Continental Normal Fault Research Articles

Modeling of Continental Normal Fault Earthquakes

AbstractThe magnitude of earthquakes on continental normal faults rarely exceeds 7.0 Mw. However, because of their vicinity to large population centers they can be highly destructive. Long recurrence time, relatively small deformations, and limited observations hinder our understanding of the deformation patterns and mechanisms controlling the magnitude of events. Here, this problem is addressed with 2D thermomechanical modeling of normal fault seismic cycles. The 2020 Samos, Greece Mw7.0 earthquake is used as an example as it is one of the largest and most studied continental normal fault earthquakes. The modeling approach employs visco‐elasto‐plastic rheology, compressibility, free surface, and a rate‐and‐state friction law for the fault. Modeling of the Samos earthquake suggests the pore fluid pressure ratio on the fault ranges from 0 to 0.7. The model demonstrates that most of the deformation during interseismic and coseismic periods, besides on the fault, occurs in the hanging wall and footwall below the seismogenic part of the fault. The largest vertical surface displacement during the earthquake is the subsidence of the hanging wall in the vicinity of the fault, while the uplift of the footwall and remote part of the hanging wall is significantly smaller. Modeling of the seismic cycles on normal faults with different setups shows the dependency of the magnitude on the thermal profile and dipping angle of the fault; low heat flow and low dipping angle are favorable conditions for the largest events, while steep normal faults in the areas of high heat flow tend to have the smallest magnitudes.

Why do continental normal fault earthquakes have smaller maximum magnitudes?

Continental normal fault earthquakes have been reported to have smaller maximum magnitudes (Mmax) than continental earthquakes with other fault geometries. This difference has significant implications for understanding seismic hazards in extensional regions. Using the Global Centroid Moment Tensor (GCMT) catalog, we examine how Mmax varies with fault geometry in continental regions, whether these trends are robust, and potential physical reasons for the smaller magnitudes of continental normal fault earthquakes.We find that the largest continental normal fault earthquakes are in the low Mw 7 range whereas other fault geometries can reach ~Mw 8. The continental normal fault earthquake magnitude-frequency distribution has a lower corner magnitude (a parameterization of Mmax) than other fault geometries. The observed smaller continental normal fault Mmax is not an artifact of classification criteria or catalog length. Probability calculations indicate that the GCMT catalog is long enough to capture differences in Mmax due to fault geometry. Additionally, our analysis indicates that neither fault length nor width is limiting the size of continental normal fault earthquakes. Fault complexity can limit rupture extent, but it is likely not the primary reason for the smaller continental normal fault Mmax.Rather, lithosphere yield stress (strength) appears to be the main factor controlling Mmax. In extension, lithosphere is weaker, failing at lower yield stresses than in compression. Although this yield stress difference is consistent with smaller continental normal fault earthquakes, it appears inconsistent with the occurrence of large oceanic normal fault earthquakes. However, the largest oceanic normal fault earthquakes occur near subduction zones where the lithosphere is bending. Laboratory studies indicate that bending lithosphere likely has a higher yield stress than lithosphere in pure extension, which may allow for larger oceanic normal fault earthquakes. Therefore, yield stress—rather than fault geometry alone—appears to be the key factor limiting an earthquake's maximum magnitude.

How fast can low-angle normal faults slip? Insights from cosmogenic exposure dating of the active Mai’iu fault, Papua New Guinea

Abstract Is there an upper limit to normal fault slip rates? The Mai’iu fault, located within the rapidly extending Woodlark Rift, Papua New Guinea, is one of few active continental low-angle normal faults (LANFs) globally. There is ongoing debate regarding how commonly normal faults slip at shallow (<30°) dips, and at what rates. We present a global compilation of reported slip rates on active and inactive extensional detachments that suggests that such faults may slip at >10–20 mm/yr—faster than any reported high-angle normal fault. Cosmogenic nuclide exposure dating (10Be in quartz) of the lowermost Mai’iu fault scarp supports this finding, indicating slip at 11.7 ± 3.5 mm/yr over the past ∼5.5 k.y. Our results highlight the long-term viability of LANFs, and show that the Mai’iu fault represents one of Earth’s fastest active continental normal faults. Rapid and large-displacement slip is likely enabled by extremely low fault frictional strength.

Growth and deformation mechanisms of talc along a natural fault: a micro/nanostructural investigation

This paper documents the occurrence of large amounts of talc within a continental normal fault. The talc-in reaction is deformation-enhanced and occurs by the interaction between dolostones and silica-rich hydrothermal fluids. In the high-strain, foliated fault core, talc forms an interconnected network of oriented (001) lamellae, 200–300 nm thick, locally associated with minor tremolite fibres, up to 300 nm in diameter. The talc structure is affected by several strain-induced defects, among which (001) interlayer delamination that produces talc “sublamellae” down to 10–30 nm thick. Micro/nanostructural observations definitely point to a predominant deformation mechanism of (001) frictional sliding, further enhanced by pervasive delamination that gives rise to an almost infinite number of possible sliding surfaces. These effects have fundamental implications in fault mechanics, resulting in significant fault weakening.

