Changes in Apparent Resistivity in the Late Preparation Stages of Strong Earthquakes

The study of earthquakes plays a key role in order to minimize human and material losses when they inevitably occur. Chapters in this book will be devoted to various aspects of earthquake research and analysis. The different sections present in the book span from statistical seismology studies, the latest techniques and advances on earthquake precursors and forecasting, as well as, new methods for early detection, data acquisition and interpretation. The topics are tackled from theoretical advances to practical applications.


Introduction
In China, a large-scale observation network that is comprised of a number of apparent resistivity (for short, AR) stations has been established for the purpose of earthquake (EQ) monitoring and prediction since the 1966 Ms7.2 Xingtai EQ in Hebei Province. Presently, over 70 AR stations are in observation in seismically active belts in densely populated areas and nearby some of large, medium-sized cities. The 2008 Ms8.0 Wenchuan great EQ in Sichuan province occurred in the AR station network that was located in the border area of both Sichuan and Gansu provinces. In the 1970s-1980s, more than 110 AR stations had been in observation, and in those years, several great earthquakes (EQs) occurred nearby AR stations, such as the 1976 Ms7.8 Tangshan EQ in Hebei province, the 1976 Ms7.2 Songpan-Pingwu EQ in Sichuan province, the 1976 Ms7. 4 Longling EQ in Yunnan province, and the 1988 Ms7.6 Lancang-Gengma EQ in Yunnan Province. At an AR station, two horizontally perpendicular observation channels or three channels, more one horizontally skewed channel (a NE or NW channel, as illustrated in Fig.1a), are employed, and for each channel, an AR observation configuration with a symmetry four-electrode array is installed (Fig.1b). For most stations, the current electrode spacing AB 1000~2000m  . Fig. 1. The observation channels (a) and symmetric four-electrode resistivity array for a channel (b) at a geo-electrical station great EQs and their recovery AR changes immediately after the two events had a similar changeable pattern in appearance. Therefore, the anomalous AR changes were believed to be the anomalies related to the two great EQs. The second problem is on the drop pattern of the two AR anomalies. It is obvious from figure 2 that the AR changes have a notable drop pattern during the imminent stage of the two impending great EQs. In general, the underground medium is abundant with water and the medium resistivity is susceptible to water, therefore, we believed that the changes of underground water resulted in these drop AR changes before the two impending EQs. Figure 3 are the raw AR daily-mean curve at CLH station and the raw curve of water level observed at underground water station [2] , Longjiadian station which was about 20 km from station CLH and nearby the Cangdong rupture zone. We can notice from figure 3 that immediately before and after the occurrence date of the EQ, the drop AR change at station CLH was well corresponding to the rise change in water level at station Longjiadian. The opposite changeable patterns between electric and water are quite significant, which indicated that the water in the underground medium nearby the focal region played an important role in AR changes. It can be seen from figure 3 that nothing was recorded before the Ms6. 9 Ninghe aftershock (Nov. 15,1976) at the geo-electrical and water stations, a possible reason for which was explained by associating with the mainly active faults in/nearby the focal region and the focal mechanism of the aftershock by Du et al. [3][4] .  (Fig.4).

Medium-term AR anomalies before the EQ
Significant AR anomalies were recorded at four stations in the medium-term stage before the Ms 8.0 Wenchuan EQ as follows.

Locally concentrated anomalies
The anomalies were recorded at CDU, JYO, GAZ and WDH during the medium-term stage before the EQ, which were in the range of 400km from the main epicenter (Fig.5, Fig.6). . AR daily mean changes observed at CDU station before the M8.0 Wenchuan EQ Furthermore, the spatial distribution of the anomalies was tectonically relevant to the Songpan-Ganzi active block. The main shock and its aftershocks occurred along the Longmen mountain nappe structure of the block. Accordingly, the anomalies were recorded at the four stations along the bordering faults around the block (Fig.4), whereas such anomalies were not recorded at the two stations MNI and XCM, which were located along the Anning river fault, beyond the block. This situation is similar to that which was observed before the 1976 Ms7.8 Tangshan EQ when anomalies appeared mostly at the stations along NE-and NW-striking conjugate faults in the Beijing-Tianjin-Tangshan areas.
According to previous statistical studies on numerous EQ cases [5][6] by other Chinese scholars, the spatially distribution of medium-term AR anomalies is about 300~400 km before an EQ with magnitude of Ms  7.0. According to paper [7], for Ms>6.0 EQs most AR anomalies distribute in the range of 400 km from epicenters and there is commonly no obvious difference between the ranges for Ms>6.0 EQs and Ms 7.0 EQs. The results show that an epicentral distance of 400km can be used as a reference for identifying medium-term AR anomalies related to Ms>6.0 EQs. Du et al. [8] studied the relationship between the spatial distribution of medium-term anomalies of other precursor observations in China, such as AR, groundwater chemical components and water level, geo-stress and geo-deformation, and the mainly active fault around the epicentral region, as a result, it was believed that the anomalies usually appeared nearby the faults around EQ focal areas. The spatial distribution of the medium-term AR anomalies before the Wenchuan great EQ, which were concentrated in the range of 400 km from the main epicenter and along the bordering faults around the Songpan-Ganzi active block, was in accordance with the foregoing research results.

