Anthropogenic subsidence in the Mexicali Valley,Baja California,Mexico, and slip on the Saltillo fault

Deep fluid extraction in the Cerro Prieto geothermal field (CPGF) has caused subsidence and induced slip on tectonic faults in the Mexicali Valley (Baja California, Mexico). The Mexicali Valley is located in the southern part of the Salton Trough, at the boundary between the Pacific and North American plates. The Valley is characterized by being a zone of continuous tectonic deformation, geothermal activity, and seismicity. Within the Cerro Prieto pull-apart basin, seismicity is concentrated mainly in swarms, while strong earthquakes have occurred in the Imperial and Cerro Prieto transform faults, that are the eastern and western bound of the basin. Since 1973, fluid extraction at the CPGF has influenced deformation in the area, accelerating the subsidence and causing rupture (frequently as vertical slip or creep) on the surface traces of tectonic faults. Both subsidence and fault slip are causing damage to infrastructure like roads, railroad tracks, irrigation channels, and agricultural fields. Currently, accelerated extraction in the eastern part of CPGF has shifted eastwards the area of most pronounced subsidence rate; this accelerated subsidence can be observed at the Saltillo fault, a southern branch of the Imperial fault in the Mexicali Valley. Published leveling data, together with field data from geological surveys, geotechnical instruments, and new InSAR images were used to model the observed deformation in the area in terms of fluid extraction. Since the electricity production in the CPGF is an indispensable part of Baja California economy, extraction is sure to continue and may probably increase, so that the problem of damages caused by subsidence will likely increase in the future.     

Fault-slip Triggering, Healing, and Viscoelastic Afterworking in Sediments in the Mexicali-Imperial Valley

—Crackmeter data from the southern tip of the Imperial fault in the Mexicali-Imperial Valley in northern Baja California, Mexico, show slip events that tend to group in episodes, or suites, that last over a few days, with months of quiescence between them. The events in these suites are apparently triggered and stopped by ground strain related to temperature changes. A characteristic decrease in peak extension velocities during the slip event suites indicates the presence of viscoelastic afterworking which contributes the strain necessary for succeeding events with diminishing driving stresses, and which can be roughly modeled as a series of Kelvin-Voigt viscoelastic solids. Occurrence of very small events towards the end of the suites plus gaps between suites indicate that for the sediments significant fault healing requires a minimum of several days without slip.     

Recent Vertical Deformation in Mexicali Valley and its Relationship with Tectonics, Seismicity, and the Exploitation of the Cerro Prieto Geothermal Field, Mexico

—The interpretation of the results of regional and local leveling which began in 1977 in the Mexicali Valley and the local short profile precision leveling which started in 1994 are discussed. The relation of vertical deformations around the Cerro Prieto Geothermal Field (CPGF) and along the Imperial fault, with local tectonics and seismicity in the Mexicali Valley, is reviewed. Also the relation between vertical deformation and fluid operation in the CPGF is analyzed. The subsidence observed in the field seems to be induced by fluid extraction. The way in which fluid production influences surface changes along the Imperial fault is not clear. The possibility that seismicity is triggering subsidence in the area and vertical movement on the Imperial fault is discussed.     

Poisson renormalized entropy as a possible precursor to large earthquakes

Journal of Seismology - The concept of Poisson renormalized entropy is presented as an observable of background seismicity with precursory possibilities. The usual way of estimating entropies...     

A Bayesian Assessment of Seismic Semi-Periodicity Forecasts

Among the schemes for earthquake forecasting, the search for semi-periodicity during large earthquakes in a given seismogenic region plays an important role. When considering earthquake forecasts based on semi-periodic sequence identification, the Bayesian formalism is a useful tool for: (1) assessing how well a given earthquake satisfies a previously made forecast; (2) re-evaluating the semi-periodic sequence probability; and (3) testing other prior estimations of the sequence probability. A comparison of Bayesian estimates with updated estimates of semi-periodic sequences that incorporate new data not used in the original estimates shows extremely good agreement, indicating that: (1) the probability that a semi-periodic sequence is not due to chance is an appropriate estimate for the prior sequence probability estimate; and (2) the Bayesian formalism does a very good job of estimating corrected semi-periodicity probabilities, using slightly less data than that used for updated estimates. The Bayesian approach is exemplified explicitly by its application to the Parkfield semi-periodic forecast, and results are given for its application to other forecasts in Japan and Venezuela.     

