Complex Fault Geometry of the 2020 MWW6.5 Monte Cristo Range, Nevada Earthquake Sequence
Christine J. Ruhl, Emily A. Morton, Jayne M. Bormann, Rachel Hatch-Ibarra, Gene Ichinose, Kenneth D. Smith
MManuscript Accepted to Seismological Research Letters Complex Fault Geometry of the 2020 M WW Christine J. Ruhl, Emily A. Morton, Jayne M. Bormann, Rachel Hatch-Ibarra, Gene Ichinose, and Kenneth D. Smith
Christine J. Ruhl, Corresponding Author
Department of Geosciences
The University of Tulsa
800 South Tucker Drive
Tulsa, Oklahoma 74104-9700 USA [email protected]
Declaration of Competing Interests
The authors acknowledge there are no conflicts of interest recorded.
ABSTRACT
On 15 May 2020 an M WW anuscript Accepted to Seismological Research Letters INTRODUCTION
The 15 May 2020 M WW anuscript Accepted to Seismological Research Letters et al. , 2020). Within two days of the mainshock, the Nevada Seismological Laboratory (NSL) deployed eight temporary seismic stations which recorded the prolific aftershock sequence (Bormann et al., 2021; Figure 2). The Monte Cristo Range earthquake is the largest to occur in Nevada since the December 1954 Fairview Peak M w w et al. , 1999; Dixon et al. , 2000; Bennett et al. , 2003). In the Central Walker Lane, this northwest-directed right-lateral strain is accommodated through a combination of three structural mechanisms: (1) left-lateral slip and clockwise block rotations between east-west-striking faults; (2) right-lateral slip and block translation on northwest-striking faults; and (3) extension on north-south-striking normal faults (see e.g., Wesnousky, 2005). anuscript Accepted to Seismological Research Letters M WW w
6+ earthquakes occurred on orthogonal and en echelon strike-slip faults (Hill, 2006). In 1984, the M w w w et al. , 2014); the long-duration 2011 M w et al. , 2011); the 2016 anuscript Accepted to Seismological Research Letters w et al ., 2019); and the 2020 Mono Lake sequence have also occurred. The left-lateral 1997 Fish Lake Valley (Ichinose et al., 2003) and the left-lateral 2013 M L w w w M L ≥ 3, 42 M L ≥ 4, and 4 M L ≥ 5 aftershocks through 31 Aug. 2020 (Figures 1 - 3). To explore the fault orientations and slip mechanisms involved in the Monte Cristo Range earthquake, we use precise earthquake relocations in combination with moment tensors for 128 of the largest events. Earthquake relocations are necessary to identify fault geometry at depth, which in recent years has been shown to be an important and complex aspect of strike-slip earthquakes in the western US (e.g., Ross et al., 2019). Precise relocations also provide information on seismogenic depth (Ruhl, Seaman, et al., e.g., Bormann et al., anuscript Accepted to Seismological Research Letters
OVERVIEW OF THE M6.5 MONTE CRISTO RANGE EARTHQUAKE SEQUENCE
The Monte Cristo Range mainshock earthquake has a w-phase moment magnitude ( M WW ) of 6.5 (USGS Event Page, https://earthquake.usgs.gov/earthquakes/eventpage/nn00725272), full-waveform moment magnitude (M w ; this study) of 6.4, and local magnitude (M L w anuscript Accepted to Seismological Research Letters anuscript Accepted to Seismological Research Letters w = 0.77M L + 0.68 and use it to convert local magnitudes of smaller events to moment magnitude. The cumulative moment magnitude of aftershocks with moment tensor solutions through 31 Aug. 2020 is M w w >5.0 aftershocks have occurred since 31 Aug 2020. The magnitude of completeness for the NSL earthquake catalog for this sequence is estimated as M L L b -value for the sequence to date is 0.87 ± 0.01 for automatic and analyst reviewed earthquakes above the magnitude of completeness and below M L L anuscript Accepted to Seismological Research Letters w WW anuscript Accepted to Seismological Research Letters
10 relocations and moment tensors to discuss the complex fault structures observed at depth before returning to the complexity of the mainshock rupture.
