COUNTRY REPORT ON Jordan seismological Observatory JSO Network

COUNTRY REPORT ON Jordan seismological Observatory (JSO) Network Name of Participant: Engineer Ahmad Algazo Name of Organization: Jordan Seismological Observatory.

• SEISMIC OBSERVATION IN JORDAN • • By: AHMAD ALGAZO Introduction • The rate of seismic activity in Jordan is moderate, however many of the strong seismic events are located along the axis of Dead Sea Rift, which is formed the northwestern boundary of the Arabian plate. Matching magnetic anomalies across the rift has indicated a total strike-slip movement of approximately 107 -km. The Jordan Seismological Observatory began routine operation in September 1983. Seismic data being transmitted via radio on assigned frequencies to the Natural Resources Authority building in Amman from 26 short periods stations cover all areas in Jordan Figure (3), also 13 stations have been installed recently in Jordan by using VSAT system two of them are Broad Band eleven are short period stations, there is Auxiliary Broad Band Station for the Comprehensive Test Ban Treaty Organization ( CTBTO ). In addition there is a Strong Motion Network which consists of 26 digital accelerographs deployed in the main cities and dams Figure (4). The earthquake location program SEISAN is used for the routine location of hypocenters, in addition to the velocity-depth model which shows a p-velocity average for the upper sedimentary of 3. 5 km/sec, and the crust is underlain by a normal mantle with a p-velocity of 8. 00 to 8. 2 km/sec. The Seismological Observatory is issued annual bulletin about the event activity (Earthquake in Jordan and Adjacent Areas), which are reported to the adjacent Observatories and to the International Seismological Center. • •

• • • Discussion The Dead Sea Rift zone extends northward from the northern end of the Red Sea for a distance in excess of 1000 km. Several linear topographic fault controlled depressions are present along a great portion of this fault zone. Within the purview of plate tectonics, the Dead Sea Rift is a transform plate boundary connecting the Red Sea, Where crustal spreading is occurring, northward to a zone of plate convergence. The Arabian plate lies to the east of the rift whereas on the west several smaller plates, the largest of which is Sinai micro plate, from a part of the larger African plate (Figure 1). Although the geology of Jordan is mapped and quite well known, the pattern of earthquake activity, in particular, its temporal and spatial characteristic and its relationship to the geologic and tectonic framework of Jordan have not well documented; this is primarily due to the absence of a seismograph network in Jordan. Studies of the Seismicity, however, suggest results which are not compatible with the inferred rate of slip that is occurring along the Dead Sea Rift plate boundary (Freund et al. , 1968). On the other hand, geological and magnetic evidence point to a relative rate of plate movement of 4 - 6 mm/yr along the rift boundary (Girdler and Styles, 1974; Hatcher et al. , 1981), and a total strike-slip movement of approximately 107 km has been indicated by matching magnetic anomalies across the rift. However, the rate of slip calculated from seismic data as a function of time yields an average slip rate of only 1 - 3 mm/yr (Kovach, 1979; Ben-Menahem, 1981). One is therefore forced to conclude that the region is probably accumulating strain energy for a future large earthquake. Other explanations, that the opening of the Red Sea has slowed significantly in historic time or that strain is being released as a seismic slip (North, 1974), or distributed slip is taking place (Kovach et al. , 1985), are considered less likely. Clues to the pattern of Seismicity can be gleaned from the historical record, and a large amount of literature is available for careful scrutiny. Three sources of data include: Earthquake listings or catalogs. descriptions of specific events, Inferences from archaeological excavations. Earthquake listings have been presented by Alsanawi and Ghalib

• 1975, Ambrayseys (1961, 1962), Ambrayses and Melville (1982), Amiran (1950, 1951), Arieh (1967, 1985), Ben-Menahem (1979), Poirer and Taher (1980), Poirer et al. , (1980). • Most of the seismic events are located along the axis of the Dead Sea Rift. These events, together with recent micro earthquake data, point to recurrence interval; of about 120 years for an Ml = 6. 0 earthquake along the rift. Verd (1978) has utilized the geologic record of total fault displacement and the rate of seismic activity to estimate that the maximum expected earthquake along the rift zone would be the range Ml = 7. 5 - 8. 0.

Structural Map of Jordan

Figure ( 1 ): Tectonic Map of the Dead Sea rift region.

• • • Seismicity The Dead Sea rift is a transform boundary between the Arabian and African plates, connecting the Red Sea spreading center in the south to the Taurus-Zagros collision zone in the north. The Dead Sea transform is about 1100 km. long, and separates the Sinai sub plate from the Arabian plate. A geological offset of about 105 -110 km. (Quennell 1959; Freund et. al. , 1970), has been accumulated along the transform since the Miocene at a mean rate of left lateral strike slip of ~ 5 mm/year. The Dead Sea transform is comprised of a series of en echelon main faults, forming pull-apart basin, among them the Dead Sea basin (Garfunkel et al. , 1981; Ben –Avraham, 1985; and many others). Although the Dead Sea transform is the main source of the seismic activity in the Dead Sea region, there are highly populated areas within the neighborhood of active fault systems that branch off the Dead Sea transform, such as Carmel, Al-Karak Al-Fiaha, Wadi El-Sirhan, Zarqa Main, and Swaqa faults. The seismicity of the Dead Sea region is considered low to moderate, nevertheless some destructive earthquakes have been occurred and destroyed most of the urban areas in Jordan and Palestine, such as 362, 748, 1034, 1070, 1201, 1759, 1837, 1927, 1995, and others earthquakes, with magnitude greater than 6. 0 degrees in Richter scale. After about 20 years of continuous operation of the seismic networks operated in the region about 9000 earthquakes were recorded in the region as shown in the seismicity map Figure. (2) , and as the map shows, most of the mapped faults in the region are active. The most populated areas of the Dead Sea region are vulnerable to strong earthquakes, that will most definitely occur in the future in this region along the Dead Sea fault system, which will be a major threat to the safety, social integrity and economics for the peoples of region. Earthquake hazard assessment no longer refers to single parameters such as the horizontal peak ground acceleration (PGA) as a quantifier of the hazard. The modern construction engineer requires information about the expected acceleration at frequencies similar to the natural frequencies of the building under design. To provide such

• information we have to know the attenuation laws of ground motion at different frequency bands. These attenuation laws should preferably be obtained experimentally. However, empirical attenuation function can only be developed in the region that well instrumented (Accelerographs) and exposed to frequent occurrence of relatively strong earthquakes. • The only remedy to earthquake loss is proper planning and building, using the right building code “Earthquakes do not kill people – houses do! ".