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.

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.

The 2019–2020 Southwest Puerto Rico Earthquake Sequence: Seismicity and Faulting

2021 ◽  
Author(s):  
Blaž Vičič ◽  
Seyyedmaalek Momeni ◽  
Alessandra Borghi ◽  
Anthony Lomax ◽  
Abdelkrim Aoudia
Keyword(s):  
Puerto Rico ◽  
Template Matching ◽  
System Analysis ◽  
Near Field ◽  
Strike Slip ◽  
Earthquake Sequence ◽  
Fault System ◽  
Normal Faulting ◽  
Small Magnitude ◽  
Multiple Faults

Abstract The 2019–2020 Southwest Puerto Rico earthquake sequence ruptured multiple faults with several moderate magnitude earthquakes. Here, we investigate the seismotectonics of this fault system using high-precision hypocenter relocation and inversion of the near-field strong motions of the five largest events in the sequence (5.6≤Mw≤6.4) for kinematic rupture models. The Mw 6.4 mainshock occurred on a northeast-striking, southeast-dipping normal fault. The rupture nucleated offshore ∼15 km southeast of Indios at the depth of 8.6 km and extended southwest–northeast and up-dip with an average speed of 1.55 km/s, reaching the seafloor and shoreline after about 8 s. The 6 January 2020 (10:32:23) Mw 5.7 and the 7 January 2020 (11:18:46) Mw 5.8 events occurred on two east–southeast-striking, near-vertical, left-lateral strike-slip faults. However, the 7 January 2020 (08:34:05) Mw 5.6 normal-faulting aftershock, which occurred only 10 min after the Mw 6.4 normal-faulting mainshock, ruptured on a fault with almost the same strike as the mainshock but situated ∼8 km farther east, forming a set of parallel faults in the fault system. On 11 January 2020, an Mw 6.0 earthquake occurred on a north–northeast-striking, westing-dipping fault, orthogonal to the faults hosting the strike-slip earthquakes. We apply template matching for the detection of missed, small-magnitude earthquakes to study the spatial evolution of the main part of the sequence. Using the template-matching results along with Global Positioning System analysis, we image the temporal evolution of a foreshock sequence (Caja swarm). We propose that the swarm and the main sequence were a response to a tectonic transient that most affected the whole Puerto Rico Island.

Flower structures in sandstones of the Paleozoic Inkisi Group (Brazzaville, Republic of Congo): evidence for two major strike-slip fault systems and geodynamic implications

2020 ◽  
Vol 123 (4) ◽  
pp. 531-550
Author(s):  
H.M.D-V. Nkodia ◽  
T. Miyouna ◽  
D. Delvaux ◽  
F. Boudzoumou
Keyword(s):  
Strike Slip ◽  
Strike Slip Fault ◽  
Fault System ◽  
Republic Of Congo ◽  
Fault Systems ◽  
Major Fault ◽  
Major Strike ◽  
Flower Structures ◽  
Dextral Strike Slip ◽  
East West

Abstract Few studies have reported field descriptions of flower structures associated with strike-slip faults. This study describes and illustrates flower structures near Brazzaville (Republic of Congo) and explains their implication for the tectonic history of the Paleozoic Inkisi Group. Field observations show that the Inkisi Group is affected by two major strike-slip fault systems. The oldest system is dominated by north-northwest–south-southeast striking sinistral strike-slip faults and minor east–west striking dextral strike-slip faults. The youngest system consists of dominant northeast–southwest striking dextral strike-slip faults and minor northwest–southeast striking sinistral strike-slip faults. Flower structures within these major strike slip faults show four types of arrangements that likely depend on fault growth, propagation and damage zones: (i) flower structures associated with wall damage zones; (ii) flower structures associated with linking damage zones; (iii) flower structures associated with tip damage zones; and (iv) “hourglass” flower structures. Paleostress analysis reveals that both major fault systems originated from two differently oriented pure strike-slip regime stress stages. The first stage, which engendered the first major fault system, developed under northwest–southeast compression (i.e, σ1 = 322°). This phase probably coincided with north–south collision in the southern part of Gondwana in the Permo-Triassic and the Late Cretaceous compression times. The second stress stage, creating the second major fault system, developed under east–west (i.e, σ1 = 078°) compression. This phase is correlated with compression from the east–west opening of the Atlantic Ocean in the Miocene times.

