Surface Rupture Kinematics and Coseismic Slip Distribution During the 2019 Mw7.1 Ridgecrest, California Earthquake Sequence Revealed by SAR and Optical Images

remote sensing

Article Surface Rupture Kinematics and Coseismic Slip Distribution during the 2019 Mw7.1 Ridgecrest, California Earthquake Sequence Revealed by SAR and Optical Images

Chenglong Li 1, Guohong Zhang 1,*, Xinjian Shan 1, Dezheng Zhao 1 , Yanchuan Li 1 , Zicheng Huang 1, Rui Jia 1,2, Jin Li 3 and Jing Nie 4

1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China; [email protected] (C.L.); [email protected] (X.S.); [email protected] (D.Z.); [email protected] (Y.L.); [email protected] (Z.H.); [email protected] (R.J.) 2 School of Resource Environment and Earth Science, Yunnan University, Kunming 650091, China 3 Automation Department, Patent Oﬃce of National Intellectual Property Administration, Beijing 100088, China; [email protected] 4 China Patent Information Center, National Intellectual Property Administration, Beijing 100088, China; [email protected] * Correspondence: [email protected]; Tel.: +86-010-6200-9095

Received: 23 October 2020; Accepted: 25 November 2020; Published: 27 November 2020

Abstract: The 2019 Ridgecrest, California earthquake sequence ruptured along a complex fault system and triggered seismic and aseismic slips on intersecting faults. To characterize the surface rupture kinematics and fault slip distribution, we used optical images and Interferometric Synthetic Aperture Radar (InSAR) observations to reconstruct the displacement caused by the earthquake sequence. We further calculated curl and divergence from the north-south and east-west components, to eﬀectively identify the surface rupture traces. The results show that the major seismogenic fault had a length of ~55 km and strike of 320◦ and consisted of ﬁve secondary faults. On the basis of the determined multiple-fault geometries, we inverted the coseismic slip distributions by InSAR measurements, which indicates that the Mw7.1 mainshock was dominated by the right-lateral strike-slip (maximum strike-slip of ~5.8 m at the depth of ~7.5 km), with a small dip-slip component (peaking at ~1.8 m) on an east-dipping fault. The Mw6.4 foreshock was dominated by the left-lateral strike-slip on a north-dipping fault. These earthquakes triggered obvious aseismic creep along the Garlock fault (117.3◦ W–117.5◦ W). These results are consistent with the rupture process of the earthquake sequence, which featured a complicated cascading rupture rather than a single continuous rupture front propagating along multiple faults.

Keywords: subpixel correlation; optical images; InSAR; complex coseismic deformations; curl and divergence; multiple-fault ruptures

1. Introduction From 4 July to 6 July 2019, a sequence of damaging earthquakes (Mw > 5) struck the northeast region of Ridgecrest, in southern California. It began with an Mw6.4 foreshock in the opening of Searles Valley, near the town of Ridgecrest on 4 July 2019 at 17:33:49 (UTC) [1]. Approximately 34 h later, the Mw7.1 mainshock occurred in Ridgecrest on 6 July 2019 at 03:19:53 (UTC) [2]. A large number of aftershocks (Mw > 2.5), including two with a moment magnitude greater than Mw 5 [3], occurred within 2 days (Figure1a). This earthquake sequence occurred within the plate boundary

Remote Sens. 2020, 12, 3883; doi:10.3390/rs12233883 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 3883 2 of 18

Remote Sens. 2020, 12, x FOR PEER REVIEW 2 of 18 region, referred to as the Eastern California Shear Zone (ECSE), but the seismicity clustered around region, referred to as the Eastern California Shear Zone (ECSE), but the seismicity clustered around more than a single fault segment. Over the past several decades, the ECSE has accommodated several more than a single fault segment. Over the past several decades, the ECSE has accommodated several large strike-slip earthquakes, such as the 1992 Mw7.3 Landers earthquakes [4] and the 1999 Mw7.1 large strike-slip earthquakes, such as the 1992 Mw7.3 Landers earthquakes [4] and the 1999 Mw7.1 Hector Mine earthquake (Figure1b) [ 5]. The 2019 Ridgecrest earthquakes activated a complex fault Hector Mine earthquake (Figure 1b) [5]. The 2019 Ridgecrest earthquakes activated a complex fault system with curved faults unconnected on the surface but intersecting each other underground. These system with curved faults unconnected on the surface but intersecting each other underground. faults were previously unmapped in the Little Lake fault zone (LLFZ), near the Airport Lake fault zone These faults were previously unmapped in the Little Lake fault zone (LLFZ), near the Airport Lake (ALFZ), the China Lake basin, and southwestern Searles Valley (Figure1a). In addition, these faults fault zone (ALFZ), the China Lake basin, and southwestern Searles Valley (Figure 1a). In addition, were not included in the U.S. Geological Survey (USGS) Quaternary Fault database (U.S. Geological these faults were not included in the U.S. Geological Survey (USGS) Quaternary Fault database (U.S. Survey and California Survey, 2010) nor in the 2010 Fault Activity Map of California (California Geological Survey and California Survey, 2010) nor in the 2010 Fault Activity Map of California Geologic Survey, 2010) [6]. (California Geologic Survey, 2010) [6].

FigureFigure 1.1.Tectonic Tectonic setting setting of the of 2019 the Ridgecrest2019 Ridgecrest earthquake earthquake sequence. sequence. (a) Spatio-temporal (a) Spatio-temporal distribution ofdistribution foreshocks of and foreshocks aftershocks. and Twoaftershocks. stars and Two beach stars balls and mark beach the balls epicenters mark the and epicenters focal mechanisms and focal ofmechanisms the 2019 Mw7.1 of the 2019 Ridgecrest Mw7.1 earthquakeRidgecrest earthquake and the Mw6.4 and the Searles Mw6.4 Valley Searles earthquake Valley earthquake (from the (from U.S. Geologicalthe U.S. Geological Survey (USGS)),Survey (USGS)), respectively. respectively. Blue and Blue pink and dots pink represent dots represen the spatialt the spatial distribution distribution and temporaland temporal evolution evolution of foreshocks of foreshocks and aftershocks and aftershocks of the of Mw7.1 the Mw7.1 mainshock, mainshock, respectively. respectively. (b) White (b) rectangleWhite rectangle shows shows the spatial the spatial coverage coverage of the Sentinel-1of the Sent ascendinginel-1 ascending (track 64)(track data 64) utilized data utilized in this in study this andstudy the and yellow the yellow one shows one shows the spatial the spatial coverage coverage of the Sentinel-1of the Sentinel-1 descending descending (track 71)(track data. 71) The data. green The rectanglegreen rectangle shows theshows spatial the coveragespatial coverage of the Sentinel-2 of the Sentinel-2 optical image. optical Black image. moment Black tensorsmoment represent tensors focalrepresent mechanisms focal mechanisms of large historical of large events historical (Mw > events6.5) that (Mw occurred > 6.5) within that southernoccurred California.within southern ALFZ: AirportCalifornia. Lake ALFZ: fault zone;Airport LLFZ: Lake Little fault Lake zone; fault LLFZ: zone; Little ECSZ: Lake Eastern fault zone; California ECSZ: shear Eastern zone; California SAF: the Sandeshear zone; Andres SAF: fault. the Sande Andres fault.

