The Seismogenic Structure of the 2017 Mw 6.9 Milin, Tibet, Earthquake: A Possible Newly Active Fault at the Eastern Himalayan Syntaxis

Abstract On 18 November 2017, an Mw 6.9 earthquake occurred in Milin, Tibet, with the epicenter at the top of the Namche Barwa syntaxis. This event did not produce surface ruptures, and its seismogenic structure remains unclear or controversial. Using the locations of the Milin mainshock and aftershocks, locations of regional small earthquakes and focal mechanism solutions from 2007 to 2009, this work analyzed the causative fault and tectonic setting of the Milin earthquake and assessed the regional seismic risk. The results suggest that the seismogenic structure of the Milin earthquake was a secondary fault, the southern branch of the XiXingla fault (XXLF). Within 28 hr after the mainshock, the aftershocks of the Milin event spread northeastward to the secondary north branch fault of the XXLF and the secondary south branch fault of the Palong–Pangxin fault. Across the top of the Namche Barwa syntaxis (Namche Barwa block) and the Chayu block in the southeast, an earthquake dense belt (EDB) has developed. This EDB has similar deep structures beneath the two blocks, in which several northeast-dipping structural planes exit, and different portions of the EDB imply a unified tectonic stress field. Combining these data with the foreshock–mainshock–aftershock data for the 1950 Mw 8.6 Chayu, Tibet, earthquake, we speculate that the structural planes produced by the EDB at depth in the two blocks have already been connected or tended to connect, resulting in a new fault system trending northwest and approximately 280 km long. The 2017 Mw 6.9 Milin earthquake occurred at the northwestern end of this fault system. At present, the development stage, maturity, and fine structure of this new fault system remain unclear but should receive additional attention. Based on its maximum rupture area, this new fault system is capable of generating an Mw 7.7 earthquake in the future.

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

10.5194/se-2021-88 ◽

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.

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa0770051579 ◽

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.

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Recent earthquake sequences at Coso: Evidence for conjugate faulting and stress loading near a geothermal field

Bulletin of the Seismological Society of America ◽

10.1785/bssa0890030785 ◽

1999 ◽

Vol 89 (3) ◽

pp. 785-795 ◽

Cited By ~ 1

Author(s):

Joydeep Bhattacharyya ◽

Susanna Gross ◽

Jonathan Lees ◽

Mike Hastings

Keyword(s):

Geothermal Field ◽

Fault System ◽

Fault Plane Solutions ◽

Temporal Decay ◽

Background Seismicity ◽

Surface Ruptures ◽

Temporal And Spatial Characteristics ◽

Coso Geothermal Field ◽

The One ◽

Earthquake Sequences

Abstract Two recent earthquake sequences near the Coso geothermal field show clear evidence of faulting along conjugate planes. We present results from analyzing an earthquake sequence occurring in 1998 and compare it with a similar sequence that occurred in 1996. The two sequences followed mainshocks that occurred on 27 November 1996 and 6 March 1998. Both mainshocks ruptured approximately colocated regions of the same fault system. Following a comparison with the background seismicity of the Coso region, we have detected evidence of stress loading within the geothermal field that appears to be in response to the 1998 earthquakes. The ML = 5.2 mainshock in the 1998 sequence occurred at 5:47 a.m. UTC and was located approximately 45 km north of the town of Ridgecrest in the Coso range. The mainshock of the 1996 sequence had an ML magnitude of 5.3. There have been no observable surface ruptures associated with either of these sequences. Though the mainshocks for both sequences were located about 900 m apart and have nearly the same local magnitudes, the sequences differ in both their temporal and spatial characteristics. An analysis of the fault-plane solutions of the mainshocks and the aftershock locations suggests that the two sequences ruptured fault planes that are perpendicular to one another. We observe a much faster temporal decay of the 1998 sequence compared to the one in 1996; moreover, while the 1996 sequence was not followed by any sizeable (i.e., ML > 4.0) aftershocks, the 1998 sequence had four such events. From an estimate of the tectonic stressing rate on the fault that produced the 1998 sequence, we infer a repeat cycle of 135 years for an earthquake of comparable magnitude at Coso.

