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

Solid Earth ◽  
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.

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 Present-day stress state in northwestern Syria

2020 ◽  
Vol 59 (4) ◽  
pp. 299-316
Author(s):  
Mohamad Khir Abdul-Wahed ◽  
Mohammed ALISSA
Keyword(s):  
Fault Plane ◽  
Eastern Mediterranean ◽  
Active Faults ◽  
Plate Boundary ◽  
Shear Zones ◽  
Stress Pattern ◽  
P Wave ◽  
Fault System ◽  
Fault Plane Solutions ◽  
Stress Regime

Northwestern Syria is a key area in the eastern Mediterranean to study the active tectonics and stress pattern across the Arabia-Eurasia convergent plate boundary. This study aims to outline the present-day stress regime in this region of Syria using the fault plane solutions of the largest events recorded by the Syrian National Seismological Network from 1995 to 2011. A dataset of fault-plane solutions was obtained for 48 events having at least 5 P-wave polarities. The tectonic regime for most of these events is extensional and produces normal mechanisms in agreement with the local configurations of the seismogenic faults in the region. Strike-slip mechanisms are more scarce and restricted to certain areas, such as the northern extension of the Dead Sea fault system. The results of the current study reveal the spatial variations of SHmax orientation across the northwestern Syria region. This spatial variation of the present-day stress field highlights the role of main geometrically complex shear zones in the present-day stress pattern of northwestern Syria. However, these results show, regardless of the relatively small magnitudes of the studied events, they provide a picture of the local stress deviations that have currently been taking place along the local active faults.

scholarly journals The key role of conjugate fault system in importing earthquakes into the eastern flank of the Red Sea

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Ali K. Abdelfattah ◽  
Chokri Jallouli ◽  
Mohamed Fnais ◽  
Saleh Qaysi ◽  
Hassan Alzahrani ◽  
...  
Keyword(s):  
Focal Mechanism ◽  
Red Sea ◽  
Vital Role ◽  
Fault System ◽  
Magnetic Data ◽  
Rift System ◽  
Stress Regime ◽  
Focal Mechanism Solutions ◽  
Eastern Flank ◽  
Conjugate Fault

AbstractThis study aims to synthesize seismic observations with gravity and magnetic data and to suggest a new scenario on the development of the Harrat Lunayyir (HL) tectonic system on the eastern Red Sea coastline, Saudi Arabia. Gravity and aeromagnetic anomalies distinctly mapped the NE and NW trends, while the InSAR data depict a small NW–SE graben and an NW–SE dyke. High-resolution relocations, which are well-consistent with the focal mechanism solutions for events with magnitudes greater than 3.0, admit two distinctly fault styles of different orientations. Thus, leading to the NE and NW fault planes’ reactivation related to the Precambrian basement faults and the Red Sea rift system, respectively. The spatiotemporal distributions of epicenters and focal mechanism solutions suggest a new seismic deformation scenario of the 2009 earthquake seismic activity. The low static frictions of 0.2–0.35 obtained from the stress inversion indicates reactivation of preexisting faults in the respective seismogenic zones. The obtained results give rise to a swarm-like sequence of tectonic implications, two activated fault styles differently oriented, and an NE conjugate fault system inherited in the region, which plays a vital role in transferring the ambient stress regime into the Red Sea’s eastern flank.

The North Gower, Ontario, Earthquake of 11 October 1983: Focal Mechanism and Aftershocks

1987 ◽  
Vol 58 (3) ◽  
pp. 65-72 ◽  
Author(s):  
Rutger Wahlstreöm
Keyword(s):  
Focal Mechanism ◽  
Fault Plane ◽  
Focal Depth ◽  
P Wave ◽  
Focal Mechanism Solution ◽  
Nodal Plane ◽  
Fault Plane Solutions ◽  
Stress Regime ◽  
Motion Data ◽  
The North

