Whittier Narrows Earthquake Research Articles

Topography effect at the critical SV-wave incidence: Possible explanation of damage pattern by the Whittier Narrows, California, earthquake of 1 October 1987

Abstract Buildings damaged by the Whittier Narrows, California, earthquake of 1 October 1987 were concentrated in several areas, most of which were located 8 to 10 km from the epicenter (corresponding to the critical incidence for SV waves) and were near topographic irregularities, such as the northern part of Whittier, just south of Puente Hills. The purpose of this study is to show the possibility that this anomalous damage pattern is due to the amplification by the topographic irregularity when SV waves are near-critical incidence. First we examined the accelerograms obtained at the USGS station (7215 Bright Ave.) nearest to downtown Whittier and conclude that the dominantly eastwest motion observed at the station is due to the influence of the building in which accelerograms were recorded. We next demonstrate that strong ground motions recorded by 12 stations in the epicentral area show a polarization direction pattern that is primarily SV -type, which is consistent with a simple thrust fault located at the hypocenter. Then we calculated the response of a two-dimensional hill with the height 0.3 km and the width 2.4 km to 1) a plane SV wave with a nearly critical angle of incidence, 2) a horizontal line force, 3) a Haskell-type 2D dislocation source, and 4) a Bouchon-type 2D multiple crack source. The results show that the amplification due to the hill relative to the flat surface is more than 1.5 for all the source models. Since this amplification is nearly independent of the source type and spectrum, we conclude that the combined effect of the topographic irregularity and critically incident SV waves might be responsible for the concentration of damage observed during the Whittier Narrows earthquake.

Response of Masonry Bed Joints in Direct Shear

This paper examines the horizontal bed joint shear failure mode and the shear load‐displacement behavior of unreinforced brick masonry during static and cyclic loading. Laboratory tests are conducted on masonry samples using a servo‐controlled direct shear apparatus. A total of 44 tests are conducted on old clay unit‐1:2:9 mortar specimens and 9 tests on new clay unit‐1:1.5:4.5 mortar specimens (laboratory specimens). In addition, three tests are conducted on field specimens collected from older brick walls damaged during the Whittier earthquake. In general, under cyclic shear loading, masonry bed joints show a peak strength for the first cycle followed by residual shear strength. Both peak and residual shear strengths are well represented by the Mohr Coulomb criterion, with friction coefficients ranging between 0.64 and 0.75 for the laboratory specimens. The field specimens show lower shear strength values. The pre‐peak response of masonry bed joints can best be represented by a hyperbolic curve and their shear stiffness depends on both the shear displacement and normal load level.

Source complexity of the October 1, 1987, Whittier Narrows earthquake

Long‐ and short‐period body waveform data recorded at regional and teleseismic distances are modeled to obtain the source parameters of the Whittier Narrows earthquake of October 1, 1987. Although the long‐period data can be satisfied by a simple thrust earthquake on a plane striking 280° at a depth of 14 km, we require a double source solution to satisfy both the long‐ and short‐period teleseismic data. The first subevent has a focal mechanism of strike (θ) = 250°, dip (δ) = 30°N, and a slip vector (λ) = 90°; the second is θ = 280°, δ = 40°N, and λ = 98°. The second event occurred 1.0 s after the first, was 5 times larger (1.2 × 1025 dyn cm for the second, 0.23 × 1025 for the first) in terms of seismic moment and was also shallower (12 km, whereas the first subevent was at 15 km). The two subevents probably occurred on a north dipping listric fault but may have occurred on two subparallel faults. These features were determined by studying a variety of phases including teleseismic P and SH and regional Pnl. The Whittier Narrows earthquake was a high stress drop event, but a single value for stress drop cannot be obtained because of the complex nature of the source. If we define the fault area as the aftershock area, we obtain a stress drop of about 750 bars.

A cross section of the Los Angeles Area: Seismically active fold and thrust belt, The 1987 Whittier Narrows earthquake, and earthquake hazard