ABSOLUTE AND RELATIVE RELOCATION OF THE AFTERSHOCK SEQUENCE
We select 18,256 events located by NSL from 1 Jan. 2020 to 31 Aug. 2020 and access P- and S-phase arrival data directly from the NSL database (see Data and Resources). Using USGS program HYPOINVERSE (Klein, 1978), we calculate precise absolute relocations (Figures S5 and S6) by applying datum and station corrections (Figure S7) to the entire sequence following Ruhl, Seaman, et al. (2016) and Ruhl, Abercrombie, et al. (2016); please see Supporting Text for more details on the absolute relocation process. This method decreases the median absolute horizontal and vertical location uncertainties from 1.15 and 1.23 km in the NSL catalog to 0.73 and 0.89 km, respectively (Figure S8). We modified the four shallowest layers of an NSL preferred velocity model developed for the Northern Walker Lane (Ruhl, Seaman, et al., 2016) to include slower velocities, which reduced the number of events locating above the surface (Table S1). To sharpen features seen in the absolute locations, we calculate waveform-based double-difference relocations. We download three-component waveform data from the IRIS Data Management Center (see Data and Resources) for all seismic stations within 150 km of each event, including waveforms for eight temporary stations rapidly deployed by NSL between 16 - 18 May 2020 (see Bormann et al., 2021). We use the double-difference relocation algorithm HypoDD (Waldhauser and Ellsworth, 2000) to pair earthquakes with 5 or more phases with up to 30 neighbors, thus forming 342,287 event pairs from 18,057 events with a total of 220,874 individual phase picks. For each phase pair, we calculate catalog differential times (2.48 million anuscript Accepted to Seismological Research Letters
11 P-waves and ~54,000 S-waves) and perform sub-sampled waveform cross-correlations using a magnitude-based bandpass filter on windows of 0.5 s and 1.0 s centered on the arrival for P- and S-waves, respectively. Please see Supporting Text for more information on location uncertainties (Figure S8), filtering, cross-correlation (Figures S9 and S10), and relocation tests (Figure S11). This approach results in ~132,000 and ~22,000 P- and S-phase cross-correlations, respectively, with cross-correlation coefficients greater than or equal to 0.6. Using a total of 2.69 million catalog and cross-correlated differential times recorded on 29 near-source and regional stations, we resolve relative relocations for 16,714 events (95%). Events are discarded if they locate above the surface (airquakes) or lose connection to other events. The relocated catalog (Dataset S1) is shown in Figure 4 and subsequent figures as well as electronic supplement Movie S1.
COMPLEX FAULT NETWORK: AFTERSHOCKS THROUGH 31 AUG. 2020
As shown by the catalog locations in Figure 2, the aftershocks extend approximately 35 km along an east-northeast-trending zone that extends eastward from the mapped surface trace of the left-lateral Candelaria fault zone (CFZ) to the southern extension of the right-lateral Petrified Springs fault system (PSFS). If the length of the aftershock zone represents the mainshock fault length, it is 1.5 to 2 times longer than expected for an Mw6.5 earthquake rupture (Wells and Coppersmith, 1994). Our relocations (Figures 4 and 5) show that the aftershock zone is composed of many distinct fault structures with various orientations. The northeast-southwest trending zone is offset near the mainshock hypocenter (Figure 4a) by a dense north-striking zone of seismicity extending from the northern terminus of the extensional Eastern Columbus Salt Marsh fault zone (ECSMF). The relocated mainshock is slightly west of the NSL catalog location, anuscript Accepted to Seismological Research Letters
12 but still occurs near the intersection of the ECSMF and the east-northeast-trending aftershock zone. Earthquakes commonly nucleate and terminate at fault intersections (e.g., King, 1983), and the mainshock occurs between two distinct fault zones. The structures defined by our relocated seismicity show distinctly different patterns to the west and east of the mainshock hypocenter. Accordingly, we describe structures in the western and eastern sections separately below.
