Structural /Subsidence 



8. Tectonic activity is an important consideration in the assessment of 

 sea- level change for areas undergoing crustal movement. Based on long- or 

 short-term time scale, vertical movement of land is due to changes in the 

 pattern of surface loads , either through changes in sea level or from movement 

 of the land. Crustal movement occurs over both long- and short-term time 

 spans. Long-term processes, such as seafloor spreading, displace ocean basin 

 volume resulting in a sea- level rise. Conversely, episodic events such as 

 earthquake and volcanic activity displace land over a short time period often 

 decreasing relative sea- level change. Ridge development and continental 

 margin subsidence also contribute to low- frequency eustatic sea- level 

 fluctuations , whereas glaciation contributes to the higher frequency portion 

 of the sea- level curve (Guidish et al . 1984). 



9. Sea- level changes that result from tectonic activity or subsidence 

 are highly variable according to geographic location. Classical case studies 

 indicating distinct trends are described for the following coastlines: 



a. South-central coast of California has experienced high uplift 

 rates of 1 to 6 millimeters/year along the marine terraces. 



b. Hicks, Debaugh, and Hickman (1965) reported uplift rates 

 between 0.37 and 3.96 centimeters/year in southeast Alaska, 

 which experiences frequent earthquake activity and glacial 

 rebound. These measurements are based on continuous sea- level 

 data collected for the period 1909-1962. 



c. Tide -gage readings along the mid- Atlantic states have high 

 rates of sea- level rise as compared with the southern east 

 coast, primarily due to subsidence along the crest of the 

 peripheral forebulge. 



d. The coastline of Japan has experienced anomalously high rates 

 (20 millimeters/year) of relative sea-level rise. Aubrey and 

 Emery (1986) applied regression and eigenanalysis to the local 

 tide gage records for the past 50 years. The resultant sea- 

 level pattern reflects the subduction activity along the 

 Pacific and Philippine plate. 



National Effects (North America) 



10. In general, most of the North American coastline is experiencing a 

 rise in relative sea level. Excellent sources of US tide gage measurements 

 are two reports published through National Oceanic and Atmospheric 