More importantly, the 2019 Ridgecrest earthquake sequence was a series of intraplate cross-fault ruptures, which are relatively rare. Two main seismogenic faults ruptured in a cross-faulting geometry (~90◦) due to the Mw6.4 foreshock and the Mw7.1 mainshock. Cross-fault ruptures were observed in the 1987 Superstition Hills earthquake sequence [7,8], the 1992 Mw7.3 Landers earthquakes, and the 1999 Mw7.1 Hector Mine earthquake in southern California. These large events are related to a large number of active strike-slip faults in southern California. The dynamic rupture process of this earthquake sequence, the multiple faults geometry, and the triggered shallow aseismic creep on the Garlock fault were analyzed utilizing seismological and geodetic observations [9,10]. The surface rupture kinematics (especially in the near-fault area) of the Mw7.1 mainshock, however, were not well demonstrated. In addition, due to the geometric complexity of multiple faults involved in this earthquake sequence [11], how multiple faults ruptured in this earthquake sequence and what mechanism dominated the cross-fault rupture cannot be well explained when a single fault model is used. In this study, we investigated the kinematic characteristics of the complex surface rupture to refine the fault geometry. Over the past decades, dramatic improvements have been made in the technology of measuring and mapping the ground deformation caused by major earthquakes, such as Interferometric Synthetic Aperture Radar (InSAR) [12] and subpixel correlation technology of optical images. The subpixel optical image correlation technology has been used to process multi-source optical images, such as the SPOT-5 image [13], SPOT-7 image [14], Pleiades image [15], Landsat-8 image, Worldview image [16], and Sentinel-2 image [17], to reconstruct the coseismic horizontal displacement field and to study the complex coseismic rupture kinematics. A high-resolution horizontal displacement field allows us to map the surface traces of cross-fault ruptures in great detail and to quantify the coseismic displacement on the surface. The horizontal displacement field reconstructed by the subpixel optical image correlation provides valuable information on the near-fault (the region within 1 km of the seismogenic fault, where SAR images usually decorrelate) deformation patterns [17], which is of great help to reveal how the fault rupture broke the surface and gives robust constraints on the shallowest part of the fault slip model [18]. Jointly utilizing InSAR line-of-sight (LOS) observations and the horizontal measurements of optical images to reconstruct surface displacements can obtain the kinematic rupture characteristics [19] and slip distribution in a fault model. In this study, we reconstructed the coseismic horizontal displacement field (north-south (NS) and east-west (EW) components) using Sentinel-2 optical images, and we computed the curl and divergence of the NS and EW components to quantitatively analyze and demonstrate the geometric complexity of the surface rupture produced by the Ridgecrest earthquake sequence. Then, we obtained the surface LOS deformation using Sentinel-1 ascending and descending SAR images and inverted the slip distribution on multiple fault segments. Finally, we used the derived multiple-fault slip model to calculate the static Coulomb stress changes, and investigated the possible triggering relationships between the seismic slip and aseismic slip involved in the earthquake sequence.

2. Data and Methods An overview of the data processing chain is given in Figure2. It describes the steps and methods that were used to reconstruct the coseismic horizontal displacement ﬁeld, compute the curl and divergence maps, and compute the InSAR coseismic deformations (both ascending and descending). Remote Sens. 2020, 12, 3883 4 of 18 Remote Sens. 2020, 12, x FOR PEER REVIEW 4 of 18

FigureFigure 2.2. Overview of the the data data processing processing chain chain used used in inthis this investigation. investigation. Sen2Cor Sen2Cor is a processor is a processor for forSentinel-2 Sentinel-2 Level Level 2A 2A (L2A (L2A level) level) product product generation generation and and formatting. MicMac: MicMac: Multi Multi Images Images CorrespondancesCorrespondances parpar MMéthodeséthodes AutomatiquesAutomatiques dede CorrCorrélation.élation. LOS: line-of-sight. DEM: DEM: digital digital elevationelevation model. model.

2.1.2.1. HorizontalHorizontal DisplacementDisplacement FieldsFields by Subpixel Correlation of Optical Images Images WeWe used used the the Sentinel-2 Sentinel-2 optical optical images images (near-infrared (near-infrared band band 8, spatial8, spatial resolution resolution of 10 of m) 10 and m) oandffset trackingoffset tracking method method to reconstruct to reconstruct the horizontal the horizontal displacement displacement field of field this of earthquake this earthquake sequence. sequence. Firstly, radiometricFirstly, radiometric calibration calib andration atmospheric and atmospheric correction correction were performedwere performed to the to pre-earthquake the pre-earthquake (28 June(28 2019)June and2019) post-earthquake and post-earthquake (8 July (8 2019) July Level2019) Leve 1C imagesl 1C images using using the Sen2Cor the Sen2Cor package package (developed (developed by the Europeanby the European Space Agency) Space Agency) to obtain to the obtain Level the 2A Le opticalvel 2A images optical [20 images]. Then, [20]. the Then, obtained the Sentinel-2obtained LevelSentinel-2 2A data Level were 2A processed data were by processed orthorectification by orth andorectification coregistration and usingcoregistration COSI-corr using software COSI-corr [21–23 ]. Finally,software we [21–23]. calculated Finally, the we EW calculated and NS displacement the EW and NS components displacement (10 components m resolution) (10 of m the resolution) coseismic horizontalof the coseismic displacement horizontal field displacement using the field MicMac using package the MicMac based package on the based subpixel on the image subpixel correlation image technologycorrelation (correlationtechnology (correlation parameter parameter setting: window setting: =window1 pixel = 1 pixel1 pixel, × 1 regularizationpixel, regularization term term= 0.5, × initial= 0.5, uncertaintyinitial uncertainty= 2.0, = merging 2.0, merging factor factor= 1 pixel, = 1 pixel, correlation correlation threshold threshold= 0.5, = 0.5, gamma gamma= 2)= 2) [24 [24––26]. The26]. MicMac The MicMac package package developed developed by the by French the French National National Geographic Geographic Institute Institute can be usedcan be to computeused to thecompute ground the displacement ground displacement field with field an accuracy with an of accu 0.1racy pixel of [14 0.1]. pixel An overview [14]. An ofoverview the data of processing the data chainprocessing is given chain in Figure is given2. in Figure 2. TheThe high-resolution high-resolution horizontal horizontal displacements displacements provide provide a good a opportunitygood opportunity to study theto geometricalstudy the complexitiesgeometrical ofcomplexities the surface ruptureof the causedsurface byrupture this earthquake caused by sequence. this earthquake The NS component sequence. (FigureThe NS3a) indicatescomponent that (Figure these earthquakes 3a) indicates ruptured that these the earthqua unmappedkes ruptured active faults the withinunmapped the LLFZ, active the faults China within Lake Basin,the LLFZ, and westernthe China Searles Lake Valley.Basin, Theand surfacewestern rupture Searlesproduced Valley. The by surface the Mw7.1 rupture mainshock produced propagated by the Mw7.1 mainshock propagated in the southeast-northwest direction between A and B in Figure 3a. The horizontal displacement increased sharply near the epicenter of the Mw7.1 event, and peaked at Remote Sens. 2020, 12, 3883 5 of 18 inRemote the southeast-northwestSens. 2020, 12, x FOR PEER directionREVIEW between A and B in Figure3a. The horizontal displacement5 of 18 increased sharply near the epicenter of the Mw7.1 event, and peaked at ~4.6 m (35.75◦ N) (Figure3a,b). Displacement~4.6 m (35.75° vectors N) (Figure were 3a,b). not consistent Displacement with azimuthvectors were of the not rupture consistent trace with near azimuth the epicenter of the (Figure rupture3c, bluetrace zone), near andthe theepicenter longer-wavelength (Figure 3c, blue displacement zone), and vectors the longer-wavelength corresponded to increasingdisplacement NS vectors and EW displacementscorresponded to here. increasing The rupture NS and propagated EW displacements northwest here. from The the rupture epicenter, propagated and the northwest azimuth offrom the rupturethe epicenter, trace suddenly and the azimuth turned northwest of the rupture and formed trace suddenly a major releasingturned northwest bend. This and part formed of the a rupture major wasreleasing notably bend. deficient, This part suggesting of the rupture that the was releasing notably bend deficient, blocks thesuggesting strike-slip that rupture. the releasing In the bend south blocks the strike-slip rupture. In the south of the blue zone (Figure 3c), the rupture was also of the blue zone (Figure3c), the rupture was also characterized by an implicit rupture trace, minor characterized by an implicit rupture trace, minor horizontal displacement component, and smoother horizontal displacement component, and smoother horizontal displacement gradient. horizontal displacement gradient.

