On the similarity of shear deformation of a granular massif and a fragmented medium in the seismically active area

In recent research, the dynamics of the medium located in the seismic region at the boundary of tectonic plates is considered as the behavior of a complex open system that is in a state of self-organized criticality. Such an approach results from the very laws of earthquake generation and the complex structure of these areas. The network of faults and cracks makes seismic zones significantly heterogeneous and fragmented. Therefore, discrete models are increasingly used to model the dynamics of these media. The basis for comparing the model and the full-scale object serves the statistical regularities of their dynamic deformation. Relying on this concept, in the paper it is modeled the shear dynamics of a granular massif composed of identical cubic granules and is compared system’s statistical characteristics with the similar characteristics obtained for the earthquake generation zone. Shear deformation is carried out by means of the box consisting of two parts — movable and immovable ones. The movable part possesses the cover which receives kinetic energy from the granular massif in the process of shear deformation. For numerical simulations of the shear dynamics, the discrete element method is applied. The numerical calculations result in the distribution of cover’s kinetic energy jumps simulating the perturbations transmitted from the granular system to an external medium. It turned out that the distribution for these perturbations is the power dependence with an exponent that is inherent in earthquakes (Gutenberg-Richter law). Before and after large perturbations it is observed the swarms of smaller perturbations which are the analogues of foreshocks and aftershocks. The distributions of element’s velocity fluctuations and the correlation of velocity fluctuations are calculated as well. It is revealed the similarity of distributions for velocity fluctuations in the model massif and in the seismically active region of California, which includes the San Andreas fault. Moreover, the similarity of corresponding correlation functions is shown. They both are the functions of the stretched exponent. The obtained result indicates that shear processes in granular massifs and natural seismic processes in the San Andreas Fault are statistically similar.

On the depth-dependent stress accumulation for earthquake generation process

<p>Dynamic rupture simulation of an earthquake mostly aims at a characteristic event, which may rupture the entire seismogenic zone of a fault system, perhaps reaching the ground surface. However, hazardous earthquakes sometimes occur along a part of the depths of a fault. Many questions arise why only this particular depth does rupture and whether the surrounding part remains hazardous. Previously, Aochi (GJI, 2018) has considered a depth-dependent stress accumulation for emphasizing the difference of reverse and normal faults under the hypothesis that stress is sufficiently and uniformly charged at all depths. We probably need to revise this hypothesis and the partially charged fault along depth would be more suitable for explaining the given question. By developing the previous simulations by Aochi (GJI, 2018), we carry out numerical simulations for demonstrating the importance of the depth-dependent stress accumulation.   </p>

A sanity check for earthquake recurrence models used in PSHA of slow deforming regions: the case of SW Iberia

<p>Probabilistic Seismic Hazard Assessment (PSHA) is the most common tool used to decide on the acceptable seismic risk and corresponding mitigation measures. One key component of these studies is the earthquake generation model comprising the definition of source zones and recurrence relationships. Slow deforming regions are particularly challenging for PSHA since the inferred return period for large earthquakes is longer than the instrumental and historical seismicity records, and the relationship between known or probable active faults and seismicity is uncertain. Therefore, in these areas PSHA results show a large variability that impairs its acceptance by the political decision-makers and the public in general. We propose two consistency tests to address the variability of earthquake generation models found in PSHA studies: i) one rule-of-thumb test where the seismic moment release from the model is converted to an average slip on a typical fault and compared with known plate kinematics or GNSS deformation field; ii) using a neotectonic model, the computed deformation is converted into seismic moment release and to a synthetic earthquake catalogue. We apply these tests to the W and SW Iberia slow deforming region, where two earthquake source areas are investigated: 1) the Lower Tagus Valley, one of the largest seismic risk zones of Portugal; and 2) the offshore SW Iberia area, considered to be the source for the 1<sup>st</sup> November 1755 event (M~8.7). Our results show that some of the earthquake source models should be regarded as suspicious, given their high/low moment release when compared to the expected values from GNSS observations or neotectonic modelling. In conclusion, PSHA studies in slow deforming regions should include a similar sanity check on their models’ evaluation, downgrading the weight of poorly compliant models.</p>

Dike Inflation Process Beneath Sakurajima Volcano, Japan, During the Earthquake Swarm of August 15, 2015

