The Holy City of Al Madinah al Munawwarah is second in importance for Muslims throughout the world after Makkah al Mukarramah. Al Madinah, a city of over 700,000 permanent inhabitants, is host to two million pilgrims and visitors annually, and is a major air and land transport hub for the Kingdom of Saudi Arabia. The discovery of a low-grade geothermal anomaly in 1985, and the occasional presence of fumaroles at Harrat Rahat, an area of basaltic lava flows immediately southeast of the city, justifies long-term microseismic studies in the area. In addition, two historic volcanic events threatened the city in the last 1,500 years, one evidenced by a small line of cinder cones southwest of the city that occurred in 641 AD, and a second, extensive flow, that occurred in 1256 AD (654 AH). At the time of writing, a ten-station short-period radio-telemetered seismic network and computer-assisted seismic recording facility were operating in the Al Madinah area for the purpose of providing accurate hypocenter locations. The network covers a region approximately 6000 km2 in size, which includes all of the Holy City, and extends southeast into Harrat Rahat. The seismic network has a total of eight single-component stations, two 3-component stations, and one repeater site. The radio-telemetry system now in place ensures accurate relative timing for computing hypocenter locations. The network uses a thin-net Ethernet system, DOS and Windows for Workgroups 3.11. A Windows interface program called TRANSFER is used to facilitate transfer of data from a computer engaged in data acquisition, to another used for data analysis. A second Windows interface program called PICK_EM is used to make earthquake phase picking and location more efficient. Pentium III-type computers are used for digital recording of seismic events and for seismic data analysis. During the calendar year 2000, the computer data acquisition system successfully recorded more than 300 local seismic events, including several teleseismic events, and numerous regional earthquakes that occurred outside the network. Of these recorded events, 46 were identified as possibly due to tectonic activity. More than 200 of the remaining events were the result of small explosions related to nearby construction northwest of Al Madinah, and along the Al Madinah to Al Qasim highway east and northeast of the city. Epicenter locations were scattered around the city and more than twenty were clustered to the northwest, but located outside the seismic network of Al Madinah. Of the 46 tectonic events analyzed, twelve events occurred within the network area and the remaining thirty-four events occurred outside. The most significant event detected during this year was a seismic storm, composed of twenty-six seismic events, which occurred between October 26th, 2000 and November 30th, 2000. They were located about 190 km northwest Al Madinah, and about 7 km south of the town of Al ‘Ays. The events associated with the seismic storm were analyzed and plotted, yielding magnitudes ranging from 2.0 to over 3.0 on the Richter scale. The number of tectonic events inside the network area decreased in 2000 in comparison to 1999. More than 60 percent of the events in 2000 occurred at moderate hypocenter depths. The magnitudes of the tremors associated with the majority of the epicenters ranged from 1.37 to 3.34 on the Richter scale. Results of the current year of operation of the network clearly suggest that there is no imminent possibility of a volcanic eruption that would threaten the Holy City. Historically, eruptions of weakly alkalic basalt elsewhere in the world, such as the 1815 eruption at Tambora, on an island east of Java, were preceded by warning signs lasting from 3 days to 3 years. However, because the present level of tectonic or volcanic-related earthquake activity is less than the previous year, Harrat Rahat cannot be considered predictable. Continued operation of the seismic network is required to determine normal background seismicity. An accurate knowledge of this background level is required for any prediction of future volcanic activity.
Kinkar, A., Showail, A., Shawali, J., 2003, Results of the microseismic monitoring subproject in the Al Madinah al Munawwarah and Harrat Rahat areas for 2000, Kingdom of Saudi Arabia: Saudi Geological Survey Open-File Report SGS-OF-2003-4, 22 p., 19 figs. 1 table, 1 app.