Results and prospects of seismological observations in the central part of the Baikal rift

This article reports the results of detailed seismological observations in the Central Baikal region conducted by the local network of seismological stations of the Buryat Division of the Geophysical Survey of the Russian Academy of Sciences. The local network was created in the 1990s. A crucial feature of the network is the combination of seismic monitoring both in the passive mode (the study of natural seismicity) and in the active mode, with a controlled vibration source of seismic waves. The study area covers the Lake Baikal region and adjacent territories characterized by high seismic activity. Here occurred several catastrophic earthquakes including the strongest one during the period of instrumental observations – the Middle Baikal’1959 earthquake. Recently here occurred the Kudarinsky earthquake on December 9, 2020 with mb=5.4. For more than twenty years the network of observations has been expanding, the equipment has been upgrading. A significant amount of seismo-logical material has been accumulated. Broadband data was processed by the receiver function method. The Moho and the lithosphere-asthenosphere boundaries for stations of the network are determined. Shear seismic wave attenuation characteristics are obtained and the possibility of energy classification of Baikal earthquakes by coda-waves total oscillations is shown.

Comparison Study of Crustal Structure in Aceh Region based on Volcanic Arc System using Receiver Function Method

Aceh region has a very complex crustal structure from the forearc ridge to the backarc basin. This study aims to determine the velocity model of P and S waves and the depth of Moho discontinuity. This research was conducted using teleseismic earthquake data (30°-90° from the station) with M>6 from four seismic stations belonging to the BMKG in Aceh region. The stations are qualified based on the volcanic arc system zone. Furthermore, the velocity model determined by result of forward modelling, while the depth of the Moho layer estimated by migrated receiver function from time domain to the depth domain. At station SNSI that represented the forearc ridge zone, the depth of Moho is ±28 km, at station TPTI represent the forearc basin is ±16 km, while at zone with higher topography, namely volcanic arc zone represented by station KCSI, the Moho depth was identified at ±38 km, and the backarc basin represented by station LASI with ±40 km depth of Moho. This variation occurs because the composition of the earth’s layers below the station is diverse also different topography for each station.

Identification of Crustal Thickness in Central Part of Sumatra Using Teleseismic Receiver Function Method

The Central part of Sumatra is a region that has a high potential for earthquakes. This research intended to determine the crustal thickness of the earth, P and S wave velocity profiles, and vp/vs value in the Central part of Sumatra using stacking H-k and inversion techniques based on the analysis of receiver function. This study utilized teleseismic earthquake data with a distance of 30° to 90° from the station and magnitude more than 6 (M>6). The stations used in this study were 3 BMKG broadband stations located in 3 zones, the fore arc ridge zone (SISI), the volcanic zone (PLSI) and the back arc zone (TPRI). The crustal thickness varies in the fore arc ridge zone (SISI) estimated 17.8 km, volcanic zone (PLSI) reaches 29.7 km and the back arc zone (TPRI) reaches 34 km. The crustal thickness is quite thick under the PLSI and thicker beneath TPRI station. These possibly due to the effect of topography and isostatic compensation in the station. However, whether there is a correlation between crustal thickness and topography needs further research using more stations. The highest vp/vs value was found in the volcanic zone of 1.9, that might be associated with the presence partial melting beneath the station. Meanwhile, the vp/vs value in the back arc zone is 1.72, indicating a relatively more homogeneous structure.

Crustal thickness estimation in Indonesia using receiver function method

The existence of seismic wave velocity difference in the Earth crust and mantle creates the possibility to use earthquake data for estimating the crustal thickness utilizing the Ps conversion phase in the boundary. The radial component signal was deconvolved from the vertical component in the frequency domain to estimate receiver function for Indonesia region. We implemented the water level deconvolution techniques with a Gaussian filter of 2.5 Hz to eliminate the high frequency noise in the receiver function. The H-k stacking technique was performed to all receiver functions from each event to predict the crustal thickness and the Vp/Vs ratio below the stations. We analyzed ten azimuthally distributed teleseismic earthquakes recorded by 108 stations of BMKG. The result shows that the crustal thickness in Indonesia varies from 20 to 39.9 km. The western part of Sumatera, northern part of Sulawesi Island, and North Maluku region show generally thinner crust with value about 20 to 25 km. The North Sumatera, Central Java, and East Java show a considerably thicker crust of up to 36 km. Furthermore, our result also reveals a difference of crustal thickness about 5 km with the previous studies.

