MUYAKAN EARTHQUAKE SEQUENCE in 2015 (Northern Baikal Region)

We present the results of studies of the seismic regime, focal mechanisms, and macroseismic data in the area of the largest Muyakan activation in 2015 (northern Baikal region). Due to the deployment of a network of temporary seismic stations, the number of registered earthquakes (KR≥3) increased significantly in 2015 and reached  30 thousand. Spatio-temporal development of the considered activation is characterized by stable low values of earthquake hypocenters and dividing the epicentral field into two clusters – eastern and north-western ones. Both clusters are connected with local stress-strain field (rift type and strike-slip, respectively), while the general regime of seismotectonic deformations of the crust in the activation area, calculated from the statistical analysis of 77 focal mechanisms of Muyakan earthquakes (KR≥9.2), demonstrates the predominance of submeri-dional horizontal extension. Macroseismic effects from the largest earthquakes of the Muyakan sequence were felt, predominantly, in Severomuysk settlement (=10–15 km, I=5). New data on strong motions, obtained from the records of the seismic station with the same name, have significantly complemented the database for the territory of the north-eastern flank of the Baikal rift zone. In general, the obtained results could be used to clarify the seismic hazard of the considered area and to improve the instrumental part of seismic scales.

The Influence of the Rate of Increase of Ground Vibration Accelerations During Earthquakes on the Value of the Observed Macroseismic Effect

Aims: It is known that along with the traditionally considered amplitudes and durations of ground vibrations, the rate of increase in the intensity of ground vibrations in time can also affect the level of macroseismic effects caused by earthquakes. According to the previously obtained correlations, the differences between the observed macroseismic effects during earthquakes with slow and fast increases in the amplitude level of oscillations can reach one point of the macroseismic scale. The purpose of these study is to obtain, on the basis of a significantly (almost 9 times) larger than before, the volume of initial data (in combination with a more effective method of analysis) new and more accurate quantitative estimates of the studied dependences, as well as their possible interpretation. Background: The work continues the research began in 1985-1989. A representative statistical material was used, including 1250 accelerograms of earthquakes that occurred in different regions of the world, with magnitudes M = 2.5-7.7, distances of 5-230 km and independent estimates of macroseismic intensities I = 3-10 points by the MSK or MMI. Objective: Correlations between the absolute and relative rates of increase of ground vibration accelerations during earthquakes with different magnitudes and distances, on the one hand, and macroseismic effects caused by these vibrations, on the other, are considered. Methods: The study was carried out in the form of a direct statistical comparison of the parameters describing the form of ground vibrations during earthquakes with the characteristics of variations in macroseismic effects caused by these vibrations. A sample was formed and analyzed, including 1250 accelerograms of sensible and strong earthquakes recorded in various regions of the world and having independent estimates of the macroseismic intensity of shaking at instrumental registration sites. Results: It is shown that the macroseismic intensity of shaking can depend on the relative rate of increase of acceleration amplitudes in the general wavetrain of ground vibrations. An increase in the macroseismic intensity of shaking was observed with an increase in the relative rate of increase of the amplitudes and, conversely, it decreases with a slowdown in the rate of increase of the acceleration intensity. Similar constructions, made according to the data of the Time-Frequency Signal Analysis (TFSA) of 50 accelerograms of earthquakes with M = 3.3-6.2, a distance of 7-139 km and a macroseismic intensity of 4-7 MMI points, showed the same dependence, but clearer and with large coefficients of regression and correlation. The difference between earthquakes with “fast” and “slow” accelerations in the intensity I can reach one MSK point. Conclusion: The results of this study indicate that the rate of increase in the acceleration of ground vibrations during earthquakes can in a certain way affect the macroseismic effects. Earthquakes with slowly increasing amplitudes of ground vibration accelerations form average less macroseismic effects than those with rapidly growing accelerations. Variations in the shaking intensity, at the same time, are quite significant and can be compared with variations associated with differences in soil-geomorphological conditions, focal mechanisms, general seismotectonic conditions and other factors that are traditionally taken into account in detailed assessments of seismic hazard. Therefore, this factor should also be taken into account when conducting such studies.

