Large near-surface block rotations at normal faults of the Iceland rift: Evolution of tectonic caves and dilatancy

Geology ◽  
2019 ◽  
Vol 47 (8) ◽  
pp. 781-785 ◽  
Author(s):  
Michael Kettermann ◽  
Christopher Weismüller ◽  
Christoph von Hagke ◽  
Klaus Reicherter ◽  
Janos L. Urai
Keyword(s):  
High Resolution ◽  
Plate Boundary ◽  
Normal Fault ◽  
Fault Zones ◽  
Normal Faults ◽  
Failure Zone ◽  
Near Surface ◽  
Modeling Studies ◽  
The Family ◽  
Aerial Vehicle

Abstract Surface ramps in normal fault zones of the Iceland plate boundary have been described in many studies, but their structure and evolution are not well understood. We show that surface ramps are manifestations of large tilted blocks (TBs) formed in dip relays of normal faults. Based on existing modeling studies, we propose three classes of TBs defined by kinematics and location of the hinge of the TB. TBs are considered a member of the family of fault relay structures that form near the surface, commonly, but not exclusively, in columnar basalts with orthotropic strength. We present high-resolution aerial vehicle–based observations of a representative set of normal faults in Iceland and compare these with geometric models we derived from modeling studies. We predict extensive tectonic cave (fluid conduit) systems under the TB, which interact with magma and groundwater flow. The general fault structure is dominated by large, subvertical open fractures reactivating cooling joints that are locally filled by basalt rubble. We propose the existence of a hybrid failure zone at larger depths before the effective vertical stress is sufficient to initiate shear fractures in intact basalt.

scholarly journals Structure of massively dilatant faults in Iceland: lessons learned from high resolution UAV data

2019 ◽  
Author(s):  
Christopher Weismüller ◽  
Janos L. Urai ◽  
Michael Kettermann ◽  
Christoph von Hagke ◽  
Klaus Reicherter
Keyword(s):  
High Resolution ◽  
Normal Fault ◽  
Lessons Learned ◽  
Normal Faults ◽  
Large Set ◽  
Recent Sediment ◽  
Field Observations ◽  
East African ◽  
Continuous Transition ◽  
Opening Width

Abstract. Normal faults in basalts develop massive dilatancy up to several tens of meters close to the Earth's surface and show corresponding interactions with groundwater and lava flow. These massively dilatant faults (MDF) are widespread in extensional settings like Iceland or the East African Rift, but their detailed geometry is not well understood, despite their importance for fluid flow in the subsurface, geohazards or geothermal energy. We present a large set of digital elevation models (DEM) of the surface geometries of MDF with 5–15 cm resolution, acquired along the Icelandic Rift zone using unmanned aerial vehicles (UAV). UAV provide a much higher resolution than aerial/satellite imagery and a much better overview than ground-based fieldwork, thus bridging the gap between outcrop scale and regional observations. Our data present representative outcrops of MDF, formed in basaltic sequences linked to the Mid Ocean Ridge. We acquired photosets of overlapping images along about 20 km of MDF and processed these using photogrammetry to create high resolution DEMs and ortho-rectified images. We use this dataset to map the faults and their damage zones to measure length, opening width and vertical offset of the faults and identify surface tilt in the damage zones. Ground truthing of the data was done by field observations. Mapped vertical offsets show typical trends of normal fault growth by segment coalescence. However, opening widths in map-view show variations at much higher frequency, caused by segmentation, collapsed relays and tilted blocks. These effects cause a commonly higher than expected ratio of vertical offset and opening width for a steep normal fault at depth. Based on field observations and the relationships of opening width and vertical offset, we define three endmember morphologies of MDF: (i) dilatant faults with opening width and vertical offset, (ii) tilted blocks (TB), and (iii) opening mode (mode I) fissures. Field observation of normal faults without visible opening invariably shows that these have an opening filled by recent sediment. TB dominated normal faults tend to have a largest opening width with respect to vertical offsets. Fissures have opening widths up to 15 m with throw below a 2 m threshold. Plotting opening width versus vertical offset of the fractures shows that there is a continuous transition between the endmembers. We conclude that fractures associated with MDF belong to one larger continuum and the three endmembers are thus not necessarily indicative for fracture maturity.

