Hector Mine Research Articles

Comment on “Temporal and spatial precursors in ionospheric total electron content of the 16 October 1999 Mw 7.1 Hector Mine earthquake” by Su et al. (2013)

AbstractWe review the recent paper by Su et al. (2013). Using Global Position System and Global Ionospheric Maps data, Su et al. claimed to have found ionospheric precursors a few days before the 16 October 1999 Hector Mine, California, earthquake. They proposed that this type of analysis of ionospheric data may be useful for locating forthcoming large earthquakes. In this Comment, we reexamine these data and show that ionospheric anomalies reported by Su et al. were not precursors to the Hector Mine earthquake. Therefore, their proposed analysis is not useful in the context of earthquake prediction.

Aftershock sequences of three seismic crises at southern California, USA, simulated by a cellular automata model based on self-organized criticality

Several properties of aftershock series related to the main shocks of Landers, Northridge and Hector Mine (southern California, USA) are reproduced by the Dynamic Fiber Bundle Model, DFBM. Optimum values for the three parameters governing DFBM are determined by searching for the best agreement of real aftershock series properties and those of synthetic sequences generated by this model. The analysis of the model parameter values provides details on the underlying physical mechanism of the aftershock sequence generation. First, the ratio of seismic energy radiated as seismic waves and transferred as stress-strain to adjacent faults; second, the degree of stress heterogeneity reproducing the complex behavior of real aftershock series. Additionally, the results of simulations support the coexistence of two types of relaxation processes. One of them is associated with the well-known modified Omori’s (MO) law, which involves elapsed times between consecutive aftershocks monotonically increasing; the other is manifested by episodes of sudden stress release, with inter-event times much shorter than those predicted by MO law. These episodes are assumed to be a consequence of the complex distribution of tectonic stresses and fault geometry. The first process is associated to events designed as leading aftershocks, LA. The second process generates series of events which are designed as cascades, CA. It is worth of mention that several properties concerning CAs can be reasonably related to critical changes on stress field along the simulation process.

Near‐prospective test of Coulomb stress triggering

AbstractNumerous studies retrospectively observed a strong association between earthquake location and increased static Coulomb stress. In addition to confirming these results for southern California seismicity, we observed prospective forecasting potential in static stress evolution. With 141 seismic and 98 aseismic stress sources, we considered alternate choices of stress components and receiver fault orientations, examined the effect of Coulomb triggering on earthquake magnitude, calculated stress histories for receiver quakes to see if some occurred after their stress had peaked, examined whether stress changes from the Hector Mine earthquake alone changed the earthquake rate as expected, and estimated the conditional distribution of earthquake occurrence given resolved Coulomb stress change. We found that with 95% confidence, M ≥ 2.8 receiver earthquakes nucleate in areas of shear or Coulomb stress change increase. On average, 59% of earthquakes occurred within stress‐enhanced zones, regardless of the choice of rupture plane or type of stress change. The 1999 Hector Mine, California, earthquake increased the seismicity in regions of positive and negative stress change but more so in the positive regions. Earthquakes frequently occur after their calculated peak stress has occurred, indicating that elastic calculations do not tell the whole story. We found no significant evidence that stress change affects the magnitude of receiver quakes. Thus, instantaneous Coulomb and shear stress change apparently influence the locations but not the magnitudes of future earthquakes.

Rheological properties of the mantle lid beneath the Mojave region in southern California

