| 1. |
- Amini, Samar, et al.
(författare)
-
Directivity analysis of the 2017 December Kerman earthquakes in Eastern Iran
- 2020
-
Ingår i: Journal of Seismology. - : Springer. - 1383-4649 .- 1573-157X. ; 24, s. 531-547
-
Tidskriftsartikel (refereegranskat)abstract
- Using an empirical Green’s function (EGF) approach and data from local to regional distances we analyzed rupture propagation directivity in the three mainshocks (ML 6.0–6.1) and in six of the largest aftershocks (ML 5.0 – 5.5) of the 2017 Kerman, Iran, seismic sequence. The EGF procedure was based on data from smaller events (ML 4.0 – 4.8). Deconvolution was applied separately to P and S phases. Using the P-wave data, we calculated relative source-time functions and examined azimuthal variations in rupture duration. In the S-wave analysis, we investigated along strike rupture directivity of the mainshocks and the largest aftershocks by evaluating azimuthal variation of the amplitude spectra. Two of the mainshocks and four of the aftershocks clearly showed rupture propagation from the south-east toward the north-west. The third mainshock and one of the aftershocks suggested almost bilateral rupture propagation, and one aftershock showed rupture directivity to the southeast. It seems that the rupture propagation direction in the area is generally to the north-west and the events which have different propagation directions are located within the NW and SE ends of the faulting area. We suggest that the general rupture propagation direction in the area is steered by regional tectonic stress field regarding the faulting orientations which have been affected by stress redistribution around a restraining bend.
|
|
| 2. |
- Amini, Samar, et al.
(författare)
-
Fault slip and identification of the second fault plane in the Varzeghan earthquake doublet
- 2018
-
Ingår i: Journal of Seismology. - : SPRINGER. - 1383-4649 .- 1573-157X. ; 22:4, s. 815-831
-
Tidskriftsartikel (refereegranskat)abstract
- An intraplate earthquake doublet, with 11-min delay between the events, devastated the city of Varzeghan in northwestern Iran on August 11, 2012. The first Mw 6.5 strike-slip earthquake, which occurred after more than 200 years of low seismicity, was followed by an Mw 6.4 oblique thrust event at an epicentral separation of about 6 km. While the first event can be associated with a distinct surface rupture, the absence of a surface fault trace and no clear aftershock signature makes it challenging to identify the fault plane of the second event. We use teleseismic body wave inversion to deduce the slip distribution in the first event. Using both P and SH waves stabilize the inversion and we further constrain the result with the surface rupture extent and the aftershock distribution. The obtained slip pattern shows two distinct slip patches with dissimilar slip directions where aftershocks avoid high-slip areas. Using the estimated slip for the first event, we calculate the induced Coulomb stress change on the nodal planes of the second event and find a preference for higher Coulomb stress on the N-S nodal plane. Assuming a simple slip model for the second event, we estimate the combined Coulomb stress changes from the two events on the focal planes of the largest aftershocks. We find that 90% of the aftershocks show increased Coulomb stress on one of their nodal planes when the N-S plane of the second event is assumed to be the correct fault plane.
|
|
| 3. |
- Dineva, Savka, et al.
