The foreshock-aftershock sequence of the March 20 1966 earthquake in the Republic of Congo

1970 ◽  
Vol 60 (4) ◽  
pp. 1245-1258 ◽  
Author(s):  
John Lahr ◽  
Paul W. Pomeroy
Keyword(s):  
Dynamic Range ◽  
B Value ◽  
High Gain ◽  
Aftershock Sequence ◽  
Confidence Limits ◽  
B Values ◽  
Aftershock Sequences ◽  
Successful Prediction ◽  
Seismograph Station ◽  
The Republic

abstract The activity associated with the Congo earthquake of March 20 1966 (mb = 6.5 to 7) was studied with emphasis on the time and magnitude distributions. The data were recorded at the Abéché, Chad, seismograph station operated by Lamont-Doherty Geological Observatory. Over a period of about 70 days, 815 earthquakes with magnitude (mb) greater than or equal to 3.3 were recorded, and they form the basis for this study. The aftershocks are distributed with magnitude (mb) according to the formula long n = a - bm with b = 1.05 ± 0.07 at the 95 per cent confidence limits. The foreshocks have b = 1.06 ± 0.35 at the 95 per cent confidence limits. These b values are in general agreement with b values derived from other aftershock sequences throughout the world. Some authors have suggested that foreshocks may have a lower b value than background activity and that this difference might be used in earthquake prediction. In this paper, an evaluation is made of the limitations of this method of prediction. Assuming that such a difference in b values does exist, it is found that a closely spaced network of high-gain seismographs with wide dynamic range would be required to assure successful prediction.

Two Foreshock Sequences Post Gulia and Wiemer (2019)

2020 ◽  
Vol 91 (5) ◽  
pp. 2843-2850 ◽  
Author(s):  
Kelian Dascher-Cousineau ◽  
Thorne Lay ◽  
Emily E. Brodsky
Keyword(s):  
Puerto Rico ◽  
Real Time ◽  
Exact Result ◽  
Monte Carlo Sampling ◽  
B Value ◽  
Aftershock Sequence ◽  
Traffic Light ◽  
B Values ◽  
Aftershock Sequences ◽  
Abrupt Changes

Abstract Recognizing earthquakes as foreshocks in real time would provide a valuable forecasting capability. In a recent study, Gulia and Wiemer (2019) proposed a traffic-light system that relies on abrupt changes in b-values relative to background values. The approach utilizes high-resolution earthquake catalogs to monitor localized regions around the largest events and distinguish foreshock sequences (reduced b-values) from aftershock sequences (increased b-values). The recent well-recorded earthquake foreshock sequences in Ridgecrest, California, and Maria Antonia, Puerto Rico, provide an opportunity to test the procedure. For Ridgecrest, our b-value time series indicates an elevated risk of a larger impending earthquake during the Mw 6.4 foreshock sequence and provides an ambiguous identification of the onset of the Mw 7.1 aftershock sequence. However, the exact result depends strongly on expert judgment. Monte Carlo sampling across a range of reasonable decisions most often results in ambiguous warning levels. In the case of the Puerto Rico sequence, we record significant drops in b-value prior to and following the largest event (Mw 6.4) in the sequence. The b-value has still not returned to background levels (12 February 2020). The Ridgecrest sequence roughly conforms to expectations; the Puerto Rico sequence will only do so if a larger event occurs in the future with an ensuing b-value increase. Any real-time implementation of this approach will require dense instrumentation, consistent (versioned) low completeness catalogs, well-calibrated maps of regionalized background b-values, systematic real-time catalog production, and robust decision making about the event source volumes to analyze.

scholarly journals Statistical analysis of aftershock sequences related with two major Nepal earthquakes: April 25, 2015, MW 7.8, and May 12, 2015, MW 7.2

2016 ◽  
Vol 59 (5) ◽  
Author(s):  
Prasanta Chingtham ◽  
Babita Sharma ◽  
Sumer Chopra ◽  
Pareshnath SinghaRoy
Keyword(s):  
Scaling Laws ◽  
Main Shock ◽  
B Value ◽  
Temporal Variations ◽  
Aftershock Sequence ◽  
Earthquake Catalog ◽  
Study Region ◽  
Nepal Himalaya ◽  
B Values ◽  
Aftershock Sequences