scholarly journals Structure of massively dilatant faults in Iceland: lessons learned from high-resolution unmanned aerial vehicle data

Solid Earth ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 1757-1784 ◽  
Author(s):  
Christopher Weismüller ◽  
Janos L. Urai ◽  
Michael Kettermann ◽  
Christoph von Hagke ◽  
Klaus Reicherter
Keyword(s):  
High Resolution ◽  
Normal Fault ◽  
Lessons Learned ◽  
Normal Faults ◽  
Large Set ◽  
Recent Sediment ◽  
Field Observations ◽  
East African ◽  
Continuous Transition ◽  
Opening Width

Abstract. Normal faults in basalts develop massive dilatancy in the upper few hundred meters below the Earth's surface with corresponding interactions with groundwater and lava flow. These massively dilatant faults (MDFs) are widespread in Iceland and the East African Rift, but the details of their geometry are not well documented, despite their importance for fluid flow in the subsurface, geohazard assessment and geothermal energy. We present a large set of digital elevation models (DEMs) of the surface geometries of MDFs with 5–15 cm resolution, acquired along the Icelandic rift zone using unmanned aerial vehicles (UAVs). Our data present a representative set of outcrops of MDFs in Iceland, formed in basaltic sequences linked to the mid-ocean ridge. UAVs provide a much higher resolution than aerial/satellite imagery and a much better overview than ground-based fieldwork, bridging the gap between outcrop-scale observations and remote sensing. We acquired photosets of overlapping images along about 20 km of MDFs and processed these using photogrammetry to create high-resolution DEMs and orthorectified images. We use this dataset to map the faults and their damage zones to measure length, opening width and vertical offset of the faults and identify surface tilt in the damage zones. Ground truthing of the data was done by field observations. Mapped vertical offsets show typical trends of normal fault growth by segment coalescence. However, opening widths in map view show variations at much higher frequency, caused by segmentation, collapsed relays and tilted blocks. These effects commonly cause a higher-than-expected ratio of vertical offset and opening width for a steep normal fault at depth. Based on field observations and the relationships of opening width and vertical offset, we define three endmember morphologies of MDFs: (i) dilatant faults with opening width and vertical offset, (ii) tilted blocks (TBs) and (iii) opening-mode (mode I) fissures. Field observation of normal faults without visible opening invariably shows that these have an opening filled with recent sediment. TB-dominated normal faults tend to have the largest ratio of opening width and vertical offset. Fissures have opening widths up to 15 m with throw below a 2 m threshold. Plotting opening width versus vertical offset shows that there is a continuous transition between the endmembers. We conclude that for these endmembers, the ratio between opening width and vertical offset R can be reliably used to predict fault structures at depth. However, fractures associated with MDFs belong to one larger continuum and, consequently, where different endmembers coexist, a clear identification of structures solely via the determination of R is impossible.

scholarly journals Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults

Solid Earth ◽  
2016 ◽  
Vol 7 (3) ◽  
pp. 789-815 ◽  
Author(s):  
Michael Kettermann ◽  
Sebastian Thronberens ◽  
Oscar Juarez ◽  
Janos Lajos Urai ◽  
Martin Ziegler ◽  
...  
Keyword(s):  
Microscopic Study ◽  
Thickness Distribution ◽  
Normal Fault ◽  
Fault Zones ◽  
Unconsolidated Sediments ◽  
Normal Faults ◽  
Low Permeability ◽  
Lignite Mine ◽  
Log Normal ◽  
Open Cast