FigureFigure 3. 3.Horizontal Horizontal displacementdisplacement fieldfield ofof thethe 2019 Ridgecrest earthquake sequence. ( (aa)) North-south North-south (NS)(NS) component component of of the the horizontal horizontal displacement displacement derived derived from from Sentinel-2Sentinel-2 opticaloptical imagesimages (28(28 JuneJune 2019–82019– July8 July 2019). 2019). The The amplitude amplitude of the of displacementthe displacement is negative is negative where where the motion the motion is southward is southward and positive and wherepositive the where motion the is motion northward; is northward; (b) East-west (b) East-west (EW) component (EW) component of the displacement of the displacement field derived field fromderived Sentinel-2 from Sentinel-2 optical images optical (28 images June 2019–8(28 June July 2019–8 2019). July The 2019). amplitude The amplitude of displacement of displacement is negative is wherenegative the where motion the is westwardmotion is andwestward positive and where positi theve motionwhere isthe eastward. motion is The eastward. red star The indicates red star the epicenterindicates ofthe the epicenter Mw7.1 event;of the (Mw7.1c) Displacement event; (c) Displacement vectors (blue arrows)vectors (blue determined arrows) from determined the profile from AB ofthe the profile NS and AB EW of horizontalthe NS and displacements. EW horizontal Red displacements. dashed line indicates Red dashed the surfaceline indicates rupture the trace surface of the Mw7.1rupture event. trace of the Mw7.1 event.

2.2.2.2. Coseismic Coseismic DeformationDeformation fromfrom InSARInSAR WeWe obtained obtained the coseismic coseismic deformation deformation fields fields using using pre-earthquake pre-earthquake (ascending (ascending track, track, 4 July 42019; July 2019;descending descending track, track, 4 July 4 2019) July 2019)and post-earthquake and post-earthquake (ascending (ascending track, 10 track, July 10 2019; July descending 2019; descending track, track,16 July 16 July2019) 2019) Sentinel-1 Sentinel-1 SAR SAR images. images. The The time time resolution resolution (revisiting (revisiting period) period) of of the the Sentinel-1 Sentinel-1 ascendingascending data datais is 66 days.days. WeWe processedprocessed interferogramsinterferograms using GAMMA [27]. [27]. The The topographic topographic effects effects werewere removed removed usingusing aa filledfilled 3-arc-sec3-arc-sec (90(90 m)m) resolutionresolution Shuttle Radar Radar Topography Topography Mission Mission (SRTM) (SRTM) digitaldigital elevationelevation modelmodel (DEM)(DEM) releasedreleased byby NASANASA [[2288].]. In In order order to to reduce reduce the the phase phase noise noise of of the the interferogram,interferogram, a a 10 10 × 22 multilookingmultilooking process was applied in the the range range and and azimuth azimuth directions directions during during × thethe processing. processing. TheThe accuracyaccuracy ofof coregistrationcoregistration in the azimuth was was better better than than 0.001 0.001 of of a a pixel, pixel, which which cancan avoid avoid phasephase jumpsjumps betweenbetween adjacentadjacent bursts in the TOPS (Terrain Observation by by Progressive Progressive Scan)Scan) data data modemode [29[29].]. AfterAfter thethe highhigh accuracyaccuracy coregistration,coregistration, we filtered filtered the the interferometric interferometric phase phase usingusing an an adaptive adaptive spectralspectral filteringfiltering [[30]30] andand unwrapped it utilizing the the minimum minimum cost cost flow flow (MCF) (MCF) algorithmalgorithm [ 31[31].]. TheThe unwrappedunwrapped interferograminterferogram image was finally finally geocoded geocoded to to the the WGS-84 WGS-84 geographic geographic coordinatescoordinates system. system. The The ascending ascending and descendingand descen coseismicding coseismic interferograms interferograms and the LOS and deformation the LOS fieldsdeformation are shown fields in Figureare shown4. in Figure 4. Remote Sens. 2020, 12, 3883 6 of 18 Remote Sens. 2020, 12, x FOR PEER REVIEW 6 of 18

FigureFigure 4.4. AscendingAscending and descendingdescending coseismic interferograms (a,c) andand deformationdeformation fieldsfields ((b,,d)) ofof thethe 20192019 RidgecrestRidgecrest earthquakeearthquake sequence.sequence. A A color cyclecycle in interferograms corresponds to a 1010 cmcm deformationdeformation inin line-of-sightline-of-sight (LOS)(LOS) direction.direction. NegativeNegative LOS displacementdisplacement values inin thethe deformationdeformation fieldfield suggestsuggest movementmovement awayaway fromfrom thethe satellite.satellite. LOS direction isis indicated byby the black arrow. Active faultsfaults areare plottedplotted as as black black thin thin lines. lines. The The purple purple lines lines in in (b (,bd,)d denote) denote the the profile profile locations locations A1-A2, A1-A2, B1-B2, B1- C1-C2,B2, C1-C2, and and D1-D2. D1-D2. The coseismic deformation fields reconstructed by Sentinel-1 data have two asymmetric lobes The coseismic deformation fields reconstructed by Sentinel-1 data have two asymmetric lobes (Figure4b,d) and a four-quadrant distribution pattern, which are typical for a strike-slip earthquake. (Figure 4b,d) and a four-quadrant distribution pattern, which are typical for a strike-slip earthquake. The fringe patterns (Figure4a,c) are characterized by more fringes on the western wall of the rupture, The fringe patterns (Figure 4a,c) are characterized by more fringes on the western wall of the rupture, indicating that the seismogenic fault plane dips west. The maximum LOS displacement (~0.96 m) indicating that the seismogenic fault plane dips west. The maximum LOS displacement (~0.96 m) appears on the western wall of the ascending deformation field. On the west of the fault, the positive appears on the western wall of the ascending deformation field. On the west of the fault, the positive LOS displacement of the ascending deformation field (Figure4a,c) indicates a range increase, as the LOS displacement of the ascending deformation field (Figure 4a,c) indicates a range increase, as the satellite LOS direction (almost northward) is nearly perpendicular to the strike of the fault, which is satellite LOS direction (almost northward) is nearly perpendicular to the strike of the fault, which is a right-lateral strike-slip with possible uplift. In contrast, the Mw6.4 foreshock was dominated by a a right-lateral strike-slip with possible uplift. In contrast, the Mw6.4 foreshock was dominated by a left-lateral strike-slip motion (blue zone shown in Figure5). left-lateral strike-slip motion (blue zone shown in Figure 5). Remote Sens. 2020, 12, 3883 7 of 18 Remote Sens. 2020, 12, x FOR PEER REVIEW 7 of 18