Synchronous medium-term AR anomalies
These AR anomalies started to appear from about Aug. 2006 at CDU, JYO, GAZ and WDH.
In other wards, they appeared synchronously before the main shock at the four stations which were located along the bordering faults of the Songpan-Ganzi active block. The behavior was similar to previous research results. According to Du et al. [8] , the mediumterm AR anomalies along a actively geological structure around an EQ focal area usually started to appear synchronously or quasi-synchronously. In fact, the medium-term anomalies in observation of groundwater chemical components and water level, geo-stress and geo-deformation along a same structure also have such behavior as seen in AR observation [8] .

Mostly drop-type AR anomalies
The drop-type anomalies were recorded at stations CDU, GAZ and JYO. Only at station WDH, was a rise-type anomaly recorded, yet this was not isolated. Stations WDH and Tianshui (TSE), in Gansu province, were all located to the north of the main epicenter, and station TSE was to the north of station WDH, which was nearby the NWW-striking western Qinling rupture belt and was 452 km from the main epicenter. At station TSE, a rise anomaly beyond 1% appeared during two months period preceding the main shock, which was the most prominent AR change in the last ten years at the station. The phenomenon of mostly drop-type AR anomalies coincided with previous researches, and the spatial distribution of the medium-term anomalies at the four stations well tallied, in the range of -400 km, with that before EQs with magnitude Ms>7.0. For change of patterns, a drop-type or rise-type change, of the medium-term AR anomalies which were processed by using the normalized variation rate method (NVRM) [9,7] , Du et al. [10,7] got the following statistic results: for Ms  7.0 EQs, about 100% of the anomalies in the range of 150 km from epicentral areas are negative (a drop-type pattern) and about 71% of the anomalies in the range of 400 km are still negative. The reasons on the change patterns of the anomalies was theoretically explained by papers [10,7].

Large-amplitude anomalies
At station CDU, which was the nearest station to the EQ epicenter, the anomaly amplitude reached up to -5.5%. At stations GAZ and WDH, which were farther away from the EQ epicenter than station CDU, the anomaly amplitudes reached up to -5.3%--4.9% and 2.9%, respectively. The mean of the anomaly amplitudes was larger than that before the1976 M7.8 Tangshan EQ, and the anomaly amplitudes of the three stations decreased with the increase of epicentral distance. The anomaly recorded at JYO was small in amplitude, only -1.1%, although it was nearby the main EQ epicenter area (according to an investigation after the EQ, the measuring instrument at the station was not in good operation at that time, with a fixed error). The relationship between AR anomaly amplitude and EQ magnitude has been studied by numerous scholars [5][6][7] . The anomaly amplitude before the Wenchuan great EQ was in accordance with the previous researches.

