Paleoseismology

Paleoseismology Lisa B. Grant University of Cafifornia, Irvine, USA

1. Introduction Paleoseismology is the study of earthquakes decades, centuries or millennia after their occurrence (Yeats and Prentice, 1996). Paleoseismic studies typically focus on prehistoric or preinstrumental earthquakes (Sieh 1981; Wallace, 1981; McCalpin and Nelson, 1996) to supplement the historic record of seismicity. The time between surface ruptures of active faults is on the order of decades for the fastest-moving faults to thousands or tens of thousands of years for more numerous but lessactive faults (see Table 1 in Chapter 29 by Sibson). Modem scientific observation and analysis of earthquakes have occurred for a small fraction of this time, so there are few observations on the spatial and temporal characteristics of fault ruptures over multiple seismic cycles (Sieh, 1996). Paleoseismic investigations provide data on earthquake occurrence and the seismogenic behavior of individual faults that complement modem observations of earthquakes, with important implications for scientific models and practical applications for seismic hazard assessment (Reiter, 1995). Moderate to large earthquakes may generate surface rupture or induce permanent changes in the landscape and local environment. Under suitable conditions, evidence of earthquakes is preserved in the geologic record. Paleoearthquakes are recognized by detailed observations and analyses of geologic or environmental conditions within fault zones or in tectonically or seismically active regions. Paleoseismic investigations yield information about the recency of fault movement, dates of previous earthquakes, recurrence times, average slip rate, and earthquake effects over time intervals ranging from decades to thousands of years. Paleoseismic data are applied toward estimating the magnitudes of past earthquakes, forecasting the magnitudes of future earthquakes (WGCEP, 1988; Reiter 1991), and identifying areas susceptible to fault rupture. Paleoseismology is a subspecialty of seismology in that it focuses on earthquakes. However, the primary methods of paleoseismic data collection and analysis are drawn from geology and are distinctly different from analyses of seismic INTERNATIONAL HANDBOOKOF EARTHQUAKEAND ENGINEERINGSEISMOLOGY,VOLUME81A Copyright ( 2002 by the Int'l Assoc. Seismol. & Phys. Earth's Interior, Committee on Education. All rights of reproduction in any form reserved.

waves conducted by geophysicists. Therefore, paleoseismology is sometimes considered to be a subspecialty of active tectonics or earthquake geology. Paleoseismology began to emerge as a distinct discipline in the late 1960s to mid-1980s. Summaries, compendia or review articles focusing on paleoseismology have been generated by Wallace (1981, 1986), Sieh (1981), Crone and Omdahl (1987), Vittori et al. (1991), Pantosti and Yeats (1993), Prentice et al. (1994), Serva and Slemmons (1995), Yeats and Prentice (1996), and Pavlides et al. (1999). Yeats et al. (1997) provide a comprehensive text on the geology of earthquakes with global coverage of active tectonics and paleoseismic studies. For a comprehensive text and reference on paleoseismology, the reader is referred to McCalpin (1996). The purpose of this chapter is to summarize methods, contributions, and issues in paleoseismology. Topics include paleoseismic investigation methods, models, and applications. Issues in research and application are discussed in each section. Examples and references are chosen to highlight major concepts and findings rather than attempting to provide a comprehensive or geographically inclusive summary. Most examples are from western North America, and heavy emphasis is placed on studies of the San Andreas fault system. Discussions of faulting and seismic hazard assessment are treated more fully by Sibson (Chapter 29) and Somerville and Moriwaki (Chapter 65).

2. Investigation Goals and Methods With few exceptions, earthquakes are generated by the movement of faults (Bolt, 1999). Therefore, paleoseismic research is directly or indirectly a study of faults and their surface expression. Paleoseismic investigation methods employ techniques from several geologic subdisciplines, including stratigraphic analysis, Quaternary geology, soil science, geomorphology, engineering geology, geochronology, and structural geology. Major objectives of paleoseismic studies include identification of recently active or seismogenic faults,

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measurement of displacement (coseismic and long-term average), and establishing catalogs of paleoearthquakes. This section is organized by common goals of paleoseismic studies, rather than by disciplinary techniques, because most investigations employ a variety of techniques.

2.1 Fault Identification Because earthquakes are generated by faults, the first step in any paleoseismic investigation is identification of a fault for study. An active fault is a fault that may have displacement within a future period of concern to humans (Wallace, 1981). Identification of active faults requires recognizing previous displacement and constraining the age of displacement. Therefore, an active fault is usually identified by associating it with tectonically deformed Quaternary-age materials or surfaces (e.g., Ziony and Yerkes, 1985).

2.1.1 Recognition of Tectonic Deformation Paleoseimic investigation methods are only applicable to faults that cause recognizable tectonic deformation or environmental changes (e.g., submergence, erosion, anomalous deposition, or mortality) at or near the Earth's surface. Faults may be classified by average slip rate (see Table 1 in Chapter 29 by Sibson), type of displacement (strike slip, dip slip normal, dip slip reverse, thrust, or oblique slip), and depth (surface or blind). The depth, rate, and type of fault slip determine the amount and nature of fault-induced deformation at the surface. The faster the slip rate, the more active the fault and the more likely the fault can be recognized and characterized by paleoseismologists (Slemmons and dePolo, 1986). Faults that reach the surface are easier to recognize than blind faults, but they may rupture to the surface infrequently if they have low slip rates (see Chapter 29 by Sibson).

