Pure and Applied Geophysics

Kossobokov, Vladimir; Nekrasova, Anastasia

doi: 10.1007/s00024-018-2071-ypmid: N/A

The three clusters of the epicenters of the nine recent (1993–2018) earthquakes of magnitude 7.0 or larger in New Zealand are located in three different tectonic environments of the Australia–Pacific Plate boundary, including the southern part of the Kermadec Trench (showing rapid westward subduction), the oblique collision zone between the Pacific Plate and Indo-Australian Plate with the dominant Alpine Fault (showing right-lateral strike-slip movement), and the Puysegur Trench (showing eastward oblique subduction). From the viewpoint of the unified scaling law for earthquakes (USLE), these regions are characterized by different levels of seismic rate (A), earthquake magnitude exponent (B), and fractal dimension of epicenter loci (C). The recent major earthquakes exemplify different scenarios of aftershock sequences in terms of either the dynamics of interevent time (τ) or the USLE control parameter (η = τ × 10 B×(5−M) × L C ), where τ is the time interval between two successive earthquakes, M is the magnitude of the second one, and L is the distance between them. We find the existence, in the long term, of different, intermittent levels of rather steady seismic activity characterized by near-constant values of mean η (〈η〉), which, in the mid-term, switch between one another at times of critical transitions, including those associated with all but one magnitude 7.0 or larger earthquake. At such a transition, seismic activity may follow different scenarios with interevent time scaling of different kinds. Evidently, although these results based on analysis of an individual series do not support the presence of universality in seismic energy release, they provide constraints on modeling realistic seismic sequences for earthquake physicists and supply decision-makers with information for improving local seismic hazard assessments.

Heidarzadeh, Mohammad; Muhari, Abdul; Wijanarto, Antonius

doi: 10.1007/s00024-018-2065-9pmid: N/A

The 28 September 2018 Sulawesi tsunami has been a puzzle because extreme deadly tsunami waves were generated following an Mw 7.5 strike-slip earthquake, while such earthquakes are not usually considered to produce large tsunamis. Here, we obtained, processed and analyzed two sea level records of the tsunami in the near-field (Pantoloan located inside the Palu Bay) and far-field (Mamuju located outside the Palu Bay) and conducted numerical simulations to shed light on the tsunami source. The two tide gauges recorded maximum tsunami trough-to-crest heights of 380 and 24 cm, respectively, with respective dominating wave periods of 3.6−4.4 and 10 min, and respective high-energy wave duration of 5.5 and >14 h. The two observed waveforms were significantly different with wave amplitude and period ratios of ~16 and ~3, respectively. We infer tsunamigenic source dimensions of 3.4–4.1 km and 32.5 km, for inside and outside of the Palu Bay, respectively. Our numerical simulations fairly well reproduced both tsunami observations in Pantoloan and Mamuju; except for the arrival time in Mamuju. However, it was incapable of reproducing the maximum reported coastal amplitudes of 6–11 m. It is possible that these two sources are different parts of the same tectonic source. A bay oscillation mode of ~85 min was revealed for the Palu Bay through numerical modeling. Actual sea surface disturbances and landslide-generated waves were captured by two video recordings from inside the Palu Bay shortly after the earthquake. It is possible that a large submarine landslide contributed to and intensified the Sulawesi tsunami. We identify the southern part of the Palu Bay, around the latitude of -0.82oS, as the most likely location of a potential landslide based on our backward tsunami ray tracing analysis. However, marine geological data from the Palu Bay are required to confirm such hypothesis.

Kumar, Vikas; Kumar, Dinesh; Chopra, Sumer

doi: 10.1007/s00024-018-1972-0pmid: N/A

The present study estimates source parameters and proposes scaling relationships for moderate size earthquakes (m b 3.7–5.8) in seismically active North–East India region. The study is based upon the spectral analysis of high-quality waveforms comprising of P- and S-waves obtained from strong ground motion (SMA) records of 50 earthquakes that has occurred in the region. A two-step procedure is adopted to estimate the earthquake source parameters. It has been observed that the average seismic moment and source radii vary from 1.05 × 1015 to 1.99 × 1017 N-m and 500 to 2000 m, respectively. The average corner frequency ratio [f C(P)/f C(S)] of P-wave and S-wave is found to be 1.2, which shows the shift in the corner frequency. The total estimated energy varies between 9.22 × 1010 and 1.42 × 1014 J, while the average stress drop varies from 1.8 to 29.4 MPa. One of the major outcome of this study is that the stress drop does not vary significantly with the magnitude and self-similarity exist among the earthquakes in North–East India region. The scaling relation between the seismic moment and the corner frequency is $$ M_{\text{o}} f_{\text{c}}^{ 3} = 1.35\; \times \; 10^{17} N{\text{ - ms}}^{ - 3} $$ M o f c 3 = 1.35 × 10 17 N - ms - 3 . The median stress drop value for NE India region is found to be about 9.2 MPa. The earthquake source parameters and the scaling relations developed in this study will be useful for carrying scenario based seismic hazard analysis studies in the NE India region.

