the block model of southern california based on of gps ...dynamo.geol.msu.ru › personal › vsz...

ISSN 0145�8752, Moscow University Geology Bulletin, 2015, Vol. 70, No. 3, pp. 201–209. © Allerton Press, Inc., 2015.Original Russian Text © D.A. Simonov, V.S. Zakharov, 2015, published in Vestnik Moskovskogo Universiteta. Geologiya, 2015, No. 3, pp. 21–30.

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INTRODUCTION

The Earth’s crustal blocks are usually relativelyrigid segments that are confined by deformationzones. They can be both complex zones of deforma�tion and minor discontinuity faults, depending on thesizes of the blocks. Determining the kinematics of theblocks of a different order is one of the most importantproblems of geodynamics. With the development ofthe methods of satellite geodesy and global position�ing, GPS and GLONASS, and the increase in thenumber of constant monitoring stations that recordthe coordinates of the earth’s surface highly accu�rately, as well as their change with time, the possibilityof assessing the modern kinematics not only of largelithospheric blocks, such as tectonic plates, but alsothe discrete movements of relatively small crustalblocks. However, we note that such studies are possibleonly for territories with a high density of networks ofsatellite�positioning stations.

Scientists have performed numerous works in thisarea. We can identify two main groups of methods toreveal discrete Earth’s crustal blocks and to determinetheir modern kinematics. The first group of methods(Nyst and Thatcher, 2004; Meade and Hager, 2005;McCaffrey, 2005) includes those that are used first toidentify relatively rigid (in some cases rigidly�elastic)blocks and their boundaries using geological, tectonic,geomorphological and other data and then the GPSpoint data from a certain block are analyzed, and kine�matic characteristics of movement (rotation poles,angular velocity, and relative movement) are calcu�lated.

The second group of methods (Simonov et al.,Zakharov and Simonov, 2010; Zubovich et al., 2006,

2007; Bogomolov et al., 2007) includes the methodswhere the block identification mainly involves theanalysis of kinematics or the relationship of the mutualdisplacements of points of GPS networks in order toconstruct discrete kinematic models of particularregions. In this case, kinematically homogeneousblocks whose boundaries must be drawn in accordancewith geological and geomorphological data areassumed as rigid homogeneous Earth’s crustal blocksin the first place.

Thus, the long�standing approaches to solving thisproblem have not yet been formed, and the results ofthe works that have been carried out using bothapproaches leave some uncertainty as expressed in theintroduction of errors of determination of the values ofthe modern velocity of movement of the blocks whenreferring a point to a certain block and also in theuncertainty of the geological boundaries of the blocks.The fact that a good network of stations of global posi�tioning should exist in the regions under study hereand that these regions must be studied in detail geolog�ically adds complexity to the development of a proce�dure for the block identification and to the identifica�tion of their modern kinematics. We use this method inthe regions that have worse observation networks andhave been less studied geologically only after the pro�cedure is tested in these reference regions.

Southern California is undoubtedly a referenceregion. The geology of this extremely active region hasbeen studied very well and in detail. The network ofglobal positioning stations (GPS) here is one of thedensest in the world. The seismological service ofSouthern California provides very accurate and fullcatalogs of earthquakes, including catalogs with thesolutions of foci mechanisms, which makes it possible

The Block Model of Southern CaliforniaBased on of GPS Data Analysis

D. A. Simonov and V. S. ZakharovDepartment of Geology, Moscow State University, Moscow, Russia

e�mail: [email protected]; [email protected] November 12, 2014

Abstract—Using the author’s original procedure for GPS data analysis, a consistent model of the SouthernCalifornia block structure that corresponds to the general structural plan and geodynamics of the area exam�ined was constructed. The block model that was constructed differs from all those that were developed earlierand, in our opinion, shows the kinematic features of the modern blocks better. It is shown that the mappedfaults do not always indicate the boundaries of kinematically uniform crustal segments at the present stage.

Keywords: geodynamics, kinematics, GPS, satellite geodesy, crustal blocks, relative movements, SouthernCalifornia

DOI: 10.3103/S0145875215030084

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to compare the movements that are identified based onthe data of global positioning and the field of stressesduring comparable periods of time. In addition to this,there is a large volume of detailed digital data about thegeological structure of the regions, including a geolog�ical map, a map of quaternary faults, different geolog�ical fields, high resolution digital models of the area,and other data, which makes it possible to performgeomorphological and morphometric analysis of theterritory.