Paleoseismic data provide an incomplete record of earthquakes and fault rupture. Tectonic deformation due to fault slip may be difficult to recognize if rupture does not reach the surface, especially in intraplate regions (e.g., Lettis et al., 1997). Earthquakes on blind faults may induce subtle surface deformations over broad areas (Stein and Yeats, 1989) or distributed evidence of shaking in disrupted sediments (Reiter, 1995). The existence of an active blind fault can be detected by geomorphic analysis (e.g., Bullard and Lettis, 1993; Keller and Pinter, 1996), by mapping Quaternary sediments and identifying areas of uplift (e.g., Grant et al., 1999) or by recognizing effects of shaking that are not associated with known surficial faults (Reiter, 1995). Effects of tectonic deformation can be enhanced or obscured by anthropogenic or natural processes such as weathering and erosion. If tectonic changes cannot be readily distinguished from changes induced by other mechanisms, then the results of paleoseismic studies may be biased toward higher or lower estimates of tectonic displacement. Such problems have been noted in a variety of tectonic and human environments (e.g., Ricci Lucchi, 1995; Obermeier, 1996; Nur, 2000).

2.1.2 Application of Geochronology Geochronology is dating of earth materials, surfaces, and processes. Geochronology is essential for paleoseismology because it constrains dates of paleoearthquakes and average rates of fault displacement. The most useful geochronologic methods for paleoseismic investigations yield high-resolution ages for common, late Quaternary materials such as soils or buried flora and fauna (Lettis and Kelson, 2000). Table 1 summarizes geochronologic methods for dating Quaternary materials in fault zones. A detailed discussion of this complex topic is beyond the scope of this chapter. A comprehensive

TABLE 1 Classification of Quaternary Geochronologic Methods (Adapted from Noller et al., 2000a) Sidereal

Isotopic

Radiogenic

Dendrochronology

Radiocarbon

Fission track

Uranium series

Thermoluminescence

2~~

Optically stimulated luminescence

Varve chronology

Chemical and Biologic

Geomorphic

Correlation

Amino acid racemization Rock-varnish cation ratio Obsidianand tephra hydration

Soil-profile development Rock-varnish development Scarp morphologyand landform modification

Stratigraphy

Soil chemistry

Rate of deformation

~~ in soils Lichenometry

Rate of deposition

U-Pb, Th-Pb Historical records

Sclerochronology and growth rings

Electron-spin resonance K-Ar and 39Ar-4~ Cosmogenic isotopes

Infrared stimulated luminescence

Rock and mineral weathering Geomorphic position

Paleomagnetism Tephrochronology Paleontology Tectites and microtectites Climate correlation Astronomical correlation Stable isotopes Archeology

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compilation and summary of Quaternary geochronologic methods and applications is provided by Noller et al. (2000b). The most commonly used methods are also described by McCalpin (1996) and Yeats et al. (1997). Radiocarbon dating is the most widely used method for dating Holocene and latest Pleistocene earthquakes. The halflife of radioactive 14C (5730y) limits the application of radiocarbon dating to organic matter formed from carbon fixed within the last 50 000-60 000 y (Trumbore, 2000). The amount of ~4C in atmospheric CO2 has varied in the past, particularly in the last few centuries due to anthropogenic emissions. To compensate for this variation, radiocarbon ages are calibrated to correspond to calendar ages (absolute ages). Calibrations based on tree rings and glacial varves extend back to the early Holocene. Calibration curves are not linear. Plateaus in the calibration curves limit the precision of radiocarbon dating (Trumbore, 2000). This problem is acute for the last few centuries. For example, Yeats and Prentice (1996) note that the two largest historic ruptures of the San Andreas fault in California, which occurred in 1857 and !906, are indistinguishable using radiocarbon dating. All methods listed in Table 1 have limitations and uncertainties. Uncertainty in geochronologic methods is a major source of uncertainty in paleoseismic data (Lettis and Kelson, 2000). In addition to uncertainty in calibration and accuracy of analysis, errors may be introduced in the selection, collection, and interpretation of field samples. Therefore, most paleoseismic studies employ multiple methods of dating to reduce uncertainty and allow cross-checking of results. Methods such as radiocarbon dating that yield accurate, high-precision ages of common or widely distributed materials are preferred. Recent improvements in geochronology methods and in the statistical treatment of dates have reduced uncertainties in previously published ages (e.g., Sieh et al., 1989; Biasi and Weldon, 1994).

2.2 Chronologies of Earthquakes Large earthquakes create features that may be preserved in the stratigraphic record and recognized by geological, geomorphic, environmental, or archeological analysis. Major effects of earthquakes include surface rupture, ground deformation, and ground failures due to shaking. Chronologies of earthquakes are developed by identifying and dating evidence of multiple paleoearthquakes along a specific fault, or by documenting evidence of shaking or tectonic deformation in a seismically active region. The longest chronology of paleoearthquakes spans 50000y, with a mean recurrence time of ~ 1 6 0 0 y (Marco et al., 1996). For most Quaternary faults, the dates of prehistoric earthquakes are not known. Well-studied faults such as the San Andreas fault in California, and the North Anatolian fault in Turkey, have chronologies of only a dozen or so paleoearthquakes. Nonseismic processes can create features that appear

very similar to those generated by earthquakes (Ricci Lucchi, 1995; Obermeier 1996). The most commonly reported and least ambiguous paleoseismic data are derived from observations of surface ruptures.