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Roman to Middle Age Earthquakes Sourced by the 1980 Irpinia Fault: Historical, Archaeoseismological, and Paleoseismological Hints

Geosciences ◽

10.3390/geosciences10080286 ◽

2020 ◽

Vol 10 (8) ◽

pp. 286

Author(s):

Paolo Galli

Keyword(s):

Middle Age ◽

Fault System ◽

Historical Period ◽

Seismogenic Structure ◽

Working Hypothesis ◽

First Millennium ◽

The Common ◽

Common Era ◽

Written Sources

The Italian seismic compilations are among the most complete and back-in time extended worldwide, with earthquakes on record even before the Common Era. However, we have surely lost the memory of dozen strong events of the historical period, mostly in the first millennium CE. Given the lack of certain or conclusive written sources, besides paleoseismological investigations, a complementary way to infer the occurrence of lost earthquakes is to cross-check archaeoseismic evidence from ancient settlements. This usually happens by investigating collapses/restorations/reconstructions of buildings, the general re-organization of the urban texture, or even the abrupt abandonment of the settlement. Exceptionally, epigraphs mentioning more or less explicitly the effects of the earthquake strengthened the field working hypothesis. Here, I deal with both paleoseismological clues from the Monte Marzano Fault System (the structure responsible for the catastrophic, Mw 6.9 1980 earthquake) and archaeoseismological evidence of settlements founded in its surroundings to cast light on two poorly known earthquakes that occurred at the onset and at the end of the first millennium CE, likely in 62 and in 989 CE. Both should share the same seismogenic structure and the size of the 1980 event (Mw 6.9).

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Surface Faulting Caused by the 2016 Central Italy Seismic Sequence: Field Mapping and LiDAR/UAV Imaging

Earthquake Spectra ◽

10.1193/111417eqs236mr ◽

2018 ◽

Vol 34 (4) ◽

pp. 1585-1610 ◽

Cited By ~ 11

Author(s):

Stefano Gori ◽

Emanuela Falcucci ◽

Fabrizio Galadini ◽

Paolo Zimmaro ◽

Alberto Pizzi ◽

...

Keyword(s):

Imaging Techniques ◽

Central Italy ◽

Normal Fault ◽

Fault System ◽

Surface Rupture ◽

Field Mapping ◽

Central Italy Earthquake ◽

Levels Of Detail ◽

Main Fault ◽

Surface Ruptures

The three mainshock events (M6.1 24 August, M5.9 26 October, and M6.5 30 October 2016) in the Central Italy earthquake sequence produced surface ruptures on known segments of the Mt. Vettore–Mt. Bove normal fault system. As a result, teams from Italian national research institutions and universities, working collaboratively with the U.S. Geotechnical Extreme Events Reconnaissance Association (GEER), were mobilized to collect perishable data. Our reconnaissance approach included field mapping and advanced imaging techniques, both directed towards documenting the location and extent of surface rupture on the main fault exposure and secondary features. Mapping activity occurred after each mainshock (with different levels of detail at different times), which provides data on the progression of locations and amounts of slip between events. Along the full length of the Mt. Vettore–Mt. Bove fault system, vertical offsets ranged from 0–35 cm and 70–200 cm for the 24 August and 30 October events, respectively. Comparisons between observed surface rupture displacements and available empirical models show that the three events fit within expected ranges.

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Paleo-surface ruptures at both sides of a pressure ridge along Alhama de Murcia Fault (SE Spain)

10.5194/egusphere-egu2020-3117 ◽

2020 ◽

Author(s):

Octavi Gómez-Novell ◽

María Ortuño ◽

Julián García-Mayordomo ◽

Eulàlia Masana ◽

Thomas Rockwell ◽

...

Keyword(s):