Upper-plate structural controls on the segmentation of the Kefalonia Fault (Ionian Sea, Greece)

<p>The NNE-SSW, right-lateral Kefalonia Transform Fault (KTF) marks the western termination of the subducting Hellenic slab, which is a part of the oceanic remnant of the African plate. The inception of the KTF, described as a STEP fault, is placed in the Pliocene. KTF is considered to be the most active earthquake source in the Eastern Mediterranean. During the last two decades, four significant earthquakes (M>6.0) have been associated with the KTF. These events are attributed to the reactivation of different segments of the KTF, which are (from North to South) the North Lefkada, South Lefkada, Fiskardo, Paliki and Zakynthos segments: the North Lefkada segment ruptured in the 2003 earthquake, the 2014 Kefalonia events are associated with the Paliki segment and the 2015 Lefkada earthquake with the South Lefkada (and possibly the Fiskardo) segments.</p><p>The upper plate structure in the islands of Lefkada and Kefalonia is characterized by the Ionian Unit, thrusted over the Paxi (or Pre-Apulian) Unit. The Ionian Thrust, which brings the Ionian over the Paxi Unit, is a main upper-plate NNW-SSE, NE-dipping structure. It runs through the island of Lefkada, to be mapped onshore again at the western coast of Ithaki and at SE Kefalonia. Two other major thrusts are mapped on this island: the Aenos thrust, which has a WNW-ESE strike at the southern part of the island and gradually curves towards NNW-SSE in the west and the Kalo Fault in the northern part. These Pliocene (and still active) structures developed during the late-most stages of thrusting in the Hellenides, strike obliquely to the KTF and appear to abut against it.</p><p>We suggest that these thrusts control not only the deformation within the upper plate, but also the earthquake segmentation of the KTF. This suggestion is corroborated by the spatio-temporal distribution and source parameters of the recent, well-documented earthquake events and by the macroseismic effects of these earthquakes. The abutment of the Ionian thrust against the KTF marks the southern termination of the Lefkada earthquake segment, which ruptured in the 2003 earthquake, while the Aenos, (or the Kalo) thrust mark the southern end of the Fiskardo segment. The spatial distribution of the Earthquake Environmental Effects related to the four significant events in the last 20 years displays a good correlation with our interpretation: most of the 2003 macroseismic effects are located in the northern part of Lefkada, which belongs to the upper block of the Ionian thrust; similarly, the effects of the 2014 earthquakes of Kefalonia are distributed mainly in the Paliki Peninsula and the southern part of the island that belong to the footwall of the Aenos thrust and the 2015 effects are found in SW Lefkada, which is part of the footwall of the Ionian thrust.</p><p>We suggest that correlation between upper-plate structure and plate boundary faulting can provide insights in the understanding of faulting pattern in convergent settings, therefore contributing to earthquake management plans.</p>

Focal parameter analysis of earthquakes of the S-SE of the Iberian Peninsula (1900-1923)

<p>The aim of this study is to make a review, actualization and homogenization of the seismic parameters of the Seismic Catalogue of the National Seismic Network of Spain, which belongs to the National Geographic Institute. Our analysis focusses on the region that spans from 36.0 to 39.5° N and from 3.25° W to 1° E, which is a seismically very active region. The studied time period refers to earthquakes occurred between 1900 and 1923, where most information comes from macroseismic data and macroseismic effects.</p><p>The study begins by searching and collecting information from seismic bulletins and seismic catalogues, seismograms, seismic surveys, photographs, specific studies, historical newspapers and different digital archives. Then, the achieved information from all the different sources were reviewed and, whenever possible, the seismic parameters such as localization, seismic intensity and magnitude were recalculated.</p><p>The objective of this work is, from one hand, to establish the study methodology that allow to develop an overall review of all the earthquakes occurred in Spain from 1900 to date, and on the other hand, to provide good quality seismic data (improving the completeness and homogeneity of this seismic catalogue). Seismic data is important because it is used to make seismic hazard maps, studies of seismic risk, to update the seismic building standards and it is also used to make seismic characterization of the territory.</p>

