We analyse seismicity during the 6-year period 2012-2017 in the new time domain termed natural time in the Chiapas region where the M8.2 earthquake occurred, Mexico's largest earthquake in more than a century, in order to study the complexity measures associated with fluctuations of entropy as well as with entropy change under time reversal. We find that almost three months before the M8.2 earthquake, i.e., on 14 June 2017, the complexity measure associated with the fluctuations of entropy change under time reversal shows an abrupt increase, which, however, does not hold for the complexity measure associated with the fluctuations of entropy in forward time. On the same date, the entropy change under time reversal has been previously found to exhibit a minimum [Physica A 506, 625-634 (2018)]; we thus find here that this minimum is also accompanied by increased fluctuations of the entropy change under time reversal. In addition, we find a simultaneous increase of the Tsallis entropic index q.
Resistivity imaging (electric t o m o g r a p h y ) is a m e t h o d o l o g y for defining lateral variations of resistivity associated with structural anomalies such as caves, water contamination, and fractured zones a m o n g others. Important urban areas in M e x i c o City are currently at high risk of collapse. Cavities and shallow fractures h a v e been created as a result of mining in hilly regions beneath several of today's urbanized n e i g h b o r h o o d s located in the southwestern portion of M e x i c o City. Selected areas h a v e been investigated. C a v e s and tunnels w e r e found for depths ranging b e t w e e n 5 m u p to 15 m. Diameters of such features w e r e b e t w e e n 5 m u p to 30 m. S o m e of these structures are across p a v e d roads and beneath buildings and houses, running for several tens of meters in length. Topographic effects w e r e also studied in o n e profile. It s h o w e d important inaccuracies in resolving the g e o m e t r y of the cave, mainly in terms of depth to the top, w h e r e differences of 2 m to 3 m w e r e encountered. G P R ( G r o u n d Penetrating Radar) w a s also used to confirm the results obtained in a resistivity profile surveyed on a test site. Results are very encouraging, demonstrating that the resistivity i m a g i n g helped to locate and characterize m i n e d areas, jointly with other geophysical m e t h o d s . Unfortunately, true resistivity is not well resolved, b e c a u s e of inherent ambiguity of the inverse m e t h o d used. I n t r o d u c t i o n M e x i c o City constitutes one of the largest concentra tions of h u m a n activities in the world, with a population of about 20 million people. T h e City w a s founded originally on a small island in a lake surrounded by volcanic ranges. T h e confined nature of this intramontane valley directly af fects air quality, water supply and urban development. Therefore, administering services for the inhabitants of this city is a big challenge ( C a m p o s et al, 1997). O n top of these p r o b l e m s , c o m e s the insufficient housing and shelter ing for the population, a p r o b l e m that has increased during the past 4 0 years. During the forties and the fifties, m o s t of the material used for construction purposes w a s obtained from mines excavated in the western Sierras. Such m a t e rials were e m p l o y e d to build houses, apartment and gov ernmental Yam&mgs m d o w n t o w n M e x i c o . W o w e v e t , mos>Y of these sites were exploited illegally. W h e n this industry c a m e to an end at the beginning of the sixties, m o s t of the cavities resulting from m i n i n g were a b a n d o n e d or refilled with debris. S o m e caves w e r e later used as shelters for entire fam ilies, or had other uses by the people in the neighborhood. N o w a d a y s , the location of most of these structures is un k n o w n . L o w -, middle-and even high middle-neighbor hoods have settled on top of m i n e d zones for the past 30 years. M a n y accidents h a v e since occurred, caves...
It has been shown that some dynamic features hidden in the time series of complex systems can be unveiled if we analyze them in a time domain termed natural time. In this analysis, we can identify when a system approaches a critical point (dynamic phase transition). Here, based on natural time analysis, which enables the introduction of an order parameter for seismicity, we discuss a procedure through which we could achieve the identification of the occurrence time of the M8.2 earthquake that occurred on 7 September 2017 in Mexico in Chiapas region, which is the largest magnitude event recorded in Mexico in more than a century. In particular, we first investigated the order parameter fluctuations of seismicity in the entire Mexico and found that, during an almost 30-year period, i.e., from 1 January 1988 until the M8.2 earthquake occurrence, they were minimized around 27 July 2017. From this date, we started computing the variance of seismicity in Chiapas region and found that it approached the critical value 0.070 on 6 September 2017, almost one day before this M8.2 earthquake occurrence.
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