Anisotropic AR changes
Two observation channels, along N58 o E and N49 o W directions, are employed at station CDU. The anomaly amplitude recorded through the N58 o E channel was -5.5%, whereas no anomaly was recorded through the N49 o W channel. According to previous works [2,11] , this anisotropic AR changes roughly indicated that the underground media here had been under the action of the maximum compressive stress in the NW-SE direction during the period from about Aug. 2006 to the occurrence of the main shock. At station GAZ, two channels, along N30 o E and N60 o W directions, are employed. The anomaly amplitude recorded through the N30 o E channel was -5.3%, and that recorded through the N60 o W channel was -4.9%. This indicated that the media here had been under the maximum compressive stress in the NWW-SEE direction during the period. The maximum compressive stress directions that were revealed by the AR changes at the two stations basically corresponded to the Paxis azimuth of the main shock [12] . Station JYO was near the northern aftershock area of the great EQ, and two channels, along the N70 o W and N10 o E directions, are employed at the station. The comparison between anomaly amplitudes recorded through the two channels was still credible as the measuring instrument at the station had just a fixed error. The anomaly amplitude recorded through the N70 o W channel was -1.1% and no anomaly was recorded through the N10 o E channel. This indicated that the media here had been under the maximum compressive stress close to the NS direction during the period, which well corresponded to the P-axis azimuths of most strong aftershocks in the northern aftershock area.
In summary, the behaviors of these medium-term AR anomalies, such as their spatial distribution within 400 km, their tectonic relevance to the Songpan-Gazi active block, and their amplitude attenuation with increasing distance, as well as their synchronous, mostly descending, large-amplitude and anisotropic changes, strongly support that these anomalies are indeed related to the focal processes of the main shock and strong aftershocks. Such locally concentrative AR anomalies as recorded at the four stations have not been succeeded in identifying in areas beyond 400 km from the EQ epicenter.

