Aftershock Sequence and Statistics of the 2017 Mw 5.5 Pohang, South Korea, Earthquake: Implications of Fault Heterogeneity and Postseismic Relaxation

2020 ◽  
Vol 110 (5) ◽  
pp. 2031-2046
Author(s):  
Jeong-Ung Woo ◽  
Minook Kim ◽  
Junkee Rhie ◽  
Tae-Seob Kang
Keyword(s):  
Spatial Distribution ◽  
Stress Field ◽  
South Korea ◽  
Aftershock Sequence ◽  
Fault System ◽  
Postseismic Deformation ◽  
Earthquake Catalog ◽  
B Values ◽  
Complex Fault ◽  
Anisotropic Expansion

ABSTRACT The sequence of foreshocks, mainshock, and aftershocks associated with a fault rupture is the result of interactions of complex fault systems, the tectonic stress field, and fluid movement. Analysis of shock sequences can aid our understanding of the spatial distribution and magnitude of these factors, as well as provide seismic hazard assessment. The 2017 Mw 5.5 Pohang earthquake sequence occurred following fluid-induced seismic activity at a nearby enhanced geothermal system site and is an example of reactivation of a critically stressed fault system in the Pohang basin, South Korea. We created an earthquake catalog based on unsupervised data mining and measuring the energy ratio between short- and long-window seismograms recorded by a temporary seismic network. The spatial distribution of approximately 4000 relocated aftershocks revealed four fault segments striking southwestward. We also determined that the three largest earthquakes (ML>4) were located at the boundary of two fault segments. We infer that locally concentrated stress at the junctions of the faults caused such large earthquakes and that their ruptures on multiple segments can explain the high proportion of non-double-couple components. The area affected by aftershocks continues to expand to the southwest and northeast by 0.5 and 1  km decade−1, respectively, which may result from postseismic deformation or sequentially transferred static coulomb stress. The b-values of the Gutenberg–Richter relationship temporarily increased for the first three days of the aftershock sequence, suggesting that the stress field was perturbed. The b-values were generally low (<1) and locally variable throughout the aftershock area, which may be due to the complex fault structures and material properties. Furthermore, the mapped p-values of the Omori law vary along strike, which may indicate anisotropic expansion speeds in the aftershock region.

scholarly journals Machine-Learning-Based High-Resolution Earthquake Catalog Reveals How Complex Fault Structures Were Activated during the 2016–2017 Central Italy Sequence

2021 ◽  
Vol 1 (1) ◽  
pp. 11-19
Author(s):  
Yen Joe Tan ◽  
Felix Waldhauser ◽  
William L. Ellsworth ◽  
Miao Zhang ◽  
Weiqiang Zhu ◽  
...  
Keyword(s):  
Central Italy ◽  
Normal Fault ◽  
Earthquake Sequence ◽  
Fault System ◽  
Earthquake Catalog ◽  
Earthquake Triggering ◽  
Aftershock Activity ◽  
Arrival Times ◽  
Complex Fault ◽  
Fault Structures

Abstract The 2016–2017 central Italy seismic sequence occurred on an 80 km long normal-fault system. The sequence initiated with the Mw 6.0 Amatrice event on 24 August 2016, followed by the Mw 5.9 Visso event on 26 October and the Mw 6.5 Norcia event on 30 October. We analyze continuous data from a dense network of 139 seismic stations to build a high-precision catalog of ∼900,000 earthquakes spanning a 1 yr period, based on arrival times derived using a deep-neural-network-based picker. Our catalog contains an order of magnitude more events than the catalog routinely produced by the local earthquake monitoring agency. Aftershock activity reveals the geometry of complex fault structures activated during the earthquake sequence and provides additional insights into the potential factors controlling the development of the largest events. Activated fault structures in the northern and southern regions appear complementary to faults activated during the 1997 Colfiorito and 2009 L’Aquila sequences, suggesting that earthquake triggering primarily occurs on critically stressed faults. Delineated major fault zones are relatively thick compared to estimated earthquake location uncertainties, and a large number of kilometer-long faults and diffuse seismicity were activated during the sequence. These properties might be related to fault age, roughness, and the complexity of inherited structures. The rich details resolvable in this catalog will facilitate continued investigation of this energetic and well-recorded earthquake sequence.

