The Storfjorden, Svalbard, Earthquake Sequence 2008–2020: Transtensional Tectonics in an Arctic Intraplate Region

2021 ◽  
Author(s):  
Lars Ottemöller ◽  
Won-Young Kim ◽  
Felix Waldhauser ◽  
Norunn Tjåland ◽  
Winfried Dallmann
Keyword(s):  
Moment Tensor ◽  
Normal Component ◽  
Strike Slip ◽  
Earthquake Sequence ◽  
Fault System ◽  
Double Difference ◽  
Offshore Area ◽  
Svalbard Archipelago ◽  
Complex Fault ◽  
East West

Abstract An earthquake sequence in the Storfjorden offshore area southwest of Spitsbergen in the Svalbard archipelago initiated with a 21 February 2008 magnitude Mw 6.1 event. This area had previously not produced any significant earthquakes, but between 2008 and 2020, a total of ∼2800 earthquakes were detected, with ∼16 of them being of moderate size (ML≥4.0). Applying double-difference relocation to improve relative locations reveals that the activity is linked to several subparallel faults striking southwest–northeast that extend across the entire crust. The southwest–northeast trend is also found as a possible fault plane from regional moment tensor inversion. The solutions range from oblique normal in the center of the cluster to pure strike slip farther away and are consistent with the compressional σ1 axis roughly in the east–west direction and plunging 57°, and the extensional σ3 axis subhorizontal trending north–south. The mainshock fault is steeply dipping to the southeast, but several other faults appear to be near vertical. The existence of oblique, right-lateral strike-slip motion on southwest–northeast-trending faults with a normal component and pure normal faulting events in between suggests transtensional tectonics that in and around Storfjorden result in activation of a complex fault system.

scholarly journals The Alland earthquake sequence in Eastern Austria: Shedding light on tectonic stress geometry in a key area of seismic hazard

2019 ◽  
Vol 112 (2) ◽  
pp. 182-194
Author(s):  
Sven Schippkus ◽  
Helmut Hausmann ◽  
Zacharie Duputel ◽  
Götz Bokelmann ◽  
_ _
Keyword(s):  
Moment Tensor ◽  
Earthquake Sequence ◽  
Fault System ◽  
Double Difference ◽  
Reverse Fault ◽  
Vienna Basin ◽  
Regional Stress ◽  
3D Velocity Model ◽  
Waveform Cross Correlation ◽  
Eastern Austria

AbstractWe present our results on the fault geometry of the Alland earthquake sequence in eastern Austria (Eastern Alps) and discuss its implications for the regional stress regime and active tectonics. The series contains 71 known events with local magnitudes 0.1 ≤ ML ≤ 4.2 that occurred in between 2016 and 2017. We locate the earthquakes in a regional 3D velocity model to find absolute locations. These locations are then refined by relocating all events relative to each other using a double-difference approach, based on relative travel times measured from waveform cross-correlation and catalogue data. We also invert for the moment tensor of the ML = 4.2 mainshock by fitting synthetic waveforms to the recorded seismo-grams using a combination of the L1- and L2-norms of the waveform differences. Direct comparison of waveforms of the largest events in the sequence suggests that all of them ruptured with very similar mechanisms. We find that the sequence ruptured a reverse fault, that is dipping with ~30° towards ~north-northeast (NNE) at 6–7 km depth. This is supported by both the hypocentres and the mainshock source mechanism. The fault is most likely located in the buried basement of the Bohemian massif, the “Bohemian Spur”. This (reverse) fault has a nearly perpendicular orientation to the normal-fault structures of the Vienna Basin Transfer Fault System further east at a shallower depth, indicating a lateral stress decoupling that can also act as a vertical stress decoupling in some places. In the west, earthquakes (at a larger depth within the upper crust) show compressive stresses, whereas the Vienna Basin to the east shows extensional (normal-faulting) stress. This provides insight into the regional stress field and its spatial variation, and it helps to better understand earthquakes in the area, including the “1590 Ried am Riederberg” earthquake.

