  Carson City Chico Monterey  Stover & Coffman (1993) and NGDC epicenter | |
| UTC time | 1898-03-31 07:43[1][2] |
|---|---|
| USGS-ANSS | ComCat |
| Local date | March 30, 1898 |
| Local time | 23:43 |
| Magnitude | 5.8–6.4Mw[3] |
| Epicenter | 38°12′N122°24′W / 38.2°N 122.4°W /38.2; -122.4[1][2] |
| Type | Unknown |
| Areas affected | San Francisco Bay Area Northern California United States |
| Total damage | $350,000[1] |
| Max. intensity | MMI IX (Violent)[4] |
| Tsunami | Possible[5] |
The1898 Mare Island earthquake occurred inNorthern California onMarch 30 at 23:43 local time with amoment magnitude of 5.8–6.4 and a maximumMercalli intensity of VIII–IX (Severe–Violent). Its area of perceptibility included much of northern and central California andwestern Nevada. Damage amounted to $350,000 (about $10,700,000 inflation adjusted to 2018) and was most pronounced onMare Island, apeninsula in northernSan Francisco Bay. While relatively strong effects there were attributed to vulnerable buildings, moderate effects elsewhere in the San Francisco Bay Area consisted of damaged or partially collapsed structures, and there were media reports of a small tsunami and mostly mild aftershocks that followed.
The mechanism of the shock is unknown, but several independent investigations focused on different aspects to gain a better understanding of the intensity, magnitude, source fault, and epicenter of this pre-instrumental event. Most investigators placed it under or to the north ofSan Pablo Bay, though two earthquake catalogs gave specific coordinates that place it within the confines of theSan Pablo Bay National Wildlife Refuge. One of the numerousstrike-slip faults of the San Andreas Fault System in theNorth Bay are most often named as the source fault, but one seismologist's paper detailed how an unnameddip-slip fault may have been responsible. Several more recent studies gave alternate perspectives that named specific faults as the origin.
TheSan Andreas Fault system (SAF) is a network of right-lateralstrike-slip faults that form a portion of a large complex anddiffuse plate boundary. Thefaults span on and off shore along the California portion of thePacific Rim, and in the area near San Francisco Bay, the extent of the various fault strands are limited to about 80 kilometers (50 mi) wide from east to west. This system of faults terminates in the north at theMendocino triple junction where the north-northwest trending SAF meets the east trendingMendocino fracture zone. It terminates in the south in a more gradual fashion at theSalton Sea where displacement transitions to a series oftransform faults andspreading centers along theGulf of California Rift Zone.[6]
The shock was felt over an area of 120,000 km2, fromChico in the north to Monterey in the south, and toCarson City inwestern Nevada.Toppozada et al. 1992 compared theisoseismal map from the event to those of earthquakes that occurred during the instrumental period to resolve the magnitude. The intensity VIisoseismals for the1969 Santa Rosa and1984 Morgan Hill earthquakes were markedly smaller; only the1989 Loma Prieta earthquake covered a larger area than that of the 1898 event. The isoseismals for intensity V, VII, and VIII were also compared for additional calibration, and 6.7 was presented as the magnitude of the event, which was presumed to have occurred on the southernRodgers Creek Fault. This estimate was later reduced to M6.4 inToppozada & Branum 2002.[1][4][7]
Bakun 1999 also analyzed intensity details to resolve for magnitude and location, resulting in M6.3 and a location at the northeast end of San Pablo Bay. A technique calledBayesian inference used by Wesson, Bakun, & Perkins 2003 showed that the northern Hayward Fault was the most likely source, with the Rodgers Creek, southern Green Valley,Concord, and northern Green Valley faults as the next most likely. Media reports of a small non-destructive tsunami led Parsons et al. 2003 to conclude that a normal fault (rather than a strike-slip fault) may have been responsible for the shock. Another viewpoint fromHecker et al. 2005 was that the Rodgers Creek Fault was not likely the source because theirtrench investigation showed that its most recent event had a maximum slip of 2 m (6 ft 7 in) and that the 1898 event was too small to result in that much displacement.[8]