Early Results of Time Domain Receiver Function Data Processing in Mt Merapi and Mt Merbabu

The receiver function method is a method to image the earth subsurface by utilizing Ps conversion waves that are formed due to the velocity boundary. In general, the receiver function method estimates depth of structures such as the mantle-crust boundary by deconvoluting the vertical component from the horizontal component. Typical receiver function data processing is done in the frequency domain where the deconvolution process can be seen as a division between two components. In this study, we tried to reprocess the data using a deconvolution technique in time domain, popularly known as iterative time-domain deconvolution. The principle of iterative time domain deconvolution consists of iterative cross-correlation between the horizontal and vertical component. We use data from the DOMERAPI seismic station network located in the vicinity of Mt Merapi and Mt Merbabu. Mt Merapi is one of the most active volcanoes in the world with frequent eruptions and located at the ring of fire chain volcano in Indonesia. Note that the previous receiver function study in this region showed complex signals at some stations that may be related to sediment at shallow sediment and possible layers of low velocity zone that interfering main signal for a crust-mantle boundary. Our current results show iterative time domain RFs have clearer and smoother signal than the frequency domain that help interpreting the waveform signals. We estimate a range of crust thickness between 26-31 km near Mt Merapi. However, we noticed that iterative time domain calculation requires longer computation time and input signal.

Filling the Moho gap: High resolution crustal structure of the Eastern Alps

<p>The dense SWATH-D seismic network in the Central-Eastern Alps gives an unprecedented window into the collision of the Adriatic and European plates. Previous studies have suggested a Moho gap overlying a subduction polarity switch. This switch, from European subduction in the west to Adriatic subduction in the east, was suggested by teleseismic tomography where low velocity zones in the mantle were interpreted as two slabs with opposite subduction polarity. The TRANSALP profile at 12°E indeed showed a gently southward dipping European Moho truncated by a nearly flat Adriatic Moho in receiver function (RF) images, which clearly indicated southward directed subduction. In contrast, RF images derived from the EASI profile at 13.3°E were interpreted to show Moho topography consistent with underthrusting Adriatic Moho, which would support the hypothesized polarity switch, but the image is actually ambiguous. </p><p>We apply the receiver function method to stations in the dense SWATH-D broadband seismic network, covering approximately the area from 45-49°N and 10-15°E, supplemented by the AlpArray Seismic Network and the EASI data. We construct common conversion point stacks in order to pick the Moho conversion and its multiples.  The 15 km average station spacing has allowed us to fill in areas where previously the Moho was too weak to image. In this more comprehensive image, the asymmetry of the Moho in the TRANSALP profile can be traced to continue to at least the longitude of the EASI profile, suggesting continued southward-directed underthrusting of the European crust along the extent of the Eastern Alps, in conflict with the popular polarity switch hypothesis. At the eastern border of our study area we capture a sharp transition from European to extended Pannonian crust. Here the Adriatic Moho retreats and dips below the Pannonian Moho as it continues beneath the Dinarides.</p>

No polarity switch? Continental subduction of European crust below the Eastern Alps imaged by receiver functions

<p>In the Eastern Alps, teleseismic tomography suggests that there is a switch from European subduction in the west to Adriatic subduction in the east. The dense SWATH-D seismic network is located in the central-eastern Alps between around 10°E and 14.5°E where a change in the dip direction was suggested to occur (e.g. Lippitsch et al. 2003; Mitterbauer et al. 2011). The receiver function method is particularly sensitive to velocity contrasts and so is suited to imaging the interfaces associated with subduction. New receiver function migrations from SWATH-D stations (supplemented by the AlpArray Seismic Network and the EASI profile) show no evidence for Adriatic subduction in the Eastern Alps. Instead, a southward dipping interface [or pair of interfaces with opposite polarity] which we interpreted as subducting  European lower crust can be traced below the Eastern Alps to a minimum depth of 120 km along the extent of SWATH-D. This suggests that in the Alps the polarity flip in subduction does not occur or is located east of our study region beyond 14.25°E, much further east than tomography suggests.</p>