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.

scholarly journals First evidence of active transpressive surface faulting at the front of the eastern Southern Alps, northeastern Italy. Insight on the 1511 earthquake seismotectonics

2018 ◽  
Author(s):  
Emanuela Falcucci ◽  
Maria Eliana Poli ◽  
Fabrizio Galadini ◽  
Giancarlo Scardia ◽  
Giovanni Paiero ◽  
...  
Keyword(s):  
Strike Slip ◽  
Southern Alps ◽  
Strike Slip Fault ◽  
Fault System ◽  
The Past ◽  
Fold And Thrust Belt ◽  
Northeastern Italy ◽  
Dextral Strike Slip Fault ◽  
Seismic Lines ◽  
Dextral Strike Slip

Abstract. We investigated the eastern corner of northeastern Italy, where the NW-SE trending dextral strike-slip fault systems of western Slovenia intersects the south-verging fold and thrust belt of the eastern Southern Alps . The area suffered the largest earthquakes of the region, among which are the 1511 (Mw 6.3) event and the two major shocks of the 1976 seismic sequence, with Mw = 6.4 and 6.1 respectively. The Colle Villano thrust and the Borgo Faris-Cividale strike-slip fault have been first analyzed by interpreting industrial seismic lines and then by performing morpho-tectonic and paleoseismological analyses. These different datasets indicate that the two structures define an active, coherent transpressive fault system that activated twice in the past two millennia, with the last event occurring around the 15th–17th century. The chronological information, and the location of the investigated fault system suggest its activation during the 1511 earthquake.

Quaternary and active faulting observed on the 1985 Academia Sinica - Royal Society Geotraverse of Tibet

1988 ◽  
Vol 327 (1594) ◽  
pp. 337-363 ◽  
Keyword(s):  
Average Rate ◽  
Strike Slip ◽  
Fault System ◽  
Ground Moraine ◽  
Normal Faulting ◽  
Stream Channels ◽  
Quaternary Period ◽  
Terminal Moraine ◽  
Northern Half ◽  
Southern Half

Active and recent faulting along the main north—south road in Tibet is dominated by normal faulting occurring on northerly-trending planes and by strike-slip faulting, both of which reflect an east-west extension of the plateau. Normal faulting is prevalent in the southern half of the plateau, but we saw no evidence for any major graben in the northern half. Strike-slip faulting on roughly easterly-trending structures is m ore prevalent in the northern half, but conjugate faulting, with right-lateral slip on northwesterly-trending planes and left-lateral slip on northeasterly-trending planes, is common in the southern half. In two areas, we also observed components of thrust faulting, apparently in association with young strikeslip faulting. Our most important results are bounds on the rates of slip on the two main strands of the Kunlun strike-slip fault system, which trends east-w est through the Kunlun range. Ground moraine containing boulders of pyroxenite is separated by 30 km from the nearest outcrop of such rock, implying that amount of displacement in the last 1.5 to 3 M a. Therefore the average rate of slip during the Quaternary period has been between 10 and 20 mm/a , with a likely value of 13 mm/a . Abundant fresh tension cracks and mole tracks imply continued slip on the main strand, the Xidatan -Tuosuohu-Maqu fault, and the likely occurrence of a major earthquake in the last few hundred years. Consistent offsets of gullies and dry stream channels of about 10 m may reflect slip of that amount during such an earthquake, and possible multiple offsets at one site suggest that slip may occur by large displacements of 10 m during infrequent great earthquakes. Along the other strand, the Kunlun Pass fault, offsets of roughly 50 to 150 m of, apparently, post-glacial valleys and of one glacier and its terminal moraine suggest a Holocene rate of slip between 5 and 20 mm/a , and most likely about 10 mm/a , on this fault. These rapid rates of displacement imply that Tibet is being extruded rapidly eastward, at a rate com parable to the rate at which India is penetrating into Eurasia, and therefore that, at present, a substantial fraction of this penetration is being absorbed by the eastward extrusion of Tibet.

The Border Ranges fault system in Glacier Bay National Park, Alaska: evidence for major early Cenozoic dextral strike-slip motion

1996 ◽  
Vol 33 (9) ◽  
pp. 1268-1282 ◽  
Author(s):  
Kevin J. Smart ◽  
Terry L. Pavlis ◽  
Virginia B. Sisson ◽  
Sarah M. Roeske ◽  
Lawrence W. Snee
Keyword(s):  
National Park ◽  
Strike Slip ◽  
Fault System ◽  
Glacier Bay ◽  
Oblique Convergence ◽  
Glacier Bay National Park ◽  
Wrangellia Terrane ◽  
Early Cenozoic ◽  
Dextral Strike Slip ◽  
Slip Motion