Abstract An earthquake near Ottawa (45.20°N, 75.75°W, focal depth 12 km) of unusually large size for the region's seisrhicity (mb(Lg) = 4.1) provided good P-wave first-motion data for a focal-mechanism solution. The maximum intensity was V(MM) and the area of perceptibility in Canada about 80,000 km2. The first and largest recorded aftershock occurred nine minutes after the main shock with magnitude mb(Lg) = 1.7. Two further small aftershocks ware recorded by a field network. The mechanism is thrust faulting with a predominantly horizontal pressure axis trending 154°. Thrust mechanisms have been found for other earthquakes in southeastern Canada and the northeastern U.S., but orientations of their stress axes are different, and so the North Gower earthquake may reflect a local and not a regional stress field. The nodal planes have strike 71°, dip 75°, and strike 221°, dip 17°. The spatial distribution of aftershocks suggests the gently-dipping nodal plane could be the fault plane. There is uncertainty about the seismotectonics of the region, and the orientation of neither nodal plane correlates with known geological features. More earthquake fault-plane solutions are required to interpret the seismotectonics and the stress regime.

scholarly journals 20 July 2017 Bodrum-Kos Earthquake (Mw:6.6) in southwestern Anatolia, Turkey

2021 ◽  
Vol 25 (3) ◽  
pp. 309-321
Author(s):  
Semir Över ◽  
Süha Özden ◽  
Esra Kalkan Ertan ◽  
Fatih Turhan ◽  
Zeynep Coşkun ◽  
...  
Keyword(s):  
Fault Plane ◽  
Regional Scale ◽  
Aegean Sea ◽  
Horizontal Stress ◽  
Focal Mechanisms ◽  
Stress Axis ◽  
Small Scale ◽  
Normal Type ◽  
Fault Plane Solutions ◽  
Stress Regime

In the Aegean Sea, the western part of Gökova Gulf, Kos and Bodrum were struck by a 6.6 (Mw) earthquake on July 20, 2017. The fault plane solution for the main shock shows an E-W striking normal type fault with approximately N-S (N4°E) tensional axis (T-axis). Fault plane solutions of 33 aftershocks show two groups of normal type fault with E-W and NE-SW to ENE-WSW orientations. The inversion of the focal mechanisms of the aftershocks yields two different normal faulting stress regimes: one is characterized by an approximately N-S (N5°E) σ3 axis (minimum horizontal stress axis). This extension is obtained from 13 focal mechanisms of aftershocks with approximately E-W direction. The other is characterized by approximately NW-SE (N330°E) σ3 axis. The latter is calculated from 21 seismic faults of aftershocks with approximately NE-SW direction. These aftershocks occurred on relatively small-scale faults that were directed from NE-SW to ENE-WSW, and possibly contributed to expansion of the basin in the west. The 24 focal mechanisms of earthquakes which occurred since 1933 in and around Gökova Basin are introduced into the inversion analysis to obtain the stress state effective in a wider region. The inversion yields an extensional stress regime characterized by an approximately N-S (N355°E) σ3 axis. The E-W directional metric faults, measured in the central part of Gökova Fault Zone bordering the Gökova Gulf in the north, also indicate N-S extension. The NE-SW extension obtained from NE-SW aftershocks appears to be more local and is responsible for the expansion of the western part of the asymmetric Gökova Basin. This N-S extension which appears to act on a regional-scale may be attributed to the geodynamic effects related to the combined forces of the southwestward extrusion of Anatolia and the roll-back process of African subduction beneath Anatolia.

