An Earthquake Nest in Cascadia

2019 ◽  
Vol 109 (5) ◽  
pp. 2021-2035
Author(s):  
Reid Merrill ◽  
Michael Bostock
Keyword(s):  
Strike Slip ◽  
Focal Mechanisms ◽  
Regional Study ◽  
Geographical Isolation ◽  
Stress Regime ◽  
Plate Models ◽  
Juan De Fuca ◽  
Reverse Faulting ◽  
Subducted Oceanic Crust ◽  
Normal Compression

Abstract We investigate an isolated cluster of temporally persistent, intraslab earthquakes (ML<3.2) at >60  km depth below the Georgia Strait in southern British Columbia that is unique in Cascadia and meets the criteria for identification as an earthquake nest. A total of 129 relocated hypocenters define two northwest‐dipping structures in the subducting Juan de Fuca mantle within an ∼30×10×10  km3 volume. Focal mechanisms for 15 events represent a mix of strike‐slip and reverse faulting, and a stress regime of down‐dip tension and plate‐normal compression, consistent with a previous regional study. Converted seismic phases inferred to originate at the boundaries of subducted oceanic crust are observed at several receivers and are consistent with a local slab depth of ∼45  km, shallower than some JdF plate models. The geographical isolation of the nest within the confines of an extrapolated propagator wake suggests that its location is controlled by this pre‐existing and presumably hydrated structure.

A methodology for unstructured damped stress inversion of microseismic focal mechanisms: Application to the Vaca Muerta Formation, Argentina

Geophysics ◽  
2020 ◽  
Vol 85 (2) ◽  
pp. KS39-KS50
Author(s):  
Bing Q. Li ◽  
Jing Du
Keyword(s):  
Stress State ◽  
Unstructured Grid ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Instability Criterion ◽  
Regular Grid ◽  
Stress Inversion ◽  
Stress Regime ◽  
Vaca Muerta ◽  
Vaca Muerta Formation

Current methodologies for stress inversion from microseismic focal mechanisms require the assignment of events to a regular grid and then solving for the stress state at each grid node. This approach can lead to irregularities in the solution because some nodes may contain few or even no events. To address this issue, we modified the algorithm to solve for stresses on an irregular (unstructured) grid. We first use the [Formula: see text]-means algorithm to split the data into suitably sized groups. The centroids of these groups are then considered as the nodes of an unstructured grid, and we simultaneously solve for the stress state in each group using damped inversion. To account for the irregularity of the unstructured grid, we use the reciprocal square distance between nodes as weights, as opposed to the existing method in which a weight of unity is assigned between adjacent nodes on a regular grid. Focal planes are selected from the auxiliary plane using the fault instability criterion. The method is first applied to synthetic data sets in which we simulate and subsequently invert for the stress field around a mode-I fracture at depth, in a strike-slip and in a normal faulting stress regime. Results indicate a stress orientation error of 10° and a stress ratio error between 1% and 10%. We then consider focal mechanism data from an unconventional shale play in the Vaca Muerta Formation in Argentina, and our results suggest the presence of a preexisting strike-slip faulting stress regime. We also find that the unambiguous focal plane picks suggest that the apparent dip-slip focal mechanisms are indeed dip-slip movement along subvertical natural fractures, which correlate well with image log data. We suggest that these dip-slip events are caused by stress changes induced by the opening of the hydraulic fractures.

scholarly journals Distributed earthquake focal mechanisms in the Aegean Sea

2018 ◽  
Vol 40 (3) ◽  
pp. 1125 ◽  
Author(s):  
A. Kiratzi ◽  
C. Benetatos ◽  
Z. Roumelioti
Keyword(s):  
Aegean Sea ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Normal Faulting ◽  
Small Magnitude ◽  
The North ◽  
Earthquake Focal Mechanisms ◽  
Reverse Faulting ◽  
Different Types ◽  
Transform Fault Zone

