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.

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 Box Counting Fractal Dimension and Frequency Size Distributon of Earthquakes in the Central Himalaya Region

2021 ◽  
Vol 26 (2) ◽  
pp. 127-136
Author(s):  
Ram Krishna Tiwari ◽  
Harihar Paudyal
Keyword(s):  
Fractal Dimension ◽  
High Stress ◽  
High Strength ◽  
B Value ◽  
Earthquake Catalog ◽  
Central Himalaya ◽  
Study Region ◽  
B Values ◽  
Box Counting ◽  
Earthquake Distribution

To establish the relations between b-value and fractal dimension (D0) for the earthquake distribution, we study the regional variations of those parameters in the central Himalaya region. The earthquake catalog of 989 events (Mc = 4.0) from 1994.01.31 to 2020.10.28 was analyzed in the study. The study region is divided into two sub-regions (I) Region A: 27.3°N -30.3°N and 80°E -84.8°E (western Nepal and vicinity) and (II) Region B: 26.4°N -28.6°N and 84.8°E -88.4°E (eastern Nepal and vicinity). The b-value observed is within the range between 0.92 to 1.02 for region A and 0.64 to 0.74 for region B showing the homogeneous nature of the variation. The seismic a-value for those regions ranges respectively between 5.385 to 6.007 and 4.565 to 5.218. The low b-values and low seismicity noted for region B may be related with less heterogeneity and high strength in the crust. The high seismicity with average b-values obtained for region A may be related with high heterogeneity and low strength in the crust. The fractal dimension ≥1.74 for region A and ≥ 1.82 for region B indicate that the earthquakes were distributed over two-dimensional embedding space. The observed correlation between D0 and b is negative for western Nepal and positive for eastern Nepal while the correlation between D0 and a/b value is just opposite for the respective regions. The findings identify both regions as high-stress regions. The results coming from the study agree with the results of the preceding works and reveal information about the local disparity of stress and change in tectonic complexity in the central Himalaya region.

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.

Rampart seismic zone of central Alaska

1983 ◽  
Vol 73 (3) ◽  
pp. 813-829
Author(s):  
P. Yi-Fa Huang ◽  
N. N. Biswas
Keyword(s):  
Main Shock ◽  
Fault Plane ◽  
B Value ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Lithospheric Plate ◽  
Normal Faults ◽  
Seismic Zone ◽  
The West ◽  
Fault Plane Solutions

abstract This paper describes the characteristics of the Rampart seismic zone by means of the aftershock sequence of the Rampart earthquake (ML = 6.8) which occurred in central Alaska on 29 October 1968. The magnitudes of the aftershocks ranged from about 1.6 to 4.4 which yielded a b value of 0.96 ± 0.09. The locations of the aftershocks outline a NNE-SSW trending aftershock zone about 50 km long which coincides with the offset of the Kaltag fault from the Victoria Creek fault. The rupture zone dips steeply (≈80°) to the west and extends from the surface to a depth of about 10 km. Fault plane solutions for a group of selected aftershocks, which occurred over a period of 22 days after the main shock, show simultaneous occurrences of strike-slip and normal faults. A comparison of the trends in seismicity between the neighboring areas shows that the Rampart seismic zone lies outside the area of underthrusting of the lithospheric plate in southcentral and central Alaska. The seismic zone outlined by the aftershock sequence appears to represent the formation of an intraplate fracture caused by regional northwest compression.

