Active fault of Eurasia database (explanatory note)
Principles and methods
3.3. Characteristics of evaluated attributes
The evaluated attributes RATE, SNS1, SNS2, SIDE, and CONF are indexed, i.e. their possible values are selected from a limited list of indices-symbols. The advantage of these attributes is their convenience for comparing objects, software processing and compiling maps according to legends. The selection of evaluated attributes is carried out on the base of justifying attributes according to the expert estimate of the compilers of the database. The compilers choose the most reliable solution, if the justifying data contradict to each other. If the choice is impossible or the justifying data are absent, the compilers estimate the evaluated attributes themselves. The lack of information in the justifying fields reduces the assessment of the reliability of the object. Attributes of the evaluated type are the final result of the work of the compilers of the database.
. The kinematic characteristics of the database object are represented by three attributes. The SNS1 attribute (the main component of movements) is obligatory, because one or another type of movements is inherent to any database object. The SNS2 attribute (minor component) is optional and its field remains blank if the main component is the only one. The SIDE attribute (indicator of relatively uplifted side) is filled in only if SNS1 or SNS2 indicate the presence of a vertical component of the movement. All these attributes are evaluative, because to select their values, the whole complex of justifying attributes relating to the kinematics of the object and contextual materials as well as structural relations with adjacent objects are analyzed.
The sense of the SNS1 and SNS2 attributes is shown by the indices: D – right lateral, S – left lateral, T – thrust, R – reverse, N – normal, E –extension, V – a vertical displacement is present, but its normal or reverse type is not defined, and U – kinematics is unknown. The uplifted side of the SIDE attribute is indexed as a following: + N – northern; + NE – northeastern; + E – eastern; + SE – southeastern; + S – southern; + SW – southwestern; + W –western; + NW –northwestern. The absence of the SIDE attribute is valid only for strike-slip and extension faults.
The main criterion for assessing the kinematic parameters is accurate and reliable information from source materials. The second criterion is the expression of the fault in detailed topographic models and space images. With a lack of accurate data and poor expression in topography, the kinematic parameters of the object are estimated based on the characteristic features of the system of active faults including the object. Its kinematics must correspond to the structural-kinematic regularities of distribution of adjacent faults of different types. In any case, it should not contradict the kinematic situation within the analyzed area on a working scale. If the source information on the kinematics of the object contradicts the kinematic relationships established in the area, this information is recorded in the justifying fields, but the corresponding evaluated attributes can remain underestimated or only the vertical component of movement is shown.
Rank of rate of movement
. Since the presence of late Quaternary movements determines the inclusion of the faul into the database, the RATE attribute is obligatory and a certain rank should be assigned to it, even if the rank is evaluated by indirect indicators. The rank of RATE attribute is estimated not only on the base of justifying data on the rate of movements along the fault, but also taking into account the indirect signs reflecting the deformation intensity. The values of the RATE attribute characterize the rate of movements on active faults and are divided into three grades: <1 mm / year, 1–5 mm / year, and >5 mm / year, which are indicated by indices 3, 2, and 1, respectively. However, in the source materials, only a small portion of the faults is provided with reliable values of the rate of movements according to the results of field measurements, and these measurements relate only to some segments of the fault zones. With a lack of accurate data, the assessment of the RATE attribute is based on the analysis of structural position of the object in the fault zone including it. For example, a segment of a fault zone may be assigned an increased value of the RATE attribute compared to neighboring ones if it is formed by the joining of several faults into a single master fault.
Thus, the RATE attribute reflects both the rate of relative movements of the fault sides and the degree of concentration of deformation within the fault as an element of zone of deformation. In the latter case, the RATE attribute refers, rather, to the rank of the object in the system of ruptures that is estimated with taking into account the general level of activity of the region and the saturation of the territory with active faults. Nevertheless, we consider that the RATE attribute reflects usually the real rate of movements, for example, an object with the value of the attribute RATE = 1 indeed have a rate of movements > 5 mm / year.
