Fixed point deformation observation, with a broad application prospect, plays an important role both in monitoring crustal deformation and capturing possible seismic precursors. Due to advantages of high precision, broad bandwidth and relative good continuity, fixed-point deformation observation, together with InSAR and GPS, has become one of the major methods in crustal deformation researches.
During the daily observation, it is usual to encounter various load disturbances, such as heavy rain, reservoir, river, building constructions etc. However, when quantitatively evaluating the effects of such load disturbance, a number of studies were conducted without taking into account the rock mass integrity in the study area, i.e. simply using general rock mechanical parameters rather than specific rock mass mechanical parameters, which therefore greatly affect the final interpretation and the elimination of disturbance, or even the identification of seismic precursory anomalies.
In this study, the strain survey data at Xiaoxinzhuang seismic station in Jixian County of Tianjin are taken as an example to illuminate that the integrity or fragmentation of rock mass in each specific site should be considered when conducting quantitative analysis, only thus can the influence of external load on deformation observation be evaluated more accurately.
Firstly, a detailed joint statistic of the rock mass around Xiaoxinzhuang and Yuqiao reservoir was carried out; then the elastic modulus of rock mass of the study area was derived from the equation between rock mass volumetric joints and elastic modulus; finally the theoretical compressive strain at Xiaoxinzhuang station produced by Yuqiao reservoir was calculated by an irregular load model, and the influence of Yuqiao reservoir on the strain of Xiaoxinzhuang station was also evaluated.
Geological surveys, combined with numerical simulation indicate that: 1)The exposed rocks are dominated by Middle Proterozoic dolomites and dolomites with minor limestone at Xiaoxinzhuang seismic station and its vicinity, and the corresponding volumetric joint counts of rock mass are more than 32, the highest values of 41 and 39 occurred at Ligezhuang and south of Yuqiao reservoir, respectively, which means the bedrocks are moderately fragmented; 2)The elastic modulus of rock mass in the study area is only 0.10~0.19 times(an averaged value of 0.14)of that of integral rock, which means that under the same stress the effect of external load disturbance will be magnified about 5~10 times(an averaged value of 7 times); 3)During the impoundment period of Yuqiao reservoir from September to November in 2019, the theoretical compressive strain at Xiaoxinzhuang station in the north-south direction induced by Yuqiao reservoir impounding is about(93.76~200.91)×10^-8, calculated by an irregular load model, accounting for about 10%~21% of the actual observed strain; 4)Therefore, it is concluded that the earlier claimed anomaly is not an earthquake precursory anomaly, but probably the effect of water level change of Yuqiao reservoir.

Recent studies have confirmed that the bedrock temperature changes when the crustal stress changes, and the information of dynamic change in crustal stress can be obtained through the observation of bedrock temperature. Moreover, there are abundant fluids in the shallow crust, and the deformation of the crust will inevitably lead to the migration of fluids, which will change the bedrock temperature. The temperature change of bedrock is equivalent to the secondary fluid thermal effect caused by crustal stress change and may be an another indirect sensitive index of crustal stress dynamic change. The bedrock temperature data of Xianshuihe fault zones show that the variation of groundwater flow rate after the Kangding M_S6.4 earthquake is consistent with the zoning characteristics of co-seismic volumetric strain in the strike-slip earthquake, indicating that the variation of near-field fluid migration characteristics is probably related to the variation of co-seismic static stress change. Moreover, the response form of bedrock temperature to the dynamic change of crustal stress and its secondary fluid effect is not consistent, as the former shows step-rise characteristics, while the latter shows exponential variation. The observation of bedrock temperature itself can obtain the dynamic change information of crustal stress and the information of shallow crustal fluid migration. Compared with crustal stress change, the variation range of fluid secondary heat effect caused by stress change may be significantly magnified(approximately an order of magnitude), which is more conducive to capturing signals, and thus may even obtain precursory fluid change information.
On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi, which happened in the bedrock temperature observation network. In particular, the Xike’er observation station is about 1.3 kilometers away from the epicenter, providing an opportunity to analyze bedrock temperature changes before and after the earthquake. The results showed that: 1)Obvious changes in bedrock temperature were found before and during the M_S6.4 earthquake. The appearance of co-seismic response means that these changes before the earthquake are related to the earthquake and may be precursory signals. 2)In terms of time, the bedrock temperature before the Jiashi earthquake first changed abnormally on the stable background, and the change reached the peak, and then fell back. When the earthquake was impending, there was a significant acceleration of the change, and the earthquake occurred after some time. The acceleration characteristics of change impending earthquake may be related to the meta-instability process of earthquakes. 3)Spatially, changes in temperature before the earthquake occurred in or near the seismogenic fault, and no obvious abnormal information was observed at the measurement points far away from the seismogenic fault, indicating that short-term and impending precursors are more likely the “near field” information; From the perspective of depth, the change in temperature before the earthquake was observed only at the local depth range. This implies that there is obvious uncertainty in the depth in precursor observation. Upon this, the ideal situation should be to carry out multi-depth joint observation, so as not to miss important precursor information. 4)Combining with the Kangding M_S6.3 earthquake on November 22, 2014, a comparative analysis is made. Similar to the Jiashi earthquake, the temperature at measurement points located in or nearby seismogenic fault of Kangding M_S6.3 earthquake shows significant changes. This means that change in the temperature before the Jiashi M_S6.4 earthquake is not an isolated case, and is a representative of universal phenomenon that occurs before strong earthquake. In a word, the change of bedrock temperature before and after the earthquake shows that the precursor information has the characteristics of near field, structural correlation and sensitive to stress change.

