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Ross S. Stein

Publications and source records attributed to Ross S. Stein.

27 records · Page 2Linked to original sources
USGS2004

Preliminary report on the 22 December 2003, M 6.5 San Simeon, California earthquake

The M w 6.5 San Simeon earthquake struck the central California coast on 22 December 2003 at 19:15:56 UTC (11:15:56 am local time.) The epicenter was located 11 km northeast of the town of San Simeon, and 39 km west-northwest of Paso Robles ( Figure 1 ), as reported by the California Integrated Seismic Network (CISN, the California region of the Advanced National Seismic System [ANSS]). The mainshock nucleated at 35.702°N, 121.108°W and a depth of 7.1 km, and the rupture propagated unilaterally to the southeast. The strong directivity of the rupture resulted in a concentration of damage and aftershock activity to the southeast of the hypocenter. The worst earthquake damage occurred in Paso Robles, where two people died in the collapse of an unreinforced masonry building. The accurate and rapid earthquake information provided in near real-time by CISN/ANSS to the Governor's Office of Emergency Services made it possible to focus emergency response in the source area, although the earthquake was felt from San Francisco to Los Angeles. The San Simeon earthquake occurred on a reverse fault striking northwest and most likely dipping to the northeast. Although motion along the Pacific-North America plate boundary in California is dominantly strike-slip, there is a small compressional component through central California. Repeated thrust earthquakes such as the San Simeon event accommodate this compression and build the Coast Ranges. Other recent thrust earthquakes in central California include the 1983 Coalinga (M 6.4) and the 1985 Kettleman Hills (M 6.0) earthquakes. Prior earthquakes in the vicinity of the San Simeon event include a M 5-6 earthquake in 1853, a M 5.7 earthquake in 1906, and the M L 6.2 Bryson earthquake of 1952 ( Figure 1 ) ( McLaren and Savage, 2001 .) The San Simeon earthquake occurred on a previously unknown blind thrust fault. No surface rupture associated with the earthquake has been identified. A number of roads, including State Highway 46, buckled due to the earthquake, but this deformation appears mainly to be failure of road fill due to ground shaking and not the result of tectonic surface rupture. Extrapolation of the fault plane to the surface would roughly align with the surface trace of the Oceanic Fault, but this is thought to be a vertical strike-slip fault. Two models for the kinematics of the region have previously been proposed. The first is a fault-propagation fold model developed by Namson and Davis ( 1990 ) for the Santa Lucia mountains ∼30 km to the southeast of the San Simeon sequence. The mainshock geometry is similar to, although more steeply dipping than, the main blind thrust of this model, implying that this model may be applicable to the San Simeon region as well. The second is the model of McLaren and Savage ( 2001 ), in which the region is dominated by strike-slip faulting with shortening on high-angle reverse faults. This model also may be applicable, although the dip of the San Simeon mainshock is shallower than predicted. The San Simeon earthquake was followed by a vigorous aftershock sequence, with 165 events above M 3 reported by CISN within the first week of the mainshock. Although the event triggered many aftershocks, it did not significantly impact the seismicity rates of other nearby faults such as the San Andreas Fault and the San Simeon-Hosgri fault zone. The only triggered seismicity seems to be a few small events within the mainshock coda at the Geysers geothermal area, north of San Francisco. The San Simeon earthquake did, however, trigger shallow creep on the San Andreas Fault at Parkfield and hydrologic changes in hot springs in Paso Robles.

California↗
USGS2002

Response of the San Andreas fault to the 1983 Coalinga-Nuñez earthquakes: an application of interaction-based probabilities for Parkfield

The Parkfield-Cholame section of the San Andreas fault, site of an unfulfilled earthquake forecast in 1985, is the best monitored section of the world's most closely watched fault. In 1983, the M = 6.5 Coalinga and M = 6.0 Nuñez events struck 25 km northeast of Parkfield. Seismicity rates climbed for 18 months along the creeping section of the San Andreas north of Parkfield and dropped for 6 years along the locked section to the south. Right-lateral creep also slowed or reversed from Parkfield south. Here we calculate that the Coalinga sequence increased the shear and Coulomb stress on the creeping section, causing the rate of small shocks to rise until the added stress was shed by additional slip. However, the 1983 events decreased the shear and Coulomb stress on the Parkfield segment, causing surface creep and seismicity rates to drop. We use these observations to cast the likelihood of a Parkfield earthquake into an interaction-based probability, which includes both the renewal of stress following the 1966 Parkfield earthquake and the stress transfer from the 1983 Coalinga events. We calculate that the 1983 shocks dropped the 10-year probability of a M ∼ 6 Parkfield earthquake by 22% (from 54 ± 22% to 42 ± 23%) and that the probability did not recover until about 1991, when seismicity and creep resumed. Our analysis may thus explain why the Parkfield earthquake did not strike in the 1980s, but not why it was absent in the 1990s. We calculate a 58 ± 17% probability of a M ∼ 6 Parkfield earthquake during 2001–2011.