Retrodeformable cross sections across the Los Angeles area interpret the Pliocene to Quaternary deformation to be a developing basement‐involved fold and thrust belt. The fold and thrust belt is seismically active as evidenced by the 1987 Whittier Narrows earthquake (ML = 5.9) and the 1971 San Fernando earthquake (MW = 6.6). The structural geology of the Los Angeles area is dominated by three major compressional uplift trends: (1) the Palos Verdes anticlinorium and western shelf, (2) the Santa Monica Mountains anticlinorium, and (3) the Verdugo Mountains‐San Rafael Hills and the San Gabriel Mountains. These trends result from major thrust ramps off a detachment(s) at 10–15 km depth. Thrusts of the Verdugo Mountains‐San Rafael Hills and the San Gabriel Mountains reach the surface; the other two uplifts are associated with blind thrusts. Compressional seismicity is concentrated along these thrust ramps. The 1987 Whittier Narrows earthquake probably occurred on the Elysian Park thrust which underlies the Santa Monica Mountains anticlinorium. The thrust interpretation accounts for the geometry of the anticlinorium, the seismological characteristics of the earthquake, and the geometry of coseismic uplift. The earthquake and aftershocks occurred within a structurally complex, narrow zone of Miocene and Pliocene northwest trending faults that cross the anticlinorium at a high angle. These northwest trending faults are interpreted to be reactivated faults now behaving as tears in the Elysian Park thrust and not the result of active right‐lateral deformation extending into the Whittier Narrows area. Our analysis suggests the Whittier Narrows earthquake sequence occurred within a structurally weakened zone along the Elysian Park thrust. We also suggest that the Whittier fault is not an important Quaternary structure and may not be seismogenic. The regional cross section is a nonunique solution, and other possible solutions are considered. Multiple solutions arise from the presence of two intersecting compressional belts in the Los Angeles area: the Transverse Ranges and the northern Peninsular Ranges. The belts may be due to one or more regional detachments and the northern Peninsular Ranges may be northeast or southwest vergent. The deformed top of the crystalline basement along the regional cross section requires a minimum of 15.0 km of north‐south convergence between the Palos Verdes Hills and the San Andreas fault regardless of the structural solution. Restoration of our cross section solution requires 21.4 km of north‐south convergence on top of the crystalline basement (including 6.4 km of slip continuing offshore to make structures of the continental borderland) and 29.7 km of convergence on the basal detachment. Geologic relationships suggest major shortening started between early and late Pliocene time (2.2–4.0 Ma) which yields a minimum convergence rate of 3.8–6.8 mm/yr between the Palos Verdes Hills and the San Andreas fault. Convergence rates for our solution range from 5.4 to 13.5 mm/yr between the edge of the continental borderland and the San Andreas fault. Slip rate estimates for the Elysian Park thrust along the eastern portion of the Santa Monica Mountains anticlinorium are 2.5–5.2 mm/yr. If the 1987 Whittier Narrows earthquake is the characteristic earthquake along the 170‐km‐long anticlinorium, then recurrence intervals are estimated to be 5.6–12.1 years. This recurrence is higher than the historic seismic record along the Santa Monica Mountains anticlinorium and suggests that (1) our slip rates are too high, (2) some crustal shortening is taken up aseismically, and/or (3) earthquakes larger than the 1987 Whittier Narrows earthquake will occur along the anticlinorium, but less frequently.

Source parameters of the October 1, 1987 Whittier Narrows Earthquake from crustal deformation data

Offsets in the regional strain field, generated by the October 1, 1987, Whittier Narrows earthquake, were recorded with large amplitudes on two deep‐borehole dilational strainmeters at distances of 46.7 and 65.5 km from the hypocenter and marginally on instruments at greater distances in the Parkfield area and at Pinon Flat, where laser extensometers also recorded small offsets. These data are insufficient to solve for the location and physical parameters of the earthquake, but by also using the measured elevation changes in the epicentral area, we are able to invert for source models consistent with all available observations of crustal deformation. The source models obtained indicate that slip extends to a depth of about 30 km, well below the recorded aftershock zone. The requirement for deeper (and presumably aseismic) slip derives from the large negative dilatation experienced by the nearest strainmeter (PUBS), but high‐frequency data from the same site exclude any significant slow component of moment release. By ignoring PUBS we obtain a moment of about 0.7 × 1018 N m on a fault that has a strike and dip of about N60°W and 40° down to the north, respectively, and extends to a depth of about 16 km. The most likely reason for the anomalous offset at PUBS appears to be sympathetic slip triggered on a nearby fault by the main shock. Precursive strain during the month prior to the earthquake is not apparent at the nanostrain level in the data from the closest instrument, but the event was accompanied by a change in strain rate at the two nearer sites.

