Al Madinah al Munawwarh is the second most important Holy City for Muslims throughout the world after Makkah al Mukarramah. Al Madinah, a city of over 700,000 permanent inhabitants, and annually hosts two million pilgrims and visitors, specially during the month of Ramadan and Al Hajj period, 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 Holy City of Al Madinah al Munawwarah, justifies long-term microseismic studies in the area. In addition, two historic volcanic events threatened the Holy City of Al Madinah al Munawwarah 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, which occurred in 1256 AD (654 AH). A Ten station short-period radio-telemetry seismic network and computer-assisted seismic recording facility operate in the Al Madinah-Harrat Rahat area under the management of the Saudi Geological Survey for the purpose of providing accurate hypocenter locations. The network now covers a region approximately 6000 km2 in size, which extends southeast into Harrat Rahat, site of a volcanic eruption in 1256 AD that threatened the Holy City itself. The Al Madinah-Harrat Rahat 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 data transfer from the acquisition computer to the analysis computer. 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. Throughout 2001, the computer data acquisition system successfully recorded more than 280 local seismic events, including several teleseismic events and regional earthquakes that occurred outside the network. In addition, the seismic network appeared more efficiency for detected numbers of the seismic foci onshore and offshore at different areas. Of the 280 events, thirty-four were identified as possibly due to tectonic activity. About 230 of the remaining events were the result of small explosions related to nearby construction northwest of the Holy City and due to the Yanbu-Al Madinah-Al Qasim highway construction project southwest, east and northeast of the Holy City of Al Madinah. The total number of events related to highway construction increased compared to the previous year. The epicenter locations were scattered around the Holy City and a few were clustered to the north and northwest of Al Madinah. Of the 34 tectonic events analyzed, ten events occurred within the network area, but were located outside the city boundaries of Al Madinah, whereas the remaining twenty-four events occurred outside the network area. Thirteen seismic events were deemed significant. Chronologically, the first event occurred on January 25, 2001 about 76 km northeast of Al Madinah area, with a coda magnitude of 3.64 on the Richter scale, and felt by the local people of Al Uqaylah, Al Matawi and Al Khayal villages. The second event was located at Harrat Kish, about 65 km to the north of Al Radwan town near the Ar Riyadh – At Ta’if expressway. It occurred on March 3, 2001, and had a coda magnitude of 3.59. The third event, with a coda magnitude of 3.25, occurred on March 9, 2001, and was located about 120 km to the northeast of Jeddah city. The fourth event occurred on June 30, 2001, northeast of Al Madinah and about 57 km northeast of Al Ishash town, and had a coda magnitude of 3.46. The remaining nine events occurred on May 21st, September 13, 26, and November 20, 2001, and were located about 200 km northwest of Al Madinah area in two clusters north-northwest and southwest of Al ‘Ays town. The magnitudes of these events ranged between 2.76 and 3.23 on the Richter scale. The number of tectonic events inside the network arreaof Al Madinah–Harrat Rahat decreased in 2001 in comparison with 2000. More than 65 percent of the events in 2001 occurred at moderate hypocenter depths. The magnitudes of the tremors associated with the majority of the epicenters ranged from 1.25 to 3.76 on the Richter scale. We conclude that there is no imminent likelihood of a volcanic eruption that would threaten the Holy City. A full array of monitoring procedures are in place to detect any changes in this situation. Historically, eruptions of weakly alkalic basalt elsewhere in the world have been preceded by anywhere from a few days to up 3 years of warning signs, such as the 1815 eruption at Tambora, on an island east of Java. The Harrat Rahat district cannot be considered predictable, however, because the level of tectonic or volcanic-related earthquake activity so far is variable. It is important to continue the operation of the seismic network and monitoring efforts 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 2001, Kingdom of Saudi Arabia: Saudi Geological Survey Open-File Report SGS-OF-2003-5, 25 p., 20 figs. 1 table, 1 app.