Combinatorial Framework for Obtaining the Optimal Spatial Distribution of Earthquakes on Complex Fault Systems

2021 ◽  
Author(s):  
Eric Geist ◽  
Tom Parsons
Keyword(s):  
Spatial Distribution ◽  
Combinatorial Optimization ◽  
Objective Function ◽  
Slip Rate ◽  
Optimization Methods ◽  
Fault System ◽  
Sequential Algorithm ◽  
Power Law Distribution ◽  
Frequency Distributions ◽  
Complex Fault

<p>A critical component of seismic hazard analysis is understanding the frequency and spatial distribution of earthquakes with different magnitudes on nearby faults.  A framework for determining the optimal spatial distribution of earthquakes on a complex fault system is developed using combinatorial optimization methods. Input to the framework is a millennia-scale sample of earthquakes taken from a regional Gutenberg-Richter (G-R) relation.  We then determine the optimal spatial arrangement of each earthquake in the fault system according to an objective function and constraints.  Our previously published results focus on minimizing the total misfit in slip rates as the objective function; constraints were maximum and minimum slip rate values that incorporate uncertainty in slip-rate values for each fault.  Both global and local combinatorial optimization methods have been developed to solve these problems: integer programming and the greedy sequential algorithm, respectively. Resulting on-fault magnitude distributions cannot be simply classified as being either purely characteristic or G-R. For example, faults may exhibit multiple “characteristic” magnitudes or a power-law distribution of magnitudes over a restricted range. Current research involves adapting the general combinatorial framework to include other and multiple objective functions, including minimizing the variation in accumulated stress over millennia.  The framework can also accommodate branching and step-over connections for the slip-rate objective, while current research is underway to include interaction stress loading among the different faults in the fault system for stress-based objectives.  Results from these methods are valuable for verifying the assumed magnitude-frequency distributions for faults in probabilistic seismic and tsunami hazard analyses.</p>

scholarly journals Mapping the pre and post seismic crustal stress heterogeneity analogous to 2016, Mw 7.8, Kaikoura quake, New Zealand

2021 ◽  
Author(s):  
Anupama M. ◽  
Sunil P.S.
Keyword(s):  
New Zealand ◽  
Stress Field ◽  
B Value ◽  
Rupture Zone ◽  
Fault System ◽  
Primary Role ◽  
Epicentral Area ◽  
B Values ◽  
The North ◽  
Seismic Stress

Abstract Heterogeneity of pre and post seismic stress states associated to any earthquake play a primary role in understanding the earthquake mechanism and hazard assessment of a seismically dynamic region. The Mw 7.8, November 14, 2016 Kaikoura, New Zealand earthquake offer an unprecedented possibility to observe the heterogeneity in stress field over a very complex fault system wherein subduction zone converges with the strike slip faults system. Here we report the pre and post seismic stress field asperity first time in terms of spatial and temporal variations of b-values associated to the Kaikoura main-shock. Pre seismic spatial disparity of b-value indicates the existence of two prominent low b-value clusters, one towards southwest closer to the epicenter and other to the north of the rupture zone. During co seismic period, owing to the stress release near the epicentral area, the pattern of prominent low b-value pattern has become negligible in the post seismic period. However, the pattern of low b-value in the north of the rupture zone remains similar in the post seismic period indicates the unreleased strain energy in the province. The temporal evaluation of the earthquakes frequency magnitude distributions over a period of two decades also showed an analogous pattern that the b-values were decreased considerably before the large earthquakes in the expanse, which could spawn a larger future earthquakes in the vicinity.

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 Modelling the Spatial Distribution of ASF-Positive Wild Boar Carcasses in South Korea Using 2019–2020 National Surveillance Data

Animals ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 1208
Author(s):  
Jun-Sik Lim ◽  
Timothée Vergne ◽  
Son-Il Pak ◽  
Eutteum Kim
Keyword(s):  
Spatial Distribution ◽  
Risk Factor ◽  
South Korea ◽  
Wild Boar ◽  
African Swine Fever Virus ◽  
Significant Risk ◽  
African Swine Fever ◽  
Credible Interval ◽  
Imperfect Detection ◽  
Wild Boars

