Journal of Geodynamics 29 (2000) 387±392Active, capable, and potentially active faults Ð a paleoseismic perspective p Michael N. Machette U.S. Geological Survey, Central Region, Geologic Hazards Team, Denver, CO 80225, USA Abstract Maps of faults (geologically de®ned source zones) may portray seismic hazards in a wide range of completeness depending on which types of faults are shown. Three fault terms Ð active, capable, and potential Ð are used in a variety of ways for dierent reasons or applications. Nevertheless, to be useful for seismic-hazards analysis, fault maps should encompass a time interval that includes several earthquake cycles. For example, if the common recurrence in an area is 20,000±50,000 years, then maps should include faults that are 50,000±100,000 years old (two to ®ve typical earthquake cycles), thus allowing for temporal variability in slip rate and recurrence intervals. Conversely, in more active areas such as plate boundaries, maps showing faults that are <10,000 years old should include those with at least 2 to as many as 20 paleoearthquakes. For the International Lithosphere Programs' Task Group II2 Project on Major Active Faults of the World our maps and database will show ®ve age categories and four slip rate categories that allow one to select diering time spans and activity rates for seismic-hazard analysis depending on tectonic regime. The maps are accompanied by a database that describes evidence for Quaternary faulting, geomorphic expression, and paleoseismic parameters (slip rate, recurrence interval and time of most recent surface faulting). These maps and databases provide an inventory of faults that would be de®ned as active, capable, and potentially active for seismic-hazard assessments. Published by Elsevier Science Ltd. 1. Introduction Throughout the world, scienti®c and engineering studies are conducted on a daily basis to locate, study, and characterize surface-rupturing faults (and folds) that are associated with p A product of International Lithosphere Program Task Group II-2. E-mail address:
[email protected] (M.N. Machette). 0264-3707/00/$ - see front matter Published by Elsevier Science Ltd. PII: S 0 2 6 4 - 3 7 0 7 ( 9 9 ) 0 0 0 6 0 - 5 ) It seems that the big dierence between a capable fault and a potentially capable (or potentially active) fault is that one may or may not have the data available to assess whether a fault is active or not. 1990). Terminology These types of regulatory statutes commonly include speci®c language that de®nes "active faults. As a result. especially in more-populated areas with high rates of seismic activity (Lettis and Kelson. These studies range from regional in extent (e.388 M. This time limit is somewhat awkward. For example. a fault may not be capable..e. capable. Machette / Journal of Geodynamics 29 (2000) 387±392 large earthquakes
M b 6). blind thrusts. Thus. one having the ability for movement. even though the orientation in the present tectonic regime may be favorable for it being capable. one capable of being or becoming active.. In both end-member cases. 1. and requires mandatory set-backs when such faults are known or found by special (geologic and paleoseismic) studies (see Hart. trenching." However. one demonstrating current movement or action. detailed mapping.e. "Potential Ð capable of being or becomingF F F " A potentially active fault. in California the term active fault is de®ned as one associated with surface-rupturing earthquakes (EQs) in the past 11. and fault-related folds) in the United States. 2. Discussion These three terms Ð active. i. having been established when the Holocene was considered . the 1972 Alquist-Priolo Special Studies Zones Act (and later revisions) places limits on the proximity of engineered structures to all potentially and recently active faults.. for design parameters on engineering projects). faults. and potential Ð are used in a variety of ways for dierent reasons or applications. historic. i. Often the studies are conducted to satisfy regulatory statutes.). the various de®nitions of fault-activity terms are the source of some confusion and discussion both in the literature and in practice.e..e. the actual paleoseismic data come from detailed site-speci®c studies (i. (This de®nition is very similar to a capable fault. (Can't most properly oriented faults be capable under certain stress regimes?).. in California.N.. "Active Ð characterized by current activity F F F " An active fault. 1996). each state may legislate dierently. but we cannot prove this because the strata overlying the fault are not there. for seismichazards assessments) to local in extent (e. (What is current: contemporary. For example. i.g..e. For example. Holocene or Quaternary?). and dierent regulatory bodies within the Federal government commonly use diering criteria Ð often dependent upon their speci®c goals or objectives.g. "Capable Ð having the abilityF F F " A capable fault. 3. etc.000 years. classi®cation is strongly in¯uenced by the amount and type of data that are available. 