Microearthquake seismicity and tectonics of Coastal Northern California

1970 ◽  
Vol 60 (5) ◽  
pp. 1669-1699 ◽  
Author(s):  
Leonardo Seeber ◽  
Muawia Barazangi ◽  
Ali Nowroozi
Keyword(s):  
High Frequency ◽  
Fault Plane ◽  
San Andreas Fault ◽  
High Gain ◽  
Strike Slip ◽  
Fault System ◽  
Northern California ◽  
Fault Plane Solutions ◽  
Sharp Bend ◽  
San Andreas

Abstract This paper demonstrates that high-gain, high-frequency portable seismographs operated for short intervals can provide unique data on the details of the current tectonic activity in a very small area. Five high-frequency, high-gain seismographs were operated at 25 sites along the coast of northern California during the summer of 1968. Eighty per cent of 160 microearthquakes located in the Cape Mendocino area occurred at depths between 15 and 35 km in a well-defined, horizontal seismic layer. These depths are significantly greater than those reported for other areas along the San Andreas fault system in California. Many of the earthquakes of the Cape Mendocino area occurred in sequences that have approximately the same magnitude versus length of faulting characteristics as other California earthquakes. Consistent first-motion directions are recorded from microearthquakes located within suitably chosen subdivisions of the active area. Composite fault plane solutions indicate that right-lateral movement prevails on strike-slip faults that radiate from Cape Mendocino northwest toward the Gorda basin. This is evidence that the Gorda basin is undergoing internal deformation. Inland, east of Cape Mendocino, a significant component of thrust faulting prevails for all the composite fault plane solutions. Thrusting is predominant in the fault plane solution of the June 26 1968 earthquake located along the Gorda escarpement. In general, the pattern of slip is consistent with a north-south crustal shortening. The Gorda escarpment, the Mattole River Valley, and the 1906 fault break northwest of Shelter Cove define a sharp bend that forms a possible connection between the Mendocino escarpment and the San Andreas fault. The distribution of hypocenters, relative travel times of P waves, and focal mechanisms strongly indicate that the above three features are surface expressions of an important structural boundary. The sharp bend in this boundary, which is concave toward the southwest, would tend to lock the dextral slip along the San Andreas fault and thus cause the regional north-south compression observed at Cape Mendocino. The above conclusions support the hypothesis that dextral strike-slip motion along the San Andreas fault is currently being taken up by slip along the Mendocino escarpment as well as by slip along northwest trending faults in the Gorda basin.

Aftershocks of the February 21, 1973 Point Mugu, California earthquake

1976 ◽  
Vol 66 (6) ◽  
pp. 1931-1952
Author(s):  
Donald J. Stierman ◽  
William L. Ellsworth
Keyword(s):  
Fault Zone ◽  
Main Shock ◽  
Fault Plane ◽  
Body Wave ◽  
P Wave ◽  
Fault System ◽  
Wave Radiation ◽  
Fault Plane Solutions ◽  
Complex Deformation ◽  
Transverse Ranges

abstract The ML 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P-wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.

Rapid earthquake source characterization in the context of tsunami early warning

2021 ◽  
Author(s):  
Alberto Armigliato ◽  
Martina Zanetti ◽  
Stefano Tinti ◽  
Filippo Zaniboni ◽  
Glauco Gallotti ◽  
...  
Keyword(s):  
Focal Mechanism ◽  
Early Warning ◽  
Fault Plane ◽  
Early Warning Systems ◽  
Slip Distribution ◽  
Generation Process ◽  
Tsunami Early Warning ◽  
Earthquake Focal Mechanism ◽  
Run Up ◽  
Seismic Networks