Influence of focal mechanism on peak accelerations of strong motions of the Whittier Narrows, California, earthquake and an aftershock

Focal mechanisms affect the pattern of the peak accelerations of the October 1, 1987, Whittier Narrows earthquake and its October 4 aftershock. The peak accelerations observed on 21 Strong Motion Instrumentation Program and 22 U.S. Geological Survey accelerograms correlate well with the ratio of shear wave amplitude computed from the thrust mechanism of the main shock and the strike‐slip mechanism of the aftershock. This correlation means that seismic energy is radiated from the fault with close to the standard double‐couple radiation pattern at the frequencies 3–6 Hz corresponding to the peak accelerations.

The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis

The October 1, 1987, Whittier Narrows earthquake (ML = 5.9) was located at 34°2.96′N, 118°4.86′W, at a depth of 14.6±0.5 km in the northeastern Los Angeles basin. The focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25°±5° and 65°±5°, respectively. The aftershocks define an approximately circular surface that dips gently to the north, centered at the hypocenter of the mainshock with a diameter of 4–6 km. Hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25° north dipping thrust fault striking west and is confined to depths from 10 to 16 km. Although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting, they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock. A cluster of reverse faulting events on north striking planes occurred within hours after the mainshock, 2 km to the west of the mainshock. The largest aftershock (ML = 5.3) occurred on October 4 and showed mostly right‐lateral faulting on the same north‐northwest striking plane within the hanging wall. Similarly, several left‐lateral focal mechanisms are observed near the eastern edge of the mainshock rupture. The earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays. When relocating the blast using the new models and delays, the absolute hypocentral location bias is less than 0.5 km. The mainshock was followed by nearly 500 locatable aftershocks, which is a small number of aftershocks for this magnitude mainshock. The decay rate of aftershock occurrences with time was fast, while the b value was low (0.67±0.05) for a Los Angeles basin sequence.

Analysis of regional broadband recordings of the 1987 Whittier Narrows, California, earthquake

Recordings of the Whittier Narrows earthquake of October 1, 1987 (BRK ML = 6.0), and the large aftershock of October 4 (BRK ML = 5.5) from broadband seismographs (10 mHz to 10 Hz) at six stations in the Berkeley Seismograph Network in northern California at 430 < Δ < 825 km demonstrate the range of information provided on source mechanisms in such circumstances. Analysis of the waveforms shows that estimation of source mechanisms in this case requires incorporation of the significantly different velocity structures along the Coast Ranges and the Sierra Nevada indicated by the broadband signals. Inversion of the transverse component waveforms, after allowance for regional wave dispersion, provides the following source estimations for the main shock: seismic moment M0 of (1.0±0.06) × 1025; preferred fault plane (strike, rake, and dip) of (259°±33°, 128°±16°, 11°±15°). The main shock mechanism is interpreted as a shallow north dipping reverse fault with the hanging wall moving southeasterly relative to the footwall. Sources of error and of differences with the more robust solution from locally recorded first motions include wave mixing and depth bias at the source. The broadband recordings indicate clearly that the mechanism of the aftershock (M0 of (1.5±0.11) × 1024 dyn cm), although not well resolved by these regional measurements, is quite different from the main shock mechanism.

The 1987 Whittier Narrows, California, earthquake: A Metropolitan shock

Just 3 hours after the Whittier Narrows earthquake struck, it became clear that a heretofore unseen geological structure was seismically active beneath metropolitan Los Angeles. Contrary to initial expectations of strike‐slip or oblique‐slip motion on the Whittier fault, whose north end abuts the aftershock zone, the focal mechanism of the mainshock showed pure thrust faulting on a deep gently inclined surface [Hauksson et al., 1988]. This collection of nine research reports spans the spectrum of seismological, geodetic, and geological investigations carried out as a result of the Whittier Narrows earthquake.Although unseen, the structure was not unforeseen. Namson [1987] had published a retrodeformable geologic cross section (meaning that the sedimentary strata could be restored to their original depositional position) 100 km to the west of the future earthquake epicenter in which blind, or subsurface, thrust faults were interpreted to be active beneath the folded southern Transverse Ranges. Working 25 km to the west, Hauksson [1987] had also found a surprising number of microearthquakes with thrust focal mechanisms south of the Santa Monica mountains, another clue to a subsurface system of thrust faults. Finally, Davis [1987] had presented a preliminary cross section only 18 km to the west of Whittier Narrows that identified as “fault B” the thrust that would rupture later that year. Not only was the earthquake focus and its orientation compatible with the 10–15 km depth and north dipping orientation of Davis' proposed thrust, but fault B appears to continue beneath the northern flank of the Los Angeles basin, skirting within 5 km of downtown Los Angeles, an area of dense commercial high‐rise building development. These results are refined and extended by Davis et al. [this issue].

