Lessons from a Small Local Earthquake (Mw 3.2) That Produced the Highest Acceleration Ever Recorded in Mexico City

2020 ◽  
Vol 91 (6) ◽  
pp. 3391-3406
Author(s):  
Shri Krishna Singh ◽  
Luis Quintanar-Robles ◽  
Danny Arroyo ◽  
Victor Manuel Cruz-Atienza ◽  
Victor Hugo Espíndola ◽  
...  

Abstract A reliable estimation of seismic hazard-facing Mexico City from local earthquakes has suffered from poor seismic instrumentation, complex crustal structure, large and variable site amplification, and lack of knowledge of recurrence period of earthquakes on the mapped faults. Owing to recent improvement in local seismic networks, an earthquake swarm activity, which occurred in June–August 2019, was well recorded. The largest event of the sequence, an Mw 3.2 earthquake, caused panic in the city and produced peak ground acceleration (PGA) exceeding 0.3g at the closest station (MHVM) about 1 km away. An analysis of the event shows that it had normal-faulting focal mechanism, consistent with northeast–southwest-oriented mapped faults in the region. It was located at a depth of ∼1  km and had a low stress drop (∼0.1  MPa). We find that the high PGA for this low stress-drop event resulted from high-frequency amplification at MHVM (about factor of ∼6 around 13 Hz), likely due to topographic site effects, superimposed on a pervasive broadband amplification of seismic waves at hill-zone sites in the Valley of Mexico (up to ∼10 in the frequency band of 0.2–10 Hz). Simulation of ground motion for a scenario Mw 5.0 earthquake, using an empirical Green’s function technique, reveals that such an event may give rise to significant seismic intensities in the lake-bed zone of Mexico City. The results emphasize the need to re-evaluate the seismic hazard to Mexico City from local crustal earthquakes in the Valley of Mexico.

Author(s):  
Ф.Ф. Аптикаев

Задание сейсмических воздействий в отечественных строительных нормах практически не меняется в течение последних 60 лет. Накопленные эмпирические данные по сильным движениям позволяют коренным образом усовершенствовать методику расчета зданий и сооружений на сейсмостойкость. Ожидается снижение погрешностей расчета примерно вдвое. Цель работы. В последнее время много внимания уделяется проблемам построения карт сейсмической опасности в ускорениях. Однако по традиции в нашей стране такие карты оценивают сейсмическую опасность в баллах шкалы сейсмической интенсивности. В большинстве стран сейсмическая опасность оценивается именно в ускорениях. Строились такие карты и в нашей стране. В частности, карты ОСР-97 и ОСР-2012 имели вариант и в ускорениях. Построение карт сейсмической опасности в ускорениях не имеет принципиальных трудностей. Проблема в том, что ускорения не являются адекватной мерой сейсмических воздействий. Более половины века тому назад американские ученые на эмпирическом материале показали, что связь ускорений с баллами, а, следовательно, и с повреждаемостью зданий неоднозначна: шкалы сейсмической интенсивности различны для разных расстояний и грунтов. Ошибка в оценке последствий землетрясения по ускорениям грунта может достигать 2 баллов. Следовательно, расчет ожидаемых воздействий следует производить с учетом других характеристик сейсмических волн. К тому же, попытки построения карт сейсмической опасности строились без учета данных инженерной сейсмологии и с нарушениями правил теории вероятностей и поэтому обладают не только определенными достоинствами, но и серьезными недостатками. Некоторые исследователи считают, что скорости колебаний лучше коррелируются с повреждениями сооружений, по крайней мере, многоэтажных зданий и подземных трубопроводов. Методы работы. Однако анализ эмпирических данных показал, что использование ускорений, скоростей и смещений характеризуется примерно одинаковой точностью. Рассмотрены способы построения карт общего сейсмического районирования. В действующей шкале сейсмической интенсивности ГОСТ Р 57546.2017 приведены оценки корреляции повреждаемости зданий с различными параметрами сейсмических колебаний: ускорениями, скоростями, смещениями, мощностью колебаний грунта. Оценено влияние продолжительности колебаний. Результаты работы. Показано, что дальнейшее повышение надежности расчетов объектов на сейсмостойкость связана с представлением сейсмических воздействий не с амплитудами колебаний, а с энергетическими характеристиками сейсмических волн The specification of seismic effects in domestic building codes has remained practically unchanged over the past 60 years. The accumulated empirical data on strong ground notions make it possible to radically improve the methodology for calculating buildings and other structures for seismic resistance. It is expected that the calculation errors will be reduced by about half. Aim. Recently, much attention has been paid to the problems of developing seismic hazard maps in accelerations. However, by tradition in our country, such maps assess the seismic hazard in terms of the seismic intensity scale. In most countries, seismic hazard is assessed in terms of accelerations. Such maps were also built in our country. In particular, OSR-97 maps also had a variant in acceleration. The construction of seismic hazard maps in accelerations has no fundamental difficulties. The problem is that accelerations are not an adequate measure of seismic effects. More than half a century ago, American scientists, using empirical material, showed that the relationship between accelerations and points, and, consequently, with the damage to buildings, is ambiguous: the seismic intensity scales are different for different distances and grounds. The error in assessing the consequences of an earthquake in terms of ground acceleration can reach 2 points. Therefore, the calculation of the expected impacts should be based on other characteristics of the seismic waves. In addition, attempts to construct seismic hazard maps were built without taking into account the data of engineering seismology and with violations of the rules of probability theory and therefore have not only certain advantages, but also serious drawbacks. Some researchers believe that vibration velocities correlate better with structural damage, at least in multi-storey buildings and underground pipelines. However, the analysis of empirical data showed that the use of accelerations, velocities and displacements is characterized by approximately the same accuracy. Methods. Methods for constructing maps of general seismic zoning, which have a higher accuracy in comparison with existing maps, are considered. In the current scale of seismic intensity GOST R 57546.2017 estimates of the correlation of damage to buildings with various parameters of seismic vibrations are given: accelerations, velocities, displacements, power of ground vibrations. The influence of the duration of the oscillations is estimated. Results. It is shown that a further increase in the reliability of calculations of objects for seismic resistance is associated with the representation of seismic effects not with vibration amplitudes, but with the energy characteristics of seismic waves