(författare)
-
Source parameters of four strong earthquakes in Bulgaria and Portugal at the beginning of the 20th century
- 2002
-
Ingår i: Journal of Seismology. - 1383-4649 .- 1573-157X. ; 6:1, s. 99-123
-
Tidskriftsartikel (refereegranskat)abstract
- Using original seismograph records and bulletin data were-determined the origin time, location, seismic moment (M0) and magnitudes (MS and Mw) for the four earthquakes in the beginning of the 20th century. These are two strong earthquakes April 4, 1904 near Krupnik, Bulgaria (Mw = 6.8, MS = 7.2 respectively), the April 23 1909 earthquake near Benavente, Portugal (MS = 6.3), and the June 14, 1913 earthquake near Gorna Orjahovitza, Bulgaria (MS = 6.3). Twenty-nine traces from original records have been analysed, a large number of original station bulletins have been consulted and a consistent methodology for analysing these early 20th century instrumental information is presented. In spite of a thorough effort in re-assembling and quality control of the original data, large inaccuracies remain in the improved instrumental epicentre locations and origin times. The seismic moment estimates we obtained (2.3 1018 ≤ M0 ≤ 3.9 1019 Nm) are the first ever determined for these events. The magnitude estimates (6.3 ≤ MS ≤ 7.2 and 6.2 ≤ Mw ≤ 7.0) are robust and systematically lower than most of previous estimates for all earthquakes (Gutenberg and Richter, 1954; Christoskov and Grigorova, 1968; Karnik, 1969). For the largest Krupnik event our estimates agree with those of Abe and Noguchi (1983b) and Pacheco and Sykes (1992). The studied earthquakes all occur in moderately seismic active regions, therefore our results may have significant consequences for hazard estimates in those regions.
|
|
| 4. |
- Enferadi, S., et al.
(författare)
-
Feasibility study of earthquake early warning in Tehran, Iran
- 2021
-
Ingår i: Journal of Seismology. - : Springer Nature. - 1383-4649 .- 1573-157X. ; 25:4, s. 1127-1140
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, we examine the scientific feasibility of an Earthquake Early Warning System in Tehran, Iran, by the integration of the Tehran Disaster Mitigation and Management Organization (TDMMO) accelerometric network and the PRobabilistic and Evolutionary early warning SysTem (PRESTo). To evaluate the performance of the TDMMO-PRESTo system in providing the reliable estimations of earthquake parameters and the available lead-times for The Metropolis of Tehran, two different approaches were analyzed in this work. The first approach was assessed by applying the PRESTo algorithms on waveforms from 11 moderate instrumental earthquakes that occurred in the vicinity of Tehran during the period 2009-2020. Moreover, we conducted a simulation analysis using synthetic waveforms of 10 large historical earthquakes that occurred in the vicinity of Tehran. We demonstrated that the six worst-case earthquake scenarios can be considered for The Metropolis of Tehran, which are mostly related to the historical and instrumental events that occurred in the southern, eastern, and western parts of Tehran. Our results indicate that the TDMMO-PRESTo system could provide reliable and sufficient lead-times of about 1 to 15s and maximum lead-times of about 20s for civil protection purposes in The Metropolis of Tehran.
|
|
| 5. |
- Fischer, Tomáš, et al.
(författare)
-
Microseismic events on the Åknes rockslide in Norway located by a back-projection approach
- 2019
-
Ingår i: Journal of Seismology. - : Springer Science and Business Media LLC. - 1383-4649 .- 1573-157X.
-
Tidskriftsartikel (refereegranskat)abstract
- The Åknes rockslide in Western Norway is characterised by a steady movement of a rock mass with an extent of about 1 km2 and a yearly deformation rate in the range of 2 to 4 cm. A seismic network consisting of 8 three-component geophones records tens to hundreds of local and distant seismic events daily. Depending on their character, local seismic events show a variety of waveforms, both with abrupt and emergent onsets. Along with the relatively low frequencies of the waveforms, standard location procedure using arrival time measurements is difficult to apply. Based on the event envelopes, we classify local events and obtain their approximate location by stacking STA/LTA ratios of back-projected waveforms. To suppress the influence of complex surface morphology and related complicated wave propagation, we constrain hypocentres to the surface of the slope and determine only horizontal coordinates. The method was successfully tested by locating two types of ground truth data: calibration shots and a block collapse. The test proved the ability of the method to determine the position of sliding events with an uncertainty of less than 36 m, which allows to distinguish amongst several foci of rockslide activity. Application of the method to 8 years of monitoring data shows continuous seismic activity, which is concentrated in the centre and at the western edge of the monitored area. Most likely, microseismic events recorded by the seismic network originate within the body of the rock slope and are related to its disintegration or potentially to sliding on the detachment fault.
|
|
| 6. |
- Gibbons, Steven J., et al.