Besutiu, Lucian; Diaconescu, Mihail; Zlăgnean, Luminita; Craiu, Andreea

doi: 10.1007/s00024-018-1956-0pmid: N/A

The Galati-Izvoarele region, located in eastern Romania, came into the public attention relatively recent, when hosted an unusual intense earthquake sequence. During September–November 2013, several hundred shallow crust earthquakes were recorded in the area. There were several attempts to explain the phenomenon, and hypotheses more or less documented have been formed. Some speculations occurred on the potential connection between the seismic swarm and oil industry activities. The paper summarizes the results of local geophysical surveys conducted in the area, jointly analysed with the observed seismicity. The main results of the study reveal the Galati-Izvoarele region as a seismic-active area belonging to the northwest prolongation of North Dobrogea mobile zone. The earthquakes were generated mainly in a highly fragmented local graben-like structure, transversally superposed on the descending slope of the North Dobrogea Promontory. The graben fault system was and might be (re)activated each time when tectonic forces acting in the Carpathians foreland intensify. Changes in the intensity of tectonic forces may reflect in the slip acceleration along Peceneaga–Camena Fault, as observed at the Baspunar Geodynamic Observatory. For example, the paroxysmal phase of the Galati-Izvoarele swarm was shortly preceded by a significant increase of the Peceneaga–Camena Fault slip rate recorded at Baspunar Geodynamic Observatory. To conclude, the Galati-Izvoarele region must be seen as a seismic-prone area and any significant increase of the tectonic stress in the Carpathians foreland may generate another seismic sequence in an unpredictable future.

Chen, Shunyun; Liu, Peixun; Guo, Yanshuang; Liu, Liqiang; Ma, Jin

doi: 10.1007/s00024-018-1933-7pmid: N/A

In this paper, we report the co-seismic temperature response of the Ms6.3 Kangding earthquake in Sichuan, China, which occurred within a bedrock temperature monitoring station network constructed in the east of the Qinghai-Tibet Plateau. Results indicate that two kinds of co-seismic responses of bedrock temperature, exponential and step-rise ones, were recorded at different stations. As for the former, the amplitude of co-seismic response is about 5.6–11.5 mK (the average value is 7.33 mK), while the latter is about 0.31–0.98 mK (the average value is 0.52 mK). These two kinds of responses would be indicative of the two different mechanisms for change in temperature. On the one hand, the exponential responses represent the effects of fluid advection along the borehole wall, which are associated with a finite quantity of heat produced by earthquake-driven transient fluid flow. The step-rise responses, on the other hand, represent the direct response of temperature to change in the crustal stress accompanying with earthquake. We still tested relationship between stress variation and temperature response on the rock samples taken from the boreholes, and the average value is 0.77 mK/MPa. Upon thus, these step-rise temperature responses imply that co-seismic change in mean stress is about 0.40 MPa. We simplify effects of fluid by presuming a one-dimensional model where the flow is either up or down based on a pressure change due to compression or tension. The stress state, compression or tension, deduced by the direction of the fluid flow is qualitatively accordant with the direct response of bedrock temperature to stress change. In summary, characteristic changes in bedrock temperature occurred coincidentally with the earthquake at different stations, agreeing generally with regional stress adjustments accompanying the earthquake. Changes in bedrock temperature do seem to offer a useful means of monitoring dynamic changes in shallow crustal stress.

Ramezani, Ali; Ali Abbaspour, Rahim; Mojarab, Masoud

doi: 10.1007/s00024-018-1973-zpmid: N/A

The casualties and financial losses caused by large earthquakes have led to an awareness of prediction importance of such earthquakes. The earthquake prediction is divided into four categories: long term, intermediate term, short term, and immediate. The M8 algorithm is one of the intermediate-term middle-range prediction algorithms primarily used to predict earthquake of magnitude 8 or more and is applied later for smaller magnitudes. The Iranian Plateau is less exposed to earthquake with magnitude 8 or more and it is observed that the seismicity rate in this region is generally low. Thus; the original M8 is not suitable for applying in this region. The objective of this study is to modify the M8 algorithm for Prediction of Major M7.0+ Earthquakes in the Iranian Plateau. The major earthquake of magnitude 7 or more in the Iranian plateau from 1975 to 2018 is considered as the target earthquakes. The hit rate times 1 minus alarm rate is defined as objective function and the particles swarm optimization meta-heuristic algorithm is used to maximize it. The optimum M8 could predict 14 out of 17 large earthquakes in the Iranian plateau while occupying 31.7% of the spatio-temporal space as the alarm. The results show that by employing an optimization algorithm, we can modify the M8 algorithm for efficient prediction of the target magnitudes less than 8 in the regions with low seismicity rate.