2015 ◽  
Vol 31 (4) ◽  
pp. 2325-2349 ◽  
Author(s):  
James R. Gingery ◽  
Ahmed Elgamal ◽  
Jonathan D. Bray

Site amplification studies and building code provisions recognize that soil liquefaction can alter the characteristics of ground shaking at a site. However, guidance as to how the amplitudes of spectral accelerations are modified is lacking. In this paper, available recorded ground motions from shallow crustal earthquakes at sites that exhibited evidence of liquefaction are investigated. Analysis of residuals computed relative to Next Generation Attenuation (NGA) estimates reveal positive bias at longer periods, slight negative bias at intermediate periods, and slight positive bias at short periods. Trends with V S30, NGA-estimated peak ground acceleration (PGA), and moment magnitude are also observed. A model is developed that removes the initially observed residual bias and reduces uncertainty. The proposed model can be used to adjust NGA-estimated acceleration response spectra to account for the effects of liquefaction on ground shaking.


2018 ◽  
Vol 57 (2) ◽  
Author(s):  
Francisco Córdoba-Montiel Córdoba-Montiel ◽  
Srhi Krishna Singh ◽  
Arturo Iglesias ◽  
Xyoli Pérez-Campos ◽  
K. Sieron

Ground motions in Xalapa, Veracruz, Mexico, during the earthquake of January 4, 1920 (M~6.4), and three significant intraslab earthquakes (Mw7.0) of the last century were estimated. These events are reasonable scenario earthquakes for Xalapa. Towards this goal, portable broadband seismographs at nine sites in the city and an additional one at a reference hard site outside the city were deployed. Peak ground acceleration (Amax) and peak ground velocity (Amax) in Xalapa were estimated based on Brune w -2 source model and the site effect, obtained from earthquake recordings by using the standard spectral ratio (SSR) technique, and the application of a stochastic method. During the 1920 Xalapa earthquake the estimated Amax values corresponding to a stress drop, Ds, of 50 bar are between 100 and 250 cm/s2, except at two sites where the site effect is very large and Amax values reach 300 and 600 cm/s2. Estimated Vmax values are between 10 and 20 cm/s, except at the site with the largest site effect where it is ~ 40 cm/s. Ds of 30 and 100 bar produce about half and twice of these peak values, respectively. The main uncertainty in the present estimations is due the Ds value, because although a range of 30 to 100 bar for crustal earthquakes in the Trans-Mexican Volcanic Belt (in which Xalapa is located) seems reasonable, it is not constrained by the data. The mean stress drop for intraslab events, ~ 300 bar, is better constrained from previous studies. A median Amax of ~ 30 cm/s2 and a median Vmax of 4 cm/s in Xalapa during the 1973 (Orizaba) and 1999 (Tehuacán) earthquakes was estimated; the corresponding values during the 1980 (Huajuapan) earthquake are ~ 10 cm/s2 and 2 cm/s. The uncertainty in the estimation is probably within a factor of 2 to 3.The ground motion prediction equations developed from data in the forearc region with less attenuation (than the backarc region) and recorded at hard sites appear to work reasonably well for Xalapa sites, which lie in the back arc. This observation suggests that the seismic waves from intraslab earthquakes, traveling through the mantle wedge before arriving Xalapa, suffer relatively large attenuation. However, these waves get amplified due to local site effects. It seems that in Xalapa these two effects, roughly, balance each other.