FigureFigure 5.5. ProfilesProfiles ofof the Ridgecrest earthquakeearthquake sequence.sequence. The line-of-sight (LOS)(LOS) displacementdisplacement profileprofile a b ofof ((a)) the the ascending ascending deformation deformation field field and and ( )( theb) the descending descending deformation deformation field field along along A1-A2; A1-A2; The LOS The displacement profile of (c) the ascending deformation field and (d) the descending deformation field LOS displacement profile of (c) the ascending deformation field and (d) the descending deformation along B1-B2; (e–h) jointly reveal the triggered creep on the Garlock fault perpendicular to profiles field along B1-B2; (e–h) jointly reveal the triggered creep on the Garlock fault perpendicular to profiles C1-C2 and D1-D2. C1-C2 and D1-D2. Additionally, from the ascending and descending coseismic deformation fields derived from Additionally, from the ascending and descending coseismic deformation fields derived from Sentinel-1 data, we observed the creep slip triggered by the Mw7.1 mainshock on the Garlock fault, Sentinel-1 data, we observed the creep slip triggered by the Mw7.1 mainshock on the Garlock fault, which has not been found by geodetic measurements [32]. The across-Garlock fault displacement which has not been found by geodetic measurements [32]. The across-Garlock fault displacement profiles, C1-C2 and D1-D2 in Figure3b, indicate a more evident LOS displacement gradient alteration profiles, C1-C2 and D1-D2 in Figure 3b, indicate a more evident LOS displacement gradient alteration (Figure5e–g) when LOS displacement profiles passed through the Garlock fault (Figure5, purple (Figure 5e–g) when LOS displacement profiles passed through the Garlock fault (Figure 5, purple region). However, despite the profile D1-D2 of the descending deformation field (Figure5h) measuring region). However, despite the profile D1-D2 of the descending deformation field (Figure 5h) only a minor fault-perpendicular LOS slip offset, the ascending profile C1-C2, the descending profile measuring only a minor fault-perpendicular LOS slip offset, the ascending profile C1-C2, the C1-C2, and the ascending profile D1-D2, respectively, measured the fault-perpendicular LOS slip offset descending profile C1-C2, and the ascending profile D1-D2, respectively, measured the fault- of up to ~0.9 cm, ~1.2 cm, and ~2.0 cm. The triggered creep slip (peaking at 20 mm) extended between perpendicular LOS slip offset of up to ~0.9 cm, ~1.2 cm, and ~2.0 cm. The triggered creep slip (peaking 117.3 W and 117.5 W along the Garlock fault. These results are consistent with the results got by at 20◦ mm) extended◦ between 117.3° W and 117.5° W along the Garlock fault. These results are Ross et al. [9] and Barnhart et al. [10]. consistent with the results got by Ross et al. [9] and Barnhart et al. [10]. 3. Revealing the Geometric Complexities of Ruptures by Curl and Divergence 3. Revealing the Geometric Complexities of Ruptures by Curl and Divergence It is hard to investigate the geometric features of the Mw7.1 rupture utilizing only the horizontal displacementIt is hard components,to investigate due the geometric to the indistinct features rupture of the trace.Mw7.1 Therefore, rupture utilizing we introduced only the curl horizontalω and divergencedisplacementδ fromcomponents, the NS anddue EWto the displacement indistinct rupture components trace. Therefore, to analyze we the introduced surface deformation curl ω and δ patterns,divergence including from the near-faultNS and EW deformations displacement where compon SARents images to analyze usually the decorrelate. surface deformation Curl ω delineatespatterns, including the fault tracethe near-fault clearly and deformations conforms towhere the right-hand SAR images rule. usually Negative decorrelate.ω corresponds Curl ω todelineates clockwise the rotation. fault trace Hence, clearly negative and confω atorms the to fault the traceright-hand is equivalent rule. Negative to right-lateral ω corresponds strike-slip to clockwise rotation. Hence, negative ω at the fault trace is equivalent to right-lateral strike-slip rupture [15]. Positive divergence δ is nicely consistent with the opening cracks in the uplifted region (net extension) [26]. Curl ω and divergence δ are computed by the following functions:

𝜕𝑣 𝜕𝑢 𝜔=𝛻𝐹⃗ = (1) 𝜕𝑥 𝜕𝑦 Remote Sens. 2020, 12, 3883 8 of 18 rupture [15]. Positive divergence δ is nicely consistent with the opening cracks in the uplifted region (net extension) [26]. Curl ω and divergence δ are computed by the following functions:

Remote Sens. 2020, 12, x FOR PEER REVIEW ∂v ∂u 8 of 18 ω = →F = (1) ∇ × z ∂x − ∂y 𝜕𝑢 𝜕𝑣 𝛿=𝛻 ∙𝐹⃗ =∂ u +∂v (2) δ = →F = 𝜕𝑥+ 𝜕𝑦 (2) ∇h· ∂x ∂y where 𝐹⃗ is the displacement field; u and v are the EW and NS displacement components from → whereSentinel-2F is theoptical displacement images, respectively; field; u and v arex is the the EW east-west and NSdisplacement direction (E); components and y is the from north-south Sentinel-2 opticaldirection images, (N). respectively; x is the east-west direction (E); and y is the north-south direction (N). WeWe foundfound some some interesting interesting surface surface deformation deformation patterns patterns at these at faults these from faults curl fromω and curl divergence ω and δdivergence(Figure6b,c). δ (Figure A typical 6b,c). scenario A typical of the scenario distributed of the rupture distributed (a zone rupture width (a of zone distributed width of cracks) distributed was showncracks) inwas the shown horizontal in the displacement horizontal displacement profile (Figure profile7a), curl (Figureω profile 7a), curl (Figure ω profile7d), and (Figure divergence 7e), andδ profiledivergence (Figure δ profile7g), where (Figure the 7h), surface where deformation the surface distributes deformation within distributes the surface within rupture the surface zone (several rupture hundredzone (several meters). hundred The meters). curl ω and The divergencecurl ω and divergenceδ profiles vary δ profiles seriously vary (from seriou positivesly (from to positive negative) to withinnegative) the distributedwithin the rupturedistributed zone, indicatingrupture zone, that theindicating surface deformation that the surface is accommodated deformation in theis extensionalaccommodated and compressionalin the extensional cracks and of thecompressional shallow surface cracks without of the any shallow detectable surface displacement without any on thedetectable fault. Figure displacement7b,e,h present on the anfault. example Figure of 7b,f,i the secondpresent scenario an example where of the surfacesecond scenario slip is entirely where localizedthe surface on slip the is fault entirely (a linear, localized single on rupturethe fault trace (a linear, by visual single analysis rupture on trace curl by field). visual Curl analysisω and on divergencecurl field). Curlδ sharply ω and increasedivergence from δ sharply zero to increase the maximum from zero within to the a verymaximum narrow within zone. a very At the narrow fault, the negative curl ω peaking at 0.045 corresponds to the right-lateral strike-slip motion and the zone. At the fault, the negative curl− ω peaking at −0.045 corresponds to the right-lateral strike-slip positivemotion and divergence the positiveδ reaching divergence 0.044 δ is reaching also consistent 0.044 is with also extension consistent that with occurred extension at thethat main occurred fault locally.at the main In addition, fault locally. cross-fault In addition, profile EF cross-faul shows theret profile is another EF shows fault there II (Figure is another7c,f,i). Thefault horizontal II (Figure displacement7c,g,j). The horizontal swath profile displacement EF (Figure swath7c) shows profile the EF far-field (Figure slip 7c) oshowsffset of the 1.92 far-field m, near-field slip offset slip of o ff1.92set ofm, 1.54near-field m (24% slip smaller offset thanof 1.54 the m far-field(24% smaller slip o thanffset), the and far-field a slip ofslip ~0.49 offset), m atand fault a slip II. Theof ~0.49 off-fault m at deformation,fault II. The off-fault which meansdeformation, that earthquake which means deformation that earthquake is not concentrated deformation onis not the concentrated fault, is related on tothe several fault, is factors, related such to several as the immaturityfactors, such of as the the fault immaturity and the typeof the of fault material and atthe the type surface of material through at whichthe surface the fault through cuts. which Fault IIthe is fault dominated cuts. Fault by the II is right-lateral dominated strike-slipby the right-lateral motion and strike-slip ruptures motion 6 km withand ruptures a striking 6 of km 311.7 with◦ (~10 a striking◦ anti-clockwise of 311.7° rotation (~10° anti-clockwise compared with rotation the main compared rupture), with thethe meanmain sliprupture), offset with of 0.5 the m. mean slip offset of 0.5 m.

