APPENDIX D: GEOLOGIC TRENDS SUPPLEMENT

Relative Sea-Level Rise and Vertical Land Motions, U.S. West Coast

Vivien Gornitz, December 1997

Introduction

Uplift or subsidence rates, as inferred from sea-level data (Woodworth 1995; Spencer and Woodworth 1993) or geodetic measurements (e.g., Mitchell et al. 1994) represent one important factor in assessing the vulnerability of a coastal segment to future sea-level rise. This appendix briefly outlines a procedure for estimating uplift or subsidence rates and correcting tide gauges for geologic factors caused by vertical land motions. Geologic data on rates of vertical land motion along the West Coast are summarized in Table 1 and Figure 1. Historical data from tide gauges are shown in Table 2 and Figure 2.

Data registered on West Coast tide gauges represent relative sea-level changes. These include long-term geologic trends of tectonic and possible glacio-isostatic origin, also more recent neotectonic motions (interseismic uplift; Mitchell et al. 1994), in addition to the mean global sea level rise of 1-2 mm/yr attributed to worldwide warming over the last 100 years (Houghton et al. 1996). The relative sea-level trend, in mm/yr, at any tide-gauge station can be expressed as:

(1)

SLR = SLG + I + E + Ts

where:

SLR

=Recent sea-level curve from the tide-gauge data.

SLG

=Long-term geologic trend (late Quaternary <125,000 years), recording uplift from raised marine terrace data, or subsidence from Holocene marsh data. To conform with the signs used for relative sea-level trends, the geologic trend is taken as positive for land subsidence and negative for landuplift. Note that this convention is opposite to that used by geologists and geodesists.

I

= Glacio-isostatic component (uplift, as in Canada; subsidence, as along the U.S. East Coast).

E

=Recent (<100-150 years) global eustatic/steric component (absolute sea-level rise), taken here as 1.5 mm/yr (after Houghton et al. 1996).

Ts

=Short-term land movements, including neotectonic motions (interseismic deformation) and anthropogenically-induced subsidence, such as caused by withdrawal of subsurface gas, oil, or groundwater.

Relative Sea-Level Trends (SLR)

Sea-level data for the U.S. West Coast are obtained from 16 tide-gauge stations with records at least 20 years in length (although some records may contain discontinuities; Woodworth 1995; Spencer and Woodworth 1993). The sea-level trends are derived by fitting a linear least-squares regression line to the time series of mean annual sea-level elevations for each of the 16 tide-gauge stations (Table 2; Figure 2). The average relative sea-level trend for the West Coast is 1.39 1.48 mm/yr. This value implies the prevalence of global sea-level rise and land subsidence; however, the high variability points to the presence of localized uplift (negative relative sea-level trends) in some areas, particularly in Neah Bay, WA, Astoria, OR, and Crescent City, CA. The spatial pattern of relative sea-level changes observed from tide gauges is consistent with geodetic surveys (Mitchell et al., 1994). The much higher rates of subsidence or uplift from recent relative sea-level data (Table 2; Figure 2) as compared with long-term geologic trends derived from the raised marine terraces from the same localities (Table 1; Figure 1), indicates accumulated interseismic strain and points to potential earthquake hazards (Mitchell et al. 1994).

Uplift or Subsidence Trends (SLS)

The local uplift or subsidence trend (SLS) is the difference between the relative sea-level trend (SLR) recorded by the tide gauges and the mean global eustatic (E) trend of 1.5 mm/yr.

(2)

SLS = SLR 1.5

The SLS term is a composite of long-term tectonic (SLG) and isostatic components (I), as well as more recent neotectonic motions (Ts).

Assessments of coastal vulnerability are concerned with the relative sea-level rise, inasmuch as this parameter is directly related to flood hazard. But since the SLR term contains local uplift or subsidence trends in addition to the global trend, the SLS term (as estimated by Equation 2) needs to be isolated, in order to adjust the local response to future global sea-level rise. However, the further breakdown of the individual components to the total vertical land motion is not essential for hazards assessment. As shown in Table 4 of this NDP, coastal segments with local subsidence trends of +2 mm/yr or more are at greater risk.

Long-term Geologic Trends (SLG)

The coastal region north of the Mendocino triple junction, California, marked by the convergence of three tectonic plates (e.g., the North American, Pacific, and Juan de Fuca-Gorda Plates), is characterized by oblique, offshore crustal subduction, with convergence rates of 30-50 mm/yr. In contrast, the coast to the south of the triple junction is dominated by right-lateral strike-slip motion (up to 56 mm/yr) associated with the San Andreas fault system. Despite this pronounced difference in tectonic style of deformation, raised Quaternary marine terraces indicate uplift along most of the West Coast, both north and south of the triple junction (Figure 1), except where active transverse structures deform the coast (Goldfinger et al. 1992), often resulting in subsidence (Table 1). Evidence for subsidence caused by a number of discrete seismic events, one as recent as 1700, is recorded in sediments from marshes and bays (e.g., Atwater 1987; Atwater et al. 1991; Darienzo et al. 1994; Peterson et al. 1997; Table 1).

Late Quaternary (<125,000 years) raised marine terraces, which occur along much of the West Coast, integrate the permanent uplift caused by multiple earthquake cycles over the last few hundred thousand years. Knowing the present elevation and age of at least one raised terrace, and the paleosealevel position at that time relative to the present mean sea level, one can calculate an average long-term uplift rate (Lajoie et al. 1991). Thus, raised terrace data can be used to derive a long-term average uplift trend. On the whole, late Quaternary uplift rates along the Pacific coast from Washington to California are low (<0.6 mm/yr; Figure 1; Table 1), except in the vicinity of the Mendocino triple junction, the Ventura anticline and Cape Blanco.

Correction of Relative Sea-Level Trends

The corrected sea-level trend (SLC) for each tide-gauge is obtained by subtracting the long-term geologic trend (SLG), the glacial isostatic trend (I), and short-term (neotectonic) movements (Ts) from the relative sea-level trend (SLR).

(3)

SLC = SLR SLG I Ts

In passive plate margin settings such as the U.S. East Coast, the corrected sea-level trend (SLC) provides an approximation of the recent eustatic sea-level rise (Gornitz and Seeber 1990; Gornitz 1995).

Glacial isostatic trends (I) (based on the ICE-3G model of Tushingham and Peltier 1991; Douglas 1991) are not available for all of the tide-gauge stations in Table 2. Furthermore, the residual isostatic motions predicted by the ICE- 3G model are not in agreement with geological field observations which suggest little or no isostatic movements within the last 6000-7000 years in the Pacific Northwest (Mathews et al. 1970; Dethier et al. 1995). An improved glacial isostatic model, ICE-4G (Peltier 1994), has not yet been applied to tide-gauge data. For these reasons, no isostatic corrections are made, and I is assumed to be zero. Thus, SLC = SLR SLG Ts.

On the tectonically-active West Coast, the SLC term may still include a recent neotectonic component, Ts. Satellite-based geodetic techniques, such as GPS, are being used to resolve the inherent ambiguity between sea-level variations and vertical crustal motions, by establishing the absolute land motion with respect to the earth's center (Baker 1993). These space-geodetic methods will yield an independent measure of the total vertical motion, including Ts.

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