Recently deformed mantle peridotite xenoliths derived from Moho depths constrain the geothermal gradient, stress magnitude, effective viscosity, and degree of localization in the uppermost mantle within the tectonically active Mojave region of California. Microstructural observations and water content measurements in the xenoliths indicate that upper mantle deformation is accommodated by dislocation creep of modestly hydrated olivine. Differential stress measured in the xenoliths using olivine paleopiezometry is 13–17 MPa, which is at least one order of magnitude less than peak stresses estimated for the gabbroic lower crust and the brittle upper crust. Similarly, the mean effective viscosity of ∼3×1019 Pas for the uppermost mantle is one to two orders of magnitude less than the mean viscosity estimated for the lower crust, consistent with recent models of postseismic relaxation following the Landers (1992) and Hector Mine (1999) earthquakes. These results support a rheological model for the Mojave region in which the peak stress resides in the crust, rather than within the lithospheric mantle (consistent with the ‘crème brûlée’ model of lithospheric strength). Temperatures and pressures recorded in the xenoliths indicate a high geothermal gradient of at least ∼30 °C/km, which may explain the regional weakness of the mantle lid. Strain rates calculated for the uppermost mantle using the xenolith data and olivine flow laws for wet dislocation creep are ∼7–70 times faster than bulk strain rates estimated across the central Mojave region from GPS-constrained surface velocities. This suggests that faults at the eastern border of the Eastern California Shear Zone persist through the seismogenic zone and retain their identities as narrow ductile shear zones into the mantle beneath the Moho. The observation that recent deformation is localized into only 10–25% of the rock body may explain why upper mantle seismic anisotropy in the Mojave region is highly oblique to present-day plate boundary motion.

Temporal and spatial precursors in ionospheric total electron content of the 16 October 1999 Mw7.1 Hector Mine earthquake

AbstractIn this paper, temporal and spatial analyses are employed to detect seismo‐ionospheric precursors (SIPs) in the ionospheric total electron content (TEC) during the 16 October 1999 Mw7.1 Hector Mine earthquake. To discriminate anomalies caused by global effects, such as solar radiations, magnetic storms, etc., and local effects, such as earthquake, we cross‐examine the GPS TECs and their gradients in the eastward and northward directions at epicenter/centers of the Hector Mine area and the other two reference areas at similar magnetic latitudes in Europe and Japan. Temporal variations of the northward TEC gradient suggest SIPs most likely appearing on days 6–5 before the earthquake. A global search by using the TEC of the global ionosphere map shows that the TEC increase and decrease anomalies continuously and specifically appear around the epicenter on day 5 before the earthquake.

California foreshock sequences suggest aseismic triggering process

Foreshocks are one of the few well‐documented precursors to large earthquakes; therefore, understanding their nature is very important for earthquake prediction and hazard mitigation. However, the triggering role of foreshocks is not yet clear. It is possible that foreshocks are a self‐triggering cascade of events that simply happen to trigger an unusually large aftershock; alternatively, foreshocks might originate from an external aseismic process that ultimately triggers the mainshock. In the former case, the foreshocks will have limited utility for forecasting. The latter case has been observed for several individual large earthquakes; however, it remains unclear how common it is and how to distinguish foreshock sequences from other seismicity clusters that do not lead to large earthquakes. Here we analyze foreshocks of three M>7 mainshocks in southern California. These foreshock sequences appear similar to earthquake swarms, in that they do not start with their largest events and they exhibit spatial migration of seismicity. Analysis of source spectra shows that all three foreshock sequences feature lower average stress drops and depletion of high‐frequency energy compared with the aftershocks of their corresponding mainshocks. Using a longer‐term stress‐drop catalog, we find that the average stress drop of the Landers and Hector Mine foreshock sequences is comparable to nearby swarms. Our observations suggest that these foreshock sequences are manifestations of aseismic transients occurring close to the mainshock hypocenters, possibly related to localized fault zone complexity, which have promoted the occurrence of both the foreshocks and the eventual mainshock.

Clustering of GPS velocities in the Mojave Block, southeastern California

AbstractWe find subdivisions within the Mojave Block using cluster analysis to identify groupings in the velocities observed at GPS stations there. The clusters are represented on a fault map by symbols located at the positions of the GPS stations, each symbol representing the cluster to which the velocity of that GPS station belongs. Fault systems that separate the clusters are readily identified on such a map. The most significant representation as judged by the gap test involves 4 clusters within the Mojave Block. The fault systems bounding the clusters from east to west are 1) the faults defining the eastern boundary of the Northeast Mojave Domain extended southward to connect to the Hector Mine rupture, 2) the Calico‐Paradise fault system, 3) the Landers‐Blackwater fault system, and 4) the Helendale‐Lockhart fault system. This division of the Mojave Block is very similar to that proposed by Meade and Hager []. However, no cluster boundary coincides with the Garlock Fault, the northern boundary of the Mojave Block. Rather, the clusters appear to continue without interruption from the Mojave Block north into the southern Walker Lane Belt, similar to the continuity across the Garlock Fault of the shear zone along the Blackwater‐Little Lake fault system observed by Peltzer et al. []. Mapped traces of individual faults in the Mojave Block terminate within the block and do not continue across the Garlock Fault [Dokka and Travis, ].

