The focal mechanisms and depths of small earthquakes as determined from Rayleigh-wave radiation patterns

1979 ◽  
Vol 69 (5) ◽  
pp. 1379-1390
Author(s):  
Alan L. Kafka ◽  
Donald J. Weidner
Keyword(s):  
Body Wave ◽  
Moment Tensor ◽  
Focal Mechanisms ◽  
Wave Radiation ◽  
Seismic Moment Tensor ◽  
Plate Boundaries ◽  
Single Event ◽  
Event Pair ◽  
Wave Data ◽  
Amplitude Data

abstract Many earthquakes of body-wave magnitude less than about 5.5 are located in key positions for defining tectonic processes at plate boundaries and in the interiors of plates. Due to the sparsity of observed body-wave data from these events, their focal mechanisms and depths cannot be well determined from body-wave data alone. Earthquakes with mb as low as 4.5 radiate surface-wave trains which can be observed at teleseismic distances. In this study, Rayleigh waves observed at WWSSN stations have been inverted to obtain focal mechanisms and depths of two small earthquakes (mb = 4.7 and mb = 5.1) in the Caribbean plate region. An event pair inversion patterned after Weidner and Aki (1973) is contrasted with a single event inversion which makes use of the seismic moment tensor formalism (Mendiguren, 1977; Gilbert, 1970) to linearize the relationship between observed signal and model. The event pair approach requires master events whose focal parameters are well known and which are located near the smaller events. The single event approach, on the other hand, does not require master events, but is limited to amplitude data only. Phase information can be utilized by considering a pair of events located near each other and recovering the difference in phase for the event pair. For the events studied, the single event analysis yields the same solution as the event pair analysis. In general, amplitude is more sensitive to mechanism than phase. However, noise in amplitude increases more rapidly than noise in phase as the size of the event is reduced.

scholarly journals A focal mechanism catalogue of earthquakes that occurred in the southeastern Alps and surrounding areas from 1928–2019

2021 ◽  
Vol 13 (5) ◽  
pp. 2245-2258
Author(s):  
Angela Saraò ◽  
Monica Sugan ◽  
Gianni Bressan ◽  
Gianfranco Renner ◽  
Andrea Restivo
Keyword(s):  
Focal Mechanism ◽  
Moment Tensor ◽  
Active Regions ◽  
Focal Mechanisms ◽  
Seismic Moment Tensor ◽  
Fault Plane Solutions ◽  
First Arrival ◽  
Populated Region ◽  
Surrounding Areas ◽  
Catalogue Of Earthquakes

Abstract. We present a focal mechanism catalogue of earthquakes that occurred in the southeastern Alps and surrounding areas from 1928 to 2019. The area involved in the process of convergence between the Adria microplate and Eurasia is one of the most seismically active regions in the Alpine Belt. The seismicity is minor, with the Ms= 6.5 Friuli earthquake being the strongest event recorded in the area, but the seismic risk is relevant because it is a highly populated region. For this reason, numerous studies have been carried out over time to investigate the stress field and the geodynamic characteristics of the region using focal mechanisms. To provide a comprehensive set of revised information, which is challenging to build quickly because the data are dispersed over many papers, we collected and revised the focal mechanisms that were previously published in the literature. Additionally, depending on the data quality and availability, we computed new focal mechanisms by first arrival polarity inversion or seismic moment tensor. Finally, we merged all the fault plane solutions to obtain a catalogue for a selection of 772 earthquakes with 1.8≤M≤6.5. For each earthquake, we reported all the available focal mechanisms obtained by different authors. We also suggested a preferred solution for users who need information provided expeditiously. The catalogue (https://doi.org/10.5281/zenodo.4660412; Saraò et al., 2021) is given as the Supplement of this paper and will be updated periodically (https://doi.org/10.5281/zenodo.4284970).

Methods for determining focal mechanisms in laboratory experiments

2021 ◽  
Author(s):  
Natalia Shikhova ◽  
Andrey Patonin
Keyword(s):  
Laboratory Experiments ◽  
High Efficiency ◽  
Moment Tensor ◽  
Focal Mechanisms ◽  
Half Period ◽  
Seismic Moment Tensor ◽  
Incoming Wave ◽  
Directional Diagram ◽  
P Waves ◽  
First Arrival

