A morpho-tectonic approach to the study of earthquakes in Rome

Rome has the world’s longest historical record of felt earthquakes, with more than 100 events during the last 2,600 years. However, no destructive earthquake has been reported in the sources and all of the greatest damage suffered in the past has been attributed to far-field events. While this fact suggests that a moderate seismotectonic regime characterizes the Rome area, no study has provided a comprehensive explanation for the lack of strong earthquakes in the region. Through the analysis of the focal mechanism and the morphostructural setting of the epicentral area of a "typical" moderate earthquake (ML = 3.3) that recently occurred in the northern urban area of Rome, we demonstrate that this event reactivated a buried segment of an ancient fault generated under both a different and a stronger tectonic regime than that which is presently active. We also show that the evident structural control over the drainage network in this area reflects an extreme degree of fragmentation of a set of buried faults generated under two competing stress fields throughout the Pleistocene. Small faults and a present-day weaker tectonic regime with respect to that acting during the Pleistocene explain the lack of strong seismicity and imply that a large earthquake could not reasonably occur.

Shear strength behaviour of liquefiable sand of petobo on treated by agarose under direct shear test

Liquefaction process is associated with the loss of the shear strength of the saturated loose sands caused by strong earthquakes. Due to mitigitation of liquefaction hazard, an appropriate mitigation of liquefaction using environmentally friendly methods is critical and becoming increasingly important and unavoidable. The laboratory investigation was carried out to study the shear strength behaviour of liquefiable sand of Petobo treated by agarose on different concentration 1%,3% 5%. A series of direct shear test were conducted under three level of vertical stress 10 kPa, 20 kPa, and 30 kPa on the specimen. It was found that the optimum content of agarose which can be considered is at 1%-3%, using stress ratio (τ/σv) analysis shows that stress ratio decreases with increasing the vertical stress on the same agar content. The implication this result that the application of this method must consider variation of material source and characteristic, and the suitable level of vertical stresses.

Assessment of the seismotectonic potential of the East Caucasus blocks

Восточный Кавказ является самым сейсмоактивным регионом европейской части России, в сферу повышенных сейсмических воздействий которого попадают крупные энергетические объекты Сулакского каскада ГЭС, высоковольтные линии электропередач, основные транспортные коммуникации, нефте- и газопроводы федерального и республиканского значения, аэро- и морской порты и крупные, разросшиеся города и поселки региона. Отсутствие исследований по проблеме оценки сейсмотектонической и геотектонической ситуации значительно ослабляет готовность региона к предупреждению обширных экологических и техногенных катастроф. Современные сейсмически активные зоны Восточного Кавказа в условиях позднеальпийского тектогенеза характеризуются иными геодинамическими и сейсмотектоническими условиями по сравнению с герцинской и киммерийской. Альпийский этап тектогенеза характеризуется значительным максимумом своей активизации, с которым связаны современные геодинамические и сейсмотектонические процессы и повышенная современная сейсмическая активность региона. Уровень сейсмотектонического потенциала, как сейсмического и геодинамического, является важнейшим показателем оценки степени сейсмической опасности. Цель исследования. Оценка уровня сейсмотектонического потенциала блоков земной коры северо-восточного сегмента Восточного Кавказа и выделение потенциальных зон ожидания возможных очагов сильных землетрясений региона (ВОЗ). Методы исследования. Анализ пространственно-временного распределения сейсмичности за инструментальный период наблюдений и экспертная оценка сейсмотектонического потенциала блоков земной коры по комплексу сейсмологических показателей, таких как мощность сейсмоактивного слоя, сейсмическая активность и наклон графика повторяемости землетрясений, максимальная отмеченная (наблюденная) магнитуда, период последней активизации и тектоническая активность. Результаты исследования. Закартировано в условных единицах изменение сейсмотектонического потен­циала на исследуемой территории. Составлена схема зон возможных очагов землетрясений региона. Распределение гипоцентров сильных землетрясений демонстрирует расслоенность геологической среды региона. В интервале глубин, в среднем от 33 до 42 км, выделяется так называемая «зона молчания» The Eastern Caucasus is the most seismically active region of the European part of Russia, where large energy facilities of the Sulak cascade of hydroelectric power plants, high-voltage power lines, main transport communications, oil and gas pipelines of federal and republican significance, air and sea ports and large, sprawling cities and towns of the region fall into the sphere of increased seismic impacts. The lack of research on the problem of assessing the seismotectonic and geotectonic situation can lead to undesirable environmental consequences. Modern seismically active zones of the Eastern Caucasus in the conditions of Late Alpine tectogenesis are characterized by different geodynamic and seismotectonic conditions compared to the Hercynian and Cimmerian. The Alpine stage of tectogenesis is characterized by a significant maximum of its activation, which is associated with modern geodynamic and seismotectonic processes and increased modern seismic activity of the region. The level of seismotectonic potential, both seismic and geodynamic, is the most important indicator of assessing the degree of seismic danger. Aim. Assessment of the level of the seismotectonic potential of the crustal blocks of the north-eastern segment of the Eastern Caucasus and identification of potential waiting zones for possible foci of strong earthquakes in the region (PFE). Methods. Analysis of the spatiotemporal distribution of seismicity over the instrumental observation period and expert assessment of the seismotectonic potential of the Earth's crust blocks according to a set of seismological indicators, such as the power of the seismoactive layer, seismic activity and the slope of the earthquake recurrence graph, the maximum marked (observed) magnitude, the period of the last activation and tectonic activity. Results. The change in the seismotectonic potential in the studied territory is mapped in conventional units. A diagram of the zones of possible earthquake foci in the region has been compiled. The distribution of hypocenters of strong earthquakes demonstrates the stratification of the geological environment of the region. In the depth range, on average from 33 to 42 km, the so-called "zone of silence" is allocated.