Tectonic position of Southern California. SouthernCalifornia is a part of the extended boundary betweenthe Pacific and North American plates. The dominantstructure of this boundary is represented by the SanAndreas northwest striking network of faults that aremainly of right strike–slip lateral kinematics, alongwhich in general the Pacific and North Americanplates relatively move. The San Andreas fault that con�nects the northern tip of the East–Pacific rise in thesouth and the Chuan�de�Fuca and the Gorda Ridgesin the north is a special element of the Pacific and

North American boundary of the plates that consistsof a right�lateral ridge–ridge type of transform fault inthe opinions of several researchers (Tevelev, 2005).

The system occurred in the middle–late Holocene.The total right�lateral strike–slip displacements forthe recent 30 myr equal several hundred kilometers,for example, V.E. Khain (2001) estimated the strike–slip at 315 km. However, we note that in fact this zoneof the interface between the Pacific and North Ameri�can tectonic plates consists of a belt that is approxi�mately 100 km wide and ~1300 km long, whoseboundaries were not clearly identified. Moreover, inaccordance with the GPS data, the field of velocities(Fig. 1) shows that the strike–slip displacements,which are typical of the boundaries between the NorthAmerican and Pacific plates, are recorded consider�ably eastward when attenuating. The belt is representedby a series of blocks that move in parallel and have dif�ferent shapes according to various authors. The bound�aries of the blocks are more often identified by the con�jugated discontinuous faults that are found by the geo�

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Santa Ana

Santa Monica

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Fault zone CleghornH

elendale

Elsinor

San Clemente fault zone

Sierra Madre overthrust zone

Santa Cruz—Santa Catarina

Hollywood

Fig. 1. The source GPS field of velocities that was constructed based on the CMM3 data (grey arrows) and discontinuous faults(fine black lines) according to (Digital…, 2005).


THE BLOCK MODEL OF SOUTHERN CALIFORNIA 203

logical and seismological methods (McCaffrey, 2005;Meade and Hager, 2005).

The velocity of the modern absolute displacementsis in general equal to or greater than 30–80 mm/yr.However, the displacements along the differentstrike–slips and in various locations occur at unequalvelocities, which also changes in various periods oftime. The direction of the displacement can changetoo, but as a whole this is a right�lateral strike–slip. Insome segments, the displacement is continuous, inanother ones it is jump wise (Khain, 2001). The latterfact makes the problem of identifying the boundariesof the blocks and their kinematics more urgent in lightof the possibility of predicting strong earthquakes.

The major discontinuous faults that form the mod�ern structural plan in the Southern California segmentof the San Andreas system are represented first of all bythe Mojave segment in the San Andreas fault, as wellas by large branching or subparallel faults, such as theSan Gabriel, San Jacinto, and Elsinor faults. In theMojave Desert, we mention the large right�lateralHelendale and Lockhart strike–slips. Among themajor faults that intersect the San Andreas fault wemention the Garlock fault, which has left�lateralstrike–slip kinematics, the Big Pine fault, which issubparallel to Garlock, and the left�lateral PintoMountain strike–slip, which intersects the SanAndreas fault southward of the Mojave ridges. Thereare also overthrusts and upthrows of different scalesup, for example, the right�lateral Santa Monicatranspressional fault, the Sierra Madre overthrustzone, and San Caetano fault system. The normal faultsoccur less frequently, but we note the quite large left�lateral Santa Inez transtentional fault (Fig. 1).

Source data. The data on the horizontal displace�ments of the Earth’s crust in Southern California (Fig. 1)were obtained from the CMM3 data base (CrustalMotion Model v.3, www.scec.org/resources/data), aproject that is supported by the Southern CaliforniaSeismological Center (SCEC). Version 3 was releasedin 2003. It includes not only the data that have beenobtained from the new own SCEC (survey�mode data)stations and the permanent Southern California Inte�grated GPS Network (SCIGN) stations, but also thedata from the USGS Crustal Strain project (1970–1992) and the data of NASA Crustal Dynamics Pro�gram (1980–1994). The data of GPS observationscover a period from 1986 to 2001. For the area of study,366 points were selected. To identify the blocks, weused a digital map of Quaternary and modern faults ofCalifornia (Digital…, 2005), as well as the SRTM dig�ital model of the relief and Landsat ETM+ satelliteimages.

