Focal Mechanisms Of Small Earthquakes Research Articles

Refined Earthquake Focal Mechanism Catalog for Southern California Derived With Deep Learning Algorithms

AbstractEarthquake focal mechanisms, determined with P‐wave polarities and S/P amplitude ratios, are primary data for analyzing fault zone geometry, sense of slip, and the crustal stress field. Solving for the focal mechanisms of small earthquakes is often challenging because phase arrivals and first‐motion polarities are hard to be separated from noise. To overcome this challenge, we implement convolutional‐neural‐network algorithms (Ross, Meier, & Hauksson, 2018, Ross, Meier, Hauksson, & Heaton, 2018, https://doi.org/10.1029/2017jb015251, https://doi.org/10.1785/0120180080) to detect additional phases and polarities. Using both existing and these new data, we build a high‐quality focal mechanism catalog of 297,478 events that occurred from 1981 to 2021 in southern California with the HASH method of Hardebeck and Shearer (2002), https://doi.org/10.1785/0120010200, Hardebeck and Shearer (2003), https://doi.org/10.1785/0120020236. The new focal mechanism catalog is overall consistent with the standard catalog (Yang et al., 2012, https://doi.org/10.1785/0120110311) but includes 40% more focal mechanisms, and is more consistent with moment tensor solutions derived using waveform‐fitting methods. We apply the new catalog to identify changes in focal mechanism properties caused by the occurrences of large mainshocks such as the 2010 Mw7.2 El Mayor‐Cucapah and 2019 Mw7.1 Ridgecrest earthquakes. Such changes may be associated with co‐seismic stress drops, post‐seismic deformation processes, and static stress changes on a regional scale. The new high‐resolution catalog will contribute to improved understanding of the crustal stress field, earthquake triggering mechanisms, fault zone geometry, and sense of slip on the faults in southern California.

Simultaneous Seismic Phase Picking and Polarity Determination with an Attention-Based Neural Network

AbstractEarthquake focal mechanisms present information on fault plane and stress direction, which is crucial for understanding tectonics and seismicity. Source focal mechanisms of small earthquakes are often difficult to determine from waveform modeling but feasible to infer from initial polarities. Here, we employ a state-of-the-art neural network infused with an attention mechanism to simultaneously pick arrivals and determine the first-motion polarity. The model is trained and tested with data from southern California. Compared with polarity inference with manual picks in the catalog, predicted polarity inference can help obtain more focal mechanism solutions in southern California. We test this model with data from different regions and observe high generalizability. The predicted arrival and polarity data are consistent with the labeled arrival and polarity data in Japan. The average-picking error is 0.04 s, and the accuracy of polarity classification is 99%. We infer the focal mechanisms from the predicted polarity in Oklahoma. The derived focal mechanisms are consistent with referencing focal mechanisms. This method allows routinely obtaining arrival and polarity data, and deriving focal mechanism solutions for events.

S/P Amplitude Ratios Derived from Single-Component Seismograms and Their Potential Use in Constraining Focal Mechanisms for Microearthquake Sequences

Abstract Focal mechanisms, which reflect the sense of slip in earthquakes, provide important constraints for understanding crustal tectonics and earthquake source physics, including the interactions among earthquakes during mainshock–aftershock sequences or seismic swarms. Focal mechanisms of small (magnitude ≲3.5) earthquakes are usually determined by first-motion P-wave polarities, sometimes supplemented by the ratio of S-wave to P-wave amplitudes (S/P). However, focal mechanisms of such events can be difficult to reliably constrain, particularly with sparse recording networks or very small magnitude events. Here, we describe a method for deriving S/P amplitude ratios from P/P and S/S amplitude ratios measured on single seismic components between pairs of nearby events, as is often performed during correlation-based earthquake detection and relocation. These measurements can be transformed into relative S/P amplitude ratios, or they can be combined with a smaller number of traditional S/P amplitude ratios to provide a single-channel estimation of full S/P ratios, even for low signal-to-noise-ratio events not routinely cataloged and not amenable to traditional S/P ratio processing. This approach has the potential to greatly expand the applicability of S/P amplitude ratios, providing additional constraints for focal mechanisms of small earthquakes, particularly for spatially concentrated seismicity sequences.