The Border Ranges fault system of southern Alaska, the fundamental break between the arc basement and the forearc accretionary complex, is the boundary between the Peninsular–Alexander–Wrangellia terrane and the Chugach terrane. The fault system separates crystalline rocks of the Alexander terrane from metamorphic rocks of the Chugach terrane in Glacier Bay National Park. Mylonitic rocks in the zone record abundant evidence for dextral strike-slip motion along north-northwest-striking subvertical surfaces. Geochronologic data together with regional correlations of Chugach terrane rocks involved in the deformation constrain this movement between latest Cretaceous and Early Eocene (~50 Ma). These findings are in agreement with studies to the northwest and southeast along the Border Ranges fault system which show dextral strike-slip motion occurring between 58 and 50 Ma. Correlations between Glacier Bay plutons and rocks of similar ages elsewhere along the Border Ranges fault system suggest that as much as 700 km of dextral motion may have been accommodated by this structure. These observations are consistent with oblique convergence of the Kula plate during early Cenozoic and forearc slivering above an ancient subduction zone following late Mesozoic accretion of the Peninsular–Alexander–Wrangellia terrane to North America.

scholarly journals A reappraisal of active tectonics along the Fethiye-Burdur trend, southwestern Turkey

2021 ◽  
Author(s):  
Edwin Nissen ◽  
Mussaver Didem Cambaz ◽  
Élyse Gaudreau ◽  
Andrew Howell ◽  
Ezgi Karasözen ◽  
...  
Keyword(s):  
Active Tectonics ◽  
Normal Fault ◽  
Recent Change ◽  
Strike Slip ◽  
Gravitational Potential Energy ◽  
Normal Faults ◽  
Normal Faulting ◽  
Arrival Times ◽  
Anatolian Plateau ◽  
Recent Earthquakes

We investigate active tectonics in southwestern Turkey along the trend between Fethiye, near the eastern end of the Hellenic subduction zone, and Burdur, on the Anatolian plateau. Previously, regional GPS velocity data have been used to propose either (1) a NE-trending zone of strike-slip faulting coined the Fethiye-Burdur Fault Zone, or (2) a mix of uniaxial and radial extension accommodated by normal faults with diverse orientations. We test these models against the available earthquake data, updated in light of recent earthquakes at Acıpayam (20 March 2019, Mw 5.6) and Bozkurt (8 August 2019, Mw 5.8) — the largest in this region in the last two decades — and at Arıcılar (24 November 2017, Mw 5.3). Using Sentinel-1 InSAR and seismic waveforms and arrival times, we show that the Acıpayam, Bozkurt and Arıcılar earthquakes were buried ruptures on pure normal faults with subtle or indistinct topographic expressions. By exploiting ray paths shared with these well-recorded modern events, we relocate earlier instrumental seismicity throughout southwestern Turkey. We find that the 1971 Mw 6.0 Burdur earthquake likely ruptured a NW-dipping normal fault in an area of indistinct geomorphology near Salda Lake, contradicting earlier studies that place it on well-expressed faults bounding the Burdur basin. Overall, the northern Fethiye-Burdur trend is characterized by orthogonal normal faulting, consistent with radial extension and likely responsible for the distinct physiography of Turkey's 'Lake District'. The southern Fethiye-Burdur trend is dominated by ESE-WNW trending normal faulting, even though most faults evident in the topography strike NE-SW. This hints at a recent change in regional strain, perhaps related to eastward propagation of the Gökova graben into the area or to rapid subsidence of the Rhodes basin. Overall, our results support GPS-derived tectonic models that depict a mix of uniaxial and radial extension throughout southwestern Turkey, with no evidence for major, active strike-slip faults anywhere along the Fethiye-Burdur trend. Normal faulting orientations are consistent with a stress field driven primarily by contrasts in gravitational potential energy between the elevated Anatolian plateau and the low-lying Rhodes and Antalya basins.

scholarly journals Pull-apart basin tectonic model is structurally impossible for Kashmir basin, NW Himalaya

2016 ◽  
Author(s):  
A. A. Shah
Keyword(s):  
Strike Slip ◽  
Strike Slip Fault ◽  
Fault System ◽  
Nw Himalaya ◽  
Tectonic Model ◽  
Pull Apart Basin ◽  
Piggyback Basin ◽  
Kashmir Basin ◽  
Dextral Strike Slip Fault ◽  
Dextral Strike Slip

Abstract. Kashmir Basin in NW Himalaya is considered a Neogene-Quatermary piggyback basin that was formed as result of the continent-continent collision of Indian and Eurasian plates. This model however is recently challenged by a pull-apart basin model, which argues that a major dextral strike-slip fault through Kashmir basin is responsible for its formation. And here it is demonstrated that the new tectonic model is structurally problematic, and conflicts with the geomorphology, geology, and tectonic setting of Kashmir basin. It also conflicts, and contradicts with the various structural features associated with a typical dextral strike-slip fault system where it shows that such a major structure cannot pass through the middle of the basin. It is demonstrated that such a structure is structurally, and kinematically impossible, and could not exist.

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