Short-term AR anomalies before the EQ
During the short-term period before the occurrence date of the great EQ, no obvious anomaly, like these recorded at station CLH (72km) in Hebei province before the 1976 M7.8 Tangshan EQ and at station WDH (110 km) before the 1976 M7.2 Songpan-Pingwu EQs ( Fig.2), was recorded at the six stations within 400 km from the EQ epicenter. Upward AR changes commenced at station CDU from March 2008 and at station JYO from April 2008 ( Fig.5a-b and Fig.6). The patterns consisting of an initial medium-term drop change followed by a short-term rise change before the EQ are consistent with these of the electrical resistivity change within an EQ focal area that are forecasted by DD model [13][14] . In fact, such patterns appeared often nearby epicentral areas of previous strong EQs [9,2,7] . However, the rise change, upon which the main shock occurred, did not satisfy the anomaly criterion in amplitude, so no sufficient precursory information could be confidently detected in the period when approaching the EQ. Du et al [3,15] studied the spatial distribution characteristics of imminent AR anomalies before the moderate, strong EQs that occurred in the continent of China. As a result, it was got that the spatial distribution of the anomalies was influenced by mainly active faults around EQ focal areas and focal mechanisms. The influences include: (1) the anomalies appear mostly along or nearby the faults; (2) most anomalies are distributed in the two areas that are symmetrical about an epicenter and that azimuthally tie to the P-or T-axis areas that correspond to the focal fault movement; (3) If there is an active fault between a station and an epicentral area, and the fault strike is along or close to the azimuth of the foregoing P-or T-axis, then, no imminent anomaly is usually recorded at the station in the period when approaching the EQ, or an anomaly is generally weak in amplitude. For example, in the Songpan-Pingwu area in Sichuan Province, three EQs with magnitude of Ms7.2, Ms6.7 and Ms7.2 occurred successively within 8 days in August 1976 at almost the same location (Fig.7), under the action of the compressive stresses in the NE direction (for the previous Ms7.2 EQ) and near to the EW direction (for the following Ms6.7/7.2 EQs) [16] .
And the P-axis azimuths of the three EQs were 63°, 101° and 95°, respectively, resulting in an right-lateral movement of the NNW-striking Huya fault (i.e., the focal fault) when the previous Ms7.2 EQ occurred, and then an left-lateral movements of the fault when the following Ms6.7 and Ms7.2 EQs occurred [16] . At that time, three stations were in observation within a range of ~190 km from the three epicenters: station Songpan (SPN, 45km, and it was abandoned shortly afterwards) in Sichuan Province and station Lixian (LXN, 190 km, and abandoned in the 1990s) and station WDH (110 km), in Gansu Province. There were obvious differences between AR changes recorded at station SPN and those recorded at station LXN (Fig. 8). Station SPN was located on the west side of the Minjiang River fault (MRF) with the NNE strike, whereas the epicenters were to the east of the fault. As a result, no imminent AR anomaly was recorded at station SPN when approaching the occurrence date of the first Ms7.2 EQ, though the station was only 45 km from the epicenter, whereas at LXN station, a drop anomaly with amplitude of -0.8% was recorded during 4 days before the EQ in despite of being 190 km away. Contrarily, for the subsequent Ms6.7 and Ms7.2 EQs, a drop anomaly with amplitude of -3.5% was clearly captured at station SPN during 3-4 days before them. This anomaly amplitude was much greater than that recorded at station LXN before the first Ms7.2 EQ. No imminent anomaly appeared at station LXN for the latter EQs. At station WDH, which was located between station LXN and the three epicenters and was near to the north side of the White Dragon River fault (WDRF) with the EW strike, the AR changes recorded before the three events were similar to those seen at station LXN. According to papers [4,17], imminent anomalies recorded at underground water level and water chemistry stations before the moderate, strong EQs which occurred in the continent of China also demonstrated such spatial distribution as AR imminent anomalies, which was related to mainly active faults of epicentral areas and EQ focal mechanisms. From loading experiments of rock sample [18] , the crust medium nearby active faults are susceptive to stress disturbances during the loading processes, therefore, the AR and underground-water anomalies which is related to the geo-stress changes are easy recorded nearby the faults. Paper [19] calculated the strain distribution in a geologic body model with a fault using the elasto-plastic 2-D finite element method, and then the calculated strain values are converted into relative AR changes. It can be seen from the spatially non-uniform distribution of these calculated AR changes that the spatial distribution of AR changes which are related mainly active faults around epicentral areas and EQ focal mechanisms, as described in papers [3,15,7], can be well explained. In fact, the spatially non-uniform distribution of imminent anomalies in observation of underground water level and water chemistry, as described in papers [4,17], can be well explained based on the calculated strain distribution by paper [19] also. Based on the above-mentioned research works, the lack of imminent AR anomalies before the Wenchuan great EQ can be roughly explained. The main shock and strong aftershocks occurred to the west of the NE-striking Doujiang Weir-An county fault; station GAZ was located on the southwestern side of the NW-striking Xianshui River fault, and station WDH was located on the northern side of the NWW-striking WDRF ( Fig.4 and Fig.7). Hence, the reason why no anomaly was recorded at stations GAZ and WDH during the period when approaching the great EQ can be explained. The reason why no anomaly was seen at stations MNI and XCM can be explained also. Stations CDU and JYO were located to the east of the Doujiang-Weir-An county fault, near the main epicenter and in the area liable to record the anomaly related to the main shock, but at the two stations only weak upward changes were recorded during the period. The reason for this remains to be explained and requires additional research.-A possible reason is affected by secondary faults nearby the two stations.  Figure 9 shows AR normalized variation rate curves of three channels of this station from Aug. of 1988 to Oct. of 2002, based on monthly AR averages, which are processed by the normalized variation rate method (NVRM) [9,7] in Nov. of 2002. It can be seen from the curves that notable medium-term or short-term AR anomalies which were up to the identification criterion for NVRM anomaly, beyond the threshold value of  2.4, appeared before several EQs with magnitude of Ms  5.0 around the station from 1990 to 2001. In 2002, two droptype AR anomalies appeared again through channels EW and NW of the station, and a risetype anomaly did through channel NS. These anomalies were new, following the rise-type anomalies of three channels in the end of 2001 which corresponded to the Nov. 2011 Kunlunshan mountain Ms8.1 EQ far-one type of anomalies which had nothing with the focal process of the great EQ [20,[7][8] . According to the past EQ cases of the station where AR anomalies corresponded to EQs around and the research results in papers [8,10] Yuanmou (YNM) in Yunnan province. The AR data, monthly mean data, observed by the two stations were processed using NVRM. Figure 10 gives the NVRM curves of PAH station which were reprocessed in 2004. This station was settled in 1970s, around which many EQs with magnitude of Ms  5.0 have occurred since then, accordingly, at this station AR anomalies have been recorded before the EQs. According to the previous EQ monitoring efficiency  (Fig.11). In figure 11, the two forecast regions were marked in green color, painted in Nov.  figure 12 that the medium-term drop-type to short-term rise-type AR changes are similar to the AR changes in the focal area as foretold by the DD model (Dilatancy-Diffusion Model) [13][14] in appearance, a pattern consisting of an initial mediumterm fall followed by a short-term rise. People may ask why the May 12, 2008, Ms8.0 EQ was not forecasted in the medium-term period before the great EQ, which just occurred around/nearby 6 AR stations in this area? In fact, authors had no time to process and analyze the AR data observed at the 6 AR stations in those days, and the AR changes of the stations were analyzed and studied only after the great EQ [7] .