(författare)
-
Enhanced detection and estimation of regional S-phases using the 3-component ARCES array
- 2019
-
Ingår i: Journal of Seismology. - : SPRINGER. - 1383-4649 .- 1573-157X. ; 23:2, s. 341-355
-
Tidskriftsartikel (refereegranskat)abstract
- In September 2014, the ARCES array in Northern Norway was upgraded to consist of 3-component instruments at each of its 25 sites. It is thereby the first array station of the International Monitoring System to be fully 3-component. S-phases are of paramount importance for the detection and location of seismic events at regional distances, especially given sparse station coverage, and it is important that these phases are both detected confidently and attributed accurate slowness estimates. The slowness estimates both identify the phase and provide phase association algorithms with the information necessary to form a high-quality seismic event hypothesis. Prior to September 2014, ARCES had 3-component sensors at four sites only. While these 3-component seismometers were highly beneficial for the detection of regional S-phases, often using incoherent beams, it was often more reliable to perform f-k analysis on the 25 vertical sensors rather than the small number of available horizontal waveforms. In addition to providing an increased signal-to-noise ratio, signals on the horizontal traces are more coherent for the S-phases and display a high measure of semblance over the array. This improves the stability of f-k analysis, since the estimates are less susceptible to the effects of scattering and incoherence. The improved coherence of S-phases across the array aperture on the horizontal components provides the basis for superior secondary phase detection capability using F-detectors and other coherence-based algorithms. Analysis of regional signal coda on the 3-component array indicates different characteristics of the apparent slowness on the different components. The 27 January 2018 earthquake on Novaya Zemlya was the first seismic event close to the former Soviet nuclear test-site to be recorded using the upgraded 3-component ARCES array. We examine the evolution of the slowness pattern for on the different components of the wavetrain and speculate that a continuous f-k detector on three components may be able to detect small seismic events in the eastern Barents Sea with increased robustness and sensitivity. We review the development of the instrumentation at ARCES and explain the motivation behind the hybrid response sensors currently deployed.
|
|
| 7. |
- Gospodinov, Dragomir, et al.
(författare)
-
On the applicability of the RETAS model for forecasting aftershock probability in underground mines (Kiirunavaara Mine, Sweden)
- 2022
-
Ingår i: Journal of Seismology. - : Springer. - 1383-4649 .- 1573-157X. ; 26:5, s. 1023-1037
-
Tidskriftsartikel (refereegranskat)abstract
- Aftershock series of even comparatively small seismic events can pose a risk to the mining operation or the personnel in deep underground mines as the main shocks and some of the aftershocks can cause damage in the rock mass. Stochastic modeling was applied in this study for the analysis of the temporal evolution of aftershock occurrence probability during a M1.85 aftershock sequence in Kiirunavaara Mine, Sweden. The Restricted Epidemic-Type Aftershock Sequence (RETAS) model was chosen for estimation of the aftershock occurrence probability. This model considers all events with magnitude above the magnitude of completeness M0 and has the advantage of including the Modified Omori Formula (MOF) model and Epidemic-Type Aftershock Sequence (ETAS) model as its end versions, considering also all intermediate models. The model was applied sequentially to data samples covering cumulative periods of time, starting from the first 2 h after the main event and increasing them by 2 h until the period covered the entire 72-h sequence. For each sample, the best-fit RETAS version was identified and the probability of a M ≥ 0.5 aftershock for every next 2 h was determined through Monte Carlo simulation. The feasibility of the resulting probability evolution for suspension and re-starting of the mining operations was discussed together with possible prospects for future development of the methodology.
|
|
| 8. |
- Hamed, A. A., et al.