Parts of the consecutive apparent resistivity monitoring stations of China have recorded clear diurnal variations. The relative amplitudes of diurnal variations at these stations range from 1.3‰ to 5.8‰. The daily accuracies of apparent resistivity observation are better than 1‰, because the background electromagnetic noise is rather low at these stations. Therefore, the diurnal variations of apparent resistivity recorded at these stations are real phenomena. The diurnal variation shapes can be divided into two opposite types according to their characteristics. One type is that the apparent resistivity data decreases during the daytime but increases during the nighttime(Type 1). The other type is the apparent resistivity data increases during the daytime but decreases during the nighttime(Type 2). There is a correspondence between the diurnal and annual variation patterns of apparent resistivity. For the monitoring direction with diurnal variation of Type 1, the apparent resistivity decreases in summer and increases in winter. However, for the monitoring direction with diurnal variation of Type 2, the apparent resistivity increases in summer and decreases in winter.
We take an analysis on the mechanism of apparent resistivity diurnal variation, combining the influence factors of water-bearing medium's resistivity, the electric structure of stations, and the apparent resistivity sensitivity coefficient(SC)theory. Intuitively, diurnal variation of apparent resistivity is caused by diurnal variation of medium resistivity in the measured area. The diurnal variation of medium resistivity will inevitably be caused by the factors with diurnal variation. Among the possible factors, there is diural variation in earth tide and temperature.
Our analysis displays that apparent resistivity diurnal variation is not caused by the usually-believed earth tide, but by the ground temperature difference between daytime and nighttime. The earth tide strain is too small to cause remarkable effects on the apparent resistivity data. On the other hand, the daily tide strain has two peak-valley variations, and its phase and amplitude has a period of approximate 28 days. However, the apparent resistivity data do not show these corresponding features to earth tide. Furthermore, the detection range of current apparent resistivity stations is within a depth of several hundred meters. Within this depth range, the medium deformation caused by solid tide can be regarded as uniform change. Therefore, all monitoring directions and all stations will have the same pattern of diurnal variation.
In general, the temperature increases in the daytime but decreases in the nighttime. For most water-bearing rock and soil medium, its resistivity decreases as temperature increases and increases as temperature decreases. Diurnal temperature difference affects about 0.4m of soil depth. Therefore, resistivity of this surface thin soil layer decreases in the daytime while increases in the nighttime. Under layered medium model, SC of each layer represents its contribution to the apparent resistivity. For the stations with positive SC of surface layer, apparent resistivity decreases in the daytime but increases in the nighttime. While for the stations with negative SC of surface layer, apparent resistivity diurnal variations display the opposite shape.

Mud volcano is a conical sedimentary body formed by high-pressure mud and gas-dominated fluid migrated to the surface through faults and other channels deep underground, which looks like a volcanic cone formed by magma-volcanism. As a product of crustal movement, mud volcano can bring a large amount of valuable information from deep to the surface when erupting. Therefore, mud volcano is called "god-given borehole" with a depth of 7~12km. Mud volcanoes are the result of upthrust of trapped gases released by the pressure in the stratum and also the channel for the upward migration of gases in the earth. The submarine mud volcano is one of the signs of hydrate and the living evidence of hydrate. The Wusu mud volcanoes are located in the northern Tianshan tectonic belt. Since the mud volcamoes locate in the active part of the tectonic belt and are well connected to the underground, their active degree has a good correlation with the seismicity. The earthquake cases studies based on the 7a long real-time macroscopic monitoring data and the more than 3a long geochemical monitoring data of the Wusu mud volcanoes show that in the earthquake cases of M_S ≥ 5.0 within the range of 300km around the Wusu mud volcanoes, the abnormal mud gushing quantity obviously increased by macroscopic monitoring before 9 out of 13 earthquakes. The geochemical microcosmic monitoring data showed obvious abnormal changes before 3 out of 6 earthquakes. The anomalous duration from the emergence of the anomaly to the occurrence of the earthquake is mainly of the mid-term(6~12 months). Before the Jinghe M_S6.6 earthquake on August 9, 2017, the Wusu mud volcanoes spewed violently and the chemical components showed an obvious high value anomaly. In January 2017, there was a significant increase in the amount of mud spewing in Aiqigou No.1mud volcano and Baiyanggou No.1mud volcano, and one month before the earthquake, there was the phenomenon that mud gushing amount of Aiqigou No.2 mud volcano gradually increased and the volcano was from dormant to active. There were obvious high values appearing before the earthquake in F^-and SO₄^2- in the Aiqigou No.1mud volcano and in F^-, CO₃^2-, SO₄^2-, Rn(gas), CH₄, Ar and N₂ in Baiyanggou(No.1 and 2)mud volcanoes. The values of F^-, CO₃^2-, SO₄^2-, Ar and N₂ showed short-term anomalies, while CH₄ and Rn(gas)showed medium term anomalies. Giggenbach triangular diagram (Na-K-Mg) indicates that the water-rock reaction of Baiyanggou mud volcanoes is complete and little disturbed by the outside. The water-rock reaction of the Aiqigou mud volcanoes is still ongoing, which can explain why the precursor anomaly of the chemical components of the Baiyanggou mud volcanoes is more obvious than that of the Aiqigou mud volcanoes. The geothermal reservoir temperature of the study area is estimated by using a cationic (Na-K, K-Mg, Na-K-Ca) geothermometer. The geothermal reservoir temperature of the Wusu mud volcanoes is about 70℃, and the circulation depth is about 3km. In the process of earthquake preparation, the mud carries deep chemical components to the ground surface due to the effect of compression stress(the result of focal mechanism)or the concentration of regional tectonic stress with earthquake preparation; Or the rock strata in or near the seismogenic area are deformed, the depth of liquid circulation will increase, and the water-rock reaction will be accelerated, which will increase the concentration of some ionic components, and the squeezing process will cause a large number of mud to gush out of the ground, carrying geochemical components. Therefore, the gushing quantity and some chemical components of the mud volcanoes were obviously abnormal before the earthquake.