Seismicity Patterns in the Western Section
In the western section (i.e., west of the ECSMF and mainshock hypocenter), seismicity defines a near-vertical fault at depth which broadens towards the surface in a negative-flower-structure-like network of en echelon dipping normal and obliquely-slipping faults (Figure 4c). This interpretation is supported by moment tensors, which are shown as lower-hemisphere in mapview figures and back-hemisphere projections in the cross-sections in Figures 4 and 5. In the fault-parallel cross-section A-A’, there is shallow zone with relatively fewer aftershocks (between 10 to 25 km distance and above ~3 km depth in Figure 4b) that may represent an area of high slip in the mainshock, although it is not correlated with left-lateral east-northeast-striking surface ruptures. Dense seismicity and pure left-lateral moment tensors cluster deeper than and to the southwest of the mainshock hypocenter (Figure 4b). We infer that this densely-populated vertical fault section is the primary left-lateral fault plane that slipped during the mainshock (Figures 4a-c, 5d-e). The west-dipping ECSMF structure is below the mainshock in oblique cross-sections in Figures 4b and 4d. Only one moderate magnitude aftershock occurred on this structure, but it is well defined by smaller magnitude seismicity. Inclusion of first-motion focal mechanisms for anuscript Accepted to Seismological Research Letters
13 smaller events will improve the kinematic interpretation of fault structures identified via seismicity lineaments. The narrow near-vertical, left-lateral fault zone extends from a approximately 12 km to 6 km depth, and strike-slip moment tensors align with the geodetically-derived fault plane at these depths (Figures 4c, 5d-e). Above 6 km depth, seismicity bifurcates into two seismicity zones highlighted by ellipses oriented at 60°SE and 70°NW in Figures 4c and 5c-e. Surface rupture and fracture zone locations are indicated by the blue lines on the surface of cross-sections in Figures 4 and 5. Seismicity zones that shallow towards the northwest consistently project towards surface ruptures identified by Koehler et al. (2021) and Dee et al. (2020); see Figures 4c and 5c-e. The shallowest seismicity appears to abut the surface ruptures, and the strike of the ruptures matches closely the northeast-strike of the nodal planes in the normal and oblique moment tensors (Figure 4). We interpret the seismicity as a broad fault-fracture mesh which extends towards the surface but is not necessarily directly connected to the mapped surface ruptures. Several shallow, northeast-striking planar seismicity clusters in the western section show distinct separation in a right-stepping en echelon pattern. This is particularly apparent when seismicity is viewed in three dimensions, and we therefore include a fly-through movie in the electronic supplement (Movie S1). Some of these planar structures appear to be subparallel, but we identify dips towards both the west-northwest and east-southeast (see Figures 4 and 5). Many of the moment tensors associated with these dipping features show both normal and oblique strike-slip motion and we interpret them as a set of discontinuous normal faults that together accommodate left-lateral motion and form the relatively broader east-northeast trending aftershock zone seen in the western section (Figure 4a). anuscript Accepted to Seismological Research Letters
14 One of the most obvious dipping structures is the nearly north-striking extension of the ECSMF near the mainshock hypocenter. We estimate the dip of this seismicity as approximately 70°W by measuring it on a vertical cross-section perpendicular to the ECSMF strike. All west- and northwest-dipping ellipses in Figures 4 and 5 are oriented at 70°, while the southeast-dipping ellipses are at 60°. Together, the slightly southeast-dipping mainshock fault plane(s) and the west-dipping ECSMF form a southwest-facing wedge that bounds the down-dropped Columbus Salt Marsh tectonic basin (gray dashed line on Figure 3). Shallow seismicity directly above these well-defined structures and within geodetic fault plane boundaries shown in Figure 4b is notably sparse, but shallower events cluster towards the end of the aftershock zone (2-6 km distance in Figure 4b). We observe short S-P times (<0.5 s) for some of these earthquakes on the temporary near-source stations, which suggests that these events may indeed be that shallow.
