Surface Wave Amplitude Spectra Research Articles

SEISMICITY of the BAIKAL REGION and TRANSBAIKALIA in 2018–2019

We considered a character of the seismic process in the Baikal region and Transbaikalia in 2018–2019. During this time period, 14545 earthquakes with КR≥5.6 were registered. More than 94 % of the seismic events were localized in the Baikal rift zone. Within it, relatively strong earthquakes occurred in Khubsugul-Tunka area (March 29, 2019, Mw=4.8) and in the junction zone of the northeastern flank of the Baikal rift and the Olekma-Stanovoy orogenic system (September 28, 2019, Mw=5.1). 49 earthquakes caused felt macroseismic effects which most often manifested themselves in the settlements of the region with intensity of 3–4 and 4. Intensity of 5–6 was observed in few cases. Earthquake source parameters were determined for 81 seismic events with КR≥9.1. Most of them occurred in the seismotectonic regime of subhorizontal extension. 61 focal mechanisms were determined from P-wave first-motion polarities. Source parameters for 13 earthquakes were estimated using a joint inversion of P-wave first-motion polarities and surface wave amplitude spectra. In general, 2018–2019 are characterized by a moderate level of seismic activity.

SEISMICITY of the BAIKAL REGION and TRANSBAIKALIA in 2016–2017

We consider a character of the seismic process in the Baikal region and Transbaikalia in 2016–2017. During the considered period, 17869 earthquakes with KR≥5.6 were registered; 95 % of these seismic events were localized in the Baikal rift zone. The largest earthquakes occurred at the north-eastern flank of the Baikal rift on November 22, 2016 (KR=13.9, Mw=5.0) and April 3, 2017 (KR=13.5, Mw=4.8). Felt effects, not exceeding 5, were observed for 47 regional earthquakes. Focal mechanisms were determined from P-wave first-motion polarities for 73 seismic events (KR≥9.4) and seismic moment tensors (scalar seismic moments, moment magnitudes, focal mechanisms) were calculated from surface wave amplitude spectra and regional P-wave first-motion polarities for 12 seismic events (KR≥11.9, Mw≥4.2). It was found that normal faults and combined types of motions were realized in sources of 94 % of the earthquakes with the obtained focal mechanisms. In general, moderate seismic activity was observed in 2016–2017.

THE SEPTEMBER 2, 2015, КR=14.0, Mw=5.1, I0=7–8 TALLAY EARTHQUAKE at the NORTH-EASTERN FLANK of the BAIKAL RIFT

We consider September 2, 2015, Mw=5.1 Tallay earthquake occurred in the previously aseismic area of the North-Muya Ridge adjoining to the Muya-Kuanda basin from the north. Instrumental and macroseismic data on this seismic event are presented. Its seismic moment tensor is calculated from surface wave amplitude spectra. New data on strong ground motions are obtained within the north-eastern flank of the Baikal rift. The Tallay earthquake is found to be connected with seismogenic renewal of the second-order multidirectional faults activated in the rift stress field.

Source process of the September 21, 2020 Mw 5.6 Bystraya earthquake at the South-Eastern segment of the Main Sayan fault (Eastern Siberia, Russia)

We study the Mw 5.6 Bystraya earthquake, occurred on September 21, 2020 at the SW flank of the Baikal rift zone in the area, in which only few small seismic events have been detected during the whole period of instrumental observations. Source parameters of the mainshock and its largest aftershock (Mw 4.7) are inverted from intermediate-period surface wave amplitude spectra. Hypocentral depths and seismic moment tensors in a double-couple approximation are calculated for both earthquakes, while integral source characteristics, describing its spatio-temporal development, are estimated only for the mainshock. For the Bystraya earthquake, directivity effects (rupture direction and velocity) are also modelled. The obtained results show that the study seismic event can be associated with activation of the SE segment of the Main Sayan fault. The faults of the mainshock and its largest aftershock are formed under the influence of the NW-SE extension and NE-SW compression. The predominance of left-lateral strike-slip motions agrees well with previous studies at the SW flank of the Baikal rift zone. A small thrust-fault component could reflect the inversion uplift of some blocks of the Bystraya basin bottom as well as the Bystrinskaya Sopka massif. The dipping of the earthquake fault plane could indicate flattening of the considered fault segment at some depth or the existence of differently oriented structural discontinuities inside the fault zone of a finite width. The long rupture of the Bystraya earthquake could be connected with its almost pure strike-slip focal mechanism and orientation of the rupture plane approximately along the Main Sayan fault strike.