2.2.1 Surface Rupture Investigations Evidence of surface rupture can be observed directly in natural exposures or, more commonly, in trench excavations (see Figs. 1A and 1B on the Handbook CD-ROM, under the directory \30Grant). Paleoseismologists look for trench sites with distinct evidence of multiple ruptures and abundant material for dating. Trenches are generally excavated with a backhoe or similar excavation equipment. Shallow trenches in unconsolidated sediments may be dug by hand. Deep trenches must be stabilized with shoring, benching or sloping. Stabilization methods are chosen to maximize wall exposure. The maximum practical depth for typical trench investigations is 5 m. Trench exposures are cleaned by scraping and brushing to highlight stratigraphy and faults. Geologists map the exposures with the aid of a string grid or survey control. The maps are called trench logs. Trench logs are supplemented with descriptive notes of observations and objective records such as photographs. Samples of datable material are collected after their locations have been documented on the trench log. A good trench log documents observations of stratigraphy and structure as objectively as possible. However, it is impossible to make an entirely objective log because construction of a log is partly an act of interpretation.

2.2.1.1 Recognition and Dating of Ruptures The goal of logging is to recognize and record the number of surface ruptures and their stratigraphic position relative to datable material so that each paleoearthquake can be analyzed and dated as accurately as possible. Evidence of surface rupture is commonly called a paleoseismic event. The stratigraphic unit that was the ground surface at the time of the earthquake is referred to as the event horizon. Figure 1 shows typical stratigraphic and structural relationships that are used for recognizing and dating previous earthquakes (Lettis and Kelson, 2000). Figure 2 shows an example of a trench log from the San Andreas fault, with several paleoseismic events and event horizons marked with letters. The most useful criteria for identifying ruptures include multiple fault terminations at a single stratigraphic horizon, tilted and folded strata overlain by less-deformed strata, and colluvial wedges draped over fault scarps (Yeats and Prentice, 1996). Fault strands may die out and terminate below the surface, or be poorly expressed at the event horizon (Bonilla and Lienkaemper, 1990). Therefore, multiple exposures of a fault should be examined whenever possible to identify the event horizon with maximum confidence. In some areas a fault cannot be examined with excavations because of a high water table, the presence of human

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FIGURE 1 Diagrams illustrating stratigraphic and structural criteria used to identify the occurrence and timing of paleoearthquakes. Stratigraphy, faults, and soils are shown schematically. Dated samples at locations marked "A" predate the earthquake. Dated samples at locations "B" postdate the earthquake. Dated samples at "C" are not helpful for deciphering the chronology of past earthquakes because the sample may predate or postdate the earthquake. Where evidence of multiple earthquakes is present, locations A1 predate the earlier event and locations A2 postdate the earlier event and predate the later event. (From Lettis and Kelson, 2000.)

structures, or burial by younger sediments. Subsurface investigation methods such as cone penetrometer testing (CPT) and large diameter borings may be applied for these conditions (Grant et al., 1997; Dolan et al., 2000). These methods must be used in combination with other subsurface investigation methods such as seismic imaging, ground penetrating radar, and standard borings. There are greater

FIGURE 2 Example of a portion of a trench log from the San Andreas fault at Bidart Ranch in the Carrizo Plain, California. Letters indicate the approximate location of evidence for several paleoearthquakes (A, B, D, E?, F). Query indicates uncertainty. (From Grant and Sieh, 1994.) uncertainties in the identification and dating of paleoearthquakes from subsurface investigations than from surface exposures. The chronology of surface ruptures revealed by paleoseismic methods is a subset of the actual earthquake history. The paleoseismic record is spatially and temporally incomplete because the conditions required for preservation of earthquakes are not present at all times on all active faults. The minimum earthquake magnitude associated with surface rupture is about ME 5 (Bonilla, 1988), so ME < 5 earthquakes are rarely recognized in trench exposures. Surface faulting is commonly associated with M > 6 earthquakes (Wells and Coppersmith, 1994). Therefore, most paleoseismic events recognized in trenches represent M > 6 earthquakes. Nor have all faults been studied by paleoseismologists. Early paleoseismic studies focused on examining evidence of larger earthquakes (M > 7) because the data were relatively easy to interpret. Therefore, the published paleoseismic record of earthquakes contains less data on the occurrence of smaller (M < 7) earthquakes (McCalpin, 1996). 2.2.2

Regional

Coseismic

Deformation

Earthquakes that generate regional deformation may be recognized and catalogued. Vertical deformation can induce changes in local rates of deposition and erosion that provide evidence of a paleoearthquake, particularly in fluvial (Schumm et al., 2000) and coastal environments (Carver and McCalpin, 1996). Indicators of coseismic uplift in coastal areas include shorelines, platforms, or flights of terraces above mean sea level. Formation of elevated terraces have been reported following historic earthquakes in coastal regions (e.g., Plafker

