California State University, Long Beach GeoDiversity Active Tectonics on Convergent Margins Dr. Ramírez, Geological Sciences

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In order to assess earthquake hazard, it is essential to reconstruct the recurrence patterns of large magnitude earthquakes during prehistoric time. Earthquake activity along coastlines has unique effects. Such coasts may rise or fall instantaneously during earthquakes (coseismic uplift or subsidence). Historic accounts of sudden uplift of coastlines during large earthquakes suggest that higher, older shorelines were also raised coseismically during recurrent earthquakes. If so, abandoned or relict shorelines record past earthquakes, and where two or more successive abandoned shorelines can be dated, the periodicity (earthquake recurrence) and relative magnitude of great earthquakes can be estimated. Abandoned shorelines are reflected on the landscape as uplifted marine terraces, wave-cut surfaces, beach ridges and marine notches. Landscape response to active tectonics (earthquakes) has been observed along the Pacific coast in South, Central and North America. Emergent Quaternary marine terraces along the California coast have been extensively studied to assess locations, styles and rates of Quaternary deformation (Hanson et al., 1994).

The Pacific coastal region of southern Mexico, which extends parallel to the Mexican subduction zone where the Cocos plate is consumed beneath the North America plate, presents a history of large magnitude earthquakes (M > 7) and is well suited for the study of coastal tectonics and ancient shorelines (Fig. 1a). The Oaxaca coast has experienced some of the largest and most damaging earthquakes recorded in the history of Mexico, more recently the September 30, 1999 earthquake (Fig. 1b). My research on the Pacific coast of southern Mexico has demonstrated that coastal landscape reflects active tectonic uplift along only certain segments of the coast. This landscape differentiation is correlated with lateral variations in tectonism and seismicity along the Mexican subduction zone (Ramirez-Herrera and Urrutia-Fucugauchi, 1999; Ramirez-Herrera et al., 1999; Ramirez-Herrera and Zamorano, submitted). The focus of my research, after accomplishing a reconnaissance study of the southern Pacific coast in Mexico and a detailed survey of the Michoacan coast, is to precisely measure and date marine terraces on the Oaxaca coast. This proposal is a continuation of this year's research (summer 2001), which allowed me to map and date ancient shorelines on the southern and northern sector of the Jalisco coast. I propose to extend my work to the Oaxaca coast to produce an integrated history of Quaternary uplift and deformation on this segment of the Pacific coast of Mexico. During the Summer 2002, I expect to interpret remotely sensed imagery of the Oaxaca coast in order to document features in the field and identify mapping targets and strategy.

Fig. 1.a.) Tectonic setting of the Pacific Coast, southern Mexico. Numbers equal years of large magnitude earthquakes (M>7), arrows show direction of plate movement. Fig. 1.b.) Oaxaca large earthquakes, ellipsoids show rupture zones.

During a 15 day-field season a Community College geologist, a student from CSULB Geology, and I will go to the field to survey marine terraces and search for samples suitable for radiocarbon and/or Uranium-Thorium series dating. If organic material is found, we will prepare the samples for radiocarbon and/or Uranium-Thorium dating. We aim to add more data on the chronologic record of ancient shorelines along the southern Mexico coast. Instruments required for aerial photo analysis (mirror stereoscope) and sample analysis (X-ray diffraction and microscope) are available in the department. If all goes well, preliminary results for dates for marine terraces and estimated rates of Quaternary deformation on the northern sector of the Oaxaca coast will be ready for presentation at the December 2002 meeting of the American Geophysical Union. However, due to amount of data to be analyzed, the completed research results will not be ready for publication until Summer 2003.

I would like to work with colleagues--geologists interested in this project. It is desirable, but not absolutely necessary, to have field-work experience, particularly in Quaternary geology and/or structural geology. This pioneering study of active coastal deformation in southern Mexico will provide research experience to the Community college-High School geologist on a quantitative field-based study and on precise dating of uplifted coasts (marine terraces, notches and elevated wave-cut platforms). It also has the potential to provide important insights into studies of sea-level changes, plate tectonics and earthquake potential along the Mexican subduction zone. This work should eventually result in several presentations at professional conferences and publication of 1-2 papers in such journals as Quaternary Research and Geology.