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| Titel |
Small earthquakes on the San Andreas fault that have magnitudes controlled by slip alone |
| VerfasserIn |
R. M. Harrington, E. E. Brodsky |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250020908
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| Zusammenfassung |
| Ordinarily, earthquake magnitude is controlled by both rupture length and slip variation. Here
we show that a special population of earthquakes has a constant rupture length, but varying
slip. We compare the source time function pulse widths of 25 earthquakes on the San Andreas
fault, and 11 earthquakes on surrounding secondary faults to show that the earthquakes on the
San Andreas fault near Parkfield have an approximately constant duration in this group with
magnitudes ranging from 1.4 to 3.7. We determine the source time function of the
earthquakes via an empirical Green’s function deconvolution. Examination of the direct
P-wave arrivals in the raw data confirms the constant duration observation determined
by the empirical Green’s function method. In contrast, earthquakes on secondary
faults indicate the more usual source parameter scaling suggestive of a constant
stress drop, i.e. they have an increase in duration with magnitude. The earthquakes
on the San Andreas fault are located approximately 20 km to the northeast of the
1966 mainshock epicenter, along the fault, to approximately 5 km south of the
2004 epicenter. Unlike previously studied repeating sequences, the magnitudes are
not constant, nor is the repeat time regular. The secondary faults are located at
distances of 5 km or greater from the trace of the San Andreas fault, and are almost
certainly not part of the active or historically active plate boundary fault system.
The constant source duration observation for the earthquakes on the San Andreas
fault suggests that fault area stays constant over the magnitude range of our data
set.
A repetitive rupture of a small, locked asperity in a creeping fault can explain the constant
duration. The dimension of the asperity could pre-determine the fault area. Therefore the
observation directly measures the scale of the heterogeneities on the fault. We observe
heterogeneities of 120, and 160 m in diameter.
Calculated stress drop values of the earthquake population on the San Andreas fault range
from 0.18 MPa to 58 MPa, and values on secondary faults range from 0.76 MPa to 14 MPa.
The differences in duration scaling between the events on the San Andreas fault and on
secondary faults suggest that earthquakes on the San Andreas fault are inherently different.
Cumulative slip values on the secondary faults are negligible in comparison to cumulative
slip values on the San Andreas fault. We speculate that faults with more cumulative
displacement have earthquakes which may rupture differently. Differences in rupture
dynamics between the two populations might be explained by differences in fault surface
roughness. |
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