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Paleoseismology

and Rubin, 1978; Matsuda et al., 1978; Pirazzoli, 1991). Sequences of similar terraces and shorelines at higher elevations provide evidence of multiple episodes of tectonic uplift (Fig. 3; and Figs. 4, 5 on the Handbook CD-ROM). In shallow tropical waters, the growth of corals responds to uplift or subsidence relative to sea level and may preserve a record of vertical coseismic motion (e.g., Zachariasen et al., 2000). Tectonic subsidence induces sedimentation that can record and preserve evidence of an earthquake (Carver and McCalpin, 1996). Buried flora or fauna may provide material for dating the earthquake. For example, in the Cascadia region of northwestern USA, tsunami deposits, drowned forests, and grasslands along the coastline provided material for high precision dating of subduction zone earthquakes (e.g., Atwater et al., 1995). The date of the most recent earthquake (AD 1700) was resolved from radiocarbon dating of submerged deposits, analysis of tree rings (dendroseismology), and historic accounts of tsunami recorded in Japan (Nelson et al., 1995; Satake et al., 1996). Measurement and interpretation of vertical deformation in coastal regions must be based on knowledge of sea level at the time of uplift. Sea level was 100-150m below modem levels during the last glacial maximum 15000-20000y ~p (Lajoie, 1986). After rising rapidly in the early Holocene, sea level stabilized in the mid-Holocene. However, sea level has fluctuated slightly (-+-2m) during the late Holocene (Lajoie, 1986) and is currently at different levels in different parts of the world (Pirazzoli, 1991). Fluctuations and regional variations have been sufficiently large to complicate interpretations of coseismic uplift or subsidence in areas with low vertical deformation rates.

FIGURE 3 Photo of shorelines and wave erosion platforms along rocky coastline of San Joaquin Hills, California. The person is standing on the active erosion platform. An elevated platform intersects the sea cliff at the paleoshoreline. An older paleoshoreline forms a notch in the sea cliff near the top of the photo. The paleoshorelines were formed by tectonic uplift of the San Joaquin Hills, southern California, USA (Grant and Ballenger, 1999).

2.2.3 Ground Shaking and Secondary Effects Surface rupture and regional deformation are considered primary effects of earthquakes because they can be directly associated with tectonic movement of specific faults. Secondary effects of earthquakes such as liquefaction, ground failure, slope failure, tsunami, and seismic seiche are induced by shaking. A history of seismic shaking is an indirect history of regional fault activity and is useful for seismic hazard assessment. If secondary effects disturb the environment, and if they are preserved, then these effects can be catalogued to develop a chronology of paleoearthquakes. The dates of shaking events may be constrained by geochronology methods. Evidence of liquefaction, inundation by tsunami, and seismically induced ground failures may be observed in natural or artificial exposures of young sediments, or in sediment cores. Liquefaction is a loss of bearing strength that occurs when saturated cohesionless sediments are subjected to strong shaking or cyclical loading. Fluidized sediment may mobilize during liquefaction to form distinctive sedimentary or geomorphic structures such as sand blows, sand dikes and craters (see Fig. 6 on the Handbook CD-ROM; Obermeier, 1996). Ground failures such as lateral spreading, slumping, lurching, and cracking may occur in soft sediment. In submarine or lacustrine environments, deposition of turbidites (Obermeier, 1996) or mixed-layer clays (Marco et al., 1996; Doig, 1998) can provide evidence of a paleoearthquake. Nonseismic events can create structures that are virtually indistinguishable from seismically deformed sediments, or seismites (Ricci Lucchi, 1995). Therefore, paleoseismologists must correlate candidate seismites over regions and rule out nontectonic origins before concluding that an earthquake occurred. For example, the widespread presence of features attributed to liquefaction and ground failure provide strong evidence of the AD 1700 Cascadia earthquake (Adams, 1990; Obermeier and Dickinson, 2000). Earthquake ground motions often trigger slope failures. Mountainous regions with steep slopes or unstable slope materials are particularly susceptible to seismically induced slope failures. Slope failures have been responsible for many fatalities in historic earthquakes (Bolt, 1999). In seismically active regions, slope failures over large areas may be linked to paleoearthquakes. Several methods have been developed to date paleoearthquakes by measuring the age of seismically induced landslide deposits. For example, lichenometry has been applied to date seismically induced rockfalls along the Alpine fault zone, New Zealand (Bull, 1996). The criteria for determining seismic origin, and methods for dating are summarized by Jibson (1996).

2.3 M e a s u r e m e n t of Slip Measurement of slip, or surface displacement, across a fault yields information about the magnitude of paleoearthquakes

Grant

480 and the average rate of deformation. The slip rate of a fault provides an upper bound for the rate of seismic moment release (Youngs and Coppersmith, 1985; WGCEP, 1995). Slip from paleoearthquakes can be used in combination with slip rate to estimate the recurrence intervals between earthquakes and to constrain the magnitude of past ruptures. 2.3.1

