FMC—Earthquake focal mechanisms data management, cluster and classification

Álvarez‐Gómez, José A.

doi:10.1016/j.softx.2019.03.008

Cited by 70 publications

(33 citation statements)

References 60 publications

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“…Each pole of the diagram represents one of the three pure styles of deformation (pure strike-slip, pure normal or pure reverse motion). For each focal mechanism, Kaverina diagrams (e.g., Alvarez-Gómez, 2019)…”

Section: Focal Mechanism Computationmentioning

confidence: 99%

Present-day geodynamics of the Western Alps: new insights from earthquake mechanisms

Mathey¹,

Sue²,

Pagani³

et al. 2020

Preprint

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Abstract. Due to the low to moderate seismicity of the European Western Alps, few focal mechanisms are available to this day in this region, and the corresponding current seismic stress and strain fields remain partly elusive. The development of dense seismic networks in the past decades now provides a substantial amount of seismic records down to low magnitudes. The corresponding data, while challenging to handle due to their amount and relative noise, represent a new opportunity to increase the spatial resolution of seismic deformation fields. The aim of this paper is to quantitatively assess the current seismic stress and strain fields within the Western Alps, from a probabilistic standpoint, using new seismotectonic data. The dataset comprises more than 30,000 earthquakes recorded by dense seismic networks since 1989 and more than 2200 focal mechanisms newly computed in a consistent manner. The global distribution of P and T axes plunges confirms a majority of transcurrent focal mechanisms in the overall alpine realm, combined with pure extension localized in the core of the belt. We inverted this new set of focal mechanisms through several strategies, including a seismotectonic zoning scheme and grid procedure, revealing extensional axes oriented obliquely to the strike of the belt. The Bayesian inversion of this new dataset of focal mechanisms provides a probabilistic continuous map of the style of seismic deformation in the Western Alps. Extension is found clustered, instead of continuous along the backbone of the belt. Compression is robustly retrieved only in the Po plain, which lays at the limit between the Adriatic and Eurasian plates. High frequency spatial variations of the seismic deformation are consistent with surface horizontal GNSS measurements as well as with deep lithospheric structures, thereby providing new elements to understand the current 3D dynamics of the belt. We interpret the ongoing seismotectonic and kinematic regimes as being controlled by the joint effects of far-field forces –imposed by the counterclockwise rotation of Adria with respect to Europe- and of buoyancy forces in the core of the belt, which together explain the high frequency patches of extension and of marginal compression overprinted on an overall transcurrent tectonic regime.

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Section: Focal Mechanism Computationmentioning

confidence: 99%

Present-day geodynamics of the Western Alps: new insights from earthquake mechanisms

Mathey¹,

Sue²,

Pagani³

et al. 2020

Preprint

View full text Add to dashboard Cite

show abstract

“…The RMS values of earthquakes were estimated from the average of the P and T direction variability shown in Table S2 and the supporting information “focal_mechanisms_uncertenties.Rar.” The Events 12, 17, and 20 are composite focal mechanism, for details see methodological section 3.3. (b) Kaverina plot of the focal mechanisms, where the number of each mechanism is the same as in (a) (this plot was made using the FMC software of Alvarez‐Gomez, 2019). (c) The meaning of each area in the Kaverina plot.…”

Section: Resultsmentioning

confidence: 99%

“…An important tectonic issue is defining the faulting type of focal mechanisms and the tectonic regime of an area. To determine the faulting type of focal mechanisms objectively, we employed Kaverina et al's plot classification (Kaverina et al, 1996) using the FMC open source software (Alvarez‐Gomez, 2019). This classification depends on the plunge of the P , T , and B axis of each focal mechanism and allows the faulting type to be described as normal, reverse, strike‐slip, or a mixture between them—either normal strike‐slip or strike‐slip normal depending on the predominant displacement component.…”

