SCSN Moment Tensors and Focal Mechanisms
Moment Tensor Solutions
A moment tensor is a mathematical representation of the movement on a fault during an earthquake. The tensor depends on the source strength and fault orientation. The moment tensor is represented visually by what are commonly called "beachball diagrams" and show the style of faulting and fault orientation. For more general information about moment tensors see ANSS ComCat Moment Tensor Description
The SCSN Moment Tensor Solution is based on the method developed at UC Berkeley by Doug Dreger. SCSN automatically calculates moment tensor solutions on all local events with Ml>3.0, and all regional events with Ml>=3.5 identified by the SCSN real-time system. If the quality of waveform fits is good enough, and the event is within the SCSN reporting region, it is automatically distributed to SCEDC moment tensor catalog and the ANSS ComCat. The solution can also be modified by analysts and re-distributed. The software is also able to run on catalog and has generated solutions for events from 1999-present.
A focal mechanism is a representation of the direction of earthquake slip and the orientation of the fault on which it occurs. It is derived from the observed direction of P wave first motions at a station.
The SCSN calculates focal mechanisms based on the HASH method (https://doi.org/10.1785/0120020236) which uses S/P amplitude ratios. If the quality of the solution is good enough and the event is within the SCSN reporting region, it is automatically distributed to the SCEDC focal mechanism catalog and the ANSS ComCat. SCEDC also hosts a number of researcher generated focal mechanism catalogs in the "EQ Catalogs->Alternate Catalogs" page. The solution can also be modified by analysts and re-distributed. Prior to 2013, focal mechanisms were calculated by the fpfit method.
For more general information on moment tensors and focal mechanisms, see USGS article on Focal Mechanisms
Moment Magnitudes and Moment Tensor Solutions are available for events from September, 1999 to present.
The date parameters are specified in the following format:
year= year (1999-present)
mm = month (1-12)
dd = day-of-month(1-31)
Output and input times are in UTC time.
hr = hour(1 - 24)
min = min(1 - 60)
sec = sec(1 - 60)
The real time moment tensor code runs on all local events with M>3.0 and all regional events with M>=3.5 identified by the SCSN real-time system.
Latitude and Longitude values are specified in decimal degrees, e.g., 34.50 for 34 and one-half degrees. Latitude values can range from -90 to 90, and longitude values can range from -180 to 180. Remember that longitude for California is WEST, and therefore should be specified as NEGATIVE (eg -121.5 for 121.5 degrees West).