Figure 6. Curl and divergence of the surface ruptures due to the mainshock. (a) Coseismic horizontal Figure 6. Curl and divergence of the surface ruptures due to the mainshock. (a) Coseismic horizontal displacement field derived from the NS and EW displacement components. Amplitude of displacement displacement field derived from the NS and EW displacement components. Amplitude of is negative for southward motion. (b) Curl ω and (c) divergence δ derived from the NS and EW displacement is negative for southward motion. (b) Curl ω and (c) divergence δ derived from the NS displacement components. Across-fault profiles (AB, CD, and EF) reveal detailed characteristics of the and EW displacement components. Across-fault profiles (AB, CD, and EF) reveal detailed surface rupture zone (both near-field and far-field). Geometrical complexities of the surface rupture characteristics of the surface rupture zone (both near-field and far-field). Geometrical complexities of contributed by the Mw7.1 event are indicated in bend b1–b5. the surface rupture contributed by the Mw7.1 event are indicated in bend b1-b5. Remote Sens. 2020, 12, 3883 9 of 18 Remote Sens. 2020, 12, x FOR PEER REVIEW 9 of 18

Figure 7. Near-field and far-field measurements. (a–c) Horizontal displacement measurements along profilesFigure 7. AB, Near-field CD, and and EF far-fi (7-km-longeld measurements. and 200-m-wide) (a–c) Horizontal of Figure5 ,displacement respectively. measurements ( d–f) Curl profiles along (AB,profiles CD, AB, and CD, EF) and show EF three(7-km-long peaks, and corresponding 200-m-wide) to of the Figure main 5, rupture respectively. at the site.(d–f) In Curl both profiles cases, (AB, curl delineatesCD, and EF) the show fault tracethree clearly, peaks, and corresponding the right-lateral to the strike-slip main rupture motion at results the site. in clockwise In both rotationcases, curl at thedelineates fault and the hence fault negativetrace clearly, curl andω.( gthe–i) right-latera Divergencel profilesstrike-slip (AB, motion CD, and results EF) in reveal clockwise three positiverotation peaksat the fault (δ > 0), and corresponding hence negative to curl net extensionω. (g–i) Divergence at the fault. profiles (AB, CD, and EF) reveal three positive peaks (δ > 0), corresponding to net extension at the fault. The Mw7.1 event produced a 55 km-long main fault (segments b1–b5) with the mean striking of 320.0The◦ andMw7.1 two event secondary produced faults a 55 (fault km-long I and main II shown fault in(segments Figure6). b1-b5) The maximum with the mean horizontal striking slip of is320.0 ~4.6° m and (35.75 two◦ secondaryN, at the segment faults (fault b2) onI and the II horizontal shown in displacementFigure 6). The field, maximum and the horizontal result is highly slip is consistent~4.6 m (35.75 with° N,the at horizontal the segment slip ~4.2b2) on 0.5the m horizontal (35.78 N) di reportedsplacement by thefield, University and the Consortiumresult is highly for ± ◦ Satelliteconsistent Navigation with the horizontal Systems and slip Crustal ~4.2 ± 0.5 Deformation m (35.78° N) Observations reported by (UNAVCO). the University To Consortium investigate thefor geometricSatellite Navigation complexities Systems and rupture and Crustal kinematics Deformation of the multiple Observations faults in(UNAVCO). the Mw7.1 event,To investigate those faults the weregeometric divided complexities into five segmentsand rupture (b1–b5, kinematics Figure of6) the according multiple to faults the geometric in the Mw7.1 characteristics event, those of faults the surfacewere divided rupture into derived five segments from curl (b1-b5,ω and Figure divergence 6) accordingδ (Table to1 the). Thegeometric Mw7.1 characteristics event produced of the a 55surface km-long rupture main derived fault (segments from curl b1–b5)ω and divergence with the mean δ (Table striking 1). The of320.0 Mw7.1◦ and event two produced secondary a 55 faults km- (faultlong main I and fault II shown (segments in Figure b1-b5)6). Thewith maximum the mean horizontalstriking of slip320.0 is° ~4.6 and mtwo (35.75 secondary◦ N, at faults the segment (fault I b2)and on II theshown horizontal in Figure displacement 6). The maximu field,m and horizontal the result slip is is highly ~4.6 m consistent (35.75° N, with at the the segment horizontal b2) slip on 4.2the horizontal0.5 m (35.78 displacementN) reported field, by and UNAVCO. the result Theis highly negative consistent curl ω withextracted the horizontal from the slip fault 4.2trace ± 0.5 ± ◦ (Tablem (35.781) corresponds° N) reported to by the UNAVCO. clockwise rotation, The negative hence curl the right-lateralω extracted strike-slipfrom the fault motion trace dominated (Table 1) segmentscorresponds b1–b5 to the and clockwise caused the rotation, secondary hence faults the right-lateral to rupture (Table strike-slip1). Divergence motion dominatedδ at the fault segments trace (Tableb1-b5 1and) reveals caused a certain the secondary near-fault faults vertical to rupture rupture (Table component 1). Divergence (extension δ uplift at the cracks) fault trace at segments (Table1) b2reveals and b4a certain (Figure near-fault4c, Table 1vertical). We furtherrupture inferred component that (extension segments uplift b2 and cracks) b4 were at segments dominated b2 byand theb4 (Figure right-lateral 4c, Table1). strike-slip, We further with a significantinferred that vertical segments rupture b2 and component b4 were on dominated the east-dipping by the right- fault. Thelateral vertical strike-slip, separation with a at significant segment b2 vertical (35.73 rupturN) measurede component by UNAVCO on the east-di is ~3.7pping0.2 fault. m. The surfacevertical ◦ ± ruptureseparation of theat segment Mw7.1 event b2 (35.73 terminated° N) measured in front by of theUNAVCO bedrock is mountain ~3.7 ± 0.2 southwestm. The surface of Searles rupture Lake of (35.5774the Mw7.1◦ N) event and onlyterminated about 5in km front from of the bedr centralock Garlock mountain fault southwest (Figure8 of). InSearles addition, Lake we (35.5774 extracted° N) twoand surfaceonly about rupture 5 km traces from (fault the central IV and Garlock V) produced fault by(Figure the Mw6.4 8). In eventaddition, (Figure we8 extracted) from the two horizontal surface displacementrupture traces field (fault (Figure IV 6anda). FaultV) produced IV and V ruptureby the alongMw6.4 striking event 224(Figure◦, and 8) intersect from the with horizontal the main faultdisplacement of the Mw7.1 field event (Figure at segment6a). Fault b5 IV (Figure and V8 rupture), forming along a cross-faulting striking 224° geometry, and intersect (~90 ◦with). The the Mw7.1 main earthquakefault of thenucleated Mw7.1 event about at 10 se kmgment northwest b5 (Figure of the 8), Mw6.4 forming event. a cross-faulting The fault II trace geometry extracted (~90 from°). The the curlMw7.1 could earthquake be seen as nucleated the initial about path 10 that km propagates northwest the of the Mw6.4 Mw6.4 rupture event. about The 6fault km II to trace the northwest extracted (Figurefrom the8). curl The could 2019 Ridgecrestbe seen as earthquakethe initial path sequence that propagates ruptured multiple the Mw6.4 faults rupture that were about unconnected 6 km to the onnorthwest the shallow (Figure surface 8). The (Figure 20198 )Ridgecrest with the Airport earthquake Lake sequence fault zone, ruptured the China multiple Lake basin,faults thethat Little were Lakeunconnected fault zone, on andthe shallow the western surface Searles (Figure Valley 8) with (Figure the 8Airport). Lake fault zone, the China Lake basin, the Little Lake fault zone, and the western Searles Valley (Figure 8).

Remote Sens. 2020, 12, x FOR PEER REVIEW 10 of 18

Table 1. The geometry and rupture kinematics of multiple faults derived from curl and divergence.

