Miguel A. Santoyo

The Seismic Alert System for Mexico City: An Evaluation of Its Performance and a Strategy for Its Improvement

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 ( A red) from root-mean-square acceleration ( A rms) at a field station. The relation permits specification of an A red threshold, given the peak acceleration at cu ( A cu) 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 A cu ≥ 2 gal (for 0.2–1.0 Hz bandpass-filtered accelerograms) with 1% probability of failure. To accomplish this we must set A red ≥ 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.

Tectonic significance of an earthquake sequence in the Zacoalco half-graben, Jalisco, Mexico

Abstract We studied a sequence of small earthquakes that occurred during the months of April and May of 1997, in Jalisco, southwestern Mexico. The earthquakes were located along a set of active faults that form the Zacoalco half-graben (La Lima fault system), west of Lake Chapala, within the rift–rift–rift triple junction. A total of 33 events were located, with magnitudes ranging from 1.5 to 3.5, recorded by a portable array of broadband seismographs. We identified two groups of events: one corresponding to a shallow normal fault, synthetic to La Lima fault system, and another group associated with a deeper fault. The events that occurred on the synthetic fault show normal faulting oriented on a NW–SE plane, dipping shallowly towards the SW. The other group of mechanisms showed either a normal fault oriented NW–SE and dipping steeply to the NE, or a very shallow-dipping normal fault, dipping to the SW. Earthquake distribution and fault plane solutions suggest that the Zacoalco half-graben developed from blocks that rotate as slip occurs on listric faults. These mechanisms could represent the type of motion expected for larger earthquakes in the area, like the one that occurred in 1568.

Space–Time Clustering of Large Thrust Earthquakes along the Mexican Subduction Zone: An Evidence of Source Stress Interaction

The spatiotemporal plot of epicenters of large ( M s ≥6.9) subduction earthquakes in Mexico (1900 to present day) suggests that these earthquakes cluster in space and time. In this work we test the hypothesis that the coseismic stress transfer may lead to this clustering. For the analysis we estimate the spatial extent of the coseismic Coulomb stress change for these large events and then perform a statistical analysis using the χ 2 goodness-of-fit test for the interevent time intervals. We find that there are, at least, two groups of time intervals where the observed frequencies are much higher than that expected from a Poisson model, indicating a bimodal pattern. For the first mode, the observed frequencies for the 0- to 5-year interval becomes about 2.1 times the expectation, with a probability of occurrence of about 30%. These results show that large thrust Mexican earthquakes between 1900 and 2003 are clustered in space and time probably due to stress interactions among them. The second mode includes the time interval of 30–50 years. In the interval of 30–40 years, the observed frequencies become about 1.7 times the expectation and about 1.2 times the expectation for the 40- to 50-year interval. This second mode could be associated with a reloading interval of tectonic stress due to the plate convergence and appears consistent with the long-term recurrence periods of large thrust earthquakes in the Mexican subduction zone.

Finite fault analysis and near-field dynamic strain and rotation estimates due to the 11/05/2011 (Mw5.2) Lorca earthquake, south-eastern Spain

The 11/5/2011 Lorca, Spain earthquake (Mw

Assessing the Reliability of the Single Circular-Array Method for Love-Wave Ambient-Noise Surveying

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A theory for microtremor H/V spectral ratio: application for a layered medium

=5.2) and related seismicity produced extensive damage in the town of Lorca and vicinity. During these earthquakes, evidence of rotations and permanent deformations in structures were observed. To analyze these aspects and study the source properties from the near-field, the displacement time histories were obtained including the static component at Lorca station. Displacement time histories were computed by an appropriate double time integration procedure of accelerograms. Using these data, the foreshock and mainshock slip distributions were calculated by means of a complete waveform kinematic inversion. To study the dynamic deformations, the 3D tensor of displacement gradients at Lorca station was first estimated by a single station method. Using the finite fault inversion results and by means of a first order finite difference approach, the dynamic deformations tensor at surface was calculated at the recording site. In order to estimate the distribution of the peak dynamic deformations, the calculation was extended to the close neighboring area of the town. The possible influence of the near-field deformations on the surface structures was analyzed.

The silent earthquake of 2002 in the Guerrero seismic gap, Mexico (Mw=7.6): Inversion of slip on the plate interface and some implications

Regional seismograms of intermediate-depth earthquakes (50≤H≤80 km) that occur below the Central Mexican Plateau show a phase which, at epicentral distances of 150 to 450 km, arrives about 15 to 20 sec after the P wave and 5 to 12 sec before the S wave. This phase was previously interpreted as a seismic wave refracted from a dipping interface below the source, thus apparently providing direct evidence of the structural location of the subducted Cocos plate below Mexico. The phase was called the plate wave. Recent intermediate-depth events have given rise to better quality data recorded by some newly-installed broadband seismographs. An analysis of these and previous data strongly suggests that the phase is an S-to-P converted phase at the free surface and, therefore, provides no information regarding the subducted plate.

Coseismic and postseismic stress changes in a subducting plate: Possible stress interactions between large interplate thrust and intraplate normal‐faulting earthquakes

Abstract The single circular array (SCA) method is a spatial autocorrelation (SPAC)–like technique for ambient noise exploration. Its main feature is the possibility of calculation of Love-wave dispersion curves by using centerless circular arrays of 3-component seismometers, allowing independent processing of each circle. Situations in which Rayleigh-wave and Love-wave arrivals or waves coming from different azimuths are mutually correlated are also correctly dealt with in this method. An algorithm for practical calculation of the SCA coefficient B is described. The algorithm includes averaging over a set of time windows and minimizes the number of spectral ratios to be computed for the purposes of stability. Numerical tests show that SCA coefficients estimated in this way have quite a robust behavior. Bias due to use of a finite number of sensors, as well as to effects of nonpropagating incoherent noise, has been theoretically studied in both the deterministic and the stationary random-field formulations. Using a finite number of stations is a cause of bias even under isotropic illumination conditions. Nevertheless, its effect can be neglected for wavelengths-to-radius ratios above a threshold that depends on the number of evenly distributed sensors. By contrast, uncorrelated noise may affect the whole frequency band and is behind the limitations of the method at low frequencies. Finally, we present the first real data test of this method, consisting of a comparison between theoretical and experimental Love-wave dispersion curves for a site where the structure is known. In practice, the minimum wavelength for direct velocity retrieval for a pentagonal array with radius r was approximately , although this value depends on the signal-to-noise ratio. Experiments demonstrate that the usable range can be extended, mainly toward shorter wavelengths, if the effects of noise and of the finite number of sensors are included in the analysis.