The indices of the RATE attribute do not always correspond to the values of movement rates given in the PARM and TEXT attributes with references to sources of information. This takes place in the following cases: (1) information in the justifying attributes from different sources varies, forming a range of values of comparable reliability; (2) the large fault zone consists of several faults, and the characteristics of the zone that are indicated in the source are assigned only to the most reliable of them; (3) the rate indicated in the source seems doubtful in the context of relationships with adjacent faults. In these cases, the RATE attribute is evaluated based on the structural position of the object.
A large fault forms a single object of the database only at a certain lavel of generalization. At the more detailed level, it can be represented by a fault zone including several faults and plastic deformation that realizes a portion of total displacement. It is not correct to assign the characteristics measured on some element of such zone to the zone as a whole. Its rate of movements can be higher. On the other hand, the high rate on some strand of the zone is not the ground to assign the same values to other faults of the same zone that would lead to an unjustified overestimation of the rate of movements on the whole fault zone.
Large fault zones are divided to several objects in the database. The rate values are assigned only to those objects that are most likely to correspond to this values according to the context materials and justifying data. If the large fault zone is represented by several objects in the working scale of the database, the rate of movements on the zone is determined by the values of RATE attribute of the segments with the higher their values. Ceteris paribus, maximum rates are assigned to segments located in the center of the zone or are formed by joining of several its strands. If the source materials contain reliable and accurate data on rates at all segments and branches of the zone, the RATE attribute of the zone is estimated by these values. If high rate of movements on really active fault is determined for the Pleistocene or Pliocene-Pleistocene time interval, this estimate is most likely assigned to the Late Pleistocene-Holocene movements.
Reliability of fault activity
. The CONF attribute determines a possibility of including of an object to the database. This is the most typical evaluated attribute, the value of which is determined by the compilers of the database on the base of analysis of the data of source materials on the age of the last movements on the fault, taking into account the whole data on the object and its structural-kinematic position. The CONF attribute shows the reliability of evidence of the Late Pleistocene-Holocene fault movements on the fault that partly depends on its studies. According to the validity of the Late Quaternary activity, active faults are usually divided into proved and supposed. Such separation is often subjective, since the evidence of activity cited by the authors is based on different approaches. The assessing of a fault as active has different degree of hypotheticalness depending on the used information and the characteristics of the region. Therefore, it is necessary to detailize the scale of estimation of active fault reliability to ensure comparability of estimates in territories with different tectonic regimes and degrees of fault studies, to expand the possibilities of the database using, to identify the inheritance of active faults from the earloier ones, and to assess seismic hazard. According to the reliability of signs of activity, the faults are divided into four categories: A, B, C and D.
The assignment of an object to category A means that several independent obvious signs of its activity were revealed, and the object was well studied in terms of its kinematics and internal structure. The grounds for including of the object to this category are the deformation of young landforms found in the field or clearly expressed on detailed remote sensing data and dated displacements of the Late Pleistocene-Holocene deposits, as well as manifestation of strong earthquakes and/or paleo-earthquakes in the fault zone.
The inclusion of an object to category B requires the manifestation of some of the above signs of activity, but their worse expression is allowed. In particular, signs of young offsets on remote sensing materials may be sufficient.
Object is included into category C, if the signs of activity listed above, or at least one of them are present, but they are unclear, incomplete and inaccurate or may have different interpretations. At the same time, the Pliocene-Quaternary offsets on the fault are doubtless.
Category D contains objects with the Pliocene-Quaternary offsets, but the signs of the Late Quaternary displacements have not yet been found on them. Most often, these are elements of the Pliocene-Quaternary fault zones that were poorly understood for the Late Quaternary activity. However, indirect signs give reason to believe that the signs of this activity can be detected with a more detailed focused study. Indirect signs are clear expression of a fault on space images and other remote sensing materials, location of epicenters of earthquakes in the fault zone, as well as the regular structural-kinematic position of the object relative to adjacent certainly active faults. The category D objects cannot be moved into the higher categories of reliability until they receive some justification of their activity.
Criteria for evaluation of the CONF attribute
. The most reliable criterion is the presence of accurate field data on displacements and deformation of the Late Pleistocene-Holocene deposits, topographic forms, drainage system elements, and human constructions in the source material. If such evidence exists, it is included into the justifying attributes, and the object clearly belongs to the CONF category A.