STRONG SEISMIC EVENTS on the SOUTH-WESTERN FLANK of the BAIKAL RIFT in 2014: November 1, 2014, Кp=13.6, Mw=4.6, I0=7–8 URIK EARTHQUAKE and December 5, 2014, Кp=13.9, Mw=4.9, I0=7–8 HOVSGOL EARTHQUAKE

We consider two earthquakes occurred at the south-western flank of the Baikal rift zone (BRZ): Urik, November 1, 2014 (Mwreg=4.6) and Hovsgol, December 5, 2014 (Mwreg=4.9). First of them is localized within the area of the Main Sayan fault, the second one is located at the north of the Hovsgol Lake. Seismic moment tensors (focal mechanisms, scalar seismic moments, moment magnitudes and hypocentral depths) of the study seismic events were calculated based on surface wave amplitude spectra. Earthquake hypocenters were found to be situated in the middle crust (h=14–21 km). Both events occurred under the strike-slip stress-strain field. The strike-slip was combined with a normal fault component in the source of the Urik earthquake and with a thrust fault component in the source of the Hovsgol earthquake. In both cases, shaking intensity in the nearest settlements (=42–124 km) was less than 4–5. Analysis of historical seismicity, seismological data on the Urik and Hovsgol earthquakes and the tectonic position of their sources demonstrates that the considered events are typical for the south-western flank of the BRZ and confirms the existence of the transition zone from rift structures at the central parts of the BRZ to regional compression structures in Northern Mongolia.

MUYAKAN-II EARTHQUAKE on MAY 23, 2014 with КР=14.3, Mw=5.5, I0=7–8 (Northern Baikal region)

We consider the May 23, 2014 Muyakan earthquake (Mw=5.5) occurred in the Muyakan Range at the north-east of the Baikal rift zone near the eastern tunnel portal of the Baikal-Amur Mainline. This event was followed by numerous aftershocks (КР=5.6–9.9) which number exceeded 2000 by the end of the year. Spatio-temporal analysis of the Muyakan seismic sequence shows that its epicentral field consists of two isolated clusters: eastern and western ones. All the main events including the foreshocks, main shock and the strongest aftershocks (Mw=4.4; 4.5) occurred in the eastern cluster while only small seismic events (КР<10.0) were recorded in the western one. Seismic moment tensor was calculated for the Muyakan earthquake from surface wave amplitude spectra. As a result, we obtained information about the rift type focal mechanism, scalar seismic moment (M0=1.9•1017 Nm), moment magnitude (Mw=5.5) and hypocentral depth (h=24 km). From regional data, hypocentral depths of the main shock and the major number of the following earthquakes (80 %) were distributed in the depth range h=3–11 km. Maximum intensity of the main shock (4–5 according to MSK-64) was felt in Severomuysk urban village (=29 km). The obtained results could be used for seismic hazard assessment of the crucial part of the Baikal-Amur Mainline.

BOGUCHAN EARTHQUAKE on JANUARY 17, 2014 with КР=13.3, Mw=4.3, I0=7 (Siberian Platform)

We consider the Boguchan January 17, 2014 earthquake(Mw=4.3) occurred on the Siberian Platform in the area of the sublongitudinal part of the Angara River. Instrumental and macroseismic data on this seismic event, tectonic features and deep structure of the crust within its source area are analyzed. Seismic moment tensor and hypocentral depth of the earthquake are calculated from its surface wave amplitude spectra. It has been shown that the concentration and relaxation of the tectonic stress in the source area are likely to be conditioned by velocity heterogeneities in the upper and middle crust. The obtained facts evidence that the study earthquake, most probably, is not connected with filling of the Angara cascade of water reservoirs.

Geological, Geophysical and Macroseismic Data on the September 28, 2019, Mw= 5.1 Earthquake That Occurred at the Northeast of the Baikal Rift

