The Wister Mud Pot Lineament: Southeastward Extension or Abandoned Strand of the San Andreas Fault?

We present the results of a survey of mud pots in the Wister Unit of the Imperial Wildlife Area. Thirty-three mud pots, pot clusters, or related geothermal vents (hundreds of pots in all) were identified, and most were found to cluster along a northwest-trending line that is more or less coincident with the postulated Sand Hills fault. An extrapolation of the trace of the San Andreas fault southeastward from its accepted terminus north of Bombay Beach very nearly coincides with the mud pot lineament and may represent a surface manifestation of the San Andreas fault southeast of the Salton Sea. Additionally, a recent survey of vents near Mullet Island in the Salton Sea revealed eight areas along a northwest-striking line where gas was bubbling up through the water and in two cases hot mud and water were being violently ejected

The slip history of the 1994 Northridge, California, earthquake determined from strong-motion, teleseismic, GPS, and leveling data

We present a rupture model of the Northridge earthquake, determined from the joint inversion of near-source strong ground motion recordings, P and SH teleseismic body waves, Global Positioning System (GPS) displacement vectors, and permanent uplift measured along leveling lines. The fault is defined to strike 122° and dip 40° to the south-southwest. The average rake vector is determined to be 101°, and average slip is 1.3 m; the peak slip reaches about 3 m. Our estimate of the seismic moment is 1.3 ± 0.2 × 10^(26) dyne-cm (potency of 0.4 km3). The rupture area is small relative to the overall aftershock dimensions and is approximately 15 km along strike, nearly 20 km in the dip direction, and there is no indication of slip shallower than about 5 to 6 km. The up-dip, strong-motion velocity waveforms are dominated by large S-wave pulses attributed to source directivity and are comprised of at least 2 to 3 distinct arrivals (a few seconds apart). Stations at southern azimuths indicate two main S-wave arrivals separated longer in time (about 4 to 5 sec). These observations are best modeled with a complex distribution of subevents: The initial S-wave arrival comes from an asperity that begins at the hypocenter and extends up-dip and to the north where a second, larger subevent is centered (about 12 km away). The secondary S arrivals at southern azimuths are best fit with additional energy radiation from another high slip region at a depth of 19 km, 8 km west of the hypocenter. The resolving power of the individual data sets is examined by predicting the geodetic (GPS and leveling) displacements with the dislocation model determined from the waveform data, and vice versa, and also by analyzing how well the teleseismic solution predicts the recorded strong motions. The general features of the geodetic displacements are not well predicted from the model determined independently from the strong-motion data; likewise, the slip model determined from geodetic data does not adequately reproduce the strong-motion characteristics. Whereas a particularly smooth slip pattern is sufficient to satisfy the geodetic data, the strong-motion and teleseismic data require a more heterogeneous slip distribution in order to reproduce the velocity amplitudes and frequency content. Although the teleseismic model can adequately reproduce the overall amplitude and frequency content of the strong-motion velocity recordings, it does a poor job of predicting the geodetic data. Consequently, a robust representation of the slip history and heterogeneity requires a combined analysis of these data sets

Behavior of the Superstition Hills fault during the past 330 years

We have investigated the recent prehistoric behavior of the Superstition Hills fault by examining its effect on the beach deposits of ancient Lake Cahuilla. Excavation of these sediments in three dimensions where they are cut by the fault has enabled determination of total offset since the latest highstand of the lake, about 330 years ago. As of 3 March 1988, total dextral offset was 1106 ± 50 mm. About 609 mm of this amount can be attributed to one or more slip events before 1987. The remaining slip occurred during the moderate earthquake of 24 November 1987 and as subsequent aftercreep. Additional aftercreep here might produce a total of 1210 ± 100 mm for that event. Thus, if afterslip continues according to our prediction, slip associated with the penultimate seismic slip event on the Superstition Hills fault was only about half of the co-seismic slip and predicted afterslip associated with the 1987 earthquake. This difference, established at a single site on the fault, may reflect a larger size for the 1987 event than for its predecessor. We calculate that the slip rate of the Superstition Hills fault zone, averaged over the past 330 years, is between about 2 and at least 6 mm/yr at this site. During this time, the average interval between large surface slip events on the Superstition Hills fault has been between about 150 and 300 years. The pre-1987 slip event documented in our excavations could have occurred at any time between A.D. 1660 and about A.D. 1915. These results are significant for understanding earthquake recurrence and patterns of earthquakes in the Imperial and Coachella Valleys