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To measure a slip rate, a geologist must find a feature that crosses the fault, is well defined, and is datable. Such features are called piercing lines. The points where they intersect a fault zone are called piercing points. Fault slip is measured from the displacement of piercing points and deformation of piercing lines. (Fig. 7 on the Handbook CD-ROM shows a map of an excavated piercing line across the San Andreas fault.) Geological piercing lines include streams, gullies, and linear sedimentary features. Anthropogenic piercing lines include walls, row crops, rice paddy boundaries, fences, and roads. The ideal piercing line crosses the fault at a right angle, is perfectly linear, has the same trend on either side of the fault zone, and is of precisely known age. Such conditions rarely exist. Therefore, most slip rates have considerable measurement uncertainty. For example, a compilation of fault slip rates in California (Peterson and Wesnousky, 1994) shows significant uncertainty in slip rate measurements. Uncertainty in slip rates are caused by uncertainties in measurements of displacement and in estimates of the time interval over which the displacement occurred. Aperture of measurement affects slip rates because deformation may occur beyond the main fault zone (McGill and Rubin, 1999; Rockwell et al., 2002) and have only subtle effects on piercing lines (Grant and Sieh, 1993; Grant and Donnellan, 1994; Sieh, 1996). Therefore, geologically measured slip rates may provide only minimum values if measured over short apertures. For blind faults, the displacement on the fault plane must be inferred from deformation far outside the fault zone, and the resulting slip rates have even larger uncertainties. The length and variation of time intervals between surface ruptures may affect measurements of slip rate or slip per event. For faults that do not creep, surface displacements are generated episodically by earthquakes. If the slip rate of a fault is low and the average time between surface ruptures is l o n g ~ the case for most faults~measurements of slip rate may be significantly greater or less than the average rate over multiple rupture cycles. It is generally assumed that slip rate is constant over the interval of measurement, but for most faults there are insufficient data to test this assumption. Geomorphic analysis of normal fault scarps in the Basin and Range province, USA,

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Paleoseismology

481

reveals significant temporal variation in slip rates for several faults (Slemmons, 1995). 2 . 3 . 2 Slip per Event

The amount of slip per event can be used to estimate the magnitude of previous earthquakes and the time between earthquakes (if the average slip rate is known). The distribution of slip may also reveal the location of segment boundaries, and areas of characteristic slip, if any are present (Ward, 1997). To measure slip per paleoearthquake, it is necessary to identify a piercing line that has been displaced by a known number of earthquakes. Ideally, this is done by measuring the displacement of a piercing line of known age in an area where the dates of individual surface ruptures are well constrained. For normal and reverse faults, the vertical component of displacement can be measured directly by excavating a trench across the scarp (Fig. 4) because material deposited by collapse or decay of a vertical scarp typically buries and preserves evidence of the rupture. Changes in slope or height of a scarp may indicate multiple ruptures. Variation in the amount of lateral displacement of piercing lines along a strike-slip fault can indicate multiple surface ruptures. In coastal areas, the vertical component of displacement can be measured from elevated marine shorelines (see Fig. 5 on the Handbook CD-ROM) (Matsuda et al., 1978; PlaNer and Rubin, 1978). For strike-slip faults, the best measurements are obtained from three-dimensional excavation using multiple trenches (Weldon et al., 1996). An alternate, but less-reliable, method is to measure displacement of surficial

features such as laterally offset channels. The total number of earthquakes is then inferred from peaks in the frequency distribution of measurements (Fig. 5), or by the assumption of constant (or "characteristic") slip per event (see Section 3.2). Uncertainty in the resulting slip per event measurements is unconstrained (see Fig. 10 on the Handbook CD-ROM).

2.4 Magnitude of Paleoearthquakes In general, the effects of an earthquake are proportional to its size (Slemmons and dePolo, 1986). Therefore, the amount of slip, tectonic deformation, or secondary effects preserved in the paleoseismic record helps to constrain the size of a paleoseismic event. The magnitude of paleoearthquakes can be estimated from measurements of slip per event or from rupture length inferred by correlating dates of paleoseismic events at different sites. Empirical regression relationships between historic earthquake magnitude and rupture length, average slip, or maximum slip (Wells and Coppersmith, 1994) can be applied to estimate the magnitude of paleoearthquakes (Table 2). Hemphill-Haley and Weldon (1999) developed empirical relationships specifically for estimating magnitude from paleoseismic measurements of slip. Their method provides uncertainties based on the number of measurements and percentage of rupture length sampled. For faults with chronologies of earthquakes from multiple paleoseismic investigations, surface rupture lengths can be

TABLE 2 Hierarchical Classification of Paleoseismic Features (Modified from McCalpin, 1996) On Fault Coseismic

On Fault Postseismic

Off Fault Coseismic

Off Fault Postseismic

Geomorphic expression

9 9 9 9 9

9 Colluvial aprons 9 Afterslip contribution

9 Tilted surfaces 9 Uplifted shorelines 9 Drowned shorelines

9 Tectonic alluvial terraces 9 Afterslip contribution

Stratigraphic expression

9 Faulted strata 9 Folded strata

9 Colluvial wedge 9 Fissure fill 9 Unconformity

9 Tsunami deposits

Abundance of similar nonseismic features

Few

Few

Some

Common

Geomorphic expression

9 Sand blows 9 Landslides 9 Disturbed trees

9 Retrogressive landslides

9 9 9 9

9 Retrogressive landslides

Stratigraphic expression

9 Sand dikes

Primary Fault scarps Fissures Folds Moletracks Pressure ridges

Secondary Sand blows Landslides Fissures Subsidence from compaction 9 Rapidly deposited lake or estuarine sediments

9 Sand dikes 9 Filled craters 9

9 Rapidly deposited lake or estuarine sediments

Soft-sediment deformation

9 Turbidites Abundance of similar nonseismic features

Some

Very common

Some

Very common

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482

estimated by assuming that ruptures with overlapping time windows occurred at the same time (see Section 3.4). However, there are large uncertainties in correlating ruptures between paleoseismic investigation sites, so the inferred rupture length and resulting magnitudes are poorly constrained. Segmentation models (see Section 3.1) have also been applied to estimate paleoearthquake rupture length and magnitude.

