LARSE I Progress Report

LARSE I Progress Report

Compiled by Robert W. Clayton

Introduction

The Los Angeles Region Seismic Experiment (LARSE) was conceived in 1993. The first part of LARSE, now called LARSE I, was executed in the Fall of 1993 and 1994. The original project consisted of two lines (Lines 1 and 3 in Figure 1 ), and a third line (Line 2) was added as a result of the Northridge earthquake

The primary emphasis of LARSE I was on Line 1, where five related seismic surveys were run: 1) a passive survey from Azusa to Barstow, 2) a marine MCS survey from Seal Beach to San Clemente Isl., 3) an onshore-offshore survey across the inner borderland 4) an active source survey across the San Garbriel Mtns., and 5) an OBS survey along the MCS profile. In addition, onshore-offshore and OBS surveys were run on Line 2, and an onshore-offshore survey on Line 3. The complete project for Line 1 also called for an active source profile in the Los Angeles Basin itself, but this was not run for financial and logistical reasons. For Line 2, the missing component is the active source survey.

LARSE I involved more than 10 institutions, 100 volunteers and cost approximately $600K for the data acquisition and basic data reduction.

Scientific Results For LARSE I.

The scientific goals for LARSE I were to image the crust and upper mantle from the Inner Borderland to the Mojave Desert. The particular targets of interest were: the Los Angeles and San Gabriel sedimentary basins, the San Gabriel Mtns, thrust faults such as the Whittier and Sierra Madre Faults, the San Andreas Fault, and the horizontal detachment surface. The results to date have produced structural images, models, and tectonic models that are relevant to all these targets.

The following scientific projects have been completed or nearly completed over the past two years:

1) Crustal Model, Los Angels Basin to the San Gabriel Mtns

The results of imaging the explosion data for Line 1 are shown in Figure 2 . The reflective portion of the image shows a set of bright reflections at ~20 km depth that can be interpreted as the horizontal detachment zone. The image also shows a slight change in character of the detachment at the location of the San Andreas Fault. Also shown are velocity images of the Los Angeles and San Gabriel Basins.

Images of thrust faults are shown in Figure 3 . In this particular shot gather, a north-dipping structure is imaged at the location of the Sierra Madre Fault (SMF), and a south dipping structure is imaged at the location of the Punch Bowl and Vincent Thrust Faults. There is no discernible image associated with the San Andreas Fault.

2) Inner Borderland Model for Lines 1 and 2

The onshore-offshore data recorded during the LARSE experiment allows the imaging of the crust and sub-crustal structure in the Inner Borderland region. The data require substantial travel time corrections to remove the effect of the undulating seafloor topography, the marine micro-basin structure, and the onshore static corrections. A novel technique was developed to do this, and its application to both Line 1 and Line 2 reveal laterally coherent refractors beneath the inner borderland. Figure 4 shows the results for Line 1 and Line 2 respectively. The dips and layer velocities are constrained by being able to logically reverse the refraction profile.

The important conclusion of this study is that there is a laterally coherent oceanic slab beneath the borderland. On Line 1, this is indicated by the 7.4 km/s layer, which is a reasonable velocity for oceanic crust, but not for continental crust. The velocity is constrained to be +/- 0.1 km/s. On Line 2, a deeper refractor is evident which we are interpreting as oceanic Moho. We are interpreting the presence of this arrival on Line 2 and not Line 1 as due to the better quality of the Line 2 data.

3 Lower Crust Model (Crustal Root) For the San Gabriel Mtns

The explosion data from LARSE Line 1 has been used to determine the depth and orientation of the Moho beneath the San Gabriel Mountains. The results are shown in Figure 5 and Figure 6 . Figure 5 is stacked section of the explosion data and an image of the Moho dipping from a depth in the Mojave Desert of 30 km to a depth of approximately 45 km beneath the high point of the San Gabriel Mountains. Figure 6 shows a model for the Moho depth that include PmP picks made from several shot gathers distributed from the Mojave to Seal Beach. The question marks indicate the lack of an observable PmP reflection to use for locating the Moho depth. The 15 km of relief on the Moho is agrees with the result of Kohler and Davis from teleseismic arrivals seen on the LARSE-93 array. The 15 km root would be sufficient to isostatically compensate the topography if it were not for the excess density inferred from the lack of correlation between the travel time residuals and topographic correction.