Anisotropic AR changes related to strong EQ
Кraev studied the calculating method of AR in a homogeneous anisotropy medium [22] . Later, Brace et al. [23] , Chen et al. [24] and Lu et al. [25] reported that during the loading process of most rock (soil) samples, AR changes showed the directional changes which were associated with the maximum loading direction. In recent years, people try to explore relatedearthquake anisotropic AR changes, which would have an important significance to understand the stress status in/nearby the EQ focal region, to explain the reason of relatedearthquake AR changes and to forecast an EQ. However, it is very difficult to detect the anisotropic changes from actual EQ cases. Such researches were reported only in China because China has long made a lot of fixed-site AR observations. Qian et al. [26] and Du et al. [9,11] reported that the anisotropic AR changes that were associated with the maximum compressional stress (P-axis) azimuths of EQ focal mechanism solutions, and Du et al. [2,27] try to explain the anisotropic AR changes in theory.

Analysis of EQ cases on anisotropic AR changes
The studied EQ cases are picked out according to the following three principles: strong EQs with magnitude of Ms  5.5, EQs nearby AR stations, and AR changes in the later stages of EQ preparation. From the formula to estimate the focal body radius L [29] , Ms=3.3+2.1 Log L, for Ms5.5 EQ L is no more than 15 km, for Ms 6～7 EQ L is no more than 60 km and for Ms7.8 EQ L is about 140 km. According to Du et al. [7,10] , AR anomalies within tens km for Ms 6~6.9 EQs and within 150 km for Ms  7.0 EQs are mostly characterized by drop-type changes in the medium-term stages of EQ preparation. The underground medium commonly contains rich water, so the concentrative range of the drop-type AR changes is well coincident with the focal body radius L. This indicates how we pick out the EQs nearby stations. It is very important in study of anisotropic AR changes to distinguish the precursory AR anomaly related to strong EQs from observational AR data. Therefore, NVRM is usually used to process data. Du et al. [11] have studied anisotropic AR changes, using NVRM, recorded at 41 stations before 27 Ms  5.5 EQs that occurred in the Chinese mainland. The results show that for over 95% of the stations, the anisotropic AR changes appeared during the later stages of EQ preparation, which were obviously related to the P-axis azimuth of EQ focal mechanism solution. The behaviors of these anisotropic changes are that the AR change recorded through the channel perpendicular (or almost perpendicular) to the azimuth is a maximum in amplitude, whereas that recorded through the channel along (or close to) the azimuth is a minimum.
For example, the two observation channels, N20°E and N70°W channels, are installed at station PGU that had epicentral distances of 111 and 140 km for the 1976 Ms7.8 Tangshan EQ and Ms7.1 Luanxian aftershock; the P-axis azimuths of the two events were 75° and 297°, respectively, roughly in the EW direction. As a result, the medium-term drop-type and short-term rise-type AR changes recorded through N20°E channel (with a near NS direction) prior to the EQs were greater in amplitude than those through N70°W channel (with a near EW direction) (Fig.13). As another example, three channels, NS, EW and N45°W channels, are installed at station SHD that had an epicentral distance of 43 km for the 2003 Ms6.1 Minle-Shandan EQ; the P-axis azimuth of this EQ was 65°. As a result, the medium-term drop-type and short-term rise-type AR changes recorded through N45°W channel prior to the EQ were greater in amplitude than those through EW channel (Fig. 12). It is obvious that the relationship between the anisotropic AR changes and the P-axis azimuth from actual EQ cases agrees well with the relationship between the directional AR changes and the maximum loading direction in most experiments of water-bearing rock (or soil) samples. This proves that such AR changes are just related to the EQ preparation process.

