IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS006 Poster presentation 1944
NEotectonic derived models for crustal deformation in a stable continental
region setting: insight from the Southwest Of western Australia.
Dr. Mark Leonard
Geoscience Australia Geohazards IASPEI
Dan Clark
In recent times, high resolution digital elevation models (DEMs) have emerged as an important tool for
finding and characterising earthquake related geomorphology, and particularly fault scarps. The results
of a reconnaissance investigation of two DEM datasets covering a large portion of southwest and central
Western Australia are presented. A total of thirty-three new fault scarps of probable Quaternary age
have been identified, bringing the total number of neotectonic features in the area to sixty. The scarps
are spatially isolated and range in length from ~15 km to over 45 km, and from ~1.5 m to 20 m in
height. Most scarps where a displacement sense could be determined from the DEM data suggest
reverse displacement on the underlying fault. In the few instances where high-resolution aeromagnetic
data is coincident with a scarp location the ruptures are seen to exploit pre-existing crustal weaknesses.
Twenty-one of the features have been verified as fault scarps by ground-truthing, and range in
apparent age from perhaps less than a thousand years to many tens of thousands of years. Only four
have been quantitatively examined to determine source parameters (e.g. timing of events, recurrence,
magnitude). However, three important characteristics are revealed in the extant data: 1) recurrence of
surface breaking earthquakes on an individual fault is typical (ie. areas hosting active fault scarps are
earthquake-prone), 2) temporal clustering of events is apparent on many faults (ie. large earthquake
recurrence in active phases might be much less than during inactive phases), and 3) significantly larger
events than have been seen in historic times (MW>7.2) might be expected in the future, Australia-wide.
This rich neotectonic record also provides an opportunity to understand the characteristics of intraplate
deformation at the scale of the entire Precambrian shield region (the Yilgarn Craton). An uniform
distribution of the northerly trending scarps suggests that strain is uniformly accommodated over the
Yilgarn Craton at geologic timescales, and that the easterly-trending compressive contemporary stress
field has pertained for hundreds of thousands of years or more. This evidence supports a model
whereby the lower, ductile part of the lithosphere is uniformly strong and deforms uniformly, and the
upper (seismogenic) layer accommodates this large-scale flow by localised, transient and recurrent
brittle deformation in zones of pre-existing crustal weakness. The proposed model implies uniform
seismic hazard across the southwest of Western Australia, but at a timescale much greater than useful
for most seismic hazard assessment applications. Palaeoseismological data on individual faults is playing
an important role in bridging the gap between this seismicity model and the historic record of seismicity
in , upon which all current seismic hazard assessments are based.
Keywords: neotectonic, paleoseismicity, earthquake