NBMG Talks – Geoff Blewitt – Monday Sept 23
Title: Observed Solid Earth Contributions to Sea Level Variation in Western US
Speaker: Geoff Blewitt, Nevada Bureau of Mines and Geology, University of Nevada, Reno
Date: Monday September 23, 2013 (lecture at 4:00, picnic to follow at Idlewild Park)
Place: DMS (Davidson Math and Science Center) Room 102 on the UNR campus
Abstract: Satellite altimetry over the last two decades has measured variations in geocentric sea level (GSL), allowing us to test models of physical oceanography and global climate change. Global mean GSL is currently rising the level of ~3 mm/yr, and appears to be accelerating. The societal impacts of sea level change however relate to variations in local sea level (LSL), relative to the land at the coast. Therefore, assessing the impacts of sea level change requires coastal measurements of vertical land motion (VLM). Indeed, ΔLSL = ΔGSL -ΔVLM, with subsidence mapping 1:1 into LSL.
Here GPS geodetic data are used within 15 km of the US west coast to infer regional, secular VLM, which is found to have different behavior north and south of the Mendocino Triple Junction (MTJ). The California coast has a coherent regional pattern of subsidence averaging 0.5 mm/yr, with an increasing trend to the north. This trend generally matches GIA model predictions of forebulge collapse. Around San Francisco Bay, the observed coastal subsidence of 1.0 mm/yr coherently decreases moving away from the Pacific Ocean to very small subsidence on the east shores of the bay. This gradient is likely caused by San Andreas-Hayward Fault tectonics, and possibly by differential surface loading across the bay and Sacramento-San Joachim River Delta. Thus in addition to the trend in subsidence from GIA going northward along the California coast, tectonics may also play a role where the plate boundary fault system approaches the coast.
In contrast, VLM of the coast north of the MTJ in Oregon and Washington is found to have the opposite sign (uplift) and varies with distance between the coast and the trench. The uplift peaks at 2-3 mm/yr in northern Washington. This pattern can be explained by elastic strain accumulation at the locked subduction zone, coupled with a contrast in rheological structure affecting GIA.
In terms of LSL and hence societal impact, the observed mean California subsidence of 0.5 mm/yr approximately cancels with GIA predictions of global GSL lowering at a similar rate. This GSL lowering is caused by “continental levering,” in which the volume of ocean basins is increasing today as the mantle flows away from under the oceans in isostatic response to >100 m of sea level rise following Pleistocene deglaciation. So the net LSL in California caused by coastal VLM plus GSL lowering by GIA is ~0.5 mm/yr LSL rise to the north, and ~0.5 mm/yr lowering to the south. In northern Washington, the net LSL is inferred to be lowering at a rate of ~3 mm/yr.
This methodology establishes a base level of LSL on top of which will be the imprint of global sea level rise from global climate change, including ice-sheet melt, thermal expansion, and changes in ocean and atmospheric circulation. Our observed regional variation in land motion are of similar magnitude as the effects of global change on sea level, and so we conclude that VLM must be taken into account. Given that UNR’s GPS time series are selected from a globally consistent set from >11,000 stations (the largest such dataset in the world), this methodology could be applied in a seamless way over coastlines across the globe, depending on available station coverage.
Upcoming Dept. of Geology talks at UNR:
9/30/2013 GRC/RMR No Seminar
10/7/2013 Tobias Fischer TBA Univ. of New Mexico
10/14/2013 Alex Morlean / Sabina Kraushaar Graduating MS student presentations
10/21/2013 Corne Kreemer Global Strain Rate Map UNR / NBMG – more info coming soon
10/28/2013 GSA No Seminar
11/4/2013 Erica Keys(Helton) / Lyndsay Hazelwod Graduating MS student presentations
11/11/2013 Veterans Day Holiday No Seminar
11/18/2013 Juliet Crider TBA University of Washington
11/25/2013 Tom Rockwell TBA San Diego State University
12/2/2013 Eric Cowgill TBA UC Davis
12/9/2013 AGU No Seminar
12/16/2013 Kyle Gray Graduating MS student presentations
For more information on these talks, please contact Wendy Calvin:
Prof. Geophysics & Remote Sensing
Director, Great Basin Center for Geothermal Energy
Univ. Nevada – Reno, email@example.com, 775-784-1785