MONTHLY DINNER MEETING
Date: Tuesday, June 8, 2004
Location: Steven's Steak House, 5332 Stevens Place, Commerce, California
Time: 6:00 p.m.-Social Hour; 7:00 p.m.-Dinner; 7:45
p.m.-Presentation
Cost: $25 per person with reservations, $30 at the door, $12 for students with a valid Student ID
Reservations: Call (949) 253-5924 ex 564, or email Brian Villalobos,
By noon, Monday, June 7, 2004
SPEAKER: Dr. Maria Teresa Ramirez
TOPIC: "Active tectonics, the Mexican subduction zone and the Pacific coast of Mexico: the Guerrero Gap and Jalisco Coast"
ABSTRACT
The Guerrero gap has a remarkable tectonic setting within a complex convergent margin - the Rivera-Cocos
subduction zone. This tectonically active margin is characterized by the Rivera-Cocos plate subduction beneath the North American
plate in southern Mexico. Subduction of the Rivera-Cocos plate under southern Mexico has generated more than 20 Ms> 7 earthquakes
recorded during the last century. Rapid 5-7 cm/yr convergence generates major earthquakes on the subduction megathrust at 30-100
year intervals. The potential for major seismic events is confirmed by the great 1932 (Ms= 8.1) Jalisco earthquake, the 1995
(Mw =8.0) Colima-Jalisco event, and by the 1985 (Mw= 8.1) Michoacan earthquake that devastated densely populated areas of Mexico
City (population ~20 million). The Michoacan earthquake caused ~ 3000 deaths and economic losses due to property damage amounted
to hundreds of millions of dollars. The amplification of seismic waves in the lacustrine sediment-filled basin produced long
period waves in Mexico City, located more than 300 km from the epicenter. Currently, the Guerrero seismic gap stands out as
region of high seismic potential. Major seismic events are known to occur in Guerrero, but none have occurred since the major
Guerrero 1911 (Ms=7.6) earthquake. Since the 1911 event, > 5m of plate convergence has generated only a few Ms<6 events near the
boundaries of this segment. The Guerrero seismic gap is therefore currently considered most likely to experience a severe
earthquake of estimated Mw = 8.1 to 8.4, or several smaller events.
Stratigraphic and geochemical indicators of paleosalinity (Br > Sr > Ca >>Na > Cs >U) indicate changes in elevations relative to
sea level, and suggest suddenness of these changes (e.g. coseismic events). Seven marine inwash events, and possibly three
tsunami events are recorded by stratigraphic/sediment, and paleosalinity indicators. Microfossil analyses (in progress) are being
used to confirm whether these horizons record marine inwash events (tsunamis). Radiometric measurements of 210Pb, 136Cs and 14C
isotopes are used to date these events. Radiocarbon dates imply a sedimentation and subsidence rate of ~ 1 mm/yr.
The Pacific coast of Jalisco, SW Mexico, has been the site of two of the largest earthquakes to occur in Mexico this century:
the 1932 (Mw 8.2) Jalisco earthquake, and the 1995 (Mw 8.0) Colima earthquake. Measurement and radiocarbon dating of emergent
paleoshorelines along the Jalisco coast provide the first constraints upon the timing for tectonic uplift. Along this coastline,
uplifted Holocene marine notches and wave-cut platforms occur at elevations ranging from ca. 1 to 4.5 m amsl. In situ intertidal
organisms dated with radiocarbon, the first dates ever reported for this area, and elevations of marine notches record tectonic
uplift during at least the past 1300 YBP at an average rate of about 3 mm/yr.
BIO
Dr. Ramirez is a geomorphologist and an expert in active tectonics
in central and southern Mexico. She has particular experience in the investigation and assessment of active faults, landslides,
and coastal erosion. She received her PhD from the University of Edinburgh, a M.Sc. from the National Autonomous University of
Mexico (UNAM), and a B.Sc. and M.Sc. from the State University of Moscow "M.V. Lomonosov." She has worked internationally in
the USA, Russia, Mexico, Argentina and Cuba. In addition to her duties as an Assistant Professor of geomorphology at CSULB, she
has been the principal investigator for research studies funded by the National Science Foundation, National Geographic Society,
the Mexican National Council for Science and Technology, and The Carnegie Trust.