3. Models and Uncertainty Paleoseismic data provide information about earthquakes over scales of time and magnitude that are useful for seismic hazard assessment and essential for understanding the long-term rupture patterns of faults. Therefore, paleoseismic data have been influential in the development and testing of models that describe fault behavior over multiple rupture cycles, and in forecasting earthquakes. This section describes models that depend critically on observations of paleoearthquakes, and discusses uncertainty in paleoseismic data.

TABLE 3 Selected Empirical Relationships between Moment Magnitude (M), Average Displacement (AD), Maximum Displacement (MD), and Surface Rupture Length (SRL)a Slip Type c

a (sa) d

b (sa) d

Standard deviation

Correlation coefficient

M= a § b x log(AD)

SS Re N All

7.04(0.05) 6.64(0.16) 6.78(0.12) 6.93(0.05)

0.89(0.09) 0.13(0.36) 0.65(0.25) 0.82(0.10)

0.28 0.50 0.33 0.39

0.89 0.10 0.64 0.75

M= a+ bx log(MD)

SS Re N All

6.81 (0.05) 6.52(0.11) 6.61(0.09) 6.69(0.04)

0.78(0.06) 0.44(0.26) 0.71(0.15) 0.74(0.07)

0.29 0.52 0.34 0.40

0.90 0.36 0.80 0.78

M =a + b x log(SRL)

SS Re N All

5.16(0.13) 5.00(0.22) 4.86(0.34) 5.08(0.10)

1.12(0.08) 1.22(0.16) 1.32(0.26) 1.16(0.07)

0.28 0.28 0.34 0.28

0.91 0.88 0.81 0.89

Equation b

aFrom Wells and Coppersmith (1994). bDisplacement in meters. Surface rupture length in kilometers. cSS, strike-slip; R, reverse; N, normal; All, all fault types. dCoefficients and standard errors. eRelationship is not significant at the 95% confidence level.

3.1 Segmentation Segmentation models assume that faults are divided into discrete, identifiable sections that behave distinctively over multiple rupture cycles (e.g., Schwartz and Coppersmith, 1986; Slemmons, 1995). Segment boundaries are thought to control the termination and initiation of fault ruptures and therefore limit the magnitude and rupture pattern of an earthquake. Fault segments are defined based on structural discontinuities, changes in strike, and rheology (Allen, 1968) as well as by paleoseismic data (Chapter 29 by Sibson; Schwartz and Coppersmith, 1984; Sieh and Jahns, 1984; WGCEP, 1988, 1990; Schwartz and Sibson, 1989). Segmentation models are attractive because they simplify fault behavior. For example, in seismic hazard assessment it is desirable to estimate the size of the largest earthquake that can occur on a fault (Yeats et al., 1997). The maximum magnitude earthquake can be estimated from empirical relationships (Table 3) and the estimated length of maximum rupture (Wells and Coppersmith, 1994). This would be either the maximum length of the fault, or of the segment. Similarly, the size of a paleoearthquake can be estimated by assuming that the entire segment ruptured. Testing of segment models requires defining segments and then observing multiple ruptures. This would take decades to centuries for the fastest slipping faults, and thousands of years for most faults. Therefore, few segment models have been tested against rupture patterns of historic earthquakes (Sieh, 1996). Results are mixed. For example, two large earthquakes on strike-slip faults in the western US (M7 1989 Loma Prieta and M7.3 1992 Landers) propagated through previously identified segment or fault boundaries (WGCEP, 1990; Sieh et al., 1993). However, several historic ruptures of normal faults in the Great Basin have terminated at or near fault

discontinuities (Zhang et al., 1999). More observations are needed to test segmentation hypotheses and their utility for seismic hazard assessment.

3.2 Characteristic Earthquakes The characteristic earthquake model (Schwartz and Coppersmith, 1984) is probably the most influential model of fault rupture and earthquake recurrence developed from paleoseismic data. The basic tenet of the model is that most surface slip on a fault occurs in characteristic earthquakes. Characteristic earthquakes are the result of characteristic slip. Figure 6c illustrates the concept of characteristic slip; at a specific location along a fault, the displacement (slip) is the same in successive characteristic earthquakes. This implies that characteristic earthquakes have similar rupture patterns and that a fault can be divided into segments that behave characteristically. Each segment would have a distinctive or "characteristic" rupture pattern and magnitude. Characteristic slip requires variable slip rate along a fault to account for different amounts of displacement. If characteristic slip occurs on a. fault, then most seismic moment is released by repetition of characteristic earthquakes of approximately the same magnitude. Characteristic slip causes a kink in the frequency magnitude relationship, known as characteristic recurrence (Fig. 7), due to the dominance of relatively large magnitude characteristic earthquakes (Schwartz and Coppersmith, 1984). The characteristic recurrence curve appears to fit the frequency-magnitude distribution for some faults (Wesnousky, 1994) but does not fit global seismicity rates as well as other models (Kagan, 1993).

