Geology topics

K.B. Olsen

Publications and source records attributed to K.B. Olsen.

4 recordsLinked to original sources

USGS2010

The ShakeOut earthquake scenario: Verification of three simulation sets

This paper presents a verification of three simulations of the ShakeOut scenario, an M w 7.8 earthquake on a portion of the San Andreas fault in southern California, conducted by three different groups at the Southern California Earthquake Center using the SCEC Community Velocity Model for this region. We conducted two simulations using the finite difference method, and one by the finite element method, and performed qualitative and quantitative comparisons between the corresponding results. The results are in good agreement with each other; only small differences occur both in amplitude and phase between the various synthetics at ten observation points located near and away from the fault-as far as 150 km away from the fault. Using an available goodness-of-fit criterion all the comparisons scored above 8, with most above 9.2. This score would be regarded as excellent if the measurements were between recorded and synthetic seismograms. We also report results of comparisons based on time-frequency misfit criteria. Results from these two criteria can be used for calibrating the two methods for comparing seismograms. In those cases in which noticeable discrepancies occurred between the seismograms generated by the three groups, we found that they were the product of inherent characteristics of the various numerical methods used and their implementations. In particular, we found that the major source of discrepancy lies in the difference between mesh and grid representations of the same material model. Overall, however, even the largest differences in the synthetic seismograms are small. Thus, given the complexity of the simulations used in this verification, it appears that the three schemes are consistent, reliable and sufficiently accurate and robust for use in future large-scale simulations. ?? 2009 The Authors Journal compilation ?? 2009 RAS.

Geophysical Journal International↗

USGS2009

The 1911 M ~6.6 Calaveras earthquake: Source parameters and the role of static, viscoelastic, and dynamic coulomb stress changes imparted by the 1906 San Francisco earthquake

The occurrence of a right-lateral strike-slip earthquake in 1911 is inconsistent with the calculated 0.2-2.5 bar static stress decrease imparted by the 1906 rupture at that location on the Calaveras fault, and 5 yr of calculated post-1906 viscoelastic rebound does little to reload the fault. We have used all available first-motion, body-wave, and surface-wave data to explore possible focal mechanisms for the 1911 earthquake. We find that the event was most likely a right-lateral strikeslip event on the Calaveras fault, larger than, but otherwise resembling, the 1984 M w 6.1 Morgan Hill earthquake in roughly the same location. Unfortunately, we could recover no unambiguous surface fault offset or geodetic strain data to corroborate the seismic analysis despite an exhaustive archival search. We calculated the static and dynamic Coulomb stress changes for three 1906 source models to understand stress transfer to the 1911 site. In contrast to the static stress shadow, the peak dynamic Coulomb stress imparted by the 1906 rupture promoted failure at the site of the 1911 earthquake by 1.4-5.8 bar. Perhaps because the sample is small and the aftershocks are poorly located, we find no correlation of 1906 aftershock frequency or magnitude with the peak dynamic stress, although all aftershocks sustained a calculated dynamic stress of ???3 bar. Just 20 km to the south of the 1911 epicenter, we find that surface creep of the Calaveras fault at Hollister paused for ~17 yr after 1906, about the expected delay for the calculated static stress drop imparted by the 1906 earthquake when San Andreas fault postseismic creep and viscoelastic relaxation are included. Thus, the 1911 earthquake may have been promoted by the transient dynamic stresses, while Calaveras fault creep 20 km to the south appears to have been inhibited by the static stress changes.

Bulletin of the Seismological Society of America↗

USGS2008

3D crustal structure and long-period ground motions from a M9.0 megathrust earthquake in the Pacific Northwest region

We have developed a community velocity model for the Pacific Northwest region from northern California to southern Canada and carried out the first 3D simulation of a M w 9.0 megathrust earthquake rupturing along the Cascadia subduction zone using a parallel supercomputer. A long-period (<0.5 Hz) source model was designed by mapping the inversion results for the December 26, 2004 Sumatra–Andaman earthquake (Han et al., Science 313(5787):658–662, 2006) onto the Cascadia subduction zone. Representative peak ground velocities for the metropolitan centers of the region include 42 cm/s in the Seattle area and 8–20 cm/s in the Tacoma, Olympia, Vancouver, and Portland areas. Combined with an extended duration of the shaking up to 5 min, these long-period ground motions may inflict significant damage on the built environment, in particular on the highrises in downtown Seattle.

Journal of Seismology↗

USGS2001

Fault zone amplified waves as a possible seismic hazard along the Calaveras fault in central California

The Calaveras fault lies within a low velocity zone (LVZ) 1-2 km wide near Gilroy, California. Accelerographs G06, located in the LVZ 1.2 km from the Calaveras fault, and G07, 4 km from G06, recorded both the M 6.2 1984 Morgan Hill and the M 6.9 1989 Loma Prieta earthquakes. Comparison of the ground motions shows that a large 0.6-1.0 Hz velocity pulse observed at G06 during the Morgan Hill event may be amplified by focussing caused by the LVZ. Such amplified waves might be a mappable seismic hazard, and the zone of increased hazard can extend as much as 1.2 km from the surface trace of the fault. Finite-difference simulations of ground motions in a simplified LVZ model show a zone of amplified motion similar to the observations.

Geophysical Research Letters↗