3. The following terms (as adapted from common English-language dictionaries) are most commonly associated with seismogenic structures (i. 2. the marine isotope stage V/VI boundary. Other groups concerned with short-term hazards consider a fault as "active" if it has historic seismicity. However. Holocene active fault: a fault that has moved in the last 10. whereas 10.000 years ago. geologic evidence for paleoseismic activity is usually the result of M b 6 EQs for surface ruptures and M b 5 EQs for liquefaction features. the Western States Seismic Policy Council (WSSPC is a consortium of state and private scientists in the western US) recommended de®ning active faults in the Basin and Range province (WSSPC. the NRC has chosen time ranges for faulting events that are exceedingly dicult to date. whereas TL (thermoluminescence) dating is generally limited to about 100. recognizing that all degrees of fault activity exist and it is the prerogative of the user to decide the degree of anticipated risk and what degree of fault activity is considered dangerous": 1. The California regulations are used to zone all types of construction.. To quote. and the start of the Quaternary). Eurasia. Their capable fault has one demonstrable movement (oset) in the past 50.S. Ar40/Ar39 dating and tephrochronologic methods for Late Quaternary volcanic deposits).g. The Basin and Range province is characterized by a tectonic regime dominated by faults having relatively slower slip rates and longer recurrence intervals (Machette. Nuclear Regulatory Commission (NRC).e. which may mean intervals as short as 150 years (typical of the western USA) to as long as 2000 years (possible in parts of Europe.000 years.000 years. Late Quaternary active fault: a fault that has moved in the last 130. waste-storage facilities). 1994.g. de®nes a capable fault strictly on a calendar basis. except in certain special circumstances (e.000 years (NRC.000-year target is beyond the practical limit of radiocarbon dating (the most commonly used dating method). in most intraplate .M. the Holocene boundary. Alquist-Priolo Special Studies are not required for residential construction if the development is for less than four single-family homes.. Japan).000 years. Although unstated. Recently. reactors.6 million years ago). but come into play most commonly with new construction on commercial developments or large residential developments.600. such as around the Paci®c Rim. 2.N. which regulates the licensing and construction of nuclear-material facilities (e. and thus requires a dierent de®nition of active fault to be relevant to seismic hazard assessments. WSSPC justi®es these time-based de®nitions largely on the basis of their association with signi®cant climatic events that are commonly recognizable in the geologic record (i.000 years is now the commonly accepted beginning of the Holocene. 1998). Thus.000 years or multiple movements in the past 500. Conversely. 1996). "Active faults can be categorized as follows. Machette / Journal of Geodynamics 29 (2000) 387±392 389 to have begun 11. the U. They also argue that a late Quaternary criterion (130. 3. Quaternary active fault: a fault that has moved in the last 1.000 years. The 50. China.. This perspective is common in regions where seismicity is associated with major plate-boundary faults. The State of California de®nes a potentially active fault as one associated with surface-rupturing EQs in the Quaternary (since 1.000 years) encompasses many (probably most) of the average recurrence intervals in the province. 1997). This fairly accurate depiction of paleoearthquake activity is a result of high rates of surface faulting (recurrence intervals of 250±5000 years and slip rates >0. Tondi. Canada EQ (Ms 6. neotectonic might be de®ned as <500. a map of Holocene faults along the West Coast of the USA will include most faults that are "active" and can cause surface deformation. the current stress regime may have started in the Middle Quaternary (ca. such as episodes of clustered activity on long-recurrence faults. seismic-hazard assessments must grapple with newly recognized fault characteristics. such as active plate boundaries and passive intraplate regions.) have evidence of Holocene activity. In the nearby Basin and Range province. but only a fraction are active by most de®nitions. For example. Thus. the one presently causing EQs and surface deformation in an area. contagion behavior (temporal patterns). Tondi (1998) considers faults having movement within the past 700.D. The region's 140-year observation window (1860 to present) is clearly inadequate to sample the past 10. Neotectonic faults are thought of herein as those formed during the current stress regime. Thus. de Polo and Slemmons (1997) showed that only 6 of the 17 faults that have ruptured in historic time (post-1860 A.000 years) recurrence intervals and short (<10 years) aftershock sequences. Thus. Conversely. and stress transfers leading to changes in "time to failure" on a fault.390 M. This type of sliding variable scale suits a continental region that is aected by plate interaction of the West Coast.2 to >10 mm/year). neotectonic faults are both capable and potential. 1992). This later point was well illustrated by the 1989 Ungava. that is.3).000 years to be active. low slip rates (<0. a region's tectonic setting often has a strong in¯uence on the perception of the citizens and governments concerning active faulting and potential seismic hazards.000.000 years for intraplate North America. such of Australia and the central and eastern portion of North America. historic seismicity does not have a clear relation to Quaternary faults. . Thus.000 years of fault activity. Paleoseismic studies of intraplate faulting (Crone et al.. a map of Holocene faults would not predict almost two-thirds of the historic faults in the Basin and Range province. In addition to these semantic issues. Similarly.000 years (the late Pleistocene and Holocene). and compressional styles of deformation are typical of most stable continental interior regions. and comparatively less vigorous compressional tectonics east of the Rocky mountains. Machette / Journal of Geodynamics 29 (2000) 387±392 regions of the world. Similarly. spatial migration of faulting. all but one (16 of 17) of the historic faults in the province have had prior movement in the past 130. 