Active Faults ◽

Great Part ◽

Slip Rate ◽

Fault System ◽

Radiocarbon Dates ◽

Seismic Parameters ◽

Central Segment ◽

Pressure Ridge ◽

Combining Data ◽

Surface Ruptures

The Alhama de Murcia Fault (AMF) is one of the most seismically active faults in the Iberian Peninsula, with important associated historical and instrumental seismicity (e.g. the 1674 IEMS VIII and 2011 Mw 5.1 Lorca earthquakes), and numerous geomorphic and paleoseismic evidence of paleoearthquakes. It is an oblique left-lateral strike slip fault within the Eastern Betics Shear Zone (EBSZ), a nearly 500 km long fault system that absorbs a great part of convergence between the Nubian and Eurasian plates. Previous paleoseismic studies have mainly focused on the southwestern and especially the central segment of the fault and yielded slip rate values ranging from 1.0 up to 1.7 mm/yr. In the central segment (Lorca-Totana), the fault splays into several branches, the two frontal ones forming a pressure ridge. Paleoseismic trenches have exclusively been dug in the northwestern fault of the pressure ridge, where most of the displacement is along strike, while the expected reverse southeastern branch has never been directly observed.We present the first results of paleoseismic trenching across a complete transect of the pressure ridge in the Lorca-Totana segment of AMF. To do so we excavated an exceptionally large trench (7 m deep) in the NW branch and 5 trenches in the SE branch. We have been able to: a) extend the paleoearthquake catalogue in the NW branch by interpreting a total of 13 paleoearthquakes, 6 of which were not identified in previous studies. A restoration analysis has been performed; b) unveil the existence and recent activity (Holocene) of the thrust that bounds the pressure ridge to the SE. We have interpreted at least 5 surface ruptures, with the last one being younger than 8-9 kyr BP, based on new radiocarbon dates.The study of these two sites allows for the refinement of the seismic parameters of the fault, formerly inferred from the study of a single branch. In this sense, the more complete paleoearthquake catalogue will allow for reassessment of the recurrence intervals assigned to the fault and new slip rate estimates will be inferred by combining data from the two studied sites. Furthermore, forthcoming OSL dates may allow us to prove or reject the synchronicity of surface ruptures on both sides of the pressure ridge, shedding light on the rupturing style of this fault system during the Late Quaternary. We discuss how these new data on fault-interaction may affect several seismic parameters and their repercussion in source modelling for fault-based probabilistic seismic hazard assessments (PSHA) of the region.

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Seismic and Geodetic Crustal Moment-Rates Comparison: New Insights on the Seismic Hazard of Egypt

Applied Sciences ◽

10.3390/app11177836 ◽

2021 ◽

Vol 11 (17) ◽

pp. 7836

Author(s):

Rashad Sawires ◽

José A. Peláez ◽

Federica Sparacino ◽

Ali M. Radwan ◽

Mohamed Rashwan ◽

...

Keyword(s):

Active Regions ◽

Seismic Source ◽

Satellite System ◽

Dead Sea ◽

Fault System ◽

Gulf Of Aqaba ◽

Seismic Sources ◽

Earthquake Catalog ◽

Focal Mechanism Solutions ◽

Rate Ratios

A comparative analysis of geodetic versus seismic moment-rate estimations makes it possible to distinguish between seismic and aseismic deformation, define the style of deformation, and also to reveal potential seismic gaps. This analysis has been performed for Egypt where the present-day tectonics and seismicity result from the long-lasting interaction between the Nubian, Eurasian, and Arabian plates. The data used comprises all available geological and tectonic information, an updated Poissonian earthquake catalog (2200 B.C.–2020 A.D.) including historical and instrumental datasets, a focal-mechanism solutions catalog (1951–2019), and crustal geodetic strains from Global Navigation Satellite System (GNSS) data. The studied region was divided into ten (EG-01 to EG-10) crustal seismic sources based mainly on seismicity, focal mechanisms, and geodetic strain characteristics. The delimited seismic sources cover the Gulf of Aqaba–Dead Sea Transform Fault system, the Gulf of Suez–Red Sea Rift, besides some potential seismic active regions along the Nile River and its delta. For each seismic source, the estimation of seismic and geodetic moment-rates has been performed. Although the obtained results cannot be considered to be definitive, among the delimited sources, four of them (EG-05, EG-06, EG-08, and EG-10) are characterized by low seismic-geodetic moment-rate ratios (<20%), reflecting a prevailing aseismic behavior. Intermediate moment-rate ratios (from 20% to 60%) have been obtained in four additional zones (EG-01, EG-04, EG-07, and EG-09), evidencing how the seismicity accounts for a minor to a moderate fraction of the total deformational budget. In the other two sources (EG-02 and EG-03), high seismic-geodetic moment-rates ratios (>60%) have been observed, reflecting a fully seismic deformation.

