Seismotectonics

Seismotectonics is the synthesis of earthquake, geophysical, geodetic and geological data to deduce the tectonic framework of a region [Scholz, 1990]. This approach has been applied successfully to active tectonic regions such as plate boundaries, regions of intraplate seismicity, and active volcanoes throughout the world

Absence of Evidence for a Shallow Magma Chamber Beneath Long Valley Caldera, California, in Downhole and Surface Seismograms

A downhole seismometer at 900-m depth and a temporary network of surface stations were deployed to use rays from local microearthquakes to study the upper and middle crust beneath the Long Valley caldera. The downhole seismograms show S waves with high apparent amplitudes from earthquakes located 2–20 km to the south of the downhole seismometer. In contrast, S waves from earthquakes located in the distance range 20–30 km to the south have low apparent amplitudes. If P and S amplitudes are normalized relative to the respective coda amplitudes, the S to coda amplitude ratios appear to remain constant but the P to coda amplitude ratios vary significantly with takeoff angle. A comparison of the calculated radiation patterns for a double couple in a uniform halfspace and focal mechanisms of the recorded earthquakes suggest that the observed variations in P and S amplitudes are caused by radiation pattern effects. Reanalysis of possible travel time delays found by Elbring and Rundle (1986), who used a subset of the borehole data analyzed in this study, shows that they underestimated the epicentral distances to three of the earthquakes and hence generated an artificial kink in the reduced travel time versus depth curve. Synthetic calculations of reduced travel time versus depth suggest that an apparent velocity of 5.7 km/s gives less scatter than 6.0 km/s used by Elbring and Rundle (1986). Plots of ts/tp versus depth show that contrary to the findings of Elbring and Rundle (1986), the Vp/Vs ratio stays fairly constant with depth and a small (<3-km diameter) magma chamber cannot easily be resolved. Furthermore, combined analysis of downhole and surface data shows that neither data set requires a low-velocity zone or a zone of anomalously high Vp/Vs at depth below the Casa Diablo area

State of Stress from Focal Mechanisms Before and After the 1992 Landers Earthquake Sequence

The state of stress in the Eastern California Shear Zone (ECSZ) changed significantly because of the occurrence of the 1992 M_w 6.1 Joshua Tree and the M_W 7.3 Landers earthquakes. To quantify this change, focal mechanisms from the 1975 Galway Lake sequence, the 1979 Homestead Valley sequence, background seismicity from 1981 to 1991, and the 1992 Landers sequence are inverted for the state of stress. In all cases, the intermediate principal stress axis (S2) remained vertical, and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes (S_1 and S_3) and small variations in the value of the relative stress magnitudes (ϕ). In general, the stress state in the ECSZ has S_1 trending east of north and ϕ = 0.43 to 0.65, suggesting that the ECSZ is a moderate stress refractor and the style of faulting is transtensional. South of the Pinto Mountain fault, in the region of the 1992 Joshua Tree earthquake, the stress state determined from the 1981 to 1991 background seismicity changed on 23 April and again on 28 June 1992. In the central zone, S_1 rotated from N14° ± 5°E to N28° ± 5°E on 23 April and back again to N16° ± 5°E on 28 June. Thus, the Landers mainshock in effect recharged some of the shear stress in the region of the M_w 6.1 Joshua Tree earthquake. Comparison of the state of stress before and after 28 June 1992, along the Landers mainshock rupture zone, showed that the mainshock changed the stress orientation. The S1 trend rotated 7° to 20° clockwise and became progressively more fault normal from south to north. Along the Emerson-Camp Rock faults, the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor. The complex distribution of P and T axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock. The San Bernardino Mountains region of the M_w 6.2 Big Bear earthquake has a distinctively different state of stress, as compared to the Landers region, with S_1 trending N3° ± 5°W. This region did not show any significant change in the state of stress following the 1992 M_w 6.2 Big Bear sequence