Kim, So; Gitterman, Yefim; Lee, Seoung-kyu

doi: 10.1007/s00024-018-1958-ypmid: N/A

North Korea has conducted six underground nuclear explosions so far. In this study, we determined source depth and characterization for the 2016J, 2016S and 2017S tests which were conducted on January 6, 2016 (2016J), September 9, 2016 (2016S) and September 3, 2017 (2017S), respectively. It has been difficult to ascertain the accurate depth of North Korean nuclear explosions due to paucity of data and information. We explore the depth calculation for the North Korean nuclear tests based on the detailed depth phases using teleseismic and regional arrays. We present the coherent spectral nulls from the average spectra of pP + P/sP + P and pPn + Pn/sPn + Pn which correlate with the depth phases showing 180° phase reversals with the P-wave arrivals. We estimated the burial depths at 2.12, 2.10, 1.98 km for the 2016J, 2016S and 2017S nuclear tests, respectively, We anticipate our absolute findings to be significant since in the past depth estimates for North Korean nuclear tests have been inconclusive and unclear owing to not only paucity of data but also trade-offs of the relative assessment based on the satellite images between the true source location and the tunnel entrance for the 2006 nuclear test used as a reference event.

Mulabisana, T.; Midzi, V.; Manzunzu, B.

doi: 10.1007/s00024-018-1959-xpmid: N/A

Site response analysis is conducted at 37 seismic stations located in the Gauteng, North West and Free State provinces in South Africa, using the Nakamura H/V spectral ratio technique on records of the Orkney 5 August 2014 earthquake. The earthquake, of magnitude M L = 5.5, led to the unfortunate death of one person and damage of more than 600 houses. Intensity data collected soon after the event showed that the effects of the earthquake appeared to vary significantly across the region. This motivated the authors to conduct a more detailed investigation of the effects of site conditions on seismic station records in the region. Resonance frequency values obtained from the H/V ratios were observed to vary strongly across the region and also within seismic station clusters. Similar behaviour was observed with the peak amplitude of the ratios at the resonance frequency, except for the Johannesburg area whose results showed a relatively simple shape of the ratios implying less complex velocity structure. All the H/V ratios exhibit dominant peaks at resonant frequencies that varied between 0.5 and 35 Hz. The average observed resonant frequency was f = 7.9 Hz. The amplitude of the dominant peaks also varied strongly from 1.66 to 11.69, with only two sites exhibiting maximum peaks with amplitude smaller than 2. These results serve as a strong motivation or justification for the on-going microzonation studies in South Africa, where a detailed study of the velocity structure will be used to obtain reliable information on site amplification and resonance.

Zhang, Pan; Wu, Ru-Shan; Han, Liguo

doi: 10.1007/s00024-018-2025-4pmid: N/A

When the source lacks low-frequency information, the linear deconvolution method based on the convolution signal model can only extract effective information within the bandwidth of the source. The envelope inversion method based on the modulation signal model can reconstruct the low-wavenumber components of the subsurface media. However, both methods have problems because seismic reflection data always show a poor S/N at low frequencies. In this article, we propose a hybrid scale separation method that combines the linear reconstruction and non-linear demodulation methods to recover the ultra-low-frequency information contained in seismic data. First, we propose an improved linear scale separation method, which can extract effective low-frequency information in the presence of strong low-frequency noises. Then, we conduct demodulation on seismic data after the improved linear scale separation, which we call hybrid scale separation. The analysis of the modulation signal model theory demonstrates that the hybrid scale separation method is good for refining the smooth Green function contained in the direct demodulation result. Based on the hybrid scale separation, we propose a new envelope inversion strategy with a strong anti-noise property. We first conduct the improved reconstruction using the synthetic observed data to suppress low-frequency noises and determine the effective signal bandwidth. Then, we calculate the envelope using the reconstructed data. By changing the parameters in the improved linear scale separation process, we can conduct envelope inversion in different scales. Our method can handle the inversion using seismic data that show a poor S/N at low-frequency components. The numerical examples using the Marmousi data set with different S/N illustrate the effectiveness of our method.

Guo, Xuebao; Liu, Hong; Shi, Ying; Wang, Weihong; Zhang, Zhen; Jing, Hongliang

doi: 10.1007/s00024-018-1968-9pmid: N/A

Under the same propagation operator, the precision of the seismic wavelet determines whether synthetic data can match field data accurately. By constructing the convolution wavefield objective function, the source difference between simulated and observed data is ignored, thus avoiding the source wavelet estimation. Theoretically, this process has no restriction on the accuracy of the wavelet, and a multi-scale inversion strategy can be implemented by using a source wavelet with different dominant frequencies. When the propagation operator used does not conform with reality, even if both the wavelet and the parameter mode are accurate, it is impossible to simulate the full waveforms of the observed data. Meanwhile, the difference in the Green function brings new discrepancy to the convolution wavefield objective function and affects the final inversion results. The method presented in this paper is based on the convolution wavefield objective function. On the basis of an original single reference trace, we discuss the performance of the convolution wavefield-type objective function under multiple reference traces selected from different offsets. After introducing the changed wavefield information, the objective function has the ability to adapt to different types of data. The analysis shows that nonlinearity is significantly increased after introducing the different wavefield information and also increases with inversion frequency. Even for anisotropic data, it is still possible to give a relatively accurate structure at the low-frequency stage, which shows that the wavefield information from different offsets can help to weaken the artifacts introduced by operator mismatch, thus providing more possibilities for the application of acoustic wave equation inversion.