A Reappraisal of the Seismicity of Sardinia, Italy

In popular opinion, Sardinia is the only nonseismic region of Italy. Most researchers are likely to agree, up to a point. Geology-wise, the Sardinia–Corsica block is among the stablest areas of the Mediterranean. History-wise, up to 2011, only one Mw 5.1 event located offshore Sardinia was listed by Italian seismic catalogs (13 November 1948). Seismic networks record only a few, low energy (Mw<5) events, mostly located offshore and with little or no effects on land. Seismic hazard in Sardinia is very low. “Low,” yes, but not “totally lacking.” We present the results of a recent reappraisal of Sardinian seismicity. We gathered information on three major earthquakes (1616, 1771, and the 1948–1949 sequence). Another sequence (January–March 1901) was re-evaluated, identifying its previously unknown main event. It was confirmed that some earthquakes (1870, 1906, 1922, and 1924) had low magnitudes and scarce to nil macroseismic effects, whereas some other turned out either very doubtful or wholly fictitious (1835, 1838, 1855, and 1898). The seismic hazard of Sardinia can now be reassessed on a sounder basis than before. We hope that our work will help the people of Sardinia to improve their awareness of living in a seismic land, if with a low level of seismicity.

TESTING THE MACROSEISMIC INTENSITY ATTENUATION RELATIONSHIPS FOR VRANCEA (ROMANIA) SUBCRUSTAL EARTHQUAKES IN RELATION WITH DAMS SITUATED IN EXTRA-CARPATHIAN AREA

The aim of the present paper is to test intensity attenuation relationships for subcrustal earthquakes occurred in Vrancea (Romania) seismogenic zone in relation with some important dams situated in extra-Carpathian area. During centuries, the Romanian territory has been shaken by strong earthquakes, most of them being centered within Vrancea Zone, which is situated at the bending area of the South-Eastern Carpathians. Most of the zones from extra-Carpathian area are affected by the subcrustal seismic events, where many hydro-technical structures exist, being also exposed to earthquakes action. A detailed analysis of the intensity attenuation laws developed for subcrustal seismic sources was performed using the most recent and complete intensity datasets. We use an extended and combined intensity data including historical and modern, qualitative and quantitative data, i.e. a number of 11 earthquakes occurred during the period 1738-2009 with epicentral/maximum intensities ranging from VII-X MSK degrees, and magnitude Mw from 5.4 to 7.9. All the input data used for testing are resulted after the reevaluation and evaluation of the macroseismic effects produced by the seismic events included in the present study (8697 IDP). The selected attenuation laws were tested for different values of epicentral intensity and with reference to twelve and twenty four azimuthal directions. Besides the testing of the relationships, isoseismal maps based on the selected attenuation laws were accomplished, associated to the biggest possible earthquake (worst scenario) for the Vrancea subcrustal zone, also highlighting the calculated intensities in the selected dam sites. Brief description of the study and used methods. Brief description of the study and used methods.

Toward a Unified Near-Field Intensity Map of the 2015 Mw 7.8 Gorkha, Nepal, Earthquake

We develop a unified near-field shaking intensity map for the 25 April 2015 Mw 7.8 Gorkha, Nepal, earthquake by synthesizing intensities derived from macroseismic effects that were determined by independent groups using a variety of approaches. Independent assessments by different groups are generally consistent, with minor differences that are likely due in large part to differences in spatial sampling. Throughout most of the near-field region, European Macroseismic Scale (EMS-98) intensities were generally close to 7 EMS. In the Kathmandu Valley, intensities were somewhat higher (6.5–7.5) along the periphery of the valley and in the adjacent foothills than in the central valley, where they were ≈6. The results are consistent with instrumental intensity values estimated from available data using a published relationship between peak ground acceleration (PGA) and intensity. Using this relationship to convert intensities to PGA, we estimate strong-motion PGA de-amplification factors of ≈0.7 in the central Kathmandu Valley, with amplification of ≈1.6 in adjacent foothills. The results support the conclusion that the Kathmandu Valley experienced a pervasively nonlinear response during the Gorkha main shock.