Focal mechanisms of small earthquakes beneath the Japanese islands based on first-motion polarities picked using deep learning

SUMMARY Knowledge of crustal stress fields is essential for understanding tectonics and earthquake generation. One approach for estimating the crustal stress field is based on the focal mechanisms of earthquakes. This study investigated the focal mechanisms of approximately 110 000 microearthquakes in the area of the Japanese islands that occurred at a depth shallower than 20 km, based on the first-motion polarities picked by a simple neural network model. The model was first trained using a data set of mainly moderate to large earthquakes throughout Japan. Following on, the model was re-trained using a data set of microearthquakes in two regions of Japan. The threshold of the confidence score from the neural network model was chosen to maximize the overall quality of the focal mechanism solutions. The P- and T-axes of the numerous focal mechanism solutions provided more detailed distributions of the crustal stress field. For example, in the Chugoku region, small differences were observed in the trend of P-axes azimuths between the northern and southern areas, spatially corresponding to geodetic observations. The results of this study are useful for revealing the crustal stress field, and, as such, for assessing past and current tectonic activities and potential future earthquake generation.

Focal mechanisms and stress field in the Nobi fault area, central Japan

In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P- and T-axes were N97° ± 23° E and N6° ± 32° E, and the averages of the dips of the P- and T-axes were 11° ± 10° and 32° ± 25°, respectively. These variations in the P- and T-axes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (σ 1) is oriented E–W over almost the entire study area. The stress ratio R, which is defined as R = (σ 1 – σ 2)/(σ 1 – σ 3), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating σ 2 ≈ σ 3, and thus the dominant stress in this region is a uniaxial compression in the direction of σ 1. The direction of the σ 1-axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released.

Visco-elastic stress triggering model of Tangshan earthquake sequence

We calculated the Coulomb failure stress change generated by the 1976 Tangshan earthquake that is projected onto the fault planes and slip directions of large subsequent aftershocks. Results of previous studies on the seismic failure distribution, crustal velocity and viscosity structures of the Tangshan earthquake are used as model constraints. Effects of the local pore fluid pressure and impact of soft medium near the fault are also considered. Our result shows that the subsequent Luanxian and Ninghe earthquakes occurred in the regions with a positive Coulomb failure stress produced by the Tangshan earthquake. To study the triggering effect of the Tangshan, Luanxian, and Ninghe earthquakes on the follow-up small earthquakes, we first evaluate the possible focal mechanisms of small earthquakes according to the regional stress field and co-seismic slip distributions derived from previous studies, assuming the amplitude of regional tectonic stress as 10 MPa. By projecting the stress changes generated by the above three earthquakes onto the possible fault planes and slip directions of small earthquakes, we find that the “butterfly” distribution pattern of increased Coulomb failure stress is consistent with the spatial distribution of follow-up earthquakes, and 95% of the aftershocks occurred in regions where Coulomb failure stresses increase, indicating that the former large earthquakes modulated occurrences of follow-up earthquakes in the Tangshan earthquake sequence. This result has some significance in rapid assessment of aftershock hazard after a large earthquake. If detailed failure distribution, seismogenic fault in the focal area and their slip features can be rapidly determined after a large earthquake, our algorithm can be used to predict the locations of large aftershocks.

Using S/P Amplitude Ratios to Constrain the Focal Mechanisms of Small Earthquakes

We test whether S-wave/P-wave amplitude ratio data can improve the computed focal mechanisms of small earthquakes, using events from two southern California aftershock sequences. The observed S/P ratios are generally consistent with the expected mechanisms, implying that S/P ratios can in fact be useful in constraining the focal mechanisms of small events. However, we also find that noise in the observations leads to scatter in the S/P ratios of factors of 2, and sometimes higher. This scatter limits the usefulness of the S/P ratios in two ways: (1) the focal mechanism cannot simply be fit to S/P amplitude data alone without accounting for the noise in a more sophisticated focal mechanism inversion process; (2) while the amplitude ratios may improve poorly constrained mechanisms, they are less useful in refining solutions that are already relatively well constrained.

