Earthquake Occurrence in the Haradh Area, Eastern Saudi Arabia: Correlation with Gravity Data
Ian C.F. Stewart
Over the past year a number of earthquakes have occurred in the Eastern Province in the general vicinity of some of the main oil fields. The largest event had a magnitude of about 4.4, which while large enough to cause some alarm would have been unlikely to result in any significant damage to property. Earthquakes have been recorded instrumentally in the region over at least the previous 10 years, and the largest magnitude previously was around 4.1. Some of the earthquakes lie on the margins of known basement structures underlying petroleum reservoirs, and hence it may be possible to associate the seismicity with faults bounding these uplifted basement blocks. At present there is no indication that any of the seismicity is related to oil production. Recently the gravity data set covering most of the area has become available, and hence it is also now possible to correlate the earthquake locations with other geophysical data. The gravity data set contained significant errors due to misties between data traverses as well as noise inherent in the observations. Substantial further processing and editing of the gravity data has now produced a much-improved data set for further analysis. Marked improvements were achieved in the region of the Ghawar and Khurais fields and now enable various enhancement procedures to be applied to the gravity to highlight some of the structural trends. This then permits some tentative correlation of the recent seismicity with structure to be achieved using the gravity data. The gravity coverage in an area 500 km by 550 km centered on the recent earthquake activity is shown in Figure 1, and includes 135698 stations. It can be seen that there is a fairly good uniform coverage over most of the region, with an exceptionally large number of gravity stations acquired in conjunction with the 2D and 3D seismic surveys. Generally the newer data from the seismic surveys supersedes the earlier stand-alone gravity data where the stations coincide. A Bouguer reduction density of 2.3 gm cm-3 was used for all the gravity data within the Phanerozoic sediments as studies of well logs indicated that this was a good compromise for the near-surface lithologies. Small topography-related effects may occur in some places due to this density being incorrect locally, and terrain corrections were not applied due to insufficient topographic data, but generally the resulting Bouguer gravity is reasonably satisfactory. The coordinates for all the data and diagrams shown here are based on the WGS84 spheroid. The maps use a Lambert Cartesian projection with a central meridian of 48oE and standard parallels of 17oN and 33oN.
Stewart, I.C.F., 2006, Earthquake occurrence in the Haradh area, eastern Saudi Arabia: Correlation with gravity data: Saudi Geological Survey Data-File Report SGS-DF-2006-3, 9 p., 6 figs.