Seismic Sources CEE 431ESS 465 Seismic Sources Identification

Seismic Sources CEE 431/ESS 465

Seismic Sources Identification Geologic evidence Field reconnaissance Trench logging Test pits, borings Airphoto interpretation Remote sensing Geophysics Historical seismicity

Seismic Sources Fault types

Seismic Sources Source zones May consist of mapped fault (known fault geometry) Areas of high seismicity Areas of shallow or outcropping bedrock Areas of sparse surficial vegetation California May consist of diffuse zone (unknown fault geometry) Areas of low seismicity Areas with significant sediment cover Washington Areas with dense vegetation cover

Seismic 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 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 L Reverse Mw = 5. 00 + 1. 22 log L Normal Mw = 4. 86 + 1. 32 L Wells andlog Coppersmith (1994) All Mw = 5. 08 + Strike 1. 16 log L M = 3. 98 + 1. 02 log A slip w Reverse Mw = 4. 33 + 0. 90 log A Normal Mw = 3. 93 + 1. 02 log A All Mw = 4. 07 + 0. 98 log A

Seismic Sources Segmentation Earthquake segments - those parts of a fault that have ruptured during individual earthquakes Wasatch fault zone - Utah 8 segments Provo segment (70 km) longest

Seismic Sources Segmentation Segment length (or area) can constrain magnitude Segments bounded by discontinuities Geometric discontinuities - abrupt changes in strike, stepovers, gaps Structural discontinuities - fault bifurcations, zones of increased structural complexity, intersections with other structures Behavioral discontinuities - changes in slip rates, senses of displacement, creeping vs. locked behavior

Seismic Sources Puget Sound area Shallow events Deep events

Seismic Sources Puget Sound area Known crustal faults

Seismic Sources Puget Sound area Seattle fault

Seismic Sources Puget Sound area Intraplate source

Seismic Sources Puget Sound area Interplate source