The Whittier Narrows, California Earthquake of October 1, 1987—Performance of Buildings at California State University, Los Angeles

The Whittier Narrows earthquake (Richter Magnitude 5.9), which struck Southern California in the morning of October 1, 1987, caused some $20.5 million damage to various engineered buildings at California State University, Los Angeles and the death of a student due to a falling precast concrete panel. Peak ground acceleration of 0.39 g and a building response of 0.48 g were recorded in one of the affected buildings. The damage was both structural (cracking of reinforced concrete beams, columns and shear walls) and non-structural (cracked plaster and partition walls, dislodged and broken ceiling tiles, lights and other fixtures, damage to the science, computer and mechanical equipment and to utilities). A fire broke out in one chemistry laboratory and toxic combinations of chemicals were created by the spills in another chemistry laboratory. Shaking from the earthquake also aggravated existing friable asbestos contamination, forcing closure of one and portions of other buildings.

The Whittier Narrows, California Earthquake of October 1, 1987—Response of Elevators

The Whittier Narrows earthquake has been the first earthquake to affect a significant number and many of the same elevator systems that now conform to the more stringent code provisions introduced after the 1971 San Fernando earthquake. While the relative damage potential of the two earthquakes is not now known, much less manchine room and rope damage was observed. The incidence of counterweights coming out of their guide rails was much less than it was from the San Fernando earthquake but still quite common. The performance and operation of elevator earthquake protection devices, seismic switches and counterweight derail detection devices, did not fully meet expectations but they did significantly reduce damage and the potential for injury. There was some inconsistency in the triggering of seismic switches. The seismic performance of elevators suggest that several changes to codes should be considered including a requirement to report earthquake damage and prevent seismic devices from being reset by unauthorized personnel.

The Whittier Narrows, California Earthquake of October 1, 1987—Performance of Tilt-up Buildings

In the 1971 San Fernando earthquake the general performance of tilt-up structures was poor, and a number of collapses occurred. After this event substantial changes were made to seismic code provisions governing the design and construction of these buildings. The October 1, 1987 Whittier Narrows earthquake produced strong shaking (up to Modified Mercalli Intensity VIII) in an area containing thousands of tilt-up buildings of various ages. This recent earthquake thereby provided the first significant test of these new provisions.

The Whittier Narrows, California Earthquake of October 1, 1987—Ticor Title Insurance Building Damage

The Ticor Title Insurance Building was a recently designed building that was significantly damaged by the recent October 1, 1987 Whittier Narrows Earthquake. The building experienced shear wall failures, floor and roof diaphragm failures, as well as failures of spandrel panel anchorages. The damage is chronicled by photographs in this article. This building, which is constructed chiefly of precast elements, shows the importance of tying together adequately different parts of a building. Special attention to building configuration, building deflection, connection and anchorage details is recommended.

The Whittier Narrows, California Earthquake of October 1, 1987—Evaluation of Strengthened and Unstrengthened Unreinforced Masonry in Los Angeles City

On November 15, 1987, the City of Los Angeles' Earthquake Division, launched a study of the performance of its strengthened unreinforced masonry buildings during the Whittier Narrows Earthquake. The objective of the study was twofold: (1) To analyze the damage to unstrengthened as well as to strengthened and tension-anchored-only buildings, and from that analysis (2) to determine the most effective ways of improving the design standards for strengthening unreinforced masonry buildings. The initial part of that objective has been completed, and the findings of that study are the primary basis for this article. Observed damage demonstrated a clear-cut need to improve certain aspects of the Code design standards for strengthened buildings; more importantly, however, it sent out warning signals to owners of unstrengthened buildings and only to a slightly lesser extent to owners of tensioned-anchors-only buildings, of the very serious need to fully strengthen their buildings and to improve on the low probability of those buildings surviving a major earthquake. The second part of the study's objective will require additional work, and the combined effort of the City of Los Angeles and the SEAOSC.

The Whittier Narrows, California Earthquake of October 1, 1987—Preliminary Evaluation of the Performance of Strengthened Unreinforced Masonry Buildings

This report presents preliminary case studies of the performance of selected unreinforced masonry buildings during the Whittier Narrows Earthquakes. Attention is focused on buildings located in the downtown Los Angeles area which have been rehabilitated to conform with the Los Angeles hazard reduction Ordinance. There was no life loss or major injuries attributed to the response of unreinforced masonry buildings to the earthquakes. However, there was a significant number of both rehabilitated and non-strengthened buildings for which masonry peeled off upper story walls. This was mainly due to separation of the outer whythe of brick, or out-of-plane bending failure, and/or in-plane shear failures of wall piers, particularly at building corners. The falling bricks associated with these failures did present an injury hazard so it is important to investigate economical measures to minimize these hazards in future earthquakes.