High-Resolution Topography along Surface Rupture of the 16 October 1999 Hector Mine, California, Earthquake (M_w 7.1) from Airborne Laser Swath Mapping

In order to document surface rupture associated with the Hector Mine earthquake, in particular, the area of maximum slip and the deformed surface of Lavic Lake playa, we acquired high-resolution data using relatively new topographic-mapping methods. We performed a raster-laser scan of the main surface breaks along the entire rupture zone, as well as along an unruptured portion of the Bullion fault. The image of the ground surface produced by this method is highly detailed, comparable to that obtained when geologists make particularly detailed site maps for geomorphic or paleoseismic studies. In this case, however, for the first time after a surface-rupturing earthquake, the detailed mapping is along the entire fault zone rather than being confined to selected sites. These data are geodetically referenced, using the Global Positioning System, thus enabling more accurate mapping of the rupture traces. In addition, digital photographs taken along the same flight lines can be overlaid onto the precise topographic data, improving terrain visualization. We demonstrate the potential of these techniques for measuring fault-slip vectors

Slip pulse and resonance of the Kathmandu basin during the 2015 Gorkha earthquake, Nepal

Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse ~20 kilometers in width, ~6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at ~3.3 kilometers per second over ~140 kilometers. The smooth slip onset, indicating a large (~5-meter) slip-weakening distance, caused moderate ground shaking at high frequencies (>1 hertz; peak ground acceleration, ~16% of Earth’s gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of 4 to 5 seconds caused the collapse of tall structures, including cultural artifacts

Fault‐Slip Distribution of the 1999 M_w 7.1 Hector Mine Earthquake, California, Estimated from Postearthquake Airborne LiDAR Data

The 16 October 1999 Hector Mine earthquake (M_w 7.1) was the first large earthquake for which postearthquake airborne Light Detection and Ranging (LiDAR) data were collected to image the fault surface rupture. In this work, we present measurements of both vertical and horizontal slip along the entire surface rupture of this earthquake based on airborne LiDAR data acquired in April 2000. We examine the details of the along‐fault slip distribution of this earthquake based on 255 horizontal and 85 vertical displacements using a 0.5 m digital elevation model derived from the LiDAR imagery. The slip measurements based on the LiDAR dataset are highest in the epicentral region, and taper in both directions, consistent with earlier findings by other works. The maximum dextral displacement measured from LiDAR imagery is 6.60±1.10  m, located about 700 m south of the highest field measurement (5.25±0.85  m). Our results also illustrate the difficulty in resolving displacements smaller than 1 m using LiDAR imagery alone. We analyze slip variation to see if it is affected by rock type and whether variations are statistically significant. This study demonstrates that a postearthquake airborne LiDAR survey can produce an along‐fault horizontal and vertical offset distribution plot of a quality comparable to a reconnaissance field survey. Although LiDAR data can provide a higher sampling density and enable rapid data analysis for documenting slip distributions, we find that, relative to field methods, it has a limited ability to resolve slip that is distributed over several fault strands across a zone. We recommend a combined approach that merges field observation with LiDAR analysis, so that the best attributes of both quantitative topographic and geological insight are utilized in concert to make best estimates of offsets and their uncertainties

Evidence for prehistoric earthquakes on the Superstition Hills fault from offset geomorphic features

Offset geomorphic features along the Superstition Hills fault show evidence for at least one slip event prior to the 1987 surface rupture, and possibly as many as four to five earlier prehistoric earthquakes. We documented several geomorphic features that appeared offset by multiple events by making detailed topographic maps. Offset features were abundant along reaches of the fault with high topographic relief and large displacement. Slip distribution for the penultimate event, as recorded by offset rills, streams, and shrub-coppice dunes, is very similar to the slip distribution from the 1987 earthquake through April 1988. This similarity may prove to be fortuitous if afterslip from the 1987 event continues to increase the total slip for this earthquake. But if afterslip associated with the 1987 event ceases in the near future, then the past two earthquakes were nearly identical in slip, and the Superstition Hills fault may be expected to produce characteristic earthquakes of roughly magnitude 6½