Models of the block structure of Southern Califor�nia. There are several models of the block structure ofSouthern California. Here, we study two of the rela�tively recently published models, since they were con�structed using the same data that we used but via otherapproaches to block identification.

In one of these models (McCaffrey, 2005), closepolygons on the Earth’s surface were taken as theEarth’s crustal blocks; each point inside the polygonsrotates at an identical angular velocity. Despite the factthat discontinuity faults were considered as the mainboundaries of the blocks, this author could not use thenetwork of the faults in full to construct the blockmodel (Fig. 2). He drew part of the boundaries for theblocks mainly based on the difference in the values ofthe GPS velocity at the neighboring points. Theseblocks included the eastern and western blocks of theregion of Ridges and Basins (in Fig. 2 EBNR andWEBR, respectively) and two blocks in accordancewith the Calico and Black Water fault system in theMojave Desert subject to restrictions. The movementsof the block were calculated for the block model rela�tive to the North American plate that was constructedusing the geological data.

Another model of the block structure of the South�ern California (Meade and Hager, 2005) was devel�oped by its authors in order to give a quantitative esti�mation of the strike–slip components along moderndiscontinuity faults; therefore, the boundaries of theblocks were considerably coarser. The network ofmajor faults served as the basis for identifying the blockboundaries, even to a greater extent than in the previ�ous model (Fig. 3). Moreover, in order to identify thesimplified boundary of the block for several areas, includ�ing the vicinity of Los Angeles, the authors of the statedwork integrated a more complex network of faults to asingle fault zone, which is a boundary of the block. Thisapproach can be justified during the construction of asimplified kinematic model, but in our opinion, leads toa loss of information concerning the actual block kine�matics that can be of considerable interest.

Thus, these models show that the boundaries of theblocks cannot always be unambiguously associatedwith discontinuity faults. Kinematic homogeneity ofblocks identified this way causes even more questionsto arise. This makes the problem of identifying kine�matically homogeneous movable crustal blocks withthe subsequent confirmation of their boundariesaccording to the geological data more urgent.

The technique of the studies. The technique wedeveloped for establishing discrete movements relativeto minor crustal blocks were presented earlier in(Simonov et al., 2006; Zakharov and Simonov, 2010).However, since these studies used an algorithm thatimplies certain parts of the methods we developed, it isnecessary to turn our attention to this algorithm.

The movements of the blocks, especially minorones, are quite complex, and they can be representedas rotation around different poles, rotation around aninternal axis, or as a combination of both types ofmovement. Most of the sources present the data on thevelocity of the movement of the points on the Earth’ssurface that were obtained from the satellite geodesydata in a so�called local Cartesian system of coordi�nates, which has three components, viz., the northern(n), eastern (e), and vertical (d) components. This sys�

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tem can be written as V = (Vn, Ve, Vd). However, not allof the sources use a vertical component of velocity. Inthe case of determining the horizontal velocity, a verti�cal component may be neglected and accepted to beequal to zero (Vd = 0). To continue the analysis cor�rectly, we are required to convert the velocity for theglobal system of coordinates V = (Vx, Vy, Vz), related tothe Earth’s center and inversely for the local system(Cox and Hart, 1989), since direct vector operationswith the velocity values of points in the local system ofcoordinates can lead to significant errors in analyzingthe differential movements.

