The Evolution of Preseismic Patterns Related to the Central Crete (Mw6.0) Strong Earthquake on 27 September 2021 Revealed by Multiresolution Wavelets and Natural Time Analysis

On 27 September 2021, a shallow earthquake with focal depth of 10 km and moment magnitude Mw6.0 occurred onshore in central Crete (Greece). The evolution of possible preseismic patterns in the area of central Crete before the Mw6.0 event was investigated by applying the method of multiresolution wavelet analysis (MRWA), along with that of natural time (NT). The monitoring of preseismic patterns by critical parameters defined by NT analysis, integrated with the results of MRWA as the initiation point for the NT analysis, forms a promising framework that may lead to new universal principles that describe the evolution patterns before strong earthquakes. Initially, we apply MRWA to the interevent time series of the successive regional earthquakes in order to investigate the approach of the regional seismicity towards critical stages and to define the starting point of the natural time domain. Then, using the results of MRWA, we apply the NT analysis, showing that the regional seismicity approached criticality for a prolonged period of ~40 days before the occurrence of the Mw6.0 earthquake, when the κ1 natural time parameter reached the critical value of κ1 = 0.070, as suggested by the NT method.

Modified data on geoeffective solar flares and seismic noise variations

The problem of the relationship between strong magnetic swarms caused by solar flares and variations in seismicity is considered. The data on the temporal dependences of the parameters of seismic noise (average level, and standard deviation, RMS) recorded by the stations of the KNET seismic network have been used as the output data of monitoring the territory of the Bishkek geodynamic proving ground (Northern Tien Shan). The signatures of the influence of a magnetic swarm that occurred after an ultra-strong solar flare on September 6, 2017 have been established. The results obtained on the increase in seismic noise after this super-strong eruptive event are consistent with the results of studies on the influence of magnetic swarms on changes in regional seismicity.

Improved Seismic Monitoring with OBS Deployment in the Arctic: A Pilot Study from Offshore Western Svalbard

The mid-ocean ridge system is the main source of earthquakes within the Arctic region. The earthquakes are recorded on the permanent land-based stations in the region, although smaller earthquakes remain undetected. In this study, we make use of three Ocean Bottom Seismographs (OBSs) that were deployed offshore western Svalbard, along the spreading ridges. The OBS arrival times were used to relocate the regional seismicity using a Bayesian approach, which resulted in a significant improvement with tighter clustering around the spreading ridge. We also extended the regional magnitude scales for the northern Atlantic region for OBSs by computing site correction terms. Besides location and magnitude improvement, the OBS network was able to detect hundreds of earthquakes, mostly with magnitude below Mw=3, including a swarm activity at the Molloy Deep. Our offshore observations provide further evidence of a low velocity anomaly offshore Svalbard, at the northern tip of Knipovich ridge, that was previously seen in full waveform inversion. We conclude that even a single permanent OBS near the ridge would make a significant difference to earthquake catalogs and their interpretation.

Perspectives on Clustering and Declustering of Earthquakes

Clustering is a fundamental feature of earthquakes that impacts basic and applied analyses of seismicity. Events included in the existing short-duration instrumental catalogs are concentrated strongly within a very small fraction of the space–time volume, which is highly amplified by activity associated with the largest recorded events. The earthquakes that are included in instrumental catalogs are unlikely to be fully representative of the long-term behavior of regional seismicity. We illustrate this and other aspects of space–time earthquake clustering, and propose a quantitative clustering measure based on the receiver operating characteristic diagram. The proposed approach allows eliminating effects of marginal space and time inhomogeneities related to the geometry of the fault network and regionwide changes in earthquake rates, and quantifying coupled space–time variations that include aftershocks, swarms, and other forms of clusters. The proposed measure is used to quantify and compare earthquake clustering in southern California, western United States, central and eastern United States, Alaska, Japan, and epidemic-type aftershock sequence model results. All examined cases show a high degree of coupled space–time clustering, with the marginal space clustering dominating the marginal time clustering. Declustering earthquake catalogs can help clarify long-term aspects of regional seismicity and increase the signal-to-noise ratio of effects that are subtler than the strong clustering signatures. We illustrate how the high coupled space–time clustering can be decreased or removed using a data-adaptive parsimonious nearest-neighbor declustering approach, and emphasize basic unresolved issues on the proper outcome and quality metrics of declustering. At present, declustering remains an exploratory tool, rather than a rigorous optimization problem, and selecting an appropriate declustering method should depend on the data and problem at hand.