483

Paleoseismology

Observations

9 Variable displacement per event at a point 9 Constant slip rate along length 5

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FIGURE 6 Schematic representations of (a) variable slip, (b) uniform slip, (c) characteristic slip, (d) overlap, and (e) coupled slip models. (From Schwartz and Coppersmith, 1984; Berryman and Beanland, 1991. Reprinted from McCalpin, 1996.)

The characteristic earthquake model is convenient for seismic hazard assessment because it assumes that faults can be divided into identifiable segments that rupture with characteristic slip and recurrence. Many studies of active faults

have focused on identifying characteristic rupture segments of faults and their properties (e.g., Wesnousky, 1986; Peterson and Wesnousky, 1994; WGCEP 1988, 1990, 1995). For example, earthquake forecasts for the San Andreas fault

484

Grant 100

_:i---li--l----li----l---ii---l--i-

seismic parameters of an active fault (see Chapter 65 by Somerville and Moriwaki). Paleoseismic investigations are the main source of data for measuring or estimating Tr. Individual recurrence times or average recurrence intervals (the average time interval between ruptures) can be measured directly by dating successive surface ruptures. For well-studied faults such as the San Andreas, average recurrence intervals and their variance are available for several locations along the fault (e.g., WGCEP, 1995, 1999). For most faults, there is insufficient information to describe variability in recurrence times. If dates of paleoearthquakes are not available, recurrence time may be estimated for a fault using the relationship:

10

AI 1.0 ..(3

E

_-

t--

Seismicity~ data

>

"5 0.1 E

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-

\

0

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FIGURE 7 Characteristic recurrence model for cumulative frequency-magnitude distribution of seismicity on a specific fault. (From Schwartz and Coppersmith, 1984.)

have been based on a combination of segmentation, timepredictable, and characteristic earthquake models. The Parkfield segment of the San Andreas fault was predicted to break before 1993 with repetition of characteristic earthquake (Bakun and Lindh, 1985; WGCEP, 1988). To date, the expected Parkfield earthquake has not occurred, so the pattern of rupture cannot be tested against the models. The Working Group on California Earthquake Probabilities (WGCEP, 1988, 1990, 1995) divided the San Andreas fault into characteristic segments and issued probabilistic rupture forecasts. In 1989, the M7 Loma Prieta earthquake occurred on portions of two segments defined prior to the earthquake (WGCEP, 1990) rather than on a single segment. Hecker and Schwartz (1994) analyzed paleoseismic data for many faults and concluded that most faults exhibit characteristic slip. However, recent paleoseismic studies of the San Andreas fault reveal complexity in spatial and temporal rupture patterns that was not evident when the characteristic earthquake model was first proposed (Grant, 1996). Debate about the applicability of the characteristic earthquake model for forecasting earthquakes is likely to continue (Yeats et al., 1997; Yeats, 2001). 3.3 Time Predictable and Recurrence Models Recurrence time, Tr, is the time interval between successive ruptures of the same fault. T~ is useful for describing the

(2)

where V is the slip rate and D is average displacement (slip). Assuming the slip rate is constant over the period of observation and there is no creep, then the recurrence time is a linear function of displacement. If displacement occurs in constantsize (characteristic) earthquakes, then the recurrence time can be "predicted". Thus Eq. (2) is referred to as the timepredictable model, after Shimazaki and Nakata (1980) who developed it from observations of uplifted coastal terraces in Japan. If the characteristic earthquake model and timepredictable model are combined, the result is a periodic recurrence model. Analysis of historic earthquakes and paleoseismic data from several regions shows that occurrence of large earthquakes is irregular (Goes, 1996). Surface ruptures of the San Andreas fault have been proposed to occur irregularly (Jacoby et al., 1988; Sieh et al., 1989) or in clusters (Grant and Sieh, 1994). Temporal and/or spatial clustering of surface ruptures has also been proposed to describe surface ruptures in the Basin and Range (Wallace, 1981; Slemmons, 1995; McCalpin, 1996) and eastern California shear zone, USA (Rockwell et al., 2000). Historic ruptures of the North Anatolian fault zone in Turkey and patterns of late Holocene seismicity in Iran suggest that seismicity may be triggered by stress shadows from previous ruptures to form temporal clusters or sequences of damaging earthquakes (Stein et al., 1997; Berberian and Yeats, 1999). For most faults, the number of documented paleoearthquakes and the precision of their dates are inadequate to test hypotheses of clustered or triggered earthquakes. Larger paleoseismic data sets are needed to conduct statistically significant tests of recurrence models. 3 . 4 R u p t u r e P a t t e r n s in S p a c e a n d T i m e Patterns of fault rupture in space and time yield insights about the physics of faulting (Ward, 1997) and provide templates for estimating the location and size of future earthquakes. Several models of fault behavior (shown in Fig. 6) are based on multiple paleoseismic measurements of slip at different sites along

485

Paleoseismology Timing of moderate to large surface rupturing events on Mojave black faults i

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(

Trench site

Reference

Age of events (in thousands of years)

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No. Johnson Valley

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Johnson Valley/ Landers/Kickapoo

Batdorf House

So. Johnson Valley

i (

Hando Rd.

10

0 i

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15

20

25

This paper

This paper

This paper

!

Johnson Valley/

'

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t i! i

Bodick Rd.

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This paper

This paper

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Hecker et al., 1993

S. Camp Rock

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Rubin and Sieh, 1997

Old Woman Springs Helendale

!