As rigid crustal blocks of the Earth, we took a rigidkinematic blocks or sets (“clusters”) of points on theEarth’s surface that belong to the common Euler pole, P,and have an equal angular velocity ω (within the spec�ified permissible errors). Here, coordinated move�ment was the major criterion of the block integrity.The data on the fault tectonics of the region were con�sidered to be of secondary importance and could beused for the geological confirmation of the boundariesof the blocks, since the discontinuity faults at that par�ticular moment could be consolidated and inactive,due to which they would not be the boundary of a con�

sistently moving block. The errors of rotation parame�ters in identifying the clusters are assigned to excludethe measurement errors and to minimize errors thatare related to internal rotation, as well as to elastic orplastic deformation of the blocks, since the proposedprocedure cannot take these parameters into account.It is evident that the most correct results will beobtained during the work with elongated blocks. Thespecified permissible errors took on quite large values(±10° for the direction of the velocity vector and±10% for the angular velocity). This choice of param�eters made it possible to neglect insignificant internalrotations, which substantially simplified the analysis,although there was a possibility of the loss of isometricminor blocks. The procedure of clusterization wastested for sensitivity to the choice of the initial point.It was found that the final result of clusterization doesnot depend on that choice and is quite stable.

The used algorithm of “clusterization” or identify�ing rigid kinematic blocks is based on the enumerationof the available data and consists in the following.

We choose two arbitrary points, T1 and T2, thathave the velocities V1 and V2, respectively, from the ini�tial set of data with respect to N observation stations,

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EMOJ

ENBR

MOJA

NOAM

SGAB

ANZA

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WBAJ

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O

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CATA

Fig. 2. A model of the Southern California block structure, according to (McCaffrey, 2005). The blocks: NOAM, the NorthAmerican plate, PACI, the Pacific plate; ENBR, the Eastern block of Ridges and Basins; MOJA, Northern Mojave; EMOJ, East�ern Mojave; SMOJ, Southern Mojave; SWMO, the Southwestern Mojave; ANZA, Anza; VENT, the block of Transverse ridges;SGAB, San Gabriel; SALT, Salton, WBAJ, Western Baja; EBAJ, Eastern Baja; SANA, Santa Ana; CATA, Catalina; SALI, Salin�ian; GVTB, the block in the Great Valley overthrust belt; SNEV, Sierra Nevada; INYO, Inyo; PANA, Panamint.



their Euler pole has not yet been identified. We deter�mine the position of the Euler pole P(φp, λp) for thepoints T1 and T2 with respect to the velocities V1 and V2;for each point T1 and T2, we formally calculate the val�ues of the instantaneous angular velocity ω1 and ω2using the values of the linear velocity and the calcu�lated position of the Euler pole, P, as described in(Zakharov and Simonov, 2010).

If the values ω1 and ω2 are relatively equal, i.e.,|ω1 – ω2| ≤ Δω, where Δω is the previously prescribedaccuracy (the maximum permissible error) of deter�mining the angular velocity ω, the Euler pole P is con�sidered to be correct and is stored and the points T1and T2 are attributed to this Euler pole, P1.

If the difference is |ω1 – ω2| ≥ Δω, another point,T3, is selected from the data set, whose Euler pole hasnot yet been established, point T2 is replaced by T3,and the previous operations are repeated until at leasttwo points that relate to one pole of rotation P1 arefound.

We choose an arbitrary point, Ti, from the data set,whose Euler pole has not yet been determined, and usethe obtained value ω to calculate the model velocity

= [ωri], where r = (rx, ry, rz) is the radius vector ofthe point Ti (in the global system of coordinates), and

the azimuth of movement at this point. We also cal�culate the values of the angular velocity ωi that wouldbe found during the rotation of this point around theearlier determined pole, P1. These calculated (model)values of the azimuth and the angular velocity arecompared with the actual ones. If the differencebetween the calculated ( ) and the actual (Ai) azi�muths is less than the earlier specified error of azimuthΔA(| – Ai| ≤ ΔA) and ωi and ω1 are relatively equalwithin Δω(|ω1 – ωi| ≤ Δω), the point Ti refers to thesame pole, P (belongs to this block), if not, the rest ofthe points from the entire data set are checked.

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VB

Fig. 3. A model of the Southern California block structure according to (Meade and Hager, 2005). The faults are designated byfine black lines according to (Jennings, 1994); the boundaries of the block are shown by the thick white lines. The blocks: BA,Baja; BB, Blackwater; BP, Big Pine; CI, Block of Coastal Islands; CR, Block of Coastal Ridges; DV, Death Valley; EL, Elsinor;EM, Eastern Mojave; HB, Helendale; KC, Kern County; LA, Los Angeles; MJ, Mojave; NA, North America; OB, Oak Ridge;OV, Owens Valley; NV, Nevada; PA, Pacific; PV, Palos Verdes; CO, San Bernardino; SD, San Diego; SG, San Gabriel; SL, Sal�ton, SN, Sierra Nevada; VB, Block of Ventura Basin.