Orientation Length Strike Segment 𝛚𝒎𝒆𝒂𝒏 𝛅𝒎𝒆𝒂𝒏 Surface Rupture Kinematics Remote Sens. 2020, 12, 3883(°N) (km) (°) 10 of 18 35.7988– Right-lateral + minor vertical b1 16.4 315.5 −0.0157 0.0047 35.8825 motion Table 1. The geometry and rupture kinematics of multiple faults derived from curl and divergence. 35.7305– Right-lateral + vertical b2 10.2 325.1 −0.0291 0.0116 Orientation35.7988 Surfacemotion Rupture Segment Length (km) Strike (◦) ωmean δmean 35.7155–(◦N) Kinematics b3 2.3 — — — — Right-lateral + minor b1 35.7988–35.882535.7305 16.4 315.5 0.0157 0.0047 35.6833– − Right-lateralvertical + motion vertical Right-lateral + vertical b4b2 35.7305–35.7988 4.710.2 321.2 325.1 −0.0250 0.02910.0103 0.0116 35.7155 − motionmotion b3 35.7155–35.730535.5774– 2.3 — — — — b5 21.1 318.3 −0.0052 ≈0 MinorRight-lateral right-lateral+ vertical b4 35.6833–35.715535.6833 4.7 321.2 0.0250 0.0103 − motion b5 35.5774–35.6833 21.1 318.3 0.0052 0Highly Minor discontinuous right-lateral fault I — — 311.0 — − — ≈ Highly discontinuous fault I — — 311.0 — — ruptures 35.7094– ruptures faultfault II II 35.7094–35.7460 6.06.0 311.7 311.7 −0.0184 0.0184 ≈0 0Minor Minor right-lateral right-lateral 35.7460 − ≈

Figure 8. TheThe surface surface rupture rupture traces of the 2019 Ridgecrest earthquake sequence. Purple lines indicate the faults recorded in thethe U.S. Geological Survey (USGS) QuaternaryQuaternary Fault database (U.S. Geological Survey andand California California Survey, Survey, 2010) 2010) and and in the in 2010the Fault2010 ActivityFault Activity Map of Map California of California (California (California Geologic GeologicSurvey, 2010). Survey, Black 2010). lines Black indicate lines the indicate surface the rupture surface traces rupture of the trac maines of faultthe main and somefault secondaryand some secondaryfaults identified faults fromidentified displacement from displacement field and curl. field Blue and beachcurl. ballsBlue showbeach theballs focal show mechanisms the focal mechanismsof the large foreshockof the large (from foreshock USGS). Red(from lines USGS). indicate Red the lines surface indicate rupture the tracessurface produced rupture bytraces the producedmainshock by obtained the mainshock by field investigations.obtained by field Green inve rectanglesstigations. indicate Green rectangles the regions indicate with the the horizontal regions slip ~4.2 0.5 m and vertical separation ~3.7 0.2 m reported by the University Consortium for Satellite ± ± Navigation Systems and Crustal Deformation Observations (UNAVCO). Remote Sens. 2020, 12, 3883 11 of 18

4. Kinematic Characteristics of the Coseismic Rupture

4.1. Multiple Faults Slip Distribution To obtain the rupture process, the surface deformation fields from the Sentinel-1 ascending and descending data were utilized jointly to invert the fault slip distribution on buried fault planes. The surface deformation fields associated with this earthquake sequence were modeled as a series of dislocations buried in an elastic half space. The InSAR deformation fields were subsampled by the Quadtree algorithm to obtain a reasonable amount of data in the inversion [33]. However, by modeling the event as one occurring on a single continuous fault with varying geometric parameters, we did not obtain a well fitted fault model. Therefore, we fitted InSAR observations with a more spatially complex model of multiple-fault planes and slightly adjusted the surface rupture trace extracted from the horizontal displacement fields, curl ω and divergence δ, and considered them as seismogenic fault traces. In addition, we added a 15 km-long fault plane (the secondary fault III) with a striking of 319◦ in the western part of the Mw7.1 main fault (2 km from segment b5) [9] to improve the model fitting. Finally, we approximated the model of this earthquake sequence by six faults, which are the main faults of the Mw7.1 event, three secondary faults of the Mw 7.1 event, and two faults of the Mw6.4 event. The geometric complexities of the surface rupture derived from the horizontal displacement fields, curl ω and divergence δ, allowed us to constrain the multiple-fault model in orientation, strike, and length [17]. The multiple-fault geometry shows a varying strike along the ruptures that reproduces the five segments b1–b5. For determining the fault dips, we tested the fault geometries of east dipping and west dipping, and found an east dipping of 84◦ to constrain the Mw7.1 main fault. Fault V ruptured in the Mw6.4 event and has a dip angle of 78◦ according to USGS. Faults I, II, and III are vertical planes with a dip of 90◦ and depth of 5 km. Moreover, we constrained the rake angle of the multiple faults more flexibly in right-lateral (Mw7.1) and left-lateral (Mw6.4) strike-slip motion. We then determined the size of the Mw7.1 main fault plane to be 58 km 15 km and discretized it into × 29 subfaults in the strike direction and 8 subfaults in the dip direction. The size of each subfault is 2 km 2 km. The multiple-fault model includes the fault V plane (18 km 15 km), which is discretized × × into 72 subfaults (2 km 2 km), and the secondary fault planes, which are discretized into a series of × subfaults (1 km 1 km). Based on the Okada elastic half-space homogeneous model [34], we utilized × the steepest descent method (SDM) to evaluate the optimal geometric parameters of the seismogenic faults and calculated the multiple-fault slip distribution (Figure9)[ 35]. The predicted displacements from our multiple-fault slip distribution fit the observations well (Figure 10). The RMS of the misfits were ~0.15 m and ~0.28 m for ascending and descending deformations, respectively. Moment tensor resolutions for the Mw6.4 foreshock are strike = 224◦, dip = 78◦, and rake = 5◦ for the left-lateral event and those of the Mw7.1 main shock are strike = 320 , dip = 81 , and rake = 174 ◦ ◦ − ◦ for right-lateral event. Our results indicate that the Mw7.1 mainshock was dominated by a right-lateral strike-slip with a maximum strike-slip of ~5.8 m near the hypocenter and accompanied by a significant dip-slip component peaking at ~1.8 m on an east-dipping fault (Figure 11a, segment b2 in Figure6). The main fault slip distribution of the Mw7.1 event indicates that coseismic slip was concentrated in the upper crust, shallower than 10 km, and the maximum rupture occurred at a depth between 4 and 8 km (Figure9a). The total seismic moment is 5.18 1019N m, which is equivalent to Mw7.12. × · In addition, the derived multiple faults model of the Mw7.1 mainshock contains three secondary faults, which are fault I located on the northwest of the epicenter (Figure 11g, releasing bend shown in the Figure2); fault II with the strike of 310 ◦, length of 10 km, and maximum slip 4 m distributed between the Mw7.1 and the Mw6.4 epicenter (Figure 11h); and fault III (Figure 11i) with a length of 15 km, strike of 319◦, and the slip peaking at 2 m west of the Mw7.1 main fault (distance surface rupture b5 of 2 km). The fault of the Mw 6.4 event slipped at a greater depth (2–14 km deep), with the maximum strike-slip of ~3.9 m rupturing at the depth of 12 km (Figure9c). The strike-slip (Figure 11e) and dip-slip (Figure 11f) components indicate that the Mw6.4 event’s ruptured minute dip-slip was Remote Sens. 2020, 12, 3883 12 of 18

dominated by pure left-lateral strike-slip with a strike of 224◦ and rake of 5◦. In contrast, the fault IV of theRemoteRemote Mw6.4 Sens.Sens. event 20202020,, (Figure1212,, xx FORFOR 11 PEERPEERj) ruptured REVIEWREVIEW at a shallower depth (2–7.5 km), with the maximum slip 12 12 of of of ~2 18 18 m.

FigureFigureFigure 9. The 9.9. TheThe slip slipslip distribution distributiondistribution model modelmodel of ofof the thethe multiple multiplemultiple faults: faults: ( (aa)) Mw7.1Mw7.1 Mw7.1 mainmain main fault,fault, fault, ((bb ()b) Mw7.1)Mw7.1 Mw7.1 secondarysecondary secondary faults,faults,faults, and andand (c) (( Mw6.4cc)) Mw6.4Mw6.4 main mainmain fault. fault.fault.

