Shallow Dipping Plane Research Articles

The 12 October 2021, MW\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_W$$\\end{document}=6.4, Zakros, Crete earthquake

We study the strong 12 October 2021, MW\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_W$$\\end{document}=6.4, offshore Zakros, Crete earthquake, and its seismotectonic implications. We obtain a robust location (azimuthal gap equal to 17∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}) for the mainshock by combining all freely available local, regional and teleseismic phase arrivals (direct and depth phase arrivals). Based on our location and the spatial distribution of the poor aftershock sequence we parameterise the fault area as a 30 km ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 20 km planar surface, and using three-component strong motion data we calculate slip models for both earthquake nodal planes. Our preferred solution shows a simple, single slip episode on a NE-SW oriented, NW shallow-dipping fault plane, instead of a N-S oriented, almost vertical nodal plane. An anti-correlation of the aftershocks spatial distribution versus the maximum slip (∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document} 27 cm) of our model further supports this, although the accuracy of the aftershock hypocentral locations could be somewhat questionable. Coulomb stress changes calculated for both kinematic models do not show substantial differences, as the aftershock seismicity within the first 3 months after the mainshock is distributed along the stress shadow zone and over the stress enhanced areas developed at the southern fault edge, induced by the mainshock. The Kasos island tide gauge record analysis shows a small signal after the earthquake, but it can hardly demonstrate the existence of tsunami waves due to the low signal-to-noise ratio. Tsunami simulations computed for the two nodal planes do not yield conclusive evidence to highlight whether the causative fault plane is NE-SW oriented, NW shallow-dipping plane, or the N-S oriented plane, nevertheless, the power spectrum analysis of the NW shallow-dipping nodal plane matches the spectral peak at 8 s period and is overall closer to the spectrum of the tide gauge record. A USGS Shakemap was also produced with all available local strong motion data and EMSC testimonies. This was also investigated in an effort to document the responsible fault. The overall analysis in this study, slightly suggests a rather westward, shallow-dipping offshore fault zone, being antithetic to the main Zakros almost vertical normal fault which shapes the coast of eastern Crete and is perpendicular to the direction of Ptolemy Trench in this area. This result agrees with seismotectonic and bathymetric evidence which support the existence of approximately N-S trending grabens, east and northeast of Crete.

Finite‐fault slip model of the 2016 Mw 7.5 Chiloé earthquake, southern Chile, estimated from Sentinel‐1 data

AbstractSubduction earthquakes have been widely studied in the Chilean subduction zone, but earthquakes occurring in its southern part have attracted less research interest primarily due to its lower rate of seismic activity. Here I use Sentinel‐1 interferometric synthetic aperture radar (InSAR) data and range offset measurements to generate coseismic crustal deformation maps of the 2016 Mw 7.5 Chiloé earthquake in southern Chile. I find a concentrated crustal deformation with ground displacement of approximately 50 cm in the southern part of the Chiloé island. The best fitting fault model shows a pure thrust‐fault motion on a shallow dipping plane orienting 4° NNE. The InSAR‐determined moment is 2.4 × 1020 Nm with a shear modulus of 30 GPa, equivalent to Mw 7.56, which is slightly lower than the seismic moment. The model shows that the slip did not reach the trench, and it reruptured part of the fault that ruptured in the 1960 Mw 9.5 earthquake. The 2016 event has only released a small portion of the accumulated strain energy on the 1960 rupture zone, suggesting that the seismic hazard of future great earthquakes in southern Chile is high.

Ext≐nsion post-orogénique et zones de cisaillement. Étude d'une brèche tectonique située le long d'un niveau de décollement à Tinos (Cyclades, Grèce)

We describe a tectonic breccia which formed during extension at the base of the seismogenic upper crust. This breccia belongs to a large low dipping shear zone active from the ductile to the brittle crust. The main characteristics of this breccia are a high density of fracture planes whose dips are very variable, as well as a strong fluid activity as shown by spectacular talc, carbonate and quartz precipitations. The characteristics of the breccia are in agreement with geophysical data concerning the base of the brittle crust, which suggest the existence of a shallow dipping plane where microseisms and important fluid circulations concentrate.