Seismicity Patterns in the Eastern Section
In the eastern section (i.e., east of the mainshock), vertical, moderately dipping, and obliquely-crossing structures are visible in map view and in cross-section (Figure 4). At the eastern edge of the aftershock zone, seismicity trends to the northwest towards the southern extension of the Petrified Springs fault system (Figures 3-5, parallel to cross-section 4e, Figure 4a D-D’ ). The northwest-trending zone at the eastern edge appears to be composed of many short, discontinuous faults, suggesting a fault-fracture mesh rather than a well-defined, through-going northwest-trending right-lateral fault (Figure 4e and Figure 5g). Events are concentrated at shallower depths on both the western and eastern edges of the aftershock zone (Figures 4b, 5b, and 5g). anuscript Accepted to Seismological Research Letters
15 Just east of the mainshock hypocenter, seismicity broadens spatially (Figure 4a) and concentrates on multiple steeply dipping oblique structures at depths between 3 and 10 km (Figure 4a, 4b, and 4d). Between the C-C’ and D-D’ profile lines (Figure 4a), the seismicity cloud collapses into a narrow (<1 km) near-vertical fault strand with small orthogonal cross-faults (Figure 4a, 4b, 5f). Northwest-striking seismicity lineaments align with right-lateral moment tensor nodal planes and show similar length and orientation to mapped surface rupture zones with right-lateral strike-slip (Figure 4a, vertical ellipse in Figure 4f; Koehler et al., 2021; Dee et al., 2020). The remarkable agreement, and yet kinematic inconsistency, between mapped surface ruptures, seismicity trends, and moment tensor lineaments suggests that the Monte Cristo Range mainshock ruptured multiple faults with different senses of slip to accommodate overall left-lateral shear.
MAINSHOCK RUPTURE: COMPLEX FAULT(S) WITH POSSIBLE SUBEVENTS
Waveforms of the mainshock are complex, with phases indicating two overlapping earthquakes or sub-ruptures (Figure 6). In the raw waveforms, there appears to be a smaller initial earthquake followed by a larger earthquake 2 - 3 s later. This observation is supported by the automatic source time function generated by the Institut de Physique du Globe de Paris (IPGP; Figure 6 inset; Vallée et al., 2011; Vallee et al., 2013). The IPGP teleseismic body-wave moment tensor has a half-duration of 4.2 s, typical of an M6.5 earthquake, but for the Monte Cristo Range earthquake source time function, the centroid minus hypocenter time is 10.0 s - more than twice the half-duration. The IPGP source time function shows a smaller and shorter initial pulse followed by a larger and longer pulse approximately 3 s after initiation (Figure 6 anuscript Accepted to Seismological Research Letters
16 inset). This further supports that the Monte Cristo Range earthquake was a complex, multiple subevent rupture. To further explore the hypothesis of complex multi-fault rupture during the mainshock, we discuss the spatiotemporal distribution of aftershocks in relation to the mainshock. The aftershock distribution extends bilaterally from the mainshock hypocenter extending to the full 35-km length of the rupture zone within one day of the mainshock (Figure 5). In cross-section A-A’ of Figure 4 and discussed in the previous sections, seismicity concentrates on numerous faults and leaves large seismicity voids. For example, there is a decrease in earthquake density where the aftershock zone crosses the region between the southern Benton Springs fault and the northern Eastern Columbus Salt Marsh fault (approximately 18 km distance in Figure 4b). The majority of large aftershocks, both east and west of the mainshock, occur within the cross-sectional area of the simple fault model independently developed by Hammond et al. (2020) using geodetic surface displacements (green rectangle in Figure 4b). Large magnitude aftershocks (those with moment tensors) cluster on the deeper vertical fault in the western section and at similar shallow depths as the mainshock in the eastern section (Figure 4b). To show this, we calculate cumulative number and cumulative moment release density plots of aftershocks along the entire fault zone (Figure 7). We bin the relocated hypocenters spatially and in depth using nonoverlapping 2.5 km-by-2.5 km bins which is roughly the size of an average M w anuscript Accepted to Seismological Research Letters