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SIMONOV, ZAKHAROV

If as a result of the enumeration of all the points,there are those for which the rotation pole is not yetdetermined, the algorithm is repeated until the Eulerpoles, P2, P3, P4, etc. are not found for the other blocks.

All the points are checked once again to ensure thatthey belong to the pole that provides the minimumerrors. The errors of the estimate of the azimuth andangular velocity for the rotation around each of thefound poles, P1, P2, P3, etc. are calculated. The pointfinally belongs to the pole for which the stated errorsare minimum and less than the assigned ΔA and Δω. Atthis stage, sets of points can occur within the identifiedclusters, which formally belong to the same pole butare actually surrounded by points that have anotherpole. This is explained by the fact that formally thesepoints conform to the prescribed conditions, i.e., theerrors are minimum and less than ΔA and Δω, but infact, although with insignificant errors (also less thanΔA and Δω, they can be included in other clusters thatare more suitable from the geological structural pointof view. This feature requires the attention of researchexperts and manual correction of errors of this type.

Based on the calculated parameters of movement,the relative movements of separate blocks are deter�mined. We used one of the four methods for determin�ing relative movements that was described in (Simo�nov et al., 2006; Zakharov and Simonov, 2010). Themethod has the following algorithm: a certain block 1is selected (point T that belongs to this block) with theEuler pole P1 and a vector of the angular velocity ω1,relative to which the movements of other blocks arecalculated, and a point that refers to the block withEuler pole P2(φ2, λ2) and a vector of angular velocityω2, whose relative movement is calculated. Then, theangular velocity, ω' = ω2 – ω1, of the block relative toblock 1 is computed. Next, the relative linear velocityof the point ti is calculated in the global system of coor�dinates V ' = [ω'r], where r is the radius vector of thepoint ti, and then the velocity that is obtained for a rel�ative movement in the global system of coordinates ispresented in the local system of coordinates. In thismethod, it is considered that the points within theblock are immovable relative to each other within aprescribed error. Although the method does not takethe rotations of the blocks into account, the results arethe most correct due to the assumptions.

The block structure of Southern California based onthe analysis results. As a result of the analysis of theinitial GPS data from the CMM3 catalog within theselected region using the described procedures, weidentified 17 blocks of different sizes and differentshapes (Fig. 4). They were mainly elongated blocksextending northwest, subparallel to the San Andreasfault system. Several blocks with a more isometricshape were detected northeast of the San Andreas faultin the Mojave Desert. Where possible, the boundariesof the blocks were drawn with respect to the network ofQuaternary and modern discontinuity faults(Digital…, 2005); however, we will show below that thediscontinuity faults cannot always be represented

everywhere as the boundaries of kinematically homo�geneous crustal blocks that are obtained using theabove procedure. Where it was impossible to outlinethe boundaries of the blocks along the mapped discon�tinuous faults, the boundaries were drawn taking therelief morphology and the results of the interpretationof Landsat ETM+ space images into account, how�ever this analysis should be considered to be only pre�liminary; more accurate drawing of the boundariesrequires more detailed studies using the complex anal�ysis of the geological structure, morphology, and seis�micity of the region. In connection with this, theboundaries were defined as broad zones. The values ofvelocity and the directions of the movement of theblocks in the presented model were calculated relativeto the San Andreas fault in its Mojave segment near thearea of Palmdale.

The Pacific block (PACI) is a margin of the Pacificplate. We drew its northeastern boundary convention�ally in the marine part along the fault zones: SanDiego–Santa Cruz–Santa Catalina. In accordancewith the GPS data, the relative movement amounts to~10 mm/yr and is recorded between the San Clementeand the Santa Catalina islands. Since there are noother data, the southern part of the boundary can alsobe drawn along the San Clemente fault system. North�ward of Santa Cruz Island the boundary comes to anarea near Santa Barbara and again returns to the oceaneither along the Santa Ines River fault system or alongthe Baseline–Lion’s Head faults. The velocity of theblock movement relative to the San Andreas fault inthe Mojave segment is ~25 mm/yr.

