whether faults are reactivated by glacially induced stresses depends to a large degree on the crustal stress field, fault properties and fluid pressures. The background (tectonic and lithostatic) stress field has a major effect on the potential for reactivation, as the varying stresses induced by the ice sheet affects the state of stress around the fault, bringing the fault to more stable or more unstable conditions. Here, we describe the effect of glacially induced stresses on fault reactivation under three potential background stress regimes of normal, strike-slip and thrust/reverse faulting. The Mohr diagram is used to illustrate how glacially induced stresses affect the location and the size of the Mohr circle. We review these different cases by applying an analysis of the stress state at different time points in the glacial cycle. In addition, we present an overview of fault properties that affect the reactivation of glacially induced faults, such as pore-fluid pressure and coefficient of friction.

Place, publisher, year, edition, pages
Cambridge: Cambridge University Press, 2021
Keywords
Geomechanics, Glacially Induced Stress, Coulomb Failure Stress, Fault Stability Margin, Forebulge, Stress Regime, Optimally Orientated Fault, Stress Migration, Background Stress, Mohr Diagram
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-504796 (URN)10.1017/9781108779906.004 (DOI)9781108779906 (ISBN)9781108490023 (ISBN)
Available from: 2023-06-15 Created: 2023-06-15 Last updated: 2023-08-21Bibliographically approved
Wu, P., Steffen, R., Steffen, H. & Lund, B. (2021). Glacial isostatic adjustment models for earthquake triggering. In: Holger Steffen; Odleiv Olesen; Raimo Sutinen (Ed.), Glacially-Triggered Faulting: (pp. 383-401). Cambridge: Cambridge University Press
Open this publication in new window or tab >>Glacial isostatic adjustment models for earthquake triggering
2021 (English)In: Glacially-Triggered Faulting / [ed] Holger Steffen; Odleiv Olesen; Raimo Sutinen, Cambridge: Cambridge University Press, 2021, p. 383-401Chapter in book (Refereed)
Abstract [en]

To model glacial triggering of earthquakes, it is necessary to obtain the spatio-temporal variation of glacial isostatic adjustment-induced stress during a glacial cycled. This can be computed efficiently using commercial Finite Element codes with appropriate modifications to include the important effects of ‘pre-stress advection’, ‘internal buoyancy’ and ‘self-gravity’. The modifications described in Wu (2004) are reviewed for incompressible and so-called materially compressible flat-earths. When the glacial isostatic adjustment-induced stress is superimposed on the background tectonic stress and overburden pressure, the time variation of earthquake potential at various locations in the Earth can be evaluated for any fault orientation. To model more complex slip and fault behavior over time, the three-stage Finite Element model approach of Steffen et al. (2014) is reviewed. Finally, selected numerical examples and their results from both modelling approaches are shown.

Place, publisher, year, edition, pages
Cambridge: Cambridge University Press, 2021
Keywords
Glacial Isostatic Adjustment, Equation of Motion, Stress Migration, Coulomb Failure Stress, Mohr Circle, Finite Element Modelling, Fault Stability, Earth Model, Boundary Conditions, Fault Slip
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-504800 (URN)10.1017/9781108779906.029 (DOI)9781108779906 (ISBN)9781108490023 (ISBN)
Available from: 2023-06-15 Created: 2023-06-15 Last updated: 2023-08-21Bibliographically approved
Ask, M., Kukkonen, I., Olesen, O., Lund, B., Fagereng, Å., Rutqvist, J., . . . Lorenz, H. (2021). Proposed Drilling into Postglacial Faults: The Pärvie Fault System. In: Holger Steffen, Odleiv Olesen, Raimo Sutinen (Ed.), Glacially-Triggered Faulting: (pp. 151-174). Cambridge: Cambridge University Press
Open this publication in new window or tab >>Proposed Drilling into Postglacial Faults: The Pärvie Fault System
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2021 (English)In: Glacially-Triggered Faulting / [ed] Holger Steffen, Odleiv Olesen, Raimo Sutinen, Cambridge: Cambridge University Press , 2021, p. 151-174Chapter in book (Refereed)
Abstract [en]

Postglacial faults in northern Fennoscandia have been investigated through geophysical methods, trenching, and mapping of brittle deformation structures. Very little is known about postglacial faults through direct measurements. A few short, up to 500 m deep, boreholes exist. Plans for a scientific drilling program were initiated in 2010. The drilling target has been identified: The Pärvie Fault system is the longest known postglacial fault in the world and has been proposed to have hosted an M8 earthquake near the end or just after the last glaciation. Further, this fault system is still microseismically active. The drill sites are north of the Arctic Circle, in a sparsely populated area. Existing site survey data, established logistics, and societal relevance through the fault’s proximity to mining and energy operations make this fault system an appropriate target. The International Continental Scientific Drilling Program approved a full drilling proposal in October 2019. This chapter presents an abbreviated version of the approved proposal.

Place, publisher, year, edition, pages
Cambridge: Cambridge University Press, 2021
Keywords
Borehole, Coring, DAFNE, Deep Biosphere, Drilling, Fault Zone Identification, ICDP, Logging, Pärvie Fault, Postglacial Fault
National Category
Geosciences, Multidisciplinary Geophysics Geology
Identifiers
urn:nbn:se:uu:diva-470459 (URN)10.1017/9781108779906.012 (DOI)9781108779906 (ISBN)
Available from: 2022-03-24 Created: 2022-03-24 Last updated: 2022-05-30Bibliographically approved
Gregersen, S., Lindholm, C., Korja, A., Lund, B., Uski, M., Oinonen, K., . . . Keiding, M. (2021). Seismicity and sources of stress in Fennoscandia. In: Holger Steffen; Odleiv Olesen; Raimo Sutinen (Ed.), Glacially-Triggered Faulting: (pp. 177-197). Cambridge: Cambridge University Press
Open this publication in new window or tab >>Seismicity and sources of stress in Fennoscandia
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2021 (English)In: Glacially-Triggered Faulting / [ed] Holger Steffen; Odleiv Olesen; Raimo Sutinen, Cambridge: Cambridge University Press, 2021, p. 177-197Chapter in book (Refereed)
Abstract [en]

This chapter investigates the Fennoscandian uplift area since the latest Ice Age and addresses the question if glacial isostatic adjustment may influence current seismicity. The region is in an intraplate area, with stresses caused by the lithospheric relative plate motions. Discussions on whether uplift and plate tectonics are the only causes of stress have been going on for many years in the scientific community.

This review considers the improved sensitivity of the seismograph networks, and at the same time attempts to omit man-made explosions and mining events in the pattern, to present the best possible earthquake pattern. Stress orientations and their connection to the uplift pattern and known tectonics are evaluated. Besides plate motion and uplift, one finds that some regions are affected stress-wise by differences in geographical sediment loading as well as by topography variations. The stress release in the present-day earthquakes shows a pattern that deviates from that of the time right after the Ice Age. This chapter treats the stress pattern generalized for Fennoscandia and guides the interested reader to more details in the national chapters.

Place, publisher, year, edition, pages
Cambridge: Cambridge University Press, 2021
Keywords
Stress Field, Seismicity, Fennoscandian Shield Intraplate, Postglacial Fault, Glacial Isostatic Adjustment, Fault Zone, Plate Boundary, Upper Crust, Caledonides
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-504801 (URN)10.1017/9781108779906.014 (DOI)9781108779906 (ISBN)9781108490023 (ISBN)
Available from: 2023-06-15 Created: 2023-06-15 Last updated: 2023-08-21Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-0789-5949

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