17 aftershocks and the cumulative moment of aftershocks spatially across this zone. Dashed boxes in Figure 7d, marking the regions with the highest number of events from Figure 7c, show that the greatest moment release is not coincident with the greatest number of events. Significantly more earthquakes occurred in the western section, but the cumulative moment release of all M w ≥ 3.0 earthquakes is concentrated in the eastern section (Figures 4 and 7). This relationship may be related to heterogeneous mainshock slip across the fault zone, preexisting fault heterogeneity, and/or secondary postseismic processes like fluid migration and aseismic slip. Finally, we discuss the depth distribution of seismicity as it relates to the mainshock rupture. There is weak evidence for downward migration of seismicity through time (Figure 5), but hypocentral depth uncertainties are higher for aftershocks in the first few days of the sequence before the deployment of near-source temporary stations (Bormann et al., 2021). We split the data into half-week (3.5-day) bins and plot the distribution of events in 1-km depth bins through time (Figure 8). Shallow events persist, even after the installation of temporary stations (Figure 8a). The majority of events occur between 3 and 8 km depth (Figure 8c), and the largest magnitude events occur primarily above 8 km depth (Figure 8b). DISCUSSION ON COMPLEX EARTHQUAKES
Ruptures spanning multiple faults have become an increasingly common observation of moderate magnitude continental earthquakes. In the western US, complex ruptures are dominated by M5-7 strike-slip earthquakes rupturing immature and often discontinuous fault zones in the diffuse parts of the plate boundary system (i.e, the Eastern California Shear Zone and the Walker Lane). We introduced many of these examples which occurred near the Monte anuscript Accepted to Seismological Research Letters
18 Cristo Range earthquake at the start of this paper, but larger magnitude examples also exist. One of the classic examples is the 1992 M w en echelon, right-stepping, right-lateral faults in the Eastern California Shear Zone (Haukkson et al., 1993). More recently, the 2019 M w w anuscript Accepted to Seismological Research Letters
19 subduction plate boundary had ruptured; inconsistencies existed between geodetic deformation, surface fault offsets, seismological observations of the mainshock, and subsurface seismicity locations including the mainshock hypocenter being significantly offset from the moment centroid (Furlong and Herman, 2017). By incorporating these data sets along with tsunami propagation modeling, it was shown that the mainshock involved synchronous slip on multiple upper crustal strike-slip faults as well as slip on the plate interface at depth (Bai et al., 2017; Furlong and Herman, 2017). Without additional constraints on fault slip at depth, it was difficult to reconcile seismogenic observations with the distributed deformation observed at the surface. These examples highlight the need for using multiple surface and subsurface observations to model complex earthquakes and to assess seismic hazard. By considering multiple observations of the Monte Cristo Range earthquake and its aftershocks, a model for the complex kinematic relationship between surface ruptures and left-lateral slip at depth can be developed. The left-lateral strike-slip moment tensor of the M WW anuscript Accepted to Seismological Research Letters SUMMARY
The M WW anuscript Accepted to Seismological Research Letters
21 Instead, our results imply that this immature left-lateral fault system does not extend to the surface and therefore left-lateral slip at depth is distributed complexly over a wide area in the top five kilometers above the fault. Based on our structural interpretation and considering the complex waveforms and double-pulse source time function of the mainshock, it is likely that the M6.5 Monte Cristo Range earthquake simultaneously ruptured multiple faults at different orientations. By considering multiple observations of the Monte Cristo Range earthquake and its aftershocks, a model for the complex kinematic relationship between surface ruptures and left-lateral slip at depth can be developed. Detailed spatiotemporal, fault kinematic, and dynamic rupture analyses are needed to truly understand how these sub faults interacted during and after the mainshock rupture.