Seismotectonic Analysis of the 2019–2020 Puerto Rico Sequence: The Value of Absolute Earthquake Relocations in Improved Interpretations of Active Tectonics

2021 ◽  
Author(s):  
Copeland W. Cromwell ◽  
Kevin P. Furlong ◽  
Eric A. Bergman ◽  
Harley M. Benz ◽  
Will L. Yeck ◽  
...  
Keyword(s):  
Puerto Rico ◽  
Moment Tensor ◽  
Active Tectonics ◽  
Strike Slip ◽  
Fault System ◽  
Normal Faulting ◽  
Rupture Area ◽  
Extensional Faulting ◽  
Dextral Strike Slip ◽  
East West

Abstract We present a new catalog of calibrated earthquake relocations from the 2019–2020 Puerto Rico earthquake sequence related to the 7 January 2020 Mw 6.4 earthquake that occurred offshore of southwest Puerto Rico at a depth of 15.9 km. Utilizing these relocated earthquakes and associated moment tensor solutions, we can delineate several distinct fault systems that were activated during the sequence and show that the Mw 6.4 mainshock may have resulted from positive changes in Coulomb stress from earlier events. Seismicity and mechanisms define (1) a west–southwest (∼260°) zone of seismicity comprised of largely sinistral strike-slip and oblique-slip earthquakes that mostly occurs later in the sequence and to the west of the mainshock, (2) an area of extensional faulting that includes the mainshock and occurs largely within the mainshock’s rupture area, and (3) an north–northeast (∼30°)-striking zone of seismicity, consisting primarily of dextral strike-slip events that occurs before and following the mainshock and generally above (shallower than) the normal-faulting events. These linear features intersect within the Mw 6.4 mainshock’s fault plane in southwest Puerto Rico. In addition, we show that earthquake relocations for M 4+ normal-faulting events, when traced along their fault planes, daylight along east–west-trending bathymetric features offshore of southwest Puerto Rico. Correlation of these normal-faulting events with bathymetric features suggests an active fault system that may be a contributor to previously uncharacterized seismic hazards in southwest Puerto Rico.

Inversion of regional surface-wave spectra for source parameters of aftershocks from the 1992 Petrolia earthquake sequence

1995 ◽  
Vol 85 (3) ◽  
pp. 705-715
Author(s):  
Mark Andrew Tinker ◽  
Susan L. Beck
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Source Parameters ◽  
Strike Slip ◽  
Earthquake Sequence ◽  
Fault System ◽  
Accretionary Wedge ◽  
Wave Spectra ◽  
The North ◽  
Gorda Plate

Abstract Regional distance surface waves are used to study the source parameters for moderate-size aftershocks of the 25 April 1992 Petrolia earthquake sequence. The Cascadia subduction zone had been relatively seismically inactive until the onset of the mainshock (Ms = 7.1). This underthrusting event establishes that the southern end of the North America-Gorda plate boundary is seismogenic. It was followed by two separate and distinct large aftershocks (Ms = 6.6 for both) occurring at 07:41 and 11:41 on 26 April, as well as thousands of other small aftershocks. Many of the aftershocks following the second large aftershock had magnitudes in the range of 4.0 to 5.5. Using intermediate-period surface-wave spectra, we estimate focal mechanisms and depths for one foreshock and six of the larger aftershocks (Md = 4.0 to 5.5). These seven events can be separated into two groups based on temporal, spatial, and principal stress orientation characteristics. Within two days of the mainshock, four aftershocks (Md = 4 to 5) occurred within 4 hr of each other that were located offshore and along the Mendocino fault. These four aftershocks comprise one group. They are shallow, thrust events with northeast-trending P axes. We interpret these aftershocks to represent internal compression within the North American accretionary prism as a result of Gorda plate subduction. The other three events compose the second group. The shallow, strike-slip mechanism determined for the 8 March foreshock (Md = 5.3) may reflect the right-lateral strike-slip motion associated with the interaction between the northern terminus of the San Andreas fault system and the eastern terminus of the Mendocino fault. The 10 May aftershock (Md = 4.1), located on the coast and north of the Mendocino triple junction, has a thrust fault focal mechanism. This event is shallow and probably occurred within the accretionary wedge on an imbricate thrust. A normal fault focal mechanism is obtained for the 5 June aftershock (Md = 4.8), located offshore and just north of the Mendocino fault. This event exhibits a large component of normal motion, representing internal failure within a rebounding accretionary wedge. These two aftershocks and the foreshock have dissimilar locations in space and time, but they do share a north-northwest oriented P axis.