FigureFigureFigure 10. 10.10.Coseismic CoseismicCoseismic LOS LOSLOS observations observationsobservations from fromfrom ascendingascendingascending Sentinel-1Sentinel-1 interferogramsinterferograms ((aa (),a), ), predictionspredictions predictions fromfromfrom our ourour slip slipslip model modelmodel (b (),(bb), and), andand residuals residualsresiduals between betweenbetween of ofof them themthem (((cc).). Observations Observations fromfrom from descendingdescending descending Sentinel-1Sentinel-1 Sentinel-1 interferogramsinterferogramsinterferograms (d ),((dd predictions),), predictionspredictions from fromfrom our ourour slip slipslip model modelmodel ( (e(ee),),), and andand residualsresidualsresiduals between between themthem them ((f (f).f).). Remote Sens. 2020, 12, x FOR PEER REVIEW 13 of 18

Moment tensor resolutions for the Mw6.4 foreshock are strike = 224°, dip = 78°, and rake = 5° for the left-lateral event and those of the Mw7.1 main shock are strike = 320°, dip = 81°, and rake = 174° for right-lateral event. Our results indicate that the Mw7.1 mainshock was dominated by a right-lateral strike-slip with a maximum strike-slip of ~5.8 m near the hypocenter and accompanied by a significant dip-slip component peaking at ~1.8 m on an east-dipping fault (Figure 11a, segment b2 in Figure 6). The main fault slip distribution of the Mw7.1 event indicates that coseismic slip was concentrated in the upper crust, shallower than 10 km, and the maximum rupture occurred at a depth between 4 and 8 km (Figure 9a). The total seismic moment is 5.18 10N∙m, which is equivalent to Mw7.12. In addition, the derived multiple faults model of the Mw7.1 mainshock contains three secondary faults, which are fault I located on the northwest of the epicenter (Figure 11g, releasing bend shown in the Figure 2); fault II with the strike of 310°, length of 10 km, and maximum slip 4 m distributed between the Mw7.1 and the Mw6.4 epicenter (Figure 11h); and fault III (Figure 11i) with a length of 15 km, strike of 319°, and the slip peaking at 2 m west of the Mw7.1 main fault (distance surface rupture b5 of 2 km). The fault of the Mw 6.4 event slipped at a greater depth (2–14 km deep), with the maximum strike-slip of ~3.9 m rupturing at the depth of 12 km (Figure 9c). The strike-slip (Figure 11e) and dip-slip (Figure 11f) components indicate that the Mw6.4 event’s ruptured minute dip-slip was dominated by pure left-lateral strike-slip with a strike of 224° and rake of 5°. In contrast, theRemote fault Sens. IV2020 of, 12 the, 3883 Mw6.4 event (Figure 11j) ruptured at a shallower depth (2–7.5 km), with13 ofthe 18 maximum slip of ~2 m.

Figure 11. Slip distribution on the multiple faults. The distribution of (a) the total-slip, (b) strike-slip, Figure 11. Slip distribution on the multiple faults. The distribution of (a) the total-slip, (b) strike-slip, and (c) dip-slip on the Mw7.1 main fault. The distribution of (d) total-slip, (e) strike-slip, and (f) dip-slip and (c) dip-slip on the Mw7.1 main fault. The distribution of (d) total-slip, (e) strike-slip, and (f) dip- on the Mw6.4 main fault. (g–j), The total-slip distribution on (g) fault I, (h) fault II, and (i) fault III of slip on the Mw6.4 main fault. (g–j), The total-slip distribution on (g) fault I, (h) fault II, and (i) fault the Mw7.1 event and (j) fault IV of the Mw6.4 foreshock. Bends b1–b5 correspond to surface ruptures III of the Mw7.1 event and (j) fault IV of the Mw6.4 foreshock. Bends b1-b5 correspond to surface b1–b5 of the Mw7.1 event. ruptures b1-b5 of the Mw7.1 event. 4.2. Static Coulomb Stress Changes 4.2. Static Coulomb Stress Changes The coseismic rupture of faults will inevitably cause stress volume change and redistribution in the surroundingThe coseismic region, rupture which affects of faults the spatial-temporalwill inevitably cause distribution stress volume of the subsequent change and earthquakes redistribution [36–38 in]. theTherefore, surrounding we investigated region, thewhich static affects stresschanges the spatia on thel-temporal multiple-fault distribution planes by of the the slip subsequent distribution earthquakesmodel to understand [36–38]. Therefore, the rupture we processesinvestigated of thethe Mw7.1static stress mainshock changes encouraged on the multiple-fault by the foreshock planes bysequence, the slip and distribution further analyzed model to whether understand the Garlock the rupture left-lateral processes fault triggeredof the Mw7.1 shallow mainshock folding. encouragedThe static Coulomb by the foreshock stress changes sequence, on and multiple-fault further analyzed planes whether were calculated the Garlock by theleft-lateral Coulomb3.3 fault triggeredsoftware [39shallow–41]. Forfolding. simplicity, The westatic only Coulomb concentrated stress on changes the triggering on multiple-fault relationship planes between were the Mw6.4 foreshock and the Mw7.1 mainshock, so the earthquakes between these two events were not considered. We set a fixed static coefficient of friction of 0.4, which is a typical value of inland strike slip faults. The shear modulus and Poisson’s ratio were assumed as 33 GPa and 0.25, respectively [42]. We then calculated the static Coulomb stress changes of each subfault to track the stress changes induced by the Mw6.4 foreshock on the Mw7.1 fault plane and the Garlock fault plane, and these earthquakes on the Garlock fault plane. The Coulomb stress change model indicates that the Mw6.4 foreshock triggered the Mw7.1 mainshock and increased the Coulomb stress (0.5–1 bar) in the nucleation zone but decreased the Coulomb stress ( 2 bar) in the main fault (Figure8, segment b3 and b4) of the Mw7.1 event, leading − to slight unclamping of the fault. Furthermore, the Coulomb stress loading is up to 0.5–2.0 bar on fault II, which is located between the Mw6.4 and Mw7.1 nucleation zones. Therefore, we inferred that fault II played a significant role in the triggering process. The spatial distribution of aftershocks of the Mw6.4 event also confirms our result that all aftershocks that occurred in the area between the Mw6.4 and the Mw7.1 nucleation zone ruptured along the fault II with a strike of 321◦ (Figure 12). We also modeled the Coulomb stress changes on the Garlock left-lateral fault induced by the Mw6.4 foreshock (Figure 12a) and the Mw7.1 mainshock (Figure 12b). A 20 km-long area with increased Coulomb stress (+0.5 bar) generated by the Mw6.4 event appeared at the Garlock fault between 117.5◦ W and 117.3◦ W. Remote Sens. 2020, 12, x FOR PEER REVIEW 14 of 18 calculated by the Coulomb3.3 software [39–41]. For simplicity, we only concentrated on the triggering relationship between the Mw6.4 foreshock and the Mw7.1 mainshock, so the earthquakes between these two events were not considered. We set a fixed static coefficient of friction of 0.4, which is a typical value of inland strike slip faults. The shear modulus and Poisson’s ratio were assumed as 33 GPa and 0.25, respectively [42]. We then calculated the static Coulomb stress changes of each subfault to track the stress changes induced by the Mw6.4 foreshock on the Mw7.1 fault plane and the Garlock fault plane, and these earthquakes on the Garlock fault plane. The Coulomb stress change model indicates that the Mw6.4 foreshock triggered the Mw7.1 mainshock and increased the Coulomb stress (0.5–1 bar) in the nucleation zone but decreased the Coulomb stress (−2 bar) in the main fault (Figure 8, segment b3 and b4) of the Mw7.1 event, leading to slight unclamping of the fault. Furthermore, the Coulomb stress loading is up to 0.5–2.0 bar on fault II, which is located between the Mw6.4 and Mw7.1 nucleation zones. Therefore, we inferred that fault II played a significant role in the triggering process. The spatial distribution of aftershocks of the Mw6.4 event also confirms our result that all aftershocks that occurred in the area between the Mw6.4 and the Mw7.1 nucleation zone ruptured along the fault II with a strike of 321° (Figure 12). We also modeled the Coulomb stress changes on the Garlock left-lateral fault induced by the Mw6.4 Remoteforeshock Sens. 2020(Figure, 12, 3883 12a) and the Mw7.1 mainshock (Figure 12b). A 20 km-long area with increased14 of 18 Coulomb stress (+0.5 bar) generated by the Mw6.4 event appeared at the Garlock fault between 117.5° W and 117.3° W. This area appeared again when the Mw7.1 mainshock struck. The positive Coulomb Thisstress area increased appeared up to again 1–2 whenbar and the lead Mw7.1 to unclamping mainshock of struck. the Garlock The positive fault, thereby Coulomb triggering stress increased shallow aseismicup to 1–2 folding bar and on lead the to Garlock unclamping fault. ofThe the Mw7.1 Garlock ma fault,inshock thereby altered triggering the coulomb shallow stress aseismic of the foldingcentral aseismicon the Garlock folding fault. zone The from Mw7.1 positive mainshock (unclamping) altered the to coulomb negative stress (clamping), of the central resulting aseismic infolding slight unclampingzone from positive on the (unclamping)fault. to negative (clamping), resulting in slight unclamping on the fault.