Theoretical analysis on anisotropic AR changes
According to DD model, the micro cracks inside the underground medium fast and nonlinearly develop immediately before the main rupture within an EQ focal area, their strikes align predominately in a certain direction and underground water fast come in them. Barsukov [29] interpreted a larger amplitude change in resistivity on the assumption that the micro crocks are tortuously linked each other, and underground water comes in them, as a result, the conductive aisles inside the medium are formed. Mei et al. [30] deduced that in the later preparation stages of strong EQs a number of micro-cracks inside the medium of an EQ focal region is increased sharply. The fact that in the medium-term statges before moderate, strong EQs of the Chinese mainland, for larger-magnitude EQs the AR anomaly amplitude increases fast, whereas the anomaly duration increases more slowly [10] supports Mei's deduction. According to Crampin et at. [31] , the maximum compressional stress inside the near-surface crust is commonly horizontal, and because of hydrostatic pressure of rock the horizontal cracks are closed and the erect cracks are developed, with the normal of the erect cracks being usually mostly horizontal and the strikes of the cracks being predominantly along in the direction of the maximum compressional stress axis. As a result, this forms an EDA medium with conductive fluid. According to the above-mentioned discussions, we presume that in the later preparation stages of strong EQs there probably exist two physical behaviors inside the medium in and nearby the focal area: (a) the erect micro-cracks is well developed, their number is non-linearly increased, and their strikes are predominantly along the direction of the maximum compressional stress axis. (b) in above-mentioned processes, the macro-cracks are linked each other and the underground water with low resistance comes fast in them. This forms the conductive aisles in the medium. As the result of its sensitivity to water, the electrical resistivity of the medium, therefore, undergoes significant changes. Based on the two physical behaviors, we approximately regard the medium in/nearby the EQ focal region as a homogeneous azimuthal anisotropic medium. We establish a Descartes coordinate system 123 ox x x  on the ground where 1 x , 2 x and 3 x are three electrical principal axes, respectively, which 2 x is along the horizontal loading direction (this means the P-axis direction of EQ focal mechanism solution in study of EQ cases), 1 x is horizontal and perpendicular to the direction and 3 x is downwards vertical to the ground surface. 1  , 2  and 3  are true resistivity (TR) along the three axes, respectively, and 1  > 2  = 3  .  is the angle between ground observation channel X and the axis 1 x . Another ground observation channel Y is perpendicular to channel X . Based on the calculating formula for the relative AR changes ( ss ) in the anisotropic medium by Du et al. [2] (after referring to  (3), in which sx   is the AR variation rate of channel X and sy   is that of channel Y. From the inequality and formulae (1) and (2) as well as the calculating formula for s  in the anisotropic medium, the following inequality is obtained [11] : Where 1   and 2   are TR variation rates along two electrical principle axes, and a true anisotropic coefficient In the case of 0.5 1.0   (viz., 12 14    ), we get the following inequality: If the AR changes along the loading direction and perpendicular to the direction are all increased the sigh ">"will replace "<" in inequalities (5) and (7), from inequality (4). According to inequalities (5) and (7) and their derivation conditions, we can have the following understandings: anisotropic TR changes in which the change along the loading direction is more prominent than that perpendicular to the direction cause anisotropic AR changes in which the change perpendicular to the direction is more prominent than that in the direction. It is obviously that there are a difference of 90  between the directions of both the most prominent AR change (perpendicular to the loading direction) and the most prominent TR change (along the loading direction). This result distinctly explained the relationship between the most loading direction and the anisotropic AR changes when approaching the main rupture of most rock (or soil) samples and theoretically supported the research results from actual EQ cases.

A Reason for anisotropic AR changes
From the TR relationship among three principal axes in the anisotropic medium, 1  > 2  = 3  , we assume the shape of a single micro crack as a schistose ellipsoid that takes 1  axis as its rotation axis, and whose radius is a in 2 x direction, b in 3 x direction and c in 1 x direction and a=b>c (an erect crack). And then, we assume that the resistivity in saturated-water cracks (water is rich within the underground medium) is f  , that of framework is   and the crack ratio is  . Using Кraev's result [22] we have the approximate formulae of  versus : underground water comes fast in. The physical processes induce the TR changes which drop-type pattern occurs in a medium dilatancy stage and rise-type pattern does in a closure stage of micro cracks, and also induce the anisotropic TR changes in which the most prominent change appears along the maximum compressional stress direction. 4. In a homogeneous anisotropic medium, the AR change is in agreement with the TR change in drop-type or rise-type pattern, hence, a drop-type pattern of AR change appears in a medium dilatancy stage and a rise-type pattern appears in a closure stage of micro cracks. Because of the directional discrepancy with 90  angle between the most prominent TR change and the most prominent AR change, the anisotropic AR changes, in which the most prominent change appears perpendicular or nearly perpendicular to the maximum loading direction, just appear. The study of earthquakes plays a key role in order to minimize human and material losses when they inevitably occur. Chapters in this book will be devoted to various aspects of earthquake research and analysis. The different sections present in the book span from statistical seismology studies, the latest techniques and advances on earthquake precursors and forecasting, as well as, new methods for early detection, data acquisition and interpretation. The topics are tackled from theoretical advances to practical applications.

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