Abstract. Clay smears in normal faults can form seals for hydrocarbons and groundwater, and their prediction in the subsurface is an important problem in applied and basic geoscience. However, neither their complex 3-D structure, nor their processes of formation or destruction are well understood, and outcrop studies to date are mainly 2-D. We present a 3-D study of an excavated normal fault with clay smear, together with both source layers, in unlithified sand and clay of the Hambach open-cast lignite mine in Germany. The faults formed at a depth of 150 m, and have shale gouge ratios between 0.1 and 0.3. The fault zones are layered, with sheared sand, sheared clay and tectonically mixed sand–clay gouge. The thickness of clay smears in two excavated fault zones of 1.8 and 3.8 m2 is approximately log-normal, with values between 5 mm and 5 cm, without holes. The 3-D thickness distribution is heterogeneous. We show that clay smears are strongly affected by R and R' shears, mostly at the footwall side. These shears can locally cross and offset clay smears, forming holes in the clay smear, while thinning of the clay smear by shearing in the fault core is less important. The thinnest parts of the clay smears are often located close to source layer cut-offs. Locally, the clay smear consists of overlapping patches of sheared clay, separated by sheared sand. More commonly, it is one amalgamated zone of sheared sand and clay. A microscopic study of fault-zone samples shows that grain-scale mixing can lead to thickening of the low permeability smears, which may lead to resealing of holes.

STRUCTURAL AND SEISMIC DEFORMATIONS ALONG NORMAL FAULTS IN THE EASTERN VENEZUELAN BASIN

Geophysics ◽  
1956 ◽  
Vol 21 (2) ◽  
pp. 368-387 ◽  
Author(s):  
Hans P. Laubscher
Keyword(s):  
Fault Zone ◽  
Normal Fault ◽  
Fault Zones ◽  
Fault Analysis ◽  
Structural Deformation ◽  
Normal Faults ◽  
Seismic Deformations ◽  
Structural Deformations ◽  
Fault Structures ◽  
Seismic Reflections

Seismic reflections in normal fault zones in the Eastern Venezuelan basin usually appear distorted. Studies of reflections over fault structures delineated by drilling indicate that this is due to the similar effects of two entirely different phenomena: 1. True structural deformation of beds on the downthrown side. 2. Purely seismic distortion of reflections from underneath the fault. Analysis indicates that the structural deformations form an integral part of the fault zone; the purely seismic distortion is caused by passage of the wave through this zone of deformation.

scholarly journals Mechanisms of clay smear formation in unconsolidated sediments – insights from 3D observations of excavated normal faults

2016 ◽  
Author(s):  
Michael Kettermann ◽  
Sebastian Thronberens ◽  
Oscar Juarez ◽  
Janos Lajos Urai ◽  
Martin Ziegler ◽  
...  
Keyword(s):  
Thickness Distribution ◽  
3D Structure ◽  
Normal Fault ◽  
Fault Zones ◽  
Unconsolidated Sediments ◽  
Normal Faults ◽  
Low Permeability ◽  
Lignite Mine ◽  
Log Normal ◽  
Open Cast

Abstract. Clay smears in normal faults can form seals for hydrocarbons and groundwater, and their prediction in the subsurface is an important problem in applied and basic geoscience. However, neither their complex 3D structure, nor their processes of formation or destruction are well understood, and outcrop studies to date are mainly 2D. We present a 3D study of an excavated normal fault with clay smear, together with both source layers, in unlithified sand and clay of the Hambach open cast lignite mine in Germany. The faults formed at a depth of 150 m, and have Shale Gouge Ratios between 0.1 and 0.3. The fault zones are layered, with sheared sand, sheared clay and tectonically mixed sand-clay gouge. Thickness of clay smears in two excavated fault zones of 1.8 and 3.8 m2 is approximately log-normal, with values between 5 mm and 5 cm, without holes. The 3D thickness distribution is heterogeneous. We show that clay smears are strongly affected by R- and R'-shears, mostly at the footwall side. These shears can locally cross and offset clay smears, forming holes in the clay smear, while thinning of the clay smear by shearing in the fault core is less important. Thinnest parts of the clay smears are often located close to source layer cutoffs. Locally, the clay smear consists of overlapping patches of sheared clay, separated by sheared sand. More commonly, it is one amalgamated zone of shared sand and clay. Microscopic study of fault zone samples shows that grain-scale mixing can lead to thickening of the low permeability smears, which may lead to resealing of holes.