In September 2019, African swine fever (ASF) was reported in South Korea for the first time. Since then, more than 651 ASF cases in wild boars and 14 farm outbreaks have been notified in the country. Despite the efforts to eradicate ASF among wild boar populations, the number of reported ASF-positive wild boar carcasses have increased recently. The purpose of this study was to characterize the spatial distribution of ASF-positive wild boar carcasses to identify the risk factors associated with the presence and number of ASF-positive wild boar carcasses in the affected areas. Because surveillance efforts have substantially increased in early 2020, we divided the study into two periods (2 October 2019 to 19 January 2020, and 19 January to 28 April 2020) based on the number of reported cases and aggregated the number of reported ASF-positive carcasses into a regular grid of hexagons of 3-km diameter. To account for imperfect detection of positive carcasses, we adjusted spatial zero-inflated Poisson regression models to the number of ASF-positive wild boar carcasses per hexagon. During the first study period, proximity to North Korea was identified as the major risk factor for the presence of African swine fever virus. In addition, there were more positive carcasses reported in affected hexagons with high habitat suitability for wild boars, low heat load index (HLI), and high human density. During the second study period, proximity to an ASF-positive carcass reported during the first period was the only significant risk factor for the presence of ASF-positive carcasses. Additionally, low HLI and elevation were associated with an increased number of ASF-positive carcasses reported in the affected hexagons. Although the proportion of ASF-affected hexagons increased from 0.06 (95% credible interval (CrI): 0.05–0.07) to 0.09 (95% CrI: 0.08–0.10), the probability of reporting at least one positive carcass in ASF-affected hexagons increased from 0.49 (95% CrI: 0.41–0.57) to 0.73 (95% CrI: 0.66–0.81) between the two study periods. These results can be used to further advance risk-based surveillance strategies in the Republic of Korea.

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.

scholarly journals Assembly of a large earthquake from a complex fault system: Surface rupture kinematics of the 4 April 2010 El Mayor–Cucapah (Mexico) Mw 7.2 earthquake

Geosphere ◽  
2014 ◽  
Vol 10 (4) ◽  
pp. 797-827 ◽  
Author(s):  
John M. Fletcher ◽  
Orlando J. Teran ◽  
Thomas K. Rockwell ◽  
Michael E. Oskin ◽  
Kenneth W. Hudnut ◽  
...  
Keyword(s):  
Large Earthquake ◽  
Fault System ◽  
Surface Rupture ◽  
Complex Fault ◽  
Rupture Kinematics ◽  
Complex Fault System

scholarly journals GEOLOGICAL AND MAGNETIC SUSCEPTIBILITY MAPPING OF MOUNT GIOUCHTA (CENTRAL CRETE)

2017 ◽  
Vol 43 (1) ◽  
pp. 289 ◽  
Author(s):  
E. Kokinou ◽  
E. Kamberis ◽  
A. Sarris ◽  
I. Tzanaki
Keyword(s):  
Magnetic Susceptibility ◽  
Spatial Distribution ◽  
Stress Field ◽  
Magnetic Measurements ◽  
Strain Analysis ◽  
Geological Structure ◽  
Early Pleistocene ◽  
Middle Pleistocene ◽  
Western Slope ◽  
Strain Pattern

Giouchta Mt. is located south of Heraklion city, in Crete. It is an N-S trending morphological asymmetric ridge, with steep western slope whilst the eastern slope represents a smoother relief, composed of Mesozoic limestone and Eocene- lower Oligocene flysch sediments of the Gavrovo -Tripolis zone. The present study focuses on the geological structure of Mt. Giouchta. Field mapping and tectonic analysis is performed for this purpose. The dominant structures are contractional in nature, deformed by normal faulting related to the extensional episodes initiated in Serravallian times. The strain pattern in the area is revealed from strain analysis. It is inferred that the orientation of the stress field in the area has changed several times: the N-S, stress field which was dominant during Late Serravallian times changed to NE-SW (in Late Serravallian? - Early Tortonian) and subsequently to WNW-ESE (Early to Middle Tortonian) to become NW-SE in Late Tortonian. This orientation changed also during the Quaternary times trending from NW-SE (Early Pleistocene) to ENE-WSW (Middle Pleistocene-Holocene). In addition to the above, surface soil samples were collected in the wider area of mount Giouchta and they were analyzed in order to determine the magnetic susceptibility. GIS techniques were used for mapping the spatial distribution of the geological features and the magnetic measurements on the topographic relief of the area. Statistical analysis techniques were also applied in order to investigate the relation of faulting and magnetic susceptibility. Maps representing the spatial distribution of the above measurements were created by using appropriate interpolation algorithms.

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