The Lorna Prieta, California, Earthquake of October 17, 1989-Hydrologic Disturbances STUART ROJSTACZER, Editor STRONG GROUND MOTION AND GROUND FAILURE THOMAS L. HOLZER, Coordinator - U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1551-E UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994 CONTENTS By Stuart Rojstaczer The origin of the tsunami excited by the earthquake- faulting or slumping ------- ------------ ----------- --------- By Kuo-Fong Ma, Kenji Satake, and Hiroo Kanamori Stream-channel adjustment in Fern Canyon near Watsonville, California, after the earthquake ---------- 11 By Deborah R. Harden and Dennis Fox Sources and magnitudes of increased streamflow in the Santa Cruz Mountains for the 1990 water year after the earthquake---------------------------------------- 31 By Robert R. Curry, Brett A. Emery, and Tom Gentry Kidwell I Hydrologic changes associated with the earthquake in the San Lorenzo and Pescadero drainage basinsÑ--- 51 By Stuart Rojstaczer and Stephen Wolf THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: STRONG GROUND MOTION AND GROUm FAILURE HYDROLOGIC DISTURBANCES INTRODUCTION By Stuart Rojstaczer, Duke University Seismic events have long been known to cause changes upstream of the fault. The postearthquake surveys of this in the level of oceans, streams, lakes, and the water table. channel suggest that earthquakes without significant surface The great San Francisco earthquake of 1906 induced sig- rupture may induce changes in stream-channel morphology nificant hydrologic changes (Lawson, 1908) that were that, over geologic time, can produce the offset streams qualitatively similar to those changes observed for the characteristic of the San Andreas fault. Tectonic deforma- Loma Prieta earthquake. What is different is that the hy- tion of stream channels occurs at a faster rate than the abil- drologic data sets collected from the Loma Prieta event ity of the channel to readjust by aggradation or erosion. have enough detail to enable hypotheses on the causes for Changes in the magnitude of streamflow in response to these changes to be tested. The papers in this chapter doc- the Loma Prieta earthquake are well documented and are ument changes in ocean level, stream morphology and the subject of three papers in this volume. Briggs examines flow, water table height, and ground-water flow rates in changes in stream flow in Waddell Creek. The earthquake response to the earthquake. Although hydrologic distur- induced a transient increase in discharge in the Creek and bances may have occurred about 1 hour before the main associated springs that exponentially declined over a period shock (Roeloffs, 1993), the papers in this chapter deal of one to two months following the earthquake. Spring strictly with postevent hydrologic changes. The hydrologic flow at higher elevations ceased at an earlier time than responses reported here reflect changes that are not the re- spring flow at lower elevations suggesting that the water sult of surface rupture. They appear to be the result of table elevation declined over time. landslides, the static displacements induced by the earth- On a regional scale, Curry and others state that verifi- quake, and changes in the permeability of the near surface. able changes in stream discharge were noted as far away The Monterey Bay tsunami is examined by Ma and oth- as 88 km from the epicenter. Increases in flow that persist- ers. They use elastic half-space models of the earthquake ed for several months following the earthquake were re- in conjunction with a finite difference model of tsunami stricted to the Santa Cruz Mountains and San Francisco motion in Monterey Bay to examine the origin of the tsu- peninsula. The source of this water appears to be the nami. The waveform of the tsunami cannot be explained ground-water system. Chemical analyses of about 100 sur- solely by the static displacement of the ocean floor in- face water and ground-water samples demonstrated mod- duced by the earthquake. To mimic the observed response, est increases in ionic concentration in some, but not all, Ma and others include earthquake-induced slumping of sampled waters. 