Observations of the Coyote Lake, California earthquake sequence of August 6, 1979

1980 ◽  
Vol 70 (2) ◽  
pp. 559-570 ◽  
Author(s):  
R. A. Uhrhammer
Keyword(s):  
San Francisco ◽  
Fault Zone ◽  
Strong Earthquake ◽  
Main Shock ◽  
B Value ◽  
Aftershock Sequence ◽  
The South ◽  
The North ◽  
Temporal And Spatial Characteristics ◽  
Largest Aftershock

abstract At 1705 UTC on August 6, 1979, a strong earthquake (ML = 5.9) occurred along the Calaveras fault zone south of Coyote Lake about 110 km southeast of San Francisco. This strong earthquake had an aftershock sequence of 31 events (2.4 ≦ ML ≦ 4.4) during August 1979. No foreshocks (ML ≧ 1.5) were observed in the 3 months prior to the main shock. The local magnitude (ML = 5.9) and the seismic moment (Mo = 6 × 1024 dyne-cm from the SH pulse) for the main shock were determined from the 100 × torsion and 3-component ultra-long period seismographs located at Berkeley. Local magnitudes are determined for the aftershocks from the maximum trace amplitudes on the Wood-Anderson torsion seismograms recorded at Berkeley (Δ ≊ 110 km). Temporal and spatial characteristics of the aftershock sequence are presented and discussed. Some key observations are: (1) the first six aftershocks (ML ≧ 2.4) proceed along the fault zone progressively to the south of the main shock; (2) all of the aftershocks (ML ≧ 2.4) to the south of the largest aftershock (ML = 4.4) have a different focal mechanism than the aftershocks to the north; (3) no aftershocks (ML ≧ 2.4) were observed significantly to the north of the main shock for the first 5 days of the sequence; and (4) the b-value (0.70 ± 0.17) for the aftershock sequence is not significantly different from the average b-value (0.88 ± 0.08) calculated for the Calaveras fault zone from 16 yr of data.

Short-term retrospective forecasting of earthquakes based on temporal variations of the b-value of the magnitude-frequency distribution

2020 ◽  
Author(s):  
Paolo Gasperini ◽  
Emanuele Biondini ◽  
Antonio Petruccelli ◽  
Barbara Lolli ◽  
Gianfranco Vannucci
Keyword(s):  
Frequency Distribution ◽  
High Probability ◽  
High Stress ◽  
B Value ◽  
Temporal Variations ◽  
Short Term ◽  
Differential Stress ◽  
B Values ◽  
Probability Of Occurrence ◽  
Seismic Catalog

<p>In some recent works it has been hypothesized that the slope (b-value) of the magnitude-frequency distribution of earthquakes may be related to the differential stress inside the crust.  In particular, it has been observed that low b-values are associated with high stress values and therefore with high probability of occurrence of strong seismic shocks. In this paper we formulate a predictive hypothesis based on temporal variations of the b-value. We tested and optimized such hypothesis retrospectively based on the homogenized Italian instrumental seismic catalog (HORUS) from 1995 to 2018. A comparison is also made with a similar predictive hypothesis based on the occurrence of strong foreshocks.</p><p> </p>

Seismotectonics of the northern Longitudinal Valley, Taiwan, inferred from aftershock sequences of 2018 Mw6.4 and 2019 Mw6.2 Hualien earthquakes

2020 ◽  
Author(s):  
Wei-Fang Sun ◽  
Hao Kuo-Chen ◽  
Zhuo-Kang Guan ◽  
Wen-Yen Chang
Keyword(s):  
Main Shock ◽  
Surface Deformation ◽  
Plate Boundary ◽  
Aftershock Sequence ◽  
Tectonic Structures ◽  
Seismicity Rate ◽  
Transitional Area ◽  
Main Shocks ◽  
Aftershock Sequences ◽  
Seismic Networks