Nearly 2,000 earthquake focal mechanisms in the Aegean Sea and the surroundings for the period 1912- 2006, for 1.5 <M<7.5, and depths from 0 to 170 km, indicate a uniform distribution and smooth variation in orientation over wide regions, even for the very small magnitude earthquakes. ~ 60% of the focal mechanisms show normal faulting, that mainly strikes ~E-W. However, a zone ofN-S normal faulting runs the backbone of Albanides-Hellenides. Low-angle thrust and reverse faulting is confined in western Greece (Adria-Eurasia convergence) and along the Hellenic trench (Africa-Eurasia). In the central Aegean Sea the effect of the propagating tip of the North Anatolian Fault into the Aegean Sea is pronounced and strike-slip motions are widely distributed. Shearing does not cross central Greece. Strike-slip motions reappear in the Cephalonia-Lefkada Transform Fault zone and in western Péloponnèse, which shows very complex tectonics, with different types of faulting being oriented favourably and operating under the present stress-field. Moreover, in western Péloponnèse the sense of the observed shearing is not yet clear, whether it is dextral or sinistral, and this lack of data has significant implications for the orientation of the earthquake slip vectors compared to the GPS obtained velocity vectors.

A seismological study of two Attica, New York earthquakes

1978 ◽  
Vol 68 (3) ◽  
pp. 641-651 ◽  
Author(s):  
Robert B. Herrmann
Keyword(s):  
New York ◽  
Source Parameters ◽  
Focal Depth ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
The Other ◽  
Nodal Plane ◽  
Reverse Faulting ◽  
Epicentral Intensity ◽  
East West

abstract The Attica, New York earthquakes of January 1, 1966 and June 12, 1967 are studied in detail to obtain their focal mechanisms, depths and seismic moments. Both events have similar source parameters with one nodal plane striking about 120° and dipping 60°S and the other nodal plane striking about 20° and dipping 70°E. The fault motion on the NNE nodal plane has a component of right lateral strike slip and one of reverse faulting. Though this nodal plane parallels the Clarendon-Linden structure, the possibility of associating the other nodal plane with a diffuse east-west seismicity trend cannot be excluded. The shallow focal depth of 2 to 3 km for these two events can be used as an explanation of the relatively high epicentral intensity VIII of the Attica event of 1929.

Reassessing the in-situ stress regimes of Australia's petroleum basins

2012 ◽  
Vol 52 (1) ◽  
pp. 415 ◽  
Author(s):  
Rosalind King ◽  
Simon Holford ◽  
Richard Hillis ◽  
Adrian Tuitt ◽  
Ernest Swierczek ◽  
...  
Keyword(s):  
Late Miocene ◽  
Normal Fault ◽  
In Situ Stress ◽  
Strike Slip ◽  
Strike Slip Fault ◽  
Normal Faults ◽  
Reverse Fault ◽  
Stress Regime ◽  
Reverse Faulting

Previous in-situ stress studies across many of Australia’s petroleum basins demonstrate normal fault and strike-slip fault stress regimes, despite the sedimentary successions demonstrating evidence for widespread Miocene-to-Recent reverse faulting. Seismic and outcrop data demonstrate late Miocene-to-Recent reverse or reverse-oblique faulting in the Otway and Gippsland basins. In the Otway Basin, a series of approximately northeast to southwest trending anticlines related to reverse-reactivation of deep syn-rift normal faults, resulting in the deformation of Cenozoic post-rift sediments are observed. Numerous examples of late Miocene-to-Recent reverse faulting in the offshore Gippsland Basin have also been observed, with contractional reactivation of previously normal faults during these times partially responsible for the formation of anticlinal hydrocarbon traps that host the Barracouta, Seahorse and Flying Fish hydrocarbon fields, adjacent to the Rosedale Fault System. A new method for interpreting leak-off test data demonstrates that the in-situ stress data from parts of the Otway and Gippsland basins can be reinterpreted to yield reverse fault stress regimes, consistent with the present-day tectonic setting of the basins. This reinterpretation has significant implications for petroleum exploration and development in the basins. In the Otway and Gippsland basins, wells drilled parallel to the orientation of the maximum horizontal stress (σH) represent the safest drilling directions for both borehole stability and fluid losses. Faults and fractures, striking northeast to southwest, previously believed to be at low risk of reactivation in a normal fault or strike-slip fault stress regime are now considered to be at high risk in the reinterpreted reverse fault stress regime.