Semi-Periodic Sequences and Extraneous Events in Earthquake Forecasting. II: Application,Forecasts for Japan and Venezuela

In order to analyze observed seismicity in central Japan and Venezuela, we applied a new method to identify semi-periodic sequences in the occurrence times of large earthquakes, which allows for the presence of multiple periodic sequences and/or events not belonging to any sequence in the time series. We also explored a scheme for diminishing the effects of a sharp cutoff magnitude threshold in selecting the events to analyze. A main four-event sequence with probability P _c  = 0.991 of not having occurred by chance was identified for earthquakes with M ≥ 8.0 in central Japan. Venezuela is divided, from West to East, into four regions; for each of these, the magnitude ranges and identified sequences are as follows. Region 1: M ≥ 6.0, a six-event sequence with P _c  = 0.923, and a four-event sequence with P _c  = 0.706. Region 2: M ≥ 5.6, a five-event sequence with P _c  = 0.942. Region 3: M ≥ 5.6, a four-event sequence with P _c  = 0.882. Region 4: M ≥ 6.0, a five-event sequence with P _c  = 0.891. Forecasts are made and evaluated for all identified sequences having four or more events and probabilities ≥0.5. The last event of all these sequences was satisfactorily aftcast by previous events. Whether the identified sequences do, in fact, correspond to physical processes resulting in semi-periodic seismicity is, of course, an open question; but the forecasts, properly used, may be useful as a factor in seismic hazard estimation.     

Semi-Periodic Sequences and Extraneous Events in Earthquake Forecasting: I. Theory and Method,Parkfield Application

We present a new method to identify semi-periodic sequences in the occurrence times of large earthquakes, which allows for the presence of multiple semi-periodic sequences and/or events not belonging to any identifiable sequence in the time series. The method, based on the analytic Fourier transform, yields estimates of the departure from periodicity of an observed sequence, and of the probability that the sequence is not due to chance. These estimates are used to make and to evaluate forecasts of future events belonging to each sequence. Numerous tests with synthetic catalogs show that the method is surprisingly capable of correctly identifying sequences, unidentifiable by eye, in complicated time series. Correct identification of a given sequence depends on the number of events it contains, on the sequence’s departure from periodicity, and, in some cases, on the choice of starting and ending times of the analyzed time window; as well as on the total number of events in the time series. Some particular data combinations may result in spectra where significant periods are obscured by large amplitudes artifacts of the transform, but artifacts can be usually recognized because they lack harmonics; thus, in most of these cases, true semi-periodic sequences may not be identified, but no false identifications will be made. A first example of an application of the method to real seismicity data is the analysis of the Parkfield event series. The analysis correctly aftcasts the September 2004 earthquake. Further applications to real data from Japan and Venezuela are shown in a companion paper.     

Occurrence Apparent Velocities for Identification and Quantification of Space–Time Clustering Precursory to a Large Earthquake. Application to Large (M > 7.0) Earthquakes in Southern California and Northern Baja California

Space–time seismic clusters, localized bursts of seismic activity, are a feature of background seismicity before the occurrence of large earthquakes, a feature that agrees with observations of diminishing Gutenberg–Richter b-value, fractal dimension, and entropy, and is therefore suggestive of high stress. However, identification and quantification of these space–time clusters, particularly when they are small, is not an easy task and requires a priori assumptions. A novel method for space–time cluster identification, based on an extension of the concept of apparent velocities, is proposed because space–time clusters in the background seismicity have a particular signature in the apparent velocity domain. The contents of histogram peaks due to clusters in the apparent velocity histogram can be used to quantify the cluster activity compared with null hypothesis levels. Identification of the earthquakes corresponding to the apparent velocities in the peaks allows identification of cluster activity in time and space. Apparent velocity peaks do appear in real catalog data for southern California and northern Baja California before the Landers 1992 M = 7.3, Hector Mine 1999 M = 7.1, El Mayor-Cucapah 2010 M = 7.2, and Ridgecrest 2019 M = 7.1 earthquakes, and they appear only within 15 to 25 years before the occurrence of large earthquakes. They are not observed either long before the large earthquakes or after them, and hence could be related to high local states of stress and be of value as a possible precursory observable.

     

Multi-sensor DInSAR applied to the spatiotemporal evolution analysis of ground surface deformation in Cerro Prieto basin,Baja California,Mexico, for the 1993–2014 period

The combined effects of active tectonics and anthropogenic activities, primarily geothermal resources exploitation for electricity production in Cerro Prieto geothermal field, influence the ground surface deformation in Cerro Prieto basin, Baja California, Mexico. In this study, a large set of multi-sensor C-band SAR images have been employed to reconstruct the spatiotemporal evolution of aseismic ground surface deformation that has affected Cerro Prieto basin from 1993 to 2014. Conventional DInSAR together with the interferograms stacking procedure was applied. The results showed that the study area presented considerable surface deformation (mainly subsidence) during the entire time of the investigation. The main changes in rate and pattern of surface deformation have a good correlation in time and space with the changes in production in the Cerro Prieto geothermal field. Comparison of LOS displacement maps from different viewing geometries, and decomposition (where possible) of LOS displacement into vertical and horizontal (east–west) components, revealed considerable horizontal displacement which mostly reflects the ground movement at and beyond the margin of the subsidence basin toward the areas of highest subsidence rates. In addition, the validation of the DInSAR results by comparing them against measurements from leveling surveys was performed, confirming the high reliably of satellite interferometry for the ground surface deformation rate mapping in the study area.