Real-time moment tensor simulation movie

Charles Mégnin, Fumiko Tajima, Doug Dreger and Barbara Romanowicz


We present a simulation of the response of our real-time moment tensor system for the 10/30/1998 Truckee, CA event (Ml=5.35, origin time 09:53:30).

Synopsis

We perform continuous inversions of moving windows of the seismic wavefield for moment tensor (DC and CLVD) and centroid depth, for 160 virtual epicenter locations on a 0.5 by 0.5 degree grid (the circles) overlaying northern and central California. The fit of the moment tensor solution to the data at each grid point is expressed as a variance reduction, its value for each window is indicated in front of 'VR' in the top legend and is color-coded on the maps. A variance reduction above 75% indicates the occurence of a seismic event.

In this simulation, the time windows are five minutes long and each successive window is shifted by 20 seconds with respect to the previous one, simulating data recorded by the seismic stations in real-time. The starting time of each moving window is given by the value t0 in the legend near the top of each frame.
The data is shown for 6 broadband `Berkeley Digital Seismic Network (BDSN)' stations used (yellow triangles on the map at BKS, CMB, HOPS, ORV, SAO and YBH) at the bottom of the plot. Each trace is scaled to its maximum amplitude. The traces on the left are the raw data and those on the right are in the bandpass between 20 and 50 mHz (i.e. those that are used in the inversion). Only the vertical component of each seismic trace is shown, but we invert 3 component data.

We show for comparison the `Rapid Earthquake Data Integration (REDI)' ( Gee et al., 1996) moment tensor solution with the (fixed) blue beach ball and the REDI epicenter location with the red star (1). The best solution corresponding to each time window is represented by the red beach ball, which points to the optimal epicentral location.
The best automated moment tensor solution for this event yielded a variance reduction of 93% (color-coded orange), achieved in the five successive five-minute time windows beginning between 09:52:20 and 09:53:40.

Results

In the first few frames, not enough data is present in the inverted time window to yield a correct solution. Consequently, the variance reduction is low everywhere and the solution displays spatial variability with time. Starting with the 12th frame, most of the seismic wavefield generated by the event is present in the window and the solution starts converging to the REDI location/moment tensor. The variance reduction in the 13th frame is above the 75% threshold and the automated moment tensor algorithm `zooms in' on the area 80 km around the best solution, computing a new set of moment tensors on a 0.2 by 0.2 degree grid, which is allowed to migrate outside of the region covered by the initial grid (frame 14). In frame 15, a better solution is obtained on the coarse grid (92.5% variance reduction, centroid location 23 km from the REDI epicenter) and a second zooming in takes place (frame 16), slightly improving the variance reduction (93%). The spatial accuracy of the new solution is about 12 km, improving the location on the coarser grid by 11 km. Frames 17 to 20 yield the same solution as 16, and since the variance reduction is not improved, the solution retained is that of frame 16.
The automated moment tensor algorithm therefore allows the computation of earthquake source characteristics in real time, providing a spatial solution of about 0.2 degree accuracy. (1) The REDI system is currently used at UC Berkeley Seismological Laboratory for fast determination of earthquake parameters.

This simulation is available in mpeg format. The mpeg file viewer (and encoder) can be freely downloaded from this site.

Download gzipped mpeg file simulation (7.3 Mb).

Suggested viewing rate : 1 frame / second or less (mpeg_play -framerate 1 nocal.6.mpeg)

Please address questions to Charles Mégnin