scholarly journals High-resolution surface faulting from the 1983 Idaho Lost River Fault Mw 6.9 earthquake and previous events

2021 ◽  
Vol 8 (1) ◽  
Author(s):  
Simone Bello ◽  
Chelsea P. Scott ◽  
Federica Ferrarini ◽  
Francesco Brozzetti ◽  
Tyler Scott ◽  
...  
Keyword(s):  
High Resolution ◽  
Scaling Laws ◽  
Normal Fault ◽  
Fault System ◽  
Pixel Resolution ◽  
High Resolution Mapping ◽  
Digital Elevation ◽  
Aerial Vehicle ◽  
Resolution Mapping ◽  
Fault Length

AbstractWe present high-resolution mapping and surface faulting measurements along the Lost River fault (Idaho-USA), a normal fault activated in the 1983 (Mw 6.9) earthquake. The earthquake ruptured ~35 km of the fault with a maximum throw of ~3 m. From new 5 to 30 cm-pixel resolution topography collected by an Unmanned Aerial Vehicle, we produce the most comprehensive dataset of systematically measured vertical separations from ~37 km of fault length activated by the 1983 and prehistoric earthquakes. We provide Digital Elevation Models, orthophotographs, and three tables of: (i) 757 surface rupture traces, (ii) 1295 serial topographic profiles spaced 25 m apart that indicate rupture zone width and (iii) 2053 vertical separation measurements, each with additional textual and numerical fields. Our novel dataset supports advancing scientific knowledge about this fault system, refining scaling laws of intra-continental faults, comparing to other earthquakes to better understand faulting processes, and contributing to global probabilistic hazard approaches. Our methodology can be applied to other fault zones with high-resolution topographic data.

Mixed-Mode Seismic Slip and Aseismic Creep on a Highly Active Low-Angle Normal Fault System in Papua New Guinea

2020 ◽  
Author(s):  
James Biemiller ◽  
Laura Wallace ◽  
Luc Lavier
Keyword(s):  
Papua New Guinea ◽  
Normal Fault ◽  
New Guinea ◽  
Fault System ◽  
Normal Faults ◽  
Fault Mechanics ◽  
Fault Rocks ◽  
Near Surface ◽  
Geological Evidence ◽  
Seismic Slip

<p>Whether low-angle normal faults (LANFs; dip < 30°) slip in large earthquakes or creep aseismically is a longstanding problem in fault mechanics. Although abundant in the geologic record, active examples of these enigmatic ‘misoriented’ structures are rare and extension rates across them are typically less than a few mm/yr. As such, geodetic and seismological observations of LANFs are sparse and can be difficult to interpret in terms of earthquake cycles. With a long-term slip rate of ~1 cm/yr, the Mai’iu fault in Papua New Guinea may be the world’s most active LANF and thus offers an outstanding natural laboratory to evaluate seismic vs. aseismic behavior of LANFs. Here, we use new results from a campaign GPS network to determine the degree of locking vs. aseismic creep on the Mai’iu fault and evaluate these results in the context of geological evidence for mixed seismic and aseismic slip in exhumed Mai’iu fault rocks.</p><p>We derive velocities from GPS measurements with 3-4 km station spacing above the shallowest portions of the fault, which dips 21-25° at the surface. Dislocation modeling of these velocities is consistent with 6-8 mm/yr of horizontal extension, corresponding to ~1 cm/yr dip-slip rates on a 27-35°-dipping fault. Strain rates and vertical derivatives of horizontal stress rates derived from these velocities confirm localized extension across the fault. We compare and evaluate two interseismic locking models that fit the data best: one in which the fault deforms by shallow near-surface creep updip of a deeper zone of increased interseismic coupling which soles into a steadily creeping shear zone at depth, and one in which the fault creeps steadily downdip of a shallowly locked patch. These results combined with field and microstructural evidence from the exhumed fault rocks suggest that the fault slips by a mixture of brittle frictional (seismic slip, fracturing, and cataclastic creep) and viscous (stress-driven dissolution-precipitation creep, or pressure solution) processes. Using depth-constrained mechanical properties and stress conditions inferred from exhumed fault rocks, we model the time-dependent competition between frictional slip and viscous creep to assess where and how elastic strain accumulates along the Mai’iu fault, and whether the fault is capable of hosting or nucleating earthquakes.</p>

scholarly journals SEGMENTATION AND INTERACTION OF NORMAL FAULTS IN CENTRAL GREECE

