Empirical relationships between instrumental ground motions and observed intensities for two great Chilean subduction zone earthquakes

Abstract:

We determine empirical relationships between instrumental peak ground motions and

observed intensities for two great Chilean subduction earthquakes: the 2010 Mw8.8 Maule

earthquake and the 2014 Mw8.2 Iquique earthquake. Both occurred immediately offshore on the

primary plate boundary interface between the Nazca and South America plates. They are among the

largest earthquakes to be instrumentally recorded; the 2010 Maule event is the second largest

earthquake to produce strong motion recordings.

Ground motion to intensity conversion equations (GMICEs) are used to reconstruct the distribution of shaking for historical earthquakes by using

intensities estimated from contemporary accounts. Most great (M>8) earthquakes, like these, occur

within subduction zones, yet few GMICEs exist for subduction earthquakes. It is unclear whether

GMICEs developed for active crustal regions, such as California, can be scaled up to the large M of

subduction zone events, or if new data sets must be analyzed to develop separate subduction

GMICEs.

To address this question, we pair instrumental peak ground motions, both acceleration

(PGA) and velocity (PGV), with intensities derived from onsite surveys of earthquake damage made

in the weeks after the events and internet-derived felt reports. We fit a linear predictive equation

between the geometric mean of the maximum PGA or PGV of the two horizontal components and intensity,

using linear least squares. We use a weighting scheme to express the uncertainty of the pairings

based on a station's proximity to the nearest intensity observation.

The intensity data derived from the onsite surveys is a complete, high-quality investigation of the earthquake damage. We perform

the computations using both the survey data and community decimal intensities (CDI) calculated

from felt reports volunteered by citizens (USGS "Did You Feel It", DYFI) and compare the results.

We compare the GMICEs we developed to the most widely used GMICEs from California and central US

earthquakes, and global earthquakes. Existing GMICEs consistently over-predict intensity for these two subduction events. This may be a regional difference, or a magnitude-dependent

effect. Currently, however, there is not enough data from these great subduction earthquakes to

prefer one interpretation over the other.