Debated Seismogenic Sources

 

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The compilers of the DISS database guarantee the existence of all listed seismogenic sources, both Individual and Composite, and strive to describe them with the best possible accuracy. Nevertheless, they are well aware that the literature contains hypotheses and descriptions concerning a significantly larger number of potential seismogenic sources than those currently listed in the database. In contrast with the aleatoric uncertainty normally associated with the inherent variability in the parameters of the expected earthquake rupture, the uncertainty associated with the mere existence of a large fault is epistemic. This uncertainty may become crucial in deterministic seismic hazard assessment, or even in a probabilistic approach dominated by large potential earthquake sources; yet it is very difficult to assess and may vary substantially depending on the region and on different investigators. No matter how careful the work is, this type of uncertainty is generally not properly conveyed to the potential end-users of a seismogenic source database (e.g. Woo, 2005).

The compilers of DISS addressed this major source of uncertainty in seismic hazard assessment practice by identifying and describing a number of "Debated Seismogenic Sources"; these are active faults that have been proposed in the literature as potential seismogenic sources but were not considered reliable enough to be included in the database. They may include:

  • faults for which only minimal surface evidence is supplied in the literature;
  • faults based on inherently ambiguous geological evidence;
  • faults for which the literature offers highly contrasting views;
  • faults that occur in low or very low seismicity areas;
  • faults whose characteristics are in open contrast with those of nearby, better known and established seismogenic sources, or that violate established tectonic and seismological evidence.

Similarly to the other categories of DISS sources, each Debated Seismogenic Source is identified by the code CCDS###, where:

  • CC is the two-letter ISO 3166-1 code for names of officially recognized countries;
  • DS identifies specifically the Debated Seismogenic Sources;
  • ### is an ordinal between 1 and 999 (including leading zeroes).

The Debated Seismogenic Sources are represented in the database as free-form polygons that contain, with large margins, the potentially active structure. We use this representation, and not a more accurate symbol, to overcome any possible ambiguity and error derived from the process of georeferencing the published maps of the active structures.

The reliability of DISS seismogenic sources is assessed by the database compilers and hence it is their sole responsibility. The database, however, was designed to allow quantification of all uncertainties involved in its compilation. Therefore, in addition to supplying formal uncertainty estimates for all sources that do appear in the current release of the Database, the compilers wanted to share with the end-users the reasons why any given Debated Seismogenic Source was not considered reliable enough for consideration in the Database. To this end they developed and compiled a questionnaire that helps focusing the attributes of any seismogenic source, either proposed by the compilers themselves or discussed in the literature, and assessing its reliability in a formalized step-by-step procedure. Each attribute was ranked as positive, negative or "not applicable", depending on whether that attribute increases or decreases the overall reliability of the potential seismogenic source, or if it is undetermined. The questionnaires prepared for each Debated Seismogenic Source are supplied as support information to that source.

See the full version of the Questionnaire below.

References


Questionnaire

  • A: Perceptibility of the fault at the surface
    • A1 Was it mapped at all?
    • A2 Was it mapped at the scale of the entire fault?
    • A3 Was it mapped using remote sensing images?
    • A4 Was it mapped at the outcrop scale?
    • A5 Was it mapped through original field work?
    • A6 Is a fault plane exposed in the bedrock?
    • A7 Are there scarps at/near the contact between bedrock and loose deposits?
    • A8 Are the fault scarps of positively tectonic origin, and can any other natural (e.g. gravitational) or anthropogenic processes be ruled out?
  • B Geometric, dimensional and kinematic compatibility
    • B1 Are parameters describing fault geometry and kinematics supplied at all?
    • B2 Are the width and depth of the fault estimated?
    • B3 Are the geometry and kinematics compatible with regional tectonic data?
    • B4 Are the geometry and kinematics compatible with other large active faults nearby?
    • B5 Are the proposed fault length and width consistent with common empirical relationships?
    • B6 Is the proposed fault size compatible with the size of other known active faults in the region?
  • C: Compatibility with long-term strains and landscape evolution
    • C1 Is there any recent basin/syncline or range/anticline associated with the inferred fault and geometrically compatible with it?
    • C2 Is the architecture of exposed young deposits compatible with the geometry and kinematics of the inferred fault?
    • C3 Is the architecture of exposed older deposits/rocks compatible with the geometry and kinematics of the inferred fault?
    • C4 Is the inferred fault or are the deposits associated with it imaged by subsurface and/or geophysical data?
    • C5 Is the architecture of buried young deposits/rocks compatible with the geometry and kinematics of the inferred fault?
    • C6 Is the architecture of buried older deposits/rocks compatible with the geometry and kinematics of the inferred fault?
    • C7 Has a quantifiable offset been observed/described?
    • C8 Can any landscape rejuvenation/differential erosion phenomena be confidently ruled out?
    • C9 Is the drainage pattern/behavior compatible with the geometry and kinematics of the inferred fault?
    • C10 Is the large-scale topography compatible with the geometry and kinematics of the inferred fault?
    • C11 Is the location and geometry of any fluvial/marine terraces compatible with the geometry and kinematics of the inferred fault?
    • C12 If analytical modeling of any fluvial/marine terraces or drainage pattern was performed, does it support the inferred fault?
  • D: Compatibility with associated seismicity
    • D1 Has anyone associated a specific earthquake with the inferred fault?
    • D2 If a specific earthquake is associated with the fault, is its inferred fault size compatible with Mw?
    • D3 Is there instrumental evidence for an earthquake of M > 5.5?
    • D4 Is the inferred fault size compatible with an earthquake of M > 5.5?
    • D5 Is there a historical and unequivocal surface rupture?
    • D6 Is there trenching evidence for surface ruptures?
    • D7 Is the size of the associated earthquake compatible with the known thickness of seismogenic layer?
    • D8 Is the earthquake damage pattern compatible at all with the inferred fault?
    • D9 Is the earthquake damage pattern well compatible with the inferred fault?
    • D10 Is the earthquake hypocenter compatible with the inferred fault?
    • D11 Does background seismicity delineate the fault with confidence?

 

 



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