Numerical Simulation Of Mechanism Of Multiple-physical Fields For Thermal Anomalies Before Earthquakes

It has become a highly focused issue that thermal anomalies appear before majorearthquakes in a large area around epicenters. The previous research results indicatethat: There are diversified manifestations of thermal anomalies before a majorearthquake, such as abnormal increase of thermal infrared signals, variations of latentheat flux and sensible heat flux. The thermal anomaly area roughly coincides withthe epicenter, or aligned with the strikes of the fault zones. The anomaly regioncontracts inward immediately before the earthquake.There have been many studies on the mechanism of thermal anomalies beforeearthquakes at home and around the world, and various hypotheses and assumptionsfor it have been proposed. The main viewpoints are as follows: The earth deflatedresults in greenhouse effect, electric field excitation drives up the temperature, stressgenerates temperature rise, thermal material from the deep Earth ascends and causestemperature rising. Because of lacking of evidence from the long-term and enoughobservations, the very small values of thermal anomalies, the very slow process ofrock heat transfer, these hypotheses are still difficult to use in reality and require to befurther researched..Gestation and occurrence of a major earthquake is related with the interaction ofmulti-physical fields. The underground fluid plays a very important role inseismogenic processes and formation of seismic precursors, especially when fluids athigh temperature surge into the surface, which is very likely to be the reason for thethermal anomaly. Some questions remain to be answered, such as whether thegeothermal energy can transfer to the surface or not, which way of the conduction andthe convection is dominant, and how the crustal structures of blocks with differentfeatures influence geothermal anomalies, all of which are involved in the temperature field, the seepage field and the stress field. Besides, rock deformation and thermaleffect can lead to changes of fluid pore pressure and permeability. Fluid-solid heattransfer and rock deformation heat can also cause temperature changes. Meanwhile,heat stress and pore fluid pressure changes can pose effects on deformation of rock.The2008Ms8.0Wenchuan earthquake is one of the largest evens in the lastdecade in China mainland. Post-seismic investigations indicate that distinguishablethermal anomalies occurred several days before the earthquake, especially anomalousincrease of surface heat latent flux. These anomalies have the followingcharacteristics:1) Significance, the heat anomaly value near the epicenter is more thantwo times the average of its surrounding areas.2)Isolating, these anomalies appearedonly near the epicenter.3) Continuity, most of these anomalies continued more thanthree days.4) Relation to tectonics, these anomalies are distributed along theLongmen Shan fault zone.Based on geological and geophysical data, this thesis attempts to study themechanism of these thermal anomalies by numerical simulation. First, a dynamicmodel of coupled stress, seepage and thermal fields （HTM model） is constructed.Then using the COMSOL multi-physics filed software, this work makes simulation ofthe dynamic process of the medium heat conduction and the pore fluid heatconvection at depth and analyzes how geologic construction conditions and geologicalparameters affect the fluid motion, tectonic deformation and the heat field. Especiallythis thesis tries to reveal the generation process and distribution patterns of thermalanomalies prior to thrust-type major earthquakes. The contents of this thesis are asfollows:（1） Establishment of a HTM dynamic model during the earthquake gestation.A2D rectangle model containing porous media is constructed, which crosses theLongmenshan fault, with a120km length. The model consists of several structuraldomains （the upper-middle crust of highland in west Sichuan and Sichuan Basin） andthe Longmenshan fault zone. It is assumed that the eastward motion of the highland inwest Sichuan is the primary reason for stress built up on the Longmenshan fault zone.Based on topography data, GPS observations and trench data, this thesis analysedthe motion speed of the western Sichuan highland and discussed three different situations V1=V2,3V1=V2and V1<V2, here V1and V2are speeds of upper crust andmiddle crust, respectively. Finally this work adopts V1=6mm/a, V2=10mm/a whichmeet the actual situations well.（2）Study of the boundary conditions of the HTM coupling of Longmenshan faultzoneAccording the recurrence interval of great earthquakes on the Longmenshan faultzone, earthquakes, this thesis estimated the time required by gestation of one shock onthe Longmenshan fault zone. On a crustal scale, ignoring effects of temperature andpressure on density of fluids and solids, considering a series of processes such asstress changes rock porosity and permeability, water flow, thermal convection andthermal exchange in solid rock, thermal stress, and solid phase property related withtemperature, a simplified equation set is established for the HTM model.（3） Numerical simulation of the crustal structure deformation under tectonicstressAs the highland in west Sichuan moves to east and is obstructed by the SichuanBasin, the crust beneath the Longmenshan fault zone is deformed. This thesissimulated such deformation in the case of different velocities of upper and middlecrust, comparing the deformation in the orthogonal directions with the horizontaldirection, comparing the horizontal velocity among the three faults in theLongmenshan, and then compares the results with the observation data of actualsituation.（4） Simulate the deep fluid migration along the faults and blocks under tectonicstressTime step is10years and a total of3000years under the ambient field. Incomparison, time step is3days when microfracture occurred. Through simulating themovements of the fluid along the faults, by changing the fault’s permeability(3×10^-16m²,3×10^-15m²,3×10-14m²,3×10^-12m²), changing the extrusion speed （10times,100times）, varying the push time and so on, this thesis analyzes the flow rates andvelocities within and outside the faults. Through the analysis and comparison ofseveral different fault models, the influencing factors of fluid percolation are studied.Then the numerical calculation is validated by comparing the results with the observation data.（5） Study of the thermal anomaly by the simulating coupled HTMThis work simulates the change of the crust thermal field （heat flux, temperature）under the ambient field and during the period when microfracture occurs.This work simulates the dynamic change of the fluid percolation, heat flux andtemperature within and outside the faults when microfracture occurs, and analyzeswhen the thermal anomaly occurs, how long the time duration lasts, what the thermalanomalous value is.The simulation objects also include the surface thermal effect within and outsidethe faults influenced by changing the fault’s permeability, modifying the extrusionspeed, and varying the push time. These results allow me to build a dynamic2Dporous medium model, to show vividly one of the most likely formation mechanismsof thermal anomalies before earthquakes.From simulation and analysis of the2D saturated porous media models, this workattains the following insights:（1）The distribution of seepage velocity, deformation amount, heat flux andtemperature in the faults are different from the surrounding areas in the ambient field.The values of heat flux in faults are higher than outside. The surrounding water flowsto faults. The water velocity in the deeper subsurface is less than the shallow, and thatin the rock mass is less than within the fault zone. The hanging wall and foot walldiffer in deformation amounts forming an imbricated thrust napped structure.（2） The pore volume ratio and fluid migration in faults increase whenmicrofracture occurs. With the fault’s pore volume ratio increasing:①the seepage flow velocity in faults increases obviously and its accelerationslows down with the expansion of fluid controlled scope.②the convective flux in faults increases perceptibly and reaches its maximumnear the hanging wall.③The fluid seepage and heat flux in faults are not only related with itspermeability or extrusion speed, but also influenced by geometrical conditions of thefault. The fluid and thermal anomalies in the Sichuan basin have a lagging effect.（3）The rocks continue compression and deformation, and the seepage velocity increases linearly with the improve extrusion speed in the pushing process. Thevarious extrusion speed is the main cause to change the seepage velocity.（4）When the microfracture occurred in faults, the seepage velocity, heat flux infaults increase perceptibly and reache its maximum which last several months veryfast. The temperature in faults will increase slowly and reach several centigrade highanomaly in the after months.