F. Ramón Zúñiga

Most and Least Likely Locations of Large to Great Earthquakes Along the Pacific Coast of Mexico, Estimated from Local Recurrence Times Based on b-values

We mapped T L and P L (local recurrence time and local probability density, respectively) for M max 7.2 earthquakes along the plate margin of the Pacific coast of Mexico in order to test two hypotheses. The first is that the minima in T L (or maxima in P L), extracted probabilistically from the frequency-magnitude distribution map asperities. The second hypothesis is that recurrence times for characteristic events are estimated more correctly by local recurrence times than by the overall recurrence times of the zone. Comparing these results to the observed recurrence times for events of M ≥7.0, we find good agreement. The coast of Guerrero between longitude west 100.5° and 101.5° and the segment off the coast of southern Chiapas show the shortest T L estimated as about 20 yr; the location at longitude 98.5° (near the boundary of Guerrero with Oaxaca, the Ometepec segment) follows with estimates of about 30 yr as the next shortest T L; along the coast of Guerrero, between 99° and 100.5° longitude, T L is estimated as about 40 yr; and finally, in the segments off the Oaxacan coast near 95.7° and 97.7° longitude, T L ranges from 40 to 60 yr. These volumes we define as asperities. Long local recurrence times are observed for the areas offshore from the isthmus of Tehuantepec, on the Pacific plate off the coast of Guerrero, on land along the coast of most of Oaxaca, and along the northernmost 40 km of the Guerrero coast. If our ideas are correct, then major moment release should emanate from the volumes we defined as asperities, and relatively minor moment release should emanate from the volumes identified by the relatively long local recurrence times.

A general overview of the catalog of recent seismicity compiled by the Mexican Seismological Survey

We have analyzed the catalog of recent (1974-1998) seismicity in Mexico, based on the automatically compiled reports of the Servicio Sismológico Nacional (Mexican Seismological Survey). To this end, we employed various tools developed for the analysis of seismic catalogs as well as for detailed studies of seismicity characteristics. Such tools comprise both newly developed techniques and traditional methods which deal with the subject of artificial and natural variations of seismicity. We found that the time characteristics of reporting suffered a drastic change during 1988. Before 1988 routine procedures involved locating a large number of events and assigning magnitude only to the largest. After 1988 a decrease in the location of small events and a significant increase in the number of magnitude determinations were noted. The minimum magnitude of completeness for the period 19881998 is Md = 4.3. A comparison of the frequency-magnitude distributions for mb (PDE) and Md (SSN) indicates that mb magnitudes are basically equivalent to Md for the range M ≤ 5.0. These results are important in that they show that when using statistics which do not take into consideration the period of reporting, serious biases could be introduced. Conversely, our results provide means to correct for the unhomogeneity of the catalog. In terms of the most significant seismicity variations in both space and time which could be due to natural causes we found several cases that need to be studied individually.

Holocene paleo-earthquakes recorded at the transfer zone of two major faults: The Pastores and Venta de Bravo faults (Trans-Mexican Volcanic Belt)

We present evidence of five late Holocene earthquake ruptures observed at two paleoseismological trenches in the Laguna Bani sag pond (Trans-Mexican Volcanic Belt, central Mexico). The trenches exposed two fault branches of the western termination of the Pastores fault, one of the major fault systems within the central Trans-Mexican Volcanic Belt. The site was studied by combining geomorphological and structural approaches, volcanic mapping, ground-penetrating radar, and paleoseismological analysis. The study revealed that coseismic surface rupture was noncharacteristic, and that the exposed fault branches had not always moved simultaneously. The fault tip has ruptured at least 5 times within the past 4 k.y., and the rupture events followed and preceded the deposition of an ignimbrite. The close temporal relationship of the seismic rupture with the volcanic activity of the area could be the result of volcanism triggered by faulting and its associated seismicity. The relatively high recurrence of seismic events (1.1–2.6 k.y.) and the noncharacteristic fault behavior observed at this tip of the Pastores fault suggest that the fault might have been active as a primary fault rupturing along segments of variable length or depth, and/or that the fault ruptured eventually as a secondary fault. The secondary ruptures would likely be related to earthquakes produced at major neighboring faults such as the Acambay fault, which moved during the 1912 Acambay earthquake, or the Venta de Bravo fault. A relatively large slip rate estimated for this fault branch (0.23–0.37 mm/yr) leads us to contemplate the possible connection at depth between the Pastores and the Venta de Bravo faults, increasing the maximum expected magnitude for central Mexico.

A first-order seismotectonic regionalization of Mexico for seismic hazard and risk estimation

The purpose of this work is to define a seismic regionalization of Mexico for seismic hazard and risk analyses. This seismic regionalization is based on seismic, geologic, and tectonic characteristics. To this end, a seismic catalog was compiled using the more reliable sources available. The catalog was made homogeneous in magnitude in order to avoid the differences in the way this parameter is reported by various agencies. Instead of using a linear regression to converts from mb and Md to Ms or Mw, using only events for which estimates of both magnitudes are available (i.e., paired data), we used the frequency-magnitude relations relying on the a and b values of the Gutenberg-Richter relation. The seismic regions are divided into three main categories: seismicity associated with the subduction process along the Pacific coast of Mexico, in-slab events within the down-going COC and RIV plates, and crustal seismicity associated to various geologic and tectonic regions. In total, 18 seismic regions were identified and delimited. For each, the a and b values of the Gutenberg-Richter relation were determined using a maximum likelihood estimation. The a and b parameters were repeatedly estimated as a function of time for each region, in order to confirm their reliability and stability. The recurrence times predicted by the resulting Gutenberg-Richter relations obtained are compared with the observed recurrence times of the larger events in each region of both historical and instrumental earthquakes.