Identification of ionospheric earthquake precursors in Kamchatka region based on correlation analysis

Сейсмическая активность является одним из источников изменчивости ионосферы. В данной работе на основе методики [1] исследовано изменение концентрации электронов в ионосфере, предшествующее наступлению сильных землетрясений с M ≥ 6.0 в Камчатском регионе. Данная методика основана на вычислении коэффициента корреляции между значениями критической частоты foF2 двух ионосферных станций, одна из которых расположена внутри зоны подготовки землетрясения, а другая – за ее пределами. Рассмотрены данные, полученные на двух станциях PETROPAVLOVSK (PK553) и EARECKSON (EA653) за период 01.09.2018–30.04.2021 гг. Статистический анализ критических частот foF2 показал, что для 6 из 8 землетрясений с M ≥ 6.0, произошедших на глубинах до 100 км и эпицентральных расстояниях до 500 км от расположения PK553, за 1–12 суток до их наступления предшествовало заметное уменьшение коэффициента корреляции. Seismic activity is one of the sources of ionospheric variability. In this work, we investigate electron concentration change in the ionosphere, preceding the onset of strong earthquakes with M ≥ 6.0 in Kamchatka region. The research technique is based on calculating the correlation coefficient between the critical frequency foF2 values at two ionospheric stations. One of them is located inside the earthquake preparation zone, and the other is outside it. The data obtained at two stations PETROPAVLOVSK (PK553) and EARECKSON (EA653) for the period 01.09.2018–30.04.2021 are considered. Statistical analysis of the critical frequencies foF2 showed that a noticeable decrease in the correlation coefficient was observed 1–7 days before the earthquakes for 6 out of 8 events with M ≥ 6.0 that occurred at depths of up to 100 km and epicentral distances of up to 500 km from the PK553 location.

Modeling of relative shear deformation zones before strong earthquakes in Kamchatka from 2018-2021

Представлено сравнительное моделирование зон относительных сдвиговых деформаций для четырех камчатских землетрясений с Mw ≥ 4.8, произошедших в период с декабря 2018 г. по март 2021 г., основанное на статической модели деформационного поля в рамках теории упругости. Земная кора рассмотрена как однородное изотропное упругое полупространство, в котором присутствуют различные источники напряжения, описывающие очаг землетрясения: точечный источник в виде единичной силы, точечный источник в виде комбинации девяти двойных сил, распределенный источник в виде прямоугольной площадки. We present a comparative modeling of the zones of relative shear deformation for four Kamchatka earthquakes Mw≥4.8 that occurred between December 2018 and March 2021. Modeling based on a static model of the deformation field in the framework of the theory of elasticity. The Earth’s crust is considered as a homogeneous isotropic elastic half-space, in which there are different sources of stress describing the source of the earthquake: a point source in the form of a single force, a point source in the form of a combination of nine double forces, a distributed source in the form of a rectangular area.

Combining a Locally Restricted Anisotropic Spatial Kernel with an ETAS Incomplete Model for Better Forecasts of the 2019 Ridgecrest Sequence

Strong earthquakes cause aftershock sequences that are clustered in time according to a power decay law, and in space along their extended rupture, shaping a typically elongate pattern of aftershock locations. A widely used approach to model seismic clustering is the Epidemic Type Aftershock Sequence (ETAS) model, that shows three major biases: First, the conventional ETAS approach assumes isotropic spatial triggering, which stands in conflict with observations and geophysical arguments for strong earthquakes. Second, the spatial kernel has unlimited extent, allowing smaller events to exert disproportionate trigger potential over an unrealistically large area. Third, the ETAS model assumes complete event records and neglects inevitable short-term aftershock incompleteness as a consequence of overlapping coda waves. These three effects can substantially bias the parameter estimation and particularly lead to underestimated cluster sizes. In this article, we combine the approach of Grimm (2021), which introduced a generalized anisotropic and locally restricted spatial kernel, with the ETAS-Incomplete (ETASI) time model of Hainzl (2021), to define an ETASI space-time model with flexible spatial kernel that solves the abovementioned shortcomings. We apply different model versions to a triad of forecasting experiments of the 2019 Ridgecrest sequence, and evaluate the prediction quality with respect to cluster size, largest aftershock magnitude and spatial distribution. The new model provides the potential of more realistic simulations of on-going aftershock activity, e.g.~allowing better predictions of the probability and location of a strong, damaging aftershock, which might be beneficial for short term risk assessment and desaster response.

SEISMICITY of KAMCHATKA and COMMANDER ISLANDS in 2015

The seismicity review of Kamchatka and surrounding territories for 2015 is given. In the Kamchatka earthquake catalogue, the minimum local magnitude of completeness is MLmin=3.5, and for earthquakes with h≥350 km under the Okhotsk sea MLmin=3.6. The Kamchatka earthquake catalogue for 2015 with ML3.5, published in the Appendix to this issue, includes 1213 events. 92 earthquakes of the catalogue with ML=3.0–6.5 were felt in Kamchatka and surrounding areas with seismic intensity I=2–6 according to the MSK-64 scale. For all events with ML5.0 that occurred in 2015 in the KB GS RAS area of responsibility, an attempt to calculate the seismic moment tensor (SMT) was made. There are 32 such events in the regional catalogue. For 28 earthquakes the SMT and depth h of the equivalent point source were calculated successfully. The calculations were performed for the SMT double-couple model using a nonlinear algorithm. In 2015, a typical location of the earthquake epicenters was observed in the Kamchatka zone. In 2015, the seismicity level in all selected zones and in the region as a whole correspond to the background one according to the “SESL’09” scale. The number of recorded events with ML3.5 and strong earthquakes with ML5.0 is close to the average annual value. Anomalous and significant events were not recorded.