Present-day tectonic stress regime in Egypt and surrounding area based on inversion of earthquake focal mechanisms

2013 ◽  
Vol 81 ◽  
pp. 1-15 ◽  
Author(s):  
H.M. Hussein ◽  
K.M. Abou Elenean ◽  
I.A. Marzouk ◽  
I.M. Korrat ◽  
I.F. Abu El-Nader ◽  
...  
Keyword(s):  
Tectonic Stress ◽  
Focal Mechanisms ◽  
Surrounding Area ◽  
Stress Regime ◽  
Earthquake Focal Mechanisms

Seismic doublets and a complex seismic sequence controlled by the rotation of the Juan Fernández microplate

2021 ◽  
Author(s):  
Simone Cesca ◽  
Carla Valenzuela Malebrán ◽  
José Ángel López-Comino ◽  
Timothy Davis ◽  
Carlos Tassara ◽  
...  
Keyword(s):  
Source Parameters ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Joint Analysis ◽  
Seismic Sequence ◽  
Local Data ◽  
Study Region ◽  
Coulomb Stress Changes ◽  
Earthquake Source Parameters ◽  
Stress Changes

&lt;p&gt; A complex seismic sequence took place in 2014 at the Juan Fern&amp;#225;ndez microplate, a small microplate located between Pacific, Nazca and Antarctica plates. Despite the remoteness of the study region and the lack of local data, we were able to resolve earthquake source parameters and to reconstruct the complex seismic sequence, by using modern waveform-based seismological techniques. The sequence started with an exceptional Mw 7.1-6.7 thrust &amp;#8211; strike slip earthquake doublet, the first subevent being the largest earthquake ever recorded in the region and one of the few rare thrust earthquakes in a region otherwise characterized by normal faulting and strike slip earthquakes. The joint analysis of seismicity and focal mechanisms suggest the activation of E-W and NE-SW faults or of an internal curved pseudofault, which is formed in response to the microplate rotation, with alternation of thrust and strike-slip earthquakes. Seismicity migrated Northward in its final phase, towards the microplate edge, where a second doublet with uneven focal mechanisms occurred. The sequence rupture kinematics is well explained by Coulomb stress changes imparted by the first subevent. Our analysis show that compressional stresses, which have been mapped at the northern boundary of the microplate, but never accompanied by large thrust earthquakes, can be accommodated by the rare occurrence of large, impulsive, shallow thrust earthquakes, with a considerable tsunamigenic potential.&lt;/p&gt;

scholarly journals Three-dimensional approach to understanding the relationship between the Plio-Quaternary stress field and tectonic inversion in the Triassic Cuyo Basin, Argentina

2015 ◽  
Vol 7 (1) ◽  
pp. 459-494
Author(s):  
L. Giambiagi ◽  
S. Spagnotto ◽  
S. M. Moreiras ◽  
G. Gómez ◽  
E. Stahlschmidt ◽  
...  
Keyword(s):  
Stress Field ◽  
Three Dimensional ◽  
Sedimentary Basins ◽  
Strike Slip ◽  
Stress Regime ◽  
Basin Inversion ◽  
Structural Style ◽  
Tectonic Inversion ◽  
The Impact ◽  
The Relationship

