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This report compiles hydrologic observations in southern California associated with the 1994 Mw = 6.7 Northridge, California earthquake. In southern California, the largest ground water level change was a drop of 52 cm at Crystalaire. Most of the steplike water-level changes recorded following the Northridge earthquake agreed in direction with the sign of the calculated coseismic volume strain field. A spring discharge increase at Southern Pacific Springs was reported to have preceded the earthquake by several days. Outside of southern California, water-level changes were also observed, but are not consistent in sign or size with the static strain field of the earthquake sequence. INTRODUCTION On January 17, 1994 an Mw 6.7 earthquake occurred in a northern suburb of Los Angeles, California at 1230 Greenwich Mean Time (GMT), the first earthquake since 1933 to strike directly under an urban area of the United States (Jones et al., 1994). Like other earthquakes of comparable size, the Northridge earthquake affected ground water and surface water not only in its immediate vicinity, but also at distances of hundreds of kilometers. Some of these hydrologic observations are consistent with our present understanding of the response of well-aquifer systems to strain. Other observations, however, are unexplained at this time. This purpose of this report is to present hydrologic observations related to the Northridge earthquake. Although an attempt has been made to locate and include data from as many sources as possible, the report should not be viewed as exhaustive. Where appropriate, the observations are compared with observations from other earthquakes and with the present understanding of earthquake-related hydrologic phenomena. Many of the sites described here also responded to the ML =7.3 Landers earthquake on June 28, 1992 (Roeloffs et al., 1995). For further information about the data described here, the reader is referred to the Appendix, which lists the authors' addresses and field study areas. THE NORTHRIDGE EARTHQUAKE Table 1 lists the time, magnitude, and location of the Northridge earthquake and the largest aftershock. Figure 1 is a map showing the Northridge epicenter and the observation sites referred to in this report. Table 1. Northridge earthquake parameters (Jones et al., 1994). Earthquake Date Time Magnitude Latitude Longitude Depth (UT) (UT)(km) Northridge 17 January 94 12:30 Mw =6.1 34° 12.53' 118° 32.44' 19 Mainshock Northridge 17 January 94 23:33 Mw =5.6 34° 19.58' 118° 41.90' 11 Aftershock Most of the hydrologic phenomena reported here are in response to the Northridge mainshock. One site, however, appears to have responded to the largest of the 3000 aftershocks recorded three weeks after the Northridge mainshock.

Hydrologic effects associated with the January 17, 1994 Northridge, California, earthquake