Annual Meeting - SCEC.org

Abstracts | Plenary Presentations
Coast has been in operation for almost two decades. The Japanese EEW system went operational in
October 2007. Several European regional EEW systems are at various stages of development under
the SAFER project. In the US the CISN is developing and testing three EEW algorithms in
cooperation with the USGS/ANSS and SCEC.
The 29 July 2008 Mw5.4 Chino Hills earthquake that was located in less than 50 km distance SE of
downtown Los Angeles, once again demonstrated the persistent seismic threat that people in
California are exposed to. Further, a recent USGS/CGS/SCEC study states that it is a matter of
time only until the next “Big One” will happen and preparative measures need to be developed
and implemented as soon as possible. I will describe the possible application of EEW systems,
which rapidly detect seismic waves, estimate the earthquake magnitude and associated ground
motion, and, if required, issue warnings to potential users before seismic waves arrive. Even if the
warning times are in the order of some seconds to about one minute, they can be sufficient to
trigger and execute automatic measures to decrease likely damage and numbers of fatalities during
strong earthquakes, e.g., by automatically stopping trains and elevators. Depending on the location
of the earthquake hypocenter, warning times for large cities such as Los Angeles might range up to
about one minute, which is sufficient to implement various mitigation measures to safe lives and
reduce the potential impact of the earthquake.
In Oct. 2007, the Japanese Meteorological Agency (JMA) launched the 2nd and arguably the most
comprehensive public EEW system in the world. If damaging ground shaking due to a strong
earthquake in Japan is expected, the JMA system broadcasts warnings to the public using diverse
communication links, including television, radio and e-mail. Some minor problems at the early
stage of the system, e.g., the initial underestimation of ground shaking during the Iwate-Miyagi
Inland Mw6.9 earthquake in June 2008, caused critical comments by the international press. This
user feedback is useful for understanding how to educate the public about the limitations of EEW.
The Japanese experience is also encouragement for implementing improvements in communication
technologies and EEW algorithm.
TUESDAY, SEPTEMBER 9, 2008 – 15:30
C 3 (CORRELATION OF CODA OF CORRELATIONS): IMPROVING THE
RECONSTRUCTION OF GREEN FUNCTIONS BETWEEN STATIONS OF A
NETWORK FROM NOISE RECORDS
Campillo M, Froment B, Stehly L, and Weaver RL
Our goal is to extract the Green function between every pairs of receivers of a network by taking
advantage of the simultaneous records of the noise by a large number of stations. In the case of an
even distribution of noise sources, it has been theoretically demonstrated that the correlation is the
exact Green function, including all types of waves. In most cases, the noise is far to be consisting in
an even azimuthal distribution of incoming waves. It would be therefore preferable to use multiply
scattered waves, which have the adequate equipartition property. Unfortunately we can not
separate the direct and scattered waves in the noise records as we can with a simple time window
in earthquake records. We use a procedure that allows for removing the effects of the directivity of
the noise. We consider two stations A and B for which the Rayleigh waves could not be discerned
in the in the correlation of continuous records of ambient noise. We computed all correlations
between the station A (resp. B) and all the 150 other stations located at regional distances.
Theoretically, these virtual seismograms contain direct waves and coda, although they are clearly
contaminated by the influence of the imperfect ambient noise field and most are inadequate for
direct analysis. We used these correlation functions as equivalents to seismograms produced by
sources acting at the 150 stations locations and recorded in A (resp B). We select time windows in
68 | Southern California Earthquake Center