<p>It is well known that for earthquake-generated tsunamis impacting near-field coastlines the focal mechanism, the position of the fault with respect to the coastline and the on fault slip distribution are key factors in determining the efficiency of the generation process and the distribution of the maximum run-up and inundation along the nearby coasts. The time needed to obtain the aforementioned information from the analysis of seismic records is usually too long compared to the time required to issue a timely tsunami warning/alert to the nearest coastlines. In the context of tsunami early warning systems, a big challenge is hence to be able to define 1) the relative position of the hypocenter and of the fault and 2) the earthquake focal mechanism, based only on the preliminary earthquake localization and magnitude estimation, which are made available by seismic networks soon after the earthquake occurs.</p><p>In this study, the intrinsic unpredictability of the position of the hypocenter on the fault plane is studied through a probabilistic approach based on the analysis of two finite fault model datasets (SRCMOD and USGS) and by limiting the analysis to moderate-to-large shallow earthquakes (Mw  6 and depth  50 km). After a proper homogenization procedure needed to define a common geometry for all samples in the two datasets, the hypocentral positions are fitted with different probability density functions (PDFs) separately in the along-dip and along-strike directions.</p><p>Regarding the focal mechanism determination, different approaches have been tested: the most successful is restricted to subduction-type earthquakes. It defines average values and uncertainties for strike, dip and rake angles based on a combination of a proper zonation of the main tsunamigenic subduction areas worldwide and of subduction zone geometries available from publicdatabases.</p><p>The general workflow that we propose can be schematically outlined as follows. Once an earthquake occurs and the magnitude and hypocentral solutions are made available by seismic networks, it is possible to assign the focal mechanism by selecting the characteristic values for strike, dip and rake of the zone where the hypocenter falls into. Fault length and width, as well as the slip distribution on the fault plane, are computed through regression laws against magnitude proposed by previous studies. The resulting rectangular fault plane can be discretized into a matrix of subfaults: the position of the center of each subfault can be considered as a “realization” of the hypocenter position, which can then be assigned a probability. In this way, we can define a number of earthquake fault scenarios, each of which is assigned a probability, and we can run tsunami numerical simulations for each scenario to quantify the classical observables, such as water elevation time series in selected offshore/coastal tide-gauges, flow depth, run-up, inundation distance. The final results can be provided as probabilistic distributions of the different observables.</p><p>The general approach, which is still in a proof-of-concept stage, is applied to the 16 September 2015 Illapel (Chile) tsunamigenic earthquake (Mw = 8.2). The comparison with the available tsunami observations is discussed with special attention devoted to the early-warning perspective.</p>

scholarly journals Neogene-Quaternary tectonic regime and macroseismic observations in the Tyrnavos-Elassona broader epicentral area of the March 2021, intense earthquake sequence

2021 ◽  
Vol 58 ◽  
pp. 200
Author(s):  
Dimitrios Galanakis ◽  
Sotiris Sboras ◽  
Garyfalia Konstantopoulou ◽  
Markos Xenakis
Keyword(s):  
Normal Fault ◽  
Fault Scarp ◽  
Focal Mechanisms ◽  
Fault System ◽  
Spatiotemporal Evolution ◽  
Surface Rupture ◽  
Alluvial Deposits ◽  
Epicentral Area ◽  
Ground Shaking ◽  
Macroseismic Observations

On March 3, 2021, a strong (Mw6.3) earthquake occurred near the towns of Tyrnavos and Elassona. One day later (March 4), a second strong (Mw6.0) earthquake occurred just a few kilometres toward the WNW. The aftershock spatial distribution and the focal mechanisms revealed NW-SE-striking normal faulting. The focal mechanisms also revealed a NE-SW oriented extensional stress field, different from the orientation we knew so far (ca. N-S). The magnitude and location of the two strongest shocks, and the spatiotemporal evolution of the sequence, strongly suggest that two adjacent fault segments were ruptured respectively. The sequence was followed by several coseismic ground deformational phenomena, such as landslides/rockfalls, liquefaction and ruptures. The landslides and rockfalls were mostly associated with the ground shaking. The ruptures were observed west of the Titarissios River, near to the Quaternary faults found by bore-hole lignite investigation. In the same direction, a fault scarp separating the alpidic basement from the alluvial deposits of the Titarissios valley implies the occurrence of a well-developed fault system. Some of the ground ruptures were accompanied by extensive liquefaction phenomena. Others cross-cut reinforced concrete irrigation channels without changing their direction. We suggest that this fault system was partially reactivated, as a secondary surface rupture, during the sequence as a steeper splay of a deeper low-to-moderate angle normal fault.

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.

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