Coda attenuation in the area of El Chichón volcano, Chiapas, Mexico

Abstract Coda-wave attenuation is investigated using the single scattering approximation on analog records from stations of a telemetered network situated close to El Chichon volcano in southern Mexico, as well as from temporary stations deployed during 1982. The single scattering model was applied to the envelope of coda waves to characterize the coda decay rate. We obtained values of Q c which, according to the filter settings of the seismographs, and the average period measured in the traces, correspond to frequencies of about 1.3 ± 0.3 Hz. Results indicate apparently larger attenuation than observed with similar techniques in other regions of Mexico, including the Mexican Volcanic Belt. Within the limitation imposed by the data, the spatial distribution of coda Q with respect to the volcanic edifice was investigated. For paths sampling the near vicinity of the volcano a value of approximately Q c = 30 was obtained. The highest attenuation results are obtained for volumes with a slight E-W tendency, with Q c values of 28 and 25, respectively. The location of a gravity high as well as microseismic activity correlates well with this result, which is interpreted as due to the probable presence of an intrusive magma body. Lower attenuation is obtained for volumes sampling deeper than the Moho, reaching a value of Q c ≈ 87, suggesting that rocks below this boundary play a major role in controlling the overall attenuation of paths traversed by coda waves. The results also suggest a decrease in attenuation towards the south and north possibly reflecting a decrease in magmatic processes in both of these directions.

Shape of the Magnitude–Frequency Distribution for the Canterbury Earthquake Sequence from Integration of Seismological and Geological Data

Abstract We combine geological and seismological data to describe the shape of the magnitude–frequency distribution for the Canterbury earthquake sequence. In doing so, we take the opportunity of the new datasets to address a long‐standing debate in the literature as to whether the seismicity of individual faults or fault zones is best described by the Gutenberg–Richter (GR) relationship or the characteristic earthquake (CE) model. We find that the magnitude–frequency distribution for the entire composite fault zone of the Canterbury earthquake sequence is adequately described by the GR relationship when uncertainties in the GR curve (fitted to the instrumental catalog data) and the range of paleoseismically derived recurrence intervals for large (mainshock) earthquakes on the Greendale fault are collectively considered. In contrast, the magnitude–frequency distribution for the smaller area of the Greendale fault is better described by the CE model. The difference is one of scale, in that the composite fault zone represented by the Canterbury earthquake sequence shows a GR distribution, whereas individual faults within the zone show CE distributions. Definition of magnitude–frequency distributions for seismic‐hazard modeling must therefore take scaling considerations into account, rather than simply assuming that a single magnitude–frequency distribution shape is applicable to all fault definitions.

On the Vulnerability of the Indigenous and Low-Income Population of Mexico to Natural Hazards. A Case Study: The Guerrero Region

This chapter deals with some of the issues behind the vulnerability of that part of the Mexican population who are most exposed to natural hazards due to the site and conditions of their dwellings. In the case of Mexico, as is the case in many other countries, the people most vulnerable to natural hazards, in general, are the ones with the lowest income. This fact, in turn, translates in countries such as Mexico, in a large percentage also being indigenous. The issue of ethnicity plays various roles dealing with ethics when facing measures to issue warnings or carry out contingency procedures. Although it would be intuitive to assume that low-income people are most vulnerable to natural hazards, it is necessary to quantify the degree of vulnerability compared to the rest of the population, so measures can be taken to reduce the unfair difference. In this chapter, we want to draw attention to the different causes for the vulnerability of this segment of the population in Mexico, and some of the reasons why the ways to help them improve their living conditions continue to be neglected. We also address the question of whether there is a difference in vulnerability, which corresponds directly to the ethnicity status. We estimate the degree of vulnerability by means of an index derived from figures on demography (Social Vulnerability Index). Overall, we can see a clear coincidence between the level of social marginalization in the indigenous regions with vulnerability, as expected. Our results also show that the vulnerability of the central and coastal regions is lower than that of the Montana (mountain) region and the recovery time is an important factor to be considered as a source of the difference between the three areas due to the isolation and difficulty of access to the mountain region. We estimate that the fatality rate in the rural population would be 20% larger than in the urban population in case of a hypothetical earthquake with magnitude M8.6 off the coast of Guerrero. We discuss some of the repercussions of a lack of planning strategies to mitigate damage and/or lack of enforcement of planning and regulations in cases where they exist. The recent catastrophic results of the “Manuel” storm of September 2013 are examples of poor preventing strategies and lack of enforcement of hazard mitigation practice.