<p>In laboratory experiments, acoustic emission (AE) caused by the deformation of geomaterial reflects changes in the strength and stress state of the sample. By analogy with the solution of focal mechanisms of earthquake sources, there are several methods for determining the mechanisms and types of AE sources using the amplitudes and signs of the first arrival of an elastic wave on sensors that register acoustic signals. With 16 receiving acoustic sensors, the number of polarity determinations of the incoming wave usually does not exceed 5-10, while the sign determination on some sensors is often incorrect due to the omission of the first half-period of the weak signal by the automatic registration algorithm. This reduces the reliability of determining the mechanism of the focus in laboratory tests of rocks by wellknown methods based on the distribution of signs of the first arrival of the AE wave. We propose a method for determining the directions of the axes and the values of compression and tension in the AE source. The algorithm uses information about the coordinates of events and receivers, values of amplitudes and signs of the first half-period of P-waves coming to the receivers. In this case, the model of the AE source is assumed as a quadrupole with compression and tension axes. The source-receiver distance, the directional diagram of the receiver, and the emission diagram of the source are taken into account for each of the receivers to calculate the value of displacements in the source. To test the proposed algorithm and compare it with the known methods, there was developed a program for generating an acoustic signal source of a given type with random coordinates and directions of the compression and tension axes. An array of signs and amplitudes of the first arrivals coming to the receivers was calculated from simulated data. The high efficiency of the proposed algorithm was shown. The usage of this method together with the determination of AE event types [Zang et.al., 1998] in real laboratory experiments allows us to characterize the prevailing processes of destruction during separate phases of the experiment on triaxial loading of rocks in more detail. The developed algorithm makes it possible to determine the directions of the axes and the values of compression-tension with a minimum number of signs of the arrivals of P- waves, to estimate the components of the seismic moment tensor and obtain more complete information about the mechanism of the AE source.</p><p>The work was supported partly by the mega-grant program of the Russian Federation Ministry of Science and Education under the project no. 14.W03.31.0033 and partly by the state assignment of the Ministry to IPE RAS.</p>

scholarly journals A focal mechanism catalogue of earthquakes that occurred in the southeastern Alps and surrounding areas from 1928–2019

2021 ◽  
Author(s):  
Angela Saraò ◽  
Monica Sugan ◽  
Gianni Bressan ◽  
Gianfranco Renner ◽  
Andrea Restivo
Keyword(s):  
Focal Mechanism ◽  
Moment Tensor ◽  
Active Regions ◽  
Focal Mechanisms ◽  
Seismic Moment Tensor ◽  
Fault Plane Solutions ◽  
First Arrival ◽  
Populated Region ◽  
Surrounding Areas ◽  
Catalogue Of Earthquakes

Abstract. We present a focal mechanism catalogue of earthquakes that occurred in the southeastern Alps and surrounding areas from 1928 to 2019. The area involved in the process of convergence between the Adria microplate and Eurasia is one of the most seismically active regions in the Alpine Belt. The seismicity is minor, with the Ms =thinsp;6.5 Friuli earthquake being the strongest event recorded in the area, but the seismic hazard is relevant because it is a highly populated region. For this reason, numerous studies have been carried out over time to investigate the stress field and the geodynamic characteristics of the region using focal mechanisms. To provide a comprehensive set of revised information, which is challenging to build quickly because the data is dispersed over many papers, we collected and revised the focal mechanisms that were previously published in the literature. Additionally, depending on the data quality and availability, we computed new focal mechanisms by first arrival polarity inversion or seismic moment tensor. Finally, we merged all the fault plane solutions to obtain a catalogue for a selection of 772 earthquakes with 1.8thinsp;≤thinsp;Mthinsp;≤thinsp;6.5. For each earthquake, we reported all the available focal mechanisms obtained by different authors. However, we also suggested a preferred solution for users who need expeditious information. The catalogue is available at https://doi.org/10.5281/zenodo.4284971 (Saraò et al., 2020).

scholarly journals BAIKAL and TRANSBAIKALIA

2020 ◽  
pp. 130-139
Author(s):  
V. Melnikova ◽  
N. Gileva ◽  
A. Seredkina ◽  
O. Masalskii
Keyword(s):  
Seismic Moment ◽  
Rift Zone ◽  
Moment Tensor ◽  
Normal Fault ◽  
Large Earthquake ◽  
Baikal Rift Zone ◽  
Focal Mechanisms ◽  
P Wave ◽  
Seismic Moment Tensor ◽  
High Level

We considered the character of the seismic process in the Baikal and Transbaikalia region in 2014. 8782 earthquakes with КР≥5.6 were recorded within the study territory during that year, 94 % of them were located in the Baikal rift zone. 26 seismic events were felt in the cities, towns and local settlements with the intensity not exceeding 5. The strongest of them (Mw=5.5) occurred in the Baikal-Muya region of the Baikal rift zone and was followed by a large earthquake sequence. Focal mechanisms were determines for 46 shocks from the data on P-wave first motion polarities, seismic moment tensor (focal mechanism, scalar seismic moment (M0), moment magnitude (Mw) and hypocentral depth (h)) was calculated for 11 events from the data on amplitude surface wave spectra. It has been found that normal fault movements are realized in the sources of 59 % of the earthquakes with the obtained focal mechanisms. In general, high level of seismic activity is a characteristic feature of the considered territory in 2014.

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