Earthquakes are one of the most disastrous natural phenomena resulting from the movement of the solid lithosphere at the outer part of the earth. Most earthquakes result from the rupture of these solid rocks and their displacement due to the relative movement between them, causing a sudden eruption of seismic waves with enormous energy, resulting in violent shaking of the surface of the Earth, destruction of installations and infrastructure, and the occurrence of heavy human and economic losses. Majority of scientists agree that the best way to reduce the effects of earthquakes is to properly assess seismic hazards and provide results to decision-makers, regulators, rescuers and agencies, so that seismic forces can be designed and built to withstand earthquake forces, in addition to facilitating the development of contingency and rescue plans. In this regard, the Earthquake Monitoring Center at Sultan Qaboos University has carried out an assessment of the seismic hazards at the bedrocks of all the Sultanate’s cities and presented the results of these studies to the Council of Ministers. These studies were the basis for the preparation of a preliminary building code for the resistance of seismic forces, which can be consulted and continuously developed for the study and design of buildings in the sultanate.
Therefore, proper seismic hazard evaluation requires not only a proper assessment of the seismic hazard at bedrocks, but also the computation of the impact of the soil on the ground motion and its effect on the ground-motion at the bedrocks. Because the nature of soils may vary from one site to another within a short distance, it is necessary to evaluate its effects on ground motion in close distances so that we can draw maps for the geographical variation of the amplification values and the values of the ground-motion, and the location where it is expected to be most destructive. These studies have been carried out in the Governorate of Muscat and the Earthquake Monitoring Research Center hopes to complete similar studies for all the governorates of the sultanate, and especially those in the north, as they are more vulnerable to earthquakes because they are closer to the Makran and Zagros seismic zones.
It should be noted that these studies are an important part of the sultanate’s efforts to establish an integrated system for the management and control of seismic hazards to ensure the mitigation of its effects and the safety of the sultanate’s infrastructure. Despite limited seismic activity in the sultanate, ambitious projects and rapid population growth require attention on seismic impacts, with the extreme risk of exposure to intense seismic activity from near or far seismic sources. The Earthquake Monitoring Center at Sultan Qaboos University is keen to complete the existing seismic monitoring system by establishing a strong ground motion monitoring network of 62 stations and 6 broad-band seismic stations to support the existing seismic network. Continuous monitoring will ensure the accurate mapping of all seismic sources and the continuous development of seismic hazard maps. It will also provide an accurate map of seismic shaking in the event of an earthquake based on measurements, not calculations (as the case now) and provide these measurements to the responsible authorities to support decision-makers in emergency situations. These instantaneous maps contribute to the identification of the most affected and the most vulnerable sites in a very short time. The construction of this integrated seismic monitoring system will assist decision-makers in the optimal use of land through the development of building codes and the development of necessary legislation and standards to avoid possible seismic risks in the development of areas and the issuance of building permits.
Evaluation of Seismic Hazard at Bedrock Level, Sultanate of Oman
In this study, the probabilistic and deterministic techniques utilized are the latest methods used to assess the seismic risk at the bedrock level in the sultanate. Seismic risk assessment in any method requires considerable knowledge of the geological nature of the area, its active faults and movement rates, and the long history of the seismic activity which is the cornerstone of that assessment. The earthquakes recorded by monitoring seismic stations are more accurate in terms of location and time of occurrence. Seismic monitoring stations have generally don’t have a very long history and have only been available in the sultanate for almost 15 years. Therefore, seismic history recorded by seismic stations is very short and is not enough to study the seismic activity in the sultanate. This led us to extend this seismic history by studying the historical earthquakes documented in old writing and the in historical documents of the correspondence of foreign embassies.
Based on the above, a complete and homogeneous seismic record was constructed and compiled as far as possible for the sultanate, where the research team examined all the available resources and detailed all the earthquakes with a magnitude greater than or equal to 4.0 degrees since 734 CE for all the seismic sources that could affect the sultanate. Historical earthquakes were added to those recorded by monitoring stations and were compiled based on data published by international and local seismic centers as well as numerous studies published in reputable international journals.
To obtain a homogeneous seismic record of the Sultanate of Oman and its environs, the various magnitude measurements of earthquakes have been shifted to a moment magnitude, which is the best seismic scale yet developed, as it is the only one that can express very large earthquake sizes without any saturation. Since the statistical models used in probabilistic seismic hazard studies only deal with independent events, modern methods of removing foreshocks and aftershocks from seismic records have been used to include only independent events.