Facilitated Estimation of ETAS

Abstract An approximation technique is introduced that greatly facilitates the estimation of the epidemic‐type aftershock sequence (ETAS) model introduced by Ogata (1988), which is widely used to describe earthquake catalogs. The proposed approximation greatly simplifies the integral term in the log‐likelihood, the computation of which is by far the greatest hurdle in maximum likelihood estimation. The approximation is close, provided the observed region contains most of the aftershock activity caused by earthquakes within the observation region. Through simulations and an example involving earthquakes in Hector Mine, California, the proposed method is shown to produce stable and accurate results, with rapid convergence.

Brief communication "Further comments on the ionospheric precursor of the 1999 Hector Mine earthquake"

Abstract. Pulinets et al. (2007) document anomalous changes in the ionospheric total electron content (TEC) starting one week before the 16 October 1999 Hector Mine earthquake. The authors maintain that this TEC anomalous change is a precursor of the subsequent earthquake. In a previous paper, Afraimovich et al. (2004) excluded that TEC variations, which occurred before the Hector Mine earthquake, were induced by the preparation process of the seismic event. Thomas et al. (2012) reach similar conclusions by performing new analyses of the same TEC data which were investigated by Pulinets et al. (2007). They show that the TEC changes documented by Pulinets et al. (2007) are not anomalous but normal variations on global scale, and, therefore, these changes are not related to the localised seismic activity of the Hector Mine area. This paper confirms the results of Afraimovich et al. (2004) and Thomas et al. (2012). Through the use of geomagnetic indices time series it is shown that the presumed precursor of Pulinets et al. (2007) was a normal TEC variation induced by solar-terrestrial interaction.

Feasibility of Along-Track Displacement Measurement From Sentinel-1 Interferometric Wide-Swath Mode

The European Space Agency's Sentinel-1, a C-band imaging radar mission to be launched in mid-2013, will provide a continuity of radar data for monitoring the changing Earth. The azimuth resolution of Sentinel-1's background mode, interferometric wide-swath (IW) mode, is four times lower than that of European remote-sensing satellite (ERS) and Envisat systems. Therefore, the measurement accuracy of along-track displacement from Sentinel-1 IW images presumably will be significantly reduced. In this paper, we test the feasibility of along-track displacement measurement from Sentinel-1 IW mode. We simulate Sentinel-1 IW synthetic aperture radar (SAR) images from the ERS raw data that captured the coseismic deformation of the 1999 Hector Mine earthquake in California. Along-track displacement maps are generated using multiple-aperture interferometric SAR (MAI) and intensity tracking techniques, respectively, and are compared with GPS measurements. The root-mean-square (rms) error between the synthetic Sentinel-1 MAI and GPS measurements is about 9.6 cm, which corresponds to only 0.5 % of the azimuth resolution. The rms error between the along-track displacements from synthetic Sentinel-1 offset tracking and GPS is about 27.5 cm, which is about 1.4 % of the azimuth resolution. These results suggest that the MAI method will still be useful to measure along-track displacements from Sentinel-1 IW InSAR imagery and that it would be difficult to effectively measure the along-track displacements by the Sentinel-1 offset tracking method.

Remarkable changes in the distribution of the order parameter of seismicity before mainshocks

By analyzing in natural time the Southern California Earthquake Catalog (SCEC) during the period 1979–2003, the probability distribution of the order parameter of seismicity is studied. Remarkable changes in the feature of this distribution are identified well before the occurrence of the three major mainshocks reported by SCEC: Landers in 1992, Northridge in 1994 and Hector Mine in 1999. In addition, just before the occurrence of these mainshocks, i.e., for 100 successive events of magnitude M ⩾ 2.0 before each of them, their scaled order parameter probability distributions almost collapse onto a single curve and share over two orders of magnitude a characteristic “exponential tail” reflecting non-Gaussian fluctuations.

