V R A N C E A   E A R T H Q U A K E S

The recent October 27, 2004 magnitude 5.9 earthquake in Vrancea, SE Carpathians (Fig. 1), though nondamaging has once again demonstrated the necessity to be prepared for future strong earthquakes in Romania (shakemap provided by USGS). The seismicity beneath Vrancea is dominated by intermediate depth earthquakes in a well-defined volume. The epicentral area is confined to about 40 km x 80 km (Fig. 1). Most earthquakes occur between 70 and 180 km depth within an almost vertical column. Deeper and shallower events have also been recorded but only with small magnitudes (Mw < 5.5). Depths and moment magnitudes (Mw) of all macroseismically and instrumentally recorded events are summarized in Radu (1974, 1991) and Oncescu et al. (1999).

Fig. 1:Top: Digital strong motion network of Kinemetrics K2 instruments in SE Romania and temporal stations during the Carpathian Arc Lithosphere Cross-Tomography (CALIXTO) experiment in 1999. Epicenters of Vrancea intermediate-depth earthquakes and shallow crustal events (since 1990) are marked by red circles and black crosses, respectively. Bottom: 3-component recordings of ground acceleration of the October 27, 2004 Vrancea earthquake (Mw=5.9) (epicenter marked in map by yellow star) at stations VRI (rock) and CFR (soil). Site effects at station CFR cause a significant higher level of ground shaking than expected from distant-dependent attenuation.

Four major events struck within this century: Date Depth (km) Moment Magnitude Nov. 10, 1940 155 7.7 March 4, 1977 95 7.4 Aug. 30, 1986 130 7.1 May 30, 1990 90 6.9

The earthquake catalogue provides recurrence times of 10 years for events with Mw > 6.5, 25 years for Mw > 7.0 and 50 years for Mw > 7.4. Historical data for the last 600 years show 3 earthquakes per century with Mw > 7.2 and 6 events/century with Mw#160;>#160;6.8, in good agreement with the Gutenberg-Richter relation. The 1977 event caused ground shaking in Bucharest with 0.2 g PGA and a very broad response spectrum. The Romanian literature identifies various types of buildings, classified according to the time of construction (Lungu et al., 1999). Residential buildings of pre-World War II with 8 to 14 stories were built as reinforced concrete frames with infill brick walls. They suffered most from the 1977 earthquake, because they were designed to resist vertical loads only. 32 buildings of this type collapsed, although they had survived the 1940 earthquake. The second group of buildings which suffered substantially are monolithic reinforced concrete buildings with 2 to 16 stories. Their quality ranges from poor for residential buildings to relatively good for hotels and public buildings. Rural houses with 1 or 2 stories, supported by light timber systems or adobe constructions with mud cast into framework, collapsed in large numbers mostly in the epicentral area.

Fig. 2: Macroseismic intensity pattern of the 1977 Vrancea earthquake (after Radu et al., 1979). This event was felt in great parts of South-Eastern Europe.

The 1977 earthquake caused 1570 casualties and more than 11,300 cases of injured persons were reported (90% of the victims in Bucharest). 32,900 apartments were destroyed or severely affected, and the overall financial loss was estimated more than 2 Billion US $. The foreign trade balance, following the 1977 earthquake, was severely upset, and it is likely that the earthquake impact considerably contributed to this fact.

During the past years several results were accomplished: Installation of a state-of-the-art K2 network for monitoring of strong ground motion (Fig. 1) Development of the damage simulation tool EQSIM (see Disaster Management Tool) Installation of a GPS (Global Positioning System) network for three-dimensional deformation analysis Realization of an international seismological tomographic experiment with 120 stations in and around the Vrancea region Measurement of two long-range refraction seismic profiles recording active sources across the Vrancea region Instrumentation of a multidisciplinary test site including a test building (Fig. 3) and other buildings (Fig. 4) in Bucharest.

Fig. 3: Experimental set up in the test building (multidisciplinary seismic test site, INCERC) in Bucharest. A FBA-accelerometer with thermal isolation is placed close to a rubber bearing.

Fig. 4: The sensor is placed with thermal isolation at the ceiling of the 10th floor (building: Bucharest downtown). The recorded accelerograms (top: 10th floor building, bottom: free-field) show the strong amplitude amplification after the S-onset caused by the structural response of the building. The event with M=5.4 occured on July 20th 2001 at nearly 130 km depth.