Kuvvet Atakan

Seismic Hazard in Istanbul following the 17 August 1999 İzmit and 12 November 1999 Düzce Earthquakes

Two recent destructive earthquakes that occurred along the western part of the North Anatolian Fault Zone (NAFZ), the 17 August 1999 ( M w 7.4) Izmit and 12 November 1999 Duzce ( M w 7.2) earthquakes, have caused major concern about future earthquake occurrences and their possible consequences in the Istanbul area. Probabilistic seismic hazard analyses are performed for the larger Istanbul area including the Gulf of Izmit and the Marmara Sea region. Hazard computations were done assuming different combinations of four attenuation relations and three alternative source models. The three models used are (1) Standard Poissonian earthquake occurrence with area sources, (2) renewal model (assuming characteristic earthquakes) with area and fault sources, and (3) renewal model (assuming characteristic earthquakes) with refined area and fault sources. Results are presented in twelve different maps of peak ground acceleration (PGA) with a 10% chance of exceedance in 50 yr. Among the earthquake recurrence models, the results assuming model 3, yield the highest PGA values, reaching greater than 0.3 g at the western end of the Gulf of Izmit. In general, PGA values decrease toward north and are reduced down to less than 0.2 g in central Istanbul in the Bosphorous area. The four attenuation relations examined display significant variations, and their effects become especially critical for distances less than 50 km. In addition, spectral hazard levels (corresponding to 475-yr return period) are computed and presented as uniform hazard response spectra for 5% damping. Two sites are selected, one in hard rock and the other in soft sediments. In general, these results correlate well with the Turkish Seismic Design code (TSDC) recommendations for the two site conditions. The estimates of the future earthquake hazard potential of the Istanbul area are sensitive to our present-day understanding of the behavior of the fault segments in the Marmara Sea region.

Sensitivity of Ground-Motion Simulations to Earthquake Source Parameters: A Case Study for Istanbul, Turkey

Following the disastrous earthquakes in Izmit and Duzce along the North Anatolian fault in 1999, the earthquake hazard in the Istanbul area became a great concern. In this study we simulate strong ground motions caused by a scenario earthquake (M 7.5) in the Marmara Sea, and investigate the effect of varying the input parameters on the broadband frequency ground motion. Simulations are based on a multiasperity source model that involves the combined rupture of the North Anatolian fault segments beneath the Marmara Sea. We use a hybrid model combin- ing a deterministic simulation of the low frequencies (0.1-1.0 Hz) with a semisto- chastic simulation of the high frequencies (1.0-10.0 Hz). Computation at each fre- quency range is performed separately and the total ground motion is combined in the time domain. Computations are linear and are performed at bedrock level, thereby not taking any effect of local geological conditions into account. We calculate a total of 17 earthquake scenarios corresponding to different source and attenuation param- eters to study their effect on the ground motion. The most significant parameters in terms of ground-shaking level are the rise time, rupture velocity, rupture initiation point, and stress drop. The largest variability of strong ground motions is observed in regions adjacent to asperities and is associated with frequencies higher than 5 Hz. For lower frequencies our simulated velocity spectra within the Istanbul area are fairly stable among scenarios. The average standard deviations of all ground-motion measures are less than 35% of the mean. Online material: Figures of peak ground acceleration and peak ground velocity and their differences to the reference scenario values.

Simulated Strong Ground Motions for the Great M 9.3 Sumatra-Andaman Earthquake of 26 December 2004

On 26 December 2004, a devastating earthquake of M 9.3 occurred offshore northern Sumatra. Due to the size of this earthquake and the accompanying tsunami wave, disastrous consequences have been observed in several countries around the Indian Ocean. The tectonics in the region are characterized by the oblique, north-northeast-oriented subduction of the Indian-Australian plate under the Sunda microplate with a rate of 6-6.5 cm/yr. This oblique convergence results in strain partitioning, where the trench-perpendicular thrust faulting along the subducting slab accommodates the east-west component of the motion, whereas the north-south component of the motion is probably accommodated by the right-lateral strike-slip faulting along the Great Sumatran fault and the Mentawi fault. Source parameters of the 26 December 2004 event have been used for modeling the resulting ground motions in the nearby affected regions. Results give an insight on the importance of ground shaking in the total destruction of places like Banda Aceh, northern Sumatra, Indonesia. The modeling is performed for a multiasperity finite fault using a hybrid procedure combining deterministic modeling at low frequencies and semistochastic modeling at high frequencies. Results show that strong shaking was distributed over a large area including northwestern Sumatra and its offshore islands. In Banda Aceh, which experienced significant damage, bedrock velocities reached 60 cm/sec with duration of the shaking of ca. 150 sec. The largest ground motions occurred near the strongest asperities of the fault plane, where velocities of 200 cm/sec are modeled for bedrock conditions.