2020 ◽  
Vol 36 (2_suppl) ◽  
pp. 213-237
Author(s):  
Miguel A Jaimes ◽  
Adrián D García-Soto

This study presents an evaluation of floor acceleration demands for the design of rigid and flexible acceleration-sensitive nonstructural components in buildings, calculated using the most recent Mexico City seismic design provisions, released in 2017. This evaluation includes two approaches: (1) a simplified continuous elastic model and (2) using recordings from 10 instrumented buildings located in Mexico City. The study found that peak floor elastic acceleration demands imposed on rigid nonstructural components into buildings situated in Mexico City might reach values of 4.8 and 6.4 times the peak ground acceleration at rock and soft sites, respectively. The peak elastic acceleration demands imposed on flexible nonstructural components in all floors, estimated using floor response spectra, might be four times larger than the maximum acceleration of the floor at the point of support of the component for buildings located in rock and soft soil. Comparison of results from the two approaches with the current seismic design provisions revealed that the peak acceleration demands and floor response spectra computed with the current 2017 Mexico City seismic design provisions are, in general, adequate.


2012 ◽  
Vol 2012 ◽  
pp. 1-10 ◽  
Author(s):  
Zhenming Wang ◽  
David T. Butler ◽  
Edward W. Woolery ◽  
Lanmin Wang

A scenario seismic hazard analysis was performed for the city of Tianshui. The scenario hazard analysis utilized the best available geologic and seismological information as well as composite source model (i.e., ground motion simulation) to derive ground motion hazards in terms of acceleration time histories, peak values (e.g., peak ground acceleration and peak ground velocity), and response spectra. This study confirms that Tianshui is facing significant seismic hazard, and certain mitigation measures, such as better seismic design for buildings and other structures, should be developed and implemented. This study shows that PGA of 0.3 g (equivalent to Chinese intensity VIII) should be considered for seismic design of general building and PGA of 0.4 g (equivalent to Chinese intensity IX) for seismic design of critical facility in Tianshui.


Geosciences ◽  
2018 ◽  
Vol 8 (7) ◽  
pp. 264 ◽  
Author(s):  
M. Sandıkkaya ◽  
L. Dinsever

A global dataset which is composed of more than 20,000 records is used to develop an empirical nonlinear soil amplification model for crustal earthquakes. The model also includes the deep soil effect. The soil nonlinearity is formulated in terms of input rock motion and soil stiffness. The input rock motion is defined by the pseudo-spectral acceleration at rock site condition (PSArock) which is also modified with between-event residual. Application of PSArock simplifies the usage of the site model by diminishing the need of using the period-dependent correlation coefficients in hazard studies. The soil stiffness is expressed by a Gompertz sigmoid function which restricts the nonlinear effects at both of the very soft soil sites and very stiff soil sites. In order to surpass the effect of low magnitude and long-distant recordings on soil nonlinearity, the nonlinear site coefficients are constrained by using a limited dataset. The coefficients of linear site scaling and deep soil effect are obtained with the full database. The period average of site-variability is found to be 0.43. The sigma decreases with decreasing the soil stiffness or increasing input rock motion. After employing residual analysis, the region-dependent correction coefficients for linear site scaling are also obtained.


1999 ◽  
Vol 42 (6) ◽  
Author(s):  
V. I. Ulomov ◽  
. The GSHAP Region Working Group

The GSHAP Regional Centre in Moscow, UIPE, has coordinated the seismic hazard mapping for the whole territory of the former U.S.S.R. and border regions. A five-year program was conducted to assemble for the whole area, subdivided in five overlapping blocks, the unified seismic catalogue with uniform magnitude, the strong motion databank and the seismic zones model (lineament-domain-source), which form the basis of a newly developed deterministic-probabilistic computation of seismic hazard assessment. The work was conducted in close cooperation with border regions and GSHAP regional centers. The hazard was originally computed in terms of expected MSK intensity and then transformed into expected peak ground acceleration with 10% exceedance probability in 50 years.


1999 ◽  
Vol 42 (6) ◽  
Author(s):  
B. Tavakoli ◽  
M. Ghafory-Ashtiany

The development of the new seismic hazard map of Iran is based on probabilistic seismic hazard computation using the historical earthquakes data, geology, tectonics, fault activity and seismic source models in Iran. These maps have been prepared to indicate the earthquake hazard of Iran in the form of iso-acceleration contour lines, and seismic hazard zoning, by using current probabilistic procedures. They display the probabilistic estimates of Peak Ground Acceleration (PGA) for the return periods of 75 and 475 years. The maps have been divided into intervals of 0.25 degrees in both latitudinal and longitudinal directions to calculate the peak ground acceleration values at each grid point and draw the seismic hazard curves. The results presented in this study will provide the basis for the preparation of seismic risk maps, the estimation of earthquake insurance premiums, and the preliminary site evaluation of critical facilities.


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