Aftershocks and surface faulting associated with the intraplate Guinea, West Africa, earthquake of 22 December 1983

1987 ◽  
Vol 77 (5) ◽  
pp. 1579-1601
Author(s):  
C. J. Langer ◽  
M. G. Bonilla ◽  
G. A. Bollinger
Keyword(s):  
West Africa ◽  
Focal Mechanism ◽  
Main Shock ◽  
Strike Slip ◽  
Fault System ◽  
Epicentral Area ◽  
Lateral Strike Slip ◽  
Focal Mechanism Solutions ◽  
Short Period ◽  
East West

Abstract This study reports on the results of geological and seismological field studies conducted following the rare occurrence of a moderate-sized West African earthquake (mb = 6.4) with associated ground breakage. The epicentral area of the northwestern Guinea earthquake of 22 December 1983 is a coastal margin, intraplate locale with a very low level of historical seismicity. The principal results include the observation that seismic faulting occurred on a preexisting fault system and that there is good agreement among the surface faulting, the spatial distribution of the aftershock hypocenters, and the composite focal mechanism solutions. We are not able, however, to shed any light on the reason(s) for the unexpected occurrence of this intraplate earthquake. Thus, the significance of this study is its contribution to the observational datum for such earthquakes and for the seismicity of West Africa. The main shock was associated with at least 9 km of surface fault-rupture. Trending east-southeast to east-west, measured fault displacements up to ∼13 cm were predominantly right-lateral strike slip and were accompanied by an additional component (5 to 7 cm) of vertical movement, southwest side down. The surface faulting occurred on a preexisting fault whose field characteristics suggest a low slip rate with very infrequent earthquakes. There were extensive rockfalls and minor liquefaction effects at distances less than 10 km from the surface faulting and main shock epicenter. Main shock focal mechanism solutions derived from teleseismic data by other workers show a strong component of normal faulting motion that was not observed in the ground ruptures. A 15-day period of aftershock monitoring, commencing 22 days after the main shock, was conducted. Eleven portable, analog short-period vertical seismographs were deployed in a network with an aperture of 25 km and an average station spacing of 7 km. Ninety-five aftershocks were located from the more than 200 recorded events with duration magnitudes of about 1.5 or greater. Analysis of a selected subset (91) of those events define a tabular aftershock volume (26 km long by 14 km wide by 4 km thick) trending east-southeast and dipping steeply (∼60°) to the south-southwest. Composite focal mechanisms for groups of events, distributed throughout the aftershock volume, exhibit right-lateral, strike-slip motion on subvertical planes that strike almost due east. Although the general agreement between the field geologic and seismologic results is good, our preferred interpretation is for three en-echelon faults striking almost due east-west.

