Seismic SourcesSeismic Sources

CEE 431/ESS465CEE 431/ESS465

Seismic SourcesSeismic Sources

Identification

Geologic evidenceField reconnaissanceTrench loggingTest pits, boringsAirphoto interpretationRemote sensingGeophysicsHistorical seismicity

Seismic SourcesSeismic Sources

Fault types

Seismic SourcesSeismic Sources

Source zones

May consist of mapped fault (known fault geometry)Areas of high seismicityAreas of shallow or outcropping bedrockAreas of sparse surficial vegetation

May consist of diffuse zone (unknown fault geometry)Areas of low seismicityAreas with significant sediment coverAreas with dense vegetation cover

California

Washington

Seismic SourcesSeismic Sources

Fault activity

Active fault?NRC definitions require:

Movement at or near the ground surface at least once within the past 35,000 yrs or movement of a recurring nature within the past 500,000 yrs

Macroseismicity instrumentally determined with records of sufficient precision to demonstrate a direct relationship with the fault; or

A structural relationship to a capable fault according to the previous two characteristics, such that movement on one could reasonably be expected to be accompanied by movement on the other

Seismic SourcesSeismic Sources

Magnitude indicators

M = f(E)

E should increase with increasing dimensions of rupture surface

Wells and Coppersmith (1994)Strike slip Mw = 5.16 + 1.12 log LReverse Mw = 5.00 + 1.22 log LNormal Mw = 4.86 + 1.32 log LAll Mw = 5.08 + 1.16 log L

Wells and Coppersmith (1994)Strike slip Mw = 3.98 + 1.02 log AReverse Mw = 4.33 + 0.90 log ANormal Mw = 3.93 + 1.02 log AAll Mw = 4.07 + 0.98 log A

Seismic SourcesSeismic SourcesSegmentation

Earthquake segments - those parts of a fault that have ruptured during individual earthquakes

Wasatch fault zone - Utah8 segmentsProvo segment (70 km) longest

Segment length (or area) can constrain magnitudeSegments bounded by discontinuities

Geometric discontinuities - abrupt changes in strike, stepovers, gapsStructural discontinuities - fault bifurcations, zones of increased structural complexity, intersections with other structuresBehavioral discontinuities - changes in slip rates, senses of displacement, creeping vs. locked behavior

Seismic SourcesSeismic SourcesSegmentation

Seismic SourcesSeismic SourcesPuget Sound area

Deepevents

Shallowevents

Seismic SourcesSeismic SourcesPuget Sound area

Knowncrustalfaults

Seismic SourcesSeismic SourcesPuget Sound area

Seattlefault

Seismic SourcesSeismic SourcesPuget Sound area

Intraplatesource

Seismic SourcesSeismic SourcesPuget Sound area

Interplatesource