Present study describes the statistical properties of aftershock sequences related with two major Nepal earthquakes (April 25, 2015, MW 7.8, and May 12, 2015, MW 7.2) and their correlations with the tectonics of Nepal Himalaya. The established empirical scaling laws such as the Gutenberg–Richter (GR) relation, the modified Omori law, and the fractal dimension for both the aftershock sequences of Nepal earthquakes have been investigated to assess the spatio-temporal characteristics of these sequences. For this purpose, the homogenized earthquake catalog in moment magnitude, MW is compiled from International Seismological Center (ISC) and Global Centroid Moment Tensor (GCMT) databases during the period from April 25 to October 31, 2015. The magnitude of completeness, MC, a and b-values of Gutenberg–Richter relationship for the first aftershock sequence are found to be 3.0, 4.74, 0.75 (±0.03) respectively whereas the MC, a and b-values of the same relationship for the second aftershock sequence are calculated to be 3.3, 5.46, 0.90 (±0.04) respectively. The observed low b-values for both the sequences, as compared to the global mean of 1.0 indicate the presence of high differential stress accumulations within the fractured rock mass of Nepal Himalaya. The calculated p-values of 1.01 ± 0.05 and 0.95 ± 0.04 respectively for both the aftershock sequences also imply that the aftershock sequence of first main-shock exhibits relatively faster temporal decay pattern than the aftershock sequence of second main-shock. The fractal dimensions, DC values of 1.84 ± 0.05 and 1.91 ± 0.05 respectively for both the aftershock sequences of Nepal earthquakes also reveal the clustering pattern of earthquakes and signifies that the aftershocks are scattered all around the two dimensional space of fractured fault systems of the Nepal region. The low b-value and low DC observed in the temporal variations of b-value and DC before the investigated earthquake (MW 7.2) suggest the presence of high-stress concentrations in the thrusting regimes of the Nepal region before the failure of faults. Moreover, the decrease of b-value with the corresponding decrease of DC observed in their temporal variations can primarily act as an indicator for possible prediction of major earthquakes in the study region.

scholarly journals Spatiotemporal Heterogeneity of <i>b</i> Values Revealed by a Data-Driven Approach for June 17, 2019 <i>M</i><sub>S</sub> 6.0, Changning Sichuan, China earthquake Sequence

2021 ◽  
Author(s):  
Changsheng Jiang ◽  
Libo Han ◽  
Feng Long ◽  
Guijuan Lai ◽  
Fengling Yin ◽  
...  
Keyword(s):  
Voronoi Tessellation ◽  
Source Region ◽  
Parametric Method ◽  
B Value ◽  
Aftershock Sequence ◽  
Data Driven ◽  
Earthquake Hazards ◽  
Absolute Deviation ◽  
B Values ◽  
Spatiotemporal Heterogeneity

Abstract. The spatiotemporal heterogeneity of b values has great potential for understanding the seismogenic process and assessing the seismic hazard. However, there is still much controversy about whether it exists or not, and an important reason is that the choice of subjective parameters has eroded the foundations of many researches. To overcome this problem, we used a recent developed non-parametric method based on the data-driven concept to calculate b values. The major steps of this method include: 1) perform a large number of Voronoi tessellation, Bayesian information criterion (BIC) value calculation and selection of the optimal models for the study area, and 2) use the ensemble median (Q2) and median absolute deviation (MAD) value to represent the final b value and its uncertainty. We investigated spatiotemporal variations of b values before and after the 2019 Changning MS 6.0 earthquake in Sichuan Basin, China. The results reveal a spatial volume with low pre-mainshock b values near the mainshock source region, and its size corresponds roughly with the rupture area of the mainshock. The anomalously high pre-mainshock b values distributed in the NE direction of the epicenter was interpreted to be related with fluid invasion or increased pore pressure. The decreases of b values during the aftershock sequence along with the occurrences of several strong aftershocks imply that b values could be an indicator of stress state. In addition, we found that although the distribution characteristics of b values obtained from different way of investigating are qualitatively consistent, they differ significantly in terms of their specific values, suggesting that the best way to study the heterogeneous pattern of b values is in the joint dimension of space-time rather than alone in time and space. Overall, our study emphasizes the importance of b value studies on assessing the earthquake hazards.

scholarly journals Pseudoprospective Evaluation of the Foreshock Traffic-Light System in Ridgecrest and Implications for Aftershock Hazard Assessment

2020 ◽  
Vol 91 (5) ◽  
pp. 2828-2842 ◽  
Author(s):  
Laura Gulia ◽  
Stefan Wiemer ◽  
Gianfranco Vannucci
Keyword(s):  
Real Time ◽  
Hazard Assessment ◽  
Monitoring Network ◽  
B Value ◽  
Traffic Light ◽  
B Values ◽  
Background Value ◽  
Light System ◽  
Aftershock Sequences ◽  
Aftershock Hazard