Section: Methodsmentioning

confidence: 99%

Seismicity in a Transpressional Volcanic Arc: The Liquiñe‐Ofqui Fault System in the Puyuhuapi Area, Southern Andes, Chile (44°S)

et al. 2020

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Understanding the relationship between crustal faults and volcanic activity in transpressional environments is a main goal in geosciences and could help to understand geothermal resources and evaluate geological hazards. In the Andean Southern Volcanic Zone (SVZ), Chile, recorded seismicity is scarce, and few studies have evaluated the relationship between volcanic activity and crustal faults from seismic observations. Thus, in this study, we deployed a seismic network for almost 1 year to understand the brittle deformation of the upper crust within the Puyuhuapi area, located at~44°S in the SVZ. We analyzed the location and kinematics of seismicity together with previously published field structural geological data. Considering these results, we developed an integrative tectonic model for the area and discussed which faults facilitate magma transport through the crust. Our results indicate the existence of two NNE-oriented seismogenic dextral to dextral-reverse regional faults that generate a duplex in a continental-scale fault setting. Inside the duplex, we observed normal to strike-slip normal focal mechanisms which recurrently have NE trending nodal planes. At a regional scale, a strike-slip tectonic environment has a N60°E/18°shortening direction and a N151°E/03°extension direction. We conclude that stratovolcanoes are located inside the duplex in a local transtensional environment where NE oriented normal faulting may occur. These faults facilitate magma transport since they represent the preferential orientation for dilatational fractures. Conversely, in local transpressional environments such as the Puyuhuapi fault (NNE oriented dextral to dextral-reverse kinematics), only minor eruptive centers of small volume are emplaced, suggesting a less productive magma transportation process.

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“…Since each inversion contains clusters of unique dimensions, we facilitate comparison between the earthquake‐only and the earthquake‐and‐SSE inversion results by designating groups of clusters that span similar spatial areas, labeled EI#, for groups from the earthquake‐only inversion, and ESI# for groups from the earthquake and SSE inversion. We utilize Kaverina‐type rupture classification diagrams (Kagan, 2005; Kaverina et al., 1996) generated by FMC (Álvarez‐Gómez, 2019) to visualize the rupture type of focal mechanism data to determine each group. Kaverina‐type ternary diagrams classify events into seven rupture types based on the plunges of the P, B, and T centroid moment tensor axes: (1) strike‐slip; (2) strike‐slip–normal; (3) strike‐slip–reverse; (4) normal–strike‐slip; (5) reverse–strike‐slip; (6) normal; and (7) reverse (Figure 3).…”

Section: Methodsmentioning

confidence: 99%

Stress Orientations in the Nankai Trough Constrained Using Seismic and Aseismic Slip

Thomas

2020

JGR Solid Earth

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The strength of subduction thrust faults is key to understanding seismogenesis at the provenance of Earth's largest earthquakes. Earthquake focal mechanisms are routinely inverted to constrain the stress state at seismogenic depths. However, on some megathrusts, deformation is accommodated by both earthquakes and types of slow fault slip. We employ focal mechanisms of short‐term slow slip events (SSEs), a type of slow fault slip, and earthquakes in a regional stress inversion to investigate the stress state of the Nankai Trough megathrust and interpret the results in the context of regional tectonics Previous studies using earthquake‐only stress inversions found principal stress orientations in this region that are incompatible with thrust faulting on the megathrust. When both SSEs and earthquakes are considered, the stress state of the central and eastern Nankai Trough megathrust is well oriented for thrust faulting. Our results suggest that slow fault slip source regions may appear to have misoriented stress fields if slow fault slip constitutes a substantial proportion of fault slip and the stress field is not well constrained by earthquakes. In the SSE region, we find that faults are well oriented for failure, suggesting they have strengths similar to their surroundings. Combined with low Vp/Vs ratios and sensitivity to small stress changes, our results imply that the megathrust and surroundings operate at low deviatoric stresses in the SSE source region. Further, we show that the coefficient of friction for areas hosting SSEs is low (μ=0.19–0.50), implying frictionally weak materials in the SSE source region.

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FMC—Earthquake focal mechanisms data management, cluster and classification

Cited by 70 publications

References 60 publications

Present-day geodynamics of the Western Alps: new insights from earthquake mechanisms

Present-day geodynamics of the Western Alps: new insights from earthquake mechanisms

Seismicity in a Transpressional Volcanic Arc: The Liquiñe‐Ofqui Fault System in the Puyuhuapi Area, Southern Andes, Chile (44°S)

Stress Orientations in the Nankai Trough Constrained Using Seismic and Aseismic Slip

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