DATA AND RESOURCES
We downloaded the regional earthquake catalog in Figure 1 through the ANSS Comprehensive Catalog (USGS EHP, 2017). We also accessed USGS products (e.g., DYFI reports and w-phase moment tensor) from the USGS event page for the mainshock (https://earthquake.usgs.gov/earthquakes/eventpage/nn00725272/). The SCARDEC source time function was accessed from the IPGP event page (http://geoscope.ipgp.fr/index.php/en/catalog/earthquake-description?seis=nn00725272). The Jan. 1 to Aug. 31 2020 local event locations and phase data were accessed directly from the Nevada Seismological Laboratory database, and phase picks for catalog events are also available through the ANSS Comprehensive catalog (https://earthquake.usgs.gov/data/comcat/). All Quaternary faults shown in maps are from the USGS Quaternary Faults and Folds Database anuscript Accepted to Seismological Research Letters
22 (USGS, CGS, and NBMG, 2020). Simplified surface fault ruptures and fracture zones were obtained from Seth Dee based on detailed mapping in Dee et al. (2021) and Koehler et al. (2021). The geodetic fault model is from Hammond et al. (2021). We accessed waveforms through IRIS Data Services, specifically the IRIS Data Management Center (DMC), using ObsPy, a python library for seismological analysis (Krischer et al., 2015). IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience (SAGE) Award of the National Science Foundation under Cooperative Support Agreement EAR-1851048. We make our moment tensor and relocated earthquake catalogs available in the electronic supplement (Datasets S1 and S2).
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Geophys. J. Int. , 338-358, doi:10.111/j.1365-246X.2010.04836.x. Wang, Q., Chu, R., Yang, H., Zhu, L., & Su, Y. (2018). Complex Rupture of the 2014 M s 6.6 Jinggu Earthquake Sequence in Yunnan Province Inferred from Double-Difference Relocation. Pure and Applied Geophysics, 175(12), 4253-4274. Waldhauser, F., & Ellsworth, W. L. (2000). A double-difference earthquake location algorithm: Method and application to the northern Hayward fault, California. Bulletin of the Seismological Society of America, 90(6), 1353-1368. Wells, D. L., & Coppersmith, K. J. (1994). New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bulletin of the seismological Society of America, 84(4), 974-1002. Wesnousky, S. G. (2005). Active faulting in the Walker Lane,
Tectonics , TC3009, doi 10.1029/2004TC001645. Wessel, P., and W. H. F. Smith (1991). Free software helps map and display data, Eos Trans. AGU , 441–446, doi: 10.1029/90EO00319. Wessel, P., W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe (2013). Generic Mapping Tools: Improved version released, Eos Trans. AGU , 409–410, doi: 10.1002/2013EO450001. Xu, W., Feng, G., Meng, L., Zhang, A., Ampuero, J. P., Bürgmann, R., & Fang, L. (2018). Transpressional rupture cascade of the 2016 Mw 7.8 Kaikoura earthquake, New Zealand. Journal of Geophysical Research: Solid Earth, 123(3), 2396-2409. anuscript Accepted to Seismological Research Letters Christine J. Ruhl
Department of Geosciences
The University of Tulsa
Tulsa, Oklahoma, USA [email protected]
Emily A. Morton
Jayne M. Bormann
Rachel Hatch-Ibarra
Kenneth D. Smith
Nevada Seismological Laboratory
University of Nevada, Reno
Reno, Nevada, USA [email protected] [email protected] [email protected] [email protected]
Gene Ichinose
Lawrence Livermore National Laboratory
Livermore, California, USA [email protected] anuscript Accepted to Seismological Research Letters anuscript Accepted to Seismological Research Letters LIST OF FIGURES FIGURES Figure 1. (a) Regional seismotectonic map showing the location of the M w Figure 2.