Magma intrusion usually causes seismicity and deformation in the surrounding rock and often leads to eruptions. A swarm of volcano-tectonic (VT) earthquakes associated with rapid dike intrusion in hours occurred beneath Sakurajima volcano on August 15, 2015. We determined the hypocenters and focal mechanisms of the VT earthquake swarm. The distributions of pressure (P)- and tension (T)-axes of the azimuths of the mechanisms are also obtained. The results indicate spatiotemporal changes of the distributions of the hypocenters and P- and T-axes. The hypocenters are distributed at depths of 0.3–1 km and 7:00–10:30 JST, and are located at depths of 0.3–3 km and 10:30–12:00 during which the seismic activity is the largest. At 12:00–24:00, the hypocenters are distributed in shallow and deep clusters at depths of 0.2–1 km and 1.5–3.5 km, respectively. The dike induced rapid ground deformation and is located between the shallow and deep clusters. The strike and opening directions of the dike are parallel to the NE–SW and NW–SE directions, respectively, corresponding to the regional maximum and minimum compression stress. The T-axes of the shallow cluster are distributed parallel to the opening direction of the dike. The P-axes of the deep cluster exhibit a pattern corresponding to the NE–SW direction and the T-axes are distributed in the NW–SE direction. In contrast, a 90° rotated pattern of strike-slip faulting is also observed at the deep cluster at 12:00–24:00, where the P-axes are distributed in the NW–SE direction and the T-axes are distributed in the NE–SW direction. This reflects the change in the stress field due to the dike inflation during the earthquake generation, and indicates that the alteration of stress in the vicinity of the dike due to the dike inflation and VT earthquakes are induced by the differential stress exceeding the brittle fracture strength of the rock. Future seismic and deformation observations in volcanoes will verify whether the spatiotemporal changes of the hypocenters and focal mechanism shown by this study are unique features of rapid dike intrusion.

Focal mechanisms of small earthquakes beneath the Japanese islands based on first-motion polarities picked using deep learning

SUMMARY Knowledge of crustal stress fields is essential for understanding tectonics and earthquake generation. One approach for estimating the crustal stress field is based on the focal mechanisms of earthquakes. This study investigated the focal mechanisms of approximately 110 000 microearthquakes in the area of the Japanese islands that occurred at a depth shallower than 20 km, based on the first-motion polarities picked by a simple neural network model. The model was first trained using a data set of mainly moderate to large earthquakes throughout Japan. Following on, the model was re-trained using a data set of microearthquakes in two regions of Japan. The threshold of the confidence score from the neural network model was chosen to maximize the overall quality of the focal mechanism solutions. The P- and T-axes of the numerous focal mechanism solutions provided more detailed distributions of the crustal stress field. For example, in the Chugoku region, small differences were observed in the trend of P-axes azimuths between the northern and southern areas, spatially corresponding to geodetic observations. The results of this study are useful for revealing the crustal stress field, and, as such, for assessing past and current tectonic activities and potential future earthquake generation.

Special Issue on Earthquake and Volcano Hazards Observation and Research Program

The Earthquake and Volcano Hazards Observation and Research Program (2014–2018) carried out comprehensive research to mitigate disasters related to earthquakes and volcanic eruptions. The program selected multidisciplinary research in which earth scientists who study the processes of earthquake generation and volcanic eruptions, historians, archaeologists, human and social scientists, and engineers were all involved. The program aimed to collect pre-instrumental and pre-historical earthquake and volcanic data to understand earthquake and volcano disasters, to find risk evaluation techniques, and to evaluate disaster response and preparedness. Active collaborations between researchers from different science fields inspired new ideas and have driven various research in the program. New findings from the program have also created international collaborations and recognitions. Most of the results and new findings in the program have already been published in various internationally recognized journals and have greatly influenced scientific communities. We believe that it is important to compile our findings from the last five years of the program and to publish the essence of our findings and published papers in this special issue. We hope that this special issue will be of value to researchers who are interested in multidisciplinary studies of mitigation of disasters such as earthquakes, volcanic eruptions, and related phenomena.

The Advancement of Research on Inland Earthquake Generation 2014–2018

The 2011 Tohoku-Oki Earthquake (M9.0) significantly affected inland areas of Japan. The crust and mantle response to the magathrust earthquake induced changes in the mechanical conditions of the seismogenic zone. Here we present important progress in the research into the seismogenesis of inland earthquakes. Stress, strain, strength, and structures are key parameters affecting the occurrence of earthquakes. In particular, both the spatial and temporal changes in these parameters around the focal areas of the large inland earthquakes have been detected and modeled. These results have provided spatial potential evaluation in terms of future inland earthquake occurrence. However, we clearly recognize that, in order to understand and predict the inland earthquake generation process, it will inevitably be necessary to unify the research on various spatial and temporal scales, from problems related to long-term stress loading from plate-relative motion to instant fault response.