Earthquake focal mechanisms and stress field in Sichuan-Yunnan area determined using P wave polarity and short period P and S waveform data

Based on waveform data, several methods to determine focal mechanisms of small earthquakes were developed since 1980. Kisslinger (1980) and Julian, Foulger (1996) proposed an approach to determine solution by using amplitude ratio of P and S wave. Schwartz (1995) devised a method to determine solutions by the use of polarity data and amplitudes of seismogram envelopes. Amplitudes of short period seismic waves propagating in an inhomogene-ous medium are sensitive to the variation in velocity and Q structure. Nakamura, et al (1999) took medium inhomo-geneity into account in determining focal mechanisms of small earthquakes using waveform data. If the locations of small earthquakes are concentrated in a small region, we can assume that the raypaths from the events to a given station are almost the same. So P and S wave attenuations are independent of event locations. In this case it is con-venient to determine focal mechanisms of these events by using short period P and S wave dataj. Focal mechanism solutions of small earthquakes in 5 regions, i.e., Rongchang, Mabian-Muchuan, Ya￠an, Baoxing and Mianzhu, which are covered by the Chengdu Telemetered Network, are obtained by analyzing the P polarity and short body wave amplitude data recorded in the network since 1992. According to the method proposed by Gephart and Forsyth (1984), based on well determined focal mechanism solutions in 15 sub-zones of Sichuan and Yunnan area, three principal stress tensors s1, s2, and s3, instead of averages of P, B, and T axis of the solutions, are determined to represent the regional stress field distribution.

Spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting Pacific plate beneath the northeastern Japan arc: A triple‐planed deep seismic zone

The northeastern Japan arc is located in one of the most seismically active subduction zones in the world. In this study, we relocated hypocenters and determined focal mechanisms of small earthquakes (M≤5) beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate. In the hypocenter relocation we adopted a “source region station correction method” in which station corrections vary with hypocenter locations. We have developed a new focal mechanism determination technique named “master solution method,” which is analogous to the “master event method” in hypocenter determination. We applied the method to P and SH wave amplitude data to obtain 1106 focal mechanism solutions. From the new mechanism solutions and relocated hypocenters we found that there occur both low‐angle thrust fault (LT) type and downdip compression (DC) type earthquakes at depths from 40 to 70 km near the aseismic front; the DC type events are underlying the LT‐type events. Almost all the earthquake clusters are composed of LT‐type events. The western limit of the region where LT‐type events have occurred is subparallel to the trench axis, although it undulates considerably; it delineates the westernend of the active region of interplate seismicity. Furthermore, we found that normal fault (NF) type events also occur at depths from 70 km in the upper plane of the double‐planed deep seismic zone, which is characterized mainly by DC‐type event. These NF‐type events are distributed only in a thin uppermost portion of the slab close to the plate boundary. Below these, in the lower plane, are downdip extension (DE) type events. This result indicates that the deep seismic zone in the northeastern Japan arc is not double‐planed but triple‐planed, even beneath the land area, which cannot be explained by any simple models.

A model for possible crustal deformation prior to a coming large interplate earthquake in the Tokai district, central Japan