Spatio-temporal characteristics of aftershocks and seismogenic structure of the 2011 MW9.0 Tohoku earthquake, Japan

The Tohoku megathrust earthquake, which occurred on March 11, 2011 and had an epicenter that was 70 km east of Tohoku, Japan, resulted in an estimated ten’s of billions of dollars in damage and a death toll of more than 15 thousand lives, yet few studies have documented key spatio-temporal seismogenic characteristics. Specifically, the temporal decay of aftershock activity, the number of strong aftershocks (with magnitudes greater than or equal to 7.0), the magnitude of the greatest aftershock, and area of possible aftershocks. Forecasted results from this study are based on Gutenberg-Richter’s relation, Bath’s law, Omori’s law, and Well’s relation of rupture scale utilizing the magnitude and statistical parameters of earthquakes in USA and China (Landers, Northridge, Hector Mine, San Simeon and Wenchuan earthquakes). The number of strong aftershocks, the parameters of Gutenberg-Richter’s relation, and the modified form of Omori’s law are confirmed based on the aftershock sequence data from the MW9.0 Tohoku earthquake. Moreover, for a large earthquake, the seismogenic structure could be a fault, a fault system, or an intersection of several faults. The seismogenic structure of the earthquake suggests that the event occurred on a thrust fault near the Japan trench within the overriding plate that subsequently triggered three or more active faults producing large aftershocks.

Improving empirical aftershock modeling based on additional source information

Several mechanisms are proposed to underlie earthquake triggering including static stress interactions and dynamic stress transfer. Significant differences of these mechanisms are particularly expected in the spatial distribution of aftershocks. However, testing the different hypotheses is challenging because it requires the consideration of the large uncertainties involved in stress calculations as well as the appropriate consideration of secondary aftershock triggering which is related to stress changes induced by smaller pre‐ and aftershocks. In order to evaluate the forecast capability of different mechanisms, we take the effect of smaller‐magnitude earthquakes into account by using the epidemic type aftershock sequence (ETAS) model where the spatial probability distribution of direct aftershocks, if available, is correlated to alternative source information and mechanisms. We test surface shaking, rupture geometry, and slip distributions. As an approximation of the shaking level, we use ShakeMap data which are available in near real‐time after a main shock and thus could be used for first‐order forecasts of the spatial aftershock distribution. Alternatively, we test the use of empirical decay laws related to minimum fault distance and Coulomb stress change calculations based on published and random slip models. For comparison, we analyze the likelihood values of the different model combinations in the case of several well‐known aftershock sequences (1992 Landers, 1999 Hector Mine, 2004 Parkfield). Our test shows that the fault geometry is the most valuable information for improving aftershock forecasts. Furthermore, we find that static stress maps can additionally improve the forecasts of off‐fault aftershock locations, while the integration of ground shaking data could not upgrade the results significantly.

On the reported ionospheric precursor of the 1999 Hector Mine, California earthquake

Using Global Positioning System (GPS) data from sites near the 16 Oct. 1999 Hector Mine, California earthquake, Pulinets et al. (2007) identified anomalous changes in the ionospheric total electron content (TEC) starting one week prior to the earthquake. Pulinets (2007) suggested that precursory phenomena of this type could be useful for predicting earthquakes. On the other hand, and in a separate analysis, Afraimovich et al. (2004) concluded that TEC variations near the epicenter were controlled by solar and geomagnetic activity that were unrelated to the earthquake. In an investigation of these very different results, we examine TEC time series of long duration from GPS stations near and far from the epicenter of the Hector Mine earthquake, and long before and long after the earthquake. While we can reproduce the essential time series results of Pulinets et al., we find that the signal they identified as being anomalous is not actually anomalous. Instead, it is just part of normal global‐scale TEC variation. We conclude that the TEC anomaly reported by Pulinets et al. is unrelated to the Hector Mine earthquake.