Physics-Based Simulation of Spatiotemporal Patterns of Earthquakes in the Corinth Gulf, Greece, Fault System

ABSTRACT A physics-based earthquake simulation algorithm for modeling the long-term spatiotemporal process of strong (M ≥ 6.0) earthquakes in Corinth Gulf area, Greece, is employed and its performance is explored. The underlying physical model includes the rate- and state-dependent frictional formulation, along with the slow tectonic loading and coseismic static stress transfer. The study area constitutes a rapidly extending rift about 100 km long, where the deformation is taken up by eight major fault segments aligned along its southern coastline, and which is associated with several strong (M ≥ 6.0) earthquakes in the last three centuries, since when the historical earthquake catalog is complete. The recurrence time of these earthquakes and their spatial relation are studied, and the simulator results reveal spatiotemporal properties of the regional seismicity such as pseudoperiodicity as well as multisegment ruptures of strong earthquakes. As the simulator algorithm allows the display of the stress pattern on all the single elements of the fault, we are focusing on the time evolution of the stress level before, during, and after these earthquakes occur. In this respect, the spatiotemporal variation of the stress and its heterogeneity appear to be correlated with the process of preparation of strong earthquakes in a quantitative way.

Seismicity Patterns Prior to the Thessaly (Mw6.3) Strong Earthquake on 3 March 2021 in Terms of Multiresolution Wavelets and Natural Time Analysis

On 3 March 2021, a strong, shallow earthquake of moment magnitude, Mw6.3, occurred in northern Thessaly (Central Greece). To investigate possible complex correlations in the evolution of seismicity in the broader area of Central Greece before the Mw6.3 event, we apply the methods of multiresolution wavelet analysis (MRWA) and natural time (NT) analysis. The description of seismicity evolution by critical parameters defined by NT analysis, integrated with the results of MRWA as the initiation point for the NT analysis, forms a new framework that may possibly lead to new universal principles that describe the generation processes of strong earthquakes. In the present work, we investigate this new framework in the seismicity prior to the Mw6.3 Thessaly earthquake. Initially, we apply MRWA to the interevent time series of the successive regional earthquakes in order to investigate the approach of the regional seismicity at critical stages and to define the starting point of the natural time domain. Then, we apply the NT analysis, showing that the regional seismicity approached criticality a few days before the occurrence of the Mw6.3 earthquake, when the κ1 natural time parameter reached the critical value of κ1 = 0.070.

Analyzing the Impact of Large Dams on Seismicity Patterns around Their Locations

Dam construction is one of the most popular solutions for managing water resources. In recent years, changes in patterns of regional seismicity associated with large impoundment dams have raised concerns among environmentalists. In this study, five large dams located in Iran were studied from this perspective. The Gutenberg-Richter, linear regression and T-test were used to examine the seismic changes in the radius of 100 km of each of the dams during a twenty-five-year period before and after the construction of the dams. The results revealed that the seismicity level and relative density of large and small earthquakes in three of these dams have increased after dam construction. A significant difference between the magnitude of earthquakes, as well as the number of earthquakes before and after the construction of dams in the region, was recognized. However, the results of the T-test statistical analysis indicated that the mean depth of the earthquakes and their distance from the dams before and after construction have not changed significantly. Overall, these results indicated that the construction of large impoundment dams has been associated with some changes in patterns of regional seismicity. The findings would guide researchers to further investigate the type of impacts that dam construction may have on seismicity patterns.

Influence of crustal lithology and the thermal state on microseismicity in the Wakayama region, southern Honshu, Japan

Here we investigate the influence of the lithology and thermal state of the upper crust on earthquake distributions beneath the Wakayama region, southern Honshu, Japan, to better understand the influence of crustal conditions on regional seismogenesis. The earthquakes are concentrated in the deeper sections of mafic belts and shallower sections of pelitic belts, based on a comparison of relocated hypocenters and estimated subsurface geological structures. We compare the frictional properties of pelitic rocks and basalt, as obtained from petrological experiments, with the hypocenter depth distributions in pelitic and mafic belts to assess the control of crustal lithology on the depth extent of regional seismicity. The earthquake distributions are consistent with the temperature ranges over which the respective rock types are expected to exhibit a velocity-weakening behavior, based on the petrological experiments. The results suggest that the occurrence of shallow intraplate earthquakes is controlled by the temperature- and lithology-dependent friction of the upper crust.