The coastal block (COS), which is detected by theCMM3 data, is the largest block; it is very well con�fined from all sides. This block is considerably largerthan the region that is examined in this work; itstretches both north� and southward of it. Its length isno less than 850 km, while the width in the southbeyond the region under study reaches 150 km. Theblock width ranges from 90 km in the widest part to 25km within the area under study. The block boundary ofthe identified region can be associated with the dis�continuous faults only fragmentarily, for example, themost distinct and well�expressed segment of theboundary goes along the San Caetano fault system.This segment of the boundary is transpressional,which conforms to the kinematics of the San Caetanofault and is clearly recorded by a sharp change in theorientation and the value of the vectors of the relativemovements of the GPS stations in the adjacent blocks.The interaction between the Coastal block and theOzena–Elsinor block (OZEL), which are situatednorthward, is so strong that the orientation of the con�siderable segment movement changes in this region byapproximately 3° eastward and this region is countedas the separate Ventura (VEN?) kinematic block (inFig. 4 it is marked by a white dashed line). However, atthis stage of the studies, the block did not show anymorphological features of boundaries; therefore weinterpret it as a pseudoblock that possibly shows an



elastic interaction between the Coastal block and theOzena–Elsinor block. To the north, the Coastal blockmoves away from the coast and gradually wedges out,approaching the San Andreas fault almost tightly. Thevelocity of this block movement relative to the selectedsegment of the San Andreas fault is ~15 mm/yr.

The Ozena–Elsinor block (OZEL) is a narrow (nomore than 25–30 km wide) and extended (up to550 km long) block that is located northeast of theCoastal block. As a result of the transpressional inter�action with the Coastal block, the Ozena–Elsinorblock was squeezed in the area of the San Caetanofault and was almost split into two separate blocks(Ozena and Elsinor). However, both blocks make up asingle kinematic cluster that moves around one poleuniformly and consistently; therefore it was inter�preted as a single plate. North of the Caetano fault, the

northeastern boundary of the block cannot be associ�ated with any known rupture; however, the boundaryof this block is quite confidently drawn southwardalong the Sierra Madre and Elsinor fault systems.Here, the current interaction with respect to the SierraMadre upthrows is mainly a strike–slip and is even atranstension in the Elsinor fault system (Fig. 4). Thevelocity of the block relative to the Mojave segment is~10 mm/yr.

The San Gabriel–San Jacinto block (SGSJ) is anextended (up to 300 km) block that wedges outsmoothly in the Big Bend area of the San Andreas faultin the north and has a quite sharp and almost sublat�eral closure in the south, in the area of the branchingfrom the San Jacinto fault system of the Buck Ridgefault. The narrowest part of the block is found in thearea of its intersection with the Sierra Madre Fault sys�

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Fig. 4. A kinematic model of the Southern California block structure according to the CMM3 data. The thick white lines definesthe boundaries of the blocks, the black arrows designate the vectors of velocity with respect to the Mojave segment of the SanAndreas fault, the fine black lines designate Quaternary and modern faults according to (Digital…, 2005). The blocks: PACI,Pacific; COS, Coastal; VEN?, Ventura; OZEL, Ozena–Elsinor; SGSJ, San Gabriel–San Jacinto; SAM, San Andreas Mojave;PR, Rialto Colton; SABB, San Andreas, Big Bend segment; MC, Mill Creek; MW, Western Mojave; SG, Southern Garlock;MO, Mojave; NFT, Block of Northern Thrusts; LO, Lokhart; HE, Helendale; ME, Eastern Mojave. The velocity values are cal�culated for the point shown by a white square.

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tem; however, this location concentrates four GPSmeasurement stations that are located at the differentwings of the fault. The data from these stations showuniform movement, which excludes the boundary ofthe blocks along this fault system. In the north, in thearea of the San Gabriel fault, the boundaries of theblock are not associated with the discontinuous faults;in the south, however, southward of Sierra Madre, theElsinor and the San Jacinto fault systems can be consid�ered to be block boundaries, except for the boundarywith the Rialto Colton block. The velocity of the blockmovement relative to the Mojave segment is ~7 mm/yr.

