indo–us workshop on “intraplate seismicity” - The CERI Blog

Most studies utilize a wide range of seismic data that are acquired with multiple types of
seismographs and sensors and highly variable sources (earthquakes), all of which can
significantly affect the measured PGV. In this study, we investigate differences in seismic P-­‐
wave propagation (peak ground velocity -­‐ PGV) in intraplate (central US) versus and
interplate (California) settings by comparing data in each setting using a matched set of
seismographs and matched (active) sources. For seismic sources, we utilized 3000-­‐lb
explosions in boreholes of essentially the same depth and diameter, and to record the data,
we utilized a set of seismographs with 2-­‐Hz vertical velocity sensors at a constant spacing of
1 and 2 km over distances of 150 to 200 km.
Our data show that PGV for the Central US and for California are similar within the first 100
km, but the peak velocities differ significantly beyond that range. In the first 150 km,
differences in the PGV are largely related to crustal and Moho reflections, which persist to
distances of ~150 km in the central US but only to about 100 km in California. Beyond 100
km, PGV is about 5 to 10 times greater in the central US than in the western US, with little
decrease in PGV for distances up to 200 km.
Our present observations are limited to only one profile in each type of setting, however,
these observations suggest that differences in attenuation and crustal structure are the
primary factors accounting for the differences in PGV in intraplate (central US) and
interplate (western US) settings. In general, intraplate settings have colder, less-­‐fractured
crust, which contributes to the lower attenuation (1/Q). Furthermore, the crust is generally
thicker in intraplate settings (Mooney et al., 1998), resulting in crustal and Moho reflections
that cause higher values of PGV at greater epicentral distances. These results are consistent
with studies indicating that stress drop is not a major factor in the difference between PGV
in intraplate and interplate settings.
S2.3
Tectonic Setting and Indian Shield Seismicity – Sources and Mechanisms
R K Chadha (Email:chadha@ngri.resin) and D Srinagesh (Email:srinagesh@ngri.res.in)
National Geophysical Research Institute, Hyderabad-­‐500 007, India.