Using short‐term postseismic displacements to infer the ambient deformation conditions of the upper mantle

To interpret short‐term postseismic surface displacements in the context of key ambient conditions (e.g., temperature, pressure, background strain rate, water content, creep mechanism), we combined steady state and transient flow into a single constitutive relation that can explain the response of a viscoelastic material to a change in stress. The flow law is then used to investigate mantle deformation beneath the Eastern California Shear Zone following the 1999 M7.1 Hector Mine earthquake. The flow law parameters are determined using finite element models of relaxation processes, constrained by surface displacement time series recorded by 55 continuous GPS stations for 7 years following the earthquake. Results suggest that postseismic flow following the Hector Mine earthquake occurs below a depth of ∼50 km and is controlled by dislocation creep of wet olivine. Diffusion creep models can also explain the data, but require a grain size (3.5 mm) that is smaller than the inferred grain size (10–20 mm) based on the mantle conditions at these depths. In addition, laboratory flow laws predict dislocation creep would dominate at the stress/grain size conditions that provide the best fit to diffusion creep models. Model results suggest a transient creep phase that lasts ∼1 year and has a viscosity ∼10 times lower than subsequent steady state flow, in general agreement with laboratory observations. The postseismic response is best explained as occurring within a relatively hot upper mantle (e.g., 1200–1300°C at 50 km depth) with a long‐term background mantle strain rate of 0.1–0.2μstrain/yr, consistent with the observed surface strain rate. Long‐term background shear stresses at the top of the mantle are ∼4 MPa, then decrease with depth to a minimum of 0.1–0.2 MPa at 70 km depth before increasing slowly with depth due to the pressure dependence of viscosity. These conditions correspond to a background viscosity of 1021 Pa s within a thin mantle lid that decreases to ∼5 × 1019 Pa s within the underlying asthenosphere. This study shows the utility of using short‐term postseismic observations to infer long‐term mantle conditions that are not readily observable by other means.

A unified analysis of crustal motion in Southern California, 1970-2004: The SCEC crustal motion map

[1] To determine crustal motions in and around southern California, we have processed and combined trilateration data collected from 1970 to 1992, VLBI data from 1979 to 1992, and GPS data from 1986 to 2004: a long temporal coverage required in part by the occurrence of several large earthquakes in this region. From a series of solutions for station positions, we have estimated interseismic velocities, coseismic displacements, and postseismic motions. Within the region from 31°N to 38°N. and east to 114°W, the final product includes estimated horizontal velocities for 1009 GPS, 190 trilateration, and 16 VLBI points, with ties between some of these used to stabilize the solution. All motions are relative to the Stable North American Reference Frame (SNARF) as realized through the velocities of 20 GPS stations. This provides a relatively dense set of horizontal velocity estimates, with well-tested errors, for the past quarter century over the plate boundary from 31°N to 36.5°N. These velocities agree well with those from the Plate Boundary Observatory, which apply to a later time period. We also estimated vertical velocities, 533 of which have errors below 2 mm/yr. Most of these velocities are less than 1 mm/yr, but they show 2–4 mm/yr subsidence in the Ventura and Los Angeles basins and in the Salton Trough. Our analysis also included estimates of coseismic and postseismic motions related to the 1992 Landers, 1994 Northridge, 1999 Hector Mine, and 2003 San Simeon earthquakes. Postseismic motions increase logarithmically over time with a time constant of about 10 days, and generally mimic the direction and relative amplitude of the coseismic offsets.

Markov-modulated Hawkes process with stepwise decay

This paper proposes a new model—the Markov-modulated Hawkes process with stepwise decay (MMHPSD)—to investigate the variation in seismicity rate during a series of earthquake sequences including multiple main shocks. The MMHPSD is a self-exciting process which switches among different states, in each of which the process has distinguishable background seismicity and decay rates. Parameter estimation is developed via the expectation maximization algorithm. The model is applied to data from the Landers–Hector Mine earthquake sequence, demonstrating that it is useful for modelling changes in the temporal patterns of seismicity. The states in the model can capture the behavior of main shocks, large aftershocks, secondary aftershocks, and a period of quiescence with different background rates and decay rates.