Tsunamis and Tsunami Hazards in Central America

A tsunami catalogue for Central America is compiledcontaining 49 tsunamis for the period 1539–1996,thirty seven of them are in the Pacific and twelve inthe Caribbean. The number of known tsunamis increaseddramatically after the middle of the nineteenth century,since 43 events occurred between 1850 and 1996. This isprobably a consequence of the lack of populationliving near the coast in earlier times.The preliminary regionalization of the earthquakessources related to reported tsunamis shows that, inthe Pacific, most events were generated by theCocos-Caribbean Subduction Zone (CO-CA). At theCaribbean side, 5 events are related with the NorthAmerican-Caribbean Plate Boundary (NA-CA) and 7 withthe North Panama Deformed Belt (NPDB).There are ten local tsunamis with a specific damagereport, seven in the Pacific and the rest in theCaribbean. The total number of casualties due to localtsunamis is less than 455 but this number could behigher. The damages reported range from coastal andship damage to destruction of small towns, and theredoes not exist a quantification of them.A preliminary empirical estimation of tsunami hazardindicates that 43% of the large earthquakes (Ms ≥7.0) along the Pacific Coast of Central America and100% along the Caribbean, generate tsunamis. On thePacific, the Guatemala–Nicaragua coastal segment hasa 32% probability of generating tsunamis after largeearthquakes while the probability is 67% for theCosta Rica–Panama segment. Sixty population centers onthe Pacific Coast and 44 on the Caribbean are exposedto the impact of tsunamis. This estimation alsosuggests that areas with higher tsunami potential inthe Pacific are the coasts from Nicaragua to Guatemalaand Central Costa Rica; on the Caribbean side, Golfode Honduras Zone and the coasts of Panama and CostaRica have major hazard. Earthquakes of magnitudelarger than 7 with epicenters offshore or onshore(close to the coastline) could trigger tsunamis thatwould impact those zones.

Seismic hazard in regions of present day low seismic activity: uncertainties in the paleoseismic investigations along the Bree Fault Scarp (Roer Graben, Belgium)

Earthquake hazard assessment in stable continental regions, such as northern Europe, has traditionally been evaluated on the basis of the instrumentally and historically recorded seismicity, which indicates relatively low hazard levels. Reliability of such estimates is a matter of debate as the long-term potential of large earthquakes usually cannot be determined based on short observational periods generally less than a few hundred years. A significant improvement to this lack of knowledge can be achieved by extending the past observations into the geological time scale. Paleoseismic investigations can provide valuable information to bridge this gap, where the potential for large earthquakes can be quantified both in magnitude and recurrence period, based on the observation of prehistoric earthquakes (paleoearthquakes) in the geological record (particularly in the last 20,000 years). However, using these records in seismic hazard analysis requires systematic treatment of uncertainties. Usually uncertainties are inherent to the interpretation of geological record, which leads, in the end, to the identification of paleoearthquakes. Field observations used in the analysis may satisfy several alternative interpretations. Such interpretations become useless when alternative solutions exist but not documented in detail, and especially when the relative reliability of the favored interpretation with respect to the alternative interpretations is not known. The recently introduced method using logic-tree formalism, which is based on qualitative description of the uncertainties related to the paleoseismic data and especially in its interpretation, is applied in the paleoseismic investigations performed on the Bree Fault Scarp, along the Feldbiss Fault (Roer Graben, Belgium). The cumulative uncertainties associated with the different stages of the study are computed as the combination of the preferred alternative branches in the logic-tree presentation. The final uncertainty and its relative importance in seismic hazard analysis is expressed as the paleoseismic quality factor (PQF), which indicate 0.76. This value can directly be used in seismic hazard analysis.

Empirical evaluation of site effects in the metropolitan area of San José, Costa Rica

Site effects for 11 selected locations were determined in the capital city of Costa Rica. We used a strong motion network made of eight K2 and three SSA accelerographs. The network recorded more than 60 earthquakes in the magnitude range from 2 to 5 during a period of nine months. The site effects were determined using the sediment-to-bedrock spectral ratio (SBSR) and the horizontal-to-vertical spectral ratio (HVSR) techniques and a time window 4 s beginning from the S-wave arrival. The result suggests that the amplification in the capital city is to be in the range from 2.0 to 3.0. The fundamental frequencies were found to be high in the southern and eastern part of the study area and low in the northern and western part. A possible topographic effect was also observed for one of the stations located nearby a river canyon. The results from earthquake data were compared with the ones obtained from noise data. The horizontal-to-vertical noise ratio (HVNR) technique was used to estimate the site effects using ambient noise. The fundamental frequencies were found to correlate very well between both sets of data; on the other hand, the amplitude given by the noise was observed to be always lower than the one derived from the earthquake data.