Analysis of the 15 December 2017 Mw 6.5 and the 23 January 2018 Mw 5.9 Java Earthquakes

2020 ◽  
Vol 110 (6) ◽  
pp. 3050-3063
Author(s):  
Anne Meylani Magdalena Sirait ◽  
Anne S. Meltzer ◽  
Felix Waldhauser ◽  
Joshua C. Stachnik ◽  
Daryono Daryono ◽  
...  
Keyword(s):  
Moment Tensor ◽  
Fluid Pressure ◽  
Fault System ◽  
Double Difference ◽  
Northern Coast ◽  
Eurasian Plate ◽  
Slab Geometry ◽  
Plate Interface ◽  
Upward Migration ◽  
Ground Shaking

ABSTRACT The west part of Java sits at the transition from oblique subduction of the Australian plate under the Sunda block of the Eurasian plate along Sumatra to orthogonal convergence along central and eastern Java. This region has experienced several destructive earthquakes, the 17 July 2006 Mw 7.7 earthquake and tsunami off the coast of Pangandaran and the 2 September 2009 Mw 7 earthquake, located off the coast of Tasikmalaya. More recently, on 15 December 2017, an Mw 6.5 earthquake occurred off the coast near Pangandaran, and, on 23 January 2018, an Mw 5.9 earthquake occurred offshore Lebak, between Pelabuhan Ratu and Ujung Kulon. Ground shaking and damage occurred locally and in Jakarta on the northern coast of Java. In this study, we use the double-difference technique to relocate both mainshocks and 10 months of seismicity (228 events) following the earthquakes. The relocation result improved the mainshock locations and depth distribution of earthquakes. Moment tensor of the December 2017 event located the hypocenter at ∼108  km depth within the subducting slab. The best-fit relocation places the depth at 61 km, close to the slab interface. Aftershocks occur between 68 and 86 km depth and align along a steeper plane than slab geometry models. The January 2018 event is located at ∼46  km depth. Aftershocks form a near-vertical, pipe-like structure from the plate interface to ∼10  km depth. A burst of aftershocks immediately following the mainshock shows a shallowing upward trend at a rate of ∼2  km/hr, suggesting that a fluid pressure wave released from the oceanic crust is causing brittle failure in the overriding plate, followed by upward migration of fluids. Five months later, shallow (<25  km) seismicity collocates with background seismicity, suggesting the January 2018 event activated the Pelabuhan Ratu fault system close to the coast.

scholarly journals Deformation associated with sliver transport in Costa Rica: seismic and geodetic observations of the July 2016 Bijagua earthquake sequence

2019 ◽  
Vol 220 (1) ◽  
pp. 585-597 ◽  
Author(s):  
Maria C Araya ◽  
Juliet Biggs
Keyword(s):  
Surface Deformation ◽  
Ground Deformation ◽  
Surface Displacement ◽  
Strike Slip ◽  
Central American ◽  
Earthquake Sequence ◽  
Fault System ◽  
Aseismic Slip ◽  
Volcanic Arc ◽  
Forearc Sliver

SUMMARY Tectonic slivers form in the overriding plate in regions of oblique subduction. The inner boundaries of the sliver are often poorly defined and can consist of well-defined faults, rotating blocks or diffuse fault systems, which pass through or near the volcanic arc. The Guanacaste Volcanic Arc Sliver (GVAS) as defined by Montero et al., is a segment of the Central American Forearc Sliver, whose inner boundary is the ∼87-km-long Haciendas-Chiripa fault system (HCFS), which is located ∼10 km behind the volcanic arc and consists of strike slip faults and pull apart steps. We characterize the current ground motion on this boundary by combining earthquake locations and focal mechanisms of the 2016 Bijagua earthquake sequence, with the surface ground deformation obtained from Interferometric Synthetic Aperture Radar (InSAR) images from the ALOS-2 satellite. The coseismic stack of interferograms show ∼6 cm of displacement towards the line of sight of the satellite between the Caño Negro fault and the Upala fault, indicating uplift or SE horizontal surface displacement. The largest recorded earthquake of the sequence was Mw 5.4, and the observed deformation is one of the smallest earthquakes yet detected by InSAR in the Central American region. Forward and inverse models show the surface deformation can be partially explained by slip on a single fault, but it can be better explained by slip along two faults linked at depth. The best-fitting model consists of 0.33 m of right lateral slip on the Caño Negro fault and 0.35 m of reverse slip on the Upala fault, forming a positive flower structure. As no reverse seismicity was recorded, we infer the slip on the Upala fault occurred aseismically. Observations of the Bijagua earthquake sequence suggests the forearc sliver boundary is a complex and diffuse fault system. There are localized zones of transpression and transtension and areas where there is no surface expression suggesting the fault system is not yet mature. Although aseismic slip is common on subduction interfaces and mature strike-slip faults, this is the first study to document aseismic slip on a continental tectonic sliver boundary fault.

