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Re: Volcanic Earthquakes

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Relative to Hawaii seismicity, volcanoes, and near-fault effects:

The largest earthquakes in Hawaii are associated with the movement of the mobile flanks of the active volcanoes, but are not directly caused by volcanic activity or movement of magma. The larger earthquakes of crustal origin on the island of Hawaii, such as M7.9 1868 and M7.2 1975, were caused by seaward slips of large sections of the flank of the volcano along nearly horizontal fault rupture planes(termed the decollement, a term of French origin) near the base of the volcano, at about 10 km depth. Other large earthquakes(up to M 6-7) of mantle origin can be caused by flexural readjustment of the underlying lithosphere to the changing load imposed by the volcanic mass; these events can occur throughout the Hawaiian islands. The small events produced by movement of magma are not of interest to structural design.

The new seismic hazard maps in the International Building Code 2000 include the near-field effects for the island of Hawaii. In these cases the distance to the fault rupture is near vertical(down 10 km) to a very large areal rupture zone, as opposed to the California strike-slip or blind thrust fault rupture.

You can also view the probabilistic seismic hazard maps for Hawaii at the USGS National Seismic Hazard Mapping Project website at:
http://gldage.cr.usgs.gov/eq/html/his.shtml
However, bear in mind that these maps are probabilistic, are calibrated to rock sites, and do not incorporate the deterministic enveloping rules of the NEHRP 1997/IBC 2000. Also, the documentation posted is superseded; final documentation is underway at Menlo Park(for next year).

The 1997 UBC does not incorporate any of the above mapping information, which was developed by USGS, the Hawaii State Earthquake Advisory Board(HSEAB), and BSSC in 1997-1998. However, the 1997 UBC did incorporate a zone 4 change to the island of Hawaii based on earlier studies by HSEAB. One can consider the USGS 10%/50 acceleration maps as indicative of near-field effects should you need to take this into account in a UBC manner.

Gary Chock, S.E.
HSEAB member
Martin, Bravo & Chock, Inc.

At 12:42 PM 10/14/98 -0700, my former colleague Fred Turner wrote:
>>From Bruce Bolt's book titled "Earthquakes" describes a magnitude 7.2 event
>in 1975 on the Big Island that produced more than $4 million in damage. It
>triggered a tsunami and was later followed by eruptions at Kilauea. These
>particular events were caused by fault movement where the fault appeared to
>trigger the subsequent tsunami and volcanic activity. No doubt, near source
>effects can be produced by such events.
>
>The minor, more harmonic seismic activity that Rick Ranous refered to can
>also occur in the vicinity of volcanoes as the magma moves below the
>surface. These events are used to track changes in magma movement in
>attempts at monitoring volcanoes for impending eruptions.
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Martin, Bravo, & Chock, Inc.
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