This paper considers the September 28, 2019 earthquake that occurred on the northeastern flank of the Baikal Rift Zone (BRZ), called the Lenger earthquake after its location. The seismic moment tensor and hypocentral depth of the event are calculated from data on surface wave amplitude spectra with additional information on P-wave first-arrival polarities recorded at regional stations. The hypocentral depth is 11–12 km, the moment magnitude Mw = 5.1, and the scalar seismic moment M0 = 5 × 1016 N m. The earthquake source is formed under the influence of horizontal submeridional extension and subhorizontal NE–SW compression. The latter approximately coincides with the azimuth of the fast S-wave velocity in the upper mantle obtained from SKS-splitting and surface wave tomography data. The Lenger earthquake was felt at epicentral distances of more than 600 km. Comparison of its macroseismic effects with effects of other strong seismic events in the study area demonstrates that the regional macroseismic field equation should be actualized for the territory of the northeastern flank of the BRZ. Analysis of the obtained data, along with the available information on the stress-strain field of the crust of this area indicates that the geodynamical pattern of the conjunction zone of the northeastern flank of the BRZ and the Olekma–Stanovoi orogenic system influences the low-active Transbaikalia block. However, due to low tectonic activity, the tectonic stress accumulation period sufficient for the occurrence of relatively strong seismic events is larger than in regions with high-level tectonic activity.

The March 29, 2019 Earthquake in the Western Part of the Tunka Rift Basin System: Source Parameters and Macroseismic Effects

The paper considers one of the perceptible earthquakes that occurred on March 29, 2019, in the western part of the Tunka rift basin system. The epicenter of this seismic event was localized within the Hoytogol basin. The earthquake source parameters were as follows: hypocentral depth (h = 36–37 km), scalar seismic moment (M0 = 1.6 × 1016 N m), moment magnitude (Mw = 4.8), and focal mechanism—were determined using joint inversion of surface wave amplitude spectra and polarities of P-wave first arrivals. The focal mechanism demonstrates the predominance of combined thrust and strike-slip movements, indicating uplifting tectonic movements in the Hoytogol basin. The shaking from the earthquake was felt within a vast area, with an intensity of up to I = III at epicentral distances up to 490 km. Macroseismic data were obtained using different methods, including an interactive questionnaire hosted on the website of the Baikal Branch of GS RAS. Analysis of the macroseismic data made it possible to formulate the goal of refining the regional macroseismic field equation.

New data on earthquake focal mechanisms in the Laptev Sea region of the Arctic-Asian seismic belt

We consider 16 earthquakes with Mw = 4.2–5.2 that occurred in the south-eastern part of the Laptev Sea shelf, Lena River Delta, and North Verkhoyanye (Russia) in 1990–2014. Focal mechanisms, scalar seismic moments, moment magnitudes, and hypocentral depths of the seismic events have been calculated from the data on amplitude spectra of surface waves and P wave first-motion polarities. The obtained results sufficiently implement the existing dataset on reliable earthquake source parameters for the study region and prove the change of the stress-strain state of the crust from extension on the Laptev Sea shelf to compression on the continent providing finer spatial details of the deformation field in the transition zones such as Buor-Khaya Bay and the Lena River Delta.

The earthquake of July 22, 2011 (Mw = 4.5) in a low-seismicity area of the Argun region

The paper considers the Argun earthquake of July 22, 2011 (M w = 4.5), which occurred in the Argun River valley in a low-seismicity territory in China. The focal parameters of the earthquake (depth of the hypocenter, moment magnitude, scalar seismic moment, and focal mechanism) were determined by calculating the seismic moment tensor from the amplitude spectra of surface waves and the data on the signs of the first arrivals of body waves at regional stations. The solution of the focal mechanism makes it possible to assume a relationship between the earthquake focus and a fault with a northeastern strike bordering the southeastern side of the Argun Basin (in Chinese territory). The Argun earthquake was felt in Russia with an intensity of II–III to V at the epicentral distances up to 255 km. The intensity of shaking did not exceed values suggested by new GSZ-2012 and GSZ-2014 seismic zoning maps of Russian territory. Nevertheless, the question on the possible occurrence of stronger earthquakes in the studied region remains open.

The February 1, 2011 Mw 4.7 earthquake: Evidence of local extension in western Transbaikalia (Eastern Siberia)

We consider the rare February 1, 2011 earthquake Mw 4.7 that occurred in the low active region of western Transbaikalia, Russia. Its epicenter relates to the Zagan metamorphic core complex (MCC). From geological data, MCCs are characterized by signs of regional extension. We calculated earthquake source parameters (hypocentral depth, moment magnitude, scalar seismic moment and focal mechanism) from the data on amplitude spectra of surface waves and the first body-wave arrivals recorded on regional stations. The results obtained show that the focus of this event was formed in the conjunction zone between the low-angle dipping zone of plastic flow (detachment) included in the structure of the Zagan MCC and the listric fault related to the adjacent basin. A normal fault focal mechanism proves the processes of horizontal extension near the MCC, with one nodal plane being low-angle dipping (dip 35°) that agrees with the dip of the detachment zone. As long as this zone is characterized by high rates of tectonic deformation, we suppose that normal-fault displacement in the earthquake origin is carried out along the low-angle dipping rupture plane. Taking into account that in the territory of western Transbaikalia, compression and strike-slip regimes of seismotectonic deformations dominate, we suppose that the extension in the focus of the earthquake under study has a local character, and is caused by the structure of the Zagan MCC.