4) Moho Depth Model San Gabriel Mtns to the Mojave Desert

Residuals of teleseismic travel times show a systematic variation with Mojave arrivals coming later than those on the south side of the San Gabriel Mtns by about 1/2-1 sec. In Figure 7 , a model of Moho topography that will explain this observation is shown. In this interpretation, the entire residual is modeled as a relative change in Moho depth of 15 km. This is consistent with the direct imaging of the Moho from the explosion data.

5) Integrated Lower Crust/Upper Mantle Model - Line 1

The crustal model from the onshore-offshore data and the lower crustal model obtained from the explosion data have been combined with the upper mantle model of Humphreys and Clayton (1984). to obtain the tectonic model shown in Figure 8 . The main feature of this model is the direct coupling of the "drip" beneath the Transverse Ranges with the crust through the underlying oceanic slab. The dynamic flow implies by this model would translate directly into horizontal compression across the Los Angeles and San Gabriel Basins with the strain increasing northward towards the mountains. This effect is observed in the GPS data. The model also raises the question whether compression across the basin preceded or is a result of the drip.

A joint tomographic inversion of LARSE passive and SCSN teleseismic residuals including LARSE-determined Moho variations, has provided a better resolved image of the "drip" and has found that it lies directly beneath the Transverse Ranges.

6) Updated LAB velocity model from LARSE explosion times

One important contribution of the LARSE explosion data is in providing known source point travel time data for 3D velocity inversions. An example of this is shown in Figure 9 , which is the 3D velocity model of Hauksson plotted in cross section along Line 1. The main features of the Los Angeles and San Gabriel Basins are evident in the model. The effect of the LARSE I data is shown in the Figure 10 where just the explosion travel times are used in a 2D inversion along the Line 1 profile. Of particular interest in the shape of the intrusion of basement rock into the basin caused by the Whittier Thrust Fault.

7) Marine Crustal Model for Line 1

The Ocean Bottom seismometer (OBS) and Reftek land data from nearby islands indicate large lateral variability in the velocity and velocity gradient of the shallow (7-8 km) crust of the Inner California Borderland between Catalina Ridge and Seal Beach. The results are shown in Figure 11 Most of the velocity anomalies are associated with low velocity (<3 km/s) sedimentary basins and high-velocity metamorphic rocks under Catalina Ridge (5.5-6.5 km/s) and San Clemente Ridge (5-6 km/s). Anomalies associated with the ridges suggest southward directed thrusting of the ridges over the adjacent basins to the south. The mid- to lower-crustal velocity structure below 8 km is apparently very simple and nearly featureless: velocities vary laterally between 6.3-6.7 km/s and has only a very small (<0.01 km/s/km) velocity gradient. High-amplitude PmP arrivals reveal that the Moho is relatively flat at 23-24 km. The upper mantle velocity is not determined from these recordings. As opposed to Line 2, no additional phases are observed from these data that would provide evidence for a "remnant oceanic crust' underlying the California Borderland. Although this lack may be due to the limited shot-receiver offsets along Line 1, it may be possible that this slab is locally absent or present only onshore.

8) MCS Upper crust image offshore Line 1

Multichannel seismic-reflection profiles provide dramatic images of the zone of active deformation within the San Pedro Basin (see Figure 12 ). This zone of deformation is bounded on the west by the San Pedro Basin fault, an oblique-slip fault having a significant normal dip-slip component (over 500 m) down throw to the east. The San Pedro Basin fault coincides with the western limit of a dense distribution of small to moderate magnitude (Mw 3-5) earthquakes extending to the east across the Palos Verdes and Newport-Inglewood fault zones. At present, the most intense folding and dip-slip faulting is located about 6 km east of the San Pedro Basin fault in a zone a few km wide. A narrower 1-km zone of chaotic faulting having only minimal strata offset about 6 km further east near the shelf edge may coincide with a region dominated by strike-slip faulting. This change in dip-slip faulting increases to the west across the oblique-slip fault zone.

9) Local earthquakes recorded on the Passive Array.

