METHODOLOGY

Because the historical record of earthquakes in this region is so brief and many faults have neither ruptured in this historic period nor are paleoseismic data available, for most fault zones we had to rely on set of empirical relationships between fundamental earthquake parameters which are described below. Throughout the text we will refer to a so-called standard methodology which presupposes a characteristic earthquake model for a given fault or segment of a fault [Schwartz and Coppersmith, 1984] and applies the procedures and assumptions as described below.

Fault Segmentation or Determination of Source Length (l)

The length of a potential earthquake source is an important parameter for estimating the potential size of an earthquake. Historical data on actual rupture length are most preferred, followed by paleoseismic evidence. Most faults have neither, so current practice relies strongly on a combination of analogy to similar faults or by judgment based on available data of diverse character: e.g., major bends or discontinuities in surface trace or microseismicity alignment are commonly chosen as segment boundaries. In northeastern California proximity to active volcanic centers offers distinct constraints on segment lengths. Tentative segmentation models adopted here are only intended for hazard modeling purposes. The endpoints should be treated as approximate in the seismic hazard analysis. Future paleoseismologic investigation is still required in all cases to test our currently preferred model. For fault location we primarily used a digital version of Jennings [1992] state fault map, but amended this map where greater accuracy was required (e.g., San Andreas, Hayward, Northern Calaveras, Concord and Green Valley faults) or where mapping was incomplete for our purposes (e.g., blind thrust faults of the Great Valley).

Fault Down-Dip Width (w)

Generally we assumed 12 km as a typical value of rupture depth in Northern California used to calculate down-dip width. David Oppenheimer and Ivan Wong reviewed the database and suggested revisions based on USGS Calnet catalog and special investigations. Dip values were taken from a variety of sources, both geologic and seismologic.

Magnitude (M_w)

When reliable historical values of magnitude were available, then they were used. Otherwise, we mostly relied on the empirical relation of moment magnitude to rupture area (a=lw) of Wells and Coppersmith [1994](W&C94):

Figure 1. Comparison of historically observed moment magnitudes in northern California with the empirical rupture area and length relations of Wells and Coppersmith [1994]

Average Coseismic Slip (d)

We used reliable historical or paleoseismological values of coseismic slip where available, otherwise, we derived slip from magnitude generally using the relation of moment magnitude to moment of Hanks and Kanamori [1979]:

Using the definition of seismic moment (M_o) and rigidity (µ = 3*10¹¹ dyne/cm²) gives average coseismic slip:

Only minimum values of geologic or long-term slip rates are available for some larger faults in the region, because they may not represent the entire breadth of the fault zone. Local and regional geodetic strain rates and regional plate tectonic models have been used to check and constrain geologic rates where needed. Slip rate is often the most important indicator of a fault for earthquake potential, because it is strongly related to recurrence time. For this reason the database gives a range of slip rate that may be valid as an important measure of reliability of the model.

Recurrence Time (t)

When reliable historical and paleoseismic values of average recurrence time (t) for events were available, then they were used. Otherwise, we continue to make use of the empirical relationships above and the following equation:

where d is average coseismic slip (as above) and r is slip rate (as above). When considerable aseismic slip or creep occurs on a particular fault, then both the values of average slip and long-term slip rate must reflect the fault behavior in the seismogenic zone at depth. Coseismic surface slip can be much smaller than deep coseismic slip on creeping faults [Oppenheimer and others, 1990; Lienkaemper and others, 1991; Lienkaemper and Prescott, 1989]. Recurrence is a critical parameter for the probabilistic aspect of the hazard map. Sources with recurrence times of >20,000 yr will have little impact on the final maps.