GPS campaign observations can monitor dynamic characteristic of crustal deformation near the fault zone effectively. Dynamic characteristic of crustal deformation is the manifestation of the dynamic action of the faults in deep and shallow structures. The locking and movement state of faults in deep and shallow structures can be an objective characterization of strain accumulation in seismogenic fault. So we can use dynamic GPS observations to invert fault locking and fault slip deficit rate by some models, and then judge the mid- to long-term seismic potential of the faults. Research about the faults around the Daliangshan sub-block is relatively poor, and the moderate-strong earthquakes increased significantly around the sub-block over the past decade, which makes fault locking and seismic potential around the sub-block be the problem to be urgently studied. Therefore, by using the GPS horizontal velocity field of 1999-2007, 2009-2013 and the negative dislocation model of DEFNODE, we inverted for spatial fault locking and fault slip deficit rate in the Daliangshan sub-block which contains three major fault zones, named Daliangshan, Mabian-Yanjin and Huize-Yiliang, before and after the Wenchuan earthquake. We analyzed the seismic potential characteristic of the three faults combining with the seismic gap and the spatial distribution of b value. The results show that the locking state of the three faults was basically same before and after the Wenchuan earthquake, which indicates that the earthquake probably has a very weak influence on the faults. The inversion results of two periods show that the southern segment of Daliangshan, Mabian-Yanjin and Huize-Yiliang Faults are basically completely locked except the southwestern segment of Huize-Yiliang Fault. The slip deficit rates of the three faults are not huge before and after the earthquake. Daliangshan Fault is mainly of a sinistral strike-slip deficit, Mabian-Yanjin Fault has a little amount of sinistral strike-slip and compressional deficit which was slightly enhanced after the earthquake, and Huize-Yiliang Fault is characteristic of compressional deficit with a small amount of dextral strike-slip deficit, which reduced to about zero after the earthquake. Combining with some other results, we conclude that current seismic potential for strong or major earthquakes exists on the three faults.

The vertical to horizontal response spectra ratio (V/H) for 68 near-fault records (R_rup<20km) and 48 eastern coast records (40<R_rup<70)from the 1999 Chi-Chi earthquake has been studied in this article. The concerned records are divided into groups according to stations' position(handing-wall or footwall),Slte、ondition and distance to the fault. This study focuses on the relationship between V/H and fault orientation and rupture direction,distance to fault,and local site condition. Some of the results attained in this paper are(onsistent with previous(onclusions,while others are somewhat different. The difference seems to reflect the specific(haracteristics of near fault ground motion of the studied event. The results in this article show that:1)the directions of fault slip and rupture propagation have(onsiderable effect on the V/H within 31im to the fault rupture. Mainly at short and medium periods,V/H of paralleling rupture direction is greater than that perpendicular to the rupture. In footwall and distance range of 3～20km,Slte、ondition has slight effect on V/H;2) for the Chi-Chi earthquake,the V/H in long-period range(>3s) is、ommonly larger than 2/3,the usually estimated value in references,with a noticeable peals,which means the long period component of vertical movement in this、ase is larger than that of other earthquakes;3)for recordings at footwall side,the V/H in short periods(<0.2s)do not obviously decrease as rupture distance increases. Comparing to stations having larger rupture distances(>40km),V/H of near fault stations are、ommonly larger in the、oncerned periods in engineering、ircle;4)the influence of site (onditions on the spectral ratio seems insignificant in near fault area. For the east stations with larger distance,the V/H in long period range at hard soil site is longer than that at soft soil site.