scholarly journals Machine-Learning-Based High-Resolution Earthquake Catalog Reveals How Complex Fault Structures Were Activated during the 2016–2017 Central Italy Sequence

2021 ◽  
Vol 1 (1) ◽  
pp. 11-19
Author(s):  
Yen Joe Tan ◽  
Felix Waldhauser ◽  
William L. Ellsworth ◽  
Miao Zhang ◽  
Weiqiang Zhu ◽  
...  
Keyword(s):  
Central Italy ◽  
Normal Fault ◽  
Earthquake Sequence ◽  
Fault System ◽  
Earthquake Catalog ◽  
Earthquake Triggering ◽  
Aftershock Activity ◽  
Arrival Times ◽  
Complex Fault ◽  
Fault Structures

Abstract The 2016–2017 central Italy seismic sequence occurred on an 80 km long normal-fault system. The sequence initiated with the Mw 6.0 Amatrice event on 24 August 2016, followed by the Mw 5.9 Visso event on 26 October and the Mw 6.5 Norcia event on 30 October. We analyze continuous data from a dense network of 139 seismic stations to build a high-precision catalog of ∼900,000 earthquakes spanning a 1 yr period, based on arrival times derived using a deep-neural-network-based picker. Our catalog contains an order of magnitude more events than the catalog routinely produced by the local earthquake monitoring agency. Aftershock activity reveals the geometry of complex fault structures activated during the earthquake sequence and provides additional insights into the potential factors controlling the development of the largest events. Activated fault structures in the northern and southern regions appear complementary to faults activated during the 1997 Colfiorito and 2009 L’Aquila sequences, suggesting that earthquake triggering primarily occurs on critically stressed faults. Delineated major fault zones are relatively thick compared to estimated earthquake location uncertainties, and a large number of kilometer-long faults and diffuse seismicity were activated during the sequence. These properties might be related to fault age, roughness, and the complexity of inherited structures. The rich details resolvable in this catalog will facilitate continued investigation of this energetic and well-recorded earthquake sequence.

Linking dynamic earthquake rupture to tsunami modeling for the Húsavík-Flatey transform fault system in North Iceland

2021 ◽  
Author(s):  
Fabian Kutschera ◽  
Sara Aniko Wirp ◽  
Bo Li ◽  
Alice-Agnes Gabriel ◽  
Benedikt Halldórsson ◽  
...  
Keyword(s):  
Strike Slip ◽  
Strike Slip Fault ◽  
Fault System ◽  
Deformation Pattern ◽  
Tsunami Modeling ◽  
Earthquake Rupture ◽  
Worst Case ◽  
Fault Systems ◽  
Complex Fault ◽  
The Impact