The Whittier Narrows, California Earthquake of October 1, 1987—Emergency Response

The Whittier Narrows earthquake of October 1, 1987 was the most significant seismic event to occur in southern California since the San Fernando earthquake of February 1971. As such, the 5.9 magnitude temblor provided a basis for evaluating the level of preparedness in at least a portion of the Los Angeles metropolitan region. The Whittier Narrows event provides a hint of the progress that has been made in local, state and private sector preparedness in the last six years as well as suggesting areas that need additional effort.

The Whittier Narrows, California Earthquake of October 1, 1987—CSMIP Strong Motion Data

The Whittier Narrows earthquake of October 1, 1987 generated the largest set of strong-motion records ever obtained from a single earthquake. The California Strong Motion Instrumentation Program (CSMIP) recovered 128 strong-motion records from 101 stations. Of these 101 stations, 63 are ground-response stations and 38 are extensively-instrumented structures. The structures include 27 buildings, eight dams, a suspension bridge, an airport tower, and a power plant. This paper summarizes that data set and highlights records of particular interest. The duration of strong shaking was approximately 3 to 4 seconds at most stations. The maximum peak acceleration values in the CSMIP data set are 0.62 g on the ground and 0.54 g in a structure. The largest acceleration (0.62 g) was recorded at a station near Tarzana, approximately 45 km from the epicenter. Other records of particular interest discussed here include the record from a soft-story building on the Los Angeles State University campus and the records from the Vincent Thomas suspension bridge near Long Beach. Digitization and processing of the accelerograms are underway, and accelerograms from 12 ground-response stations have been digitized as of this writing. The spectra show that the motion at the Tarzana station was dominated by 3 Hz energy. Spectra from other sites are relatively flat and do not show this spectral peak. The attenuation of peak acceleration with distance for this earthquake is compared with the relationship of Joyner and Boore (1981) derived from past earthquakes. On average, the peak acceleration data from this earthquake are higher than would be predicted by the Joyner-Boore model.

The Whittier Narrows, California Earthquake of October 1, 1987—Masonry Building Performance Survey

The observed earthquake response of unreinforced and reinforced masonry buildings during the October 1 and 4, 1987 Whittier Narrows Earthquakes was documented in a comprehensive building survey. This paper describes the extent of the survey, the type of data collected and a preliminary summary of some survey results.

The Whittier Narrows, California Earthquake of October 1, 1987—Buildings at Risk

Current methods of risk assessment require determining the incidence of damage to the elements at risk in order to establish vulnerability. Recent disasters can provide extremely useful information for calibrating various parameters of damage rates. It is necessary to produce estimates of the elements at risk and apply the best information available concerning damage. This requires considering the total physical context in which the disaster occurred. We estimate population, number of buildings, building floor area, and building replacement cost for the Modified Mercalli Intensity VI, VII and VIII areas for the Whittier Narrows Earthquake and calculate incidence rates.

The Whittier Narrows, California Earthquake of October 1, 1987—Simulation of Recorded Accelerations

We have simulated accelerograms from many of the strong motion stations close to the mainshock of the 1987 Whittier Narrows earthquake using a semi-empirical Green's function summation technique. This method allows gross aspects of the source rupture process to be treated deterministically using a kinematic model based on first motion studies, teleseismic modeling and the distribution of aftershocks. Stochastic aspects of the rupture process are then included to simulate irregularity in both rupture and slip velocity. Gross aspects of wave propagation are modeled using theoretical Green's functions calculated with generalized rays. Detailed aspects of the source radiation at high frequencies, as well as unmodeled propagational aspects such as scattering, are included empirically by using multiple recordings of a smaller Imperial Valley earthquake as empirical source functions. Our main objective is to see how well we can predict the peak ground accelerations, time histories and response spectra of ground motions of a moderate sized earthquake within the Los Angeles Basin having limited detailed source information. We find that the simulations predict the observations accurately enough to identify which phases and amplitudes in the observed data may be due to local site response rather than source or radiation effects. Comparisons between observed and simulated accelerograms for all the stations modeled are made using peak ground acceleration, and using time histories and response spectra for the stations that have been hand-digitized to date. The Bright Avenue Whittier station has the largest simulated peak acceleration, in agreement with the recorded peak acceleration data.