The Zagan earthquake of February 1, 2011, in the low-seismoactive zone of western Transbaikalia: Observations and analysis

We described the February 1, 2011 (Mw = 4.7) Zagan earthquake occurred in the area of the Zagan range. This event is one of the most significant in western Transbaikalia. Macroseismic effects of this seismic event were felt over a wide territory: the intensity of II was observed at the distance of more than 300 km. Earthquake focal parameters (hypocentral depth, seismic moment, moment magnitude and focal mechanism) calculated from the data on surface wave amplitude spectra correspond to the structural position of the origin connected with a zone of plastic flow (slip) bordering the northwest slope of the Zagan range (similar complex of the metamorphic core). The results obtained prove the existence of the continental extension processes in this area.

The M w 4.3 January 17, 2014, earthquake: very rare seismic event on the Siberian platform

We described the 2014 January 17 earthquake (M w = 4.3) occurred on the Siberian platform in the area of sublongitudinal part of the Angara river in the zone of possible influence of two large reservoirs—Ust-Ilimsk and Boguchan. This is the first event of such magnitude recorded in this previously aseismic area during the whole period of instrumental observations. A seismic moment, a moment magnitude, a hypocentral depth, and a focal mechanism of the event were calculated on the basis of surface wave amplitude spectra. Analysis of the geological and geophysical data showed that the earthquake origin is connected with high-velocity gradient zone located at the border of the Late Proterozoic cover and the Precambrian basement of the Siberian platform. Some evidences for a natural character of the earthquake were considered.

Seismotectonics of the Armutlu peninsula (Marmara Sea, NW Turkey) from geological field observation and regional moment tensor inversion

The Armutlu peninsula, located in the eastern Marmara Sea, coincides with the western end of the rupture of the 17 August 1999, İzmit MW 7.6 earthquake which is the penultimate event of an apparently westward migrating series of strong and disastrous earthquakes along the NAFZ during the past century. We present new seismotectonic data of this key region in order to evaluate previous seismotectonic models and their implications for seismic hazard assessment in the eastern Marmara Sea. Long term kinematics were investigated by performing paleo strain reconstruction from geological field investigations by morphotectonic and kinematic analysis of exposed brittle faults. Short term kinematics were investigated by inverting for the moment tensor of 13 small to moderate recent earthquakes using surface wave amplitude spectra. Our results confirm previous models interpreting the eastern Marmara Sea Region as an active transtensional pull-apart environment associated with significant NNE–SSW extension and vertical displacement. At the northern peninsula, long term deformation pattern did not change significantly since Pliocene times contradicting regional tectonic models which postulate a newly formed single dextral strike slip fault in the Marmara Sea Region. This area is interpreted as a horsetail splay fault structure associated with a major normal fault segment that we call the Waterfall Fault. Apart from the Waterfall Fault, the stress strain relation appears complex associated with a complicated internal fault geometry, strain partitioning, and reactivation of pre-existing plane structures. At the southern peninsula, recent deformation indicates active pull-apart tectonics constituted by NE–SW trending dextral strike slip faults. Earthquakes generated by stress release along large rupture zones seem to be less probable at the northern, but more probable at the southern peninsula. Additionally, regional seismicity appears predominantly driven by plate boundary stresses as transtensional faulting is consistent with the southwest directed far field deformation of the Anatolian plate.

Estimating earthquake source depths by combining surface wave amplitude spectra and teleseismic depth phase observations