Tectonic regime and slip orientation of reactivated faults

The slip orientation of reactivated faults is predicted by assuming that one principal stress direction is vertical, that faulting occurs on pre-existing planes of weakness with a slip direction along the applied shear stress, and by extending the original concept of tectonic regime so as to include the stress tensor aspect ratio. The predicted rake of slip is then analysed so as to reveal the respective influence of the fault-plane orientation (strike and dip) and of the tectonic regime. The analysis of this direct problem allows one to describe the geometry of the slip directions within any given tectonic regime, its evolution as the tectonic regime varies, and the constraints provided by slip-direction data on the tectonic regime. However, the question of what fault orientations would satisfy a failure criterion and thus be preferentially reactivated is not addressed. In any tectonic regime, pure strike-slip occurs on vertical planes and only on them, except in wrench regimes, where it also occurs on all fault planes along four strikes that are determined by the tectonic regime; pure dip-slip occurs on planes striking along one of the horizontal principal stress directions and only on them, except in radial compression and radial extension regimes, where it occurs on all planes except vertical ones; at fixed strike but varying dip, the slip direction is closest to dip-slip for zero dip, and pure strike-slip for vertical dip; at fixed dip and varying strike, the slip direction is pure dip-slip for strikes along one of the horizontal principal stress directions and closest to strike-slip for four strikes that are determined by the tectonic regime; the horizontal principal stress directions separate four quadrants: two where the slip direction is dextral and two where it is sinistral. The slip directions of all fault planes of the same dip can be easily deduced from those of subhorizontal planes and can be constructed geometrically. As the tectonic regime varies continuously from radial compression to radial extension, the domain of fault-plane orientations where steep reverse slip directions are possible shrinks and disappears when the extensional regime is entered; conversely, the domain where steep normal slip directions are possible appears when the wrench regime is entered and grows thereafter. The strike-slip domain first grows towards shallow dip until the wrench regime is entered, and then shrinks after the extensional regime is entered. Whereas in the model of failure in isotropic rocks, dip-slip or strike-slip data fully determine the original tectonic regime, in the model of failure on pre-existing planes of weakness, constraints on the extended tectonic regime require additional information. The sense of shallow slip direction data, which are to be found on steeply dipping planes, constrains the horizontal principal stress directions, and the steepness of slip-direction data on shallow dipping planes striking away from the principal stress directions constrains the extended tectonic regime. In the case of wrench regimes, an extra constraint arises from the fact that the extended tectonic regime also controls the strike at which the dip component of slip changes from normal to reverse. These constraints can be quantified by superposing a simple plot of the data rake versus strike with an abacus that is called Breddin's graph for tectonic regimes; while this graph does not replace inverse techniques, it may help in detecting tectonic phases in complex data sets. Without using this graph or resorting to inverse techniques, the transition from wrench to either compressional or extensional regime is difficult to infer from data that display a limited range of fault-plane strikes and the same sense of dip component of the slip direction.

Recent large earthquakes near Cape Mendocino and in the Gorda plate: Broadband source time functions, fault orientations, and rupture complexities

The northward migration of the Mendocino Triple Junction is associated with complex faulting within the Gorda plate and in the convergent zone between the Gorda and North American plates. This region has experienced substantial recent large earthquake activity, and quantification of these faulting processes is essential for understanding the evolution of the triple junction. Using an empirical Green function deconvolution method for teleseismic and regional surface waves and body waves, we obtain relative source time functions for the April 25, 1992, Cape Mendocino thrust earthquake (Mw = 7.2), its two large strike‐slip aftershocks on April 26, 1992 (Mw = 6.5, 6.6), and two large strike‐slip events in the Gorda plate that occurred July 13, 1991 (Mw = 6.8), and August 17, 1991 (Mw = 7.1). The removal of propagation effects using empirical Green functions provides unusually detailed source rupture information and indicates that all the ruptures are less than 16 s in duration. Analyzing the directivity effects observed in the source time functions, we resolve rupture directions and corresponding fault orientations for the four largest events and place some constraints on the smaller aftershock. Significant differences in the rupture duration and stress drop are observed between the Cape Mendocino mainshock and its two largest aftershocks. The mainshock, which ruptured westward on a shallow dipping plane either on or paralleling the interplate contact between the Gorda and North American plates, had a smooth 9‐s‐long rupture with a 0.9–4.6 MPa static stress drop. The two largest aftershocks occurred within the uppermost mantle of the Gorda plate and have more complex ruptures with total durations of 14–15 s and static stress drops of less than 0.3 MPa, perhaps reflecting a difference between mantle and crustal earthquakes in this region. The aftershocks appear to have ruptured conjugate strike‐slip faults, accounting for differences in their damage patterns. The August 17, 1991, Gorda plate event ruptured a southwest striking fault, while the July 13, 1991, event ruptured a southeast striking fault, with both events relieving north‐south compression within the deforming Gorda plate. The existence of multiple active faults in the region constitutes a significant earthquake hazard associated with the complex stress environment of the migrating triple junction.