Journal of Geophysical Research B: Solid Earth↗
USGS2002

Evidence from the AD 2000 Izu islands earthquake swarm that stressing rate governs seismicity

Magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits 1,2,3,4 that cannot be associated with the diffusion of pore fluids or heat 5 . Such ‘swarm’ seismicity also migrates with time, and often exhibits a ‘dog-bone’-shaped distribution 3,4,6,7,8,9 . The largest earthquakes in swarms produce aftershocks that obey an Omori-type (exponential) temporal decay 10,11,12 , but the duration of the aftershock sequences is drastically reduced, relative to normal earthquake activity 7,13 . Here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion 8,11,14,15 . A 1,000-fold increase in seismicity rate and a 1,000-fold decrease in aftershock duration occurred during the two-month-long dyke intrusion. We find that the seismicity rate is proportional to the calculated stressing rate, and that the duration of aftershock sequences is inversely proportional to the stressing rate. This behaviour is in accord with a laboratory-based rate/state constitutive law 16 , suggesting an explanation for the occurrence of earthquake swarms. Any sustained increase in stressing rate—whether due to an intrusion, extrusion or creep event—should produce such seismological behaviour.

Izu Islands↗
USGS1996

Damage and restoration of geodetic infrastructure caused by the 1994 Northridge, California, earthquake

We seek to restore the integrity of the geodetic network in the San Fernando, Simi, Santa Clarita Valleys and in the northern Los Angeles Basin by remeasurement of the network and identification of BMs which experienced non-tectonic displacements associated with the Northridge earthquake. We then use the observed displacement of BMs in the network to portray or predict the permanent vertical and horizontal deformation associated with the 1994 Northridge earthquake throughout the area, including sites where we lack geodetic measurements. To accomplish this, we find the fault geometry and earthquake slip that are most compatible with the geodetic and independent seismic observations of the earthquake. We then use that fault model to predict the deformation everywhere at the earth's surface, both at locations where geodetic observations exist and also where they are absent. We compare displacements predicted for a large number of numerical models of the earthquake faulting to the coseismic displacements, treating the earthquake fault as a cut or discontinuity embedded in a stiff elastic solid. This comparison is made after non-tectonic deformation has been removed from the measured elevation changes. The fault slip produces strain in the medium and deforms the ground surface. The model compatible with seismic observations that best fits the geodetic data within their uncertainties is selected. The acceptable model fault bisects the mainshock focus, and the earthquake size , magnitude, is compatible with the earthquake size measured seismically. Our fault model was used to identify geodetic monuments on engineered structures that were anomalously displaced by the earthquake.

California↗
USGS1996

The Loma Prieta, California, Earthquake of October 17, 1989: Main shock characteristics

The October 17, 1989, Loma Prieta, Calif., earthquake (0004:15.2 G.m.t. October 18; lat 37.036º N., long 121.883º W.; 19-km depth) had a local magnitude (M L ) of about 6.7, a surface-wave magnitude (M S ) of 7.1, a seismic moment of 2.2x10 19 N-m to 3.5x10 19 N-m, a source duration of 6 to 15 s, and an average stress drop of at least 50 bars. Slip occurred on a dipping fault surface about 35 km long and was largely confined to a depth of about 7 to 20 km. The slip vector had a large vertical component, and slip was distributed in two main regions situated northwest and southeast of the hypocenter. This slip distribution caused about half of the earthquake's energy to be focused toward the urbanized San Francisco Bay region, while the other half was focused toward the southeast. Had the rupture initiated at the southeast end of the aftershock zone, shaking in the bay region would have been both longer and stronger. These source parameters suggest that the earthquake was not a typical shallow San Andreas-type event but a deeper event on a different fault with a recurrence interval of many hundreds of years. Therefore, the potential for a damaging shallow event on the San Andreas fault in the Santa Cruz Mountains may still exist.

California↗
USGS1994

Static stress changes and the triggering of earthquakes

To understand whether the 1992 M = 7.4 Landers earthquake changed the proximity to failure on the San Andreas fault system, we examine the general problem of how one earthquake might trigger another. The tendency of rocks to fail in a brittle manner is thought to be a function of both shear and confining stresses, commonly formulated as the Coulomb failure criterion. Here we explore how changes in Coulomb conditions associated with one or more earthquakes may trigger subsequent events. We first consider a Coulomb criterion appropriate for the production of aftershocks, where faults most likely to slip are those optimally orientated for failure as a result of the prevailing regional stress field and the stress change caused by the mainshock. We find that the distribution of aftershocks for the Landers earthquake, as well as for several other moderate events in its vicinity, can be explained by the Coulomb criterion as follows: aftershocks are abundant where the Coulomb stress on optimally orientated faults rose by more than one-half bar, and aftershocks are sparse where the Coulomb stress dropped by a similar amount. Further, we find that several moderate shocks raised the stress at the future Landers epicenter and along much of the Landers rupture zone by about a bar, advancing the Landers shock by 1 to 3 centuries. The Landers rupture, in turn, raised the stress at site of the future M = 6.5 Big Bear aftershock site by 3 bars. The Coulomb stress change on a specified fault is independent of regional stress but depends on the fault geometry, sense of slip, and the coefficient of friction. We use this method to resolve stress changes on the San Andreas and San Jacinto faults imposed by the Landers sequence. Together the Landers and Big Bear earthquakes raised the stress along the San Bernardino segment of the southern San Andreas fault by 2 to 6 bars, hastening the next great earthquake there by about a decade.

Bulletin of the Seismological Society of America↗