Abstract. The Cacheuta sub-basin of the Triassic Cuyo Basin is an example of rift basin inversion contemporaneous to the advance of the Andean thrust front, during the Plio-Quaternary. This basin is one of the most important sedimentary basins in a much larger Triassic NNW-trending depositional system along the southwestern margin of the Pangea supercontinent. The amount and structural style of inversion is provided in this paper by three-dimensional insights into the relationship between inversion of rift-related structures and spatial variations in late Cenozoic stress fields. The Plio-Quaternary stress field exhibits important N–S variations in the foreland area of the Southern Central Andes, between 33 and 34° S, with a southward gradually change from pure compression with σ1 and σ2 being horizontal, to a strike-slip type stress field with σ2 being vertical. We present a 3-D approach for studying the tectonic inversion of the sub-basin master fault associated with strike-slip/reverse to strike-slip faulting stress regimes. We suggest that the inversion of Triassic extensional structures, striking NNW to WNW, occurred during the Plio–Pleistocene in those areas with strike-slip/reverse to strike-slip faulting stress regime, while in the reverse faulting stress regime domain, they remain fossilized. Our example demonstrates the impact of the stress regime on the reactivation pattern along the faults.

scholarly journals A functional tool to explore the reliability of micro-earthquake focal mechanism solution for seismotectonic purposes

2021 ◽  
Author(s):  
Guido Maria Adinolfi ◽  
Raffaella De Matteis ◽  
Rita De Nardis ◽  
Aldo Zollo
Keyword(s):  
Focal Mechanism ◽  
Fault Plane ◽  
Focal Mechanisms ◽  
Seismic Network ◽  
Fault System ◽  
Focal Mechanism Solution ◽  
Fault Plane Solutions ◽  
Stress Regime ◽  
Focal Mechanism Solutions ◽  
Earthquake Focal Mechanism

Abstract. Improving the knowledge of seismogenic faults requires the integration of geological, seismological, and geophysical information. Among several analyses, the definition of earthquake focal mechanisms plays an essential role in providing information about the geometry of individual faults and the stress regime acting in a region. Fault plane solutions can be retrieved by several techniques operating in specific magnitude ranges, both in the time and frequency domain and using different data. For earthquakes of low magnitude, the limited number of available data and their uncertainties can compromise the stability of fault plane solutions. In this work, we propose a useful methodology to evaluate how well a seismic network used to monitor natural and/or induced micro-seismicity estimates focal mechanisms as function of magnitude, location, and kinematics of seismic source and consequently their reliability in defining seismotectonic models. To study the consistency of focal mechanism solutions, we use a Bayesian approach that jointly inverts the P/S long-period spectral-level ratios and the P polarities to infer the fault-plane solutions. We applied this methodology, by computing synthetic data, to the local seismic network operated in the Campania-Lucania Apennines (Southern Italy) to monitor the complex normal fault system activated during the Ms 6.9, 1980 earthquake. We demonstrate that the method we propose can have a double purpose. It can be a valid tool to design or to test the performance of local seismic networks and more generally it can be used to assign an absolute uncertainty to focal mechanism solutions fundamental for seismotectonic studies.

scholarly journals The 11 May 2011 earthquake at Lorca (SE Spain) viewed in a structural-tectonic context

2011 ◽  
Vol 3 (1) ◽  
pp. 527-540 ◽  
Author(s):  
R. L. M. Vissers ◽  
B. M. L. Meijninger
Keyword(s):  
Strike Slip ◽  
Focal Mechanisms ◽  
Strike Slip Fault ◽  
Betic Cordillera ◽  
Structural Studies ◽  
Directed Motion ◽  
Se Spain ◽  
Lorca Earthquake ◽  
Fault Belt

Abstract. The Lorca earthquake of 11 May 2011 in the Betic Cordillera of SE Spain occurred almost exactly on the Alhama de Murcia fault, a marked fault that forms part of a NE-SW trending belt of faults and thrusts. The fault belt is reminiscent of a strike-slip corridor, but recent structural studies have provided clear evidence for reverse motions on these faults. Focal mechanisms of the main earthquake but also of a foreshock are strikingly consistent with structural observations on the Alhama de Murcia fault. This strengthens the conclusion that, rather than a strike-slip fault, the fault is at present a contractional fault with an oblique reverse sense of motion, presumably in response to NW directed motion of Africa with respect to Europe.

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