1997) has shown that long recurrence (>100.000 years in California. 700. 1998) owing to changes in plate dynamics.01 mm/year).000 years). The term neotectonic captures a concept well suited to EQ hazards in diering seismic regimes.000 years in the Basin and Range province and perhaps <15. in the USA. to return to the de®nitions presented above.. owing to relatively long (>10.000. a map of Holocene faults in intraplate North America would only show a fraction of those faults that have been active during the Quaternary. pervasive extensional tectonics in the Intermountain West.N. which formed a 10-km-long surface rupture by reactivating an Archean fault (Adams et al. 2. The terminology used by these regulatory and advisory agencies may be problematic because they may be used in a variety of tectonic regimes. In the Apennines of central Italy.000 years ago. Tondi.000±50. A. 1998.M.. Slemmons. J. then maps should include faults that are 50.). (Eds. 45 pp. 1992.R Lund (Ed. Geological Survey of Canada Paper 92-C.N. pp. Wetmiller. M.M. Contrasts between short.. maps showing faults that are <10.. This subdivision allows one to select diering time spans for seismic-hazard analysis depending on tectonic regime. thus allowing for temporal variability in slip rate and recurrence intervals.6 Ma). U. W.N. Robertson. E. Geologic controls on the 1989 Ungava surface rupture: a preliminary interpretation. Western States Seismic Policy Council Proceeding Volume. Geological Survey OpenFile Report 93±338. 1997. in plate-boundary regions of the West Coast. Age criteria for active faults in the Basin and Range Province.. 91. Western Hemisphere International Lithosphere Program (ILP) Project II-2: Guidelines for U. fault maps should encompass a time interval that includes several earthquake cycles. if the common recurrence interval in the Basin and Range is 20. Hart. In: W.N. Proceedings Volume of International Workshop..J.. geomorphic expression. J..M... Kelson. 25. Drysdale... Australian Journal of Earth Sciences 44 (1).R. R..J.A. M.. Bowman.R. J. Fault-rupture hazard zones in California Ð Alquist-Priolo Special Studies Zones Act of 1972 with Index to Special Studies Zones Maps. . Tondi. pp. R. For example.R. References Adams. 1993).S. In: Cello. and paleoseismic parameters (slip rate. [California] Division of Mines and Geology Special Publication 42 (Revised 1990)... 1996 Annual Meeting. Proceedings. Machette / Journal of Geodynamics 29 (2000) 387±392 391 4. Machette. 147±155. Crone. Invernizzi. W.S. These maps and databases provide an inventory of faults that would be de®ned as active..A.2 mm/year (most intraplate faults) to >5 mm/year (most plate boundary faults). G. our planned maps and database will show ®ve age categories (historic to <1.000 years old should include those with at least 2 to as many as 20 paleoearthquakes. M. G.000±100. 84±95 (Utah Geological Survey Miscellaneous Publication 98-2). 1993. In: Technical Seminar on Earthquake Engineering for Dams. C. Deiana. The maps are accompanied by a database that describes evidence for Quaternary faulting. Dart. Basin and Range Province SeismicHazards Summit. Camerino. fault maps (geologically de®ned source zones) may re¯ect seismic hazards in a wide range of completeness. 1996. K. Conclusions Depending on a region's tectonic setting and the time window used. capable..L.I. To be useful for seismic-hazards analysis. p. which we use as a proxy for fault activity.N.000 years old (two to ®ve typical earthquake cycles). For the International Lithosphere Programs' Task Group II-2 Project on Major Active Faults of the World (Trifonov and Machette. 1997. 203±214. Machette. Conversely.W. Maps of Major Active Faults. Western States Seismic Policy Council Proceeding Volume. Basin and Range Province Seismic-Hazards Summit. Haller. Haller et al. 25 pp. E.. Machette. E. Database and Map. In: Lund. Slip rate. 74±83 (Utah Geological Survey Miscellaneous Publication 98-2).). Association of State Dam Safety Ocials. Italy.. P. Percival. recurrence interval and time of most recent surface faulting)... K.B. 1990. 1993. Active fault recognition and paleoseismic investigation techniques. Lettis. 1998. C. Active and capable fault segments in the central Apennines (Italy). (Ed.B. June 3±6.. D. is classi®ed in four categories ranging from <0. de Polo.).000 years. and potentially active for seismic-hazard assessments.and long-term records of seismicity in the Rio Grande rift: important implications for seismic-hazards analysis in areas of slow extension. The episodic nature of earthquakes in the stable interior of continents as revealed by paleoseismicity studies of Australian and North American Quaternary faults. The Resolution of Geological Analysis and Models for Earthquake Faulting Studies. J. Machette. Identi®cation and Characterization of Seismic Sources and Determination of Safe Shutdown Earthquake Ground Motions. Federal Register.S.N. San Francisco. Section 100. U. Regulatory Guide 1. WSSPC (Western States Seismic Policy Council). M. Geologic and Seismic Siting Factors. NRC (U. p. 1994. Annali di Geo®sica 36 (3-4). October 17.23.392 M. V. Nuclear Regulatory Commission).165. The world map of major active faults.S. January 1996). May 22. WSSPC Policy Recommendation 97-1 White Paper. NRC (U. 1994.S. 1997. 1993. . 1997. Nuclear Regulatory Commission. 3. Active fault de®nition for the Basin and Range Province. Nuclear Regulatory Commission).N.. Draft 10 CFR Part 100. 1996. Oce of Nuclear Regulatory Research (draft.G. 225±236. Machette / Journal of Geodynamics 29 (2000) 387±392 Trifonov.. CA.