The ability to confidently estimate the depths of small-to-medium sized (3.5 ≤ mb ≤ 5.5) seismic disturbances is important both in plate tectonics studies, and when monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Seismic source depths can be determined by identification of the teleseismic depth phases pP and sP, and also by modelling surface wave amplitude spectra. The radiation pattern of the teleseismic depth phase pP and fundamental mode Rayleigh amplitudes show that the effectiveness of these methods of earthquake depth estimation is dependent on the orientation of the focal mechanism and the station locations. For some focal mechanisms, the predicted amplitude of the teleseismic depth phase pP will only be large for stations in certain locations, and the Rayleigh wave spectral nulls that tightly constrain the seismic source depth when modelling surface wave amplitude spectra often only occur for a limited range of azimuths. In this study, we show that for sources where Rayleigh wave spectral nulls are not observed and the source depth cannot be constrained using the surface wave amplitude spectra, the focal mechanism obtained by modelling Rayleigh and Love wave amplitude spectra can be used to identify the locations of stations where pP should have a large amplitude and hence be easiest for an analyst to identify. The increased global coverage of seismometer stations means that there is an increased likelihood that stations exist in the locations where the predicted amplitude of pP is large. As the identified depth phases are consistent with the focal mechanism this approach allows increased confidence to be placed in the identified depth phases and hence the estimated source depth. This approach could potentially be used with other methods of focal mechanism estimation provided that the method used to estimate the focal mechanism is independent of the source depth.

Multi-objective analysis of body and surface waves from the Market Rasen (UK) earthquake

SUMMARY The 2008 February 27 Market Rasen (UK) earthquake (mb 4.5) is used to show how earthquake source parameters can be estimated using a multi-objective optimization approach to the joint inversion of teleseismic body wave and regional surface wave observations. To estimate the source depth, short-period teleseismic seismograms are interpreted in terms of P and the depth phases pP and sP using the F-statistic and its associated probability theory. Results of this analysis are inconclusive as two possible source depths are identified. To identify our preferred depth, a method of estimating source depth and mechanism by modelling surface wave amplitude spectra is applied to data recorded at regional distances. A source depth of 22 km is found to be consistent with the observed body and surface waves. Upper and lower amplitude bounds and polarities are then placed on the phases interpreted as P, pP and sP for a source depth of 22 km. The Gaussian relative amplitude method is used to calculate a likelihood for all possible orientations of the double-couple source. A multi-objective optimization approach can then used to compare these likelihoods with calculated misfits between observed and synthetic surface wave amplitude spectra for all orientations of the double-couple source. Our preferred focal mechanism (φs= 205°, δ= 50° and λ= 15°) is found by normalizing and summing both misfits to find the focal mechanism with the lowest combined misfit. Synthetic body wave seismograms, surface wave amplitude spectra and seismograms are in good agreement with the observed data for our preferred focal mechanism and depth.

Determination of stress glut moments of total degree 2 from teleseismic surface wave amplitude spectra

A special case of the seismic source, where the stress glut tensor can be expressed as a product of a uniform moment tensor and a scalar function of spatial coordinates and time, is considered. For such a source, a technique of determining stress glut moments of total degree 2 from surface wave amplitude spectra is described. The results of application of this technique for the estimation of spatio-temporal characteristics of the Georgian earthquake, 29.04.91 are presented.

The 4.6 mb,Lg Northeastern Kentucky Earthquake of September 7, 1988

Abstract Source parameters for the September 7, 1988 northeastern Kentucky earthquake have been estimated from the analysis of surface-wave amplitude spectra. The source that best fits the observed data had a seismic moment of 2.0 × 1022 dyne-cm, a mechanism with strike = 198° ± 10°, dip = 51° ± 11°, and slip = −178° ± 17°, (T) trend = 160°, plunge = 25°, (P) trend = 55°, plunge = 28°, and source depth of 4 to 7 km. Thirty-two aftershocks were recorded during 2 weeks of monitoring following the mainshock; 23 of the aftershocks were locatable and fall on a roughly NW-SE linear trend. This trend is subparallel with the NW-SE nodal plane of the mainshock. Our analysis shows the 1988 event to be different from the July 27, 1980 mb,Lg = 5.3 earthquake located 11 km to the northwest. First, the 1988 event is considerably shallower (4 to 7 km) than the 1980 event (14 to 22 km). Second, data from the 1988 event suggest the motion is on a conjugate fault and is in contrast with the 1980 event, which had right-lateral strike-slip on a southeast-dipping plane.

Tectonic Strain Release by Underground Nuclear Explosions and its Effect on Seismic Discrimination

The analysis of surface waves from a large number of underground nuclear explosions reinforces the hypothesis of tectonic strain release. Such strain release is dependent on rock type and ambient stress levels. In harder media, such as granite, strain release is considerably greater than in soft media such as loose alluvium and salt. In the case of three events for which the tectonic component exceeded that of the explosion itself, the effect on the Ms - mb discriminant due to the added surface wave energy was not noticeable. This was probably due to the averaging of magnitude over all azimuths. A decaying pulse-type source-time function is consistent with surface wave amplitude spectra and can explain the long-period/short-period spectral ratio.