Deformation associated with a continental normal fault system, western Grand Canyon, Arizona

Reverse-drag folds are often used to infer subsurface fault geometry in extended terrains, yet details of how these folds form in association with slip on normal fault systems are poorly understood. Detailed structural mapping and global positioning system (GPS) surveying of the Frog Fault and Lone Mountain Monocline in the western Grand Canyon demonstrate a systematic relationship between elements of the normal fault system and fold geometry. The Lone Mountain Monocline, which parallels the Frog Fault, is made up of two half-monoclinal flexures: a hanging-wall fold in which dips gradually increase toward the fault over ∼1.5 km reaching a maximum dip of 25° and a footwall fold in which dips decrease away from the fault over ∼0.5 km from a maximum of 12°. The highest dips associated with folding are found where throw on the Frog Fault and a synthetic fault are at a maximum. Lower dips are found where there is less throw on the Frog Fault and antithetic faults are present. This relationship between fault and fold geometry suggests that the folding is associated with Basin and Range extension rather than Laramide contraction. Mechanical models of normal fault-related deformation predict similar patterns of folding over planar faults of finite extent and corroborate the important role of subsidiary fault geometry in the overall pattern of deformation. Application of these models in an inverse sense yields ∼1 km estimates of down-dip extent, a result that may indicate fault confinement within the sedimentary section or weakening effects associated with folding layered strata. A general analysis illustrates that reverse-drag folds of moderate dip are expected to form in association with slip on planar faults of finite extent—a result that has the potential to impact our estimates of hydrocarbon volumes, crustal extension, and earthquake hazards associated with continental normal faults.

Discussion and reply: Kinematic evolution of a large-offset continental normal fault system, South Virgin Mountains, NevadaDiscussion

[Brady et al. (2000)][1], attempting to demonstrate wider applicability of a model they developed for extensional normal faulting in the South Virgin Mountains, Nevada, presented an erroneous reinterpretation of a cross section through the Yerington district, Nevada (their Fig. 15, reproduced here

Kinematic evolution of a large-offset continental normal fault system, South Virgin Mountains, Nevada

Research Article| September 01, 2000 Kinematic evolution of a large-offset continental normal fault system, South Virgin Mountains, Nevada Robert Brady; Robert Brady 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Brian Wernicke; Brian Wernicke 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Joan Fryxell Joan Fryxell 2Department of Geological Sciences, California State University, San Bernardino, California 92407, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (9): 1375–1397. https://doi.org/10.1130/0016-7606(2000)112<1375:KEOALO>2.0.CO;2 Article history received: 15 Jun 1998 rev-recd: 20 Aug 1999 accepted: 17 Sep 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Robert Brady, Brian Wernicke, Joan Fryxell; Kinematic evolution of a large-offset continental normal fault system, South Virgin Mountains, Nevada. GSA Bulletin 2000;; 112 (9): 1375–1397. doi: https://doi.org/10.1130/0016-7606(2000)112<1375:KEOALO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The South Virgin Mountains and Grand Wash trough comprise a mid-Miocene normal fault system that defines the boundary between the unextended Colorado Plateau to the east and highly extended crust of the central Basin and Range province to the west. In the upper 3 km of the crust, the system developed in subhorizontal cratonic strata in the foreland of the Cordilleran fold and thrust system. The rugged topography and lack of vegetation of the area afford exceptional three-dimensional exposures. Compact stratigraphy and well-defined prefaulting configuration of the rocks permitted a detailed reconstruction of the system. Reconstruction of cross sections based on more than 300 km2 of detailed mapping at a scale of 1:12 000 shows that the fault system accommodated more than 15 km of roughly east-west–directed Miocene extension. Extension was initially accommodated on moderately to steeply dipping listric normal faults. As the early faults and fault blocks tilted, steeply to moderately dipping faults initiated within the fault blocks, soling into the early faults. Some of the early faults were active at dips of <20°. Isostatically driven tilting is superimposed on tilting due to active slip and domino-style rotation of the fault blocks. Collectively these processes rotated originally steeply dipping faults to horizontal orientations. The kinematics are inconsistent with the widely accepted view that many near-horizontal normal faults were rotated to their present orientations by later, crosscutting normal faults. However, reexamination of other areas suggests that the evolutionary sequence seen in the South Virgin Mountains may, in fact, be widely applicable. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geometry of continental normal faults: Seismological constraints

Teleseismic body waves from large earthquakes are used to study the downdip geometry of continental normal faults in the Aegean. Waveform modeling techniques together with rigorous statistical tests are applied to put firm bounds on the amount of downdip curvature of these faults and the role of coseismic slip on a basal detachment. Synthetic modeling shows that good azimuthal station coverage and inclusion of SH waves are necessary to resolve fault curvature. The data indicate ruptures of the Aegean events occurred on planar faults extending across the entire brittle portion of the crust. No seismogenic low‐angle detachment faulting at the base of the upper crust was detected for these events. Decoupling of the brittle upper crust from the plastic lower crust probably occurs aseismically in a ductile fashion.

Bathymetric segmentation and faulting on the Mid-Atlantic Ridge, 24°00′N to 24°40′N

Abstract This investigation of valley-wall faulting and its relationship to bathymetry focuses on the first two segments north of the Kane Fracture Zone. The valley-wall faults imaged by deep-towed sidescan sonar show aspects common to continental normal faults, and a similar erosional pattern, from a fresh fault surface, through eroded, talus covered scarps, to scarps draped with pelagic sediments. The first, southernmost segment is a symmetric graben at the segment centre, and an asymmetric half-graben at the northern end. The second, northern segment is an asymmetric half-graben for its entire length. The gross morphology is reflected in the faulting style, with multiple small faults at the symmetric graben, and one or two large faults on the eastern wall, defining the asymmetic half-graben. This difference in faulting style may reflect differences in depth to the brittle-ductile transition, with asymmetry occurring when the brittle-ductile transition is deep and brittle faults can cross the axial valley, locking faults on the other side. A gross estimate of horizontal strain accommodated by the valley-wall faults correlates well with valley-floor bathymetry, suggesting that bathymetric segmentation is partially due to tectonic thinning at segment ends.