Martín Cárdenas-Soto

Regional Path Effects on Seismic Wave Propagation in Central Mexico

We present results of the analysis of a set of seismograms recorded in a temporary array of seven broadband seismic stations located in central Mexico. Our purpose was to investigate path effects on ground motion in central Mexico for earthquakes occurring in the subduction zone, along the Pacific coast. All our stations were located on rock. Nineteen earthquakes (3.6 < M c < 4.9), recorded by all seven stations, were analyzed. The results show that path effects result in significant amplification and an increase of the duration of the seismic motion at two stations located in the central part of the Transmexican Volcanic Belt (TVB). Analysis of the records for periods larger than 5 sec shows that the 3D crustal structure acts as an efficient wave guide for surface waves coming from distances greater than 200 km, only for paths oriented north–south or east–west. In the band of 2–6 sec, we observe large amplification and an increase in the duration of ground motion, associated to the regional propagation of higher modes of surface waves. After correlating our results with recent geological and gravimetric studies, we suggest that this amplification could be due to the impedance contrast of low-velocity volcanic rocks of the TVB overlying higher velocity limestones. If we are right, the increase of ground-motion duration would result from mode conversion at the southern boundary of the TVB. These path effects in the 2- to 6-sec period band are very important for ground motion in Mexico City because the very soft surficial deposits in that basin amplify greatly ground motion in this same period band.

Performance of CMORPH, TMPA, and PERSIANN rainfall datasets over plain, mountainous, and glacial regions of Pakistan

The present study aims at the assessment of six satellite rainfall estimates (SREs) in Pakistan. For each assessed products, both real-time (RT) and post adjusted (Adj) versions are considered to highlight their potential benefits in the rainfall estimation at annual, monthly, and daily temporal scales. Three geomorphological climatic zones, i.e., plain, mountainous, and glacial are taken under considerations for the determination of relative potentials of these SREs over Pakistan at global and regional scales. All SREs, in general, have well captured the annual north-south rainfall decreasing patterns and rainfall amounts over the typical arid regions of the country. Regarding the zonal approach, the performance of all SREs has remained good over mountainous region comparative to arid regions. This poor performance in accurate rainfall estimation of all the six SREs over arid regions has made their use questionable in these regions. Over glacier region, all SREs have highly overestimated the rainfall. One possible cause of this overestimation may be due to the low surface temperature and radiation absorption over snow and ice cover, resulting in their misidentification with rainy clouds as daily false alarm ratio has increased from mountainous to glacial regions. Among RT products, CMORPH-RT is the most biased product. The Bias was almost removed on CMORPH-Adj thanks to the gauge adjustment. On a general way, all Adj versions outperformed their respective RT versions at all considered temporal scales and have confirmed the positive effects of gauge adjustment. CMORPH-Adj and TMPA-Adj have shown the best agreement with in situ data in terms of Bias, RMSE, and CC over the entire study area.

Modelling the vulnerability of groundwater to contamination in an unconfined alluvial aquifer in Pakistan

Abstract The area of Thal Doab is located in the Indus Basin and is underlain by a thick alluvial aquifer called the Thal Doab aquifer (TDA). The TDA is undergone intense hydrological stress owing to rapid population growth and excessive groundwater use for livestock and irrigated agricultural land uses. The potential impact of these land uses on groundwater quality was assessed using a DRASTIC model in a Geographic Information System environment. Seven DRASTIC thematic maps were developed at fixed scale and then combined into a groundwater vulnerability map. The resultant vulnerability index values were grouped into four zones as low, moderate, high and very high. The study has established that 76% of the land area that is underlain by the TDA has a high to very high vulnerability to groundwater contamination mainly because of a thin soil profile, a shallow water table and the presence of soils and sediments with high hydraulic conductivity values. In addition, only 2 and 22% of the total area lie in low and moderate vulnerability zones, respectively. The outcomes of this study can be used to improve the sustainability of the groundwater resource through proper land-use management.

Site response and seismic wavefield in Toluca city, Mexico, from strong motion records

Since 1993 a network of 6 accelerographs has been operating in the city of Toluca. To date, only one seismic event has been recorded by all six stations. We analyse in detail those records with the purpose of measuring site response and analyzing the recorded wavefield. We compute spectral ratios of Fourier amplitude spectra relative to a reference station and of the horizontal components relative to the vertical recorded at each site. We compare the traces in different period bands, analyse group velocity dispersion for each trace, and compute f-k (frequency-wave number) spectra. Our results show, as expected, the large similarity between ground motion in the valley of Mexico and the valley of Toluca concerning path effects. We show some evidence of late arrivals of energy coming from a direction different from the epicentral one. It is fortunate for Toluca that local site effects do not amplify ground motion as much as in Mexico City, eventhough observed path effects lead to similar long durations at intermediate periods (between 1 and 5 sec). A more detailed analysis is needed, however the quality and quantity of the available records do not allow this at present. Artículo recibido el 20 de septiembre de 2004 y aprobado para su publicación el 9 de marzo de 2006. Se aceptarán comentarios y/o discusiones hasta cinco meses después de su publicación. (1) Instituto de Ingeniería, UNAM, Apdo. Postal 70-472, Coyoacán, 04510 México D.F., México. paco@pumas.iingen.unam.mx (2) Posgrado de Geofísica, Instituto de Geofísica, UNAM. hugo.ferrer@upaep.mx (3) DICT, Facultad de Ingeniería, UNAM. martinc@servidor.unam.mx Hugo Ferrer, Martín Cárdenas-Soto, and Francisco J. Chávez-García