0.01 km3 of sediment into the ocean from Moss Landing. The ground-water and surface-water response of the San The response of stream morphology to San Andreas fault Lorenzo and Pescadero drainage basins is discussed by motion is the subject of Harden and Fox. Stream channels Rojstaczer and Wolf. Streamflow increased at most gaging in the Santa Cruz Mountains have been altered by prehis- stations within 15 minutes after the earthquake. Ground- toric earthquakes in this region. Harden and Fox have water levels in the highlands parts of the basins were local- made repeat surveys of a channel within Fern Canyon. ly lowered by as much as 21 m within weeks to months This channel was chosen for detailed study because it after the earthquake. Streamflow reduction in these basins shows evidence of geologically recent disruption by the followed an exponential rate similar to that shown by San Andreas fault and because it has a relatively straight Briggs. In the San Lorenzo basin, changes in stream chem- channel that enters the fault zone at a right angle. The sur- istry were significant. Although cationfanion ratios of the veys indicated that the channel elevation increased along a major constituents remained relatively constant, overall 30-m segment upstream of the fault. The middle 17 m of ionic concentrations and the calcite saturation index of the this reach showed an average of 28 cm of aggradation; the streamwater increased. Solute concentrations declined sig- aggradation appears to reflect a decrease in the gradient nificantly within several months after the earthquake. E2 HYDROLOGIC DISTURBANCES Although increases in stream and spring flow have been nient, and it is so generally believed that they are likely to sometimes ascribed to fluid sources from the midcrust, the be permanent, that the inhabitants of the town are begin- cause of streamflow increases examined in three of the pa- ning to build cisterns, in order to accumulate artificial res- pers described above appears to be a permeability increase ervoirs of water (p. 95). of the near-surface aquifers and aquitards. Hence, the ob- The hydrologic changes in response to the Loma Prieta served ground-water and surface-water response are not earthquake are much more local in extent than those of the apparently coupled to the earthquake-generation process. New Madrid, but the impact on communities in the region The permeability increases seem to persist for a period was significant. Unlike the hydrologic disturbances observed longer than the time of increased streamflow. Rojstaczer at the time of the New Madrid earthquakes, the causes for and Wolf present a simple diffusipnal model that mimics these changes appear to be well understood. They reflect the the observed streamflow and ground-water changes. The dynamic response of the ocean, streams, and ground water cause for the permeability increases is not known. Curry to earthquake-induced physical changes in the morphology and others speculate that the permeability increase is due and internal fabric of near-surface rocks and sediment. to microbiologic processes. Rojstaczer and Wolf propose that the permeability increase is due to seismically in- duced fracturing and microfracturing. REFERENCES CITED In the papers presented in this chapter, hydrologic changes associated with the Loma Prieta earthquake are Lawson, A.C., chairman, 1908, The California earthquake of April 18, spatially variable, but the overall character of the changes 1906; Report of the State Earthquake Investigation Commission: can be explained by relatively simple conceptual models. Carnegie Institution of Washington, Publication 87, 451 p. In a study of ground-water disturbances in South Carolina Roeloffs, E., 1993, A reported streamflow increase: U.S. Geological Sur- vey Professional Paper 1550-C, p. 47-52. associated with the New Madrid earthquakes of 181 1- 18 12 Smith, E.D., 1819, On the changes which have taken place in the wells of (Smith, 1819) the following was noted: Whatever may be water situated in Columbia, South-Carolina, since the earthquakes of the cause of this phenomenon, the effects are so inconve- 181 1-12: American Journal of Science, v, 1, p. 93-95. THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: STRONG GROUND MOTION AND GROUND FAILUW HYDROLOGIC DISTURBANCES \ 1 THE ORIGIN OF THE TSUNAMI EXCITED BY THE EARTHQUA=- FAULTING OR SLUMPING By Kuo-Fong Ma, California Institute of Technology, Kenji Satake, University of Michigan, and Hiroo Kanamori, California Institute of Technology CONTENTS amplitude of the observed tsunami, the volume of sedi- ments involved in the slumping is approximately 0.012 h3.Thus the most likely Fause of the tsunami observed Abskact ........................................................... at Monterey is the combination of the vertical uplift of the hkoduction....................................................... sea floor due to the main faulting and a large-scale slump- Dab ............................................................ --- Method .............................. -------------- -- ----------- --- ing near Moss Landing. Fault model ........................... -........................... Results----------------------me-------- ------------- ----- -------- --- Conclusions......................... -------------- -------- ----- --- INTRODUCTION References cited --------------------