Radon content of groundwater as an earthquake precursor: Evaluation of worldwide data and physical basis

The properties of a worldwide data set of 91 radon (^(222)Rn) anomalies (the frequency of occurrence, the precursor time interval, and the distribution of peak amplitudes) are correlated with earthquake data such as the respective magnitude and epicentral distance. These anomalies were reported as precursors to earthquakes in the United States, USSR, China, Japan, and Iceland. Although the data set is incomplete and limited by experimental deficiencies, several consistent patterns emerge. Radon anomalies from different tectonic regions show similar patterns. The radon anomalies occur at greater epicentral distances for earthquakes of the larger magnitude. Anomalies preceding large earthquakes (M ≥ 6) are frequently observed at a distance of 100 to 500 km. These distances are larger than several times the rupture dimensions of the future earthquakes. The time from the onset of an anomaly to the time of the earthquake (the precursor time) increases with magnitude but decreases with distance between epicenter and radon station. In addition, radon anomalies are observed more frequently prior to large earthquakes than prior to small ones, indicating that the preparation zone increases in size as magnitude increases. The peak amplitude does not scale with magnitude but forms a consistent pattern with epicentral distance in that the larger the earthquake magnitude, the farther away the largest amplitudes tend to occur. The preparation zone of the earthquake where the anomalies occur forms an almost continuous annulus that expands with time away from the future rupture zone. The outer radius of this annulus scales with the earthquake magnitude. Model calculations indicate that strain fields of at most 10^(−6) to 10^(−8) strain caused the radon anomalies. If these strains are divided by the appropriate precursor time, minimum strain rates from 10^(−7) day^(−1) to 10^(−10) day^(−1) are obtained. Such small strains and strain rates suggest that in most cases neither mechanical crack growth induced by dilatancy nor mechanical coupling between pore pressure and the rock matrix caused the anomalies. Large changes in the orientation of the local strain field, however, could occur and affect the local stress intensity factor. Since changes in the stress intensity factor can result in stress corrosion, the occurrence of radon anomalies is attributed to slow crack growth controlled by stress corrosion in a rock matrix saturated by groundwater

Crustal structure and seismicity distribution adjacent to the Pacific and North American plate boundary in southern California

New three-dimensional (3-D) V_P and V_P/V_S models are determined for southern California using P and S-P travel times from local earthquakes and controlled sources. These models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the Pacific-North America plate boundary. The models extend from the U.S.-Mexico border in the south to the southernmost Coast Ranges and Sierra Nevada in the north and have a 15-km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 22 km depth. The heterogeneity of the crustal structure as imaged by V_P and V_P/V_S models is larger within the Pacific plate than the North American plate. Similarly, the relocated seismicity deepens and shows more complex 3-D distribution in areas of the Pacific plate exhibiting compressional tectonics. The models reflect mapped changes in the lithology across major geological terranes such as the Mojave Desert, the Peninsular Ranges, and the Transverse Ranges. The interface between the shallow Mono of the Continental Borderland and the deep Moho of onshore California forms a broad zone to the north beneath the western Transverse Ranges, Ventura basin, and the Los Angeles basin and a narrow zone to the south, along the Peninsular Ranges. The near-surface increase in velocity, from the surface to up to 8 km depth, is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins. At midcrustal depths a rapid increase in V_P is imaged beneath the sediments of the large sedimentary basins, while beneath the adjacent mountain ranges the increase is small or absent