NSL Catalog location map. Earthquakes are sized and colored by local magnitude. Temporary seismic stations are shown by white triangles with labels. USGS Quaternary Faults and Folds database faults are shown in gray. Relevant fault zones are labeled (CFZ = Candelaria fault zone, ECSMF = Eastern Columbus Salt Marsh fault, PSFS = Petrified Springs fault system, and BSF = Benton Springs Fault). Columbus Salt Marsh is highlighted by white dashed polygon. The anuscript Accepted to Seismological Research Letters
33 moment tensor for the M w w Figure 3.
Earthquake statistics of the Monte Cristo Range aftershocks. (a) Comparison of NSL local magnitudes (M L ) with body-wave-based moment magnitudes (M w ) determined from moment tensor solutions. Solid line represents the best-fit relationship between the magnitude types. (b) Earthquakes in the region of the Monte Cristo Range sequence (black circles) since 15 Apr. 2020, 1 month preceding the mainshock, through 31 Aug. 2020, with corresponding local magnitudes. Orange line corresponds to the cumulative aftershock M w for 128 aftershocks M L ≥ 3 with moment tensor solutions, culminating at M w Figure 4.
High-precision earthquake relocations in map view and cross-section. (a) Map of earthquakes and moment tensors. Events are sized by magnitude and colored by depth. USGS Quaternary faults (gray), generalized surface ruptures and fracture zones (blue), and geodetic fault model (green) are also shown. The mainshock has a bold outline. The geodetic fault model is shown in green and surface ruptures are shown in blue. (b-d) Cross-sections for vertical profile lines shown in (a). Events within 1.0 km and surface ruptures within 2.5 km of profile lines are anuscript Accepted to Seismological Research Letters
34 included in the cross-sections. Fault structures discussed in the text are highlighted by dashed ellipses. Southeast-dipping and northwest-dipping ellipses are oriented at 60° and 70°, respectively.
Figure 5.
High-precision earthquake relocations in map view and cross-section. (a) Map of earthquakes and moment tensors. Events are sized by magnitude and colored by time in days since the mainshock. USGS Quaternary faults (gray), generalized surface ruptures and fracture zones (blue), and geodetic fault model (green) are also shown. The mainshock has a bold outline. The geodetic fault model is shown in green and surface ruptures are shown in blue. (b-g) Cross-sections for vertical profile lines shown in (a). Events within 1.0 km and surface ruptures within 2.5 km of profile lines are included in the cross-sections. Shallow fault structures discussed in the text are highlighted by dashed ellipses. Southeast-dipping ellipses are oriented at 60°.
Figure 6. Three component d isplacement record sections for the mainshock. Filtered waveforms (1 Hz highpass) are colored semi-transparent to show overlapping traces and labeled by corresponding seismic network code and station name. P-wave (blue) and S-wave arrivals (red) are marked on the waveforms. Vertical line at the bottom left of the east-component panel indicates 1 mm of displacement. Corresponding IPGP source time function is inset (top center).
Figure 7.
Density plots of cumulative number of relocated events (a and b) and cumulative moment release (c and d). Cumulative moment release corresponds to 128 aftershocks M L ≥ 3 with moment tensor solutions using their relocated hypocenters. Density maps (a and c) show anuscript Accepted to Seismological Research Letters
35 USGS Quaternary faults (black lines) and relocated event locations (gray dots). Depth cross-sections (b) and (d) include aftershocks within 10 km of line A-A’. Mainshock hypocenter is indicated by the black star. Dashed lines in cross-section (d) correspond to areas from cross-section (b) that had the highest numbers of aftershocks.
Figure 8.