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

Solid Earth ◽

10.5194/se-13-65-2022 ◽

2022 ◽

Vol 13 (1) ◽

pp. 65-83

Author(s):

Guido Maria Adinolfi ◽

Raffaella De Matteis ◽

Rita de Nardis ◽

Aldo Zollo

Keyword(s):

Focal Mechanism ◽

Fault Plane ◽

Normal Fault ◽

Focal Mechanisms ◽

Seismic Network ◽

Fault System ◽

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 a 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 operating in the Campania–Lucania Apennines (southern Italy) aimed to monitor the complex normal fault system activated during the Ms 6.9, 1980 earthquake. We demonstrate that the method we propose is effective and can be adapted for other case studies with 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.

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Late Eocene – Early Miocene facies and stratigraphic development, Taranaki Basin, New Zealand: the transition to plate boundary tectonics during regional transgression

Geological Magazine ◽

10.1017/s0016756818000997 ◽

2019 ◽

Vol 156 (10) ◽

pp. 1751-1770 ◽

Cited By ~ 3

Author(s):

Dominic P. Strogen ◽

Karen E. Higgs ◽

Angela G. Griffin ◽

Hugh E. G. Morgans

Keyword(s):

New Zealand ◽

Tectonic Setting ◽

Plate Boundary ◽

Late Eocene ◽

Early Miocene ◽

Fault System ◽

Initial Development ◽

Early Oligocene ◽

Well Data ◽

Latest Eocene

AbstractEight latest Eocene to earliest Miocene stratigraphic surfaces have been identified in petroleum well data from the Taranaki Basin, New Zealand. These surfaces define seven regional sedimentary packages, of variable thickness and lithofacies, forming a mixed siliciclastic–carbonate system. The evolving tectonic setting, particularly the initial development of the Australian–Pacific convergent margin, controlled geographic, stratigraphic and facies variability. This tectonic signal overprinted a regional transgressive trend that culminated in latest Oligocene times. The earliest influence of active compressional tectonics is reflected in the preservation of latest Eocene – Early Oligocene deepwater sediments in the northern Taranaki Basin. Thickness patterns for all mid Oligocene units onwards show a shift in sedimentation to the eastern Taranaki Basin, controlled by reverse movement on the Taranaki Fault System. This resulted in the deposition of a thick sedimentary wedge, initially of coarse clastic sediments, later carbonate dominated, in the foredeep close to the fault. In contrast, Oligocene active normal faulting in a small sub-basin in the south may represent the most northerly evidence for rifting in southern Zealandia, related to Emerald Basin formation. The Early Miocene period saw a return to clastic-dominated deposition, the onset of regional regression and the southward propagation of compressional tectonics.

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Late Quaternary Tectonic Activity of the Udine-Buttrio Thrust, Friulian Plain, NE Italy

Geosciences ◽

10.3390/geosciences10020084 ◽

2020 ◽

Vol 10 (2) ◽

pp. 84

Author(s):

Andrea Viscolani ◽

Christoph Grützner ◽

Manuel Diercks ◽

Klaus Reicherter ◽

Kamil Ustaszewski

Keyword(s):

Late Quaternary ◽

Tectonic Setting ◽

Surface Expression ◽

Vertical Surface ◽

Fault System ◽

Tectonic Activity ◽

Morphological Evidence ◽

Seismic Reflection Data ◽

Reflection Data ◽

Ne Italy

The NW-SE trending Udine-Buttrio Thrust is a partly blind fault that affects the Friulian plain southeast of Udine in NE Italy. It is part of a wider fault system that accommodates the northward motion of the Adriatic plate. Although seismic reflection data and morphological evidence show that the fault was active during the Quaternary, comparably little is known about its tectonic activity. We used high-resolution digital elevation models to investigate the surface expression of the fault. Measured vertical surface offsets show significant changes along strike with uplift rates varying between 0 and 0.5 mm/yr. We then analyze a topographic scarp near the village of Manzano in more detail. Field mapping and geophysical prospections (Georadar and Electrical Resistivity Tomography) were used to image the subsurface geometry of the fault. We found vertical offsets of 1–3 m in Natisone River terraces younger than 20 ka. The geophysical data allowed the identification of deformation of the fluvial sediments, supporting the idea that the topographic scarp is a tectonic feature and that the terraces have been uplifted systematically over time. Our findings fit the long-term behaviour of the Udine-Buttrio Thrust. We estimate a post-glacial vertical uplift rate of 0.08–0.17 mm/yr recorded by the offset terraces. Our results shed light on the Late Quaternary behaviour of this thrust fault in the complicated regional tectonic setting and inform about its hitherto overlooked possible seismic hazard.

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