Abstract We perform numerical simulations of a seismic cycle by applying a laboratory-derived friction law to evaluate possible crustal deformations prior to the hypothesized Tokai earthquake, which is expected to occur in the near future along the Suruga trough, central Japan. The frictional force obeying a rate- and state-dependent friction law is assumed to act on the plate interface in a 2D model of the subduction zone. We test two versions of the friction law: the so-called slowness version and the slip version. We select model parameters on the basis of historical documents about past large earthquakes in the Tokai district and the present coupling state of the plate interface beneath the Tokai district estimated by GPS observations. Simulation results for both the slowness and the slip version are as follows: (1) The coupling region of the plate interface becomes narrower with time in the interseismic period. (2) The inland crust contracts at a nearly constant rate for most of the interseismic period. The contraction rate decreases or a change from contraction to extension takes place before the occurrence of a large interplate earthquake. (3) Significant abnormal crustal deformation precedes the earthquake, and maximum amplitudes about one day before the earthquake exceed the noise levels in the case of clear weather of borehole strainmeters installed in the Tokai district. (4) The spatiotemporal variation of aseismic sliding on the plate interface perturbs the regional stress field around the source volume of the coming earthquake, leading possibly to precursory seismic quiescence in the overriding plate and a change in focal mechanisms of small earthquakes in the subducting slab. The most significant difference in the results between the slowness version and the slip version of the friction law is that amplitudes of preslip and crustal deformation just before earthquakes are several times larger for the slowness version than for the slip version. Although some ambiguity remains in the present model mainly because of the incompleteness of the friction law and the deficiency of observational data constraining model parameters, the physical model can be improved through the comparison of predicted crustal deformation with observations. Continuous and concentrated observations of crustal deformation and seismic activity are thus important in the Tokai district to understand quantitatively the sliding process preceding a large interplate earthquake there.

Focal mechanisms of small earthquakes in the southeastern Brazilian shield: a test of stress models of the South American plate

Focal mechanisms of small earthquakes with magnitudes of about 3 in the SE Brazilian shield are calculated using S/P amplitude ratios. Low attenuation (Qp from 400 to 800) in the shield upper-crustal layers allowed sharp S arrivals to be recorded up to distances of 100km. Besides P-wave polarities, SH-wave first motions were also used to constrain the nodal-plane orientations. Normal and reverse faulting mechanisms with strike-slip components were found. The inversion of four mechanisms to estimate the stress tensor indicated a strike-slip stress regime with roughly E–W-orientated σ1 and N–S σ3. Both the orientations and the shape factor (φ=0.7) of the inverted stress are in excellent agreement with theoretical predictions for that part of Brazil from the driving-force model of Coblentz & Richardson (1996). Good agreement with the nature of the stress, as well as its orientation, was also found for the model of Meijer (1995). Both of these theoretical models include spreading stresses along the continent/ocean lithospheric transition. Because the earthquakes are more than 300km from the continental shelf they should not be affected by the local flexural forces caused by sediment load in the marginal basins. The agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses.

Large-scale tectonic deformation inferred from small earthquakes

It is a long-standing question whether the focal mechanisms of small earthquakes can be used to provide information about tectonic deformation on a regional scale. Here we address this question by using a 28-year record of seismicity in the San Francisco Bay area to compare the strain released by small earthquakes with geological, geodetic and plate-tectonic measurements of deformation in this region. We show that on a small spatial scale, the strain released by small earthquakes is closely related to specific geological features. But when averaged over a regional scale, strain release more closely follows the regional pattern of tectonic deformation: this relationship holds for all but the largest earthquakes, indicating that the earthquake strain is self-similar1,2 over a broad range of earthquake magnitudes. The lack of self-similarity observed for the largest earthquakes suggests that the time interval studied is not large enough to sample a complete set of events—the fault with the highest probability3 for hosting one such missing event is the Hayward fault.

Focal mechanisms of small earthquakes and the stress field in the western Quebec Adirondack region

Focal mechanisms were determined for 42 small earthquakes (1.4 < M < 3.4) that occurred in the Adirondacks-western Quebec seismic region. The analysis demonstrates the variability of the focal mechanisms and suggests that the region can be divided into two subprovinces with distinctive seismotectonic regimes: 1. (1) in the Adirondacks, the focal mechanisms are mostly strike-slip with a comparatively smaller thrust component and indicate NNE compression: 2. (2) in the western Quebec seismic region, the stress field is more heterogeneous and is dominated by two directions (NE and NW) of horizontal compression. This variable pattern suggests that the seismicity and stress field are controlled by regional plate stresses but are affected by local structures and/or by the predominant orientation of preexisting faults.

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

AbstractThe 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 ML ≧ 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.