Depth histograms of earthquake relocations colored by (a) days since the mainshock in 3.5 day increments (gray lines) with relocation velocity model (Table S1, black line), and by (b,c) local magnitude. (b) Events with magnitudes greater than M3.5 are plotted separately from (c) events with magnitudes less than M3.5 for clarity. Note that the centroid for events larger than M4.5 may be offset from the hypocentral depths shown here. anuscript Accepted to Seismological Research Letters Figure 1. (a) Regional seismotectonic map showing the location of the M w anuscript Accepted to Seismological Research Letters
37 extent of maps in subsequent figures. (b) Inset map shows the study location (blue box) in context of larger Pacific/North American plate boundary in Nevada and California. Abbreviations: SAF - San Andreas fault, SN - Sierra Nevada microplate, WL - Walker Lane, ECZS - Eastern California Shear Zone, and BRP - Basin and Range Province.
Figure 2.
NSL Catalog location map. Earthquakes are sized and colored by local magnitude. Temporary seismic stations are shown by white triangles with labels. USGS Quaternary Faults and Folds database faults are shown in gray. Relevant fault zones are labeled (CFZ = Candelaria fault zone, ECSMF = Eastern Columbus Salt Marsh fault, PSFS = Petrified Springs fault system, and BSF = Benton Springs Fault). Columbus Salt Marsh is highlighted by white dashed polygon. The moment tensor for the M w w anuscript Accepted to Seismological Research Letters Figure 3.
Earthquake statistics of the Monte Cristo Range aftershocks. (a) Comparison of NSL local magnitudes (M L ) with body-wave-based moment magnitudes (M w ) determined from moment tensor solutions. Solid line represents the best-fit relationship between the magnitude types. (b) Earthquakes in the region of the Monte Cristo Range sequence (black circles) since 15 Apr. 2020, 1 month preceding the mainshock, through 31 Aug. 2020, with corresponding local magnitudes. Orange line corresponds to the cumulative aftershock M w for 128 aftershocks M L ≥ 3 with moment tensor solutions, culminating at M w anuscript Accepted to Seismological Research Letters
39 best fitting decay corresponds to a decay parameter p=0.8. Orange line indicates the cumulative number of events through 31 Aug. 2020. anuscript Accepted to Seismological Research Letters Figure 4.
High-precision earthquake relocations in map view and cross-section. (a) Map of earthquakes and moment tensors. Events are sized by magnitude and colored by depth. USGS Quaternary faults (gray), generalized surface ruptures and fracture zones (blue), and geodetic fault model (green) are also shown. The mainshock has a bold outline. The geodetic fault model is shown in green and surface ruptures are shown in blue. (b-d) Cross-sections for vertical profile lines shown in (a). Events within 1.0 km and surface ruptures within 2.5 km of profile lines are included in the cross-sections. Fault structures discussed in the text are highlighted by dashed ellipses. Southeast-dipping and northwest-dipping ellipses are oriented at 60° and 70°, respectively. anuscript Accepted to Seismological Research Letters Figure 5.
High-precision earthquake relocations in map view and cross-section. (a) Map of earthquakes and moment tensors. Events are sized by magnitude and colored by time in days since the mainshock. USGS Quaternary faults (gray), generalized surface ruptures and fracture zones (blue), and geodetic fault model (green) are also shown. The mainshock has a bold outline. The geodetic fault model is shown in green and surface ruptures are shown in blue. (b-g) Cross- anuscript Accepted to Seismological Research Letters
42 sections for vertical profile lines shown in (a). Events within 1.0 km and surface ruptures within 2.5 km of profile lines are included in the cross-sections. Shallow fault structures discussed in the text are highlighted by dashed ellipses. Southeast-dipping ellipses are oriented at 60°.
Figure 6.
Three component displacement record sections for the mainshock. Filtered waveforms (1 Hz highpass) are colored semi-transparent to show overlapping traces and labeled by corresponding seismic network code and station name. P-wave (blue) and S-wave arrivals (red) are marked on the waveforms. Vertical line at the bottom left of the east-component panel indicates 1 mm of displacement. Corresponding IPGP source time function is inset (top center). anuscript Accepted to Seismological Research Letters Figure 7.