The San Andreas–Mojave block (SAM) stretchesimmediately along the San Andreas fault southwest�ward and northeastward, which indicates that theMojave segment in the San Andreas fault is“squeezed” for a considerable distance. This is alsoproven by the almost total aseismicity of those parts ofthe segment where the GPS stations move consistentlyat the different wings of the fault, i.e., they are immov�able relative to each other, as well as by the commoncharacter of the velocity field (Fig. 3) and several otherfactors that we will not consider here. This block origi�nates somewhat northward of the intersection of the SanAndreas fault and the Big Pine fault and runs to the southup to where the San Andreas fault splits into northern andthe southern branches. Here, the block bends to thesouth, the southern branch of the San Andreas faultbecomes its northern boundary, and the San Jacintofault system becomes its southern boundary. The blockwidth ranges from 5 to 20 km at various segments. Theblock boundaries are not associated with discontinu�ous faults along the greater length, except for theboundaries in its southern part that are describedabove, as well as at a small segment in the central partof the block in the area of Palmdale, where the SanAndreas fault is the block boundary along a length of25 km, and also excluding the northernmost segmentof the block, where the Pleito upthrow system canserve as the block boundary north of the Garlock fault.The block is immovable relative to the Mojave seg�ment of the San Andreas fault. The movements ofother blocks were obtained relative to this block. Thefeatures of the occurrence of the San Andreas faultinside the block can indicate its extreme potential seis�mic hazard within this segment.

The Rialto–Colton block (RC) is a minor triangularblock that is squeezed between the Sierra Madre faultsystem and the northern tip of the San Jacinto fault. Itssouthern boundary is not associated with discontinuousfaults. The direction of the movement of this block isquite consistent with the right�lateral strike–slip kine�matics of the San Jacinto fault and the upthrow kinemat�ics of the Sierra Madre fault; however the relative velocityof its movement is extremely small (3–4 mm/yr).

The San Andreas block, which is a segment of theBig Bend block (SABB), is a narrow (no more than10 km wide) block that stretches along the Big Bend ofthe San Andreas fault from its intersection with the BigRhine fault northward at a distance of ~80 km. It

moves northward relative to the San Andreas–Mojaveblock at a velocity of ~8 mm/yr, which can addition�ally confirm the fact that the Mojave segment of theSan Andreas fault is actually blocked.

Northward of the San Andreas fault, the blocksbecome more isometric in plan view, except for theWestern Mojave block, and their relative movement isless manifested, at least within the Mojave Desert tothe Helendale and the Lockhart faults.

The Mill Creek block (MC) is an isometric blockthat is 40�km wide, which is detected between thenorthern branch of the San Andreas fault and the sys�tem of Northern Frontal overthrusts. The data thatexist to identify it are extremely sparse. At the block,there is only one GPS station that demonstrates slow(~2 mm/yr) movement to the northeast relative to theSan Andreas–Mojave block, which is poorly consistentwith the movements of the other blocks and is possiblya measurement error; if this is true, this block can beintegrated with the San Andreas–Mojave block.

The Western Mojave block (WM) is an extendedblock that stretches in parallel to the San Andreas faultwithin the Mojave segment. In accordance with ourpreliminary estimates, its width varies from 5 to 40 km;however it is quite problematic to identify the blockboundaries, since the block is covered by large cones ofdetritus from the San Gabriel ridge.

The block boundaries cannot be associated withdiscontinuous faults along the greater length, exceptfor the described segment of the boundary with theSan Andreas–Mojave block in the area of Palmdaleand the southern boundary we drew (mostly due toinsufficiency of data) along the Cleghorn fault. Thevelocity of the southeast movement of this block rela�tive to the San Andreas–Mojave block is ~5 mm/yr.

The South Garlock block (SG) is a minor block, thenorthward extension of the Western Mojave blockintersecting the southern part of the Garlock fault tothe Pleito fault system and the Spring fault. Kinemat�ically, this block differs from the Western Mojave blockin a somewhat increased velocity, up to 8 mm/yr, rela�tive to the San Andreas–Mojave block. This blockmight be considered to be a part of the Western Mojaveblock; the increased velocity of its movement isexplained by the interaction with the adjacent blocks.This assumption is supported by the fact that the Teh�achapi block (TH) that lies northward is included in thesame kinematic group as the Western Mojave block.