The second important criterion for assessing reliability is the peculiaritirs of expression of the object in detailed topograpic models, space images and aerial photographs. Modern remote sensing materials have a high enough resolution to identify an object as an active fault and sometimes to identify its kinematics by using characteristic features. The results of interpretation alone are usually not enough to give the highest reliability to the object. However, it can justify the category B in combination with less reliable criteria.
The other four criteria of the CONF attribute assessment are less reliable, indirect, and therefore cannot justify the highest values of this attribute. However, they are taken into account, especially for assessing of the supposed activity of objects of categories C and D.
The third criterion is location of the earthquake hypocenters, especially if they are shallow on the fault plane. Regular relationships between earthquake foci and the fault and location of the earthquake hypocenters in or near the fault plane (with taking into account the fault dip) is a serious argument to assume that the fault is active.
The fourth criterion is the position of the object relative to the surrounding active faults. Structural-kinematic relations of adjacent elements in a system of faults impose restrictions on the possible values of object parameters. Depending on whether the fault parameters in the source materials comply with these restrictions or not, the reliability of an object increases or decreases. If the object occupies a logical place in the structure of already established active faults (according to its kinematics and expression in contextual materials), this increases the assessment of reliability of the fault.
The fifth criterion is the consistency of researchers' opinions on the activity of the fault, as well as the number of independent publications in which the object is indicated as active. Each new researcher reassesses the results of his predecessors based on his own experience and the whole complex of new information, technologies, methods and concepts, sees the old object in new aspects and introduces additional arguments in its justification. However, in itself, the plurality of links cannot serve as a reliable justification for the reliability of the object, since different researchers can rely on the same old primary data without significant additions. There are cases when different authors conduct it in different ways on a certain part of the fault. If, according to the results of decryption, both options look convincing, then both options are given in the database on such a site, each under its own authorship. The reliability assessment within the agreed interpretation will be higher than in the area where the opinions of different authors differ.
Finally, the sixth criterion for assessing the reliability of objects is generally determined by a set of indicators, including the total number and versatility of the given information about the object, the degree of occupancy of all attributes, and the number of formalized parameters. With all the conventions, this criterion reflects the general level of researchers' attention to the object under consideration, the detail and versatility of its study, which indirectly increases the assessment of its reliability.
Thus, the totality of the content of substantiating attributes affects the reliability assessment.
Some platform researchers have attempted to isolate active violations only by deciphering distance materials. When detecting signs of young movements in a limited area of a decryptable element, the authors often sought to unjustifiably extrapolate the result to the entire length of the fault. This makes us treat these results with great caution. No matter how these elements are called in source materials, they or their fragments can be included in the database only if there is at least indirect evidence in favor of their activity. The processes of platform tectogenesis are slow, and their manifestations are weak and scattered, which rarely leads to the appearance of those signs by which the violation can be interpreted as active. However, the distribution on the periphery of the crustal seismicity platforms in combination with other data allows us to identify linear zones with which indirect signs of late Quaternary movements are supposedly associated. Prior to the accumulation of more reasonable information on the activity of such zones, they are included in the database as category D. objects. The exception is platform areas where manifestations of active tectonics are amplified by glacioisostasis processes, for example, in Fennoscandia.
An important task of the compilers of the database when adding new materials to it is to coordinate the values of the estimated attributes of the objects with those already existing in the database. This coordination and, if required, reassessment of the attributes takes into account the validity of the objects, including by analyzing their structural-kinematic relationships at the site of consideration on a working scale. As a means of redistributing the significance of individual faults in the general system of database objects, the CONF activity reliability attribute is used and, in part, the RATE attribute characterizing the speed of tectonic movements.
With the consistent application of the same methodological principles of processing them to heterogeneous source materials, in the process of regular reassessment of existing data in accordance with the continuously arriving new information, the tectonic model presented in the form of a database gradually improves. Its realism increases in the process of sequential integration of various data and expert opinions with a gradual rejection by lowering the reliability assessment from those that are least consistent with the new reliable information. Thus, the database is not only a set of established facts, but also a form of integration of expert opinions made on their basis.
Next: 4. Conclusion