Another seismologist re-examined the event following the2014 South Napa earthquake and found that the heavy effects on Mare Island were the result of weak or deficient buildings. Comparing the intensity distribution of the two shocks revealed that it was indeed severe on Mare Island as it was likely close to the rupture, which may have involved both strands of the Franklin Fault (between theWest Napa and Rodgers Creek Faults). Although surface rupturing events have been documented on the Hayward–Rodgers Creek Fault System just to the west, nosurface rupture was associated with this earthquake. A magnitude range of 5.8–6.4Mw was given, with the lower bound representing an average stress drop event and the higher bound for a lower stress drop event.[3]
The isoseismal map fromToppozada & Real 1981 places the epicenter to the north ofSan Pablo Bay and shows elongated rings aligned NNW–SSE. The innermost intensity VIII (Severe) ring encompassesVallejo, Mare Island, and much of San Pablo Bay, and also includes three instances of intensity IX (Violent), but the locations cannot be determined with accuracy due to a lack of map resolution. WhileSan Francisco is within the intensity VII (Very strong) ring,Santa Cruz andSan Jose are labeled intensity VI (Strong) exceptions within the intensity V (Moderate) ring.Ukiah,Stockton,Sacramento, andGilroy also lay in the intensity V ring.[9]
Toppozada et al. 1992 presents an updated isoseismal map that focuses on the near field rather than an extended perspective, with slightly more conservative maximum intensities, and locations that are clearly labeled with specific intensities. The innermost isoseismal shows only one location that is an exception to intensity VIII. Tubbs Island, to the northwest of Mare Island, lists VIII–IX.Schellville,Lakeville, and Mare Island all show intensity VIII. A large number of locations are marked VI–VII, includingSanta Rosa,San Rafael,Oakland, and San Francisco. As opposed to showing a specific epicenter, the newer, enhanced map shows that it may have been anywhere within a broad swath centered on the southern Rodgers Creek Fault.[4]
While theNational Geophysical Data Center categorizes the overall effects of the event as moderate, significant damage occurred at theMare Island Naval Shipyard, where several buildings suffered partial or total collapse and some equipment was damaged. Other strong effects occurred at Vallejo and to the southeast inBenicia, where a cannery was damaged and inMartinez where the courthouse was damaged. Other locations inSonoma County that were severely affected included Schellville, Greenwood Estate, and near Petaluma Creek. The effects were less severe in San Francisco, but one building partially collapsed, a girls high school suffered damage valued at several thousand dollars, and soil conditions contributed to damage at the Whittier School.[1][10]
Bay Area newspapers reported on aftershocks that were felt in a number of locations. These shocks were only reported at locations that were within the intensity VII or higher isoseismals. TheSonoma Index-Tribune reported that atSonoma there were four strong aftershocks and more than twenty lighter shocks (that at most just rattled windows) later that night. TheSan Francisco Call stated that while the aftershocks were heaviest near Tubbs Island, they were more frequent near Lakeville at the Petaluma marshes, and this was interpreted as the alluvium under the island suppressing the lighter shocks. Analysis of the severity, location, and frequency of the aftershocks bolsteredToppozada et al. 1992's stance that the origin of the mainshock was the southern Rodgers Creek Fault.[4]
Numerous bay area newspapers reported on various disturbances that were experienced by mariners. In thePacific Ocean, vessels southeast ofPoint Reyes and near theFarallon Islands reported feeling the shock, and a sharp rise in sea level was reported in theSan Francisco Bay and on theNapa River. Upon examination of the event, tsunami experts declared that while the Rodgers Creek Fault is primarily strike-slip, some vertical movement is possible, which is associated with tsunami generation. A five-point scale (0–4) was used to certify whether a tsunami event was legitimate. With 0 intended for "not a valid tsunami report" and 4 being "certainly a valid report", a score of 2 was assigned to the event, meaning that it was "possibly a valid report" based on insufficient information, nonexpert observations, and unclear descriptions.[5]
