# Presentation: 078927

## An Assessment of the High-Gain Streckheisen STS2 Seismometer for Routine Earthquake Monitoring in the US

*ISSUE: Is the high-gain STS2 too sensitive to record on-scale significant earthquakes within the continental US.*

## Methods. Three different methods were used to examine the levels of input ground velocity expected during routine earthquake monitoring within the continental US.

- 1. Computing PGV expected at each ANSS backbone station from the USGS hazard maps (Frankel, 2000).
- 2. Modeling seismic wave amplitude (Brune, 1970)
- 	3. Analysis of the Parkfield M6 Earthquake, September 2004

## STS2 Sensitivity:

- 	high-gain STS2 = 20,000 volts/meter/sec
- 	standard-gain STS2 = 1500 volts/meter/sec
- Q330 channel sensitivity = 4.19e5 counts/volt
- Q330 clip level = 8.38e6 = 8388608 counts
***Input ground velocity required to clip Q330***
- 	high-gain STS2 = 0.001 m/sec
- 	standard-gain STS2 = 0.013 m/sec (factor of 13 larger)

- Standard ANSS Backbone Station

## Method I:  PGV computed from 1Hz Spectral Acceleration Map.

- Use hazard map to calculate the expected PGV at each ANSS backbone station.
***Procedure:***
- 1. For each ANSS station determine *%g* from the 1 Hz spectral acceleration map and convert to *g*
- 	(*g = 0.01* %g*) where *1g=980cm/s/s*.
- 2. Compute PGV in m/sec after (Newmark and Hall, 1982)
*	**	**PGV = 594.8 * g / (2*****).*
- 3. Compare PGV to clipping velocity of standard and high-gain recording systems.

**1 Hz spectral acceleration 10% probability of exceedance in 30 years**
- (Frankel et al., 2000)

## Method I: Results

- USGS 10% probability of exceedance in 30 years 1Hz spectral acceleration (Frankel et al., 2000)

- All stations clipped by expected local ground motion.

- Most stations clipped by expected local ground motion except regions far from active earthquakes.

## Method II: Amplitude Modeling using Brune (1970, 1971)

- 1. For each Mw (5-7)  determine Mo from: Mw = 0.667 log(Mo) – 10.7  (*Kanimori, *1977).
- 2. For each Mo determine fault dimension from:
- 		Mo = 2.29r3 dyne-cm  (*Brune *1970, 1971)
- 		where   = 1.0x108 effective stress.
- 3. For each fault dimension, determine source corner frequency from:
- 		fault dimension in cm *(Brune *1970, 1971*).*
- where	** = shear wave velocity = 3.5km/s
- 		*fc* = source corner frequency
- 4. Find angular frequency (*fm*), by bisection, where Brune source spectrum is a maximum given ANSS short-period detection filters (0.5-12Hz).
- 5. For each source-receiver distance (from 25-1100 km), calculate Shear-wave PGV amplitude (*As*) in m/sec from:
-   			*(Brune *1970, 1971*).*
- 			where	** = rigidity
- 				*fc* = source corner frequency
- 				*fm* = effective frequency where amplitude is maximum
- 				** = shear-wave velocity (3.5km/s)
*	**	**	**	**** *= source-receiver distance.
- 5. Apply attenuation to Shear-wave amplitude (As).
- 			where ** = geometric spreading (0.5)
- 				*Q* = Quality factor.
- 			Northern California (Erickson et al. 2004).
- 				*Q* = 105*fm *0.67

## 6.  Multiply the modeled amplitude by the total sensitivity of the high-gain recording system (8.38e9 counts/m/s) and plot corrected PGV as a function of epicentral distance.

## Method III: M6.0 Parkfield Earthquake, September 2004

- COR

- HOPS

- WDC

- MOD

- COR 1003km

- MOD 667km

- WDC 558km

- HOPS 421km

**Lg**

- STS1 Q380

- STS2-SG Q380

- STS1 QX-80

- STS1 Q380

- Seconds from origin time

- Parkfield M6

***Procedure:***
- 1.  Transfer from individual station instrument response to STS2-HG through Q330.
- 2.  Measure peak amplitude (PGV in m/sec) of Lg arrival on BHZ component.

## HOPS, WDC and COR would have clipped with High-Gain STS2

## BHZ

## Recommendation: 

- Based on the analysis of the three methods discussed above it is our recommendation that the standard-gain STS2 is more appropriate for routine earthquake monitoring in the US.
- Method 1 indicates that if the high-gain configuration is used at all ANSS backbone stations, all stations can be expected to clip given the local ground shaking levels expected in the next 30 years.
- Methods 2 and 3 indicate that an M6 will clip stations within approximately 875km of the epicenter.
- The high-gain STS2 may be more useful for recording teleseismic events on a global scale but not suitable for on-scale recording of moderate to large earthquakes at regional distances within the continental US.