<p>In the Hualien area, two Mw6.4 and Mw6.2 earthquakes, 20 km apart, occurred in February 2018 and April 2019 respectively. The former to the northeast, located offshore to ​​the Liwu river, triggered several earthquake clusters along the Milun fault and southward to the Longitudinal Valley, the suture of the Eurasian and the Philippine Sea plates; the latter to the southwest, located in the Central Range, also triggered several seismic swarms in the Central Range,  along the Liwu river to the northeast and at Ji'an to the southeast. Except for the Milun fault, neither GPS nor InSAR observations detects significant surface deformation after the occurrence of these two main shocks, indicating that the earthquake ruptures mainly developed within the crust. Therefore, seismic observation becomes an efficient tool for revealing the seismotectonics of the two earthquake sequences. For monitoring the aftershock sequences, two days after the main shocks, we deployed two geophone arrays, 70 Z-component RefTek 125A TEXANs for two weeks in 2018 and 47 three-component Fairfield Nodal Z-Lands for one month in 2019, with 1-5 km station spacing around the Hualien City. These earthquake swarms were well recorded and analyzed through the dense seismic networks. The numbers of aftershock sequences manually identified are two-fold more than that issued by the Central Weather Bureau, Taiwan. The seismicity of the 2018 aftershock sequence, to depths of between 5-15 km, was significantly reduced within 10 days after the main shock. however, the seismicity of the 2019 aftershock sequence, to depths of between 2-50 km, was still above background seismicity rate 30 days after the main shock. The spatial distribution of the 2018 aftershock sequence could be related to a fault zone of the plate boundary, but that of the 2019 and the relocated 1986 aftershock sequences show a conjugate thrust fault pair beneath the eastern Central Range. Our results clearly depict several local tectonic structures that have not been observed at the northern tip of the Longitudinal Valley, not only a suture but also a transitional area from collision to subduction.</p>

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 Spatial Variations in Seismicity Characteristics in and Around the Source Region of the 2019 Yamagata-Oki Earthquake, Japan

2020 ◽  
Author(s):  
Taku Ueda ◽  
Aitaro Kato ◽  
Yoshihiko Ogata ◽  
Lina Yamaya
Keyword(s):  
Source Region ◽  
B Value ◽  
Spatial Variations ◽  
Japan Meteorological Agency ◽  
Earthquake Catalog ◽  
Shear Strain Rate ◽  
Study Region ◽  
Etas Model ◽  
Seismicity Rate ◽  
Background Seismicity

Abstract The 2019 {\text{M}}_{\text{j}} 6.7 Yamagata-Oki Earthquake occurred adjacent to the northeastern edge of the source region of the 1964 {\text{M}}_{\text{j}} 7.5 Niigata Earthquake, offshore of Yamagata Prefecture, Japan. Few aftershocks occurred in the source region of the Yamagata-Oki earthquake immediately following the Niigata earthquake, and the recent seismicity rate in this region is extremely low compared with that of the surrounding region. This spatial variation in seismicity may allow us to elucidate plausible physical processes that shape the spatiotemporal evolution of these shallow-crustal environments. Here, we investigate the spatial variations in seismicity characteristics by applying the HIerarchical Space–Time Epidemic Type Aftershock Sequence (HIST-ETAS) model to an earthquake catalog compiled by the Japan Meteorological Agency for events in and around the Yamagata-Oki earthquake rupture region. We compare spatial variations in the background seismicity rate and aftershock productivity estimated from the HIST-ETAS model with the geophysical features in the study region. The background seismicity rate is high along the eastern margin of the Sea of Japan and correlates well with a previously identified zone that possesses a high geodetic shear strain rate. The two major earthquakes occurred in and around an active shear zone, suggesting that the background seismicity rate may serve as a key parameter for evaluating seismic hazard across the Japanese Archipelago. Furthermore, the source region of the Yamagata-Oki earthquake has a higher aftershock productivity, lower b-value, and lower seismic-wave velocity than that of the Niigata earthquake. We interpret this low-velocity zone to be a well-developed damaged rock that resulted in both a reduction in the b-value and an increase in aftershock productivity based on previous laboratory experiments and numerical results; this damage makes the rock more ductile at the macroscopic scale. The higher ductility in the source region of the Yamagata-Oki earthquake may have worked as a soft barrier against the propagation of dynamic rupture that occurred during the Niigata earthquake.

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.

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