Relationships between macroseismic intensity and peak ground acceleration and velocity for the Vrancea (Romania) subcrustal earthquakes

The goal of this paper is to develop a new empirical relationship between observed macroseismic intensity and strong ground motion parameters such as peak ground acceleration (PGA) and velocity (PGV) for the Vrancea subcrustal earthquakes. The recent subcrustal earthquakes provide valuable data to examine these relationships for Vrancea seismogenic region. This region is one of the most active seismic zones in Europe and it is well-known for the strong subcrustal earthquakes. We examine the correlation between the strong ground-motion records and the observed intensities for major and moderate earthquakes with Mw ≥ 5.4 and epicentral intensity in the range VI to IX MSK degrees that occurred in Vrancea zone in the period 1977-2009. The empirical relationships between maximum intensity and ground parameters obtained and published by various authors have shown that these parameters do not always show a one-to-one correspondence, and the errors associated with the intensity estimation from PGA/PGV are sometimes +/-2 MSK degree. In the present study, the relation between macroseismic intensity and PGA/PGV will be given both as a mathematical equation, but also as corresponding ground motion intervals. Because of the intensity data spreading and errors related to mathematical approximations, it is necessary to systematically monitor not only the acceleration and velocity but also all the other ground motion parameters. The mathematical relation between these parameters might be used for the rapid assessment of ground shaking severity and potential damages in the areas affected by the Vrancea earthquakes.

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.

GROUND MOTION PREDICTION AT THE DAM SITES IN SW ROMANIA FOR MODERATE VRANCEA SUBCRUSTAL EARTHQUAKES

The seismic hazard studies of the last 30 years have been largely carried out taking into account the needs of the construction engineers, by linking the specific quantities of soil movement with the physical parameters determined instrumentally, namely, with the maximum values of soil acceleration. At present, interest in the results of hazard studies has increased and has extended to other areas such as insurance or design companies, environmental protection, etc. A fundamental element in the estimation of seismic hazard is the variation of the amplitude of the movement of the soil according to the distance, magnitude and local conditions. To understand and prevent the effects of the strong Vrancea earthquakes in the dam sites located in the South West of Romania, we study the seismic waves attenuation relations using the accelerations recorded by the national network of K2 accelerometers, following the moderate Vrancea intermediate earthquakes. The study area includes the largest agglomeration of dams in Romania, with almost 100 dams out of the 250 large dams. One of the most important specific requirements towards dams' safety is the seismic risk and hazard assessment and the computation of attenuation relationships is one of the most important steps of the work. The main objective of the present work is the evaluation of the specific attenuation relationships of the seismic wave propagating from Vrancea subcrustal focus toward south-west, with direct application for the dams situated in the area.

Continental collisions and the origin of subcrustal continental earthquakes

The existence of subcrustal continental earthquakes beneath the Alpine–Himalayan Belt was recognised more than 60 years ago. There is general agreement that most of those beneath the western part of the belt in the Mediterranean result from the subduction of oceanic lithosphere. There is less agreement about the origin of those beneath Vrancea in Romania, the Hindu Kush, and the Pamir. Because there is little evidence for the former existence of oceanic lithosphere beneath these regions, many authors have argued that these seismic zones result from the separation of the mantle part of the continental lithosphere from the crust before it sinks into the mantle. However, this model has become steadily less satisfactory. Detailed studies of the depth of earthquakes beneath all stable regions of continents have shown that substantial subcrustal earthquakes, with magnitudes greater than 5.5, are rare. We show that this distribution is controlled by temperature, with material hotter than ∼600 °C being aseismic. This simple rule accounts for the distribution of almost all earthquakes in oceanic and continental lithosphere, including those in subduction zones. We argue that the subcrustal continental earthquakes must also result from the subduction of oceanic lithosphere. This proposal is not new but has generally been dismissed because of the lack of surface geological evidence that suitable pieces of oceanic lithosphere existed. However, the depth distribution of continental earthquakes makes it steadily harder to avoid.