Abstract The Mw 7.1 Ridgecrest earthquake sequence in California in July 2019 offered an opportunity to evaluate in near-real time the temporal and spatial variations in the average earthquake size distribution (the b-value) and the performance of the newly introduced foreshock traffic-light system. In normally decaying aftershock sequences, in the past studies, the b-value of the aftershocks was found, on average, to be 10%–30% higher than the background b-value. A drop of 10% or more in “aftershock” b-values was postulated to indicate that the region is still highly stressed and that a subsequent larger event is likely. In this Ridgecrest case study, after analyzing the magnitude of completeness of the sequences, we find that the quality of the monitoring network is excellent, which allows us to determine reliable b-values over a large range of magnitudes within hours of the two mainshocks. We then find that in the hours after the first Mw 6.4 Ridgecrest event, the b-value drops by 23% on average, compared to the background value, triggering a red foreshock traffic light. Spatially mapping the changes in b values, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. After the second, magnitude 7.1 mainshock, which did occur in that location as anticipated, the b-value increased by 26% over the background value, triggering a green traffic light. Finally, comparing the 2019 sequence with the Mw 5.8 sequence in 1995, in which no mainshock followed, we find a b-value increase of 29% after the mainshock. Our results suggest that the real-time monitoring of b-values is feasible in California and may add important information for aftershock hazard assessment.

scholarly journals Spatiotemporal heterogeneity of <i>b</i> values revealed by a data-driven approach for the 17 June 2019 <i>M</i><sub>S</sub> 6.0 Changning earthquake sequence, Sichuan, China

2021 ◽  
Vol 21 (7) ◽  
pp. 2233-2244
Author(s):  
Changsheng Jiang ◽  
Libo Han ◽  
Feng Long ◽  
Guijuan Lai ◽  
Fengling Yin ◽  
...  
Keyword(s):  
Source Region ◽  
Parametric Method ◽  
Information Criterion ◽  
B Value ◽  
Aftershock Sequence ◽  
Data Driven ◽  
Earthquake Hazards ◽  
Absolute Deviation ◽  
B Values ◽  
Spatiotemporal Heterogeneity

Abstract. The spatiotemporal heterogeneity of b values has great potential for helping in understanding the seismogenic process and assessing seismic hazard. However, there is still much controversy about whether it exists or not, and an important reason is that the choice of subjective parameters has eroded the foundations of much research. To overcome this problem, we used a recently developed non-parametric method based on a data-driven concept to calculate b values. The major steps of this method include (1) performing a large number of Voronoi tessellations and Bayesian information criterion (BIC) value calculation, selecting the optimal models for the study area, and (2) using the ensemble median (Q2) and median absolute deviation (MAD) value to represent the final b value and its uncertainty. We investigated spatiotemporal variations in b values before and after the 2019 Changning MS=6.0 earthquake in the Sichuan Basin, China. The results reveal a spatial volume with low pre-mainshock b values near the mainshock source region, and its size corresponds roughly with the rupture area of the mainshock. The anomalously high pre-mainshock b values distributed in the NW direction of the epicenter were interpreted to be related to fluid invasion. The decreases in b values during the aftershock sequence along with the occurrences of several strong aftershocks imply that b values could be an indicator of the stress state. In addition, we found that although the distribution characteristics of b values obtained from different methods of investigation are qualitatively consistent, they differ significantly in terms of their specific values, suggesting that the best way to study the heterogeneous pattern of b values is in the joint dimension of space-time rather than separately in time and space. Overall, our study emphasizes the importance of b-value studies in assessing earthquake hazards.

scholarly journals The East Aegean Sea strong earthquake sequence of October–November 2005: lessons learned for earthquake prediction from foreshocks

2006 ◽  
Vol 6 (6) ◽  
pp. 895-901 ◽  
Author(s):  
G. A. Papadopoulos ◽  
I. Latoussakis ◽  
E. Daskalaki ◽  
G. Diakogianni ◽  
A. Fokaefs ◽  
...  
Keyword(s):  
Seismic Analysis ◽  
Strong Shock ◽  
Aegean Sea ◽  
B Value ◽  
Aftershock Sequence ◽  
Lessons Learned ◽  
Time Interval ◽  
Aftershock Sequences ◽  
Duration Magnitude ◽  
Foreshock Activity