Record sections from local earthquakes contain high signal to noise ratio S waves which have been used to construct an S-wave crustal model. Strong SmS arrivals are observed from events north of the San Gabriel Mtns whereas SmS is not observed for events to the south, possibly because the Moho is more disturbed in the reflection region. Models have been constructed to fit P, S travel times as well as SmS amplitudes.

Publications

Papers

Fuis, G. S., Okaya, D. A., Clayton, R. W., Lutter, W. J., Ryberg, T., Brocher, T. M., Henyey, T. L., Benthien, M. L., Davis, P. M., Mori, J., Catchings, R. D., ten Brink, U.S., Kohler, M. D., Klitgord, K. D., and Bohannon, R. E., 1966, Images of crust beneath southern California will aid study of earthquakes and their effects: EOS (Transactions, American Geophysical Union), v. 77, no. 19, pp 173, 176.

Kohler, M. D. and P. M. Davis, Crustal Thickness Variations in Southern California from Los Angeles Region Seismic Experiment (LARSE) Passive Phase Teleseismic Travel Times, submitted Aug. 23, 1996 to Bull. Seismol. Soc. Am.

Video

Video tape produced jointly by SCEC and USGS: "The Los Angeles Region Seismic Experiment" (9 min).

Open File Reports:

Brocher, T., R. Clayton, K, Klitgord, R. Bohannon, R. Sliter, J. McRaney, J. Gardner, J. Keene, 1995, Multichannel seismic-reflection profiling on the R/V Maurice Ewing during the Los Angeles Region Seismic Experiment (LARSE), California: U.S. Geological Survey Open File Report 95-228, 73 pp.

Brocher, T.M., and S.L. Bilek, Merged and stacked wide-angle airgun recordings made during the Los Angeles Region seismic Experiment (LARSE), California: U. S. Geological Survey Open File Report, in review.

Kohler, M. D., Davis, P. M., Liu, H., Benthien, M. L., Gao, S., Fuis, G. S., Clayton, R. W., Okaya, D. A., and Mori, J., 1995, Data report for the 1933 Los Angeles Region Seismic Experiment (LARSE 93), Southern California: a passive study from Seal Beach northeastward through the Mojave Desert: USGS Open-File Report 96-85, 82 p..

Murphy, J., G. Fuis, T. Ryberg, D. Okaya, M. Benthien, M. Alvarez, I. Asudeh, W. Kohler, G. Glassmoyer, M. Robertson, and J. Bhowmik, 1995, Report for the explosion data acquired in the Los Angeles Region Seismic Experiment (LARSE), Los Angeles, California; USGS Open File Report (in review).

Okaya, D., J. Bhowmik, G. Fuis, J. Murphy, M. Robertson, A. Chakraborty, M. Benthien, K. Hafner, and J. Norris, Region Seismic Experiment (LARSE), California: USGS Open-File Report (in review).

Okaya, D., J. Bhowmik, G. Fuis, J. Murphy, M. Robertson, A. Chakraborty, M. Benthien, K. Hafner, and J. Norris, 1995, Report for Earthquake Data Acquired at Onshore Stations during the Los Angeles Region Seismic Experiment (LARSE), California: USGS Open-File Report (in review).

ten Brink, U. S., R. M. Drury, G. K. Miller, T. Brocher, and D. Okaya, 1996, Los Angeles Region Seismic Experiment (LARSE), California Off-shore Seismic Refraction data: USGS Open-File Report 96-27, 29 p.

Abstracts AGU Fall 95

Some Observations on the Basement Rocks Along the LARSE Line. L. T. Silver Structural framework of the Los Angeles Region. T. L. Wright

Tectonics of the Greater Los Angeles Region: Implications for the Lower Crust and Upper Mantle. E. Humphreys

Tectonophysical Setting of the Offshore Part of the Los Angeles Region Seismic Experiment (LARSE) of October 1994. R. G. Bohannon, K. Klitgord

An Overview of Preliminary Seismic Images From the Los Angeles Region Seismic Experiment (LARSE): G. S. Fuis, T. M. Brocher, K. Klitgord, D. A. Okaya, T. M. Henyey, R. W. Clayton, T. Ryberg, W. J. Lutter Overview of Shipboard Word During the Los Angeles Region Seismic Experiment (LARSE). T. M. Brocher, K. D. Klitgord, R. Bohannon, R. Sliter, R. W. Clayton, U. S. ten Brink