2017 ◽  
Vol 43 (1) ◽  
pp. 428 ◽  
Author(s):  
S. Kokkalas
Keyword(s):  
Scaling Laws ◽  
Normal Fault ◽  
Fault Zones ◽  
Segment Length ◽  
Normal Faults ◽  
Mechanical Interaction ◽  
Fault Segment ◽  
Central Greece ◽  
Fault Length ◽  
Radial Propagation

The aim of this study is to improve our understanding on the mechanical interaction and linkage process between normal fault segments. Faults grow by the process of radial propagation and the linkage of segments, as strain increases, evolving to large fault systems. For this purpose we conducted a combined field and photogeological study on two major segmented fault zones in Central Greece, the Atalanti and Arkitsa fault zones. This approach includes effects of fault size and spatial distribution, scaling laws and footwall-hanginwall topography. Throw distribution and the geometry of the segmented fault arrays were analyzed in order to investigate the complexity of fault zones, the fault linkage process and the geometric characteristics of the relay zones formed between individual segments. The correlation of fault throw with fault length (D-L) and the ratios of overlap-separation (OL-S), separation-fault segment length (S-L) and relay displacement vs. separation (Dr-S) were examined in order to give an insight for fault segment interaction and linkage .

Emergence of Low-Frequency Aftershocks of the 2019 Ridgecrest Earthquake Sequence

2022 ◽  
Author(s):  
Ayako Tsuchiyama ◽  
Taka’aki Taira ◽  
Junichi Nakajima ◽  
Roland Bürgmann
Keyword(s):  
Fluid Pressure ◽  
Plate Boundary ◽  
Low Frequency ◽  
Fault Zones ◽  
Aftershock Sequence ◽  
Earthquake Sequence ◽  
Near Surface ◽  
Hypocentral Distance ◽  
Generation Mechanisms ◽  
Stress Drops

ABSTRACT Low-frequency earthquakes (LFEs) generally have relatively stronger spectral components in the lower frequency range compared with what is expected for regular earthquakes based on their magnitude. LFEs generally occur in volcanic systems or deep (>∼15 km) in plate boundary fault zones; however, LFEs have also been observed in nonvolcanic, upper crustal settings. Because there are few studies that explore the spatiotemporal behaviors of LFEs in the shallow crust, it remains unclear whether the shallow-crustal LFEs reflect local attenuation in their immediate vicinity or differences in their source mechanism. Therefore, it is important to identify shallow-crustal LFEs and to characterize their spatiotemporal activity, which may also improve our understanding of LFEs. In this study, we focus on detecting shallow-crustal LFEs and explore the possible generation mechanisms. We analyze 29,646 aftershocks in the 2019 Ridgecrest, California, earthquake sequence, by measuring the frequency index (FI) to identify candidate low-frequency aftershocks (LFAs), while accounting for the magnitude dependency of the FI. Using small earthquakes (ML 1–3) recorded in the borehole stations to minimize the attenuation effects in near-surface layers, we identify 68 clear LFAs in total. Based on their distribution and comparisons with other seismic parameters measured by Trugman (2020), the LFAs possess distinct features from regular events in the same depths range, including low corner frequencies and low stress drops. Events in the close vicinity of LFAs exhibit lower average FI values than regular aftershocks, particularly if the hypocentral distance between an LFA and its neighbors is less than 1 km. Our results suggest that LFAs are related to local heterogeneity or a highly fractured fault zone correlated with an abundance of cross faults induced by the aftershock sequence at shallow depths. Zones of high pore-fluid pressure in intensely fractured fault zones could cause the bandlimited nature of LFAs and LFEs in general.

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