Earthquake (M w 7.1) from Airborne Laser Swath Mapping

Abstract In order to document surface rupture associated with the Hector Mine earthquake, in particular, the area of maximum slip and the deformed surface of Lavic Lake playa, we acquired high-resolution data using relatively new topographicmapping methods. We performed a raster-laser scan of the main surface breaks along the entire rupture zone, as well as along an unruptured portion of the Bullion fault. The image of the ground surface produced by this method is highly detailed, comparable to that obtained when geologists make particularly detailed site maps for geomorphic or paleoseismic studies. In this case, however, for the first time after a surface-rupturing earthquake, the detailed mapping is along the entire fault zone rather than being confined to selected sites. These data are geodetically referenced, using the Global Positioning System, thus enabling more accurate mapping of the rupture traces. In addition, digital photographs taken along the same flight lines can be overlaid onto the precise topographic data, improving terrain visualization. We demonstrate the potential of these techniques for measuring fault-slip vectors

Continuous GPS Observations of Postseismic Deformation Following the 16 October 1999 Hector Mine, California, Earthquake (M_w 7.1)

Rapid field deployment of a new type of continuously operating Global Positioning System (GPS) network and data from Southern California Integrated GPS Network (SCIGN) stations that had recently begun operating in the area allow unique observations of the postseismic deformation associated with the 1999 Hector Mine earthquake. Innovative solutions in fieldcraft, devised for the 11 new GPS stations, provide high-quality observations with 1-year time histories on stable monuments at remote sites. We report on our results from processing the postseismic GPS data available from these sites, as well as 8 other SCIGN stations within 80 km of the event (a total of 19 sites). From these data, we analyze the temporal character and spatial pattern of the postseismic transients. Data from some sites display statistically significant time variation in their velocities. Although this is less certain, the spatial pattern of change in the postseismic velocity field also appears to have changed. The pattern now is similar to the pre-Landers (pre-1992) secular field, but laterally shifted and locally at twice the rate. We speculate that a 30 km × 50 km portion of crust (near Twentynine Palms), which was moving at nearly the North American plate rate (to within 3.5 mm/yr of that rate) prior to the 1992 Landers sequence, now is moving along with the crust to the west of it, as though it has been entrained in flow along with the Pacific Plate as a result of the Landers and Hector Mine earthquake sequence. The inboard axis of right-lateral shear deformation (at lower crustal to upper mantle depth) may have jumped 30 km farther into the continental crust at this fault junction that comprises the southern end of the eastern California shear zone

Co-Seismic Displacements of the 1994 Northridge, California, Earthquake

The 17 January 1994 Northridge, California, earthquake significantly deformed the Earth's crust in the epicentral region. Displacements of 66 survey stations determined from Global Positioning System (GPS) observations collected before and after the earthquake show that individual stations were uplifted by up to 417 ± 5 mm and displaced horizontally by up to 216 ± 3 mm. Using these displacements, we estimate parameters of a uniform-slip model. Fault geometry and slip are estimated independent of seismological information, using Monte Carlo optimization techniques that minimize the model residuals. The plane that best fits the geodetic data lies 1 to 2 km above the plane indicated by aftershock seismicity. Modeling for distributed slip on a coplanar, yet larger model fault indicates that a high-slip patch occurred up-dip and northwest of the mainshock hypocenter and that less than 1 m of slip occurred in the uppermost 5 km of the crust. This finding is consistent with the lack of clear surface rupture and with the notion that the intersection with the fault that ruptured in 1971 formed the up-dip terminus of slip in the Northridge earthquake. Displacements predicted by either of these simple models explain most of the variance in the data within 50 km of the epicenter. On average, however, the scatter of the residuals is twice the data uncertainties, and in some areas, there is significant systematic misfit to either model. The co-seismic contributions of aftershocks are insufficient to explain this mismatch, indicating that the source geometry is more complicated than a single rectangular plane