Possible seismogenic shallow-dipping normal faults in the Woodlark-D'Entrecasteaux extensional province, Papua New Guinea

Research Article| December 01, 1991 Possible seismogenic shallow-dipping normal faults in the Woodlark-D'Entrecasteaux extensional province, Papua New Guinea Geoffrey A. Abers Geoffrey A. Abers 1Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Geology (1991) 19 (12): 1205–1208. https://doi.org/10.1130/0091-7613(1991)019<1205:PSSDNF>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Geoffrey A. Abers; Possible seismogenic shallow-dipping normal faults in the Woodlark-D'Entrecasteaux extensional province, Papua New Guinea. Geology 1991;; 19 (12): 1205–1208. doi: https://doi.org/10.1130/0091-7613(1991)019<1205:PSSDNF>2.3.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 SocietyGeology Search Advanced Search Abstract Active normal faults dipping less than 30° have not been documented in the seismic record, but they are commonly observed in geologic structures. One interpretation is that seismogenic low-angle faulting is physically impossible and that the geologic structures do not form at shallow dips. To better examine this problem, teleseismic body waves were inverted to determine source parameters for five large earthquakes in the Woodlark-D'Entrecasteaux region, a rare example of active continental extension associated with Pliocene-Quaternary metamorphic core complexes. The inversions show that at least two and possibly four events are consistent with normal dip slip on shallow-dipping faults. The largest earthquake (Mw = 6.8) is located near the mapped core complexes. The shallow-dipping plane for this event, determined to dip between 10° and 25°, is nearly coplanar to the faults bounding the core complexes and may represent slip on these or other parallel low-angle structures. Although slip on the conjugate high-angle plane cannot be ruled out, observations of Quaternary uplift and the similarity to the adjacent core-complex-bounding structures favor motion on the low-angle plane. Similar earthquake mechanisms have not yet been documented in most other extensional provinces, perhaps because instances of active core-complex exhumation are rare. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Source process of the great 1971 Solomon Islands doublet

Abstract Large underthrusting earthquakes in the Solomon Islands tend to occur as multiplets, separated by a few hours to several days in time and 30–100 km in space. The largest sequence, a doublet on July 14, 1971 ( M w = 8.0) and July 26, 1971 ( M w = 8.1), is particularly unusual in that it occured at the junction of two trenches, with the July 14 event in the Solomon Islands Trench preceding the July 26 rupture in the New Britain Trench. The location and large moment release of this doublet as well as an incompatibility of published body wave and surface wave focal mechanisms for the July 14 event, motivated a detailed study of the source process of the 1971 sequence. To satisfy both the surface wave and body wave data, we explored the possibility of a change in the faulting geometry during rupture for the July 14 event. A fault model that changes orientation from a northwest-southeast striking, shallow-dipping plane, similar to the mechanisms of neighboring underthrusting events in the Solomon Islands Trench, to a more north-south striking plane ∼ 70 s after rupture initiation is consistent with the observed Rayleigh and Love wave amplitude patterns. This change in mechanism is suggested by systematic variations in the mechanisms of smaller earthquakes near the cusp in the Solomon Islands plate. A simultaneous inversion of WWSSN P wave seismograms for the time, location, seismic moment and focal mechanism of individual subevents gives results in agreement with the surface wave model, although the precise change in mechanism is not well resolved. The spatiotemporal distribution of subevents reveals two major pulses of moment release. The first occurred on the initial fault near the epicenter at the origin time of the earthquake. The second occurred on the north-south striking fault, 50–70 km northwest of the epicenter ∼ 70 s after the origin time. The change in mechanism appears to reflect contortion of the slab near the trench junction. Inversion of WWSSN P wave seismograms for the rupture history of the July 26 event reveals a westerly propagating rupture extending ∼ 60 km along the New Britain Trench. For both of the 1971 earthquakes, regions of high moment release are located in the vicinity of the Solomon Islands-New Britain Trench junction. Contortion of the subducting lithosphere as it bends around the arc-arc junction may have enhanced the degree of mechanical coupling between the subducting and overriding plates, concentrating stress in this region, and in turn, promoting efficient triggering across the trench junction.

Velocity structure in the Izu‐Bonin Seismic Zone and the depth of the olivine‐spinel phase transition in the slab

