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1 Departamento de
Sismología
Instituto de Geofísica
Universidad Nacional
Autónoma de México
CU, México, DF, C.P.
04510
(A.I., S.K.S., M.A.S, J.P.)
2 Instituto de
Ingeniería
Universidad Nacional Autónoma de
México
CU, México, DF, C.P.
04510
(M.O.)
The seismic alert system (SAS) for Mexico City has now been in
operation
for about 15 years. The SAS takes advantage of the fact that the city
is located
more than 300 km from the foci of many of the potentially damaging earthquakes.
The system consists of 15 accelerometers located along the coast of the State of
Guerrero, above a segment of subduction plate boundary that is a mature seismic
gap. An algorithm estimates the magnitudes of earthquakes from the near-source
accelerograms and issues public and restricted alerts for earthquakes with
M 
6 and
5 
M < 6, respectively. An evaluation of the
SAS’s performance during 1991–2004
reveals a surprisingly high rate of failure and false alerts. This poor
performance
results from an inadequate detection algorithm and a limited areal coverage by
the
SAS. This renders the alert system of limited use.
In this article we propose an alternative strategy for detecting earthquakes
potentially
damaging to Mexico City that differs substantially from the one presently
implemented by the SAS. Although our analysis is based on
close-to-source accelerograms
of 45 Mexican earthquakes and the corresponding peak accelerations recorded
at a reference site in Mexico City (CU), there is no restriction on
the distance
to the field station, except that its location should provide sufficient alert
time to
Mexico City. Based on these data, an attenuation relation is derived to compute
expected peak acceleration at CU (Ared) from
root-mean-square acceleration (Arms) at
a field station. The relation permits specification of an
Ared threshold, given the peak
acceleration at CU (ACU) for which an
alert is desired along with prescribed probabilities
of failure and false alert. We find that the use of bandpass-filtered
(0.2–1.0 Hz)
accelerograms leads to an improved performance of the SAS. The choice
of the filter
is guided by the frequency band of amplification of seismic waves in the
lake-bed
zone of Mexico City. We think that a single level of general public alert may be
the
best option. A good choice appears to be an alert for
ACU 2 gal (for 0.2–1.0 Hz
bandpass-filtered accelerograms) with 1% probability of failure. To accomplish
this
we must set Ared 0.8 gal. The data since 1985 suggest
that such an alert would
occur about once or twice a year and the event will be felt by most persons in
the
lake-bed zone. The proposed algorithm, along with an array of sensors located 30
to
40 km apart and distributed in a roughly semicircular arc of 310-km radius
centered
at Mexico City, should considerably improve the areal coverage and performance
of
the SAS and potentially save thousands of lives.
This article has been cited by other articles:
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  S. K. Singh, M. Ordaz, J. F. Pacheco, L. Alcantara, A. Iglesias, S. Alcocer, D. Garcia, X. Perez-Campos, C. Valdes, D. Almora, et al. A Report on the Atoyac, Mexico, Earthquake of 13 April 2007 (Mw 5.9) Seismological Research Letters, November 1, 2007; 78(6): 635 - 648. [Full Text] [PDF]
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