Earthquakes, faulting, and stress in the Los Angeles Basin

Since 1920 fourteen moderate-sized (M_L = 4.9–6.4) earthquakes have been reported in the Los Angeles basin. These events are associated with both mappable surficial faults and concealed faults beneath the basin sediments. To determine the style of faulting and state of stress in the basin, single-event focal mechanisms for 244 earthquakes of M ≥ 2.5 that have occurred during 1977–1989 have been calculated. Fifty-nine percent of the events are strike-slip and are mostly located near two of the major, northwest striking right-lateral strike-slip faults in the basin, the Newport-Inglewood fault and the Palos Verdes fault. The 1988 Pasadena and the 1988 Upland earthquakes showed left-lateral strike-slip on northeast striking faults. Numerous small earthquakes in the eastern part of the basin show left-lateral strike-slip faulting and form a northeast trend near Yorba Linda. Thirty-two percent of the events have reverse mechanisms and are distributed along two broad zones. The first, the Elysian Park fold and thrust belt, coincides with anticlines along the eastern and northern flank of the Los Angeles basin extending into Santa Monica Bay. The second, the Torrance-Wilmington fold and thrust belt, coincides with anticlines mapped on the southwest flank of the basin and extends from offshore Newport Beach to the northwest into Santa Monica Bay. Oblique faulting that could be inferred by the merging of strike-slip and compressional tectonics does not occur in the basin. Instead, the coexistence of zones of thrusting and large strike-slip faults in the basin suggests that the thrust and strike-slip movements are mostly decoupled. A few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines. The trend of the maximum horizontal stress varies from N1°W to N31°E across the basin and consistently forms high angles with the fold axes. The stress field that exists along the flanks of the basin has a vertical minimum stress axis. This stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin. Today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north-south compression across the basin

Structure of the Benioff Zone beneath the Shumagin Islands, Alaska: Relocation of local earthquakes using three-dimensional ray tracing

Seismic rays are traced through a prescribed three-dimensional inhomogeneity that simulates the subducted slab below the Shumagin Islands region to calculate local station delays for a given hypocenter and a slab model. Hypocenters determined by the Shumagin seismic network are then relocated using the station delays, a flat-layered velocity structure and a standard earthquake location computer program. Station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data. A set of path corrections that is calculated for each grid point-station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km. Application of this method to data from 1982 for the digitally recording Shumagin seismic network shows the following results: (1) a previously observed apparent increase in dip of the subducted slab at depths of ≈ 100 km disappears, (2) the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45° toward north-northwest at depths between 80 and 250–300 km and has a 7% higher velocity than the surrounding mantle, (3) hypocenters determined from Shumagin network data are located only 10–20 km south of high-quality hypocenters determined from teleseismic data alone, (4) qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high-frequency coda, possible converted phases, and low amplitudes of first P arrivals for rays that travel mostly along the slab. Conversely, for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first P arrivals that are followed by an insignificant coda and low-amplitude S waves

Seismotectonics of the Newport-Inglewood fault zone in the Los Angeles basin, southern California

The Newport-Inglewood fault zone (NIF) strikes northwest along the western margin of the Los Angeles basin in southern California. The seismicity (1973 to 1985) of M_L ≧ 2.5 that occurred within a 20-km-wide rectangle centered on the NIF extending from the Santa Monica fault in the north to Newport Beach in the south is analyzed. A simultaneous full inversion scheme (VELEST) is used to invert for hypocentral parameters, two velocity models, and a set of station delays. Arrival time data from three quarry blasts are included to stabilize the inversion. The first velocity model applies to stations located along the rim and outside the Los Angeles basin and is well resolved. It is almost identical to the starting model, which is the model routinely used by the CIT/USGS southern California seismic network for locating local earthquakes. The second velocity model applies to stations located within the Los Angeles basin. It shows significantly lower velocities down to depths of 12 to 16 km, which is consistent with basement of Catalina Schist below the sediments in the western Los Angeles basin. The distribution of relocated hypocenters shows an improved correspondence to mapped surface traces of late Quaternary fault segments of the NIF. A diffuse trend of seismicity is observed along the Inglewood fault from the Dominguez Hills, across the Baldwin Hills to the Santa Monica fault in the north. The seismicity adjacent to Long Beach, however, is offset 4 to 5 km to the east, near the trace of the subsurface Los Alamitos fault. The depth distribution of earthquakes along the NIF shows clustering from 6 to 11 km depth, which is similar to average seismogenic depths in southern California. Thirty-nine single-event focal mechanisms of small earthquakes (1977 to 1985) show mostly strike-slip faulting with some reverse faulting along the north segment (north of Dominguez Hills) and some normal faulting along the south segment (south of Dominguez Hills to Newport Beach). The results of an inversion of the focal mechanism data for orientations of the principal stress axes and their relative magnitudes indicate that the minimum principal stress is vertical along the north segment while the intermediate stress is vertical along the south segment. The maximum principal stress axis is oriented 10° to 25° east of north. Reverse faulting along the north segment indicates that a transition zone of mostly compressive deformation exists between the Los Angeles block and the Central Transverse Ranges