In this study a method is presented for determining the in situ elastic wave velocities of slabs simultaneously with the relative relocations of earthquakes. The method is an extended arrival time difference (ATD) scheme that incorporates a realistic model. Several assumptions are made about the characteristics of ray paths in and around the slab in order to take advantage of the realism of the model without completely sacrificing the simplicity inherent in the ATD technique. The errors introduced by these approximate rays are estimated by comparing them with more exact rays calculated with a three‐dimensional ray‐tracing routine. In general, the comparisons suggest that these errors are usually much less than 0.1 s. The inverse method is tested on the same set of hypothetical data. The results of these tests show that the parameters that influence the arrival time differences the most are the amplitude of the slowness anomaly within the slab and the azimuths and takeoff angles for rays leaving the master event. The arrival time differences are relatively insensitive to the other parameters describing the slab (strike, dip, half width, and the position of the master event), which implies that the scheme is fairly insensitive to the particular choice of analytic function to describe the slowness variation within the slab. Convergence of the inverse routine is rapid in the tests; usually less than five iterations are required before a stable solution is reached. It appears that coupling between solutions for slab parameters and ray directions is insignificant. Unlike the standard ATD approach, solutions for hypocenters and velocities are not directly coupled in this scheme, although well‐located events are required to insure that the hypocenters are not unduly biased by variations in structure. Once the velocity structure of the slab is determined, that information can be used routinely by this scheme to relocate less well‐recorded events. This method was applied to arrival time data from earthquakes located between 200 and 600 km depth in the Izu‐Bonin subduction zone. Structure in the 200–300 km depth range could not be resolved, and results in the 500–600 km depth range proved to be too sensitive to noise. Data from events between 27°N and 31°N show that P wave velocities within the slab from 400 to 500 km depth are 4–6% higher than those of the ambient mantle. Between 300 and 400 km depth the slab structure varies regionally. North of about 32.5°N the P wave velocity within the slab is 3–4% higher than ambient from 180 to 375 km depth. In contrast, the velocities south of 32.5°N increase from 6–7% higher than ambient in the 375–410 km depth range to 8–10% higher in the 325–375 km depth range and appear to be 9% higher than ambient in the 270–325 km depth range. The determination of the high velocities in this region between 325 and 375 km depth proved to be independent of the selection of data and of the choice of starting model. Above 325 km depth, however, the velocity determinations are somewhat dependent on the choice of starting model. As a result of this dependence, no upper bound on the depth of the high velocities could be resolved. A plausible explanation for the exceptionally high P wave velocities within the slab south of 32.5°N is that the olivine‐spinel phase change boundary is elevated in this part of the slab, while north of 32.5°N it is not significantly elevated. It is not known if the phase change is elevated south of 31°N, but the velocities between 400 and 500 km depth in this region imply that it is not depressed. The high‐velocity zone south of 32.5°N lies in an area that marks the beginning of a seismic gap, increasing in breadth to the south, between deep and shallow seismicity. The dip of the slab also becomes gradually steeper to the south of this region. In contrast, the lower velocities north of 32.5°N occur in a region where the seismicity is fairly continuous and is aligned along a shallow dipping plane. If the seismic gap south of 32.5°N represents a region of high velocity, then the increasingly vertical dip of the slab in the south could be due to a substantial gravitational force provided by the excess mass extending above the ambient olivine‐spinel phase boundary. The decreasing width of the gap to the north suggests that the evolution of the Izu‐Bonin seismic zone has been governed by a propagating instability from the south.

Lithospheric deformation within the Molucca Sea arc‐arc collision: Evidence from shallow and intermediate earthquake activity

This paper presents the results of a local earthquake survey in the Molucca Sea arc‐arc collision zone of eastern Indonesia. Although intermediate depth earthquakes define zones dipping to the west beneath the Sangihe arc and to the east beneath the Halmahera volcanic arc, only shallow activity (less than 60 km depth) occurred beneath the central part of the collision zone where the majority of activity is observed. The concentration of earthquake foci in the 10‐ to 50‐km depth range in a limited region beneath the Talaud‐Mayu Ridge suggests that convergence between the arcs proceeds by shortening within basement of the intervening Molucca Sea plate rather than by slip along shallow dipping planes between the arcs and the subducted slabs. Published focal mechanism solutions suggest that high‐angle reverse faulting beneath the Talaud‐Mayu Ridge generates almost all of the seismic energy release within the collision zone. The predominance of shallow reverse and strike slip faulting at the axis of the bilaterally subducting Molucca Sea lithosphere suggests that stresses are not due to bending alone and have a large horizontal regional component perpendicular to the island arcs. The geometry and state of stress within the Molucca Sea lithosphere requires an external driving force for the convergence between the two arcs and a strong degree of coupling between the island arcs and the subducting Molucca Sea plate, at least at the present stage of collision.

Earthquake mechanisms and plate tectonics in the Himalayan region

Seven focal-mechanism solutions are obtained for earthquakes in the Kashmir, Tibet and Nepal regions of the Himalaya from P-wave first-motion directions. These solutions indicate thrust faulting. Sense of motion on the shallow dipping plane is consistent with underthrusting towards the northnortheast for the earthquakes beneath the Himalaya and towards the SW beneath the Trans-Himalaya. The tensional axes of these two groups of earthquakes dip towards each other. These observations suggest that the lithospheric blocks of the Indian and Eurasian plates underthrust towards each other.