Continued Earthquake Hazard in Northern Sumatra

The occurrence of two large earthquakes (Mw = 8.4 and Mw = 7.9) along the Sumatran west coast on 12 September 2007 as well as an Mw = 7.4 event on 20 February 2008 have again put the high earthquake hazard of this region into focus. These events are the most recent in a series of major subduction zone earthquakes that began with the great Mw = 9.3 event of 26 December 2004 followed by an Mw = 8.7 event on 28 March 2005 [Bilham, 2005; Lay et al., 2005; Stein and Okal, 2005]. The major subduction zone earthquakes have been propagating southward along the Sunda trench, and the remaining stress is expected to be released along the subduction zone in a long stretch from the Andaman Sea in the north to the southernmost extension of the recent ruptures, especially in the southernmost part close to the Sunda Strait (Figure 1). Also, there is an additional and significant hazard due to potential earthquakes along the Great Sumatran Fault (GSF), a major rightlateral strike-slip fault parallel to the western coast of Sumatra. The GSF accommodates the component of plate convergence parallel to the trench, where strain partitioning is a result of the oblique collision along the Sunda trench.

Site Response as a Function of Near-Surface Geology in the South Iceland Seismic Zone

Site response measurements provide information on the amplification of ground motions generated by local conditions. Recent studies of large destructive earthquakes have shown that damage during the earthquakes are often caused by the amplification of seismic waves in near-surface sedimentary layers. The estimation of site response is therefore critical, in order to evaluate the true seismic hazard potential of a given area. We investigated local site amplifications in the South Iceland Seismic Zone (SISZ). Nine digital seismographs were deployed, temporarily, in an area of approximately 400 km2, in the westernmost part of the SISZ. Among the 90 events recorded, 15 were used in this study, including a magnitude 3.1 (ML) event and selected aftershocks, which occurred in the northern outskirts of the village Hveragerdi. Single Station Spectral Ratios (SSSR) of the recorded earthquakes revealed some of the effects of local site conditions. Spectral amplification factors of 2–5 on average, can be expected in the SIL area, depending on the sediment type and thickness. Higher site amplifications occur in the southern part of the study region, where the thickest sedimentary cover is found. Spectral amplification, related to topographical effects, is observed at the bedrock reference station, Bjarnastadir. Standard Spectral Ratios (SSR), with respect to the bedrock reference station, Bjarnastadir, were also calculated for some stations, in order to compare the two spectral ratio results. The two methods show a good correlation at some stations, whereas at others they vary considerably. The comparison between the earthquake and ambient noise data, on the other hand, gave better correlation when the SSSR method is used.

Tectonic Processes in the Jan Mayen Fracture Zone Based on Earthquake Occurrence and Bathymetry

Jan Mayen is an active volcanic island situated along the mid-Atlantic Ridge north of Iceland. It is closely connected with the geodynamic processes associated with the interaction between the Jan Mayen Fracture Zone (jmfz) and the slowly spreading Kolbeinsey and Mohns Ridges. Despite the significant tectonic activity expressed by the frequent occurrence of medium to large earthquakes, detailed correlation between individual events and the causative faults along the jmfz has been lacking. Recently acquired detailed bathymetric data in the vicinity of Jan Mayen has allowed us to document such correlation for the first time. The earthquake of 14 April 2004 ( M w 6), which occurred along the jmfz, was studied in detail and correlated with the bathymetry. Locations of aftershocks within the first 12 hours after the mainshock outline a 10-km-long fault plane. Interactions between various fault systems are demonstrated through locations of later aftershocks, which indicate that supposedly normal fault structures to the north of the ruptured fault, in the Jan Mayen Platform, have been reactivated. Correlation of the waveforms shows that events located on these structures are significantly different from activity at neighboring structures. Coulomb stress modeling gives an explanation to the locations of the aftershocks but cannot reveal any information about their mechanisms.

The new accelerograph network for Santa Fe De Bogota, Colombia and implications for microzonation

The new accelerographic network of Santa Fe de Bogota is composed of 29 three-component stations with sensors at the surface and three additional six-component borehole stations with three sensors at the surface and three at depth (115, 126 and 184 m). In total, 32 stations have been operative in the metropolitan area of Bogota since 1999. During this period of time, a significant number of weak motion are recorded and used for a preliminary analysis of local site effects. Using the SH-wave response spectra we verify the behavior of the different seismic zones proposed by the previous microzonation study of the city. A comparison between normalized SH-wave response spectra and the normalized design spectra for each zone clearly depicts that parts of the design spectra should be revised, as well as the boundaries between different zones may require some changes. The spectral amplification levels reach up to a factor of 5. The predominant periods obtained by the amplification spectra in different stations in the city, show variability from 0.3 to 3.0 s. A comparison is also made between the predominant periods obtained using H/V spectral ratios of microtremors and those using weak motion. In general, microtremors tend to predict slightly lower values of dominant periods than those calculated by the weak motion spectra. However, there is a general correlation between the two data sets. Using the data recorded by one of the borehole station, an equivalent linear seismic response analysis was conducted. The modeled and recorded response spectra show similarities in period peaks, however, the modeled soil amplification is underestimated for periods less than 0.8 s. Since the available record is weak motion which represents mostly the linear response of the soils, further analysis is required.