Old Woman

I Springs Ranch 1 Rabbit Springs [ &Waverly Rd.

Lenwood

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Houser and Rockwell, 1996 Bryan and Rockwell, 1995 Padgett and Rockwell, 1995

i

FIGURE 8 Compilation of rupture history for several faults in the Mojave desert, California, USA. The sequence suggests regional temporal clustering of paleoearthquakes. (From Rockwell et al., 2000.)

a fault. The variable slip model (Fig. 6a) assumes a constant slip rate along the fault with variable displacement per event at a location, and variable earthquake size. The uniform slip model (Fig. 6b) is similar to the characteristic slip model (Fig. 6c) because both assume constant-size displacement at a location. The slip rate is constant along the fault in the uniform slip model. Therefore, a range of earthquake sizes is required to account for sections of fault with large or small slip per event. The overlap model (Fig. 6d) assumes constant displacement per event at a point (characteristic slip), variable slip rate along the fault, and repetition of constant size large earthquakes. Finally, the coupled model (Fig. 6e) is a uniform slip model coupled with a segmentation model. Spatial and temporal patterns of fault rupture can be analyzed by plotting the dates of paleoearthquakes on a spacetime diagram. Figures 8 and 9 show examples from a group of faults that ruptured in the 1992 Landers earthquake, and the southern San Andreas fault. Uncertainty in dates ranges from decades to more than a thousand years. Paleoearthquakes at adjacent sites with overlapping dates could have been the

same rupture, or separate ruptures that are indistinguishable within the uncertainty. Additional dates of paleoearthquakes would provide better constraints on the locations and magnitudes of past ruptures.

3.5 Uncertainty in Paleoseismic Data The quality and reliability of paleoseismic data are affected by the investigation method, the characteristics of the study site, and the perspective of the investigator. Many paleoseismic studies are destructive to the study site, so the data cannot be reproduced or compared with a control. Therefore, each paleoseismic publication becomes a repository of data that will be interpreted by scientists who will not be able to evaluate it first-hand. Collection of paleoseismic data requires recognition and interpretation of complex patterns in the geologic record and landscape. Therefore, some interpretation is automatically convolved with the data, and no deconvolution algorithm can be applied after the data are published. Paleoseismologists have developed several methods to reduce

486

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400

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Distance (km)

Space-time rupture correlation diagram for paleoseismic sites along the southern San Andreas fault, California, USA. Vertical lines show age limits for paleoearthquakes at each site. Horizontal lines represent ruptures, based on proposed correlations between sites. (From Arrowsmith et al., 1997. Modified from Grant and Sieh, 1994 and other sources.)

FIGURE 9

subjectivity and standardize uncertainty in their data. Fault exposures are generally documented with photographs as well as logs. Photos are objective records, devoid of interpretation. "Trench parties" are held to allow reviewers to examine primary field data prior to publication. When possible, care is taken to fill excavations so that they can be reopened at a later date. Uncertainty in data collection and field interpretations are often reported qualitatively. For example, Sieh (1978) and McGill and Sieh (1991) reported the apparent "quality" of their measurements of offset streams. Paleoseismologists may report high quality measurements with "high confidence". Such assessments are qualitative, and should not be confused with statistical confidence intervals.

4. Applications Paleoseismic data are used for seismic source characterization in hazard assessment and mitigation (see Chapter 65 by

Somerville and Moriwaki). Probabilistic seismic hazard assessment incorporates information on fault location and geometry, slip rates, recurrence intervals, dates of previous ruptures, and maximum earthquakes. Maximum earthquakes are estimated from segmentation models, slip per event, and lengths of previous ruptures. Deterministic hazard assessments use paleoseismic data to develop earthquake scenarios including rupture location, magnitude and surface displacement. Earthquake scenarios are used to estimate expected damage for emergency response planning. Surface rupture hazard can be mitigated by identifying fault zones that are most likely to rupture and restricting land uses within such zones. Paleoseismic investigations provide data on the recency and frequency of fault rupture, as well as the type and width of surface rupture. This information can be used to classify fault zones according to rupture potential and expected displacement in future earthquakes. Critical structures can be sited to avoid rupture hazard zones, or designed to accommodate expected displacement.

Paleoseismology

5. Future Directions Paleoseismology is a young science that focuses on understanding the long-term patterns of large earthquakes. Paleoseismic data can be applied toward understanding the behavior of faults, estimating the potential for large earthquakes to occur in populated areas, and for mitigating seismic hazard. As populations increase in seismically active areas, many paleoseismologists are moving toward predictive or applied work in seismic hazard assessment. Establishment of digital databases and standard formats for paleoseismic data allows better integration with more quantitative fields of seismology and earthquake engineering. Investigations of active faults are being extended to offshore areas (e.g., Goldfinger and Nelson, 2000) and paleoseismic data are being compiled for the World Map of Active Faults under the International Lithosphere Program. As paleoseismology matures, it is likely to play an increasingly important role in seismology and society.

Acknowledgments R. Yeats, M. Bonilla, and R. Sibson provided helpful reviews of the manuscript. This is publication no. 1 in the Department of Environmental Analysis and Design, University of California, Irvine, contribution series.

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Editor's Note Due to the space limitation, the complete set of figures for this Chapter is placed on the Handbook CD-ROM, under directory k30Grant.