Because we are dealing with statistical models in which seismic event numbers are a key factor, it is necessary to determine the start date of completeness of each seismic magnitude so that the number of earthquakes of this magnitude can be checked during the statistical analysis in proportion to the period of completion. After analysis, we found that earthquakes with a magnitude greater than or equal to 4.0 had not occurred since 1973, while earthquakes with magnitudes greater than or equal to 5.2 had not occurred since 1928.
The compilation and preparation of the seismic catalog was followed by the definition of the characteristics and boundary of seismic hazard sources that could affect the Sultanate of Oman, or the so-called seismotectonic model. This requires a study of the concentration of seismic foci, the expected maximum seismic rate, the rate of displacement on active faults, the mechanical fault of these earthquakes, as well as the distribution of these seismic foci with tectonics. Twenty-six seismic sources have been delineated between faults and active seismic zones in and around Oman (Oman Mountains, Oman Sea, Zagros Mountains, Gulf of Aden and Yemen). Because of the uncertainty of seismic source boundaries and the existence of different theories on the mechanics of seismic activity in the Makran and Zagros regions, another seismotectonic model was used and the calculation was made using both model utilizing the logic tree technique.
Because the ground motion resulting from the earthquakes is attenuated with the distance depending on the nature of the medium in which seismic waves are spread, and because all the seismic sources are located at different distances from the study area, the research team used the best models that represented how the ground motion generated in each source in order to obtain the most accurate results possible. At least three prediction equations have been selected to represent the attenuation of ground motion generated from each seismic source. Since the definition of the horizontal component of ground motion is different in some of the prediction equation models, some necessary corrections have been made in order for seismic ground motion to be the geometrical mean of ground motion in the two horizontal components. The probabilistic seismic hazard assessment was carried out utilizing the logic tree technique to solve the uncertainty problems in some of the study inputs.
Using the above, the seismic hazard at bedrocks level was estimated at 2%, 10% and 50% within 50 years, corresponding to 2475, 475 and 72 years respectively, for many important spectral periods from the engineering point of view (0.0, 0.1, 0. 2, 0.3, 1.0 and 2.0 seconds, respectively). The Sultanate of Oman was covered by a grid. The distance between each vertical or horizontal grid lines was about 20 km. The seismic hazard values were calculated at all grid points (2296 points). Using the logic-tree, the average seismic hazard at each grid point was calculated as well as the standard deviation. Using these average calculations once and adding the value of the standard deviation back to them, contour maps of the averages and 84 percentile were created showing the change in seismic hazard values and providing the expected levels of ground motion. Maps show that the highest values of ground motion occur in the northern regions where Khasab is located near major earthquakes in the Zagros Mountains and Makran, where earthquakes are more frequent than other seismic sources around Oman. Relatively low seismic hazards are shown in the western and southern regions. The relatively moderate seismic hazard values in the Muscat area are attributed to its close proximity to the Oman Mountain and Makran seismic sources. The seismic hazard of the cities of Khasab, Dibba, Sohar, Muscat, Nizwa, Sur, Salalah, Abu Dhabi and Dubai in the United Arab Emirates was calculated and the seismic hazard results are represented by hazard curves and uniform hazard spectra profiles for each city.
Deaggregation analysis shows that earthquakes occurring 90 km from Khasab city with 6.75 magnitude have the greatest contribution to seismic hazard on this city for 475 and 2475 year return periods and for all spectral periods taken into account. The earthquakes that can occur in the Oman Mountains are the second in terms of contribution to seismic hazard, especially for short spectral periods (0.0 and 0.2 seconds). For the city of Sur, which is characterized by low seismic hazard, the earthquakes that occur in the west of the Makran seismic zone at a distance of 300 km are the largest contributors to the seismic hazard curve for the return periods of 475 and 2475 years. For the remaining cities and for the return period of 475 years, distant and very large earthquakes dominate the seismic hazard for all spectral periods, while, for the return period of 2475 years, nearby small and medium earthquakes dominate the seismic hazard of short spectral periods. Large distant earthquakes (7.0) dominate the seismic hazard for long spectral periods. Therefore, the seismic sources of Makran and Zagros make a significant contribution to seismic hazards to the high rise structures in these cities.
Seismic hazard was also assessed using deterministic methods and maps were provided representing the maximum ground motion that could occur in any location within the sultanate. This method impairs the use of the largest earthquake that can occur in all seismic sources without regard to the rest of the seismic activity in this source and the likelihood of recurrence, which may require in the low-seismic activities of the Sultanate of Oman thousands or even tens of thousands of years, leading to very conservative results.
The maps presented in this study have been used to develop a seismic study in the Sultanate of Oman and can be used in land use planning, emergency reserves and insurance studies. These maps, therefore, pave the way for more integrated studies of human and property protection against this unavoidable threat, so that its effects can be remedied and mitigated.