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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS006 Oral Presentation 1933 A New Approach to Paleoseismic Event Correlation - Dr. Glenn Biasi Seismological Laboratory University of Nevada Reno Ray J. Weldon When there is more than one paleoseismic site on a fault, the issue arises of correlating events between them. As the number of paleoseismic sites increases, the number of possible correlations and combinations among them becomes unmanageable. Correlation based on dating evidence alone remains uncertain even with high quality radiocarbon evidence. These realities seriously complicate the use of paleoseismic data for estimation of recurrence intervals and seismic hazard. We present a new approach that permits estimates of seismic hazard and multiple-site earthquake recurrence without having to solve the correlation problem per se. The most important input data for our approach are reasonably complete paleoseismic records and approximate dates for the events. To begin we develop a complete list of all possible single and multiple-site ruptures allowed within the event dating uncertainties. Each individual paleoseismic event is counted as a rupture. We compare each event with the neighboring site chronology, and make another rupture of any which overlap with the first. If two overlap in time, both become new ruptures. Each rupture is extended in turn to a third site chronology to make new rupture(s). Lack of dating overlap keeps this process from expanding as the outer product of the number of events. Ruptures are also extended through sites with no evidence of the rupture, but penalized for contradicting the paleoseismic record there. The pool of all possible ruptures thus includes all the available event data for the fault. Paleoseismic displacement estimates or displacements drawn from an average displacement model are used to extrapolate ruptures beyond the outside sites in each rupture. Rupture length is used with regression relations to assign each rupture an average displacement and simple displacement profile. To constrain the rupture history of the fault we construct rupture scenarios by drawing from the pool of all possible ruptures. Sampling is constrained so that every event is included each scenario exactly once. The scenario becomes a possible rupture history for the fault at some level of probability. A large suite of scenarios is developed in this manner. To extract likely scenarios from the suite, scenarios are graded for consistency with dating evidence (do dates overlap strongly or weakly) and by comparing the displacement totaled over the ruptures at any given point on the fault with the total predicted independently from the geodetic or geologic slip rate and elapsed time since the oldest event. Other grading criteria may also be applied, such as by using perevent displacement measurements. Ensembles of scenarios become more or less likely actual histories of the fault. They may then be used directly in seismic hazard assessment using ground motion prediction methods, or indirectly to estimate the likely fractions of shorter or longer ruptures, frequency of multiple segment ruptures, etc. We illustrate the method with paleoseismic data from the southern San Andreas fault in California. Results have been provided as inputs to the most recent assessments of the Working Group on California Earthquake Probabilities. Keywords: paleoseismology, seismic hazard, san andreas fault

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS006 Oral Presentation 1934 The seismic potential of a silent and slow moving fault: the Carboneras fault (Eastern Betic Shear Zone, southern Spain) Dr. Eullia Masana Ximena Moreno, Eullia Grcia, Pere Santanach, Hector Perea, Joao Cabral, Patricia Ruano, Oriol Piqu, Raimon Palls Africa and Iberia converge at a very slow rate (4.5 to 5.6 mm/yr) through a diffuse collisional plate boundary. The seismicity is low to moderate and widespread over a very wide area suggesting a number of slow moving faults. Because of this, many of the seismogenic faults in Iberia have not yet generated a large historical earthquake and remain may hidden. We aim to detect these and characterise their seismic potential. The seismogenic behaviour of the Alhama de Murcia fault has yet been evidenced by previous paleoseismic studies. This fault belongs to the Eastern Betic Shear Zone (EBSZ), composed by several left-lateral and reverse faults that crop out from Alicante to the north down to Almeria to the south (at both tips the system is entering the Mediterranean). The morphological signature of the faults composing the EBSZ is similar to that of the Alhama de Murcia suggesting than the rest of the faults may also be active and possibly seismogenic. The Carboneras fault, the aim of this study, is the southernmost fault of the system. It shows very few instrumental earthquakes but its morphological expression indicates recent left-lateral slip. After a geomorphological study, based on aerial photograph analysis and field work, the northwestern side of La Serrata range was selected, as the most suitable area for a detailed paleoseismological study. We present the first paleoseismological results at the El Hacho site, to the south of La Serrata range. Microtopography, GPR, Electric tomography, and a detailed geomorfological analysis were used to select the best sites to trench and to understand the recent behaviour of the fault. Four trenches across the fault and a 3D trench where analysed. Evidence of a minimum of four reverse component paleoearthquakes where interpreted based on sealed fault branches and colluvial wedges. TL, U/Th and radiocarbon dating indicated that the four events have taken place since 55 ka BP and previous to 1180 yr BP. This suggests a mean recurrence time of 14 ka and a very short elapsed time. Marine studies (swath-bathymetry, TOPAS, high-resolution MCS, magnetic, gravity and core sampling) are also in course and will be integrated to the onshore data to obtain a complete image of the behaviour of the fault. The offshore change in strike of fault was interpreted as a possible segment boundary. Down to this bouondary, the maximum magnitude that the Carboneras fault could generate is Mw 7.2 although no evidence is found about the effectiveness of the mentioned segment boundary. The paleoseismic results will contribute to obtain more realistic values of the seismic hazard of this highly populated area. The methodological results are also of interest in the Iberian Peninsula where we suppose that many seismogenic faults are historically silent and at the same time they enter the sea and need to be studied integrating onshore-offshore data. Keywords: eastern betic shear zone, carboneras fault, slow moving faults

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