Abstract. The seismic sequence of October–November 2005 in the Samos area, East Aegean Sea, was studied with the aim to show how it is possible to establish criteria for (a) the rapid recognition of both the ongoing foreshock activity and the mainshock, and (b) the rapid discrimination between the foreshock and aftershock phases of activity. It has been shown that before the mainshock of 20 October 2005, foreshock activity is not recognizable in the standard earthquake catalogue. However, a detailed examination of the records in the SMG station, which is the closest to the activated area, revealed that hundreds of small shocks not listed in the standard catalogue were recorded in the time interval from 12 October 2005 up to 21 November 2005. The production of reliable relations between seismic signal duration and duration magnitude for earthquakes included in the standard catalogue, made it possible to use signal durations in SMG records and to determine duration magnitudes for 2054 small shocks not included in the standard catalogue. In this way a new catalogue with magnitude determination for 3027 events was obtained while the standard catalogue contains 1025 events. At least 55 of them occurred from 12 October 2005 up to the occurrence of the two strong foreshocks of 17 October 2005. This implies that foreshock activity developed a few days before the strong shocks of 17 October 2005 but it escaped recognition by the routine procedure of seismic analysis. The onset of the foreshock phase of activity is recognizable by the significant increase of the mean seismicity rate which increased exponentially with time. According to the least-squares approach the b-value of the magnitude-frequency relation dropped significantly during the foreshock activity with respect to the b-value prevailing in the declustered background seismicity. However, the maximum likelihood approach does not indicate such a drop of b. The b-value found for the aftershocks that followed the strong shock of 20 October 2005 is significantly higher than in foreshocks. The significant aftershock-foreshock difference in b-value is valid not only if the entire aftershock sequence is considered but also if only the segment of aftershocks that occurred within the first 24-h or the first 48-h after the mainshock of 20 October 2005 are taken into account. This difference in b-value should be examined further in other foreshock-aftershock sequences because it could be used as a diagnostic of the mainshock occurrence within a few hours after its generation.

Change in b-values during movement on cut surfaces in granite

1978 ◽  
Vol 68 (2) ◽  
pp. 333-341 ◽  
Author(s):  
J. Weeks ◽  
D. Lockner ◽  
J. Byerlee
Keyword(s):  
Constant Strain Rate ◽  
Warning System ◽  
B Value ◽  
Granite Sample ◽  
B Values ◽  
Aftershock Sequences ◽  
Slip Event ◽  
Confining Pressures ◽  
Microseismic Events ◽  
Strain Rate Loading

Abstract A large granite sample containing a saw cut, modeling a natural fault, was triaxially loaded at confining pressures up to 1000 bars. Fourteen violent slip events accompanied by foreshock and aftershock sequences occurred under constant strain rate loading. From digitally recorded acoustic emission, locations and amplitudes were determined for nearly 8,000 microseismic events. Plots of log amplitude versus log frequency of microseismic events were drawn for three periods between each slip event, termed foreshock, aftershock, and background. These plots indicate that the b-value is lower during foreshocks than for periods between events, implying increased average amplitude of microseismic activity just before slip. These experimental results suggest that it may be possible to devise an earthquake warning system based on changes in b-values in active tectonic regions. It has been suggested that according to the dilatancy-diffusion model, b-value would decrease prior to earthquakes. In our experiment, however, the rock was dry.

High gain limiting erbium-doped fibre amplifier with over 30 dB dynamic range

1991 ◽  
Vol 27 (3) ◽  
pp. 211 ◽  
Author(s):  
W.I. Way ◽  
D. Chen ◽  
M.A. Saifi ◽  
M.J. Andrejco ◽  
A. Yi-Yan ◽  
...  
Keyword(s):  
Dynamic Range ◽  
High Gain ◽  
Fibre Amplifier ◽  
Erbium Doped

The Ceres, South Africa, earthquake of September 29, 1969

1971 ◽  
Vol 61 (4) ◽  
pp. 851-859 ◽  
Author(s):  
R. W. E. Green ◽  
S. Bloch
Keyword(s):  
South Africa ◽  
Fault Plane ◽  
Large Earthquake ◽  
Aftershock Sequence ◽  
Fault System ◽  
Vertical Fault ◽  
Seismic Recording ◽  
The Republic ◽  
Aftershock Region ◽  
Considerable Damage

abstract Aftershocks following the Ceres earthquake of September 29, 1969, (Magnitude 6.3) were monitored using a number of portable seismic recording stations. Earthquakes of this magnitude are rare in South Africa. The event occurred in a relatively densely-populated part of the Republic, and resulted in nine deaths and considerable damage. Accurate locations of some 125 aftershocks delineate a linear, almost vertical fault plane. The volume of the aftershock region is 3 × 9 × 20 km3 with the depth of the aftershocks varying from surface to 9 km. Aftershocks following the September event had almost ceased when another large earthquake (Magnitude 5.7) occurred on April 14, 1970. Following this event, the frequency and magnitude of aftershocks increased, and they were located on a limited portion of the same fault system delineated by the September 29th aftershocks. Previously-mapped faults do not correlate simply with the fault zone indicated by the aftershock sequence.

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