<p>Earthquake generated tsunamis are generally associated with large submarine events on dip-slip faults, in particular on subduction zone megathrusts (Bilek and Lay, 2018). Submerged ruptures across strike-slip fault systems mostly produce minor vertical offset and hence no significant disturbance of the water column. For the 2018 Mw 7.5 Sulawesi earthquake in Indonesia, linked dynamic earthquake rupture and tsunami modeling implies that coseismic, mixed strike-slip and normal faulting induced seafloor displacements were a critical component generating an unexpected and devastating local tsunami in Palu Bay (Ulrich et al., 2019), with important implications for tsunami hazard assessment of submarine strike-slip fault systems in transtensional tectonic settings worldwide. </p><p>We reassess the tsunami potential of the ~100 km Húsavík Flatey Fault (HFF) in North Iceland using physics-based, linked earthquake-tsunami modelling. The HFF consists of multiple fault segments that localise both strike-slip and normal movements, agreeing with a transtensional deformation pattern (Garcia and Dhont, 2005). The HFF hosted several historical earthquakes with M>6. It crosses from off-shore to on-shore in immediate proximity to the town of Húsavík. We analyse simple and complex fault geometries and varying hypocenter locations accounting for newly inferred fault geometries (Einarsson et al., 2019), 3-D subsurface structure (Abril et al., 2020), bathymetry and topography of the area, primary stress orientations and the stress shape ratio constrained by the inversion of earthquake focal mechanisms (Ziegler et al., 2016).</p><p>Dynamic rupture models are simulated with SeisSol (https://github.com/SeisSol/SeisSol), a scientific open-source software for 3D dynamic earthquake rupture simulation (www.seissol.org, Pelties et al., 2014). SeisSol, a flagship code of the ChEESE project (https://cheese-coe.eu), enables us to explore simple and complex fault and subsurface geometries by using unstructured tetrahedral meshes. The dynamically adaptive, parallel software sam(oa)²-flash (https://gitlab.lrz.de/samoa/samoa) is used for tsunami propagation and inundation simulations and solves the hydrostatic shallow water equations (Meister, 2016). We consider the contribution of the horizontal ground deformation of realistic bathymetry to the vertical displacement following Tanioka and Satake, 1996. The tsunami simulations use time-dependent seafloor displacements to initialise bathymetry perturbations. </p><p>We show that up to 2 m of vertical coseismic offset can be generated during dynamic earthquake rupture scenarios across the HFF, which resemble historic magnitudes and are controlled by spontaneous fault interaction in terms of dynamic and static stress transfer and rupture jumping across the complex fault network. Our models reveal rake deviations from pure right-lateral strike-slip motion, indicating the presence of dip-slip components, in combination with large shallow fault slip (~8 m for a hypocenter in the East), which can cause a sizable tsunami affecting North Iceland. Sea surface height (ssh), which is defined as the deviation from the mean sea level, and inundation synthetics give an estimate about the impact of the tsunami along the coastline. We further investigate a physically plausible worst-case scenario of a tsunamigenic HFF event, accounting for tsunami sourcing mechanisms similar to the one causing the Sulawesi Tsunami in 2018.</p>

scholarly journals SEISMOTECTONICS OF THE SOUTHERN MARMARA REGION, NW TURKEY

2017 ◽  
Vol 50 (1) ◽  
pp. 173
Author(s):  
H. Yalçın ◽  
A. Kürçer ◽  
M. Utkucu ◽  
L. Gülen
Keyword(s):  
Stress Tensor ◽  
Moment Tensor ◽  
Strike Slip ◽  
Tensor Analysis ◽  
Fault System ◽  
Earthquake Risk ◽  
Seismic Gap ◽  
Marmara Region ◽  
Nw Turkey ◽  
Earthquake Moment

The Southern Marmara Region is an active deformation area, which is a transition zone between the strike-slip tectonics manifested by the North Anatolian Fault System and the N-S extensional regime of the Aegean Region. We have reviewed tectonic and geological structure of the region, based onseismological studies. We have obtained a total of 37 earthquake moment tensor solutions between 1953 and 2015. In addition, stress tensor analysis has been carried out using 37 earthquake moment tensor solutions. Also long term seismicity were investigated and a,b, Mc values were calculated and mapped. Moment tensor solutions indicate that the source of these earthquakes are mostly NE-trending dextral strike-slip faults and some of them are E-W trending dip-slip normal faults. The stress tensor analysis shows that the direction of the regional compressive stress is NW-SE. The temporal and spatial distrubution of the large earthquakes (1944, 1953, 1964) indicate that the ruptures unilaterally propagate from SW to NE. The 1855 earthquake had been occurred to the east of Manyas Lake. The elapsed time (160 year) and regional stress transfer suggest that the segments to the east of Manyas Lake form a probable seismic gap and this area has a high earthquake risk.

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