[S05-2] Preservation and usage of analog seismogram archives II

[S05-2-05] Observations of large earthquakes in the Mexican subduction zone over 110 years

^○Vala Hjorleifsdottir¹, Shri Krishna Singh¹, Bjorn Lund², Chen Ji³ (1.Universidad Nacional Autonoma de Mexico, Mexico City, Mexico, 2.Department of Earth Sciences, Uppsala University, Sweden, 3.Department of Earth Science, University of California, Santa Barbara, USA)

Fault slip during an earthquake is observed to be highly heterogeneous, with areas of large slip interspersed with areas of smaller or even no slip. The cause of the heterogeneity is debated. One hypothesis is that the parts of the rupture surface that have large slip during earthquakes are coupled more strongly, whereas the areas in between creep continuously or episodically, partially releasing strain energy during the interseismic period. This would lead to subsequent earthquakes having large slip in the same place, or persistent asperities. A second hypothesis is that in the absence of creeping sections, the prestress is governed mainly by the accumulative stress change associated with previous earthquakes. Assuming homogeneous frictional properties on the fault, a larger prestress results in larger slip, i.e. the next earthquake may have large slip where there was little or no slip in the previous earthquake, which translates to non-persistent asperities.

The study of earthquake cycles are hampered by short time period for which high quality, broadband seismological records, needed for detailed studies of slip distributions, are available. The earthquake cycle in the Mexican subduction zone is relatively short, with about 30 years between large events in many places. We are therefore entering a period for which we have good records for two subsequent events occurring in the same segment of the subduction zone.

In this study we compare seismograms recorded either at the Wiechert seismograph or on a modern broadband seismometer located in Uppsala, Sweden for earthquakes in the Mexican subduction zone rupturing the same patch. The Wiechert seismograph is unique in the sense that it recorded continuously for more than 80 years, without changes in the instrument response. In total we have registers from more than 20, M>6.9 earthquakes in seven along trench segments.

Our observations indicate that some asperities are persistent over time, whereas others are not.