Our recent study (*Tajima et al.*, 2001) tested the feasibility
of a new automated moment-tensor (*AMT*) determination system that
continuously monitors seismic waveforms from a sparse network of
regional broadband stations.
This system uses real-time waveforms in a time window that is
repeatedly shifted forward with a short time interval (20 sec),
and unlike the current methods, performs inversion of
moment tensors (MT's) over a grid that is set for a region of interest,
without prior knowledge of the location and origin time information.
The advantage of this system is its independency of the information
that is currently provided by dense short-period networks.
Significant improvement of the computational efficiency
is expected if the system is fully configured on a powerful
PC with multiple CPU's of 1 Gbyte memory, and the inversion
over the grid can be updated within
the time frame of time window shift (20 sec).

In the *AMT* inversion the time window of waveforms is shifted forward
by a prescribed amount . Therefore, the CPU time needed to compute
variance reductions () that is defined by

To make the *AMT* system operational and computationally
effective, path calibration is necessary to account for the 3-D
structure between virtual sources over the grid and stations.
Currently three multi-layered models are used for calculating
Green's functions (see the regionalization in Figure 22.1).

If Green's functions ( stands for a virtual source
on the grid, for a station, and for the -th elementary
moment-tesnsor) are calculated with well constrained
velocity models and stored in the memory, the computation time
will be shortened significantly, and the efficiency desired for the
*AMT* system can be achieved.
However, there is no short cut in calibrating paths for the 3D structure,
and this project is still in progress.
With Green's functions that account for 3D paths, the gridded
structure of the methodology is ideally suited to achieve better
resolution and stability.

(grid points)

for a km area. The total size of Green's functions for each station requires

.

If up to six stations should be used, the memory size will be about 750 Mb, and be slightly increased should the auto correlated Green's functions be also in the memory.

Figure 22.2 illustrates the projected time frame of
the *AMT* as compared with the present standard system.
The standard system at the *BSL* as part of the
*REDI* (*Gee et al.*, 1996; 2001) determines MT's for
events of .
The time frame between an event occurrence and MT determination
(using 2 different methods) is 8-10 min. If the
*AMT* system monitors seismic wavefield continuously, and
updates the regional MT search over the grid every 20 sec, the time
interval between the origin time and MT determination could be shortened
by several minutes or more.

A tsunami warning system can be considered for an effective
application of the *AMT* method, as the event location and MT
can be determined early enough before the tsunami waves arrive at
the coasts. Here, the interval of the P-wave first arrival
at the station () and the tsunami arrival at the coast
() is roughly estimated as

Dreger, D. S. and D. V. Helmberger, Determination of source parameters
at regional distances with three-component sparse network data,
*J. Geophys. Res., 98*, 8,107-8,125, 1993.

Dreger, D. S. and B. Romanowicz, Source characteristics of events in the
San Francisco Bay region, *U.S. Geol. Surv., Open-File Rept. 94-176*,
301-309, 1994.

Gee, L., D.S. Dreger, D. Neuhauser and B. Romanowicz, The REDI program,
*Bull. Seism. Soc. Am., 86*, 936-945, 1996.

Gee, L., D. Neuhauser, D. Dreger, M. Pasyanos, R. Uhrhammer, and B. Romanowicz,
The Rapid Earthquake Data Integration Project,
*Handbook of Earthquake and Engineering Seismology*, IASPEI, in press, 2001.

Kawakatsu, H., On the realtime monitoring of the long-period seismic wavefield,
*Bull. Earthq. Res. Inst., 73*, 267-274, 1998.

Tajima, F., D. Dreger, B. Romanowicz, Modeling of the transitional
structure from ocean to continent in the Mendocino region using
broadband waveform data, *EOS Trans. Am. Geophys. Union, 81,* 2000.

Tajima, F., C. Mégnin, D. Dreger, and B. Romanowicz, Feasibility of
real-time broadband waveform inversion for simultaneous moment tensor
and centroid location determination, *Bull. Seism. Soc. Am., in press,* 2001.

Questions or comments? Send e-mail: www@seismo.berkeley.edu

© 2001, The Regents of the University of California.