The Mojave block (MO) and the block of NorthernFrontal Thrusts (NFT) are kinematically very similar.The kinematics of the block of Northern Thrusts dif�fers from the Mojave kinematics in that the vectors ofthe relative velocity of this block deviate at approxi�mately 5°. The kinematics of the block of NorthernThrusts is based on a quite confident concentratedsampling from the velocity field; however as in the casewith the Ventura pseudoblock, it is difficult to establishits boundary with the Mojave block. It is highly prob�able that the block of Northern Thrusts is a pseudob�lock, as in the case of the Ventura block, and its kine�



matic features are explained by the elastic interactionbetween the Mojave block and the Mill Creek blockthat borders on the block of Northern Thrusts alongthe system of Northern Frontal Thrusts. The Helen�dale fault system serves as the northeastern boundaryfor both blocks. In the north, the Mojave blockstretches beyond the Garlock fault outside the areaunder study. The boundary of this block can be drawnalong the White Wolf fault system; however we notethat its movement is quite similar to the movements ofthe points that are located within the Central Valley,many of which fall within the same kinematic samplingas the points that are located inside the Mojave block.The velocity of the movement of the Mojave block andthe block of Northern thrusts relative to the SanAndreas–Mojave block is 10 mm/yr.

The Lokhart block (LO), the Helendale block (HE),and the Eastern Mojave block (EM) were identifiedaccording to quite scarce data. No clearly expressedboundary was defined between the Lokhart and theHelendale blocks. We drew the boundary with theEastern Mojave block along the Lokhart fault system.All three blocks have identical kinematics. Therefore,they should be considered as a single Eastern Mojaveblock. The Helendale fault system is an importantboundary here; northeastward of it the velocity relativeto the San Andreas–Mojave block increases notice�ably, up to 15 mm/yr.

CONCLUSIONS

Using the procedure for identifying discrete crustalblocks and determining their kinematics that wasdeveloped by the authors (Zakharov and Simonov,2010), a consistent model of the block structure wasconstructed for Southern California that conformedto the general structural plan and the geodynamics ofthe studied region. This block model differs from thosethat were designed earlier (McCaffrey, 2005; Meadeand Hager, 2005) and in our opinion, better presentsthe features of the modern block kinematics. It isshown that the mapped discontinuous faults do notalways mark the current boundaries of the kinemati�cally uniform crustal segments. Moreover, theattempts to outline the boundaries of the blocks onlyalong the discontinuous faults can lead to a loss ofespecially important information on the modern kine�matics and the stress state along the faults, which canbe used to predict catastrophic events, as is shown bythe example of the block we identified in the Mojavesegment of the San Andreas fault.

However, we note that the correct identification ofthe boundaries of the kinematically uniform blockscauses considerable difficulties and requires a carefulcomplex analysis of additional data, such as the infor�mation on the geological structure, morphology of therelief, geophysical fields, and the features of the seis�micity occurrence.

REFERENCES

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Digital Database of Quaternary and Younger Faults from theFault Activity Map of California, vs. 2.0.2005. Compiledby W.A. Bryant, California Geol. Survey, 2005. http://www.consrv.ca.gov/CGS/information/publica�tions/QuaternaryFaults_ver2.htm. Cited May 10, 2010.

Jennings, C.W., Fault activity map of California and adja�cent areas with location and ages of recent volcaniceruptions, Calif. Geol. Data Map Ser., Map. 6, Sacra�mento, California: Calif. Dept. of Conserv. Div. ofMines and Geol., 1994.

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Zubovich, A.V., Mosienko, O.I., Kuzikov, S.I., and Mel�lors, R., Study of modern tectonics of the Tien Shan onthe data of satellite geodesy, in Oblasti aktivnogo tekto�geneza v sovremennoi i drevnei istorii Zemli: Mal�lyXXXIX tektonicheskogo soveshchaniya. T. 1 (Proc.XXXIX Tecton. Conf. “Areas of Active Tectonogenesisin the Modern Ancient History of the Earth”), Mos�cow: GEOS, 2006, vol. 1, pp. 243–244.

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Translated by L. Mukhortova

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