Refraction/Wide-Angle Reflection Imaging of the Los Angeles Region Using Onshore-Offshore Data. D. Okaya, J. Bhowmik, M. Robertson, T. Henyey, G. Fuis, J. Murphy, R. Clayton, K. Hafner, J. Norris, M. Benthien, P. Davis, K. Miller

Analysis of the Offshore-Onshore Data Collected During the 1994 LARSE Survey. J. J. Norris, R. W. Clayton

Crustal Structure of the Inner California Borderland-Preliminary Results. U. S. ten Brink, R. Drury, D. Okaya, R. Bohannon, T. Brocher, G. Fuis

An Image of the Upper 5 km From Inversion of First Arrivals From the 1994 LARSE Experiment: Line 1 From Seal Beach to El Mirage Lake. W. J. Lutter, C. Thurber, G. S. Fuis

Crustal Structure Beneath the San Gabriel Mountains, California from (LARSE). K. Hafner, R. W. Clayton

Refined Three-Dimensional Velocity Model for the Central Transverse Ranges and the Los Angeles Basin. E. Hauksson, J. S. Haase

Seismic Properties of Rocks From the Mojave-San Gabriel Region in Southern California. C. L. McCaffree Pellerin, N. I. Christensen, G. Fuis

Structural Features Under Southern California From Teleseisms Recorded During the Los Angeles Region Seismic Experiment Passive Phase. M. D. Kohler, P. M. Davis, M. Benthien, H. Liu, S. Gao, G. Pei

Abstracts: GSA Fall 96

The Inner Borderland and Los Angeles Basin as Revealed by LARSE-94 Data . Norris, Julie J., Clayton, Robert W.

Crustal Structure Beneath the Eastern Transverse Ranges from LARSE-94 Data Hafner, Katrin, Clayton, Robert W.

Seismic Imaging from the Continental Borderland to the Mojave Desert, Southern California: The Los Angeles Region Seismic Experiment (LARSE) . Okaya, David A., Fuis, Gary S., Lutter, William J. Ryberg, Trond, Brocher, Thomas M., Klitgord, Kim D.

The Los Angeles Region Seismic Experiment (LARSE): Seismic Imaging from the Continental Borderland to the Mojave Desert, Southern California . Fuis, Gary S., Lutter, William J., Ryberg, Trond, Brocher, Thomas M., Klitgord, Kim D., Okaya, David A.

Abstracts AGU Fall 96

Transpressional Plate Boundaries: Geophysical Transects across Southern California (LARSE) and New Zealand (SIGHT). T. Henyey, D. Okaya, G. Fuis, T. Brocher, R. Clayton, F. Davey, T. Stern

Stacking Wide-Angle Airgun Signals from the Los Angeles Region Seismic Experiment (LARSE).S. L. Bilek, T. M. Brocher

Refraction/Wide-Angle Reflection Imaging of the LARSE Northridge Transect. D. Okaya, T. Henyey, G. Fuis, T. Brocher

Mid- and Upper-Crustal Structure of the San Gabriel Mountains: Results from the Los Angeles Seismic Experiment (LARSE). T. Ryberg, G. S. Fuis, W. J. Lutter, D. A. Okaya

Mid- and Lower-Crustal Structure Beneath the San Gabriel Mountains, California (LARSE). K. Hafner, R. W. Clayton and E. Hauksson

Evidence for Lateral Crustal Variations and a Dipping Interface in the Lower Crust Analysis of Onshore-Offshore Data from LARSE Line 1 and Line 2. J. J. Norris, R. Clayton and W. Heaton

Oblique-slip Deformation in the San Pedro Basin Offshore Southern California. K. Klitgord and T. Brocher

Abstracts: AGU Spring 97

Crustal Structure of the Inner California Borderland Offshore of the Los Angeles Basin. U. S. ten Brink and J. Zhang

Joint Nonlinear Refraction and Refraction Traveltime Tomography: Application to the Crustal Structure of the California Borderland. J. Zhang, U. S. ten Brink and N Toksoz.