Single-Station Estimates of the Focal Mechanism for Weak Earthquakes


 The focal mechanism is a parameter of an earthquake that typically demands observations surrounding the focus. If such observations are not available, only limited information can be retrieved regarding the process of rupturing (e.g., geometry, and the orientation of the fault plane and slip), not its shear versus non-shear nature. In extreme cases of monitoring that incorporate a single station, a mechanism can only be estimated if data from additional seismic phases are available. However, such is not the case for weak seismic events, in which a station often only records direct P and S waves. Under such conditions, information is severely limited, and standard synthetics-to-data-matching does not allow even the simplest and most constrained seismological source model, namely a double-couple focal mechanism, to be determined. During data review, we detected an internal ambiguity for solutions of the inverse task using single-station geometry. Nevertheless, we found that at least some important features of a focal mechanism may be retrieved. Such scenarios occur for particular configurations of focal mechanism orientation and source-station direction, as follows: (1) If a station lies in a direction within the quadrant of compressions, the T axis can be retrieved with reasonable error. However, the P axis remains ambiguous. (2) Conversely, if a station lies in a direction within the quadrant of dilatations, reasonable resolution of the P axis is possible, while the T axis remains ambiguous. (3) If the data are noisy, apart from extreme cases of contamination, the possibility of estimating the direction of the T/P axis remains intact. The closer source-station direction is to the center of the quadrant of compressions or dilatations, the better the resolution of the T/P axis. Our method was applied to selected weak earthquakes recorded by the three-component borehole seismic station MDBI, located near the Dead Sea in Israel.