Effects Of The Subsidence On The Changes Of Dominant Periods Of Soils Within Mexico City Valley

Effects of the Subsidence on the Changes of
Dominant Periods of Soils within Mexico City Valley

José A. Martínez-González, Javier F. Lermo-Samaniego,
Francisco J. Sánchez-Sesma, Joel Ángulo -Carrillo,
Rubén Valle-Orozco & Jorge Ordoñez-Alfaro
Instituto de Ingeniería, Universidad Nacional Autónoma de México, Torre de Ingeniería, 2do.
Piso, Ala Sur, Cd. Universitaria, Coyoacán, C.P. 04510,México, D.F.

Luis E. Pérez-Rocha
Instituto de Investigaciones Eléctricas, Palmira, Cuernavaca, México

SUMMARY:
In this work the variations of the dominant periods in Mexico City soil sites within a time window of 20 years
are pointed out. These changes appear to be strongly correlated to regional subsidence. Therefore, a proposition
is made to update the corresponding zon ification mapwithin the Technical Annex of Mexico City (Federal
District) Building Code. The acceleration records on 85 stations of the Mexico City Accelerometric Network
(MCAN) for an assortment of recent earthquakes were studied. Besides, 810 new measurements of microtremors
were performed at various sites. From data analysis a relationship that describes this 20-years-variation is
obtained. It revealschanges of up to 0.9s within a zone traditionally considered as Lake (zone IIId). This
relationship allowed updating the available information, reaching 1300 data points that allowed the generation of
a new seismic zonification.
Keywords: subsidence, basin of mexico, microtremor, seismic zonification

1. INTRODUCTION
As a consequence of the great Michoacán earthquake of September 19th, 1985(M8.1) that produced
unprecedented dead toll and infrastructure damage to Mexico City, the studies of dynamic soil
properties, the seismic response of the city, the seismic hazard analysis and the zonification got a
renewed interest. The city already had a zonification of three zones based on the geology: zone I (Hill
zone) located at the higher parts of the basin, its soils are typicallycharacterized by high strengths with
little compressibility; the zone II (T ransition zone) has soils composed by sa nd and silt; and the zone
III (Lake zone) was divided in four subzones (A, B, C, and D) and comprises the ancient Texcoco and
Xochimilco lakes, with soil types of volcanic origin, very soft and compressible, with very high water
content, and low shear wave velocities. All this favorthe amplification of seismic waves.
The first map of soil periods for Mexico City was issued in 1987. It was composed after data from 99
measurement points were gathered. Afterwards, measurements of both microtremors and strong
motion data in many other points were conducted. Lermo et al. (1994) collected many of such
measurements and performed new ones away from fixed stations or previousmeasure points. They
also recorded small earthquakes using a temporal seismic network.
Lermo and Chávez-García (1994) studied 81 accelerometric stations and 409 records of microtremors,
with a relatively high spatial density produced a map of dominant periods for the valley (see Fig.
1.1a). This map was the base for the isoperiod chart within the Complementary Technical Annex for
Seismic Design(NTCDS for its name in Spanish) of the Mexico City (Federal District) Building Code
(RCDF) (see Fig. 1.1b). The code specify that the limit between zones I and II is the line of dominant
period Ts=0.5s, while the limit between zones II and III corresponds to Ts=1.0s.

a)

b)

Figure 1.1 a) Map proposed by Lermo et al. (1994) for Mexico City, each dot indicates a measurement. b)
IsoperiodMap for the Federal District in Complementary Technical Annex for Seismic Design (NTCDS, 2004).

Note however, that these maps have some differences that require a comment. The NTCDS’ map
displays isoperiod curves (Ts=3.0 and Ts=4.0s) that cross the well-known P eñón de los Baños and
Peñón del Marqués topographic features. On the other hand, the Lermo and Chávez-García (1994)
maps clearly...