Geology topics

G. Anderson

Publications and source records attributed to G. Anderson.

5 recordsLinked to original sources

USGS2005

Constraints on fault slip rates of the southern California plate boundary from GPS velocity and stress inversions

We use Global Positioning System (GPS) velocities and stress orientations inferred from seismicity to invert for the distribution of slip on faults in the southern California plate-boundary region. Of particular interest is how long-term slip rates are partitioned between the Indio segment of the San Andreas fault (SAF), the San Jacinto fault (SJF) and the San Bernardino segment of the SAE We use two new sets of constraints to address this problem. The first is geodetic velocities from the Southern California Earthquake Center's (SCEC) Crustal Motion Map (version 3 by Shen et al.), which includes significantly more data than previous models. The second is a regional model of stress-field orientations at seismogenic depths, as determined from earthquake focal mechanisms. While GPS data have been used in similar studies before, this is the first application of stress-field observations to this problem. We construct a simplified model of the southern California fault system, and estimate the interseismic surface velocities using a backslip approach with purely elastic strain accumulation, following Meade et al. In addition, we model the stress orientations at seismogenic depths, assuming that crustal stress results from the loading of active faults. The geodetically derived stressing rates are found to be aligned with the stress orientations from seismicity. We therefore proceed to invert simultaneously GPS and stress observations for slip rates of the faults in our network. We find that the regional patterns of crustal deformation as imaged by both data sets can be explained by our model, and that joint inversions lead to better constrained slip rates. In our preferred model, the SJF accommodates ???15 mm yr-1 and the Indio segment of the SAF ???23 mm yr-1 of right-lateral motion, accompanied by a low slip rate on the San Bernardino segment of the SAF 'Anomalous' fault segments such as around the 1992 Mw = 7.3 Landers surface rupture can be detected. There, observed stresses deviate strongly from the long-term loading as predicted by our simple model. Evaluation of model misfits together with information from palaeoseismology may provide further insights into the time dependence of strain accumulation along the San Andreas system. ?? 2004 RAS.

Geophysical Journal International↗

USGS2004

Dynamic rupture modeling of the transition from thrust to strike-slip motion in the 2002 Denali fault earthquake, Alaska

We use three-dimensional dynamic (spontaneous) rupture models to investigate the nearly simultaneous ruptures of the Susitna Glacier thrust fault and the Denali strike-slip fault. With the 1957 Mw 8.3 Gobi-Altay, Mongolia, earthquake as the only other well-documented case of significant, nearly simultaneous rupture of both thrust and strike-slip faults, this feature of the 2002 Denali fault earthquake provides a unique opportunity to investigate the mechanisms responsible for development of these large, complex events. We find that the geometry of the faults and the orientation of the regional stress field caused slip on the Susitna Glacier fault to load the Denali fault. Several different stress orientations with oblique right-lateral motion on the Susitna Glacier fault replicate the triggering of rupture on the Denali fault about 10 sec after the rupture nucleates on the Susitna Glacier fault. However, generating slip directions compatible with measured surface offsets and kinematic source inversions requires perturbing the stress orientation from that determined with focal mechanisms of regional events. Adjusting the vertical component of the principal stress tensor for the regional stress field so that it is more consistent with a mixture of strike-slip and reverse faulting significantly improves the fit of the slip-rake angles to the data. Rotating the maximum horizontal compressive stress direction westward appears to improve the fit even further.

Alaska↗

USGS2003

The 2002 Denali fault earthquake, Alaska: A large magnitude, slip-partitioned event

The MW (moment magnitude) 7.9 Denali fault earthquake on 3 November 2002 was associated with 340 kilometers of surface rupture and was the largest strike-slip earthquake in North America in almost 150 years. It illuminates earthquake mechanics and hazards of large strike-slip faults. It began with thrusting on the previously unrecognized Susitna Glacier fault, continued with right-slip on the Denali fault, then took a right step and continued with right-slip on the Totschunda fault. There is good correlation between geologically observed and geophysically inferred moment release. The earthquake produced unusually strong distal effects in the rupture propagation direction, including triggered seismicity.

Science↗

USGS2003

Salton Trough regional deformation estimated from combined trilateration and survey-mode GPS data

The Salton Trough in southeastern California, United States, has one of the highest seismicity and deformation rates in southern California, including 20 earthquakes M 6 or larger since 1892. From 1972 through 1987, the U.S. Geological Survey (USGS) measured a 41-station trilateration network in this region. We remeasured 37 of the USGS baselines using survey - mode Global Positioning System methods from 1995 through 1999. We estimate the Salton Trough deformation field over a nearly 30-year period through combined analysis of baseline length time series from these two datasets. Our primary result is that strain accumulation has been steady over our observation span, at a resolution of about 0.05 μstrain/yr at 95% confidence, with no evidence for significant long-term strain transients despite the occurrence of seven large regional earthquakes during our observation period. Similar to earlier studies, we find that the regional strain field is consistent with 0.5 ± 0.03 μstrain/yr total engineering shear strain along an axis oriented 311.6° ± 23° east of north, approximately parallel to the strike of the major regional faults, the San Andreas and San Jacinto (all uncertainties in the text and tables are standard deviations unless otherwise noted). We also find that (1) the shear strain rate near the San Jacinto fault is at least as high as it is near the San Andreas fault, (2) the areal dilatation near the southeastern Salton Sea is significant, and (3) one station near the southeastern Salton Sea moved anomalously during the period 1987.95-1995.11.

California↗

USGS2003

Static stress transfer during the 2002 Nenana Mountain-Denali Fault, Alaska, earthquake sequence

On 23 October 2002, the Mw 6.7 Nenana Mountain earthquake occurred in central Alaska. It was followed on 3 November 2002 by the Mw 7.9 Denali Fault mainshock, the largest strike-slip earthquake to occur in North America during the past 150 years. We have modeled static Coulomb stress transfer effects during this sequence. We find that the Nenana Mountain foreshock transferred 30-50 kPa of Coulomb stress to the hypocentral region of the Denali Fault mainshock, encouraging its occurrence. We also find that the two main earthquakes together transferred more than 400 kPa of Coulomb stress to the Cross Creek segment of the Totschunda fault system and to the Denali fault southeast of the mainshock rupture, and up to 80 kPa to the Denali fault west of the Nenana Mountain rupture. Other major faults in the region experienced much smaller static Coulomb stress changes.

Geophysical Research Letters↗