Figure 12. Static CoulombCoulomb stress stress changes changes (CSC) (CSC) on on multiple multiple faults faults due due to the to 2019the 2019 earthquakes. earthquakes. (a) CSC (a) CSCon the on Mw7.1 the Mw7.1 faults faults and the and Garlock the Garlock fault plane fault causedplane caused by the Mw6.4by the Mw6.4 event; (bevent;) CSC ( onb) CSC the Garlock on the Garlockfault plane fault caused plane by caused the 2019 by th Ridgecreste 2019 Ridgecrest earthquake earthquake sequence. sequ Theence. green The line green indicates line indicates the aseismic the aseismiccreep triggered creep triggered by these by earthquakes. these earthquakes. Static Static coeﬃ cientcoefficient of friction of friction is 0.2. is 0.2. Positive Positive stress stress change change is isconsistent consistent with with unclamping unclamping of theof the faults, faults, and and a negative a negative stress stress change change leads leads to clamping to clamping of the faults.of the 5. Discussionfaults.

5.1. Multiple-Fault Slip Model Pirtro et al. used the MAI (Multiple Aperture Interferometry) technique to reconstruct the time series of the 3-D components of the 2019 Ridgcrest earthquake [43]. Our results (NS and EW displacement) are consistent with the results of Pirtro et al. Hence, the subpixel optical image correlation technology can allow us to reconstruct the coseismic displacement (NS and EW) produced by this earthquake sequence, and further investigate multiple-fault rupture traces, which is a good method to investigate multiple-fault events. Like the 1992 Landers earthquake and the 1999 Hector Mine earthquakes, the 2019 Ridgecrest earthquake sequence ruptured along a complex fault system and triggered obvious seismic and aseismic slips on multiple intersecting faults (Figure8). In this study, we modeled the Mw6.4 left-lateral foreshock and the Mw7.1 right-lateral mainshock on two orthogonal fault structures (Figures8 and9). Cross-fault ruptures have been very well researched, such as the 1987 Superstition Hills earthquake sequence, the 1992 Big Bears and Landers earthquakes, the 2010–2011 Darfield, New Zealand earthquake sequence [19], the 2010 Haiti earthquake, and the 2012 Sumatra earthquake [44]. For instance, the 2012 Sumatra earthquake ruptured along four faults with different geometry parameters. It is a struggle to investigate the fault traces, geometry parameters, and near-field rupture patterns of these faults only using seismic data and geodetic (GPS) observations. We can Remote Sens. 2020, 12, 3883 15 of 18 investigate the multiple faults ruptures and intersecting faults utilizing space geodesy techonolgy (InSAR and optical image correlation). The surface displacements derived from Sentinel-2 optical images and Sentinel-1 SAR images help us understand the surface rupture kinematics and deep slip distribution produced by this earthquake sequence. The surface traces of the ruptured faults can be extracted from the surface displacement fields, curl and divergence. The geometric complexities of the multiple-fault model were well constrained by the surface rupture kinematics, and the surface rupture kinematics and deep slip distribution should be consistent with each other, so then reducing uncertainties caused by the geometric parameters of the fault model in the process of inversion.

5.2. Cascading Rupture Process According to the surface rupture kinematics and multiple-fault slip model, the Mw6.4 event and its aftershocks triggered the Mw7.1 earthquake. The rupture process of the earthquake sequence could be a cascading rupture rather than a single continuous rupture propagating along multiple faults [45–48]. Fault II (Figure8) had a major influence on the cascading rupture process, and had a promotional e ffect on the dynamic process of the Mw6.4 foreshock, which indirectly triggered the Mw7.1 mainshock. This earthquake sequence provides a value opportunity to understand the cascading rupture model. In a cascading rupture process, the geometric complexities of the cross-faulting ruptures within a fault zone have a major influence on the earthquake rupture process, because there are many potential pathways (such as the Mw7.1 secondary fault II) to sustain the rupture propagation. This earthquake sequence demonstrated the cascading rupture process again, in which multiple highly distributed faults can cascade to produce a greater magnitude earthquake, and the more ruptures that cascade, the more faults will be produced by the coseismic ruptures. The Mw7.1 event ruptured multiple immature faults distributed within the southwest opening of Searles Valley and the China Lake basin. These immature faults may not be connected at a shallow depth, but they are physically connected at greater depths, at least partly. The cascading rupture process may accelerate the development of those faults and make them connected.

6. Conclusions The 2019 Ridgecrest, California earthquake sequence ruptured along a complex fault system, which triggered obvious seismic and aseismic slip on multiple intersecting faults. Using the surface displacement derived from the Seninel-2 optical images and Sentinel-1 SAR data, and by introducing curl and divergence, we investigated the localized and distributed surface deformation patterns. The results show that a maximum surface slip of ~4.6 m, mean strike of 320◦, and a 55 km-long surface rupture zone were produced by the Mw7.1 event. The Mw6.4 foreshock and the Mw7.1 mainshock ruptured multiple faults, and the main faults of these two events intersected each other at ~90◦ in a cross-faulting geometry. The results inferred from geodetic observations are consistent with the rupture processes of this earthquake, which featured a complicated cascading rupture rather than a single continuous rupture propagating along multiple faults. The multiple immature faults ruptured by this earthquake sequence may be connected together at depth. Due to the rupture at shallow surfaces in this cascading rupture of the event, these immature faults became connected at a shallower depth and will continue to develop at the next seismic cycle by a connected right-lateral fault. Moreover, this earthquake sequence triggered a 20 km-long shallow aseismic folding zone on the central Garlock left-lateral fault (117.3◦ W–117.5◦ W). Further investigations on the Garlock fault by geologic and geodetic observations will be signiﬁcant for understanding its contributions to seismic hazard in the Eastern California shear zone.

Author Contributions: Conceptualization, C.L.; Data curation, C.L., D.Z., Z.H., and R.J.; Formal analysis, Z.H.; Funding acquisition, G.Z. and X.S.; Investigation, C.L., R.J., and J.N.; Methodology, D.Z. and Y.L.; Project administration, C.L.; Resources, X.S.; Software, C.L.; Supervision, G.Z.; Validation, X.S. and J.L.; Writing—original draft, C.L.; Writing—review and editing, C.L., G.Z., X.S., D.Z., Z.H., R.J., J.L., and J.N. All authors have read and agreed to the published version of the manuscript. Remote Sens. 2020, 12, 3883 16 of 18

Funding: This work was co-supported by the National Key Research and Development Program of China (Grant No. 2019YFC1509204), the National Science Foundation of China (Grant No. 41631073), and the National Nonprofit Fundamental Research Grant of China, Institute of Geology, China, Earthquake Administration (Grant No. IGCEA2005). Acknowledgments: The Sentinel-1 and Sentinel-2 data were provided by the European Space Agency (https://scihub.copernicus.eu/dhus/#/home) through Sentinels Scientific Data Hub. The COSI-corr software can be downloaded from http://www.tectonics.caltech.edu/slip_history/spot_coseis/download_software.html. The MicMac software package was developed by the French National Geographic Institute, and the sources can be obtained from https://github.com/micmacIGN/micmac. The Coulomb3.3 software was developed by the U.S. Geological Survey. The Shuttle Radar Topography Mission (SRTM) data from the CGIAR-CSI SRTM 90 database (http://srtm.csi.cgiar.org) was used for InSAR data processing and topographic map generation. The figures were produced using GMT mapping software. Conflicts of Interest: The authors declare no conflict of interest.

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