(författare)
-
On the strength of the phase cross-correlation in retrieving the Green's function information in a region affected by persistent aftershock sequences
- 2021
-
Ingår i: Journal of Seismology. - : Springer Nature. - 1383-4649 .- 1573-157X. ; 25:3, s. 987-1003
-
Tidskriftsartikel (refereegranskat)abstract
- Although research on seismic interferometry is now entering a phase of maturity, earthquakes are still the most troublesome issues that plague the process in real applications. To address the problems that arise from spatially scattered and temporally transient enormous earthquakes, preference is usually given to the use of time-dependent weights. However, small earthquakes can also have a disturbing effect on the accuracy of interpretations if they are persistently clustered right next to the perpendicular bisector of the line joining station pairs or in close proximity to one of the stations. With regard to the suppression of these cluster earthquakes, commonly used solutions for dealing with monochromatic microseismic cluster events (e.g., implementing a band-reject filter around a comparatively narrow frequency band or whitening the amplitude spectra before calculating the cross-spectrum between two signals) may not have the necessary efficiency since earthquake clusters are generally a collection of events with different magnitudes, each having its own frequency and energy contents. Therefore, the only solution left in such a situation is to use stronger non-linear time-dependent weights (e.g., square of the running average or one-bit normalization), which may cause Green's function amplitude information to be lost. In this paper, by simulating the records of a benchmark earthquake M-N 5.2 with the help of empirical Green's functions (EGF) obtained after the Ahar-Varzeghan Earthquake Doublet (M-N 6.4 and M-N 6.3), it is shown that the amplitude-unbiased phase cross-correlation is a relatively efficient approach in the face of the issues concerning long-standing cluster events.
|
|
| 9. |
- Karkooti, Ehsan, et al.
(författare)
-
Investigating the role of source mechanism, surface topography, and attenuation on the observed PGA pattern in May 28, 2004, Mw 6.2 Baladeh earthquake (Iran)
- 2016
-
Ingår i: Journal of Seismology. - : Springer Science and Business Media LLC. - 1383-4649 .- 1573-157X. ; 20:2, s. 495-510
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, we use seismic waveform simulation to investigate the influence of source mechanism complexity, surface topography, and quality factor on the observed peak ground motions in May 28, 2004, moment magnitude (Mw) 6.2 Baladeh earthquake. The observed peak ground acceleration (PGA) pattern in this event, which is the biggest earthquake to hit the Central Alborz Mountains of Iran in modern instrumental era, is irregular in some respects. First, the observed PGA contours are elongated toward north-west and, second, the maximum observed PGA value of 1049 cm/s(2) on the horizontal component of Hasan Keyf station 50 km away from the epicenter is quite high and irregular for an earthquake of this magnitude, at such long distance. In this study, we employ the spectral element method, implemented in SPECFEM3D software package to simulate the 3D wave propagation from several source models in the area. Our results suggest directivity effect is the main cause of the anomalous observations in this earthquake and could account for the elongation of PGA contours and also the anomalous maximum PGA value observed at Hasan Keyf strong motion station. We show that the surface topography has minor effect on the observed peak ground acceleration and the resulting PGA maps. Also by finding the bounds of seismic quality factor effect on the peak ground acceleration values, we show that this factor could not account for the elongation of iso-acceleration contours in the north-west direction.
|
|
| 10. |
- Keshvari, Forough, et al.
(författare)
-
Upper-mantle S-velocity structure across the Zagros collision zone resolved by nonlinear teleseismic tomography
- 2011
-
Ingår i: Journal of Seismology. - : Springer Science and Business Media LLC. - 1383-4649 .- 1573-157X. ; 15:2, s. 329-339
-
Tidskriftsartikel (refereegranskat)abstract
- Non-linear teleseismic S-phase tomography across the Zagros collision zone in southwestern Iran is used to determine a high-resolution image of the upper-mantle structure. The inversion was done using 41 high-quality earthquakes recorded by 19 broad-band and medium-band stations along a 620 km long profile across the collision zone. Smearing from strong crustal velocity anomalies into the upper-mantle is suppressed by travel-time corrections calculated based on a 3D crustal model for the study area. Our results show that the relatively old and cold Arabian shield has a higher velocity (up to 6% faster, at depths between 70 and 300 km) than the younger lithosphere farther north in Central Iran. These two upper-mantle domains are separated by a sharp near-vertical transition whose surface expression coinciding with the Main Zagros Thrust.
|
|