Application of waveform modeling to determine focal mechanisms of four 1982 Miramichi aftershocks

The objective of this study is to determine the focal mechanisms of small earthquakes at very near epicentral distances and use them to predict the observed waveforms. To achieve our objectives, the following steps were adopted before attempting to match the entire waveform: (a) obtain an appropriate velocity model; (b) calculate the Green's functions and predict ground motions due to an earthquake specified by dip (δ), slip (λ), and strike ( ϕ ) of a fault; (c) select all mechanisms which satisfy the first-motion polarity at all stations; (d) select a subset of focal mechanisms from those obtained in step (c) which produce P and S amplitudes and P/SV and SV/SH amplitude ratios correctly; and (e) use focal mechanisms of step (d) to match waveforms. As an alternative approach, a least-squares inversion for the moment-tensor elements was performed and decomposition of moment tensor matrix to two double couples (major and minor) was performed. Green's functions were computed using the Cagniard-de-Hoop theory (Helmberger and Harkrider, 1978). We applied the above approaches to four aftershocks ( mbLg ≦ 3.5) of the New Brunswick earthquake of 9 January 1982 and succeeded in obtaining focal mechanisms and producing synthetic waveforms very similar to the observed waveforms. Two groups of focal mechanisms are found. Two of the earthquakes are found to have reverse fault geometry similar to that of the main event of 9 January 1982 ( mb = 5.7), while the other two have significant components of strike-slip motion on a dipping plane. The results obtained from the least-squares analysis support the results of the search method. The fault geometries determined by the search method correspond well to the results of the major double couple of the least-square inversion. For completeness of the source characteristics investigation, spectral analysis of the seismograms was performed to determine the source parameters: corner frequencies, fc , seismic moments, source dimensions, and stress drops.

Constraints on plate motions in southern Pakistan and the northern Arabian Sea from the focal mechanisms of small earthquakes

The focal mechanism and depth were determined for nine small earthquakes (M0&lt;1025 dyn cm, M&lt;5.5) that occurred in southern Pakistan and the northern Arabian Sea from an analysis of the vertical component of Rayleigh waves in combination with limited first‐motion data. Focal parameters were determined from the Rayleigh waves by using an event‐pair method of analysis. For earthquakes that are located very close to each other (&lt;≈ 50 km), the event‐pair method is able to remove a significant proportion of propagation effects at all periods in the range of interest (20–50 s). For events separated by more than ≈ 100 km the propagation effects are reduced for only the longer periods (≈ 40–50 s). The earthquakes that were studied provide evidence for a model of plate interactions in the vicinity of the southern Pakistan triple junction. The Owen fracture zone is a transform fault that accommodates right‐lateral motion between the Indian and Arabian plates. The plate boundary in the vicinity of the Murray ridge is also partially made up of transform segments that strike subparallel to the Owen fracture zone. Spreading centers may also exist in the vicinity of the Murray ridge but were not documented by seismic or other evidence. The slip azimuths for earthquakes along this boundary are significantly more northerly than those predicted by various regional and worldwide models of plate motion. The Arabian plate is being subducted beneath the Eurasian plate along the southern coast of Pakistan. Slip vectors for earthquakes along this boundary trend northnortheasterly in general agreement with predicted directions. Left‐lateral motion is documented along the boundary between the Indian and Eurasian plates in southern Pakistan. The predicted direction of relative motion between these plates is not significantly different from that observed. Two of the earthquakes studied appear to be intraplate in nature. The depth and focal mechanism of one intraplate event, which may have occurred within the accretionary prism along the Makran coast, however, are based on limited data. The other intraplate earthquake occurred within the sedimentary deposits along the western edge of the Indian subcontinent near the Indian‐Eurasian plate boundary.Appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐009; $2.50. Payment must accompany order.

Determination of focal mechanism from SV/P amplitude ratios at small distances

A computational scheme for determining focal mechanisms of small earthquakes recorded at short distances by high-gain, short-period seismographs has been developed and tested. The steps in the procedure are explained. The most serious difficulty is the inherent nonlinearity of the problem and the attendant question of determining when a convergent solution is a well-constrained representation of the real focal mechanism.