Scottcal: Created page with "All CyberShake studies through Study 24.8 utilize an earthquake rupture forecast (ERF) derived from UCERF2. Some details about the construction of this ERF for CyberShake are..."

2026-01-09T06:30:42Z

Created page with "All CyberShake studies through Study 24.8 utilize an earthquake rupture forecast (ERF) derived from UCERF2. Some details about the construction of this ERF for CyberShake are..."

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All CyberShake studies through Study 24.8 utilize an earthquake rupture forecast (ERF) derived from UCERF2. Some details about the construction of this ERF for CyberShake are described below.

== Event Types ==

There are three types of events given by the UCERF2 ERF: regular, median, and aleatory.

=== Regular ===
Regular events represent floating sources in UCERF2, about 85% of all events in CyberShake.
<ol>
<li> Compute target Magnitude Frequency Distribution (MFD) for each source as a Gutenberg-Richter (G-R) distribution up to maximum magnitude determined by two Mw-Area scaling relationships (Ellsworth-B; Hanks and Bakun, 2007). The G-R b-value depends on the fault type: b=0 for type A faults (e.g., the San Andreas), and the average of G-R distributions with b=0 and b=0.8 are used for type B faults. Then average the MFD from the two scaling relationships, which is discretized in 0.1 magnitude bins (i.e., UCERF2 magnitude) to determine the final source MFD.</li>
<li> Extend the down-dip width (DDW) of the fault to match the Somerville (2006) area for a full-fault rupture at the maximum magnitude. (CyberShake-only, not done for standard UCERF2)</li>
<li> For each magnitude bin, compute the rupture area with Somerville 2006 (or, for standard UCERF2, Hanks and Bakun, 2007).</li>
<li> Build N floating ruptures with that area.</li>
<li> Set rate of each rupture to G-R rate for magnitude divided by N (equal weight to each rupture for that magnitude bin).</li>
</ol>

=== Median ===
Median events represent the characteristic sources in UCERF2, whose rupture areas are modified to match the Somerville (2006) scaling relationship, which represent ~2% of events in CyberShake.
<ol>
<li>For each branch on UCERF2 logic tree (Ellsworth-B; Hanks and Bakun, 2007), compute median magnitude from area with given scaling relationship, discretized into 0.1 magnitude bins (i.e., UCERF2 magnitude)</li>
<li>For each source:
<ol>
<li>compute implied area from Somerville 2006 from UCERF2 magnitude</li>
<li>compute DDW correction factor as ratio of implied area to original UCERF2 (U2) area, intended to be: ddwCorrFactor = som06Area / origU2Area 
Note that there is a bug in the UCERF2 implementation such that ddwCorrFactor = som06Area / ellBArea (rather than origU2Area). This resulted in a constant ddwCorrFactor = 1.65959 for characteristic sources (this bug does not affect floating regular sources).</li>
<li>extend DDW such that the new rupture area equals the implied Somerville 2006 area: newDDW = origDDW * ddwCorrFactor (to the nearest integer) 
Note: what this does, holding magnitude constant, is extend the area such that if you were to use Somerville 2006 to compute magnitude it would equal the input UCERF2 magnitude.</li>
</ol>
</li>
</ol>

=== Aleatory ===
Aleatory events have the same rupture areas of Median events, but with varying magnitudes to capture the variations in magnitude-area relationship, which represent ~13% of events in CyberShake.
<ol>
<li>For each Median source, compute the total moment rate</li>
<li>compute moment balanced Gaussian aleatory magnitude distribution with sigma=0.12, two sided truncation at 2 sigma 
Note: this means that the resultant moment rate from this distribution will equal the moment rate from Median (1)</li>
<li>Compute magnitude discretized in 0.1 magnitude bins 
Note: the largest magnitudes in CyberShake are aleatory events that have very large stress drops, which can lead to very high seismic hazard at long return periods.</li>
</ol>

Summary: Both Regular and Median types follow the Somerville 2006 scaling relationship, hence have the same estimated stress drop values. Aleatory events, on the other hand, have a large range of stress drop values.

CyberShake UCERF2 ERF - Revision history

Scottcal: Created page with "All CyberShake studies through Study 24.8 utilize an earthquake rupture forecast (ERF) derived from UCERF2. Some details about the construction of this ERF for CyberShake are..."