Earthquakes that occur near known nuclear test sites are invaluable for evaluating the capabilities of the International Monitoring System (IMS), currently being established to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). On 13 March 2003 a seismic disturbance with magnitude m (sub b) (National Earthquake Information Center [NEIC]) 4.8 occurred near the Chinese nuclear test site at Lop Nor, southern Xinjiang. Here we attempt to identify this disturbance as an earthquake by using three types of seismic data: (1) teleseismic P waves recorded at IMS stations; (2) long-period surface waves recorded at a network of stations in Eurasia that simulates the proposed IMS seismic network in this region, and (3) long-period full waveform (P, S, and surface wave) modeling at station WMQ, which is approximately 250 km from the reported epicenter of the 13 March 2003 disturbance. We find that all impulsive teleseismic P-wave onsets show compressive first motion and that discrimination using the m (sub b) :M (sub s) criterion requires assumptions that cannot be justified on theoretical grounds. However, by combining the three data types, we conclude that the observations are consistent with a double-couple source with strike phi = 125+ or -10 degrees , dip delta = 40+ or -10 degrees , rake lambda = 90+ or -10 degrees , and moment M (sub 0) 5.5+ or -1X10 (super 15) N m. That said, consistency of the seismic wave field with a double couple does not immediately rule out an explosion source, because tectonic release accompanying an explosion can make it impossible to resolve the isotropic moment using long-period data. The strongest argument for an earthquake is that both the regional surface waves, and waveform modeling at WMQ, give an estimated focal depth of 6+ or -1 km. This result reinforces the importance of focal depth determination to CTBT monitoring, in particular, for shallow earthquakes with mechanisms that are close to perfect 45 degrees reverse-dip-slip, which are difficult to discriminate by using other methods. Fortunately, depth determination using surface waves is especially favorable for this type of earthquake, if a sufficient number of stations can be used. We therefore recommend that long-period data recorded at auxiliary seismic stations of the IMS be utilized by the International Data Centre to monitor compliance with the CTBT.