Crustal geophysics and seismicity in southern California

The geographical distribution of the (1981–2005) seismicity in southern California forms a ±150 km broad zone adjacent to the Pacific–North America plate boundary, ranging from depths of ∼1–~30 km, with the bulk of the focal depths in the range of 2–12 km. The distribution of the seismicity that includes both mainshock–aftershock sequences and background events is affected by both static and kinematic geophysical parameters of the crust. The static parameters include heat flow, topography, crustal density, V_(p)/V_(s) ratio, hypocentral fault-distance and crustal thickness from receiver functions. The tectonic loading is represented by kinematic parameters such as the crustal shear strain rate field, and the dilatational strain rate field. In our analysis, we normalize the seismicity relative to the areal density of the range of values of each of the parameters. Most of the seismicity occurs in areas of average heat flow, low to intermediate topography, average Vp/Vs and high late Quaternary fault density, and forms seismogenic zones that extend through the brittle crust. The location of late Quaternary faults, often described as zones of weakness, influences the geographical distribution of seismicity more than any other parameter. Although above or below average crustal properties such as high heat flow, thin crust or very low V_(p)/V_(s) values exist, these properties only influence the spatial distribution of seismicity in a minor way. As an example, the Salton Trough area of low topography, high heat flow, high V_(p)/V_(s), high shear strain rate and thin crust has distributed seismicity within a thin seismogenic zone. Also, somewhat surprisingly, areas of high topography, low heat flow, low V_(p)/V_(s), low shear strain rate and thick crust have low seismicity rates but a thin seismogenic zone. We determine an empirical relationship between heat flow and crustal thickness to show how the ~400 °C temperature isotherm gradually deepens with crustal thickness and forms the base of the seismogenic zone for crustal thicknesses from 22 to 37 km. For crustal thickness ranging from 37 to 43 km, the ~250 °C isotherm forms the base of the seismogenic zone, suggesting that seismic faulting in these regions is confined to the top of the upper crust (12–14 km), and thus does not accommodate plate motion

Episodic rifting and volcanism at Krafla in north Iceland: Radon (222) emission from fumaroles near Leirhnjukur

From June 1978 until late 1980 radon emission from the Leirhnjukur fumaroles was monitored within the Krafla caldera of north Iceland where episodic volcanism is occurring. Frequent sampling of the fumaroles shows that no easily identifiable short-term radon precursors occur in the days prior to subsidence of the caldera, despite an observed increase in microseismicity preceding deflation. Following the onset of subsidence, however, the radon emission of the fumaroles gradually increases and reaches a maximum 3–6 days later. The radon in the fumaroles is assumed to be transported from depth by steam and noncondensing gases that slowly escape from the geothermal water table. The cause of the co-episodic increase in radon emission appears to be a temporal rise of the water table driven by fissure closure resulting in an abbreviated transport time for radon to the surface fumaroles. Furthermore, the closing of the fissures appears to cause a transient increase in the velocity of transport, making the shape of the anomalies broader and higher than is predicted from a change in the water level alone. Changes in radon emission also coincide with fluctuations in fumarolic activity and permanent changes in the level of geothermal water that occur during periods of uplift