The 2010 M w 7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotectonics along the Mexican Pacific Margin

The El Mayor-Cucapah earthquake sequence started with a few foreshocks in March 2010, and a second sequence of 15 foreshocks of M > 2 (up to M4.4) that occurred during the 24 h preceding the mainshock. The foreshocks occurred along a north–south trend near the mainshock epicenter. The Mw 7.2 mainshock on April 4 exhibited complex faulting, possibly starting with a ~M6 normal faulting event, followed ~15 s later by the main event, which included simultaneous normal and right-lateral strike-slip faulting. The aftershock zone extends for 120 km from the south end of the Elsinore fault zone north of the US–Mexico border almost to the northern tip of the Gulf of California. The waveform-relocated aftershocks form two abutting clusters, each about 50 km long, as well as a 10 km north–south aftershock zone just north of the epicenter of the mainshock. Even though the Baja California data are included, the magnitude of completeness and the hypocentral errors increase gradually with distance south of the international border. The spatial distribution of large aftershocks is asymmetric with five M5+ aftershocks located to the south of the mainshock, and only one M5.7 aftershock, but numerous smaller aftershocks to the north. Further, the northwest aftershock cluster exhibits complex faulting on both northwest and northeast planes. Thus, the aftershocks also express a complex pattern of stress release along strike. The overall rate of decay of the aftershocks is similar to the rate of decay of a generic California aftershock sequence. In addition, some triggered seismicity was recorded along the Elsinore and San Jacinto faults to the north, but significant northward migration of aftershocks has not occurred. The synthesis of the El Mayor-Cucapah sequence reveals transtensional regional tectonics, including the westward growth of the Mexicali Valley and the transfer of Pacific–North America plate motion from the Gulf of California in the south into the southernmost San Andreas fault system to the north. We propose that the location of the 2010 El Mayor-Cucapah, as well as the 1992 Landers and 1999 Hector Mine earthquakes, may have been controlled by the bends in the plate boundary.

Steady-state laboratory flow laws alone fail to explain postseismic observations

We test whether laboratory derived steady-state flow laws for wet olivine can be used to explain postseismic relaxation in the upper mantle following the 1999 Hector Mine earthquake. This is accomplished using rate changes in GPS observed displacement time-series at far-field stations as constraints on the parameters associated with laboratory derived temperature- and stress-dependent flow laws for diffusion and dislocation creep of olivine. We primarily concentrate on the influences of temperature, grain size, and background strain rate to explain observed fast early displacement rates that slow rapidly in the 7 years of postseismic observations. We find that a reasonable fit to the observed time-series cannot be achieved by any combination of flow law parameters and plausible assumptions for the tectonic environment of the mantle beneath the Mojave Desert. We conclude that postseismic relaxation within the mantle following the Hector Mine earthquake likely includes an initial transient weakening phase, suggesting that steady-state flow laws by themselves are insufficient to characterize the flow of mantle rocks when stress conditions evolve.

On the resolution of shallow mantle viscosity structure using postearthquake relaxation data: Application to the 1999 Hector Mine, California, earthquake

Most models of lower crust/mantle viscosity inferred from postearthquake relaxation assume one or two uniform‐viscosity layers. A few existing models possess apparently significant radially variable viscosity structure in the shallow mantle (e.g., the upper 200 km), but the resolution of such variations is not clear. We use a geophysical inverse procedure to address the resolving power of inferred shallow mantle viscosity structure using postearthquake relaxation data. We apply this methodology to 9 years of GPS‐constrained crustal motions after the 16 October 1999 M = 7.1 Hector Mine earthquake. After application of a differencing method to isolate the postearthquake signal from the “background” crustal velocity field, we find that surface velocities diminish from ∼20 mm/yr in the first few months to ≲2 mm/yr after 2 years. Viscoelastic relaxation of the mantle, with a time‐dependent effective viscosity prescribed by a Burgers body, provides a good explanation for the postseismic crustal deformation, capturing both the spatial and temporal pattern. In the context of the Burgers body model (which involves a transient viscosity and steady state viscosity), a resolution analysis based on the singular value decomposition reveals that at most, two constraints on depth‐dependent steady state mantle viscosity are provided by the present data set. Uppermost mantle viscosity (depth ≲ 60 km) is moderately resolved, but deeper viscosity structure is poorly resolved. The simplest model that explains the data better than that of uniform steady state mantle viscosity involves a linear gradient in logarithmic viscosity with depth, with a small increase from the Moho to 220 km depth. However, the viscosity increase is not statistically significant. This suggests that the depth‐dependent steady state viscosity is not resolvably different from uniformity in the uppermost mantle.